Growth Hormone-Dependent Events in the Adipose Differentiation of 3T3-F442A Fibroblasts: Modulation of Macromolecular Synthesis*

Abstract

GH is necessary but not sufficient to induce adipose differentiation of 3T3-F442A fibroblasts in serum-free medium. Human (h) GH (2 nM) treatment of 3T3-F442A cells in serum-free medium caused a time-dependent (maximal at 48-72 h) and dose-dependent (EC50, approximately 0.2 nM) decrease (40-60%) in de nova protein synthesis. Insulin-like growth factor-I (IGF-I; 17 nM), PRL (2 nM), and glucagon (20 nM) did not decrease de novo protein synthesis, whereas bovine GH was equipotent with hGH. The half-lives of 35S-labeled proteins of 3T3-F442A cells were 21 and 57 h for cells maintained in serum-free medium for 3 days without or with hGH (2 nM), respectively. The total protein content of cells maintained in hGH (2 nM) for 1-4 days was unaffected compared to cells in serum-free medium alone. IGF-I (17 nM) treatment of cells for 4 days doubled the protein content of cells compared to control values in serum-free medium. hGH (2 nM) pretreatment of cells for 1-4 days had no effect on total RNA synthesis. hGH (2 nM) but not IGF-I (17 nM) treatment (3 days) resulted in a 7-fold decrease in cytoplasmic 18S rRNA content (as measured by DNA-RNA hybridization) of cells compared to that of control cells maintained in serum-free medium. When 3T3-F442A cells were transferred to serum-free medium there was a progressive decrease in DNA synthesis. The presence of hGH enhanced the rate at which DNA synthesis decreased for 3T3-F442A cells. IGF-I (17 nM) increased DNA synthesis by 6- and 8-fold after 2 and 3 days of IGF-I exposure. 3T3-F442A cells maintained in serum-free medium for 3 days responded to the addition of platelet-derived growth factor (2 U/ml) and insulin (1.6 microM) with a 56-fold increase in DNA synthesis, assayed 24 h later. 3T3-F442A cells treated with hGH (2 nM) for 3 days before platelet-derived growth factor and insulin addition exhibited a diminished DNA synthetic response, demonstrating that GH-exposed cells were partially refractory to mitogenic stimulation. GH had no effect on any aspect of macromolecular synthesis in 3T3-C2 cells, which have a low frequency of adipogenesis. Based upon these results a cell cycle model for the role of GH in the adipose differentiation of 3T3-F442A cells was proposed.