Eugene J. Barrett
With the development of multicellular organisms that have specialized tissues and organs, two major systems evolved to communicate and coordinate body functions:
1. The nervous system integrates tissue functions by a network of cells and cell processes that constitute the nervous system and all its subdivisions, as discussed in Chapters 10 through 14.
2. The endocrine system integrates organ function through chemicals that are secreted from endocrine tissues or “glands” into the extracellular fluid. These chemicals, called hormones, are then carried through the blood to distant target tissues, where they are recognized by specific, high-affinity receptors. As discussed in Chapter 3, these receptors may be located either on the surface of the target tissue, within the cytosol, or in the target cell’s nucleus. These receptor molecules allow the target cell to recognize a unique hormonal signal from among the numerous chemicals that are carried through the blood and bathe the body’s tissues. The accuracy and sensitivity of this recognition are remarkable in view of the very low concentration (10−9 to 10−12 M) at which many hormones circulate.
Once a hormone is recognized by its target tissue or tissues, it can exert its biological action by a process known as signal transduction (see Chapter 3). In this chapter, we discuss how the signal transduction cascades couple the hormone to its appropriate end responses (see Chapter 3). Some hormones elicit responses within seconds (e.g., the increased heart rate provoked by epinephrine or the stimulation of hepatic glycogen breakdown caused by glucagon), whereas others may require many hours or days (e.g., the changes in salt retention elicited by aldosterone or the increases in protein synthesis caused by growth hormone [GH]). We also examine the principles underlying the feedback mechanisms that control endocrine function. In Chapters 48 to 52, we see how the principles introduced in this chapter apply to specific endocrine systems.
PRINCIPLES OF ENDOCRINE FUNCTION
Chemical signaling can occur through endocrine, paracrine, or autocrine pathways
As shown in Figure 3-1A, in classic endocrine signaling, a hormone carries a signal from a secretory gland across a large distance to a target tissue. Hormones secreted into the extracellular space can also regulate nearby cells without ever passing through the systemic circulation. This regulation is referred to as paracrine action of a hormone (see Fig 3-1B). Finally, chemicals can also bind to receptors on or in the cell that is actually secreting the hormone and can thus affect the function of the hormone-secreting cell itself. This action is referred to as autocrine regulation (see Fig 3-1C). All three mechanisms are illustrated for individual endocrine systems in subsequent chapters. At the outset, it can be appreciated that summation of the endocrine, paracrine, and autocrine actions of a hormone can provide the framework for a complex regulatory system.
Endocrine Glands The major hormones of the human body are produced by one of seven classic endocrine glands or gland pairs: the pituitary, the thyroid, the parathyroids, the testes, the ovary, the adrenal (cortex and medulla), and the endocrine pancreas. In addition, other tissues that are not classically recognized as part of the endocrine system produce hormones and play a vital role in endocrine regulation. These tissues include the central nervous system (CNS), particularly the hypothalamus, and the gastrointestinal tract, liver, heart, kidney, and others. In some circumstances, particularly with certain neoplasms, nonendocrine tissues can produce hormones that are usually thought to be made only by endocrine glands (see the box titled Neoplastic Hormone Production).
Paracrine Factors Numerous specialized tissues that are not part of the classic endocrine system release “factors” into the extracellular fluid that can signal neighboring cells to effect a biological response. The interleukins, or lymphokines, are an example of such paracrine factors, as are several of the growth factors, such as platelet-derived growth factor (PDGF), fibroblast growth factor, and others. These factors are not hormones in the usual sense. They are not secreted by glandular tissue, and their sites of action are usually (but not always) within the local environment. However, these signaling molecules share many properties of the classic peptide and amine hormones in that they bind to surface receptors and regulate one or more of the specific intracellular signaling mechanisms described in Chapter 3.
The distinction between the hormones of the classic endocrine systems and other biologically active secreted peptides blurs even further in the case of neuropeptides. For example, the hormone somatostatin is a 28–amino acid peptide secreted by the Δ cells of the pancreatic islet, in which it can exert a paracrine action in the regulation of insulin and glucagon secretion (see Chapter 51). However, somatostatin is also made by hypothalamic neurons. Nerve terminals in the hypothalamus release somatostatin into the pituitary portal bloodstream (see Chapter 48). This specialized segment of the circulatory system then carries the somatostatin from the hypothalamus to the anterior pituitary, where it inhibits the secretion of GH. Somatostatin in the hypothalamus is one of several neuropeptides that bridge the body’s two major communication systems.
Hormones may be peptides, metabolites of single amino acids, or metabolites of cholesterol
Although the chemical nature of hormones is diverse, most commonly recognized mammalian hormones can be grouped into one of several classes. Table 47-1 is a list of many of the recognized classic mammalian hormones, which are divided into three groups based on their chemical structure and how they are made in the body.
Table 47-1 Chemical Classification of Selected Hormones
|
Peptide Hormones |
|
ACTH |
|
Atrial natriuretic peptide (ANP) |
|
AVP (ADH) |
|
Calcitonin |
|
Cholecystokinin (CCK) |
|
CRH |
|
FSH |
|
Glucagon |
|
GnRH |
|
GH |
|
GHRH |
|
Inhibin |
|
Insulin |
|
IGFs |
|
LH |
|
Oxytocin |
|
PTH |
|
PRL |
|
Secretin |
|
Somatostatin |
|
TSH |
|
TRH |
|
Vasoactive intestinal peptide (VIP) |
|
Amino Acid–Derived Hormones |
|
DA |
|
Epinephrine (adrenaline) |
|
Norepinephrine (noradrenaline) |
|
Serotonin (5-HT) |
|
T4 |
|
T3 |
|
Steroid Hormones |
|
Aldosterone |
|
Cortisol |
|
Estradiol (E2) |
|
Progesterone |
|
Testosterone |
Peptide hormones include a large group of hormones made by a variety of endocrine tissues. Insulin, glucagon, and somatostatin are made in the pancreas. The pituitary gland makes the following: GH; the two gonadotropin hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH); adrenocorticotropic hormone (ACTH); thyrotropin (also called thyroid-stimulating hormone or TSH); and prolactin (PRL). Parathyroid hormone (PTH) is made in the parathyroid, and calcitonin is made in the thyroid glands.
In addition, other peptide hormones, such as somatostatin and several releasing hormones (e.g., GH-releasing hormone [GHRH]), are made by the hypothalamus. Secretin, cholecystokinin, and other hormones are made by the gastrointestinal tract, yet these tissues are not considered classic endocrine glands.
More restricted numbers of tissues make catecholamines and steroid hormones. Synthesis of these hormones (from tyrosine and cholesterol, respectively) necessitates certain enzymatic steps. Only very specialized tissues are capable of the series of enzymatic conversions necessary to make active hormone from the starting materials. Synthesis of thyroid hormone is even more complex and is essentially entirely restricted to the thyroid gland.
Several glands make two or more hormones. Examples are the pituitary, the pancreatic islets, and the adrenal medulla. However, for the most part, individual cells within these glands are specialized to secrete a single hormone. One exception is the gonadotropin-producing cells of the pituitary, which secrete both FSH and LH.
Neoplastic Hormone Production
The ability of nonendocrine tissue to produce hormones first became apparent with the description of clinical syndromes in which some patients with lung cancer were found to make excessive amounts of AVP, a hormone usually made by the hypothalamus. Shortly afterward, people with other lung or gastrointestinal tumors were found to make ACTH, which is normally made only in the pituitary. Subsequently, many hormone-secreting neoplastic tissues were described. As the ability to measure hormones in tissues has improved and, in particular, as the capability of measuring mRNA that codes for specific peptide hormones has developed, it has become clear that hormone production by neoplastic tissue is quite common, although most tumors produce only small amounts that may have no clinical consequence.
The production of hormones by nonendocrine neoplastic cells has been most clearly defined for cancers of the lung. Several different types of lung cancer occur, each deriving from a different cell line, and yet each is capable of producing one or several hormones. The clinical syndromes that result from secretion of these hormones are often called paraneoplastic syndromes. Thus, lung cancers arising from squamous cells are sometimes associated with hypercalcemia, which results from the secretion of a protein—PTH-related peptide—that can mimic the activity of PTH (see Chapter 52). Small cell lung cancers are notorious for their ability to secrete numerous hormones, including AVP or ADH (with resultant hyponatremia; see Chapter 38 for the box on Syndrome of Inappropriate Antidiuretic Hormone Secretion), ACTH (with resultant Cushing syndrome; see Chapter 50), and many others. Still other types of lung cancer produce other paraneoplastic syndromes.
Nearly all these ectopic, neoplastic sources of hormone produce peptide hormones. Other sources of hormone production, in addition to lung cancer, include gastrointestinal tumors, renal and bladder cancer, neural tumors, unique tumors called carcinoid tumors that can arise almost anywhere in the body, and even lymphomas and melanomas. In some patients, the symptoms and signs resulting from ectopic hormone production may appear before any other reason exists to suspect an underlying neoplasm, and these symptoms may be the key clues to the correct diagnosis.
Hormones can circulate either free or bound to carrier proteins
Once secreted, many hormones circulate freely in the blood until they reach their target tissue. Others form complexes with circulating binding proteins; this use of binding proteins is particularly true for thyroid hormones (thyroxine [T4] and triiodothyronine [T3]), steroid hormones, insulin-like growth factor types 1 and 2 (IGF-1 and IGF-2), and GH.
Forming a complex with a circulating binding protein serves several functions. First, it provides the blood with a reservoir or pool of the hormone, thus minimizing minute-to-minute fluctuations in hormone concentration. Second, it extends the half-life of the hormone in the circulation. For example, more than 99.99% of T4 circulates bound to one of three binding proteins (see Chapter 49); the half-life of circulating T4 is 7 to 8 days, whereas the half-life of free T4 is only several minutes. The hormones bound to plasma binding proteins appear to be those whose actions are long term—in particular, those involving induction of the synthesis of new protein in target tissues. Hormones that play a major short-term role in the regulation of body metabolism (e.g., catecholamines, many peptide hormones) circulate freely without associated binding proteins.
The presence of plasma binding proteins can affect the total circulating concentration of a hormone without necessarily affecting the concentration of unbound or free hormone in the blood. For example, during pregnancy, the liver’s synthesis of T4-binding globulin increases. Because this protein avidly binds T4, the free T4 concentration would fall. However, the pituitary senses the small decline in free T4levels and secretes more TSH. As a result, the thyroid makes more T4, so plasma levels of total T4 rise. However, the free T4 level is unchanged.
Immunoassays allow measurement of circulating hormones
In the late 1950s, Solomon Berson and Rosalyn Yalow demonstrated that in patients who receive insulin, the body forms antibodies directed against the insulin molecule. This observation was important in two respects:
1. It advanced the principle that the body’s immune system can react to endogenous compounds; therefore, autoimmunity or reaction to self-antigens does occur. This notion is a fundamental tenet of our current understanding of many autoimmune diseases, among which are endocrine diseases such as type 1 diabetes mellitus, autoimmune hypothyroidism, and Graves disease, a common form of autoimmune hyperthyroidism. Before the description of insulin autoantibodies, it was thought that the immune system simply did not react to self-antigens.
2. Because antibodies with a high affinity for insulin were induced in patients who were treated with insulin, Berson and Yalow reasoned that these antibodies could be used to measure the amount of insulin in serum. Figure 47-1illustrates the principle of a radioimmunoassay and how it is used to measure the concentration of a hormone (or many other chemicals). If we incubate increasing amounts of a radiolabeled hormone with an antibody to that hormone, the quantity of labeled hormone that is bound to the antibody will yield a saturation plot (Fig. 47-1A). If we now add unlabeled hormone to the incubation mixture, less radioactively labeled hormone will remain complexed to the antibody as unlabeled hormone takes its place. The more unlabeled hormone we add, the less labeled hormone is bound to the antibody (Fig. 47-1B). A displacement curve is created by plotting the amount of radioactively labeled hormone complexed to the antibody as a function of the concentration of unlabeled hormone that is added (Fig. 47-1C). This displacement curve can then be used as a standard curve to estimate the amount of hormone present in unknown samples. This estimate is accurate only if two assumptions hold true: first, that nothing else in the unknown mixture binds with the antibody other than the hormone under study, and second, that nothing in the unknown sample interferes with normal binding of the hormone to the antibody.
Figure 47-1 Principles of the radioimmunoassay.
Antibodies that are highly specific for the chemical structure of interest can frequently be obtained. Moreover, these antibodies are of sufficiently high affinity to bind even the often minute amounts of hormone that is circulating in blood. Thus, radioimmunoassays—and recent adaptations that substitute chemiluminescent or enzymatic detection for radioactivity—have emerged as a potent and popular tool. Radioimmunoassays are now used for the measurement of virtually all hormones, as well as many drugs, viruses, and toxins. Much of our understanding of the physiology of hormone secretion and action has been gained by the use of immunoassay methodology. Yalow shared the 1977 Nobel Prize in Medicine or Physiology for the discovery of the radioimmunoassay (Berson died before the honor was bestowed). (See Note: Rosalyn Yalow)
Hormones can have complementary and antagonistic actions
Regulation of certain complex physiological functions necessitates the complementary action of several hormones. This principle is true both for minute-to-minute homeostasis and for more long-term processes. For example, epinephrine (adrenaline), cortisol, and glucagon each contribute to the body’s response to a short-term period of exercise (e.g., swimming the 50-m butterfly or running the 100-m dash). If any of these hormones is missing, exercise performance is adversely affected, and even more seriously, severe hypoglycemia and hyperkalemia (elevated plasma [K+]) may develop. On a longer time scale, GH, insulin, IGF-1, thyroid hormone, and sex steroids are all needed for normal growth. Deficiency of GH, IGF-1, or thyroid hormone results in dwarfism. Deficiency of sex steroids, cortisol, or insulin produces less severe disturbances of growth.
Integration of hormone action can also involve hormones that exert antagonistic actions. In this case, the overall effect on an end organ depends on the balance between opposing influences. One example is the counterpoised effects of insulin and glucagon on blood glucose levels. Insulin lowers glucose levels by inhibiting glycogenolysis and gluconeogenesis in the liver and by stimulating glucose uptake into muscle and adipose tissue. Glucagon, in contrast, stimulates hepatic glycogenolysis and gluconeogenesis. Whereas glucagon does not appear to antagonize glucose uptake directly, epinephrine (which, like glucagon, is released in response to hypoglycemia) does. Balancing of tissue function by opposing humoral effector mechanisms appears to be an important regulatory strategy for refining the control of many cellular functions.
Endocrine regulation occurs through feedback control
The key to any regulatory system is its ability to sense when it should increase or decrease its activity. For the endocrine system, this function is accomplished by feedback control of hormone secretion (Fig. 47-2A). The hormone-secreting cell functions as a sensor that continually monitors the circulating concentration of some regulated variable. This variable may be a metabolic factor (e.g., glucose concentration) or the activity of another hormone. When the endocrine gland senses that too much (or too little) of the regulated variable is circulating in blood, it responds by decreasing (or increasing) the rate of hormone secretion. This response, in turn, affects the metabolic or secretory behavior of the target tissue, which may either directly feed back to the sensing cell or stimulate some other cell that eventually signals the sensor regarding whether the altered function of the endocrine gland has been effective.
Figure 47-2 Feedback control of hormone secretion. A, A sensor (e.g., a β cell in a pancreatic islet) detects some regulated variable (e.g., plasma [glucose]) and responds by modulating its secretion of a hormone (e.g., insulin). This hormone, in turn, acts on a target (e.g., liver or muscle) to modulate its production of another hormone or a metabolite (e.g., glucose), which may affect a second target (e.g., making glucose available to the brain). In addition, the other hormone or metabolite feeds back on the original sensor cell. B, Under the influence of the cerebral cortex, the hypothalamus releases CRH, which stimulates the anterior pituitary to release ACTH, which, in turn, stimulates the adrenal cortex to release cortisol. The cortisol acts on certain effector organs. In addition, the cortisol feeds back on both the anterior pituitary and the hypothalamus.
A simple example is insulin secretion by the β cells of the pancreas. Increases in plasma [glucose] are sensed by the β cell, which secretes insulin in response. The rise in plasma [insulin] acts on the liver to decrease the synthesis of glucose and on the muscle to promote the storage of glucose. As a result, plasma [glucose] falls, and this decrease is sensed by the β cell, which reduces the rate of insulin secretion. This arrangement represents a very simple feedback system. Other systems can be quite complex; however, even this simple system involves the recognition of two circulating signals. The liver and muscle recognize the increase in plasma [insulin] as one signal, and the pancreatic β cell (the cell responsible for insulin secretion) recognizes the signal of a rise or decline in blood [glucose] as the other signal. In each case, the sensing system within a particular tissue is linked to an effector system that transduces the signal to the appropriate biological response.
Endocrine regulation can involve hierarchic levels of control
Faced with a stressor (e.g., a severe infection or extensive blood loss), the cerebral cortex stimulates the hypothalamus to release a neuropeptide called corticotropin-releasing hormone (CRH; Fig. 47-2B). Carried by the pituitary portal system (blood vessels that connect the hypothalamus to the anterior pituitary), CRH stimulates the anterior pituitary to release another hormone, ACTH, which, in turn, stimulates the adrenal cortical cells to synthesize cortisol. Cortisol regulates vascular tone as well as metabolic and growth functions in a variety of tissues.
This stress response therefore involves two glands, the pituitary and the adrenal cortex, as well as specialized neuroendocrine tissue in the hypothalamus and the CNS. This hierarchic control is regulated by feedback, just as in the simple feedback between plasma [glucose] and insulin. Within this CRH-ACTH-cortisol axis, feedback can occur at several levels. Cortisol inhibits the production of CRH by the hypothalamus, as well as the sensitivity of the pituitary to a standard dose of CRH, which directly reduces ACTH release.
Feedback in hierarchic endocrine control systems can be quite complex and frequently involves interaction between the CNS and the endocrine system. Other examples are regulation of the female menstrual cycle (see Chapter 55) and regulation of GH secretion (see Chapter 48).
Among the classic endocrine tissues, the pituitary (also known as the hypophysis) plays a special role (Fig. 47-3). Located at the base of the brain, just below the hypothalamus, the pituitary resides within a saddle-shaped cavity called the sella turcica (from the Latin sella [saddle] + turcica [Turkish]), which has bony anterior, posterior, and inferior borders and fibrous tissue that separate it from venous sinuses on either side. The human pituitary is composed of both an anterior lobe and a posterior lobe. Through vascular and neural connections, the pituitary bridges and integrates the neural and endocrine mechanisms of homeostasis. The pituitary is a highly vascular tissue. The posterior pituitary receives arterial blood, whereas the anterior pituitary receives only portal venous inflow from the median eminence. The pituitary portal system is particularly important in its function of carrying neuropeptides from the hypothalamus and pituitary stalk to the anterior pituitary.
Figure 47-3 The hypothalamopituitary axis. The pituitary (or hypophysis) is actually two glands—an anterior pituitary and a posterior pituitary (or neurohypophysis). Although in both cases the hypothalamus controls the secretion of hormones by the pituitary, the mechanisms are very different. Anterior pituitary: Small-bodied neurons in the hypothalamus secrete releasing and inhibitory factors into a rich, funnel-shaped plexus of capillaries that penetrates the median eminence and surrounds the infundibular recess. The cell bodies of these neurons are found in several nuclei that surround the third ventricle. These include the arcuate, the paraventricular and ventromedial nuclei, and the medial preoptic and periventricular regions. The capillaries (primary plexus), which are outside of the blood-brain barrier, coalesce into longportal veins that carry the releasing and inhibitory factors down the pituitary stalk to the anterior pituitary. Other neurons secrete their releasing factors into a capillary plexus that is much farther down the pituitary stalk; short portal veins carry these releasing factors to the anterior pituitary. There, the portal veins break up into the secondary capillary plexus of the anterior pituitary and deliver the releasing and inhibitory factors to the “troph” cells that actually secrete the anterior pituitary hormones (TSH, ACTH, PRL) that enter the systemic bloodstream and distribute throughout the body. Posterior pituitary: Large neurons in the paraventricular and supraoptic nuclei of the hypothalamus actually synthesize the hormones AVP and oxytocin (OT). These hormones travel down the axons of the hypothalamic neurons to the posterior pituitary where the nerve terminals release the hormones, like neurotransmitters, into a rich plexus of vessels.
The anterior pituitary regulates reproduction, growth, energy metabolism, and stress response
Glandular tissue in the anterior lobe of the pituitary synthesizes and secretes six peptide hormones: GH, TSH, ACTH, LH, FSH, and PRL. In each case, secretion of these hormones is under the control of hypothalamic releasing hormones (Table 47-2). The sources of these releasing hormones are small-diameter neurons located mainly in the periventricular portion of the hypothalamus that surrounds the third ventricle. These small-diameter neurons synthesize the releasing hormones and discharge them into the median eminence and neural stalk, where they enter leaky capillaries—which are not part of the blood-brain barrier (see Chapter 11). The releasing hormones then travel through the pituitary portal veins to the anterior pituitary. Once in the anterior pituitary, a releasing factor stimulates specialized cells to release a particular peptide hormone into the systemic bloodstream. The integrative function of the anterior pituitary can be appreciated by realizing that the main target for four of the anterior pituitary hormones (i.e., TSH, ACTH, and LH/FSH) is other endocrine tissue. Thus, these four anterior pituitary hormones are themselves releasing hormones that trigger the secretion of specific hormones. For example, TSH causes the follicular cells in the thyroid gland to synthesize and release thyroid hormones. The mechanism by which the pituitary regulates these endocrine glands is discussed in detail in Chapters 48 to 52 Chapter 49 Chapter 50 Chapter 51 Chapter 52.
Table 47-2 Hypothalamic and Pituitary Hormones
GH also acts as a releasing factor in that it regulates the production of another hormone, IGF-1. IGF-1 is made in principally nonendocrine tissues (e.g., liver, kidney, muscle, and cartilage). Nevertheless, IGF-1 in the circulation feeds back on the hypothalamus and on the pituitary to inhibit GH secretion. In this respect, the GH–IGF-1 axis is similar to axes involving classic pituitary pathways, such as the thyrotropin-releasing hormone (TRH)–TSH axis.
Regulation of PRL secretion differs from that of other anterior pituitary hormones in that no endocrine feedback mechanism has yet been identified. The pituitary secretes PRL at relatively low levels throughout life both in boys and men and in girls and women. However, its major biological action, promotion of lactation, is important only in women and only at specific times in a woman’s life. Although PRL is not part of an identified feedback system, its release is controlled. Left to its own devices, the anterior pituitary would secrete high levels of PRL. However, secretion of PRL is normally inhibited by the release of dopamine (DA) from the hypothalamus (see Chapter 56). During breast stimulation, neural afferents inhibit hypothalamic DA release, thus inhibiting release of the inhibitor and permitting lactation to proceed. PRL receptors are present on multiple tissues other than the breast. However, other physiological actions beyond lactation have not been well characterized.
The posterior pituitary regulates water balance and uterine contraction
Unlike the anterior pituitary, the posterior lobe of the pituitary is actually part of the brain. The posterior pituitary (or neurohypophysis) contains the nerve endings of large-diameter neurons whose cell bodies are in the supraoptic and paraventricular nuclei of the hypothalamus (Fig. 47-3). Recall that the hypothalamic neurons that produce releasing factors, which act on “troph” cells in the anterior pituitary, are small-diameter neurons. The large-diameter hypothalamic neurons synthesize arginine vasopressin (AVP) and oxytocin and then transport these hormones along their axons to the site of release in the posterior pituitary. Thus, like the anterior pituitary, the posterior pituitary releases peptide hormones. Also as in the anterior pituitary, release of these hormones is under ultimate control of the hypothalamus. However, the hypothalamic axons traveling to the posterior pituitary replace both the transport of releasing factors by the portal system of the anterior pituitary and the synthesis of hormones by the anterior pituitary “troph” cells. Although the posterior pituitary is part of the brain, it is one of the so-called circumventricular organs (see Chapter 11) whose vessels breach the blood-brain barrier and allow the secreted AVP and oxytocin to reach the systemic circulation.
AVP, or antidiuretic hormone (ADH), is a neuropeptide hormone that acts on the collecting duct of the kidney to increase water reabsorption (see Chapter 38). Oxytocin is the other neuropeptide secreted by the posterior pituitary. However, its principal biological action relates to stimulation of smooth muscle contraction by the uterus during parturition and by the mammary gland during suckling (see Chapter 56).
These two posterior pituitary hormones appear to have a common ancestor—vasotocin—in amphibians and other submammalian species. The two peptide hormones secreted by the posterior pituitary are each made by hypothalamic neurons as a precursor molecule that is transported along the axons of the hypothalamic neurons to the posterior pituitary. For AVP, this precursor protein is proneurophysin II, as detailed in Chapter 40 and illustrated in Figure 40-8, whereas for oxytocin it is proneurophysin I. In each case, cleavage of the precursor occurs during transport along the axons from the hypothalamus to the posterior pituitary. When the active neurohormone is secreted (e.g., AVP), its residual neurophysin is co-secreted stoichiometrically. Defects in the processing of the neurophysin precursor can lead to impaired and secretion of active hormone. In the case of AVP, the result is partial or complete diabetes insipidus.
PEPTIDE HORMONES
Specialized endocrine cells synthesize, store, and secrete peptide hormones
Organisms as primitive as fungi secrete proteins or peptides in an effort to respond to and affect their environment. In more complex organisms, peptide hormones play important developmental and other regulatory roles. Transcription of peptide hormones is regulated by both cis-and trans-acting elements (see Chapter 4). When transcription is active, the mRNA is processed in the nucleus, and the capped message moves to the cytosol, where it associates with ribosomes on the rough endoplasmic reticulum. These peptides are destined for secretion because an amino acid signal sequence present near the N terminus targets the protein to the endoplasmic reticulum while the protein is still associated with the ribosome (see Fig. 2-15).
With minor modification, the secretory pathway that is illustrated in Figure 2-18 describes the synthesis, processing, storage, and secretion of peptides by a wide variety of endocrine tissues. Once the protein is in the lumen of the endoplasmic reticulum, processing (e.g., glycosylation or further proteolytic cleavage) yields the mature, biologically active hormone. This processing occurs in a very dynamic setting. The protein is first transferred to the cis-Golgi domain, then through to the trans-Golgi, and finally to the mature, membrane-bound secretory vesicle or granule in which the mature hormone is stored before secretion. This pathway is referred to as the regulated pathway of hormone synthesis because external stimuli can trigger the cell to release hormone that is stored in the secretory granule, as well as to promote the synthesis of additional hormone. For example, binding of GHRH to somatotrophs causes them to release GH.
A second pathway of hormone synthesis is the constitutive pathway. Here, secretion occurs more directly from the endoplasmic reticulum or vesicles formed in the cis-Golgi. Secretion of hormone, both mature and partially processed, by the constitutive pathway is less responsive to secretory stimuli than is secretion by the regulated pathway.
In both the regulated and constitutive pathways, fusion of the vesicular membrane with the plasma membrane—exocytosis of the vesicular contents—is the final common pathway for hormone secretion. In general, the regulated pathway is capable of secreting much larger amounts of hormone—on demand—than is the constitutive pathway. However, even when stimulated to secrete its peptide hormone, the cell typically secretes only a very small amount of the total hormone present in the secretory granules. To maintain this secretory reserve, many endocrine cells increase the synthesis of peptide hormones in response to the same stimuli that trigger secretion.
Peptide hormones bind to cell surface receptors and activate a variety of signal transduction systems
Once secreted, most peptide hormones exist free in the circulation. As noted earlier, this lack of binding proteins contrasts with the situation for steroid and thyroid hormones, which circulate bound to plasma proteins. IGF-1 and IGF-2 are an exception to this rule: at least six plasma proteins bind these peptide growth factors.
While traversing the circulation, peptide hormones encounter receptors on the surface of target cells. These receptors are intrinsic membrane proteins that bind the hormone with very high affinity (typically, Kdranges from 10−8 to 10−12 M). Examples of several types of peptide hormone receptors are shown in Figure 47-4. Each of these receptors has already been introduced in Chapter 3. The primary sequence of most peptide hormone receptors is known from molecular cloning, mutant receptors have been synthesized, and the properties of native and mutant receptors have been compared to assess primary structural requirements for receptor function. Despite this elegant work, too little information is available on the three-dimensional structure of these membrane proteins for us to know just how the message that a hormone has bound to the receptor is transmitted to the internal surface of the cell membrane. However, regardless of the details, occupancy of the receptor can activate many different intracellular signal transduction systems (Table 47-3) that transfer the signal of cell activation from the internal surface of the membrane to intracellular targets. The receptor provides the link between a specific extracellular hormone and the activation of a specific signal transduction system. We discussed each of these signal transduction systems in Chapter 3. Here, we briefly review the various signal transduction systems through which peptide hormones act.
Figure 47-4 A to F, Receptors and downstream effectors for peptide hormones. AC, adenylyl cyclase; JAK, Janus kinase or just another kinase.
Table 47-3 Peptide Hormones and Their Signal Transduction Pathways
G Proteins Coupled to Adenylyl Cyclase cAMP, the prototypic second messenger, was discovered during an investigation of the action of glucagon on glycogenolysis in the liver. In addition to playing a role in hormone action, cAMP is involved in such diverse processes as lymphocyte activation, mast cell degranulation, and even slime mold aggregation.
As summarized in Figure 47-4A, binding of the appropriate hormone (e.g., PTH) to its receptor initiates a cascade of events (see Chapter 3): (1) activation of a heterotrimeric G protein (αs or αi); (2) activation (by αs) or inhibition (by αi) of a membrane-bound adenylyl cyclase; (3) formation of intracellular cAMP from ATP, catalyzed by adenylyl cyclase; (4) binding of cAMP to the enzyme protein kinase A (PKA); (5) separation of the two catalytic subunits of PKA from the two regulatory subunits; (6) phosphorylation of serine and threonine residues on a variety of cellular enzymes and other proteins by the free catalytic subunits of PKA that are no longer restrained; and (7) modification of cellular function by these phosphorylations. The activation is terminated in two ways. First, phosphodiesterases in the cell degrade cAMP. Second, serine/threonine-specific protein phosphatases can dephosphorylate enzymes and proteins that had previously been phosphorylated by PKA.
G Proteins Coupled to Phospholipase C As summarized in Figure 47-4B, binding of the appropriate peptide hormone (e.g., AVP) to its receptor initiates the following cascade of events (see Chapter 3): (1) activation of Gαq; (2) activation of a membrane-bound phospholipase C (PLC); and (3) cleavage of phosphatidylinositol 4, 5-biphosphate (PIP2) by this PLC, with the generation of two signaling molecules, inositol 1, 4, 5-triphosphate (IP3) and diacylglycerol (DAG); (4) the first step in the IP3 fork of the pathway is binding of IP3 to a receptor on the cytosolic surface of the endoplasmic reticulum; (5) release of Ca2+ from internal stores, which causes [Ca2+]i to rise by several-fold; (6) activation of Ca2+-dependent kinases (e.g., Ca2+-calmodulin–dependent protein kinases, protein kinase C [PKC]) by the increases in [Ca2+]i; and (7) alteration of cell function.
The first step in the DAG fork of the pathway is (4) allosteric activation of PKC by DAG (the activity of this enzyme is also stimulated by the increased [Ca2+]i) and (5) phosphorylation of a variety of proteins by PKC, which is activated in the plane of the cell membrane. An example of a hormone whose actions are in part mediated by DAG is TSH.
G Proteins Coupled to Phospholipase A2 As summarized in Figure 47-4C, some peptide hormones (e.g., TRH) activate phospholipase A2 (PLA2) through the following cascade (see Chapter 3): (1) activation of Gαq or Gα11, (2) stimulation of membrane-bound PLA2 by the activated Gα, (3) cleavage of membrane phospholipids by PLA2 to produce lysophospholipid and arachidonic acid, and (4) conversion—by certain enzymes—of arachidonic acid into a variety of biologically active eicosanoids (e.g., prostaglandins, prostacyclins, thromboxanes, and leukotrienes).
Pseudohypoparathyroidism
Inasmuch as G proteins are part of the signaling system involved in large numbers of hormone responses, molecular alterations in G proteins could be expected to affect certain signaling systems. In the disorder pseudohypoparathyroidism, the key defect is an abnormality in a stimulatory α subunit (αs) of a heterotrimeric G protein. The result is an impairment in the ability of PTH to regulate body calcium and phosphorus homeostasis (see Chapter 52). Patients with this disorder have a low serum [Ca2+] and high serum phosphate level, just like patients whose parathyroid glands have been surgically removed. However, patients with pseudohypoparathyroidism have increased circulating concentrations of PTH; the hormone simply cannot act normally on its target tissue, hence the term pseudohypoparathyroidism. These individuals also have an increased risk of hypothyroidism, as well as of gonadal dysfunction in women. These additional endocrine deficiencies arise from the same defect in signaling.
Guanylyl Cyclase Other peptide hormones (e.g., atrial natriuretic peptide) bind to a receptor (Fig. 47-4D) that is itself a guanylyl cyclase that converts cytoplasmic guanosine triphosphate to cGMP (see Chapter 3). In turn, cGMP can activate cGMP-dependent kinases, phosphatases, or ion channels.
Receptor Tyrosine Kinases For some peptide hormones, notably insulin and IGF-1 and IGF-2, the hormone receptor (Fig. 47-4E) itself possesses tyrosine kinase activity (see Chapter 3). This property is also true for other growth factors, including PDGF and epidermal growth factor. Occupancy of the receptor by the appropriate hormone increases kinase activity. For the insulin and IGF-1 receptor, as well as for others, this kinase autophosphorylates tyrosines within the hormone receptor, as well as substrates within the cytosol, thus initiating a cascade of phosphorylation reactions.
Tyrosine Kinase–Associated Receptors Some peptide hormones (e.g., GH) bind to a receptor that, when occupied, activates a cytoplasmic tyrosine kinase (Fig. 47-4F), such as a member of the JAK (Janus kinase) family of kinases (see Chapter 3). As for the receptor tyrosine kinases, activation of these receptor-associated kinases initiates a cascade of phosphorylation reactions.
AMINE HORMONES
Amine hormones are made from tyrosine and tryptophan
Four major amine hormones are recognized. The adrenal medulla makes the catecholamine hormones epinephrine and norepinephrine from the amino acid tyrosine (see Fig. 13-8C). These hormones are the principal active amine hormones made by the endocrine system. In addition to its role as a hormone, norepinephrine also serves as a neurotransmitter by the CNS (see Chapter 13) and by postganglionic sympathetic neurons (see Chapter 14). DA, which is also synthesized from tyrosine, acts as a neurotransmitter (see Chapter 13); it is synthesized in certain other tissues, but its functional role outside the nervous system is not well clarified. Finally, the hormone serotonin is made from tryptophan (see Fig. 12-8B) by endocrine cells that are located within the gut mucosa. Serotonin appears to act locally to regulate both motor and secretory function of the gut.
The human adrenal medulla secretes principally epinephrine (see Chapter 50). The final products are stored in vesicles called chromaffin granules. Secretion of catecholamines by the adrenal medulla appears to be mediated entirely by stimulation of the sympathetic division of the autonomic nervous system (see Chapter 14). Unlike the situation for many peptide hormones, in which the circulating concentration of the hormone (e.g., TSH) negatively feeds back on secretion of the releasing hormone (e.g., TRH), the amine hormones do not have such a hierarchic feedback system. Rather, the feedback of amine hormones is indirect. The higher control center does not sense circulating levels of the amine hormones (e.g., epinephrine), but rather a physiological end effect of that amine hormone (e.g., blood pressure; see Chapter 23). The sensor of the end effect may be a peripheral receptor (e.g., stretch receptor) that communicates to the higher center (e.g., the CNS), and the efferent limb is the sympathetic outflow that determines release of the amine.
Serotonin (5-hydroxytryptamine [5-HT]), in addition to being an important neurotransmitter in the CNS (see Fig. 13-7B), is a hormone made by neuroendocrine cells, principally located within the lining of the small intestine and larger bronchi. Unlike the other hormones that we discuss in this chapter, serotonin is not made by a specific gland. Little is known about feedback regulation or even regulation of secretion of this hormone. Serotonin arouses considerable clinical interest because of the dramatic clinical presentation of patients with unusual tumors—called carcinoid tumors—of serotonin-secreting cells. These individuals frequently present with a carcinoid syndromecharacterized by episodes of spontaneous, intense flushing in a typical pattern involving the head and neck, associated with diarrhea, bronchospasm, and occasionally right-sided valvular heart disease. The primary tumors involved can occur within the intestinal tract, in the bronchial tree, or more rarely at other sites.
Amine hormones act through surface receptors
Once secreted, circulating epinephrine is free to associate with specific adrenergic receptors or adrenoceptors located on the surface membranes of target cells. Numerous types of adrenoceptors exist and are generically grouped as α or β, each of which has several subtypes (see Table 15-2). All adrenoceptors, isolated from a variety of tissues and species, are classic G protein–coupled receptors. As indicated in Figure 47-5, the intracellular action of a specific catecholamine is determined by the complement of receptors present on the surface of a specific cell. For example, when epinephrine binds to the β1-adrenergic receptor, it activates a Gαs protein, which stimulates adenylyl cyclase, promotes increases in [cAMP]i, and thus enhances the activity of PKA (see Table 15-2). In contrast, when the same hormone binds to a cell displaying principally α2 receptors, it activates a Gαi protein, which inhibits adenylyl cyclase, diminishes [cAMP]i, and therefore reduces PKA activity. Thus, the response of a specific tissue to adrenergic stimulation (whether through circulating epinephrine or through norepinephrine released locally by sympathetic neurons) is determined by the receptor repertoire displayed by the cell. The same is true for DA; the DA-1 (D1) receptor is coupled to Gαs and the DA-2 (D2) receptor is linked to Gαi.
Figure 47-5 Catecholamine receptors. The β1, β2, and D1 receptors all interact with Gαs, which activates AC and raises levels of cAMP. The α2 and D2 receptors interact with Gαi, which inhibits AC. Additionally, the α1 receptor interacts with Gαq, which activates PLC, which, in turn, converts phosphoinositides in the cell membrane to IP3 and DAG.
Epinephrine has a greater affinity for β-adrenergic receptors than for α-adrenergic receptors, whereas norepinephrine acts predominantly through α-adrenergic receptors. The various signal transduction systems linked to these receptors are discussed in Chapter 3. β-Adrenergic stimulation occurs through the adenylyl cyclase system. The α2-adrenergic receptor also usually acts through adenylyl cyclase. However, α1-adrenergic stimulation is linked to Gαq, which activates a membrane-associated PLC that liberates IP3 and DAG. IP3 can release Ca2+ from intracellular stores, and DAG directly enhances the activity of PKC. Combined, these actions enhance the cellular activity of Ca2+-dependent kinases, which produce a metabolic response that is characteristic of the specific cell. The response to adrenergic agonists may be, for example, glycogenolysis in the liver or muscle (predominantly a β effect), contraction (an α1 effect) or relaxation (a β2 effect) of vascular smooth muscle, a change in the inotropic or chronotropic state of the heart (a β1 effect), or various other effects.
STEROID AND THYROID HORMONES
Cholesterol is the precursor for the steroid hormones: cortisol, aldosterone, estradiol, progesterone, and testosterone
The family of hormones called steroids shares a common biochemical parentage: all are synthesized from cholesterol. Only two tissues in the body possess the enzymatic apparatus to convert cholesterol to active hormones. The adrenal cortex makes cortisol (the main glucocorticoid hormone), aldosterone (the principal mineralocorticoid in humans), and androgens. The gonads make either estrogen and progesterone (ovary) or testosterone (testes). In each case, production of steroid hormones is regulated by trophic hormones released from the pituitary. For aldosterone, the renin-angiotensin system also plays an important regulatory role.
The pathways involved in steroid synthesis are summarized in Figure 47-6. Cells that produce steroid hormones can use, as a starting material for hormone synthesis, the cholesterol that is circulating in the blood in association with low-density lipoprotein (LDL; see Chapter 46). Alternatively, these cells can synthesize cholesterol de novo from acetate (see Fig. 46-16). In humans, LDL cholesterol appears to furnish ~80% of the cholesterol used for steroid synthesis (Fig. 47-6). An LDL particle contains both free cholesterol and cholesteryl esters, in addition to phospholipids and protein. The cell takes up this LDL particle through the LDL receptor and receptor-mediated endocytosis (see Chapter 2) into clathrin-coated vesicles. Lysosomal hydrolases then act on the cholesteryl esters to release free cholesterol. The cholesterol nucleus, whether taken up or synthesized de novo, subsequently undergoes a series of reactions that culminate in the formation of pregnenolone, the common precursor of all steroid hormones. Through divergent pathways, pregnenolone is then further metabolized to the major steroid hormones: the mineralocorticoid aldosterone and the glucocorticoid cortisol (see Fig. 50-2), the estrogen estradiol (see Fig. 55-10), and the androgen testosterone (see Fig. 54-5).
Figure 47-6 Uptake of cholesterol and synthesis of steroid hormones from cholesterol. The cholesterol needed as the starting material in the synthesis of steroid hormones comes from two sources. Approximately 80% is taken up as LDL particles through receptor-mediated endocytosis. The cell synthesizes the remaining cholesterol de novo from acetyl coenzyme A (acetyl CoA). VLDL, very-low-density lipoprotein.
Unlike the peptide and amine hormones considered earlier, steroid hormones are not stored in secretory vesicles before their secretion (Table 47-4). For these hormones, synthesis and secretion are very closely linked temporally. Steroid-secreting cells are capable of increasing the secretion of steroid hormones many-fold within several hours. The lack of a preformed storage pool of steroid hormone does not appear to limit the effectiveness of these cells as an endocrine regulatory system. Furthermore, steroid hormones, unlike peptide and amine hormones, mediate nearly all their actions on target tissues by regulating gene transcription. As a result, the response of target tissues to steroids typically occurs over hours to days.
Table 47-4 Differences Between Steroid and Peptide Amine Hormones
|
Property |
Steroid Hormones |
Peptide Amine Hormones |
|
Storage pools |
None |
Secretory vesicles |
|
Interaction with cell membrane |
Diffusion through cell membrane |
Binding to receptor on cell membrane |
|
Receptor |
In cytoplasm or nucleus |
On cell membrane |
|
Action |
Regulation of gene transcription (primarily) |
Signal transduction cascades affecting a variety of cell processes |
|
Response time |
Hours to days (primarily) |
Seconds to minutes |
Like cholesterol itself, steroid hormones are poorly soluble in water. On their release into the circulation, steroid hormones associate with specific binding proteins (e.g., sex hormone–binding globulin) that transport the steroid hormones through the circulatory system to their target tissues. The presence of these binding proteins, whose concentration in the circulation can change in response to a variety of physiological conditions, can complicate efforts to measure the amount of active steroid hormone in the circulation.
Steroid hormones bind to intracellular receptors that regulate gene transcription
Steroid hormones appear to enter their target cell by simple diffusion across the plasma membrane (Fig. 47-7). Once within the cell, steroid hormones are bound with high affinity (Kd in the range of 1 nM) to receptor proteins located in the cytosol or the nucleus. As detailed in Chapter 4, binding of steroid hormone to its receptor results in a change in the receptor conformation so that the active receptor-hormone complex now binds with high affinity to specific DNA sequences called hormone response elements or steroid response elements (SREs). These sequences are within the 5′ region of target genes whose transcription is regulated by the specific steroid hormone–receptor complex. Termination of gene regulation by the steroid hormone–receptor complex is not as well understood as initiation of the signal. The receptor protein may be modified in a manner that permits dissociation of the hormone and DNA. The receptor itself could then be recycled and the steroid molecule metabolized or otherwise cleared from the cell.
Figure 47-7 Action of steroid hormones. The activated steroid hormone receptor binds to specific stretches of DNA called SREs, thus stimulating the transcription of appropriate genes. hsp, heat shock protein.
Steroid receptors are monomeric phosphoproteins with a molecular weight that is between 80 and 100 kDa. A remarkable similarity is seen among receptors for the glucocorticoids, sex steroids, retinoic acid, the steroid-like vitamin 1, 25-dihydroxyvitamin D, and thyroid hormone. The receptors for these diverse hormones are considered part of a gene superfamily (see Chapter 3). Each of these receptors has a similar modular construction with six domains (A through F). The homology among receptors is especially striking for the C domain, particularly the C1 subdomain, which is the part of the receptor molecule that is responsible for binding to DNA.
Steroid hormone receptors dimerize on binding to their target sites on DNA. Dimerization appears essential for the regulation of gene transcription. Within the C1 DNA-binding domain of the steroid receptor monomer are two zinc fingers that are involved in binding of the receptor to DNA (see Chapter 4). Even receptors with very different biological actions have a striking sequence similarity in this domain of the receptor. Because the specificity with which genes are regulated by a specific steroid receptor arises from the specificity of the DNA-binding domain, mutations in this region can greatly alter hormone function. For example, substitution of two amino acids in the glucocorticoid receptor causes the mutated glucocorticoid receptor to bind to DNA to which the estrogen receptor normally binds. In such a system, a glucocorticoid could have an estrogen-like effect.
The activated steroid receptor, binding as a dimer to SREs in the 5′ region of a gene, regulates the rate of transcription of that gene. Each response element is identifiable as a consensus sequence of nucleotides, or a region of regulatory DNA in which the nucleotide sequences are preserved through different cell types. The effect of gene regulation by activated steroid receptors binding to an SRE is dramatically illustrated by the chick ovalbumin gene. Chicks that are not exposed to estrogen have approximately four copies of the ovalbumin mRNA per cell in the oviduct. A 7-day course of estrogen treatment increases the number of copies of message 10,000-fold. This increase in message is principally the result of an increased rate of gene transcription. However, steroid hormones can also stabilize specific mRNA molecules and increase their half-life. (See Note: Stabilization of mRNA by Estrogen)
The 5′ flanking region of the gene typically has one or more SREs upstream of the TATA box, a nucleotide sequence rich in adenine and thymine that is located near the starting point for transcription (see Chapter 4). The activated steroid hormone receptors recognize these SREs from their specific consensus sequences. For example, one particular consensus sequence designates a site as a glucocorticoidresponse element if the SRE is in a cell with a glucocorticoid receptor. This same consensus sequence in a cell of the endometrium would be recognized by the activated progesterone receptor or, in the renal distal tubule, by the activated mineralocorticoid receptor. The specificity of the response thus depends on the cell’s expression of particular steroid receptors, not simply the consensus sequence. For example, the renal distal tubule cell expresses relatively more mineralocorticoid receptors than it does progesterone receptors when compared with the endometrium. As a result, changes in plasma aldosterone regulate Na+ reabsorption in the kidney with greater sensitivity than does circulating progesterone. However, very high levels of progesterone can, like aldosterone, promote salt reabsorption.
From the foregoing, it should be apparent that the specificity of response of a tissue to steroid hormones depends on the abundance of specific steroid receptors expressed within a cell. Because all somatic cells have the full complement of DNA with genes possessing SREs, whether a cell responds to circulating estrogen (e.g., breast), androgen (e.g., prostate), or mineralocorticoid (e.g., renal collecting duct) depends on the receptors present in the cell. This specificity raises the obvious, but as yet unanswered question of what regulates the expression of specific steroid receptors by specific tissues.
Within a given tissue, certain factors control the concentration of steroid hormone receptors. In the cytosol of all steroid-responsive tissues, steroid receptor levels usually drop dramatically immediately after exposure of the tissue to the agonist hormone. This decrease in receptor level is the result of net movement of the agonist-receptor complex to the nucleus. Eventually, the cytosolic receptors are repopulated. Depending on the tissue, this repopulation may involve new synthesis of steroid hormone receptors or simply recycling of receptors from the nucleus after dissociation of the agonist from the receptor. In addition, some steroids reduce the synthesis of their own receptor in target tissues. For example, progesterone reduces the synthesis of progesterone receptor by the uterus, thus leading to an overall net reduction or downregulation of progesterone receptor concentration in a target tissue. An interesting observation in this regard is that the genes for steroid receptor proteins do not appear to have SREs in their 5′ flanking region. Thus, this regulation of receptor number probably involves trans-acting transcriptional factors (see Chapter 4) other than the steroid hormones themselves.
Other factors that affect the concentration of steroid receptors in target tissues include the state of differentiation of the tissue, the presence of other hormones that affect steroid receptor synthesis, and whether the steroid hormone has previously stimulated the tissue. For example, estrogen receptor concentrations are low in an unstimulated uterus but rise dramatically in an estrogen-primed uterus (receptor upregulation). Regulation of steroid receptor number is clearly one factor that alters overall tissue sensitivity to these hormones.
Thyroid hormones bind to intracellular receptors that regulate metabolic rate
In many respects, the thyroid gland and thyroid hormone are unique among the classic endocrine axes. In Chapter 49, in which thyroid physiology is discussed in more detail, this uniqueness begins with the structure of the thyroid gland, which is composed of follicles. Each follicle is an epithelial layer of cells encircling a lake of fluid that contains very protein-rich follicular fluid. The principal protein component of the follicular fluid is an extremely large protein, thyroglobulin. Neither of the two thyroid hormones—T4 and T3—is free in the follicular fluid. Rather, these hormones are formed by the iodination of tyrosine residues on the thyroglobulin molecule and are thus part of the primary structure of the thyroglobulin molecule.
T4 and T3 remain part of the thyroglobulin molecule in the follicle lumen until thyroid secretion is stimulated. The entire thyroglobulin molecule then undergoes endocytosis by the follicular cell and is degraded within the lysosomes of these cells. Finally, the follicular cell releases the free T4 and T3 into the circulation. Once secreted, T4 is tightly bound to one of several binding proteins. It is carried to its sites of action, which include nearly all the cells in the body. In the process of this transport, the liver and other tissues take up some of the T4 and partially deiodinate it to T3; this T3 can then re-enter the circulation.
Both T3 and T4 enter target cells and bind to cytosolic and nuclear receptors. These receptors are similar to those for steroid hormones (see Chapter 3). T3 has higher affinity than T4 for the thyroid hormone receptor. Even though it accounts for only ~5% of the circulating thyroid hormone, T3 is probably the main effector of thyroid hormone signaling. The activated thyroid hormone receptor binds to thyroid hormone response elements in the 5′ region of responsive genes and regulates the transcription of multiple target genes.
Quantitation of Steroid Receptors in Patients with Cancer
The affinity of steroid molecules for their receptors can be studied in vitro in a manner analogous to that described for the radioimmunoassay of peptide hormones (Fig. 47-8). In a typical immunoassay, an antibody with high affinity for a hormone or other compound binds to a radioactively labeled hormone or other molecule. A sample containing an unknown amount of the compound to be measured is added to the antibody-labeled hormone mixture and displaces the radioisotope from the antibody in proportion to the concentration of the unknown. The amount of unknown can be quantitated by comparison to the displacing activity of known standards.
Figure 47-8 Quantitating receptor affinity and number of receptors. A, The plot of bound hormone (i.e., hormone-receptor complex) on the y-axis versus that of free steroid concentration on the x-axis. In this example, we have assumed that the Kd for hormone binding is 3 nM and that the maximal bound hormone concentration is 0.5 nM. B, This plot is a transformation of the data in A. The colored points in this plot match the points of like color in A. Plotted on the y-axis is the ratio of [Bound hormone]/[Free hormone]. Plotted on the x-axis is [Bound hormone]. The slope of this relationship gives the −1/KD, where Kd is the dissociation constant (3 nM). The x-axis intercept gives the total number of receptors (0.5 nM).
For quantitating steroid receptors, cell extracts containing an unknown amount of steroid receptors are incubated with increasing concentrations of labeled steroid hormone. At each concentration, hormone that is bound by the receptor is separated from that remaining free in the extract. The result is a saturation curve (Fig. 47-8A), provided the tissue extract has a finite number of specific hormone receptors. This saturation curve can often be linearized by a simple arithmetic manipulation called a Scatchard plot (Fig. 47-8B). This analysis allows quantitation of the affinity of the receptor for the hormone and provides an estimate of the number of receptors (actually, the concentration of receptors) for that particular hormone.
The technique of quantitating receptor number has found an important application in determining the number of estrogen and progesterone receptors present in breast cancer cells. The number of estrogen and progesterone receptors per milligram of breast cancer tissue (obtained by biopsy of the breast or involved lymph node) is quantitated with radiolabeled estrogen (or progesterone). For postmenopausal women with estrogen receptor–positive breast cancer (i.e., a tumor with a high level of estrogen receptors), treatment with an antiestrogen (e.g., tamoxifen) is effective therapy. For premenopausal women, an antiestrogen may be used as well, or the ovaries can be removed surgically. The woman may also be given a long-acting gonadotropin-releasing hormone (GnRH) agonist, which paradoxically blocks both LH and FSH production by the anterior pituitary, thereby reducing estradiol production and accomplishing a medical oophorectomy. The continuous (as opposed to the normally pulsatile) administration of GnRH downregulates GnRH receptors in the anterior pituitary.
These therapies are not effective in patients with cancers that do not express significant numbers of estrogen receptors. The presence of abundant estrogen and progesterone receptors in breast tumors correlates with a more favorable prognosis, possibly because of the relatively advanced state of differentiation of the tumor, as well as the tumor’s responsiveness to manipulation by estrogen or progesterone therapy.
Thyroid hormone receptors are present in many tissues, including the heart, vascular smooth muscle, skeletal muscle, liver, kidney, skin, and CNS. A major role for thyroid hormone is overall regulation of metabolic rate. Because T4 affects multiple tissues, individuals affected by disorders involving oversecretion or undersecretion of thyroid hormone manifest a host of varied symptoms that reflect the involvement of multiple organ systems.
Steroid and thyroid hormones can also have nongenomic actions
A central dogma of steroid hormone and thyroid hormone action has been that all their diverse actions are secondary to genomic regulation. However, evidence accumulating over the past decade demonstrated that these receptors can also bind to and modulate the activity of cytosolic proteins and can thereby regulate their activity or behavior through a nongenomic action. The quest to explain some of the very rapid onset of steroid and thyroid hormone actions (occurring within 2 to 15 minutes) which appeared incompatible with a mechanism requiring new protein synthesis led to the discovery of their nongenomic actions.
An example of the foregoing is the association of the estrogen receptor (ERα) with phosphatidylinositol-3-kinase (PI3K; see Chapter 58). In the absence of estradiol, ERα binds weakly, if at all, to PI3K. However, in the presence of estradiol, ERα binds strongly to the 85-kDa regulatory subunit of PI3K, thus stimulating the 110-kDa catalytic subunit to phosphorylate PIP2 to phosphatidylinositol 3, 4, 5-trisphosphate (PIP3). In vascular endothelial cells, PIP3increases the activity of downstream protein kinases that phosphorylate and regulate NO synthase, which is itself an important regulator of vascular function (see Chapter 20). In addition to estrogen, thyroid hormones, testosterone, glucocorticoids, and aldosterone may also have nongenomic actions, some of which also appear to be mediated by binding to PI3K.
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