Cell Signalling Pathways

Cells must detect and respond to a vast array of signals — hormones, growth factors, neurotransmitters, cytokines, and physical stimuli — to coordinate their behaviour within tissues. Cell signalling is the molecular language that allows multicellular organisms to function as integrated systems.

Principles of Signal Transduction

Signal transduction converts an extracellular signal (first messenger) into an intracellular response via a series of molecular events. The key principles are: specificity (a receptor recognises only its cognate ligand), amplification (one receptor molecule can activate many downstream effectors), integration (cells receive multiple simultaneous signals and integrate them), adaptation (sustained signals lead to receptor desensitisation), and coordination (signalling pathways are interconnected in networks).

Extracellular signals include: endocrine hormones (travel in the blood, act at distant targets); paracrine signals (act on nearby cells); autocrine signals (act on the same cell); juxtacrine signals (require direct cell-cell contact); and neurotransmitters (released at synapses). Signalling molecules can be hydrophilic (cannot cross the plasma membrane, bind cell-surface receptors) or hydrophobic (lipid-soluble, cross the membrane, bind intracellular receptors — e.g., steroid hormones, thyroid hormone, retinoids, vitamin D).

G Protein-Coupled Receptors (GPCRs)

GPCRs constitute the largest family of cell-surface receptors (~800 genes in the human genome) and are the targets of ~34% of all approved drugs. They are seven-transmembrane (7-TM) proteins with an extracellular N-terminus and intracellular C-terminus. Ligand binding to the extracellular/transmembrane domain causes a conformational change that activates the associated heterotrimeric G protein (Gα, Gβ, Gγ). Activated Gα exchanges GDP for GTP and dissociates from Gβγ; both Gα-GTP and Gβγ regulate downstream effectors.

Gs pathway: Gαs activates adenylyl cyclase (AC) → cAMP synthesis from ATP. cAMP activates protein kinase A (PKA) by releasing its catalytic subunits from regulatory subunits. PKA phosphorylates serine/threonine residues on target proteins: glycogen phosphorylase kinase (glycogenolysis), hormone-sensitive lipase (lipolysis), cardiac L-type Ca²⁺ channels (increased contractility), CREB transcription factor (gene expression). cAMP is degraded by phosphodiesterases (PDEs) — inhibited by methylxanthines (caffeine, theophylline) and sildenafil (PDE5, cGMP-specific).

Gi pathway: Gαi inhibits adenylyl cyclase → reduced cAMP. Activated by M2 muscarinic receptors in the heart (bradycardia), α2-adrenoceptors, opioid receptors (μ, δ, κ), and dopamine D2 receptors.

Gq pathway: Gαq activates phospholipase C β (PLCβ) → cleaves PIP2 (phosphatidylinositol-4,5-bisphosphate) into two second messengers: IP3 (inositol-1,4,5-trisphosphate) + DAG (diacylglycerol). IP3 diffuses to the ER and binds IP3-gated Ca²⁺ channels (IP3R), releasing Ca²⁺ from the ER store into the cytoplasm. DAG remains in the membrane and, together with elevated [Ca²⁺]ᵢ, activates protein kinase C (PKC) → phosphorylates many substrates including MAPK pathway components and transcription factors. Receptors using Gq: M1/M3 muscarinic, α1-adrenoceptors, angiotensin AT1, vasopressin V1.

Receptor desensitisation: Sustained GPCR activation is attenuated by: (1) GRK (GPCR kinase) phosphorylation of the activated receptor; (2) β-arrestin binding to phosphorylated receptor → uncouples from G protein, prevents further signalling, and promotes receptor internalisation (endocytosis) via clathrin-coated pits; (3) receptor degradation (downregulation) or recycling to the cell surface.

Receptor Tyrosine Kinases (RTKs)

RTKs are single-pass transmembrane receptors with an extracellular ligand-binding domain and intracellular tyrosine kinase domain. Ligand binding (typically dimeric growth factors — EGF, PDGF, FGF, VEGF, insulin) induces receptor dimerisation and trans-autophosphorylation of tyrosine residues on the intracellular domain. Phosphotyrosines serve as docking sites for SH2-domain-containing adaptor proteins (GRB2, SHC, PLCγ, PI3K regulatory subunit p85).

Ras/MAPK cascade: GRB2 binds phospho-RTK → recruits SOS (a Ras-GEF) → activates Ras (small GTPase, exchanges GDP for GTP). Ras-GTP activates Raf (MAPKKK) → phosphorylates and activates MEK (MAPKK) → phosphorylates and activates ERK1/2 (MAPK). Activated ERK translocates to the nucleus and phosphorylates transcription factors (Elk-1, c-Fos) → upregulates genes for proliferation and differentiation. RAS mutations (most commonly G12V, G12D, Q61L) constitutively activate this pathway and are found in ~30% of all human cancers (KRAS in pancreatic cancer 90%, NRAS in melanoma, HRAS in bladder cancer). RAS inhibitors (sotorasib targets KRAS G12C) represent a major recent advance.

PI3K/Akt/mTOR pathway: RTK activation → p85/p110 PI3K recruited → phosphorylates PIP2 to PIP3 → PIP3 recruits PDK1 and Akt (PKB) to the membrane → PDK1 phosphorylates Akt-T308; mTORC2 phosphorylates Akt-S473 (full activation). Akt promotes cell survival (phosphorylates and inhibits pro-apoptotic BAD and FOXO), growth (activates mTORC1 via TSC1/2 inhibition → S6K and 4EBP1 phosphorylation → increased protein synthesis), and glucose metabolism (GLUT4 translocation). PTEN (phosphatase and tensin homolog) dephosphorylates PIP3→PIP2, opposing PI3K. PTEN is one of the most frequently mutated tumour suppressors (PTEN loss in prostate, endometrial, and glioblastoma). mTOR inhibitors (everolimus, temsirolimus) are used in renal cell carcinoma, breast cancer, and organ transplantation.

The JAK-STAT Pathway

Cytokines (interleukins, interferons, growth hormone, erythropoietin, G-CSF) signal through cytokine receptors that lack intrinsic kinase activity but are constitutively associated with Janus kinases (JAK1, JAK2, JAK3, TYK2). Cytokine binding → receptor dimerisation → JAK trans-phosphorylation and activation → JAK phosphorylates receptor tyrosines → docking sites for STAT (signal transducer and activator of transcription) proteins → JAK phosphorylates STAT → STAT dimerises → translocates to nucleus → activates target genes. IFN-γ → JAK1/JAK2 → STAT1 → antiviral genes (IRF1, MX1). EPO → JAK2 → STAT5 → erythroid survival/proliferation genes (BCL-XL, cyclin D). JAK2 V617F gain-of-function mutation causes polycythaemia vera, essential thrombocythaemia, and myelofibrosis — targeted by ruxolitinib (JAK1/2 inhibitor).

Second Messengers

cAMP, IP3, DAG, Ca²⁺, cGMP, and PIP3 are key second messengers that amplify and diversify signalling. cGMP is synthesised by guanylyl cyclase (soluble GC activated by NO; receptor GC activated by ANP/BNP). cGMP activates PKG (smooth muscle relaxation — vasodilation) and PDE5 (self-limiting). Sildenafil inhibits PDE5, increasing cGMP in penile corpus cavernosum → vasodilation → erection. Nitrates (GTN) are prodrugs that release NO → GC activation → vasodilation in coronary and systemic vasculature.

Downstream Signalling Integration

Signalling pathways are not linear but form interconnected networks with extensive cross-talk. PKC can activate Raf; ERK can phosphorylate and inhibit SOS (negative feedback); Akt can phosphorylate and inhibit Raf. The transcription factors activated (AP-1, NF-κB, CREB, STATs) integrate signals to determine gene expression responses. Feedforward and feedback loops determine signal duration, amplitude, and specificity.