Differential effects of retinoic acid upon early and late events in adipose conversion of 3T3 preadipocytes.

When confluent 3T3-F442A cell cultures (Day 0) were grown for 3 days in fetal calf serum-supplemented medium containing isobutyl methyl xanthine and dexamethasone (induction phase) and then shifted to serum-free hormone-defined medium (expression phase), they spontaneously exhibited a sharp rise in lipoprotein lipase activity (LPL); at the peak (Day 7) the LPL activity was about 25 times higher than in control cultures and was further enhanced by insulin. Although this expression of LPL activity was spontaneous, the emergence of glycerophosphate dehydrogenase (G3PDH) activity was completely dependent upon insulin as well as upon the expression of the differentiated phenotype. In committed cells, insulin elicited sustained DNA synthesis associated with limited cell proliferation. The addition of retinoic acid during the phase of expression inhibited insulin-dependent terminal differentiation (i.e., the emergence of G3PDH activity and acquisition of the differentiated phenotype). In addition, retinoic acid counteracted the stimulating effect of insulin upon LPL activity, but affected neither the mitotic process nor the spontaneous emergence of LPL activity. When added during the phase of induction, it prevented the overall process of adipogenic differentiation. Thus, the use of retinoic acid can indicate independent control of the mitogenic and lipogenic effects of insulin following commitment to adipogenic differentiation.