Cytokine receptors of the type I and type II receptor families engage signal transduction pathways that involve nonreceptor tyrosine kinases called Janus kinases (JAKs) and transcription factors called signal transducers and activators of transcription (STATs). The discovery of the JAK-STAT pathways came from biochemical and genetic analyses of interferon signaling. There are four known JAKs (JAKs1–3 and TYK2) and seven STATs (STATs 1–4, 5a, 5b, and 6).
The sequence of events in the JAK-STAT signaling path ways is now well defined (Fig. 1). Inactive JAK enzymes are noncovalently attached to the cytoplasmic domains of type I and type II cytokine receptors. When two receptor molecules are brought together by binding of a cytokine molecule, the receptor-associated JAKs are activated and phosphorylate tyrosine residues in the cytoplasmic portions of the clustered receptors. Some of these phosphotyrosine moieties of the receptors are then recognized by and bind SH2 domains of monomeric cytosolic STAT proteins. The STAT proteins are thus brought close to JAKs and are phosphorylated by these receptor-associated kinases. The SH2 domain of one STAT monomer is able to bind to a phosphotyrosine residue on an adjacent STAT protein. The STAT dimers that are generated migrate to the nucleus, where they bind to specific DNA sequences in the promoter regions of cytokine-responsive genes and activate gene transcription.
Fig1. JAK-STAT signaling induced by cytokines. Ligation of receptors for type I and type II cytokines results in the activation of an associated JAK tyrosine kinase, the phosphorylation of the receptor tail, and the recruitment of an SH2 domain–containing activator of transcription (STAT) to the receptor. The recruited STAT is activated by JAK phosphorylation, dimerizes, enters the nucleus, and turns on the expression of cytokine target genes.
Even though a limited numbers of JAKs and STATs are used by a large number of cytokine receptors, structural differences in the cytoplasmic tails of different cytokine receptors contribute to differences in signal outcomes from different cytokine receptor combinations. The cytoplasmic tails provide the scaffold for specifically binding, and thereby activating, different combinations of JAKs and STATs. The SH2 domains of different STAT proteins selectively bind to phosphotyrosines and flanking residues of different cytokine receptors. This is largely responsible for the activation of particular STATs by various cytokine receptors and therefore for the specificity of cytokine signaling. Several type I and type II cytokine receptors are heterodimers of two different polypeptide chains, each of which binds a different JAK. Furthermore, different STATs may heterodimerize on phosphorylation. Thus, there is a significant amount of combinatorial diversity in the signaling that can be generated from a limited number of JAK and STAT proteins. The subset of type I cytokine receptors that use the common γ chain all use the JAK3 kinase for signaling. JAK3 is the only JAK kinase that is not expressed ubiquitously. Its expression is largely restricted to immune cells, and it is only activated by γc-containing receptors. Type I cytokine receptors of the IL-6 family use JAK2 to activate STAT3. A number of other cytokines also activate STAT3. Another important source of differential signal output from different cytokine receptors is provided by other structural features of the cytoplasmic tails of different cytokine receptors, distinct from the STAT binding site. Some of these tails, once phosphorylated, also provide docking sites for different downstream enzymes and therefore for the activation of a variety of downstream signaling pathways that differ from receptor to receptor as alluded to below.
Several JAKs and STATs are relevant to human disease and are targets of therapeutic agents. Gain-of-function mutations in JAK2 are the cause of myelodysplastic syndrome with aplastic anemia and polycythemia vera. Mutations affecting the γc chain, or less frequently, JAK3, cause severe combined immunodeficiency. Dominant-negative mutations in STAT3 cause an immunodeficiency disease due to defects in Th17 responses. Activating mutations in STAT3 are characteristic features of large granular lymphocytic leukemias, malignant proliferation of cells of the NK cell or the CD8+ T-cell lineages. The elucidation of JAK-STAT signaling has also led to the development of novel therapeutic agents targeting these pathways. Small molecule JAK antagonists are approved for the treatment of acute myeloid leukemia and some other myeloid malignancies, and also for certain chronic inflammatory diseases, including rheumatoid arthritis, psoriasis, atopic dermatitis, alopecia areata, and IBD.
Cytokines activate signaling pathways and transcription factors in addition to the JAKs and STATs. For instance, the IL-2 receptor β chain activates RAS-dependent MAP kinase pathways that may be involved in gene transcription and growth stimulation. Other cytokine receptors may similarly activate other signaling pathways in concert with the JAK STAT pathways to elicit biologic responses to the cytokines. T-cell proliferation, triggered to a considerable degree by cytokines such as IL-2, is targeted by some immunosuppressive small molecules. An important downstream protein kinase that regulates protein translation, survival, and proliferation in many cell types, including T cells, is mTOR. It is, as its name implies, inhibited by rapamycin, a clinically used immunosuppressive drug.
Several mechanisms of negative regulation of JAK-STAT pathways have been identified. Proteins called suppressors of cytokine signaling (SOCS) serve as adaptors for multisubunit E3 ligase activity. They can bind to activated STATs and JAKs, and the tightly associated E3 ligases ubiquitinate the JAKs and STATs, targeting them for proteasomal degradation. SOCS protein levels can be regulated by TLR ligands, by cytokines themselves, and by other stimuli. In this way, SOCS proteins serve as negative feedback regulators of the cytokine-mediated activation of cells. Other inhibitors of JAK-STAT signaling include tyrosine phosphatases, such as SHP1 and SHP2, which can dephosphorylate and therefore deactivate JAK molecules. Another family of inhibitory proteins, called protein inhibitors of activated STAT (PIAS), binds phosphorylated STATs and pre vents their interaction with DNA. It is now known that PIAS proteins also interact with and block the function of other transcription factors associated with cytokine signaling, including NF-κB and SMADs (transcription factors downstream of members of the TGF-β receptor family).
