Apoptosis induced by ectopic expression of cyclin D1 but not cyclin E

Deregulation of the pRb/E2F pathway leads to disruption of the normal control of the G1/S transition, and is associated with transformation. However, recent accumulated evidence suggest that under certain circumstances deregulation of the pRb/E2F pathway can also lead to apoptotic cell death. Apoptosis was shown to be induced by expression of DNA tumor virus oncoproteins, knockout of the rb gene, and expression of E2F from heterologous promoter. Since phosphorylation of pRb by G1 cyclin-dependent kinases (Cdks) causes its inactivation, we examined whether deregulation of G1 Cdks, also drives apoptosis. We have used rat fibroblast cell lines capable of expressing cyclin E, cyclin D1, or both, in an inducible manner, through a tetracycline responsive promoter. We show here that ectopic expression of cyclins D1 and E in rat fibroblasts under serum starvation, leads to deregulated entry into S phase, and subsequently to apoptotic cell death. Furthermore, expression of cyclin D1 alone is sufficient to provoke apoptosis, whereas expression of cyclin E alone during serum starvation does not. Moreover, expression of either cyclins D1 and E, and cyclin D1 alone, under serum starvation led to a significant increase in the fraction of hyper-phosphorylated pRb whereas cyclin E expression alone did not. These results demonstrate that expression of cyclin D1 from heterologous promoter leads to apoptosis in serum starved cells, which may be mediated by phosphorylation of pRb.