JAK-STAT, MAPK and PI3K Signalling
On this page
Direct answer
Three kinase cascades carry most hormone and cytokine traffic from membrane to nucleus. Receptors lacking intrinsic kinase activity (growth hormone, prolactin, erythropoietin, interferons, many interleukins) associate with Janus kinases (JAK), which phosphorylate the receptor, recruit STATs, and release STAT dimers that drive gene transcription — growth hormone works through JAK2-STAT5. Receptor tyrosine kinases (growth factors, insulin) instead assemble the GRB2-SOS-RAS-RAF-MEK-ERK mitogen-activated protein kinase cascade, and simultaneously activate phosphoinositide 3-kinase (PI3K), which generates PIP3 and activates AKT — insulin's metabolic arm — restrained by the tumour suppressor PTEN and feeding into mTOR, the cell's growth-permission hub. Each cascade has a named disease: JAK2 V617F in myeloproliferative neoplasms, RAS-MAPK germline mutations in Noonan syndrome, PTEN loss in Cowden syndrome.
What you must remember
- JAK-STAT mechanics: ligand dimerises the receptor, associated JAKs trans-phosphorylate each other and the receptor tail, STATs dock via SH2 domains, are phosphorylated on a single tyrosine, dimerise and enter the nucleus; STAT5 mediates growth hormone and prolactin actions, STAT1 the interferon-gamma axis.
- JAK2 V617F is present in over 95% of polycythaemia vera and about half of essential thrombocythaemia and primary myelofibrosis — a single gain-of-function substitution making haematopoietic cells hypersensitive to cytokines; ruxolitinib inhibits JAK1/2 for symptom and spleen control.
- Negative regulation of JAK-STAT: SOCS proteins (induced by STATs themselves) and PIAS proteins — the feedback loop examiners ask about when explaining why growth hormone pulses rather than signals continuously.
- MAPK chain to memorise: RTK - GRB2 - SOS - RAS (a small GTPase, locked off by GDP) - RAF - MEK - ERK, with ERK entering the nucleus to phosphorylate transcription factors like ELK1; GTPase-activating proteins (neurofibromin) switch RAS off.
- RASopathy gallery: Noonan syndrome (PTPN11, KRAS and related genes — short stature, pulmonic stenosis, hypertrophic cardiomyopathy), Noonan-with-multiple-lentigines, Costello (HRAS) and cardio-facio-cutaneous syndromes — all germline RAS-MAPK activation, a favourite one-mark genetic association.
- PI3K-AKT-mTOR axis: PI3K converts PIP2 to PIP3; AKT phosphorylates forkhead (FOXO), GSK3 and others; mTORC1 (gated by TSC1/TSC2 hamartin-tuberin) phosphorylates S6K and 4E-BP1 to license protein synthesis; PTEN opposes everything by dephosphorylating PIP3.
- Disease pairings on the same axis: PTEN mutation causes Cowden syndrome with multiple hamartomas and cancer risk; TSC1/TSC2 loss causes tuberous sclerosis, treated in Indian centres with everolimus for subependymal giant-cell astrocytomas and renal angiomyolipomas.
One growth factor, three outputs, reasoned
Platelet-derived growth factor binding its receptor tyrosine kinase sets three programmes in parallel, and separating them explains modern oncology. The GRB2-SOS-RAS-RAF-MEK-ERK arm drives immediate-early gene transcription — cell division decisions. The PI3K-AKT arm phosphorylates and inactivates pro-apoptotic BAD and FOXO factors — cell survival decisions — while for insulin's nearly identical receptor, this same AKT drives GLUT4 translocation and glycogen synthesis through GSK3 inhibition. The PLC-gamma arm releases inositol trisphosphate and diacylglycerol — calcium and protein kinase C signalling. Now the therapeutic logic reads like the pathway diagram: BRAF V600E melanoma responds to BRAF inhibition; everolimus mTOR-inhibits tuberous sclerosis lesions; and insulin resistance in type 2 diabetes sits in the insulin receptor substrate-PI3K arm while the MAPK arm (mitogenic, pro-atherogenic) stays responsive — the "selective insulin resistance" concept that explains why hyperinsulinaemic patients both store fat and develop atherosclerosis.
How the examiner frames it
Two linkage questions recur. "Which pathway mediates growth hormone — G protein, JAK-STAT or intracellular receptor?" JAK2-STAT5; the same answer pattern puts erythropoietin on JAK2 and interferon-gamma on STAT1, whose loss causes mendelian susceptibility to mycobacterial disease — a connective tissue of biochemistry and immunology Indian postgraduate exams relish. The second is the cancer-suppressor mapping: RTK - RAS - RAF - MEK - ERK versus PI3K - AKT - mTOR with PTEN as brake; give the tumour (Cowden for PTEN, tuberous sclerosis for TSC2, Noonan for PTPN11) and the candidate names the node. Add one practical note: JAK2 mutation testing on peripheral blood has replaced older clonality assays in Indian diagnostic panels for suspected polycythaemia vera.
Frequently asked questions
How does growth hormone signal at the cellular level?
Through a JAK2-STAT5 pathway: hormone dimerises its receptor, JAK2 phosphorylates STAT5, and STAT5 dimers drive IGF-1 and growth gene transcription.
What is the significance of JAK2 V617F?
A gain-of-function mutation found in over 95% of polycythaemia vera cases, making it both a diagnostic marker and the target of ruxolitinib therapy.
Which cascade does activated RAS initiate?
The RAF-MEK-ERK MAP kinase cascade, terminating in nuclear transcription factor phosphorylation and cell-cycle entry.
How does PTEN act as a tumour suppressor?
It dephosphorylates PIP3 back to PIP2, opposing PI3K-AKT signalling; its loss in Cowden syndrome unleashes survival and growth signalling.
Which pathway does insulin use for GLUT4 translocation?
The IRS-PI3K-AKT arm, which is selectively resistant in type 2 diabetes while the MAPK mitogenic arm remains responsive.