DNA TUMOR-VIRUS ONCOPROTEINS AND RETINOBLASTOMA GENE-MUTATIONS SHARE THE ABILITY TO RELIEVE THE CELLS REQUIREMENT FOR CYCLIN D1 FUNCTION ING1

The retinoblastoma gene product (pRB) participates in the regulation o f the cell division cycle through complex formation with numerous cell ular regulatory proteins including the potentially oncogenic cyclin D1 . Extending the current view of the emerging functional interplay betw een pRB and D-type cyclins, we now report that cyclin D1 expression is positively regulated by pRB. Cyclin D1 mRNA and protein is specifical ly downregulated in cells expressing SV40 large T antigen, adenovirus E1A, and papillomavirus E7/E6 oncogene products and this effect requir es intact RB-binding, CR2 domain of E1A. Exceptionally low expression of cyclin D1 is also seen in genetically RB-deficient cell lines, in w hich ectopically expressed wild-type pRB results in specific induction of this GI cyclin. At the functional level, antibody-mediated cyclin D1 knockout experiments demonstrate that the cyclin D1 protein, normal ly required for G1 progression, dispensable for passage through the ce ll cycle in cell lines whose pRB is inactivated through complex format ion with T antigen, E1A, or E7 oncoproteins as well as in cells which have suffered loss-of-function mutations of the RB gene. The requireme nt for cyclin D1 function is not regained upon experimental elevation of cyclin D1 expression in cells with mutant RB, while reintroduction of wild-type RB into RB-deficient cells leads to restoration of the cy clin D1 checkpoint. These results strongly suggest that pRB serves as a major target of cyclin D1 whose cell cycle regulatory function becom es dispensable in cells lacking functional RB. Based on available data including this study, we propose a model for an autoregulatory feedba ck loop mechanism that regulates both the expression of the cyclin D1 gene and the activity of pRB, thereby contributing to a G1 phase check point control in cycling mammalian cells.