Inhibition of protein phosphatase 2A induces serine/threonine phosphorylation, subcellular redistribution, and functional inhibition of STAT3

Signal transducers and activators of transcription (STATs) are rapidly phos phorylated on tyrosine residues in response to cytokine and growth factor s timulation of cell surface receptors. STATs hereafter are translocated to t he nucleus where they act as transcription factors. Recent reports suggest that serine phosphorylation of STATs also is involved in the regulation of STAT-mediated gene transcription. Here, we studied the role of serine/threo nine phosphatases in STAT3 signaling in human antigen-specific CD4(+) T cel l lines and cutaneous T cell lymphoma lines, expressing a constitutively ac tivated STAT3. We show that an inhibitor of protein phosphatases (PPs) PP1/ PP2A, calyculin A, induces (i) phosphorylation of STAT3 on serine and threo nine residues, (ii) inhibition of STAT3 tyrosine phosphorylation and DNA bi nding activity, and (iii) relocation of STAT3 From the nucleus to the cytop lasm. Similar results were obtained with other PP2A inhibitors (okadaic aci d, endothall thioanhydride) but not with inhibitors of PP1 (tautomycin) or PP2B (cyclosporine A). Pretreatment with the broad serine/threonine kinase inhibitor staurosporine partly blocked the calyculin A-induced STAT3 phosph orylation, whereas inhibitors of serine/threonine kinases, such as mitogen- activated protein kinase-1 extracellular-regulated kinase-kinase, mitogen-a ctivated protein p38 kinase, and phosphatidy/inositol 3-kinase, did not. In conclusion, we provide evidence that PP2A plays a crucial role in the regu lation of STAT3 phosphorylation and subcellular distribution in T cells. Mo reover, our findings suggest that the level of STAT3 phosphorylation is bal anced between a staurosporine-sensitive kinase(s) and PP2A.