General Endocrinology

15.1.1 Introduction

The communication between different cells across various organ systems is a pre-requisite way to maintain a wide range of physiological mechanisms, which helps in governing any multi-cellular organism to function as a singular entity.

15.1.2 BriefHistory

Claude Bernard has used the word “internal secretion” to denote the secretion of glucose from liver. However, it is widely extended to denote any bio-molecule that is released into blood. The French physiologist Charles Brown-Sequard demonstrated that the ablation of adrenal gland is fatal and also claimed that injecting testicular extracts has a rejuvenating effect in men. Another exciting discovery by a British neurosurgeon named Victor Horsley was the onset of

Fig. 15.1 Types of chemical messengers. Chemical messengers released from various cells help in the communication and coordination between different tissues culminating in the execution of different physiological functions

myxoedematous signs upon removing the thyroid gland in monkeys.

15.1.3 Glands and Classification

Glands are defined as a group of cells that are structurally and functionally organised to work in unison to synthesise a prod­uct, viz. either an enzyme, sweat, saliva, milk, or hormone, and secrete into the duct or bloodstream. The presence or absence of a duct is often used as a distinguishing feature in classifying different glands into three major classes, namely:

1. Exocrine glands: Consists of secretory acini, which empty their products into a specialised duct.

E.g. Liver (Bile), salivary gland (Saliva), and mammary gland (Milk)

2. Endocrine glands: Glands that lack ducts and the synthesised products are often secreted directly into the bloodstream.

E.g. Hypothalamus (Somatostatin), pituitary gland (GH), thyroid gland (T₃ZT₄)

3. Mixed glands: Glands that perform both exocrine and endocrine functions.

E.g. Pancreas (exocrine: enzymes, endocrine: insulin), testes (exocrine: spermatozoa, endocrine: testosterone)

15.1.4 Hormones and Their General Properties

Hormones are defined as the chemical substances secreted by specialised endocrine glands or cells in minute quantities, and conveyed to distant or nearby target cells via the bloodstream or interstitial fluid respectively (components of ECF). The definition has been evolved to incorporate all the chemical messengers that are qualified as hormones irrespective of their source and site of action.

The salient properties of hormones (Fig. 15.2) are enlisted below:

1. Bind to specific receptors: Every hormone binds to a specific high-affinity receptor that ensues the formation of a hormone-receptor complex (HRC) before eliciting a target response.

2. Slow onset of action: The onset of biological effects of various hormones is slow ranging from a few hours to even days, generally attributed to their action on the genetic machinery to initiate transcription of several effec­tor genes.

3. Absence of enzymatic activity: They lack inherent enzy­matic activity and cannot catalyse any intracellular enzy­matic reaction directly.

4. Signal transduction: A phenomenon characterised by the activation of a cascade of intracellular reactions by several hormones upon binding to the target cell receptors. Hormones are widely recognised as “first messengers”

Fig. 15.2 General properties of hormones. [Synthesised in specialised endocrine cells and carried in the bloodstream, hormones bind to specific receptors present upon reaching the target cells to elicit specific biological effects]

and often lead to the synthesis of intracellular “secondary messengers” in the target cells.

5. Feedback mechanism: The rate of secretion and concen­tration of every hormone are regulated within a narrow range mostly by the means negative feedback mechanism.

6. Metabolic clearance rate: After eliciting desired biological effects, they are rendered inactive by the target tissues and/or eliminated from the circulation by excretory actions of the liver and kidney.

15.1.5 ClassificationofHormones

The source of secretion, chemical nature, physiological action, mechanism of action, and degree of solubility are the few criteria used in classifying different hormones.

1. Based on their source of secretion: Classified as pitui­tary and non-pituitary hormones based on their site of production.

(a) Pituitary hormones: Synthesised and secreted from different lobes of the pituitary gland.

E.g. Growth hormone (GH), prolactin (PRL), luteinising hormone (LH), follicle-stimulating hor­mone (FSH), adrenocorticotropic hormone (ACTH), melanocyte stimulating hormone (MSH), anti­diuretic hormone (ADH), & oxytocin.

(b) Non-pituitary hormones: Include hormones pro­duced by other endocrine glands except from the pituitary gland.

E.g. Somatocrinin (GHRH) from the hypothalamus, insulin from the pancreas, aldosterone from the adre­nal cortex, thyroid hormones (T₃ZT₄) from the thy­roid gland, etc.

2. Based on their chemical nature: Depending on the bio-chemical structure, they are categorised as protein, polypeptide, amine, steroid, and fatty acid derivatives in nature.

Table 15.1 List of amine hormones and their site of synthesis

(e) Fatty acid derivatives: Synthesised from fatty acids. Includes hormones of prostaglandin family (e.g.

(a) Protein hormones: Those hormones which are com­posed in excess of 50 amino acids are termed as protein hormones. Further, the presence of glycosylated amino acid residues is used to sub-categorise the protein hormones into simple pro­tein and glycoprotein hormones.

i. Simple protein hormones: Comprised of amino acids joined together by peptide bonds. E.g. GH, Insulin, PRL, etc.

ii. Glycoprotein hormones: A carbohydrate moiety is conjugated to the protein. E.g. LH, FSH, TSH, and chorionic gonadotropins (eCG/hCG)

(b) Polypeptide hormones: Composed of less than 50 amino acid residues. E.g. Calcitonin, TRH, glucagon, etc.

(c) Amine hormones: Hormones derived from decarboxylated amino acids (Table 15.1).

(d) Steroid hormones: Comprised of hormones derived from cholesterol. Majorly produced from gonads (e.g. oestrogen, testosterone, etc.) and adrenal cortex (e.g. aldosterone, cortisol, etc.)

Know More...

• The smallest peptide hormone is TRH (thyrotropin­releasing hormone) composed of three amino acids (Glutamate-Histidine-Proline).

• The Swedish physiologist Ulf Von Euler discovered prostaglandins in human semen (1935) and thought to be secreted from prostate gland.

3. Based on their physiological action: The integration of endocrine and nervous systems is crucial in controlling most basic physiological events such as the growth, reproduction, intermediary metabolism, stress response (fight or flight response, emotional/physical stress, envi­ronmental stress), feeding responses, water and electro­lyte balance (Fig. 15.3). Hence, the appropriate functioning of the endocrine system is vital for animal survival, optimum reproduction, production, and adapta­tion to various environmental conditions.

4. Based on their degree of solubility:

(a) Water-soluble/hydrophilic hormones: The poly- peptide/glycoprotein hormones are soluble in water and hence does not require carrier proteins for their transport in blood.

Fig. 15.3 Physiological actions of hormones. [Hormones are implicated in regulating a variety of physiological functions such growth, metabolism, appetite, reproduction, fluid and electrolyte balance. [GnRH gonadotropin­releasing hormone; GH growth hormone; LH luteinising hormone; FSH follicle­stimulating hormone; T₃ triiodo­thyronine; T₄ tetraiodothyronine; ADH anti-diuretic hormone; ANP atrial natriuretic peptide; BNP B-type natriuretic peptide; CCK cholecystokinin]

(b) Water-insoluble/lipophilic hormones: Hormones derived from cholesterol, fatty acids, and thyroid gland are insoluble in water and require carrier proteins for their transport in the systemic circula­tion. These hormones possess intracellular (cyto- solic/nuclear) receptors as they can freely cross the cell membrane. E.g. T3/T4, testosterone,

aldosterone, etc.

15.1.6 SynthesisofHormones

The altered levels of a circulating hormone or a metabolite (including ions) along with the sensory stimuli are vital factors in regulating the synthesis of different hormones. Additionally, the releasing and inhibiting hormones from the hypothalamus exert potent effects on the synthesis of various hormones. An appropriate stimulus increases the transcription and subsequent translation of the gene encoding a target hormone. However, the general mechanism for the biosynthesis and post-translational modifications varies between different peptide/protein and steroid hormones.

15.1.6.1 Peptide or Protein Hormones

During the translation, the cellular organelle rough endoplas­mic reticulum (RER) produces large precursor proteins known as “pre-prohormones” and is regarded as the initial site of synthesis for various peptide and protein hormones. The signal peptide present in a pre-prohormone is cleaved by the signal peptidases of RER to produce prohormones. Followed by the site-specific endopeptidases (also called as prohormone convertases) in the golgi apparatus (GA) finally transform prohormones into mature hormones, which are then stored as cytoplasmic vesicles. Furthermore, the enzy­matic cleavage mediated by carboxypeptidase and amino­peptidase along with simultaneous post-translational modifications (Table 15.2) occurring in the secretory vesicles is responsible for conferring the biological activity to a specific hormone. The increased intracellular concentra­tion of calcium (Ca⁺²) and cAMP produced due to a partic­ular stimulus ensues the fusion of storage vesicles with the cell membrane to release a hormone into the extracellular fluid and the whole process is commonly known as exocy- tosis (Fig. 15.4).

Table 15.2 Types of post-translational modifications seen in peptide hormones

Fig. 15.4 Steps involved in the synthesis of peptide hormones. Peptide or protein hormones are initially synthesised as large precursor molecules known as pre-pro hormones, undergoes proteolytic cleavage to yield mature hormones that are biologically active

Fig. 15.5 Biosynthesis of steroid hormones. Derived from cholesterol, steroid hormones are manufactured in the mitochondria and smooth endoplasmic reticulum. Conversion of cholesterol to pregnenolone by P450scc is the common step occurring in the steroid biosynthesis;

subsequently tissue specific hydroxylases convert pregnenolone in to different types of steroids. [StAR steroidogenic acute regulatory protein; P450scc cholesterol side chain cleavage enzyme; SC Sertoli cell; CL corpus luteum; GC granulosa cell; TC theca cell; LC Leydig cell]

Know More...

• The specific post-translational modifications seen in different peptide hormones confer or potentiate their biological activity. In addition, they play a role in determining the duration of action and half-life of peptide hormones.

• More than 50% of the mammalian hormones undergoes amidation in order to exhibit their biological activity.

15.1.6.2 Steroid Hormones

All the steroid hormones are derived from cholesterol that is obtained from the circulation or synthesised de novo from the condensation of acetyl-CoA in the smooth endoplasmic retic­ulum (SER) with the help of a rate-limiting enzyme HMG-CoA-reductase. Cells that have the ability to secrete steroid hormones have abundant SER and in turn lipid droplets in the cytosol. Most often, tropic peptide hormones stimulate the steroid hormone-producing cells by increasing the uptake, de novo synthesis of cholesterol, and the activa­tion of downstream enzymatic machinery. An appropriate stimulus triggers the steroidogenic acute regulatory protein (StAR) and elicits an acute steroidogenic response marked by a rapid mobilisation of cholesterol into the inner mitochon­drial membrane (IMM) from the outer mitochondrial mem­brane (OMM). The steroidogenesis is initiated by the transformation of cholesterol into pregnenolone by a cytochrome enzyme commonly known as the cholesterol side chain cleavage enzyme (P450scc or CYP11A1). The former step is considered as the rate-limiting step common for all classes of steroid hormones. The subsequent conver­sion of pregnenolone into different steroid hormones depends entirely on the mitochondrial enzymes and SER possessed by the particular cell (Fig. 15.5).

15.1.7 GeneralMechanismofAction

Hormones act on the target cells/tissues by binding to its specific receptor to form a HRC. The cellular localisation of receptors varies according to different hormone types. The receptors for various peptide hormones are present on the cell membrane, whereas the receptors for steroid hormones local­ise in both cytosol and nucleus. Thyroid hormones are unique that they bind to nuclear receptors in order to elicit their biological effects. The HRC initiates the downstream cellular signalling pathways that either directly or indirectly culmi­nate in modulating the cellular metabolic and transcriptional activities.

15.1.7.1 Mechanism of Action of Peptide Hormones

The hydrophilic nature of peptide and protein hormones necessitates the presence of membrane-bound receptors. They act on the target cells by binding to any of the two different types of receptors, i.e. G-protein coupled receptors (GPCRs) and receptor tyrosine kinases (RTK).

Fig. 15.6 Mechanism of activation of adenylate cyclase and phospholipase C systems by peptide hormones through GPCRs. [Peptide hormones bind to membrane-bound receptors such as GPCRs to exert biological effects on the target cells. G_s and G_q are stimulatory GPCRs, upon their activation generate the production of secondary messengers such as cAMP and Ca⁺² that initiate the downstream signalling pathways. [GPCRs G-protein coupled receptors; ATP adenosine triphosphate; cAMP cyclic adenosine monophosphate; PKA protein kinase A; PKC protein kinase C; PIP₂ phosphatidylinositol 4, 5-bisphosphate; IP₃ Inositol triphosphate; DAG di-acyl glycerol]

15.1.7.1.1 Mechanism of Hormone-Responsive GPCR Signalling

The polypeptide/protein hormones initiate the signal trans­duction predominantly by binding to the membrane-bound GPCRs. They are proteins consisting of three distinct regions, i.e. extracellular (N-terminus), transmembrane, and intracellular domains (C-terminus). The extracellular domain acts as a receptive site for binding with the hormone (first messenger). The transmembrane segment is mainly com­posed of a protein with seven membrane-spanning α-helices, responsible for linking and anchoring the other two domains. The intracellular domain, which extends into the cytoplasm, is coupled to a heterotrimeric G-protein with three sub-units, α, β, and γ. In addition, inactive GPCRs have a GDP mole­cule attached to α sub-unit of the G-protein. The formation of HRC leads to the activation of GPCRs characterised by phosphorylation of GDP molecule to GTP and consequent dissociation of GTP bound α sub-unit (G_α-GTP). The G_α- GTP then either activates or inhibits the membrane-bound enzymes such as adenylate cyclase (AC), guanylate cyclase (GC), and phospholipase C (PLC) or ion channels. The G-proteins that activate a specific membrane-bound enzyme/ion channel are known as stimulatory G-proteins (G_s, Gq) whereas the opposite is true with inhibitory G-protein (G_i). G_s stimulates the production of the secondary messenger cAMP, wherein Gq activation leads to the produc­tion of Di-acyl glycerol (DAG) and inositol triphosphate (IP₃). A surge in the production of secondary messengers leads to the activation of serine/threonine kinases such as protein kinase A (PKA) and protein kinase C (PKC). They modulate the activity of several enzymes that affect cellular metabolism, transcription, and reproduction, thereby produc­ing a well-defined target effect (Fig. 15.6).

15.1.7.1.2 Receptor Tyrosine Kinases (RTK) in Peptide Hormone Signalling

Receptor tyrosine kinases (RTK) are transmembrane receptors that includes receptors with an inherent tyrosine kinase domain (also known as tyrosine kinase receptors) and receptors associated with proteins possessing tyrosine kinase activity (also called as tyrosine kinase-associated receptors).

15.1.7.1.2.1 Tyrosine Kinase Receptors

The tyrosine kinase receptors dimerise upon binding to a hormone with the subsequent activation of the intracellular kinase domain by transphosphorylation. Subsequent to the activation of tyrosine kinase domain, the phosphorylation of tyrosine moieties residing in the receptor’s intracellular domain takes place. This ensues the binding of proteins or enzymes that contain SRC homology domain (SH2) resulting in the activation downstream signalling pathways such as Ras-MAPK and Ras/PI3K/AKT to modulate the cellular metabolism, proliferation, and differentiation. Insulin is a classic example for a peptide hormone that acts via tyrosine kinase receptors.

15.1.7.1.2.2 Tyrosine Kinase-Associated Receptors

Tyrosine kinase-associated receptors are receptors coupled with protein tyrosine kinases (PTKs) to complement the lack of an intrinsic protein kinase domain. Peptide hormones such

Fig. 15.7 Mechanism of action of steroid hormones [Steroid hormones bind to specific intracellular receptors to form HRC, which subsequently migrates to nucleus and binds to specific regions in the genome known as hormone response element (HRE) to initiate the transcription and translation process in the target cell. [HRC hormone-receptor complex]

as GH, PRL, leptin, etc. exert their biological action by binding to these receptors with a majority of PTKs belonging to the Janus kinase (JAK) family and initiate classical JAK-STAT signalling pathway. The dimerization of receptors when bound with a hormone leads to a conforma­tional change in its intracellular domain with the subsequent activation of an associated JAK. The kinase domain of an activated JAK phosphorylates the tyrosine residues present in the receptor’s intracellular region. This enables the binding, phosphorylation and dissociation of various signal transducer and activator of transcription (STAT) proteins. The phosphorylated STATs migrate in to nucleus and effect the transcription process by binding to specific gene promoter regions. In addition, an active JAK phosphorylates various cellular kinases such as SRC family kinase (SFK) that acti­vate other signalling pathways to alter the cellular metabo­lism and functions.

15.1.7.2 Mechanism of Action of Steroid Hormones

The receptors for a majority of steroid hormones reside in the cytoplasm, and few hormones such as oestrogen (E2) have receptors localised in the nucleus. These intracellular receptors are characterised by having three major domains, namely: ligand-binding domain (LBD) at C-terminus, DNA binding domain (DBD), and amino terminal domain (NBD). The LBD serves as binding site for steroid hormones and contains a region known as activation function 2 (AF2). The DBD enables them to bind to specific regions of genome known as hormone response element (HRE) or steroid response element (SRE), thence directly modulating the tran­scriptional rate of specific genes. The NTD consists of acti­vation function 1 (AF1) region that determines optimum transcriptional activity. The cytosolic steroid receptors are bound with heat shock proteins (HSP) in their inactive state. The removal of HSP along with the phosphorylation of the receptor happens when bound to a steroid hormone, subsequent migration and binding of HRC to SRE/HRE regions in the DNA contribute the classic steroid hormone- signalling pathway. However, instead of directly binding to DNA, AF2 can bind with DNA bound transcription factors such as AP1 or SP1 there by indirectly regulating the cellular transcription (Fig. 15.7).

Know More...

• Although the steroid receptors initially believed to be intracellular, recent studies on the rapid non-genomic effects of steroid hormones suggest their presence on the cell membrane affecting vari­ous cellular signalling pathways. Membrane-bound receptors for oestrogen, androgen, glucocorticoid, and mineralocorticoid play a role in activating kinases, Ca+2 influx, and G-protein activation.

• The cytosolic steroid receptors can also be stimulated in the absence of hormones on phosphor­ylation due to the activation of intracellular kinases, a phenomenon that is termed as “ligand independent activation”.

15.2