Receptor-associated constitutive protein tyrosine phosphatase activity controls the kinase function of JAK1

Abstract

Exposure of cells to protein tyrosine phosphatase (PTP) inhibitors causes an increase in the phosphotyrosine content of many cellular proteins. However, the level at which the primary signaling event is affected is still unclear. We show that Jaks are activated by tyrosine phosphorylation in cells that are brief ly exposed to the PTP inhibitor pervanadate (PV), resulting in tyrosine phosphorylation and functional activation of Stat6 (in addition to other Stats). Mutant cell lines that lack Jak1 activity fail to support PV-mediated Cytokines transmit their signals through transmembrane receptors that are physically associated with members of the Jak family of protein tyrosine kinases (PTK) (1-8). Aggregation of receptors, resulting in the association of Jaks in a receptor complex and a conformational change in the kinase domain, is an early step in cytokine receptor activation (2-11). Transphosphorylation of the conserved tyrosine residues in the kinase activation segments promotes kinase activity (11). The activated Jaks then phosphorylate the receptor-associated and downstream signaling molecules, including Stat proteins recruited to the receptor complex (1)(2)(3)(4)(5)(6)(7)(8)(12)(13)(14)(15).Protein tyrosine phosphatases (PTP) are associated with many cytokine receptors and are implicated in the downregulation of ligand-induced signaling through dephosphorylation of the activated Jaks and the receptors (15)(16)(17)(18)(19)(20). Dephosphorylation of activated Stat proteins is catalyzed by different PTP activities in the nucleus (21-23). We and others have found that cytokine receptor-associated PTP activities not only down-regulate cytokine-induced signals but also suppress the constitutive activities of Jak proteins (21,(24)(25)(26).

Inhibition of IgG1 and IgE Production by Stimulation of the B Cell CTLA-4 Receptor

“…In T cells, the Src homology 2 domain tyrosine phosphatase (SHP2) 3 binds to phosphorylated Tyr 201 of the CTLA-4 cytoplasmic tail (36,37) and dephosphorylates members of the Ras-activating pathway, which, in turn, affect mitogen-activated protein kinases involved in IB kinase activation (38). Although the specific roles of SHP2 (and SHP1) phosphatases in IL-4 signaling have not been delineated, inhibition of phosphatase activity may result in STAT6 activation, indicating a possible modulatory role of these enzymes (39). Whether these mechanisms are also involved in the effects induced by B cell CTLA-4 stimulation requires further investigations.…”

PI3K-AKT pathway negatively controls EGFR-dependent DNA-binding activity of Stat3 in glioblastoma multiforme cells

“…DNA-protein complexes detected in U87 and D54 cells contained Stat3 homodimers as a major component of SIF complexes, although Stat1 homodimers or Stat1-Stat3 heterodimers were also detectable at lower levels ( Figure 1b). To show the relative migration of SIF complexes, we used extracts of U87 and D54 cells that were exposed to 200 mM pervanadate for 30 min, which inhibited protein tyrosine phosphatase activity and induced all three SIF complexes (Haque et al, 1997). Stat5-specific antibody served as a negative control (Rahaman et al, 2002).…”

Role of the STAT1-SH2 Domain and STAT2 in the Activation and Nuclear Translocation of STAT1

“…This treatment even proved effective in the activation of STAT1 in the absence of the Jak tyrosine kinases that are required for STAT activation (44), indicating that this procedure effectively bypasses the need for receptormediated signaling. We therefore decided to explore the possibility that this treatment might also activate a STAT1 carrying the Arg 3 Gln mutation in the SH2 domain.…”

“…It has been previously shown that incubation of cells with a combination of 1 mM hydrogen peroxide and 0.1 mM orthovanadate (H/V) results in the ligand-independent activation of STAT proteins (43,44). This treatment even proved effective in the activation of STAT1 in the absence of the Jak tyrosine kinases that are required for STAT activation (44), indicating that this procedure effectively bypasses the need for receptormediated signaling.…”

“…Previous findings suggested that the SH2 domain of STAT1 was required to recruit latent STAT1 to activated receptors (29, 35, 36, 40 -42). Consequently, the reverse approach of introducing an Arg 3 Gln mutation in the SH2 domain to render it incapable of binding phosphotyrosine and preventing dimerization was unsuccessful, because a STAT1 carrying such a mutation did not become tyrosine phosphorylated in response to IFN or epidermal growth factor (34, 37).It has been previously shown that incubation of cells with a combination of 1 mM hydrogen peroxide and 0.1 mM orthovanadate (H/V) results in the ligand-independent activation of STAT proteins (43,44). This treatment even proved effective in the activation of STAT1 in the absence of the Jak tyrosine kinases that are required for STAT activation (44), indicating that this procedure effectively bypasses the need for receptormediated signaling.…”