Phorbol esters, which activate isoforms of protein kinase C, are general activators of the transcription factor activated protein 1 (AP-1). The pathway involved in this signal transduction is not very clear. Currently, little is known about whether phosphatidylinositol-3 (PI-3) kinase plays any role in phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced signal transduction. We demonstrate here that TPA not only has markedly synergistic effects on insulin-induced PI-3 kinase activity, but it also can induce PI-3 kinase activity and the PI-3 phosphates by itself. We also found that insulin, a PI-3 kinase activator, enhanced TPA-induced AP-1 trans-activation and transformation in JB6 promotion-sensitive cells. Furthermore, wortmannin and LY294002, two PI-3 kinase inhibitors, markedly decreased AP-1 activity induced by insulin, TPA, or TPA and insulin and inhibited JB6 promotion-sensitive cell transformation induced by TPA or TPA and insulin. Most importantly, constitutive overexpression of the dominant negative PI-3 kinase P85 mutants completely blocked insulin- or TPA-induced AP-1 trans-activation and TPA-induced cell transformation. All evidence from present studies suggests that PI-3 kinase acts as a mediator in TPA-induced AP-1 activation and transformation in JB6 cells.