Sample chapter hormones - title -tietz textbook of clinical chemistry & molecular diagnostics , 5e by burtis - elsevier
C HA P T E R 29 Hormones Michael Kleerekoper, M.D., F.A.C.B., F.A.C.P., M.A.C.E.*A hormone is a chemical substance produced in the reversibly bound to transport proteins (e.g., cortisol-binding body by an organ, cells of an organ, or scattered cells, globulin, sex hormone-binding globulin) with only a small having a specific regulatory effect on the activity of fraction free, or unbound available to exert physiologican organ or organs.15 Hormones are produced at one site in action.5,8,17 The half-life of steroid hormones is 30 to 90the body and exert their action(s) at distant sites through minutes. Free steroid hormones, being hydrophobic, enterwhat is called the endocrine system. It is increasingly recog- the cell by passive diffusion and bind with intracellular recep-nized that many hormones exert actions locally through what tors in the cytoplasm or the nucleus.3is termed the paracrine system. Finally, some hormones exerttheir action on the cells of origin, regulating their own syn- Amino Acid–Related Hormonesthesis and secretion via an autocrine system. The classic endo- Thyroxine and catecholamine are examples of hormones thatcrine hormones include insulin, thyroxine, and cortisol. are derived from amino acids; they are water soluble and cir-Neurotransmitters and neurohormones are examples of the culate in plasma bound to proteins (thyroxine) or free (cate-paracrine system, and certain growth factors that stimulate cholamines). Thyroxine binds avidly to three binding proteinssynthesis and secretion of true hormones from the same cell and has a half-life of about 7 to 10 days; free and unboundare examples of an autocrine system. catecholamines such as epinephrine have a very short half-life Table 29-1 lists hormones that are commonly measured of a minute or less. As do the water-soluble peptide andin clinical practice plus a few others to illustrate concepts. protein hormones, these hormones interact with membrane-Biochemical, clinical, and analytical information for specific associated receptors and use a second messenger system.hormones may be found in Chapters 26 and 46 through 57. RELEASE AND ACTION OF HORMONESCLASSIFICATION The physiologic functions of hormones have been broadlyHormones are classified as (1) polypeptides or proteins, categorized into those that (1) affect growth and develop-(2) steroids, or (3) derivatives of amino acids. ment, (2) exert homeostatic control of metabolic pathways, and (3) regulate the production, use, and storage of energy.Polypeptide or Protein Hormones The descriptions that follow illustrate examples of these func-Adrenocorticotropic hormone (ACTH), insulin, and para- tions and mechanisms of control of hormone secretion.thyroid hormone (PTH) are examples of polypeptide orprotein hormones. They are generally water soluble and cir- Growth and Developmentculate freely in plasma as the whole molecule or as active or Normal growth and development of the whole human organ-inactive fragments. The half-life of these hormones in plasma ism is dependent on the complex integrative function ofis short (≤10 to 30 minutes), and wide fluctuations in their many hormones, including gonadal steroids (estrogen andconcentration may be seen in several physiologic and patho- androgen), growth hormone, cortisol, and thyroxine. Severallogic circumstances. These hormones initiate their response pituitary hormones are responsible specifically for the growthby binding to cell membrane receptors (on or in the mem- and development of endocrine glands themselves, and thusbrane) and exciting a “second messenger” system, which con- are responsible for control of synthesis and secretion of othertinues the specific actions of these hormones. hormones. Those other hormones can provide negative feed- back on secretion of the pituitary hormones. Other regulatorsSteroid Hormones of secretion of the pituitary hormones include circadianSteroid hormones (e.g., cortisol, estrogen) are hydrophobic rhythms and a hypothalamic pulse generator that controls theand insoluble in water. These hormones circulate in plasma, Text continued on page 842*The author gratefully acknowledges the original contribution by Dr. Ronald J. Whitley on which portions of this chapterare based. 837
838 Section III ■ Analytes TABLE 29-1 Major Hormones and Frequently Measured Hormone Precursors and Cytokines Endocrine Organ Chemical Nature Major Sites and Hormone of Hormone of Action Principal Actions Hypothalamus Thyrotropin-releasing hormone Peptide (3aa, Glu-His- Anterior pituitary Release of TSH and prolactin (PRL) (TRH) Pro)a Gonadotropin-releasing Peptide (10aa) Anterior pituitary Release of LH and FSH hormone (Gn-RH) or luteinizing hormone- releasing hormone (LH-RH) Corticotropin-releasing Peptide (41aa) Anterior pituitary Release of ACTH and β-lipotropic hormone (CRH) hormone (LPH) Growth hormone-releasing Peptides (40, 44aa) Anterior pituitary Release of growth hormone (GH) hormone (GH-RH) Somatostatinb (SS) or growth Peptides (14, 28aa) Anterior pituitary Suppression of secretion of many hormone-inhibiting hormones [e.g., GH, TSH, gastrin, hormone (GH-IH) vasoactive intestinal polypeptide (VIP), gastric inhibitory polypeptide (GIP), secretin, motilin, glucagon, and insulin] Prolactin-releasing peptide Peptide (20aa) Anterior pituitary Release of PRL Prolactin-releasing/inhibiting Dopamine Anterior pituitary Suppression of synthesis and factor secretion of PRL Anterior Pituitary Lobe Thyrotropin or thyroid- Glycoprotein, Thyroid gland Stimulation of thyroid hormone stimulating hormone (TSH) heterodimerc (α, formation and secretion 92aa; β, 112aa) Follicle-stimulating hormone Glycoprotein, Ovary Growth of follicles with LH, (FSH) heterodimerc (α, secretion of estrogens, and 92aa; β, 117aa) ovulation Testis Development of seminiferous tubules; spermatogenesis Luteinizing hormone (LH) Glycoprotein, Ovary Ovulation; formation of corpora heterodimerc (α, lutea; secretion of progesterone 92aa; β, 121aa) Testis Stimulation of interstitial tissue; secretion of androgens PRL Peptide (199aa) Mammary gland Proliferation of mammary gland; initiation of milk secretion; antagonist of insulin action Growth hormone (GH) or Peptide (191aa) Liver Production of IGF-1 (promoting somatotropin growth) Liver and peripheral Anti-insulin and anabolic effects tissues Corticotropin or Peptide (39aa) Adrenal cortex Stimulation of adrenocortical steroid adrenocorticotropin (ACTH) formation and secretion β-Endorphin (β-END)b,h Peptide (31aa) Brain Endogenous opiate; raising of pain threshold and influence on extrapyramidal motor activity Chorionic gonadotropin (CG) Glycoprotein, or choriogonadotropin heterodimerc (α, 92aa; β, 145aa) α-Melanocyte-stimulating Peptide (13aa) Skin Dispersion of pigment granules, hormone (α-MSH) darkening of skin Leu-enkephalin (LEK)b,h and Peptide (5aa) Brain Same as β-endorphin met-enkephalin (MEK)b,h
Chapter 29 ■ Hormones 839TABLE 29-1 Major Hormones and Frequently Measured Hormone Precursors and Cytokines—cont’dEndocrine Organ Chemical Nature Major Sitesand Hormone of Hormone of Action Principal ActionsPosterior Pituitary LobeVasopressin or ADH Peptide (9aa) Arterioles Elevation of blood pressure; water Renal tubules reabsorptionOxytocin Peptide (9aa) Smooth muscles Contraction; action in parturition (uterus, mammary and in sperm transport; ejection gland) of milkPineal GlandSerotonin or Indoleamine Cardiovascular, Neurotransmitter; stimulation or 5-hydroxytryptamine (5-HT) respiratory, and inhibition of various smooth gastrointestinal muscles and nerves systems; brainMelatonin Indoleamine Hypothalamus Suppression of gonadotropin and GH secretion; induction of sleepThyroid GlandThyroxine (T4) and Iodoamino acids General body tissue Stimulation of oxygen consumption triiodothyronine (T3) and metabolic rate of tissueCalcitonin or thyrocalcitonin Peptide (32aa) Skeleton Uncertain in humansParathyroid GlandParathyroid hormone (PTH) or Peptide (84aa) Kidney Increased calcium reabsorption, parathyrin inhibited phosphate reabsorption; increased production of 1,25- dihydroxycholecalciferol Skeleton Increased bone resorptionAdrenal CortexAldosterone Steroid Kidney Salt and water balanceAndrostenedioned Steroid Hormone precursor Converted to estrogens and testosteroneCortisol Steroid Many Metabolism of carbohydrates, proteins, and fats; anti- inflammatory effects; othersDehydroepiandrosterone Steroids Hormone precursors Converted to estrogens and (DHEA) and testosterone dehydroepiandrostenedione sulfate (DHEAS)17-Hydroxyprogesterone Steroid Hormone precursor Converted to cortisolAdrenal MedullaNorepinephrine and Aromatic amines Sympathetic Stimulation of sympathetic nervous epinephrine receptors systemEpinephrine Liver and muscle, Glycogenolysis adipose tissue LipolysisOvaryActivin A Peptidese Pituitary, ovarian Stimulates release of FSH; enhances 2 βA subunits follicle FSH action; inhibits androgen production by theca cellsActivin B Peptidese See activin A above See activin A above 2 βB subunits betaDHEA and DHEAS Steroids Hormone precursors Converted to androstenedione Continued
840 Section III ■ Analytes TABLE 29-1 Major Hormones and Frequently Measured Hormone Precursors and Cytokines—cont’d Endocrine Organ Chemical Nature Major Sites and Hormone Endocrine Organ of Hormone Chemical Nature of Action Major Sites Principal Actions and Hormone Ovary—cont’d of Hormone of Action Principal Actions Estrogens Phenolic steroids Female accessory sex Development of secondary sex organs characteristics Bone Control of skeletal maturation et al Follistatin Peptides (288aa, Pituitary, ovarian Inhibits FSH synthesis and secretion 315aa) follicles by binding activin Inhibin A Peptide (α subunit Hypothalamus, Inhibits FSH secretion; stimulates and βA subunit) ovarian follicle theca cell androgen production Inhibin B Peptide (α subunit See inhibin A above See inhibin A above and βB subunit) Progesterone Steroid Female accessory Preparation of the uterus for ovum reproductive implantation, maintenance of structure pregnancy Relaxin Peptidef Uterus Inhibition of myometrial contraction Testis Inhibin B See above Anterior pituitary, Control of LH and FSH secretion hypothalamus Testosterone Steroid Male accessory sex Development of secondary sex organs characteristics, maturation, and normal function Placenta Estrogens See above See above See above Progesterone See above See above See above Relaxin See above See above See above Chorionic gonadotropin (CG) Glycoprotein, Same as LH Same as LH; prolongation of corpus or choriogonadotropin heterodimerc(α, luteal function 92aa; β, 145aa) Placental growth hormone Peptides (22 and Same as GH Same as GH (GH-V) 26 kDa) Chorionic Peptide (191aa) Same as PRL Same as PRL somatomammotropin (CS) or placental lactogen (PL) Pancreas Amylin Peptide (37aa) Pancreas Inhibits glucagon and insulin secretion Glucagon Peptide (29aa) Liver Glycogenolysis Insulin Peptideg Liver, fat, muscle Regulation of carbohydrate metabolism; lipogenesis Pancreatic polypeptide (PP) Peptide (36aa) Gastrointestinal tract Increased gut motility and gastric emptying; inhibition of gallbladder contraction Somatostatin (SS)h Peptide (14aa) Pancreas Inhibition of secretion of insulin, glucagon Gastrointestinal Tract Gastrinh Peptide (17aa) Stomach Secretion of gastric acid, gastric mucosal growth Ghrelinh (GHRP) Peptide (28aa) Anterior pituitary Secretion of GH Secretin Peptide (27aa) Pancreas Secretion of pancreatic bicarbonate and digestive enzymes
Chapter 29 ■ Hormones 841TABLE 29-1 Major Hormones and Frequently Measured Hormone Precursors and Cytokines—cont’dEndocrine Organ Chemical Nature Major Sitesand Hormone of Hormone of Action Principal ActionsCholecystokinin-pancreozymin Peptide (33aa) Gallbladder and Stimulation of gallbladder (CCK-PZ)h pancreas contraction and secretion of pancreatic enzymesMotilin Peptide (22aa) Gastrointestinal tract Stimulation of gastrointestinal motilityVIPh Peptide (28aa) Gastrointestinal tract Neurotransmitter; relaxation of smooth muscles of gut and of circulation; increased release of hormones and secretion of water and electrolytes from pancreas and gutGastric inhibitory peptide Peptide (42aa) Gastrointestinal tract Inhibition of gastric secretion and (GIP) motility; increase in insulin secretionGlucagon-like peptide-1 Peptide (30-31aa) Gastrointestinal tract Increase insulin and decrease glucagon secretion; inhibit gastric emptyingBombesinh Peptide (14aa) Gastrointestinal tract Stimulation of release of various hormones and pancreatic enzymes, smooth muscle contractions and hypothermia, changes in cardiovascular and renal functionNeurotensinh Peptide (13aa) Gastrointestinal tract Uncertain and hypothalamusSubstance P (SP)h Peptide (11aa) Gastrointestinal tract Sensory neurotransmitter, analgesic; and brain increase in contraction of gastrointestinal smooth muscle; potent vasoactive hormone; promotion of salivation, increased release of histamineKidney1,25-(OH)2 cholecalciferol Sterol Intestine Facilitation of absorption of calcium Bone and phosphorus; increase in bone resorption in conjunction with PTH Kidney Increase in reabsorption of filtered calciumErythropoietin Peptide (165aa) Bone marrow Stimulation of red cell formationRenin-angiotensin-aldosterone Peptides (renin, 297aa; Renin (from kidney) Ang II increases blood pressure and system Ang I, 10aa; Ang II, catalyzes stimulates secretion of aldosterone 8aa, produced from hydrolysis of (see adrenal) Ang I by angiotensinogen angiotensin (from liver, 485aa) converting enzyme) to ang I in the intravascular spaceLiverIGF-1, formerly called Peptide (70aa) Most cells Stimulation of cellular and linear somatomedin growthIGF-2 Peptide (67aa) Most cells Insulin-like activity Continued
842 Section III ■ Analytes TABLE 29-1 Major Hormones and Frequently Measured Hormone Precursors and Cytokines—cont’d Endocrine Organ Chemical Nature Major Sites and Hormone of Hormone of Action Principal Actions Thymus Thymosin and thymopoietin Peptides (28, 49aa) Lymphocytes Maturation of T lymphocytes Heart Atrial natriuretic peptide (ANP, Peptide with an Vascular, renal, and Regulation of blood volume and Atriopeptin) intrachain disulfide adrenal tissues blood pressure bond (28aa) B-type natriuretic peptide Peptide with an Vascular, renal, and Regulation of blood volume and (BNP) intrachain disulfide adrenal tissues blood pressure bond (32aa) Adipose Tissue Adiponectin Peptide oligomers of Muscle Increases fatty acid oxidation 30 kDa subunits Liver Suppresses glucose formation Leptin Peptide (167aa) Hypothalamus Inhibition of appetite, stimulation of metabolism Resistin Peptide (94aa) Liver Insulin resistance Multiple Cell Types Estrogens See above See above See above Galanin Peptide (30aa) Brain, pancreas, Regulates food intake, memory, and gastrointestinal cognition; inhibits endocrine and (GI) tract exocrine secretions of pancreas; delays gastric emptying; prolongs colonic transport times Parathyroid hormone-related Peptides (139, 141, Kidney, bone Physiologic function conjectural; peptide (PTH-RP) 173aa) PTH-like actions; tumor marker Growth factors (e.g., epidermal Peptides Many Stimulation of cellular growth growth factor, fibroblast growth factor, transforming growth factor family, platelet-derived growth factor, nerve growth factors) Monocytes/Lymphocytes/Macrophages Cytokines (e.g., interleukins 1 Peptides Many Stimulation or inhibition of cellular to 18, tumor necrosis factor, growth; other interferons)a aa, Amino acid residues.b Also produced by gastrointestinal tract and pancreas.c Glycoprotein hormones composed of two dissimilar peptides. The α-chains are similar in structure or identical; the β-chains differ among hormones andconfer specificity.d Androstenedione is also produced in the ovary and testis.e Each activin and inhibin is found in multiple forms.f Two chains linked by two disulfide bonds: α, 24aa; β, 29aa.g Two chains linked by two disulfide bonds: α, 21aa; β, 30aa.h Also produced in the brain.pulsatile secretion of gonadotropins. Examples of hormones Chapter 56). Ovarian and testicular hormones in turn reg-of the anterior pituitary gland include the following: ulate pubertal growth; development and maintenance of• Gonadotropins [luteinizing hormone (LH) and follicle- secondary sex characteristics; growth, development, and stimulating hormone (FSH)] that regulate the develop- maintenance of the skeleton and muscles; and distribution ment, growth, and function of the ovary and testis (see of body fat.
Chapter 29 ■ Hormones 843• ACTH that regulates growth of the adrenal glands and Regulation of the Production, Use, and Storage synthesis and secretion of adrenal gland hormones (see of Energy Chapters 53 and 54). Under normal conditions, regulation of energy production,• Thyroid-stimulating hormone (TSH) that regulates growth use, and storage is under tight hormonal control. Under con- of the thyroid gland and iodination of amino acids to ditions of changing demands that require more energy (e.g., produce the thyroid hormones triiodothyronine and thy- exercise, starvation, infection or trauma, emotional stress), roxine (see Chapter 55).1 many hormones are upregulated to control not only circulat- ing levels of nutrients but also the metabolism of these nutri- ents into necessary energy. This very complex activity, whichHomeostatic Control of Metabolic Pathways may involve hormones from different organs, as alreadyThe metabolic pathways under hormonal control are diverse alluded to in the preceding section, is under neurologicand complex. The following important examples illustrate the control, with numerous neuroendocrine hormones partici-feedback control of hormone secretion, which is critical for pating actively in this integrative metabolic process, whichhomeostasis: affects most organs in the body and modulates, for example,• Regulation of blood glucose: In response to a glucose load, heart rate, sweating, fertility, and reproduction. insulin is promptly released from the pancreas, which regu- lates the dispersal of glucose into cells (fat, muscle, liver, and brain) for the metabolism necessary to produce energy ROLE OF HORMONE RECEPTORS from glucose (see Chapter 26). As circulating glucose con- The “unique” or specific action of a hormone on its target centrations thus return to preload concentrations, insulin tissue is a function of the interaction between the hormone secretion slows. Several counter-regulatory hormones and its receptor. As discussed previously, several types of come into play to further regulate this process to ensure hormone-receptor interactions may occur.3,5,8,17 The hormone- that blood glucose concentrations do not become too low. receptor complex provides the very high specificity of the These include glucagon, cortisol, epinephrine, and growth action of the hormone, allowing the target tissue to accumu- hormone. Recent attention has focused on a group of gas- late the hormone from among all the molecules to which it trointestinal hormones termed incretins (see Chapter 46) is exposed. This is essential because hormones generally cir- that are released during eating and stimulate insulin secre- culate in picomolar or nanomolar concentrations (10−9 to tion from the pancreas in advance of any measurable 10−12 mol/L). increase in blood glucose. Incretins also affect the rate of Hormone receptors may be on the cell surface or may be absorption of nutrients from the gut by slowing down the intracellular within the cytoplasm or nucleus. rate of gastric emptying. Another mechanism by which incretins have a role in the regulation of blood glucose is Cell-Surface Receptors by delaying release of the counter-regulatory hormone glu- Peptide hormones bind to cell surface receptors, and the con- cagon from the alpha cells of the pancreatic islets. The most formational change resulting from this binding activates an studied incretins are glucagon-like peptide-1 (GLP-1) and effector system, which in turn is responsible for the down- gastric inhibitory peptide (GIP). stream actions of the hormone (Figure 29-1).11,12 For most• Regulation of serum calcium (see Chapter 52): The calcium- peptide hormones, the intracellular effector that is activated sensing receptor (CaSR) on the parathyroid gland recog- by the hormone-receptor interaction is a specific G-protein nizes the ambient concentration of ionized calcium, which (guanyl-nucleotide–binding protein),4,10,13,18 and the receptors in turn regulates the synthesis and secretion of PTH. When are called G-protein–coupled receptors (GPCRs; Figure ionized calcium concentrations fall (so imperceptibly that 29-2). GPCRs are hepta-helical molecules with seven most analytical methods could not detect the change), PTH membrane-spanning domains. The amino terminus is extra- synthesis and secretion are stimulated. This additional cellular, and the carboxy terminus is intracellular. The major PTH will attempt to restore serum ionized calcium by structural classes of GPCRs have been identified, each con- enhancing renal tubular reabsorption of calcium and taining receptors for specific subsets of hormones (Figure calcium efflux from the skeleton. PTH also catalyzes 29-3). Group I is the largest group, containing receptors for the synthesis of the renal hormone calcitriol (1,25- many peptide hormones and catecholamines. Group II con- dihyroxycholecalciferol), which acts on the gut to increase tains receptors for the family of gastrointestinal hormones intestinal absorption of calcium. These very rapid responses (secretin, glucagons, and vasoactive intestinal polypeptide). of PTH and calcitriol quickly restore ionized calcium to Group III contains the CaSR and the glutamate receptor. concentrations where the CaSR is no longer activated, and Stimulation of a G-protein initiates the intracellular processes PTH and calcitriol synthesis and secretion return to basal of signal transduction that characterize the specific action of rates. the hormone. G-proteins are composed of α, β, and γ sub-• Water and electrolyte metabolism is regulated by aldoste- units and are classified according to the α subunit, of which rone from the adrenal gland, renin from the kidney, and 20 have been identified to date (see Figure 29-3). G-proteins vasopressin [antidiuretic hormone (ADH)] from the pos- may stimulate adenylate cyclase (GS type of G-proteins) or terior pituitary gland (see Chapters 48, 53, and 54). may inhibit adenylate cyclase (Gi type). The many classes of
844 Section III ■ Analytes Progesterone polypeptide hormones with cell surface receptors changes the o conformation of the receptor protein, the binding of a lipid- o soluble hormone with its specific hormone-binding domain o on the intracellular receptor changes the molecular confor- mation of the intracellular receptor. This conformational o change, or activation of the receptor, enables the hormone- o receptor complex to bind to specific regulatory DNA o R sequences of a target gene, permitting control of specific gene oo expression (see Figure 29-1).7 o RR o XTyr TF P TFTyr P R ss POSTRECEPTOR ACTIONS OF HORMONES PKA Target Gene s cAMP Cell surface and intracellular receptors have different postre- XTyr P IGF-I ceptor actions. R G AC ATP AAAAA mRNAs Cell Surface Receptors Once GPCRs are occupied by a hormone, G-protein subunits ss sLH begin a cascade of activation of specific enzymes that generate Proteins molecules that serve as second messengers to effect the hormone response. The best known of these are adenylyl cyclase, which generates cyclic adenosine monophosphate (cAMP), and phospholipase C, which generates both inositol Biological Responses 1,4,5-trisphosphate (IP3) and diacylglycerol (see Figure 29-2).Figure 29-1 Hormonal signaling by cell surface and The production of second messengers and the subsequentintracellular receptors. Receptors for the water-soluble magnitude of the effect of the hormone are functions of thepolypeptide hormones, luteinizing hormone (LH) and amount of hormone bound to the GPCR. The binding of ainsulin-like growth factor (IGF)-1, are integral membrane small number of hormone molecules on the cell surface leadsproteins located at the cell surface. They bind the to the production of many molecules of the second messen-hormone using extracellular sequences and transduce a ger, thus amplifying the signal sent by the hormone (whichsignal through the generation of second messengers— can be thought of as the first messenger).cyclic adenosine monophosphate (cAMP) for the LH cAMP-dependent protein kinases (PKAs) are a family ofreceptor and tyrosine-phosphorylated substrates for the enzymes that, in the presence of cAMP, phosphorylate aIGF-1 receptor. Although effects on gene expression are number of intracellular enzymes and other proteins to acti-indicated, direct effects on cellular proteins (e.g., ionchannels) are also observed. In contrast, the receptor for vate or inactivate the function of these enzymes and proteins,the lipophilic steroid hormone progesterone resides in thereby regulating their function. As a further means of regu-the cell nucleus. It binds the hormone and becomes lating hormone action, these cAMP-dependent kinasesactivated and capable of directly modulating target consist of two catalytic and two regulatory subunits (C andgene transcription. Tf, Transcription factor; R, receptor R, respectively, in Figure 29-4). The regulatory subunits existmolecule. (From Conn PM, Melmed S. Textbook of endocrinology. as a dimer that can bind two molecules of cAMP, and theTowanta NJ: Humana Press, 1997.) binding of cAMP releases the catalytic subunits, which then are activated as phosphorylating enzymes. When cAMP is removed from the regulatory subunit, this dimer is not ableGPCRs and G-proteins briefly described in this section to associate two catalytic subunits and amplify the signal ofprovide some insight into the mechanisms responsible for the the hormone.specificity of hormone action. Some nonpeptide hormones Phospholipase C (Figure 29-2) acts on inositol phospho-also use cell surface receptors. lipids within the cell membrane to produce IP3, which opens up ion channels to facilitate entry of calcium into the cyto-Intracellular Receptors plasm, where it acts as a messenger, and diacylglycerol, whichLipid-soluble hormones such as progesterone (see Figure modulates protein kinase C activity.29-1) are transported in plasma bound to carrier proteins, The insulin receptor represents a somewhat different classwith only a small fraction of the hormone being in the free of cell surface receptors that contain intrinsic hormone-or unbound state. The free hormone enters the cell via passive activated tyrosine kinase activity and do not otherwise involvediffusion and binds to intracellular receptors in the cytoplasm a second messenger.16 The insulin receptor, the prototype ofor, more often, the nucleus (see Figure 29-1). These receptors this type of receptor, consists of two α and two β subunitsare characterized by a hormone-binding domain, a deoxyri- joined by disulfide bridges. The extracellular hormone-bonucleic acid (DNA)-binding domain, and an amino- binding domains are the α subunits, and the β subunits areterminal variable domain. Just as the interaction of protein or intracellular. They contain an ATP-binding site and a catalytic
Chapter 29 ■ Hormones 845 NH2 NH2 H H AC Gs␣ ␥ ␤ ␤␥ Gq␣ PLC ATP GTP GTP COOH COOH PI(4,5)P2 cAMP (Membrane Associated) IP3 DAG Extracellular cAMP cAMP ϩ PKA Cellular Responses PKC R Ca2ϩ ATP ATP P P ER Stores Phosphorylated Phosphorylated Substrate Substrate Calmodulin Kinases Figure 29-2 Signal transduction by cell surface receptors that are coupled to G-proteins. Two seven-transmembrane domains, coupled to different G-proteins (GS and Gq), are shown. Activation of GS leads to stimulation of the effector enzyme adenylate cyclase and production of a cyclic adenosine monophosphate (cAMP) second messenger, causing the activation of protein kinase A (PKA) and the initiation of potential phosphorylation cascades. Activation of Gq leads to stimulation of the effector enzyme phospholipase C-β and the production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) second messengers, one effect of which is to activate protein kinase C (PKC) and initiate a potential phosphorylation cascade. (From Conn PM, Melmed S, eds. Textbook of endocrinology. Towanta NJ: Humana Press, 1997.)kinase domain through which tyrosine kinase is activated phosphorylation of the hormone receptor permits inter-immediately upon insulin binding to the receptor. nalization of the complex from the cell surface into the Because hormones largely serve a regulatory function, cytoplasm where dephosphorylation occurs, permittingthere are of necessity many self-limiting steps in the previous degradation of the hormone and recycling of the GPCR to itsprocesses. Without these self-limiting processes, hormone original transmembrane location, awaiting coupling withaction would continue unabated. For cAMP, cessation of more hormone.hormone action involves the inactivation of G-protein stimu-lation of adenylate cyclase by guanosine triphosphatase Intracellular Receptors(GTPase) (Figure 29-5). In the absence of hormone interac- As noted, lipid-soluble hormones bind to the hormone-tion with the GPCR (basal or unstimulated state), GS is bound binding domain of cytosolic or nuclear receptors.11,12 Thisto guanosine diphosphate (GDP). Once the hormone is results in a conformational change that enables the hormone-bound to the receptor, GDP is released from Gs and is replaced receptor complex to bind to specific regulatory DNAby GTP, and the Gs-GTP complex activates adenylate cyclase. sequences in the 5′ end of the target gene.7 The binding speci-The Gs-GTP complex is inactivated by GTPase, restoring the ficity of the (hormone-bound) receptor for specific regions ofGS-GDP state, which cannot stimulate formation of cAMP the DNA of the target gene is determined by zinc fingeruntil further hormone binding to the GPCR takes place. structures in the receptor’s DNA-binding domain. It is theWithin a few minutes (or less) of hormone-GPCR interaction binding of the hormone-receptor complex to DNA regulatoryand the initiation of hormone action, the receptor is phos- elements that enhances or represses gene transcription. Thephorylated by protein kinase A and protein kinase C. This messenger ribonucleic acid (RNA) that is enhanced or
846 Section III ■ Analytes G Protein-Coupled Receptor (GPCR) Superfamily Family A: Receptors Related to Rhodopsin Family D: Receptors Related to the STE2 and the ␤-Adrenergic Receptor Pheromone Receptor Group I: Olfactory, Adenosine, Melanocortin Rs Group I: Alpha Factor Pheromone Rs Group II: Adrenergic, Muscarinic, Serotonin, DA Rs Group III: Neuropeptide Rs and Vertebrate Opsins Group IV: Bradykinin R and Invertebrate Opsins Family E: Receptors Related to the STE3 Group V: Peptide and GP Hormone, Chemokine Rs Pheromone Receptor Group VI: Melatonin and Orphan Rs Group I: A Factor Pheromone Rs Family B: Receptors Related to the Calcitonin and Parathyroid Hormone Receptors Family F: Receptors Related to the cAMP Receptor Group I: Calcitonin, Calcitonin-like, CRF Rs Group II: PTH and PTHrP Rs Group I: Dictyostelium cAR1-4 Rs Group III: Glucagon, Secretin, VIP, GHRH Rs Family C: Receptors Related to the Metabotropic Glutamate Receptors Group I: Metabotropic Glutamate Rs Group II: Extracellular Calcium Ion Sensor RsA Glycosylation Agonist Binding Conserved Cysteines Antagonist Binding TMD1 TMD2 TMD3 TMD4 TMD5 TMD6 TMD7 Palmitoylation P P PKA Phosphorylation Required for Catecholamine Binding and Activation Asp 113 Ser 204, 207 G-Protein Coupling COOH P P P P P P B ␤ARK Phosphorylation Figure 29-3 Classiﬁcation and basic architecture of cell surface receptors that couple to G-proteins. A, Lists the major families and groups of G-protein–coupled receptors (GPCRs). The mammalian receptors are conﬁned to families A, B, and C. Family A is the largest and includes the diverse odorant receptors and prototypic GPCRs such as rhodopsin and the β-adrenergic receptor. B, Shows a schematic structure of one of the most extensively characterized GPCRs, the β-adrenergic receptor. Major structural features are indicated and are expanded on in the text. (From Conn PM, Melmed S, eds. Textbook of endocrinology. Towanta NJ: Humana Press, 1997.)
Chapter 29 ■ Hormones 847 Ligand hormones that bind to intracellular receptors are rapid and do not depend on synthesis of protein, suggesting that these hormone-receptor complexes exert actions by mechanisms Receptor different from binding to DNA. Adenylyl GTP ␣s cyclase From these descriptions, one can begin to deduce both the ␣s ␥ complexity and the specificity of hormone action, in terms of ATP ␤ an “on and/or off ” concept and in terms of an “effect size” GDP ␣s concept. cAMP CLINICAL DISORDERS OF HORMONES R R Inactive C C PKA Although several chapters of this textbook detail a variety of cAMP cAMP endocrine disorders, a brief introduction is appropriate here. R R cAMP cAMP In general, endocrine diseases may result from a deficiency or an excess of a single hormone or several hormones, or Target C C Active from resistance to the action of hormones. Hormone defi- proteins PKA ciency can be congenital or acquired, and hormone excessFigure 29-4 Signaling by GS. The alpha, beta, and gamma can result from endogenous overproduction (from withinsubunits of Gs are shown. The alpha subunit, when bound the body) or exogenous overmedication. Hormone resistanceto GTP, activates adenylyl cyclase which catalyzes the can occur at several levels but can most simply be character-formation of cyclic AMP (cAMP) from ATP. Then cAMP ized as receptor mediated, postreceptor mediated, or at thebinds to the regulatory subunit (R) of protein kinase A level of the target tissue. The clinical manifestations will(PKA, cyclic AMP-dependent protein kinase), releasingPKA’s catalytic subunit (C) and thus leading to depend on the hormone system affected and the type ofphosphorylation of target proteins. (Image from Conn PM, abnormality.Melmed S, eds. Textbook of endocrinology. Towanta NJ: Humana Press, Diabetes mellitus (DM) is an example of an endocrine1997.) disorder; it is the most common endocrine disorder in the United States (see Chapter 46). It is classified as type 1 or type 2. DM type 1 results from failure of the pancreas to secrete Ligand insulin even though the pancreas is otherwise normal. Type 2, the most common form of DM, results from end-organ resistance to the action of insulin, which, in this case, is Effector Receptor molecule secreted from the pancreas in abundant amounts and circu- lates at high concentrations. Secondary DM occurs when a ␥ GTP ␣ nonendocrine disease such as pancreatitis destroys the pan- ␣ ␣ creas, including the insulin-secreting cells. The biochemical ␤ GDP hallmark of DM is hyperglycemia. GTP GDP In contrast to diabetes, there are uncommon, insulin- producing tumors of the pancreas (insulinomas) in which the ␥ ␥ production of insulin is not regulated by the blood glucose ␣ ␤ concentration and the biochemical hallmark of the tumors is ␤ hypoglycemia. Thus hyperglycemia can be present whenGDP there is insulin deficiency or insulin excess, and insulin excess can accompany both hyperglycemia and hypoglycemia. ThisFigure 29-5 The G-protein cycle. The alpha, beta, and simple illustration underscores the homeostatic and/or regu-gamma subunits of Gs are shown. The alpha subunit, when lating nature of the endocrine system.bound to GTP, activates the effector molecule (such asadenylyl cyclase, see Figure 29-4). GTP then is hydrolyzedto GDP, stopping the activation of the effector molecule MEASUREMENTS OF HORMONES ANDand leading to reformation of the GDP-bound state of RELATED ANALYTESthe G protein. (Image from Conn PM, Melmed S, eds. Textbookof endocrinology. Towanta NJ: Humana Press, 1997.) Hormones are measured by a variety of analytical techniques, including bioassay, receptor assay, immunoassay, and instru- mental techniques such as mass spectrometry interfaceddiminished by hormone receptor binding to the target gene with liquid or gas chromatography. A general overview ofregulates the synthesis of specific proteins that mediate the these techniques is given here. Analytical details for indi-hormone’s physiologic actions. The system is further regu- vidual hormones using such techniques are found in thelated by the presence or absence of coactivators or core- discussion of the individual hormones in their respectivepressors of gene expression. In addition, many actions of chapters.
848 Section III ■ AnalytesBioassay Techniques electrospray ionization techniques that allow sequencing ofBioassays are based on observations of physiologic responses peptides and mass determination of picomole quantities ofspecific for the hormone being measured. In vivo bioassays analytes.usually involve the injection of test materials (such as blood Compared with older methods, tandem mass spectrome-or urine from a patient) into suitably prepared animals; target try offers greater analytical sensitivity, accuracy, and speed,gland responses such as growth or steroidogenesis are then and may allow simultaneous determination of multiple hor-measured. In vitro bioassays involve the incubation of tissue, mones related to a clinical condition.14membranes, dispersed cells, or permanent cell lines in adefined culture medium, with subsequent measurement of Specimen Requirementsan appropriate hormone response. Most in vitro bioassays As can be seen from the brief descriptions of hormone actionmeasure responses proximal or distal to a second messenger given previously and amplified in the hormone-specific chap-such as stimulation of cAMP formation. Bioassays tend to be ters, particular attention must be paid to the clinical materialimprecise and are rarely necessary in clinical medicine. sent to the laboratory for assay. Some hormones are directly affected by food (e.g., insulin) or by circadian variability (e.g.,Receptor-Based Assays cortisol). In many clinical circumstances, the metabolic envi-Receptor assays depend on the in vitro interaction of a ronment plays a crucial role in hormone production, and ithormone with its biological receptor. In this type of assay, is essential to obtain a simultaneous sample for measurementunlabeled hormone displaces trace amounts of radioactively of both the hormone and the molecule(s) regulated by thatlabeled hormone from receptor sites. A second approach is hormone. An isolated measurement of plasma insulin withoutto measure a response, such as production of cAMP, when a concurrent knowledge of the plasma glucose, or measure-test sample is added to a preparation that includes the recep- ment of parathyroid hormone independent of serum calcium,tor and necessary cofactors. In general, receptor assays are is of little if any value. When a patient is evaluated for possiblesimpler to perform and have greater sensitivity than bioas- hormone deficiency or hormone excess, it is often necessarysays. Receptor assays also have an advantage over immunoas- to perform a stimulation or suppression test. Most hormonesays in that they reflect the biological function of a hormone, assays can be performed on plasma or serum, and many cannamely, the capacity to combine with specific receptor sites. be performed on urine samples, usually a 24 hour collection.By contrast, immunoassays may measure active hormone and Increasingly, saliva has become a convenient body fluid forinactive prohormone, hormone polymer, and metabolites hormone analysis, particularly for hormones secreted in awhen all share a common antigenic determinant or set of diurnal rhythm such as cortisol. Unlike blood sampling,determinants. In general, receptor assays are not as sensitive which requires the patient to present to a blood drawing facil-as immunoassays, and enzymes in the biological specimen ity, patients can be provided with salivary collection materialmay degrade the receptor or destroy the labeled tracer. The such that they can provide to the laboratory specimens col-added complexity and lability of receptor preparations also lected at multiple times during the day or at unusual (butcontribute to the limited application of these assays in the biologically very relevant) times such as 11 pm—a commonlyroutine clinical laboratory. used time for obtaining a specimen for measurement of cortisol.Immunoassay TechniquesImmunoassays employing antibodies are widely used to REFERENCESquantify hormones (see Chapter 16). Currently labeled anti- 1. Brent GA. The molecular basis of thyroid hormone action. N Engl Jbody (immunometric) assays with nonisotopic labels are the Med 1994;331:847-53.method of choice for measuring most hormones, especially 2. Chace DH. Mass spectrometry in the clinical laboratory. Chem Revpeptides and proteins. Immunometric assays use saturating 2001;101:445-77.concentrations of two or more antibodies (often monoclonal) 3. Edwards DP. 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