Signal transduction: adenylate cyclase, cAMP, insulin, cortisol

General principles of cellular signal transduction

Signaling molecules (hormones, mediators) transmit information into the cell via receptors. The receptor type and signal transduction mechanism are determined by the physicochemical properties of the ligand. Hydrophilic hormones (adrenaline, glucagon, insulin, angiotensin II, ADH) cannot cross the plasma membrane and act via membrane receptors. Hydrophobic hormones (cortisol and other steroids) freely cross the membrane and bind to intracellular receptors.

There are three main types of membrane signaling systems: adenylate cyclase, inositol phosphate (both non-catalytic, G-protein-coupled), and catalytic receptors (insulin receptor, atrial natriuretic peptide receptor).

Adenylate cyclase system

The system consists of three components: a receptor (Rs — stimulatory or Ri — inhibitory), a G protein (αβγ subunits), and the enzyme adenylate cyclase. In the inactive state, the α subunit is bound to GDP.

Activation stages:

1. The hormone binds to the Rs receptor, altering its conformation.
2. The receptor interacts with the α subunit; GDP is replaced by GTP.
3. The α-GTP complex dissociates from βγ and laterally diffuses to adenylate cyclase.
4. Adenylate cyclase is activated and catalyzes the conversion of ATP to cAMP.
5. cAMP binds to the regulatory subunits of protein kinase A (PKA), releasing the catalytic subunits.
6. Active PKA phosphorylates target proteins at serine and threonine residues using ATP.

Inactivation occurs due to the intrinsic GTPase activity of the α subunit: GTP is hydrolyzed to GDP, the α subunit returns to βγ, and adenylate cyclase is turned off.

When a hormone binds to the Ri receptor, α_i-GTP is activated, which inhibits adenylate cyclase: cAMP concentration drops, PKA reassociates, and protein phosphorylation decreases.

Epinephrine (via β-receptors) and glucagon act through the adenylate cyclase system.

Inositol phosphate system

The structure is similar to the adenylate cyclase system, but instead of the G_s protein, a G_q protein operates, and instead of adenylate cyclase, phospholipase C (PLC) is involved.

After the hormone binds to the receptor, the α subunit exchanges GDP for GTP and activates PLC. The enzyme hydrolyzes membrane phosphatidylinositol-4,5-bisphosphate (PIP₂) into two second messengers:

• Diacylglycerol (DAG) remains in the membrane;
• Inositol 1,4,5-trisphosphate (IP₃) enters the cytosol.

IP₃ opens calcium channels of the endoplasmic reticulum, and Ca²⁺ ions are released into the cytosol along their concentration gradient.

Effects of calcium ions

• Ca²⁺ binds to the protein calmodulin (4 binding sites), forming the Ca₄-calmodulin complex, which activates Ca-calmodulin-dependent protein kinases that phosphorylate proteins at serine and threonine residues.
• Ca²⁺, together with DAG and membrane phosphatidylserine, activates protein kinase C (PKC), which also phosphorylates proteins at serine and threonine residues.

Angiotensin II, ADH (on vascular smooth muscle), and epinephrine in the liver act through the inositol phosphate system.

Catalytic receptors

These receptors possess intrinsic enzymatic activity.

Insulin receptor

It consists of two α- and two β-subunits. The β-subunits possess tyrosine kinase activity. Upon binding of insulin to the α-subunits, the β-subunits undergo autophosphorylation at tyrosine residues and phosphorylate IRS proteins (insulin receptor substrates). Effects:

• increased glucose transport due to the insertion of GLUT-4 into the membrane of insulin-sensitive tissues;
• activation of phosphoprotein phosphatase, which dephosphorylates key enzymes;
• activation of phosphodiesterase, which degrades cAMP to AMP (antagonism with the adenylate cyclase system);
• alteration in the expression of specific genes.

Guanylate cyclase system

The atrial natriuretic peptide receptor is itself a guanylate cyclase. Upon ligand binding, the enzyme converts GTP into cGMP, which activates protein kinase G (PKG)—a functional analogue of PKA. PKG phosphorylates target proteins.

Intracellular receptors

Typical for hydrophobic hormones (steroids, thyroid hormones), such as cortisol. Stages:

1. The hormone diffuses through the plasma membrane.
2. It binds to a cytoplasmic receptor protein.
3. The hormone-receptor complex enters the nucleus.
4. It binds to regulatory regions of DNA — enhancers (activation of transcription) or silencers (suppression of transcription).
5. The rate of synthesis of specific proteins and, consequently, cell metabolism changes.

This pathway provides a slow but prolonged effect.

Comparison of signaling systems

• Non-catalytic systems (adenylate cyclase, inositol phosphate) are G-protein-coupled and use second messengers (cAMP, IP₃, DAG, Ca²⁺).
• Catalytic receptors are enzymes themselves (tyrosine kinase, guanylate cyclase).
• Intracellular receptors act at the level of gene transcription.
• The choice of pathway is determined by hormone solubility: hydrophilic use membrane receptors; hydrophobic use intracellular receptors.

Key points

• Adenylate cyclase system: receptor → G_s/i protein → adenylate cyclase → cAMP → PKA → protein phosphorylation.
• The α subunit of the G protein has intrinsic GTPase activity, which ensures signal turn-off.
• The inositol phosphate system generates two messengers — IP₃ (mobilizes Ca²⁺ from the ER) and DAG (activates PKC along with Ca²⁺).
• Ca²⁺ acts via calmodulin and Ca-calmodulin-dependent protein kinases, and also activates PKC.
• The insulin receptor is a tyrosine kinase; it phosphorylates IRS proteins, increases glucose transport via GLUT-4, and activates cAMP phosphodiesterase.
• The atrial natriuretic peptide receptor is a guanylate cyclase that generates cGMP and activates PKG.
• Hydrophobic hormones (cortisol) act via intracellular receptors, regulating gene transcription through enhancers and silencers.
• Epinephrine and glucagon work through the adenylate cyclase system; angiotensin II and ADH work through the inositol phosphate system.