Cyclin D1 expression is a major target of the cAMP-induced inhibition of cell cycle entry in fibroblasts

We previously described in the CCL39 hamster fibroblast cell line the inhibition of DNA synthesis reinitiation by agents that elevate cyclic AMP. Here, we show that 8Br-cAMP strongly blocks both the growth factor-induced increase in cyclin D1 protein expression and decrease in p27(KIP1) protein levels, leaving untouched the levels of cyclin D3, cdk2 and cdk4. To assess the role of cyclin D1 in the cAMP-mediated inhibition of DNA synthesis, we overexpressed the cyclin D1 gene in CCL39 and analysed the cAMP response in stable transfectants. We showed that the kinase activities associated to G(1) cyclin-cdk complexes are significantly more resistant to cAMP in cyclin D1 transfectants than in their normal counterparts, although the serum-induced p27(KIP1) disparition is still cAMP sensitive in cyclin D1 overexpressors. Interestingly, the mitogen-induced DNA synthesis reinitiation is also much less inhibited by cAMP in cyclin D1 transfectants than in control cells. These data clearly establish that the cAMP-inducible blockade of the G(1) phase of the cell cycle can be partially alleviated by overexpression of cyclin D1 in hamster fibroblasts, thus strongly suggesting that cyclin D1 protein is one of the major targets for cAMP inhibitory action in fibroblasts.