Constitutive tyrosine and serine phosphorylation of STAT4 in T‐cells transformed with HTLV‐I

STAT4 is a critical mediator of IL‐12‐stimulated gene regulation in T‐helper type 1 (Th1) cell. IL‐12 activates the Janus family tyrosine kinases JAK2 and Tyk2, which in turn phosphorylate STAT4 on tyrosine 693. The p38 mitogen‐activated protein kinase (MAPK) signaling pathway is also activated in response to IL‐12, followed by phosphorylation of STAT4 on serine 721, which is required for STAT4 full transcriptional activity. In the present study, we demonstrated constitutive activation of STAT4 in HTLV‐I‐transformed T‐cell lines MT‐2, MT‐4 and HUT102 by immunoprecipitation, Western blotting and electrophoretic mobility shift assay (EMSA). In HTLV‐I‐transformed T‐cell lines, STAT4 was constitutively phosphorylated not only on tyrosine 693 but also on serine 721, and formed a heterodimer with STAT3. Constitutive phosphorylation of its upstream activators, JAK2, Tyk2 and p38 MAPK was also observed in the cells. EMSA and transient transfection studies further showed that the high‐affinity sis‐inducible element (hSIE) preferentially binds the STAT3/STAT4 heterodimer and is constitutively transactivated in MT‐2 cells in the absence of exogenous cytokine stimulation. When STAT4 expression vector was cotransfected along with STAT3 expression vector into MT‐2 cells, STAT4 significantly synergized with STAT3 to transactivate hSIE, showing the functional importance of heterodimer formation between STAT4 and STAT3.