Synergistic Effect of Cyclic AMP and Insulin on the Expression of Cyclin A Gene in Swiss 3T3 Cells

Abstract

Cyclic AMP and insulin exert a synergistic mitogenic effect on Swiss 3T3 cells. Here, we showed that the activity of cyclin A-dependent kinase was elevated 3-fold at 24 h after cotreatment of cells with dibutyryl cAMP and insulin. The cotreatment elevated cyclin A protein levels 12-fold higher than those of insulin-treated cells without altering the levels of CDK2 and p27Kip1proteins. Interestingly, the half-life of cyclin A protein increased from 7 h in the insulin-treated cells to 22 h in the cotreated cells. Levels of cyclin A mRNA were also elevated 9-fold. Cotreatment synergistically increased the binding activities of transcription factors to the NF-Y and the E2F-like sites of the cyclin A promoter. However, neither NF-Y nor E2F was involved in the binding. These results suggest that the synergistic elevation of cyclin A protein may be achieved by both the stabilization of cyclin A protein and the recruitment of transcription factors to the NF-Y and E2F-like sites of the cyclin A promoter.