BIOCHEMISTRY: A TEXTBOOK FOR MEDICAL UNIVERSITIES - E. S. Severin - 2004

CHAPTER 11. HORMONAL REGULATION OF METABOLISM AND BODY FUNCTIONS

II. Interaction of Hormones with Receptors and Mechanisms of Hormonal Signal Transduction into Cells

The BIOLOGICAL EFFECTS OF Hormones are manifested through their interaction with receptors on target Cells. To exert biological activity, the binding of a hormone to its receptor must trigger the generation of an intracellular chemical signal that elicits a specific biological response, such as altering The rate of synthesis of Enzymes and other Proteins or modifying their activity (see Chapter 5). Target cells for a given hormone may belong to one or several tissues. By acting on a target Cell, the hormone elicits a specific response. For example, The Thyroid Gland is a specific target for thyrotropin, which stimulates the proliferation of thyroid acinar cells and increases the rate of thyroid hormone Biosynthesis. Glucagon acts on adipocytes to activate lipolysis, while in the Liver it stimulates Glycogen mobilization and Gluconeogenesis. A characteristic feature of a target cell is its ability to perceive the information encoded in the Chemical Structure of the hormone.

A. Hormone Receptors

The initial stage in the action of a hormone on a target cell is the interaction of the hormone with a cell receptor. The concentration of hormones in extracellular fluid is very low, typically ranging from 10^-6 to 10^-11 mmol/L. Target cells distinguish their specific hormone from a multitude of other molecules and hormones due to the presence of complementary receptors featuring specialized hormone-binding domains.

1. General characteristics of Receptors

Receptors for Peptide Hormones and epinephrine are located On the surface of The Cell membrane, whereas receptors for steroid and THYROID HORMONES reside inside the cell. Furthermore, intracellular receptors for certain hormones, such as glucocorticoids, are localized in the Cytosol, whereas others—such as those for androgens, estrogens, and thyroid hormones—are located in the Cell Nucleus (see Chapter 5).

Chemically, receptors are proteins and typically consist of several domains.

The structure of Membrane Receptors can be divided into three functionally distinct regions. The first domain (the recognition domain) is located in the N-terminal region of the polypeptide chain on the outer surface of the cell membrane; it contains glycosylated segments that mediate hormone recognition and binding. The second domain is the transmembrane domain. In one type of receptor, those coupled to G proteins, it consists of seven tightly packed α-helical polypeptide sequences. In another type of receptor, the transmembrane domain comprises only a single α-helical polypeptide chain (for example, both β subunits of the heterotetrameric Insulin receptor α₂β₂). The third (cytoplasmic) domain generates the intracellular chemical signal that couples hormone recognition and binding to a specific intracellular response. The cytoplasmic domain of receptors for hormones such as insulin, epidermal growth factor, and insulin-like growth factor-1 exhibits Tyrosine kinase activity on the inner face of the membrane, whereas the cytoplasmic domains of Growth Hormone, prolactin, and cytokine receptors do not possess intrinsic tyrosine kinase activity; instead, they associate with other cytoplasmic protein Kinases that phosphorylate and activate them.

Steroid and thyroid hormone receptors contain three functional regions. The C-terminal region of the polypeptide chain contains the hormone recognition and binding domain. The central portion of the receptor includes the DNA-binding domain. The N-terminal region of the polypeptide chain contains a domain known as the variable region of the receptor, which is responsible for binding to other proteins that cooperatively regulate METABOLISM/31.html">Transcription.

2. Regulation of Receptor Number and Activity

The concentration of receptors inside the cell or on its surface, as well as their affinity for a given hormone, are normally regulated by various mechanisms and can also change in disease states or upon the administration of hormones or their agonists as drugs. For example, when cells are exposed to β-adrenergic agonists for several minutes, adenylate cyclase activation ceases upon subsequent re-addition of the agonist, and the biological response disappears. Such a reduction in receptor sensitivity to a hormone (desensitization) can occur As a result of a decrease in receptor number via down-regulation. The hormone binds to the receptor, and the hormone-receptor complex enters the cell via endocytosis (internalization), where some receptors undergo proteolytic degradation by lysosomal enzymes, while others are inactivated by dissociation from other membrane components. This leads to a decrease in the number of receptors on The Plasma Membrane. For instance, in the case of insulin, glucagon, and catecholamines, this process occurs within minutes or hours. When hormone concentration declines, receptors return to the cell surface, and hormone sensitivity is restored. Receptor activity, i.e., its affinity for the hormone, can also be modulated by covalent modification, predominantly through phosphorylation. The concentration of intracellular receptors can also be regulated via Induction and Repression mechanisms.

B. Mechanisms of Hormonal Signal Transduction into Cells

Based on their MECHANISM OF ACTION, hormones can be divided into two groups. The first group comprises hormones that interact with membrane receptors (peptide hormones, epinephrine, as well as local-acting hormones such as cytokines and Eicosanoids). The second group includes hormones that interact with intracellular receptors.

The binding of a hormone (the primary messenger) to its receptor induces a conformational change in the receptor. This change is detected by other macromolecules, meaning that hormone-receptor binding couples one set of molecules to another (signal transduction). This generates a signal that regulates the cellular response by altering the activity or Abundance of enzymes and other proteins. Depending on The pathway of hormonal signal transduction, the rates of metabolic reactions change:

✵ as a result of changes in enzyme activity;

✵ as a result of changes in enzyme abundance (Table 11-3).

Class="center">Table 11-3. MAIN STAGES OF hormonal signal transduction

1. Hormonal Signal Transduction via Membrane Receptors

Hormones (primary messengers), upon binding to receptors on the cell membrane surface, form a hormone-receptor complex that converts the primary messenger's signal into fluctuations in the intracellular concentrations of specialized molecules known as secondary messengers. Secondary messengers may include the following molecules: cAMP, cGMP, IP₃, DAG, Ca²⁺, and NO.

Hormones whose interaction with target cell receptors leads to The production of cAMP act through a three-component system comprising a receptor protein, a G protein, and the enzyme adenylate cyclase. The cAMP synthesized by adenylate cyclase activates protein kinase A, which phosphorylates enzymes and other proteins (see Chapter 5). More than 200 different G proteins are known, each containing three subunits: α, β, and γ (see Chapter 5). In the absence of a hormone, the α subunit of the G protein is bound to GDP. The formation of the hormone-receptor complex induces Conformational Changes in the α subunit, the replacement of GDP with GTP, and the dissociation of the βγ dimer from α-GTP. In the case of receptors coupled to Gs proteins, the αs-GTP subunit activates adenylate cyclase (Fig. 11-3).

Fig. 11-3. Hormonal signal transduction via membrane receptors. IP₃ — Inositol 1,4,5-trisphosphate; DAG — diacylglycerol; PIP₂ — phosphatidylinositol bisphosphate; GH — growth hormone.

In the case of G_i protein-coupled receptors, the α_i-GTP subunit inhibits adenylyl cyclase. Table 11-4 lists Examples of hormones whose interaction with their respective receptors either activates or inhibits adenylyl cyclase.

Table 11-4. Activation and inhibition of adenylyl cyclase by hormones

* V₁ and \/₂ receptors are discussed below (IV, A).

Another system that generates cGMP as a secondary messenger is coupled to guanylyl cyclase. The cytoplasmic domain of this receptor type exhibits guanylyl cyclase activity, which catalyzes the formation of cGMP from GTP (similar to adenylyl cyclase). cGMP molecules can either activate Ion Channels or activate cGMP-dependent protein kinase G, which is involved in the phosphorylation of other cellular proteins. For example, phosphodiesterase, which hydrolyzes cAMP to AMP, is activated as a result of phosphorylation by cGMP-dependent protein kinase.

Certain hormones (such as vasopressin or epinephrine), by forming a complex with their corresponding receptors (the V₁ receptor for vasopressin and the α₁ receptor for epinephrine), activate phospholipase C via the activation of respective G proteins. This results in the intracellular generation of secondary messengers IP₃ and DAG. The IP₃ molecule stimulates the release of Ca²⁺ from the ER. Calcium binds to the protein calmodulin, and this complex activates a Ca²⁺-calmodulin-dependent protein kinase. Calcium Ions and DAG participate in the activation of protein kinase C (see Section 5).

Many hormones transmit signals into the cell via receptors that either possess tyrosine kinase activity or associate with cytoplasmic proteins exhibiting tyrosine kinase activity. The binding of insulin to a membrane receptor—which Functions as a tyrosine kinase and contains a phosphorylation site—initiates autophosphorylation, followed by the phosphorylation of insulin receptor substrates and other proteins (see Section 5 and Subsection III, G below).

In the case of the interaction of epidermal growth factor or insulin-like growth factor-1 with a membrane receptor, for instance, receptor dimerization and activation occur first. The thus-activated receptor homodimer, whose inner membrane domain possesses tyrosine kinase activity, undergoes autophosphorylation and induces the phosphorylation of other Proteins and Enzymes involved in activating Gene transcription factors.

Some hormones (such as growth hormone, prolactin, interferon, and cytokines) interact with membrane receptors associated with cytoplasmic protein kinases (known as Janus kinases, or JAK family kinases). Hormone binding induces receptor dimerization, recruitment of Janus kinases, and their subsequent autophosphorylation and activation. In turn, Janus kinases phosphorylate the receptor at tyrosine residues, enabling the receptor to bind other proteins, such as specific signal transducers and activators of transcription (STAT). This is followed by a tyrosine kinase-initiated phosphorylation cascade. STAT proteins are phosphorylated, form dimers, and are translocated into The Nucleus, where they bind to specific DNA regions to help regulate transcription (Fig. 11-4).

Fig. 11-4. Mechanism of signal transduction via Janus kinase (JAK)-associated membrane receptors. 1 — hormone interacts with the membrane receptor, inducing receptor dimerization. Janus kinases (cytoplasmic tyrosine kinases with two active sites) bind to the membrane receptor dimer, leading to their activation and autophosphorylation; 2 — Janus kinases (JAK) phosphorylate the receptor dimer at tyrosine residues; 3 — the complex of the phosphorylated receptor dimer and Janus kinases recruits specific cytoplasmic proteins (STAT), which are then phosphorylated by Janus kinases; 4 — phosphorylated STAT proteins are activated and form a dimer; 5 — the STAT dimer moves from the cytosol into the nucleus, binds to the promoter region of DNA, and induces gene transcription.

Nitric oxide (NO) can also serve as an Intracellular Signaling molecule. It is synthesized in the body from Arginine by the enzyme NO synthase, which is present in Nervous Tissue, vascular endothelium, platelets, and other tissues (see Section 9). NO molecules can rapidly diffuse across the membrane of the endothelial cells where they are synthesized into neighboring cells. The action of nitric oxide is short-lived, as the T_1/2 of NO ranges from 5 to 10 s. In the Blood, the molecule persists for approximately 100 ms because it rapidly reacts with molecular oxygen to form nitrite, which is subsequently converted into nit

rate and excreted in the urine. In target cells, such as smooth Muscle cells, NO interacts with the iron ion located in the active center of guanylyl cyclase (see Section 5), thereby promoting the rapid formation of cGMP. An increase in cGMP concentration in smooth muscle cells leads to the activation of

kinases, which ultimately results in the relaxation of vascular smooth muscle cells and subsequent vasodilation. The Mechanism of action of nitric oxide explains The Use of nitroglycerin as a medication for relieving acute cardiac pain, as nitroglycerin serves as a source of generated NO molecules that induce blood vessel relaxation and increase blood flow to the myocardium.

2. Signal transduction via intracellular receptors

Steroid and thyroid hormones bind to intracellular receptors and regulate the transcription rate of specific genes (Fig. 11-5).

Fig. 11-5. Hormonal signal transduction via intracellular receptors (steroid hormone receptors can be located in both the Cytoplasm and the nucleus).

In the absence of a hormone, intracellular receptors are typically bound to other proteins in the cytosol or nucleus. For example, glucocorticoid receptors form a complex with a chaperone in the cytosol, which prevents the receptor from binding to DNA molecules (Fig. 11-6).

Fig. 11-6. Regulation of steroid hormone receptor activity. 1 — in the absence of a hormone, the receptor forms a complex with a chaperone via its hormone-binding domain, preventing receptor-DNA binding; 2 — in the presence of a hormone, the receptor is released from the chaperone, forming a receptor dimer that binds to DNA and triggers transcriptional activation.

The interaction of the hormone with the binding site on the C-terminal region of the receptor polypeptide chain induces conformational changes and the release of the receptor from the chaperone. Two receptor molecules then associate to form a homodimer. The receptor dimer recognizes a specific nucleotide sequence located in the promoter region of the gene. Interaction with the specific DNA site known as the HRE (hormone response element) is mediated by the central domain of the receptor. This domain contains an Amino Acid Sequence that forms two "zinc fingers." In each zinc finger, a zinc atom is coordinated with four Cysteine residues (Fig. 11-7).

Fig. 11-7. STRUCTURE OF THE central domain of a steroid hormone receptor. 1 — amino acid residues involved in DNA binding; 2 — dimerization region. The central DNA-binding domain contains two "zinc fingers." Zinc atoms are linked to The amino acid sequence via cysteine residues. Functional regions 1 and 2 are responsible for DNA binding and receptor dimerization, respectively.

The structure of one zinc finger contains an amino acid sequence responsible for DNA binding, whereas the second zinc finger contains an amino acid sequence involved in receptor dimerization. The interaction of the hormone-receptor complex with a specific nucleotide sequence in the promoter region of DNA leads to the activation of transcription.

Thyroid hormone receptors are constitutively bound to DNA. In the absence of hormones, these receptors repress Gene Expression. Conversely, hormone binding converts them into transcriptional activators.

3. Signal transduction via ion channel-coupled receptors

Ion channel-coupled receptors are multi-subunit integral Membrane Proteins. They function simultaneously as ion channels and as receptors capable of specifically binding an extracellular effector that alters their ion conductance. Typical effectors of this type include hormones and Neurotransmitters (see Fig. 11-3).

Receptors associated with ion channels are known for A number of hormones and the majority of neurotransmitters, with the Acetylcholine Receptor being the most thoroughly studied. The acetylcholine receptor consists of five cylindrical subunits arranged in the membrane parallel to one another: α₂, β, y, δ. A Water-filled channel runs along the central axis between them. Each receptor subunit contains A large number of hydrophobic amino acid residues. Furthermore, all subunits feature a single helical transmembrane segment, whose amino acid side chains (polar uncharged residues, predominantly Serine and Threonine) line the inner surface of the central receptor channel. Leucine residues are localized in the middle region of the subunits facing the channel. In the presence of acetylcholine, lateral interactions between subunits keep the channel open, thereby facilitating ion transport. In the absence of acetylcholine, a shift in the relative orientation of the subunits causes the channel to close, as the inward-projecting leucine residues form a tight hydrophobic ring that blocks the movement of hydrated ions in this region (Fig. 11-8).

Fig. 11-8. Cytology/cytology/92.html">SCHEMATIC STRUCTURE OF the acetylcholine receptor. A — closed state of the receptor channel in the absence of acetylcholine; B — open state of the receptor channel in the presence of acetylcholine. The transmembrane helical segments of all 5 subunits contain polar uncharged amino acid residues; hydrophobic leucine residues (L), located in the middle of each helical hydrophilic segment, protrude into the central part of the channel and obstruct ion movement.