O-GlcNAc transferase regulates p21 protein levels and cell proliferation through the FoxM1-Skp2 axis in a p53-independent manner.

The protein product of the CDKN1A gene, p21, has been extensively characterized as a negative regulator of the cell cycle. Nevertheless, it is clear that p21 has manifold complex and context-dependent roles that can be either tumor suppressive or oncogenic. Most well studied as a transcriptional target of the p53 tumor suppressor protein, there are other means by which p21 levels can be regulated. In this study, we show that pharmacological inhibition or siRNA-mediated reduction of O-GlcNAc transferase (OGT), the enzyme responsible for glycosylation of intracellular proteins, increases expression of p21 in both p53-dependent and p53-independent manners in nontransformed and cancer cells. In cells harboring WT p53, we demonstrate that inhibition of OGT leads to p53-mediated transactivation of CDKN1A, while in cells that do not express p53, inhibiting OGT leads to increased p21 protein stabilization. p21 is normally degraded by the ubiquitin-proteasome system following ubiquitination by, among others, the E3 ligase Skp-Cullin-F-box complex; however, in this case, we show that blocking OGT causes impairment of the Skp-Cullin-F-box ubiquitin complex as a result of disruption of the FoxM1 transcription factor-mediated induction of Skp2 expression. In either setting, we conclude that p21 levels induced by OGT inhibition correlate with cell cycle arrest and decreased cancer cell proliferation.