ROLES OF PROTEIN-TYROSINE PHOSPHATASES IN STAT1-ALPHA-MEDIATED CELL SIGNALING

Different Stat proteins are activated through phosphorylation of uniqu e tyrosine residues in response to different cytokines and growth fact ors. Interferon-gamma activates Stat1 molecules that form homodimers a nd bind cognate DNA elements. Here we show that treatment of permeabil ized cells with 200-500 mu M peroxo-derivatives of vanadium, molybdenu m, and tungsten results in the accumulation of constitutively phosphor ylated Stat1 alpha molecules, In contrast, treatment of permeabilized cells with orthovanadate, vanadyl sulfate, molybdate, and tungstate at the same range of concentrations does not result in the accumulation of activated Stat1 alpha molecules in the absence of ligand. However, these compounds inhibit the inactivation of interferon-gamma-induced D NA-binding activity of Stat1 alpha. A 4-6-h exposure of the permeabili zed cells to orthovanadate, molybdate, and tungstate, but not vanadyl sulfate, results in a ligand-independent activation of Stat1 alpha, wh ich is blocked by the inhibition or depletion of NADPH oxidase activit y in the cells, indicating that NADPH oxidase-catalyzed superoxide for mation is required for the bioconversion of these metal oxides to the corresponding peroxo-compounds. Interestingly, Ligand-independent Stat 1 alpha activation by peroxo-derivatives of these transition metals do es not require Jak1, Jak2, or Tyk2 kinase activity, suggesting that ot her kinases can phosphorylate Stat1 alpha on tyrosine 701.