Identification of a Nuclear Stat1 Protein Tyrosine Phosphatase

Abstract

Upon interferon (IFN) stimulation, Stat1 becomes tyrosine phosphorylated and translocates into the nucleus, where it binds to DNA to activate transcription. The activity of Stat1 is dependent on tyrosine phosphorylation, and its inactivation in the nucleus is accomplished by a previously unknown protein tyrosine phosphatase (PTP). We have now purified a Stat1 PTP activity from HeLa cell nuclear extract and identified it as TC45, the nuclear isoform of the T-cell PTP (TC-PTP). TC45 can dephosphorylate Stat1 both in vitro and in vivo. Nuclear extracts lacking TC45 fail to dephosphorylate Stat1. Furthermore, the dephosphorylation of IFN-induced tyrosine-phosphorylated Stat1 is defective in TC-PTP-null mouse embryonic fibroblasts (MEFs) and primary thymocytes. Reconstitution of TC-PTP-null MEFs with TC45, but not the endoplasmic reticulum (ER)-associated isoform TC48, rescues the defect in Stat1 dephosphorylation. The dephosphorylation of Stat3, but not Stat5 or Stat6, is also affected in TC-PTP-null cells. Our results identify TC45 as a PTP responsible for the dephosphorylation of Stat1 in the nucleus.A wide variety of cytokines and growth factors activate intracellular signaling events involving Janus kinases (JAKs) and signal transducers and activators of transcription (STATs). Ligand activation of receptor-associated JAKs leads to tyrosine phosphorylation of the receptor chains, creating docking sites for STATs. In turn, STATs become phosphorylated on tyrosine, dimerize, and translocate to the nucleus to activate transcription (8). A precise regulation of both the magnitude and duration of JAK activity and of STAT activation is essential for the cytokine orchestration of many biological processes, and dysregulation of the JAK-STAT pathway has pathological implications (4, 33). Negative regulation of the JAK-STAT signaling pathway occurs through several distinct mechanisms. Cessation of signaling from the cell surface occurs through degradation of the receptor-ligand complex via the ubiquitinproteosome pathway (5, 13) and through induction of the suppressor of cytokine signaling (SOCS) family of proteins, which inhibit the activity of JAKs (14). In addition, several protein tyrosine phosphatases (PTPases), including SHP-1, SHP-2, CD45, and PTP-1B, can dephosphorylate either cytokine receptors or JAKs (17,23,33). PTPases that can dephosphorylate STATs in the cytoplasm have also been described (1, 36). In the nucleus, PIAS (protein inhibitor of activated STAT) proteins can inhibit the transcriptional activity of STATs (25). In addition, previous studies have demonstrated the existence of a nuclear phosphatase activity that can inactivate Stat1, the founding member of the STAT family. Once dephosphorylated, Stat1 is rapidly exported back into the cytoplasm and takes part in subsequent activation-inactivation cycles (3,9,12,13,22,26). The identity of the nuclear Stat1 phosphatase, however, had remained uncovered. Other STATs also undergo rapid activation-inactivation cycles (5, 20, 35), suggesting that nuclear STAT d...

Identification of Shp-2 as a Stat5A Phosphatase

“…However, the physiological role of these phosphatases in down-regulation of Stat5A is not clear. In fact, recent studies using TC-PTP-deficient cells have clearly demonstrated that TC-PTP is a Stat1, but not a Stat5 phosphatase (50). Although our findings provide direct evidence that Shp-2 is a Stat5A phosphatase in vivo, the possibility that there is another Stat5A phosphate(s) still exists.…”

“…Therefore, it is more likely that the turnover of tyrosine-phosphorylated Stat1 is mediated by a phosphatase. In fact, recent studies using cells derived from TC-PTP-deficient mice have clearly demonstrated that TC-PTP is a Stat1 phosphatase (50). In previous experiments, we demonstrated that different mechanisms are involved in regulating the inactivation of various Stat proteins (24).…”

Potential role of signal transducer and activator of transcription (STAT)3 signaling pathway in inflammation, survival, proliferation and invasion of hepatocellular carcinoma

“…Distinct importin subtypes determine trafficking of different STATs. STAT activation with nuclear accumulation terminates within minutes [145,146]. Recently, nucleocytoplasmic shuttling of STAT3 was shown by fluorescence localization, after photobleaching (FLAP), to be a dynamic process that involves constitutive shuttling of unstimulated STAT3 in the absence of cytokine stimulation [144].…”

Common polymorphic transcript variation in human disease

“…2D), one of which localizes to the nucleus and the other to the endoplasmic reticulum (Ibarra-Sanchez et al 2000). The nuclear isoform is known to dephosphorylate several important targets in T-cells, including STAT1 (ten Hoeve et al 2002), so autoimmune disease phenotypes could certainly be influenced by the ratio of these splice forms (Todd et al 2007). CLEC2D (also known as LLT1) is a C-type lectin involved in immune response, and the PTV/T1D-associated SNP (Todd et al 2007;Wellcome Trust Case Control Consortium 2007a) is at the penultimate base of its second exon (Fig.…”