Negative regulation of JAK/STAT pathway

STAT (signal transducers and activators of transcription) proteins are a family of latent cytoplasmic transcription factors that are activated by a large number of extracellular signals such as cytokines and growth factors.; Transcriptional induction by interferons requires the tyrosine and serine phosphorylation of the STAT proteins. For STAT1, arginine methylation at the amino-terminus is also required for its optimal transcriptional activity. Tyrosine dephosphorylation of STATs is critical for the downregulation of STATs transcriptional activity. Here we show that arginine methylation of STAT1 controls the rate of STAT1 dephosphorylation by modulating its interaction with PIAS1 and the nuclear tyrosine phosphatase TcPTP. Inhibition of STAT1 arginine methylation, or mutation of STAT1-R31, results in a prolonged half-life of STAT1 tyrosine phosphorylation. This effect appears to be mediated by an increased binding of PIAS1 to STAT1 in the absence of STAT1 arginine methylation, and a concomitant decrease in the association of STAT1 with TcPTP. Furthermore, inhibitors of arginine-methylation require the presence of PIAS1 in order to exert their negative regulatory effect on the dephosphorylation of STAT1.; We also demonstrate here that the small dual-specificity phosphatase, VHR selectively tyrosine dephosphorylates another member of the STAT family, STAT5, leading to the subsequent inhibition of STAT5 transcriptional activity and the transcriptional induction driven by the β-casein promoter. In contrast, VHR has no effect on the state of STAT1 tyrosine phosphorylation. We also report here that phosphorylation of VHR Tyr138 was required for its tyrosine phosphatase activity towards STAT5. A tyrosine to phenylalanine VHR Y138F mutant augmented STAT5 tyrosine phosphorylation induced by IFNβ stimulation. The tyrosine kinases, Jak1 and Tyk2, which are activated upon IFNβ stimulation and are responsible for the tyrosine phosphorylation and activation of STAT5, were shown to be able to tyrosine phosphorylate the STAT5 negative regulator, VHR at Tyr138. Furthermore, Jak1 phosphorylation of VHR was found to be Tyk2-dependent. Preliminary experiments indicate that the STAT5 SH2 domain is required for the efficient dephosphorylation of STAT5 by VHR.