Effect of differentiation on the adenylate cyclase system of 3T3-C2 and 3T3-L1 cells. Determination of choleragen substrates in differentiating 3T3-L1 and nondifferentiating 3T3-C2 cells.

3T3-L1 preadipocytes, when treated with 3-isobutyl-1-methylxanthine, dexamethasone, and insulin, differentiate into cells with the morphological and biochemical properties of adipocytes; the closely related 3T3-C2 cells, under identical conditions, exhibit a low frequency of adipocyte conversion. During differentiation, 3T3-L1 preadipocytes acquire an increased responsiveness to certain agonists (e.g. isoproterenol and adrenocorticotropic hormone) that influence lipolysis and lipogenesis through activation of adenylate cyclase, whereas 3T3-C2 cells do not. It has been suggested that changes in hormone responsiveness of 3T3-L1 cells during differentiation result from increased amounts of the guanyl nucleotide-binding protein of adenylate cyclase, as demonstrated by choleragen-catalyzed [32P]ADP ribosylation of 42 and 49-50-kilodalton particulate peptides. Particulate fractions from nondifferentiating 3T3-C2 cells, like those from 3T3-L1 cells, contained choleragen substrates of 42 and 46-47 (doublet) kilodaltons. Incubation of intact 3T3-L1 or 3T3-C2 cells with choleragen prior to preparation of particulate fractions prevented the subsequent in vitro choleragen-dependent [32P]ADP ribosylation of only these peptides. Increased incorporation of radioactivity into both the 42 and 46-47-kilodalton peptides was observed during differentiation of 3T3-L1 cells. However, a similar increase was also observed in nondifferentiating 3T3-C2 cells subjected to the differentiation protocol. Therefore, increased hormone responsiveness of 3T3-L1 adipocytes cannot be explained solely on the basis of increased labeling, and perhaps increased amounts, of the guanyl nucleotide-binding protein.