Signal Transduction Pathways

General Signal Transduction Mechanism

• Signal → Receptor binding → Intracellular cascade → Gene transcription/Protein modification → Cellular response

Key Mechanisms

• Receptor Tyrosine Kinase (RTK) Pathway – Ligand binding (e.g., EGF, FGF, PDGF) causes receptor dimerization and autophosphorylation of tyrosine residues. Phosphorylated tyrosines serve as docking sites for adapter proteins (Grb2, SOS) that activate RAS, a small GTPase. RAS-GTP recruits RAF to the membrane, initiating the MAPK/ERK cascade (RAF → MEK → ERK). Activated ERK translocates to the nucleus and phosphorylates transcription factors (e.g., c-Fos, c-Jun) to promote cell proliferation and differentiation. Alternatively, RTKs can activate PI3K/AKT pathway, leading to survival signals and metabolic changes.

• G-Protein Coupled Receptor (GPCR) Pathway – Ligand binding (e.g., epinephrine, acetylcholine, chemokines) causes conformational change in the transmembrane receptor, activating heterotrimeric G-proteins (Gαβγ). The Gα subunit exchanges GDP for GTP and dissociates from Gβγ. Gs activation increases cAMP (via adenylyl cyclase), activating PKA, which phosphorylates target proteins and transcription factors. Gi activation decreases cAMP and can activate K+ channels. Gq activation stimulates phospholipase C (PLC), generating IP3 (inositol 1,4,5-trisphosphate) and DAG (diacylglycerol). IP3 triggers Ca²⁺ release from intracellular stores; DAG and Ca²⁺ activate PKC (protein kinase C).

• Second Messenger Systems – Intracellular signaling relies on diffusible small molecules that amplify the initial signal. cAMP and Ca²⁺ are the classic second messengers. cAMP is synthesized by adenylyl cyclase and degraded by phosphodiesterases; elevated cAMP activates PKA, which phosphorylates numerous substrates including glycogen phosphorylase kinase and CREB (cAMP response element binding protein). Calcium acts through calmodulin, which activates kinases like CaMKII and phosphatases like calcineurin. These amplification cascades allow a single ligand-receptor interaction to produce many downstream effects.

• JAK-STAT Pathway – Cytokine receptors (lacking intrinsic kinase activity) bind ligands and recruit JAK kinases (Janus kinases) to their cytoplasmic tails. JAKs phosphorylate each other and the receptor, creating docking sites for STAT proteins (Signal Transducers and Activators of Transcription). STATs are phosphorylated by JAKs, dimerize, and translocate to the nucleus to regulate gene transcription. Critical for immune signaling (interferon, interleukin signaling).

• Wnt/β-Catenin Pathway – Wnt ligands bind Frizzled receptors and LRP5/6 coreceptors, inhibiting GSK-3β-mediated degradation of β-catenin. β-catenin accumulates and enters the nucleus, associating with TCF/LEF transcription factors to activate target genes (e.g., c-myc, cyclin D1). Dysregulation is critical in colorectal cancer.

• Notch Pathway – Ligand binding (Delta, Jagged) to Notch receptor causes proteolytic cleavage by γ-secretase, releasing the Notch intracellular domain (NICD). NICD translocates to the nucleus and activates CSL transcription factors, regulating cell fate decisions. Essential for developmental biology and stem cell maintenance.

• TGF-β/SMAD Pathway – TGF-β binds to serine/threonine kinase receptors, which phosphorylate SMAD2/3 proteins. Activated SMADs bind SMAD4 and translocate to the nucleus to regulate transcription. Important in fibrosis, cancer, and immune tolerance.

• Negative Regulation and Signal Termination – Phosphatases (PP2A, MKPs) remove phosphate groups; ubiquitin ligases tag proteins for proteasomal degradation; feedback inhibitors (e.g., SOCS proteins, MAPKinase phosphatases) limit pathway duration. Desensitization of GPCRs occurs via phosphorylation by GRKs and binding of arrestins. These mechanisms prevent excessive signaling.

• Oncogenic Signal Transduction – Gain-of-function mutations in RTKs (e.g., HER2 in breast cancer, EGFR in lung cancer, KIT in GIST) or RAS (30% of cancers) cause constitutive pathway activation, leading to uncontrolled proliferation, invasion, and metastasis. Patients typically present with malignancy-related symptoms (mass, constitutional symptoms).

• Loss-of-Function Signal Defects – Mutations in tumor suppressors (e.g., p53, RB, PTEN) or negative regulators (e.g., NF1 in neurofibromatosis type 1) impair growth inhibition signals. Patients develop neurofibromas, café-au-lait spots, and increased malignancy risk.

• Hormone and Neurotransmitter Dysfunction – Abnormalities in GPCR signaling or their ligands cause endocrine/neuropsychiatric diseases. Adrenergic receptor dysfunction contributes to hypertension, heart failure, and anxiety. Dopamine pathway defects underlie Parkinson's disease and schizophrenia. Thyroid hormone receptor mutations cause resistance to thyroid hormone.

• Immunological Dysregulation – JAK mutations (e.g., JAK2 V617F in myeloproliferative disorders) cause cytokine hypersensitivity. IL-6 and TNF-α overproduction via aberrant NF-κB signaling drive inflammation in rheumatoid arthritis and inflammatory bowel disease.

• Developmental Anomalies – Notch pathway mutations cause CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy). Hedgehog signaling defects (e.g., patched mutations) cause basal cell nevus syndrome and medulloblastoma.

• Molecular Genetic Testing – DNA sequencing identifies gain-of-function mutations (e.g., BRAF V600E in melanoma, HER2 amplification in breast cancer) and loss-of-function mutations (e.g., TP53, BRCA1/2, NF1). FISH, PCR, and next-generation sequencing (NGS) are standard. Critical for cancer diagnosis, prognosis, and targeted therapy selection.

• Phosphoprotein Analysis – Western blotting with phospho-specific antibodies quantifies pathway activation (e.g., phospho-ERK, phospho-AKT). **Immunohist

Match the receptor to its G protein

• Gq → PLC → IP3 + DAG: α1, M1/M3, H1, V1, AT-II, GnRH, TRH — remember "HAVe 1 M&M". IP3 releases stored Ca²⁺; DAG + Ca²⁺ activate PKC. A stem describing ↑ intracellular calcium with normal cAMP is Gq, not Gs.
• Gs (↑cAMP): β1/β2, D1, H2, V2, and most anterior pituitary/peptide hormones (TSH, ACTH, LH, FSH, PTH, glucagon, calcitonin). Gi (↓cAMP): M2, α2, D2 — the basis for clonidine and for M2-mediated bradycardia.

Toxins and G proteins (the most repeated distractor)

• Cholera toxin ADP-ribosylates Gαs, locking it GTP-bound → ↑cAMP → CFTR-mediated Cl⁻/water secretion. Pertussis toxin ADP-ribosylates Gαi, disabling inhibition of adenylyl cyclase — it also raises cAMP but does not activate Gs. Both end in ↑cAMP by opposite mechanisms.
• GNAS: activating mosaic mutation → McCune-Albright (café-au-lait, precocious puberty, polyostotic fibrous dysplasia); loss of function → Albright hereditary osteodystrophy with PTH resistance.

Kinase pathways worth memorizing

• Insulin receptor is an RTK, signaling through IRS-1 → PI3K/AKT → GLUT4 translocation. Insulin does not use cAMP; glucagon and epinephrine do. Opposing effects on glycogen phosphorylase hinge on this.
• NF1 encodes neurofibromin, a RAS-GAP: loss of function leaves RAS in the GTP-bound active state — a *loss*-of-function mutation producing gain of pathway signaling.
• JAK-STAT carries GH, prolactin, EPO, TPO, interferons, and most interleukins. JAK2 V617F is the polycythemia vera association.

Testing that changes therapy (name the guideline)

• NCCN Guidelines for Colon Cancer require RAS (KRAS/NRAS) and BRAF testing before anti-EGFR antibody therapy (cetuximab/panitumumab); a downstream RAS mutation makes upstream receptor blockade futile.
• NCCN Guidelines for Non-Small Cell Lung Cancer direct broad molecular profiling (EGFR, ALK, ROS1, BRAF) in advanced non-squamous disease to select tyrosine kinase inhibitors; ASCO/CAP HER2 testing guidelines govern IHC/FISH interpretation in breast cancer.

Second-messenger drug targets

• Phosphodiesterase inhibitors raise cyclic nucleotides: sildenafil (PDE5, cGMP), milrinone (PDE3, cAMP), theophylline (nonselective). Nitric oxide and nitrates act on soluble guanylyl cyclase; ANP/BNP act on a membrane receptor guanylyl cyclase — neither is a GPCR.