REGULATION OF DIFFERENTIATION AND PROTEIN-KINASE-C EXPRESSION IN 3T3 T-PROADIPOCYTES - EFFECTS OF TGF-BETA AND TRANSFORMATION

We are studying the mechanisms that regulate proliferation and differe ntiation of normal 3T3 T proadipocytes and neoplastically transformed clones which have lost the ability to differentiate. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) and transforming growth fa ctor beta (TGF-beta) are known inhibitors of the same step of the diff erentiation process in 3T3 T cells. Here, we examined the expression o f the phorbol ester receptor/protein kinase C (PKC) during adipocytic differentiation of 3T3 T cells and its modulation by the differentiati on inhibitor TGF-beta. PKC receptor assays were performed using a trit iated analogue of TPA and it was found that PKC receptor levels decrea sed approximately threefold during differentiation. Northern blot anal yses revealed an even greater decrease of PKC transcripts during diffe rentiation. TGF-beta inhibited not only differentiation, but the diffe rentiation-dependent decrease in PKC levels as well. Transformed 3T3 T cells which have lost the ability to differentiate were found to expr ess aberrant levels of PKC. The data suggest that TGF-beta may inhibit differentiation via a PKC-dependent pathway and that disruption of no rmal PKC levels or its regulation may be involved in the loss of diffe rentiation control in transformed 3T3 T cells.