A protein kinase C‐independent pathway leading to c‐Jun‐dependent expression of 100‐kDa Ras GTPase‐activating protein in JEG‐3 human choriocarcinoma cells

Although the 100‐kDa Ras GTPase‐activating protein (p100 RasGAP) has been reported to exist specifically in human placental trophoblasts, the molecular mechanisms responsible for regulating its expression remain unclear. In this study we used okadaic acid, an inhibitor of serine/threonine phosphatase 1 and 2 A, as a probe to explore the signaling pathway regulating the expression of p100 RasGAP in JEG‐3 human placental choriocarcinoma cells. Treatment of JEG‐3 cells with okadaic acid provoked dose‐ and time‐dependent stimulation of p100 RasGAP expression without marked modification of expression of p120 RasGAP, another isoform of RasGAP. Co‐treatment of cells with okadaic acid and the protein kinase C activator, phorbol 12‐myristate 13‐acetate, exerted an additive effect on p100 RasGAP induction. Moreover, the response of the p100 RasGAP de novo synthesis to okadaic acid was not affected by the selective inhibitor of protein kinase C, GF 109203X. Thus this study identified a novel signaling pathway regulating p100 RasGAP expression, which is independent of protein kinase C. In addition, okadaic acid treatment resulted in the activation of ERK2 (p42 MAP kinase) and the induction of both c‐Jun and c‐Fos proteins without activating JNK (c‐Jun NH 2 ‐terminal kinase). Significantly, blockade of c‐Jun expression with antisense c‐ jun oligonucleotides suppressed p100 RasGAP expression. Taken together, it is concluded that okadaic acid induces the expression of p100 RasGAP protein in JEG‐3 cells preceded by activation of ERK and AP‐1 cascade, and that this okadaic acid‐induced p100 RasGAP expression is independent of protein kinase C‐mediated pathway but requires c‐Jun/AP‐1 function.