Cytokines play a pivotal role in regulating a variety of immune responses mainly by downstream gene activation or repression. To transduce extracellular signals into intracellular responses, many cytokines utilize a common pathway - the Janus kinase (Jak)-signal transducer and activator of transcription (STAT) signaling pathway. Jak-STAT signaling pathway was first discovered through the study of gene activation in response to type I and II interferons (IFNs). Upon ligand binding, Jak family kinases, constitutively associated with specific cytokine receptors, are phosphorylated. Activated Jaks phosphorylate specific tyrosine residues on the cytoplasmic domain of cytokine receptor and SH2 domain of STATs. Activated STATs dissociate from the cytokine receptor, dimerize, and translocate to the nucleus to induce the ex
The mammalian Jak family has four members - Jakl, Jak2, Jak3 and Tyk2 (tyrosine kinase 2). Of these, Jakl, Jak2, and Tyk2 are ubiquitously expressed, while Jak3 is predominately expressed in immune and hematopoietic tissues. Jaks contain four conserved domains, which can be further divided into 7 Jak homology (JH) domains. The N-terminal FERM domain (named after its homology to band four point one, ezrin, radixin, and moesin; JH7 and JH6) is required for interaction with cytokine receptors and regulates catalytic activity. Jaks also have a SH2-like segment or JH4 domain whose function has not yet been characterized. The pseudo-kinase domain or JH2 negatively regulates the catalytic activity of Jaks, mediated by SOCS3-independent mechanism and mutations in JH2 domain of Jak3 have been detected in Jak3-SCID (severe combined immunodeficiency) patients. The C-terminal catalytic domain or JH1 is a conserved functional kinase domain. While the function of Jaks is nonredunant, Jaks all share the conserved structure. Jaks bind not only cytokine receptors but also other proteins such as STAMs. Jaks become activated secondary to tyrosine phosphorylation in response to cytokines. Subsequently Jaks initiate the signal transduction cascade by phosphorylation of tyrosine motifs present in receptor cytoplasmic domains and in receptor associated proteins. Jak are phosphorylated on multiple sites during cytokine signaling, the identity and function of most of these sites remain unknown.
STATs are a group of latent cytoplasmic transcription factors. To date, seven mammalian STATs have been reported - STAT1, STAT2, STAT3, STAT4, STAT5a, STAT5b, and STAT6. All seven mammalian STATs share six conserved domain-like structures. The N-terminal domain is a stabilizing domain, which stabilizes dimer or tetramer formation. The coiled-coil domain plays an important role in protein interaction. The DNA-binding domain has several conserved residues throughout all STATs. The linker domain has a helical structure and is important for transcription. The SH2 domain is required for receptor binding and contains a tyrosine residue that is phosphoryted by Jaks, leading to activation and dimerization of STATs. The transactivation domain (TAD) mediates interactions of the STATs with a number of transcriptional coactivators, which also facilitates chromatin modifications, and transcriptional activation. In addition, phosphorylation of a conserved serine residue in TAD is required for full transcriptional activation of STATs.
Phosphotyrosine-containing motifs in cytoplasmic domains of cytokine receptor act as docking sites for many SH2 signaling proteins, including STATs. Dimerization occurs immediately after tyrosine phosphorylation through reciprocal phosphotyrosine - SH2 interactions. STAT1, STAT3, STAT4, STAT5a, and STAT5b form homodimers, whereas STAT1 and STAT2 or STAT1 and STAT3 can form heterodimers. In addition, several STATs can form tetramers. STATs are able to be phosphorylated in both a Jak-dependent and independent manner. STATs are phosphorylated by Jaks in response to cytokines. Some hormones receptors such as EGFR and PDGFR function as a receptor tyrosine kinase to phophorylate STATs. STATs can also be phophorylated by several non-receptor oncogenic tyrosine kinases, such as v-Src and BCR-ABL. The targeted disruption of individual STATs in mice has revealed the physiological importance of STATs in the immune system. Only STAT3-deficiency in mice is embryonic lethal, indicating that STAT3 plays a pivotal role in development.
Several different kinases have been suggested to mediate Ser727 phosphorylation of STAT1, including p38, ERKs, JNKs, PI3K, PDK1 (3-phosphinositide-dependent protein kinase I), p90 ribosomal S6 kinase (p90RSK2), PKC8, CaMKII, and CK2, depending on the stimulus. It has been proposed that ISGylation is used to regulate the IFN-inducible Jak-STAT pathway in vitro. In contrast to in vitro studies, ISG15 deficient mice have intact responses to IFN signaling.
Recent studies have shown that STATs can form stable dimers in the absence of phosphorylation, although these dimers cannot bind to DNA. These STAT1 or STAT3 homodimers dynamically shuttle between the cytoplasm and nucleus independent of their phosphorylated status. Monomeric STAT1, STAT3 and STAT5b also continuously shuttle between cytoplasm and nucleus. The biological function of unphosphorylated STATs remains to be investigated but it has been suggested that unphosphorylated STATs can form complex with other proteins (STAT1/IRF1 or STAT2/IRF9 or STAT3/JUN) and may play an important role in mediating distinct gene ex