Modulation of mouse fibroblast adipocyte differentiation by X-rays and TPA

The purpose of this study was to determine whether X-rays and the tumor promoter TPA which, in combination, are more effective at transforming cells in vitro than is either alone, may also be more effective at inhibiting differentiation in combination. Previous studies with 3T3 T proadipocytes have shown that both the tumor promoter 12-o-tetradecanoylphorbol-13-acetate (TPA) and a classical initiator, UV light, can inhibit differentiation. We have now extended these studies by examining the effects of X-rays and TPA administered singly and in combination on differentiation of 3T3 T proadipocytes caused by incubation in medium containing 25% human plasma and on 5-azacytidine induced differentiation of C3H/10T1/2 cells. X-rays alone caused a dose-dependent inhibition of differentiation of 3T3 proadipocytes from 1.0 to 9.0 Gy. Low doses of X-rays (1.0 Gy) also inhibited differentiation of C3H/10T1/2 cells treated with 5-azacytidine but high doses (6.0 Gy) actually caused differentiation in the absence of 5-azacytidine treatment. TPA also inhibited differentiation of both 3T3 T and 5-azacytidine-treated C3H/10T1/2 cells at a dose of 0.1 micrograms/ml. At the lower dose of 0.001 micrograms/ml TPA, 3T3 T differentiation was inhibited only 10%, while 2.5 Gy of X-rays inhibited differentiation approximately 20%. Combined treatment of cells with X-rays and TPA caused more than 60% inhibition of differentiation, i.e., approximately twice the inhibition that would be expected if the effects of these two agents were additive. This synergistic effect suggests that in some systems there may be a closer association between the multistage process of neoplastic transformation and the inhibition of processes involved in cell differentiation than previous experiments, performed with initiators or promoters administered separately, have indicated.