OVEREXPRESSION OF CYCLIN D1 BLOCKS PROLIFERATION OF NORMAL DIPLOID FIBROBLASTS

Human cyclin D1 forms complexes with several Cdks, with proliferating cell nuclear antigen, and with a recently discovered 21-kDa inhibitor of Cdk activity. Substrates for cyclin D1/Cdks have not been identifie d in vivo, but it has been proposed that the D class of cyclins might play a role in regulating the activity of the retinoblastoma gene prod uct p105(Rb). Here we report that normal human diploid fibroblasts tha t endogenously or ectopically express high levels of cyclin D1 are una ble to enter S phase in response to normally mitogenic stimuli. Fibrob lasts that have reached the end of their in vitro life span (senescent cells) express fivefold higher levels of cyclin D1 protein than low-p assage cells and individual cells in mass culture that fail to initiat e DNA synthesis in response to serum addition have severalfold higher levels of this cyclin than proliferation-competent cells. These observ ations provide evidence that cyclin D1 is involved with the regulation of cell proliferation by more than one mechanism. (C) 1995 Academic P ress, Inc.