Hormones & Signal Transduction

1. What this chapter covers, and how NEET PG actually tests it

Signal transduction looks like a long list of hormones and messengers, and is usually studied that way.

It is far smaller than it appears, because there are only four receptor families, and every hormone belongs to one of them.

Assigning a hormone to its family immediately predicts several things: how fast it acts, what its second messenger is, whether it changes gene expression, and which drugs and toxins interfere with it.

Speed is the most useful of these. A surface receptor acts in seconds because it modifies existing proteins; an intracellular receptor acts over hours because it must alter transcription.

So a question describing an effect within seconds has already excluded the steroid and thyroid hormones, whatever else it says.

This chapter covers G protein-coupled receptors, enzyme-linked receptors including insulin, intracellular nuclear receptors, and the toxins, amplification and desensitisation mechanisms that act on these systems.

2. G protein-coupled receptors

2.1 One architecture, three outcomes

A G protein-coupled receptor spans the membrane seven times and, when occupied, activates a heterotrimeric G protein by exchanging GDP for GTP on its alpha subunit.

What happens next depends entirely on which alpha subunit the receptor is coupled to, and there are three that matter.

The Gq pathway deserves one extra sentence because it splits.

Phospholipase C cleaves membrane PIP2 into two products. IP3 is water-soluble and diffuses to the endoplasmic reticulum to release stored calcium, while DAG remains in the membrane and activates protein kinase C.

A single receptor therefore generates two simultaneous signals, which is why Gq-coupled agonists produce both rapid calcium-dependent effects and slower phosphorylation-dependent ones.

2.2 Which receptors use which

The assignments are not random, but they are numerous enough to need organising.

Gs includes the beta-adrenergic receptors, glucagon, and most of the anterior pituitary trophic hormones — adrenocorticotropic hormone, thyroid stimulating hormone, luteinising hormone and follicle stimulating hormone — plus parathyroid hormone, histamine H2 and vasopressin V2.

Gi includes alpha-2 adrenergic, muscarinic M2, dopamine D2, opioid and somatostatin receptors.

Gq is the smallest and most worth memorising directly, since it is easiest to enumerate: alpha-1 adrenergic, muscarinic M1 and M3, histamine H1, vasopressin V1, along with angiotensin II, gonadotropin-releasing hormone, thyrotropin-releasing hormone and oxytocin.

Note that vasopressin appears twice, using different receptors for different jobs. V2 receptors in the collecting duct are Gs-coupled and insert aquaporins; V1 receptors on vascular smooth muscle are Gq-coupled and cause vasoconstriction.

The same logic applies to the adrenergic receptors, where beta receptors are Gs and Gi-coupled while alpha-1 is Gq — which is why adrenaline produces opposite effects in different vascular beds.

2.3 When the G protein itself is diseased

The G protein is a switch, and it can fail in either position — with diseases at both extremes.

A constitutively active Gs mutation leaves the switch permanently on, so the cell behaves as though hormone were always present.

McCune-Albright syndrome is the classic example, and its features follow directly. A post-zygotic mutation produces mosaicism, so only some tissues are affected — giving the characteristic patchy distribution.

Wherever the mutation lands, that tissue behaves as if maximally stimulated: polyostotic fibrous dysplasia in bone, café-au-lait macules in skin, and precocious puberty that is gonadotropin-independent because the gonad no longer needs the trophic signal.

A loss-of-function Gs mutation leaves the switch stuck off, so hormone binds normally but nothing happens downstream.

Pseudohypoparathyroidism type 1A is that disease. Parathyroid hormone levels are high because the feedback loop senses the low calcium, but the target tissue cannot respond.

The biochemistry therefore looks like hypoparathyroidism while the hormone level looks like hyperparathyroidism, which is the discriminating pattern, alongside the short fourth metacarpals of Albright hereditary osteodystrophy.

The same defect impairs signalling at other Gs-coupled receptors, which is why hypothyroidism and gonadal dysfunction often accompany it.

3. Enzyme-linked receptors

3.1 Receptor tyrosine kinases and the insulin branch point

A receptor tyrosine kinase has intrinsic catalytic activity in its intracellular domain.

Ligand binding causes the receptors to dimerise, after which each phosphorylates the other — autophosphorylation — creating docking sites for downstream proteins.

Insulin, insulin-like growth factor 1, epidermal growth factor, platelet-derived growth factor and fibroblast growth factor all use this design.

Insulin's cascade then divides into two arms, and the division has real clinical significance.

The PI3-kinase and Akt arm produces the metabolic effects, most importantly translocation of GLUT4 transporters to the membrane in muscle and adipose tissue.

The RAS and MAP kinase arm produces the growth-promoting effects on gene transcription and proliferation.

Because the two arms are separable, insulin resistance can be selective. The metabolic arm may become resistant while the growth arm remains fully responsive, which is one explanation for why hyperinsulinaemic states are associated with proliferative changes such as acanthosis nigricans despite poor glucose control.

The RAS and MAP kinase arm is also the pathway most frequently hijacked in cancer, which is why so many oncogenes are components of it.

3.2 JAK-STAT

Some receptors have no intrinsic kinase activity and instead recruit a separate Janus kinase on binding.

The activated JAK phosphorylates STAT proteins, which dimerise, translocate to the nucleus and act directly as transcription factors.

This pathway is unusually short — there is no long cascade between the membrane and the gene — which suits signals that must alter transcription reliably rather than rapidly.

Its users are the cytokines and a specific group of hormones: growth hormone, prolactin, erythropoietin, thrombopoietin, the interferons and most interleukins.

Knowing this group explains a family of drugs. JAK inhibitors such as tofacitinib work across rheumatoid arthritis and myeloproliferative disease precisely because so many cytokines converge on this one pathway.

3.3 Guanylyl cyclase and cyclic GMP

Two distinct receptors generate cyclic GMP, and the difference between them is examinable.

Atrial natriuretic peptide binds a membrane-bound guanylyl cyclase, a receptor whose intracellular domain is itself the enzyme.

Nitric oxide is a gas that crosses the membrane and activates a soluble, cytoplasmic guanylyl cyclase.

Both converge on cyclic GMP and protein kinase G, producing smooth muscle relaxation.

This convergence explains an entire drug class and its most important interaction.

Nitrates work by donating nitric oxide, raising cyclic GMP. Sildenafil works by inhibiting phosphodiesterase-5, the enzyme that degrades cyclic GMP.

Combining them raises cyclic GMP by two independent mechanisms simultaneously, producing profound vasodilation and hypotension — which is why the combination is contraindicated rather than merely cautioned against.

4. Intracellular receptors

4.1 Why these hormones are slow

Steroid hormones, thyroid hormone, vitamin D and retinoic acid are lipophilic, so they cross the plasma membrane rather than binding a surface receptor.

Their receptors are intracellular transcription factors.

In the unbound state, a steroid receptor is held in the cytoplasm by heat shock proteins that prevent it entering the nucleus.

Hormone binding displaces the heat shock protein, the receptor dimerises, enters the nucleus and binds a specific hormone response element in DNA, altering transcription of nearby genes.

Everything about the resulting kinetics follows from that mechanism.

Onset takes hours, because new protein must be synthesised. Duration is long, because the protein persists after the hormone has gone.

So a question describing an effect within seconds cannot involve these hormones, and one describing an effect persisting long after the hormone has been cleared probably does.

Note two variations. Thyroid hormone receptors are already in the nucleus and bound to DNA even without hormone, acting as repressors until hormone arrives. And thyroid hormone and vitamin D receptors typically form heterodimers with the retinoid X receptor rather than homodimers.

4.2 Why one drug can act oppositely in two tissues

A nuclear receptor does not act alone. Once bound to DNA it recruits coactivator or corepressor proteins, and which of these it recruits depends on the shape the ligand imposes on the receptor.

Because tissues express different coregulators, the same drug-receptor complex can activate transcription in one tissue and repress it in another.

Tamoxifen is the standard illustration. It occupies the oestrogen receptor and acts as an antagonist in breast tissue, which is its therapeutic purpose, while behaving as a partial agonist in endometrium and bone.

That single mechanism explains both its protective effect on bone density and its association with endometrial carcinoma — effects that would otherwise seem contradictory for one drug.

Raloxifene differs in the conformation it induces, which is why it retains the bone benefit without the endometrial stimulation.

The general principle is worth stating plainly: for nuclear receptors, the ligand does not simply switch the receptor on or off but selects among several possible active shapes, and the tissue then determines what each shape does.

5. Toxins, amplification and regulation

5.1 Two toxins that raise cyclic AMP by opposite routes

Cholera toxin ADP-ribosylates the alpha subunit of Gs, locking it in the active GTP-bound state because it can no longer hydrolyse GTP.

Adenylyl cyclase therefore runs continuously, cyclic AMP accumulates, protein kinase A phosphorylates the CFTR chloride channel, and chloride with water pours into the intestinal lumen.

The resulting secretory diarrhoea is isotonic and enormous, and the mucosa is structurally intact — which is why cholera stool has no blood or inflammatory cells.

Pertussis toxin ADP-ribosylates the alpha subunit of Gi, disabling it.

Since Gi normally restrains adenylyl cyclase, removing that restraint also raises cyclic AMP.

Both toxins therefore raise cyclic AMP, but one activates the accelerator and the other cuts the brake. That contrast is the point of the pairing in examinations, and confusing the two mechanisms is the standard error.

5.2 Amplification

A single hormone molecule produces an enormous cellular response because each step of a cascade is catalytic.

One occupied receptor activates many G proteins, each activating an adenylyl cyclase that makes many cyclic AMP molecules, each activating protein kinase A, which phosphorylates many substrate molecules.

This is why hormones are effective at nanomolar concentrations while their substrates are present at millimolar levels.

Amplification also explains why signalling must be terminated actively, since an unopposed cascade would be self-sustaining.

Termination occurs through the intrinsic GTPase activity of the G alpha subunit, and through phosphodiesterases that degrade cyclic nucleotides — the enzymes that caffeine and sildenafil inhibit.

5.3 Desensitisation and down-regulation

Continued stimulation produces tolerance, by two distinct mechanisms operating on different timescales.

Desensitisation is rapid. A G protein-coupled receptor kinase phosphorylates the occupied receptor, allowing beta-arrestin to bind, which uncouples the receptor from its G protein and promotes internalisation.

Down-regulation is slower and involves an actual reduction in receptor number through degradation and reduced synthesis.

Together these explain why continuous agonist exposure loses effect while intermittent exposure does not — the clinical reason that continuous gonadotropin-releasing hormone agonist therapy suppresses gonadotropins while the natural pulsatile pattern stimulates them.

That distinction, between pulsatile and continuous stimulation of the same receptor producing opposite outcomes, is one of the most counterintuitive facts in endocrinology and follows directly from desensitisation.

7. Common exam traps

• Forgetting that Gq produces two messengers. IP3 releases calcium and DAG activates protein kinase C, from the same cleavage of PIP2.
• Treating vasopressin as having one receptor. V2 is Gs-coupled for water reabsorption; V1 is Gq-coupled for vasoconstriction.
• Confusing the two toxin mechanisms. Cholera locks Gs on; pertussis disables Gi. Both raise cyclic AMP.
• Expecting rapid effects from steroid or thyroid hormones. Transcription takes hours, so a second-scale effect excludes them.
• Assuming all tyrosine kinase signalling is a single pathway. Insulin's metabolic and growth arms diverge, which allows selective resistance.
• Overlooking JAK-STAT as the growth hormone and prolactin pathway. These are commonly assumed to be G protein-coupled.
• Missing the two sources of cyclic GMP. Atrial natriuretic peptide uses a membrane-bound enzyme, nitric oxide a soluble one.
• Assuming continuous stimulation is always stronger than intermittent. Desensitisation reverses this, and the reversal is used therapeutically.

Summary

• Four receptor families account for all hormone signalling, and assigning a hormone to its family predicts its speed, messenger and drug interactions.
• Surface receptors act within seconds by modifying existing proteins; intracellular receptors act over hours by altering transcription.
• Gs raises cyclic AMP and Gi lowers it, both acting through adenylyl cyclase and protein kinase A.
• Gq activates phospholipase C, cleaving PIP2 into IP3, which releases stored calcium, and DAG, which activates protein kinase C.
• The Gq group is worth direct memorisation: alpha-1, M1 and M3, H1 and V1, plus angiotensin II, gonadotropin-releasing hormone, thyrotropin-releasing hormone and oxytocin.
• Vasopressin uses Gs at V2 receptors for water reabsorption and Gq at V1 receptors for vasoconstriction.
• Receptor tyrosine kinases dimerise and autophosphorylate, and insulin's cascade then splits into a metabolic PI3-kinase arm and a growth-promoting RAS arm.
• Because those arms are separable, insulin resistance can be selective, sparing the proliferative effects.
• JAK-STAT is a short pathway used by cytokines, growth hormone, prolactin, erythropoietin and the interferons, which is why JAK inhibitors have such broad application.
• Atrial natriuretic peptide activates a membrane-bound guanylyl cyclase while nitric oxide activates a soluble one, both raising cyclic GMP and relaxing smooth muscle.
• Nitrates and sildenafil raise cyclic GMP by production and by blocked degradation respectively, which is why the combination is contraindicated.
• Steroid receptors are held by heat shock proteins until hormone binding releases them to dimerise and bind hormone response elements in DNA.
• Thyroid hormone receptors are already nuclear and DNA-bound, acting as repressors until hormone arrives, and typically heterodimerise with the retinoid X receptor.
• Cholera toxin locks Gs in its active state and pertussis toxin disables Gi, so both raise cyclic AMP by opposite routes.
• Catalytic amplification at every cascade step makes nanomolar hormone concentrations effective, and requires active termination by GTPase activity and phosphodiesterases.
• Desensitisation through receptor kinase phosphorylation and beta-arrestin, followed by down-regulation, explains why continuous stimulation suppresses while pulsatile stimulation sustains.
• A constitutively active Gs mutation gives McCune-Albright syndrome, with mosaic distribution and gonadotropin-independent precocious puberty.
• A loss-of-function Gs mutation gives pseudohypoparathyroidism type 1A, where parathyroid hormone is high but the tissue cannot respond, producing hypocalcaemic biochemistry with a hyperparathyroid hormone level.