Polyomavirus middle T antigen as a probe for T cell antigen receptor- coupled signaling pathways

Stimulation of the T cell antigen receptor (TCR) triggers a complex series of signaling events that culminate in T cell activation and proliferation. The complex structure of the TCR has hindered efforts to link specific signaling events induced by TCR cross-linkage to downstream activation responses, such as interleukin-2 (IL-2) gene transcription. Previous studies have shown that the polyomavirus-derived oncoprotein, middle T antigen (mT), transforms rodent fibroblasts by interacting with and activating several cytoplasmic signaling proteins (Src kinases, phospholipase C (PLC)-γ1, Shc, and phosphoinositide 3-kinase (PI3-K) implicated in cell growth control. In this study, we demonstrate that expression of mT activates Jurkat T cells, as measured by increases in IL-2 promoter- and NFAT (nuclear factor of activated T cells)-dependent reporter gene transcription. The transcriptional response provoked by mT was blocked by the immunosuppressive drug FK506, a potent inhibitor of TCR-mediated IL-2 gene expression. Mutations that disrupted the binding of mT to Src kinases or PLC-γ1 abrogated the ability of mT to deliver the signals needed for IL-2 promoter activation. In contrast, a mT mutant that failed to bind PI3-K induced a markedly elevated transcriptional response in Jurkat cells, whereas mutation of the Shc binding site in mT had little effect on the transactivating potential of this vital oncoprotein. Additional studies demonstrated that the association of mT with PLC-γ1 was necessary and sufficient to activate both Ca²⁺- and Ras-dependent signaling cascades in Jurkat cells. These results indicate that PLC-γ1 activation plays pivotal and pleiotropic roles in the stimulation of IL-2 gene expression, whereas activation of PI3-K negatively modulates this response in Jurkat T cells.