SIMIAN-VIRUS-40 SMALL T-ANTIGEN COOPERATES WITH MITOGEN-ACTIVATED KINASES TO STIMULATE AP-1 ACTIVITY

The simian virus 40 small tumor antigen (small t) specifically interac ts with protein phosphatase type 2A (PP2A) in vivo and alters its cata lytic activity in vitro. Among the substrates for PP2A in vitro are th e activated forms of MEK and ERK kinases. Dephosphorylation of the act ivating phosphorylation sites on MEK and ERKs by PP2A in vitro results in a decrease in their respective kinase activities. Recently, it has been shown that overexpression of small t in CV-1 cells results in an inhibition of PP2A activity toward MEK and ERK2 and a constitutive up regulation of MEK and ERK2 activity. Previously, we have observed that overexpression of either ERK1, MEK1, or a constitutively active trunc ated form of c-Raf-1 (BXB) is insufficient to activate AP-1 in REF52 f ibroblasts. We therefore examined whether overexpression of small t ei ther alone or in conjunction with ERK1, MEK1, or BXB could activate AP -1. We found that coexpression of small t and either ERK1, MEK1, or BX B resulted in an increase in AP-1 activity, whereas expression of eith er small t or any of the kinases alone did not have any effect. Simila rly, coexpression of small t and ERK1 activated serum response element -regulated promoters. Coexpression of kinase-deficient mutants of ERK1 and ERK2 inhibited the activation of AP-1 caused by expression of sma ll t and either MEK1 or BXB. Coexpression of an interfering MEK, which inhibited AP-1 activation by small t and BXB, did not inhibit the act ivation of AP-1 caused by small t and ERK1. In contrast to REF52 cells , we observed that overexpression of either small t or ERK1 alone in C V-1 cells was sufficient to stimulate AP-1 activity and that this stim ulation was not enhanced by expression of small t and ERK1 together. T hese results show that the effects of small t on immediate-early gene expression depend on the cell type examined and suggest that the mitog en-activated protein kinase activation pathway is distinctly regulated in different cell types.