The JAK-STAT signaling pathway transmits signals from cytokines, interferons, and growth factors to the nucleus, regulating gene expression in immune responses, inflammation, hematopoiesis, and cell growth. It provides a direct route from the cell surface to the nucleus.
JAK Kinases
The Janus kinase family has four members in mammals: JAK1, JAK2, JAK3, and TYK2. JAKs are large cytoplasmic tyrosine kinases with a unique domain structure. The FERM domain mediates association with cytokine receptors. The pseudokinase domain regulates kinase activity, and the C-terminal kinase domain catalyzes tyrosine phosphorylation. The pseudokinase domain was thought to lack catalytic activity, but it has regulatory functions and can serve as a binding platform.
JAKs are constitutively associated with the cytoplasmic tails of cytokine receptors. Receptor binding does not require activation, but the JAKs are held in an inactive conformation until ligand binds. JAK3 expression is restricted to hematopoietic cells, while the other JAKs are widely expressed. Mutations in JAK3 cause severe combined immunodeficiency, highlighting its essential role in immune system development.
Cytokine Receptors
Cytokine receptors lack intrinsic kinase activity and rely on associated JAKs for signaling. They are classified into several families based on structural features. Type I cytokine receptors include the receptors for interleukins, erythropoietin, growth hormone, and prolactin. They contain conserved extracellular WSXWS motifs and intracellular Box1 and Box2 regions required for JAK binding. Type II cytokine receptors include interferon receptors and the IL-10 receptor family.
Receptor activation involves ligand-induced dimerization or oligomerization. Some cytokine receptors are homodimers such as the erythropoietin receptor, while others are heterodimers of distinct subunits such as the IL-6 receptor. The common gamma chain is shared by receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21, and its mutation causes X-linked SCID.
STAT Activation
STAT proteins are latent cytoplasmic transcription factors. The seven mammalian STATs are STAT1, STAT2, STAT3, STAT4, STAT5A, STAT5B, and STAT6. They share conserved domains: an N-terminal domain involved in dimerization, a DNA-binding domain, an SH2 domain, and a C-terminal transactivation domain containing a conserved tyrosine that is the target of JAK phosphorylation.
Ligand binding brings receptor-associated JAKs into proximity, allowing trans-phosphorylation and activation. JAKs then phosphorylate tyrosine residues on the receptor cytoplasmic tail, creating docking sites for STAT SH2 domains. Recruited STATs are themselves phosphorylated by JAKs on the conserved C-terminal tyrosine. Phosphorylated STATs dissociate from the receptor and form dimers through reciprocal SH2-phosphotyrosine interactions.
Nuclear Translocation and Gene Regulation
STAT dimers accumulate in the nucleus, where they bind to specific DNA response elements in target gene promoters. STAT1 homodimers bind gamma-activated sequence elements in interferon-gamma target genes. STAT1-STAT2 heterodimers, together with IRF9, form the ISGF3 complex that binds interferon-stimulated response elements in interferon-alpha target genes. STAT3 regulates genes involved in acute phase response, cell survival, and proliferation.
STATs generally activate transcription, but can also repress gene expression through competition with other transcription factors or recruitment of corepressors. The transcriptional response is modulated by interactions with other transcription factors and coactivators such as p300 and CBP. STAT target genes include suppressors of cytokine signaling proteins that provide negative feedback.
Signal Termination
STAT signaling is terminated by several mechanisms. SOCS proteins are induced by STAT activation and inhibit signaling by binding to JAKs or cytokine receptors, targeting them for degradation. Protein tyrosine phosphatases such as SHP-1 and SHP-2 dephosphorylate JAKs and STATs. PIAS proteins act as STAT sumoylation E3 ligases and inhibit STAT transcriptional activity. STATs are also inactivated by dephosphorylation in the nucleus and exported to the cytoplasm.
Clinical Implications
JAK-STAT pathway dysregulation causes several diseases. JAK2 V617F mutations are found in most patients with myeloproliferative neoplasms, causing constitutive JAK2 activation. STAT3 gain-of-function mutations cause autoimmune and allergic diseases, while STAT3 loss-of-function causes hyper-IgE syndrome. Ruxolitinib, a JAK1/JAK2 inhibitor, treats myelofibrosis and polycythemia vera. Tofacitinib, a JAK3/JAK1 inhibitor, treats rheumatoid arthritis and other inflammatory conditions.
