INSULIN HAS A LIMITED EFFECT ON THE CELL-CYCLE PROGRESSION IN 3T3 L1 FIBROBLASTS

Insulin has pleiotropic effects on the regulation of cellular growth, differentiation, and metabolism. The biochemical events ultimately lea ding to cell proliferation after insulin treatment have been demonstra ted in detail by numerous research groups. However, depending on cell types, it has been shown that insulin has various effects on cell prol iferation. Therefore, we attempted to more critically evaluate the eff ect of insulin on cell proliferation in 3T3 L1 fibroblasts. In this st udy, we investigated insulin's effect on cell proliferation by using [ H-3]thymidine incorporation, flow cytometry, and cell counting. In 3T3 L1 fibroblasts studied in 0.5% serum, insulin induced a two-fold incr ease in [H-3]thymidine incorporation over at 48 h, and the maximal rat e of DNA synthesis was observed during 8-12 h incubation. The now cyto metric analysis also showed that insulin increased the cell population in the S phase. After insulin treatment for 48 h, cell numbers increa sed approximately 45% in comparison with 0.5% serum control. Cell divi sion was found to occur only once in 60 h after staining 3T3 L1 fibrob lasts with carboxyfluorescein diacetate succinimidyl ester (CFSE). Tak en together, this data indicates that insulin stimulated the transit f rom the G0/G1 to S phase, progressed the cell cycle through the G2/M p hase, and increased the cell number. However, under our experimental c onditions, cells divided only once in 60 h in the presence of insulin.