Regulation of p14ARF tumor suppressor activities and functions

The ARF protein is encoded by the alternative reading frame of the INK4a locus, one of the most frequent sites of genetic loss in human cancers. The ARF tumor suppressor function, as a sensor of hyper-proliferative stimuli, is to restrict cell proliferation through both p53-dependent and independent pathways. However, few studies started to address the possibility that ARF might promote the survival of subsets of tumors. In the past years, intensive studies have been focused not only on ARF activation at transcriptional level but also on the mechanisms regulating ARF protein turnover. Here we show that p14ARF is a PKC target. In fact, PKC activation mediates phosphorylation and stabilization of ARF endogenously expressed protein. Moreover, a phosphomimetic ARF mutant accumulates in the cytoplasm and, interestingly, despite the ability to stabilize p53 and bind MDM2, is not able to efficiently block cell growth in both human and mouse cell lines. These data could be interpreted as a way of cancer cells to escape ARF surveillance in tumorigenesis, but they might also indicate that ARF phosphorylation in tumor cells could be a mean to sustain tumor progression by conferring pro-survival properties to the cells. We thus explored the role of endogenously expressed ARF protein in some tumor and stabilized cell lines by knocking down ARF expression by siRNA. Unspectedly, we observed a block of cell proliferation and induction of DAPK mediated p53-dependent apoptosis in ARF depleted Hela and HaCat cells. Our results suggest that ARF might have a pro-survival role depending on cellular context.