The differentiation of murine proadipocytes can be induced by two different methods: 7.) by culture of high-density proad ipocytes in medium containing high concentrations of serum to which is added a variety of hormones and 2.) by culture of lowdensity proadipocytes in heparinized medium containing only human plasma. Phorbol myristate acetate influences differen tiation in both culture systems, but the biological mechanisms by which phorbol myristate exerts its effect have not been established

more »

... previously. In this regard, we have recently shown that five distinct steps mediate the integrated control of proadipocyte proliferation and differentiation (J. Cell Biol., 94: 400, 1982). Three steps are involved in the differentiation process: (a) growth arrest at a distinct state in the Gì phase of the cell cycle (Go); (b) nonterminal differentiation; and (c) terminal differentiation. Two steps antagonize the differentiation proc ess and cause proliferation. These are (a) induction of loss of the differentiated phenotype and (t>) reinitiation of proliferation. In this paper, we have therefore examined the effects of the tumor promoter phorbol myristate acetate on each step of integrated control of proliferation and differentiation in highand low-density proadipocytes. The results show that in the high-density system, phorbol myristate acetate can have the following effects under appropriate experimental conditions. It can: (a) block growth arrest at GD; (o) inhibit nonterminal differentiation; (c) promote loss of the differentiated phenotype; and (cO promote the proliferation of Go-arrested cells. In the low-density system by contrast, phorbol myristate acetate (a) does not block GD arrest and (o) does not block nonterminal differentiation but (c) does promote loss of the differentiated phenotype and (d) does promote the proliferation of Go-ar rested cells. These results show that the mechanisms by which phorbol myristate acetate affects differentiation are signif icantly influenced by a variety of factors which include changes in the culture medium, cell density, and cell-cell contact. There fore, attempts to correlate general effects of phorbol myristate acetate on the control of cell proliferation and differentiation with its action as a tumor promoter may not be justified. We suggest that only actions of phorbol myristate acetate which selectively affect initiated cells should be considered to be relevant to the mechanisms of promotion of carcinogenesis.