MYC transcription factors are potent stimulators of cell proliferation. It has been suggested that the CDK-inhibitor p27kip1 is a critical G1 phase cell cycle target of c-MYC. We show here that mouse embryo fibroblasts deficient for both p27kip1 and the related p21cip1 are still responsive to stimulation by c-MYC and can be arrested in G1 by a dominant negative mutant of c-MYC. This growth arrest can be overruled by ectopic expression of E2F or adenovirus E1A, but not by a mutant of E1A defective for binding to retinoblastoma family proteins. We show that fibroblasts with a genetic disruption of all three retinoblastoma family members (pRb, p107 and p130) are unresponsive to a dominant negative c-MYC mutant. These data indicate that p27kip1 is not the only rate limiting cell cycle target of c-MYC and suggest that regulation of E2F is also essential for c-MYC's mitogenic activity.
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