C-JUN AND MULTISTAGE CARCINOGENESIS - ASSOCIATION OF OVEREXPRESSION OF INTRODUCED C-JUN WITH PROGRESSION TOWARD A NEOPLASTIC END-POINT IN MOUSE JB6 CELLS SENSITIVE TO TUMOR PROMOTER-INDUCED TRANSFORMATION

Tumor promoters such as 12-O-tetradecanoylphorbol-13-acetate (TPA) and epidermal growth factor (ECF) induce neoplastic transformation, eleva ted c-jun protein expression, and activator protein-1 (AP-l)-dependent gene expression in JB6 mouse epidermal cells sensitive to tumor promo ters (clone 415a Pc cells). In contrast, JB6 cells resistant to tumor promoter-induced transformation (clone 307b P- cells) exhibit a greatl y reduced IPA or EGF inducible c-jun expression and AP-1 activity. We have recently shown that induced AP-1 is necessary for tumor promoter- induced transformation of Pc cells because introduction of a dominant negative c-jun mutant into Pc cells inhibits both AP-1 dependent trans activation and the transformation response to tumor promoter. The inte nt of the investigation presented here was to test the hypothesis that elevation of AP-1 activity is sufficient to cause progression to the Pc phenotype in P- cells or to the transformed phenotype in Pc cells. Clonally derived Pc and P- recipient cells transfected with a human c- jun expression construct and overexpressing c-jun protein were tested for progression by assaying for constitutive or inducible anchorage in dependent phenotype and nude-mouse tumorigenicity. Overexpression of c -jun did not produce progression in P- cells but did increase the prob ability of progression in Pc cells (two of five transfectant cell line s progressed to the tumor phenotype). In addition, c-jun overexpressio n did not increase AP-1 activity in any of the P-/-c-jun transfectants or in the two of five P+/-c-jun transfectants that acquired the trans formed phenotype. The P+/-c-jun transfectants that showed elevated AP- 1 activity did not progress to the tumor phenotype, demonstrating that an increase in AP-1 activity is insufficient for this progression. Si nce Pc-to-tumor phenotype progression occurred in cells overexpressing c-jun but not AP-1, we propose that P+-to-transformed phenotype progr ession is c-jun dependent and AP-1 independent. (C) 1995 Wiley-Liss, I nc.