Skp2 is necessary for Myc-induced keratinocyte proliferation but dispensable for Myc oncogenic activity in the oral epithelium

The proto-oncogene c-Myc encodes a transcription factor that is implicated in the regulation of cellular proliferation, differentiation, and apoptosis. Myc accelerates the rate of cell proliferation, at least in part, through its ability to down-regulate the expression of the cell cycle inhibitor p27 ^Kip1. Moreover, p27^Kip1 protein levels are regulated by ubiquitin-mediated turnover, leading to destruction by the E3 ubiquitin ligase SCF^Skp2. Therefore, we hypothesize that a lack of Skp2 expression should lead to increased p27^Kip1 levels and further inhibition of Myc-mediated proliferation and tumorigenesis. Myc expression in epithelial tissues of transgenic mice (K5-Myc) led to increased keratinocyte proliferation and the development of spontaneous tumors within the oral cavity. We generated K5-Myc- transgenic mice in an Skp2-null background. Consistent with our hypothesis, we found that Myc-mediated keratinocyte hyperproliferation was abolished by the loss of Skp2. However, Skp2 ablation did not affect Myc-driven tumorigenesis because the incidence, latency, and degree of differentiation of oral tumors were identical between K5-Myc/Skp2^+/+ and K5-Myc/Skp2^-/- mice. Altogether, these findings suggest that Skp2 and p27^Kip1 are critical for Myc-driven keratinocyte proliferation; however, Myc-mediated tumorigenesis in the oral epithelium is independent of the Skp2-p27^Kip1 axis.