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 G(1)/S transition, and is associated with transformatio n. However, recent accumulated evidence suggest that under certain cir cumstances deregulation of the pRb/E2F pathway can also lead to apopto tic 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 G(1) c yclin-dependent kinases (Cdks) causes its inactivation, we examined wh ether deregulation of G(1) Cdks, also drives apoptosis. We have used r at fibroblast cell lines capable of expressing cyclin E, cyclin D1, or both, in an inducible manner, through a tetracycline responsive promo ter. We show here that ectopic expression of cyclins D1 and E in rat f ibroblasts under serum starvation, leads to deregulated entry into S p hase, and subsequently to apoptotic cell death. Furthermore, expressio n of cyclin D1 alone is sufficient to provoke apoptosis, whereas expre ssion of cyclin E alone during serum starvation does not. Moreover, ex pression of either cyclins D1 and E, and cyclin D1 alone, under serum starvation led to a significant increase in the fraction of hyper-phos phorylated pRb whereas cyclin E expression alone did not. These result s demonstrate that expression of cyclin D1 from heterologous promoter leads to apoptosis in serum starved cells, which may be mediated by ph osphorylation of pRb.