Retinoblastoma protein modulates gankyrin-MDM2 in regulation of p53 stability and chemosensitivity in cancer cells.

MDM2 is a key ubiquitin E3 ligase for p53 and its activity is critically regulated by a set of modulators, including ARF, p300, YY1 and recently by gankyrin, an oncoprotein frequently overexpressed in human heptocellular carcinomas. We have previously shown that MDM2 binds to and promotes retinoblastoma protein (Rb) degradation. Here we show that Rb inhibits MDM2 E3 ligase activity resulting in stabilization of p53. In addition, we demonstrated that Rb inhibits MDM2-mediated p53 ubiquitination in a gankyrin-dependent manner and the Rb-gankyrin interaction is critical for Rb-induced p53 stabilization. Furthermore, acute ablation of Rb facilitates gankyrin-mediated p53 destabilization, and desensitizes cancer cells for chemotherapy-induced apoptosis. These results indicate that Rb antagonizes gankyrin to inhibit MDM2-mediate p53 ubiquitination in cancer cells and suggest that the status of both p53 and Rb is important for efficacy of cancer chemotherapy.

ARF promotes accumulation of retinoblastoma protein through inhibition of MDM2.

The INK4a/ARF locus, encoding two tumor suppressor proteins, p16(INK4a) and p14(ARF) (ARF), plays key roles in many cellular processes including cell proliferation, apoptosis, cellular senescence and differentiation. Inactivation of INK4a/ARF is one of the most frequent events during human cancer development. Although p16(INK4a) is a critical component in retinoblastoma protein (Rb)-mediated growth regulatory pathway, p14(ARF) plays a pivotal role in the activation of p53 upon oncogenic stress signals. A body of evidence indicates that ARF also possesses growth suppression functions independent of p53, the mechanism of which is not well understood. We have recently shown that MDM2 interacts with Rb and promotes proteasome-dependent Rb degradation. In this study, we show that ARF disrupts MDM2-Rb interaction resulting in Rb accumulation. Wild-type ARF, but not ARF mutant defective in MDM2 interaction, stabilizes Rb and inhibits colony foci formation independent of p53. In addition, ablation of Rb impairs ARF function in growth suppression. Thus, this study demonstrates that ARF plays a direct role in regulation of Rb and suggests that inactivation of ARF may lead to defects in both p53 and Rb pathways in human cancer development.