EFFECT OF HUMAN GROWTH-HORMONE AND INSULIN ON [H-3] THYMIDINE INCORPORATION, CELL-CYCLE PROGRESSION, AND CYCLIN-D EXPRESSION IN 3T3-F442A PREADIPOSE CELLS

Cellular growth- and cyclin D expression-regulating activities of GH and insulin were investigated in 3T3-F442A preadipose cells under serum-free culture conditions. The present report provides evidence that the proliferative potential of 3T3-F442A cells is reduced by GH in a time- and concentration-dependent manner based on [H-3]thymidine incorporation assay and cell cycle analysis. In contrast, treatment of 3T3-F442A cells with insulin resulted in cellular proliferation. The insulin-induced proliferation of 3T3-F442A cells was diminished in the presence of GH. In an effort to define biochemical events relevant to the regulatory activities of GH and insulin on the proliferation of 3T3-F442A cells, the effects of these peptides on the expression of cyclin D were studied using Western blotting. Treatment of 3T3-F442A cells with insulin led to an increase in cyclin D expression relative to that in untreated cells. The insulin-elicited expression of cyclin D was time and dose dependent. In addition, the ability of insulin to induce cyclin D expression was reduced by GH. Our experimental results indicate that proliferation of 3T3-F442A cells was regulated by GH and insulin. The regulatory effects of GH and insulin are mediated at least in part by the alternating expression of cyclin D protein.