Myc-enhanced expression of Cul1 promotes ubiquitin-dependent proteolysis and cell cycle progression

The c-Myc oncoprotein plays an important role in the growth and proliferation of normal and neoplastic cells. To execute these actions, c-Myc is thought to regulate functionally diverse sets of genes that directly govern cellular mass and progression through critical cell cycle transitions. Here, we provide several lines of evidence that c-Myc promotes ubiquitin-dependent proteolysis by directly activating expression of the Cul1 gene, encoding a critical component of the ubiquitin ligase SCFSKP2. The cell cycle inhibitor p27kip1 is a known target of the SCFSKP2 complex, and Myc-induced Cul1 expression matched well with the kinetics of declining p27kip1 protein. Enforced Cul1 expression or antisense neutralization of p27kip1 was capable of overcoming the slow-growth phenotype of c-Myc null primary mouse embryonic fibroblasts (MEFs). In reconstitution assays, the addition of in vitro translated Cul1 protein alone was able to restore p27kip1ubiquitination and degradation in lysates derived fromc-myc−/− MEFs or density-arrested human fibroblasts. These functional and biochemical data provide a direct link between c-Myc transcriptional regulation and ubiquitin-mediated proteolysis and together support the view that c-Myc promotes G1 exit in part via Cul1-dependent ubiquitination and degradation of the CDK inhibitor, p27kip1.