Mechanisms of Hormone Action

Introduction

Hormones regulate cellular activity through diverse signaling pathways. Peptide hormones act via second messengers, while steroid and thyroid hormones influence gene expression. Receptor interactions, including tyrosine kinase activation and adaptive changes like up-regulation and down-regulation, ensure precise physiological control.

Receptors

• Receptors are specific protein molecules that bind hormones and mediate their effects on target cells. They are located on the cell membrane, in the cytoplasm, or within the nucleus.
• Hormone binding forms a hormone–receptor complex, which initiates intracellular signaling pathways. These pathways involve sequential enzymatic reactions that generate second messengers.
• A key feature is signal amplification, where a small initial signal produces a large cellular response. This allows hormones to act effectively even at very low concentrations, typically 10⁻⁹ to 10⁻¹² mol per liter.
• Peptide hormones bind to membrane receptors and activate intracellular signaling cascades.
• Most amine hormones also act through surface receptors and second messengers.
• Steroid and thyroid hormones diffuse into cells and bind to intracellular receptors. These receptors regulate gene transcription, leading to long-term cellular effects.
• Receptor specificity ensures that each hormone produces a precise and regulated physiological response.

Receptor Functions

• Membrane receptors are large glycoproteins with molecular weight of about 50,000 to 200,000 daltons.
• Many receptors traverse the membrane multiple times, often forming complex transmembrane structures.
• Hormone binding forms a hormone–receptor complex, which is internalized by endocytosis.
• Inside the cell, the complex is degraded by lysosomal enzymes.
• Receptors are commonly recycled back to the membrane, although some are degraded.
• Structural defects or mutations in receptors can lead to receptor-related diseases.

Quantity and Sensitivity of Receptor

Receptor number and sensitivity are regulated by circulating hormone concentration. These adaptations include up-regulation, down-regulation, and desensitization.

Up-regulation

• Up-regulation occurs when hormone levels remain low for a prolonged period.
• The number of receptors on target cells increases.
• Receptor sensitivity to the hormone also increases. This enhances cellular responsiveness to limited hormone availability. It contributes to phenomena such as denervation hypersensitivity.

Down-regulation

• Down-regulation occurs when hormone levels are persistently elevated.
• The number of receptors on target cells decreases. This reduces cellular responsiveness and prevents excessive stimulation.

Desensitization

• Desensitization refers to reduced responsiveness despite continued hormone presence. It occurs with chronic exposure to high hormone concentrations.
• Receptor signaling efficiency declines even if receptor number remains unchanged.
• Desensitization may affect the same hormone, termed homologous desensitization. It may also affect responses to other hormones, termed heterologous desensitization.
• These regulatory mechanisms ensure stable homeostasis by preventing overstimulation or under-response of target tissues.

G Proteins

• G proteins are membrane-associated proteins that couple surface receptors to intracellular effectors.
• Most peptide hormones act through receptors linked to these proteins.
• G proteins consist of multiple subunits, each mediating specific signaling pathways. They connect receptors to enzymes or ion channels present in the cell membrane.
• When a hormone binds to its receptor, the G protein becomes activated.
• The protein exchanges GDP for GTP, initiating the signaling process.
• The activated G protein interacts with effector molecules such as adenylate cyclase or phospholipase. These effectors generate second messengers, which amplify the signal inside the cell.
• Second messengers produce specific changes in cellular activity, leading to physiological responses.
• G proteins possess intrinsic GTPase activity, which converts GTP back to GDP. This mechanism terminates the signal and restores the inactive state. Thus, G proteins play a crucial role in signal transduction, ensuring rapid and regulated hormone action.

Types of G Proteins

• G proteins are broadly classified into small and large types based on molecular size.

Small G Proteins

• Small G proteins are monomeric proteins that regulate intracellular signaling and cellular organization.
• Major families include Ras, Rab, and Rho or Rac groups.
• Many small G proteins are attached to membranes by lipid modification, while some remain in the cytosol.

GAPs and GEFs

• Their activity is controlled by regulatory proteins.
• Guanine exchange factors activate them by promoting exchange of GDP for GTP.
• GTPase activating proteins inactivate them by accelerating GTP hydrolysis.

Functions of Small G Proteins

• Small G proteins play essential roles in cell function.
• The Rab family regulates vesicle transport between organelles and the plasma membrane.
• The Rho or Rac family controls cytoskeletal organization and cell shape.
• The Ras family regulates cell growth and signal transmission from membrane to nucleus.
• These proteins are crucial for cell signaling, transport, and growth regulation.

Large G Proteins

• Large G proteins are membrane-associated proteins that couple receptors to intracellular signaling pathways. They belong to families such as Gs, Gi, Gq, Gt, and G13.
• These proteins are heterotrimeric, consisting of α, β, and γ subunits. In the resting state, the α subunit is bound to guanosine diphosphate.

Mechanism of Action

• Activation begins when a hormone binds to a G protein–coupled receptor. This causes exchange of guanosine diphosphate with guanosine triphosphate on the α subunit.
• The α subunit separates from the β–γ complex and activates effector systems.
• The β–γ complex may also participate in signaling.
• The α subunit has intrinsic GTPase activity, which hydrolyzes guanosine triphosphate to guanosine diphosphate. This terminates the signal and allows reassociation of subunits.
• Regulatory proteins enhance this inactivation process.
• Different α subunits produce specific effects.
• Gs stimulates adenylyl cyclase and increases cyclic adenosine monophosphate.
• Gi inhibits adenylyl cyclase and reduces cyclic adenosine monophosphate.
• Gq activates phospholipase C, leading to intracellular signaling via second messengers.

G Protein Coupled Receptors

• G protein–coupled receptors are transmembrane proteins with seven membrane-spanning segments.
• Binding of a ligand induces conformational change and activates multiple G proteins. This results in signal amplification and enhanced cellular response.
• Receptor activity can be reduced by phosphorylation, which limits prolonged signaling.

G Protein Diseases

• Mutations in G proteins or their receptors can alter signaling pathways.
• Such abnormalities may lead to disease states due to excessive or reduced cellular responses.

Dysfunction due to Increased G Protein Responses

• Increased G protein activity can result from mutations that reduce GTPase activity of the α subunit. This prolongs signaling and increases cyclic adenosine monophosphate production.
• Somatotroph tumors may cause acromegaly due to persistent stimulation.
• McCune–Albright syndrome involves similar mutations, leading to endocrine overactivity, hyperpigmentation, and hormonal excess.

Table 53.1: Ligands (hormones/chemicals) that act through G proteins coupled receptors (GPCRs).

Ligand Category	Examples (GPCR-mediated)
Neurotransmitters	Catecholamines (epinephrine, norepinephrine, dopamine), acetylcholine, histamine, serotonin, adenosine, opioids
Peptide hormones	Angiotensin II, ADH, oxytocin, GRP, VIP, PTH, TRH
Glycoproteins	TSH, FSH, LH, hCG
Lipid mediators	Thromboxane A₂ (arachidonic acid derivative)
Tachykinins	Substance P, neurokinin A, neuropeptide K
Miscellaneous	Endothelins, platelet-activating factor (PAF), tastants, odorants

Dysfunction due to Decreased G Protein Responses

• Type 1 pseudohypoparathyroidism results from a defect in the Gαs protein of the G protein–coupled receptor pathway.
• The impaired signaling prevents normal cellular response to parathyroid hormone.
• Despite adequate hormone levels, target tissues show resistance. This leads to biochemical and clinical features resembling hypoparathyroidism.

Second Messengers (Signal Transduction Pathways)

• Hormones act as first messengers and bind to specific cellular receptors. This interaction forms a hormone–receptor complex, initiating intracellular signaling events.
• Signal transduction leads to generation of second messengers that amplify cellular responses.
• Four principal mechanisms mediate second messenger formation.
• Receptor activation can open or close ion channels, altering membrane potential.
• Receptors may activate G proteins, which regulate downstream signaling cascades.
• Some receptors stimulate enzymes, producing molecules such as cyclic nucleotides or inositol phosphates.
• Certain pathways directly influence gene transcription, modifying protein synthesis.

Table 53.2: Broad mechanisms of ligands bringing about change in cell functions.

Mechanism of Action	Representative Ligands and Effects
Ion channel modulation	Acetylcholine (nicotinic receptors); norepinephrine alters cardiac potassium channels
Adenylate cyclase (cAMP)	Norepinephrine via β₁ increases cyclic adenosine monophosphate; via α₂ decreases it
Phospholipase C pathway	Angiotensin II, norepinephrine (α₁), antidiuretic hormone (V₁) generate diacylglycerol and inositol trisphosphate
Cyclic guanosine monophosphate	Atrial natriuretic peptide, nitric oxide
Tyrosine kinase receptors	Insulin, epidermal growth factor, platelet-derived growth factor
Serine/threonine kinase receptors	Transforming growth factor beta, activin, inhibin
Nuclear receptors	Steroid and thyroid hormones regulate gene transcription

Table 53.3: Important protein kinases in the cell.

Kinase Class	Key Examples
Serine/Threonine kinases	Calmodulin-dependent kinases (myosin light chain kinase, phosphorylase kinase, Ca²⁺–calmodulin kinases I–III); protein kinase C; cyclic nucleotide–dependent kinases (protein kinase A, cyclic guanosine monophosphate–dependent kinase)
Tyrosine kinases	Insulin receptor, epidermal growth factor receptor, platelet-derived growth factor receptor, macrophage colony-stimulating factor receptor

Role of phosphorylation:

• Phosphorylation is a key regulatory process in signal transduction.
• Protein kinases add phosphate groups to serine, threonine, or tyrosine residues.
• Protein phosphatases remove these phosphate groups, reversing the signal.
• More than 300 protein kinases have been identified, indicating pathway diversity. ·  The balance between phosphorylation and dephosphorylation functions as a molecular timing mechanism controlling signal duration and intensity.

Second Messengers

• Second messengers are intracellular signaling molecules generated after formation of the hormone–receptor complex. Their production depends on receptor type and effector cell specificity.
• Many pathways are initiated through activation of G proteins linked to membrane enzymes.
• Major second messengers include cyclic adenosine monophosphate, diacylglycerol, inositol trisphosphate, cyclic guanosine monophosphate, intracellular calcium, and phosphorylated proteins. These molecules amplify signals and regulate diverse cellular responses.
• Four principal systems mediate second messenger generation.
• The adenylyl cyclase system produces cyclic adenosine monophosphate.
• The phospholipase pathway generates diacylglycerol and inositol trisphosphate.
• The guanylyl cyclase system forms cyclic guanosine monophosphate.
• Some signals directly alter gene transcription through new messenger ribonucleic acid synthesis.

Cyclase–Cyclic AMP System

• The cyclic adenosine monophosphate system is widely used by peptide and amine hormones. It also regulates immune cell activation and secretory responses.

Steps of Signal Transduction

• Signal transduction begins when a hormone binds to a specific membrane receptor. This interaction forms a hormone–receptor complex, initiating intracellular signaling.
• The complex activates G proteins, either stimulatory or inhibitory types.
• Stimulatory G protein enhances downstream activity, whereas inhibitory G protein suppresses it.
• The enzyme adenylyl cyclase, an integral membrane protein, is then regulated.
• Activation increases cyclic adenosine monophosphate formation, while inhibition reduces its synthesis.
• Adenylyl cyclase converts adenosine triphosphate into cyclic adenosine monophosphate, a key second messenger.
• Cyclic adenosine monophosphate activates protein kinase A, a tetrameric enzyme with regulatory and catalytic subunits.
• Binding of cyclic adenosine monophosphate releases active catalytic subunits.
• These catalytic units phosphorylate serine and threonine residues on intracellular proteins.
• Phosphorylation produces phosphoproteins that alter enzyme activity and cellular responses.
• Activated protein kinase A may enter the nucleus and modify transcription factors. This leads to changes in gene expression through deoxyribonucleic acid–binding proteins.
• The coordinated effects of phosphorylation and gene regulation produce specific physiological responses.

Termination of cAMP Actions

• Cyclic adenosine monophosphate signaling is terminated by enzymatic degradation and protein dephosphorylation.
• Phosphodiesterase converts cyclic adenosine monophosphate into inactive adenosine monophosphate.
• Protein phosphatases remove phosphate groups from previously phosphorylated proteins, reversing cellular effects.

Clinical Importance

• Many hormones act through cyclic adenosine monophosphate–mediated pathways.
• Certain bacterial toxins disrupt signaling by altering cyclic adenosine monophosphate levels. ·  Cholera toxin increases cyclic adenosine monophosphate, whereas pertussis toxin reduces its regulation, leading to abnormal cellular responses.

Table 53.4: Signal transduction pathways for hormones.

Hormone/Signal	Pathway Component	Activated Process	Key Outcome
Parathyroid hormone, β-adrenergic signals	Gαs protein	Adenylyl cyclase activation	↑ cyclic adenosine monophosphate
Angiotensin II, α₂ signals	Gαi protein	Inhibition of adenylyl cyclase	↓ cyclic adenosine monophosphate
Angiotensin II, α₁ signals, vasopressin	Gαq protein	Phospholipase C activation	Inositol trisphosphate, diacylglycerol
Atrial natriuretic peptide, nitric oxide	Receptor enzymes	Guanylyl cyclase activation	Cyclic guanosine monophosphate
Growth factors, insulin	Enzyme-linked receptors	Tyrosine kinase signaling	Phosphoproteins
Thyroid, steroid hormones	Nuclear receptors	Gene transcription	Messenger ribonucleic acid
Neurotransmitters	Ion channels	Channel modulation	Calcium or potassium flux

Table 53.5: Important hormones that act by altering cAMP concentration in the cell.

Effect on cAMP	Representative Hormones
Increase in cAMP	Growth hormone–releasing hormone, corticotropin-releasing hormone, gonadotropin-releasing hormone, follicle-stimulating hormone, luteinizing hormone, β₁-adrenergic signals, vasopressin (V₂ receptor)
Decrease in cAMP	α₂-adrenergic signals, somatostatin

Stimulation of cAMP

• Cholera is an acute diarrheal disease caused by Vibrio cholerae.
• Cholera toxin modifies the stimulatory G protein by adenosine diphosphate–ribosylation using nicotinamide adenine dinucleotide. This modification persistently activates adenylyl cyclase, causing excessive cyclic adenosine monophosphate production.
• Elevated cyclic adenosine monophosphate reduces sodium absorption and increases chloride secretion in intestinal epithelial cells.
• Water follows electrolytes osmotically, producing profuse isotonic diarrhea.

Inhibition of cAMP

• Pertussis is a respiratory infection caused by Bordetella pertussis.
• Pertussis toxin enhances inhibitory G protein activity, suppressing adenylyl cyclase function. ·  Reduced cyclic adenosine monophosphate signaling disrupts immune cell responses and weakens host defense mechanisms.

Membrane Phospholipid-Phospholipase System (via IP3 and DAG)

• The phospholipase C pathway is a major mechanism for hormone signal transduction.
• A hormone binds to its receptor and forms a hormone–receptor complex. This complex activates the Gαq protein on the inner membrane surface.
• Activated Gαq stimulates phospholipase C, a membrane-associated enzyme.
• Phospholipase C hydrolyzes phosphatidylinositol 4,5-bisphosphate, a key membrane phospholipid. This reaction generates two second messengers: inositol trisphosphate and diacylglycerol.
• Inositol trisphosphate diffuses into the cytosol and releases calcium from the endoplasmic reticulum.
• Increased intracellular calcium acts as an important signaling mediator.
• Diacylglycerol remains within the membrane and activates protein kinase C.
• Protein kinase C phosphorylates target proteins, modifying enzyme activity.
• These parallel pathways form the inositol trisphosphate–calcium and diacylglycerol–protein kinase C subsystems. The combined actions regulate secretion, contraction, metabolism, and gene expression.

In IP3 Subsystem:

• In the inositol trisphosphate subsystem, inositol trisphosphate binds to receptors on the endoplasmic reticulum. This interaction releases calcium ions, raising cytosolic calcium several-fold.
• Elevated calcium binds to calmodulin and other calcium-binding proteins. This activates calcium–calmodulin–dependent protein kinases.
• These proteins modify cellular activities such as secretion, contraction, and metabolism.

In DAG Subsystem:

• In the diacylglycerol subsystem, diacylglycerol activates protein kinase C within the membrane.
• Protein kinase C phosphorylates multiple intracellular proteins.
• The resulting phosphoproteins mediate specific functional responses.
• Hormones using this pathway include norepinephrine (α₁ receptor), vasopressin (V₁ receptor), angiotensin II, and thyrotropin-releasing hormone.
• Termination involves metabolism of second messengers.
• Inositol trisphosphate is dephosphorylated to inositol.
• Diacylglycerol is converted to phosphatidic acid and then recycled to phosphatidylinositol, maintaining membrane signaling capacity.

Activation of Phospholipase A2

• Hormone binding activates Gαq protein, promoting diacylglycerol formation.
• Diacylglycerol and Gαq stimulate membrane phospholipase A2. This enzyme releases arachidonic acid from membrane phospholipids.
• Arachidonic acid is converted into prostaglandins, prostacyclin, thromboxanes, and leukotrienes. These mediators regulate inflammation, vascular tone, and cellular responses.
• Certain peptide hormones, such as thyrotropin-releasing hormone, utilize this pathway.

Guanylyl Cyclase-Cyclic GMP System

• The cyclic guanosine monophosphate system functions as an important second messenger pathway.
• A hormone binds to its membrane receptor, forming a hormone–receptor complex. This interaction activates guanylyl cyclase, which is part of the receptor’s intracellular domain.
• Guanylyl cyclase converts guanosine triphosphate into cyclic guanosine monophosphate.
• Cyclic guanosine monophosphate activates specific protein kinases and phosphatases. These enzymes regulate phosphorylation of intracellular proteins and modify cellular responses. It can also directly regulate ion channels, especially in visual pathways of retinal cells.
• Important ligands include atrial natriuretic peptide, nitric oxide, and intestinal peptides such as guanylin.
• Certain bacterial toxins also increase cyclic guanosine monophosphate, causing secretory diarrhea.

Intracellular Receptor (Transcription of mRNA) System

• In the intracellular receptor system, lipid-soluble hormones diffuse across the cell membrane. They bind to receptors located in the cytoplasm or nucleus.
• The hormone–receptor complex interacts with deoxyribonucleic acid, regulating gene transcription. This process leads to synthesis of new proteins through messenger ribonucleic acid formation.

Receptors

• Steroid hormone receptors are intracellular monomeric phosphoproteins. They share structural similarity with receptors for vitamin D, thyroid hormones, and retinoids, forming a gene superfamily. These receptors contain multiple functional domains, labeled A to E.
• The C domain is highly conserved and mediates binding to deoxyribonucleic acid.
• Some receptors, such as estrogen receptor, possess an additional domain.
• Receptors undergo dimerization after hormone binding, which is essential for transcriptional activation. They have distinct amino-terminal and carboxy-terminal regions.
• The hormone-binding domain is located near the carboxy terminal.

• A cysteine-rich deoxyribonucleic acid–binding domain contains zinc finger motifs. These structures enable precise interaction with target genes, regulating protein synthesis.

Steps of Signal Transduction

• Hormone binding to its intracellular receptor induces a conformational change in the receptor protein. This change exposes the deoxyribonucleic acid–binding domain, enabling gene interaction.
• In the inactive state, receptors are associated with heat shock proteins, which mask the binding domain.
• Hormone attachment causes dissociation of these proteins, activating the receptor.
• Heat shock proteins function as protective molecules during cellular stress.
• The activated hormone–receptor complex translocates to the nucleus if required. It binds to specific hormone response elements within deoxyribonucleic acid. This binding initiates transcription, producing messenger ribonucleic acid from target genes.
• The synthesized messenger ribonucleic acid is translated on ribosomes into new proteins.
• Newly formed proteins modify cellular activities such as metabolism, growth, and differentiation. These genomic effects are slower in onset but produce sustained physiological responses.

Other Mechanisms of Signal Generation

Tyrosine Kinase Activation

• Some peptide hormones signal through tyrosine kinase receptors without involving G proteins. These receptors possess extracellular, transmembrane, and intracellular kinase domains.
• Hormone binding induces a conformational change, exposing intracellular catalytic sites.
• The receptor undergoes autophosphorylation on specific tyrosine residues. It also phosphorylates intracellular substrate proteins. This phosphorylation initiates a cascade activating serine–threonine kinases and phosphatases. These downstream enzymes regulate metabolism, growth, and differentiation.
• Important ligands include insulin, insulin-like growth factors, and growth factors such as platelet-derived and epidermal growth factors.

JAK-STAT Pathway

• Another mechanism involves the Janus kinase–signal transducer and activator of transcription pathway.
• Hormone binding exposes intracellular sites that recruit cytoplasmic tyrosine kinases.
• Activated Janus kinases phosphorylate signal transducer and activator of transcription proteins.
• Phosphorylated signal transducer proteins dimerize and translocate to the nucleus. They bind to deoxyribonucleic acid and regulate gene transcription.
• This pathway produces rapid and sustained changes in cellular function.
• Growth hormone and several cytokines commonly utilize this signaling mechanism.

Calcium–Calmodulin System

• The calcium–calmodulin system mediates hormone signaling through intracellular calcium changes.
• Hormone binding activates G proteins, which open membrane calcium channels.
• Calcium enters from extracellular fluid, increasing intracellular concentration.
• Calcium acts as a second messenger, regulating multiple cellular processes. It is also released from intracellular stores such as the endoplasmic reticulum and mitochondria.
• Depletion of stored calcium triggers store-operated calcium entry, enhancing calcium influx. This mechanism helps restore intracellular calcium balance and sustain signaling.
• Elevated calcium binds to intracellular proteins, especially calmodulin.
• The calcium–calmodulin complex activates specific enzymes and kinases.

• These events regulate secretion, contraction, metabolism, and gene expression.

Calmodulin-dependent Kinases

• Calmodulin is a 148–amino acid protein with four calcium-binding domains.
• Binding of calcium activates calmodulin-dependent kinases, which regulate diverse cellular functions.
• Myosin light chain kinase phosphorylates myosin, facilitating muscle contraction.
• Phosphorylase kinase promotes phosphorylation of intracellular proteins involved in metabolism.
• Calcium–calmodulin kinases I and II are important for synaptic activity and neuronal signaling.
• Calcium–calmodulin kinase III participates in protein synthesis.

Calcium Binding Proteins

• Key calcium-binding proteins include calmodulin, troponin, and calbindin.
• Troponin mediates calcium-dependent contraction in skeletal muscle.
• Calcineurin, a calmodulin-activated phosphatase, dephosphorylates target proteins. It regulates calcium channel activity and plays a critical role in T lymphocyte activation and immune responses.

Rapid Actions of Steroids

• Steroid hormones typically act via intracellular receptors, producing delayed genomic effects. These effects involve messenger ribonucleic acid transcription and new protein synthesis.
• Some steroid actions occur rapidly and are termed nongenomic actions. In these cases, steroids bind to membrane-associated receptors. This interaction alters ion permeability, especially calcium, or activates second messengers such as cyclic adenosine monophosphate.
• Rapid responses are evident in neuronal and vascular tissues. These actions do not require gene transcription.
• Steroids can modulate existing signaling pathways through intracellular messengers. This enables interaction with other hormones at the second messenger level.
• For example, estrogen can influence signaling pathways shared with neurotransmitters. ·  Such cross-talk enhances the diversity and speed of physiological responses.