MDM2-Dependent Downregulation of p21 and hnRNP K Provides a Switch between Apoptosis and Growth Arrest Induced by Pharmacologically Activated p53

Abstract

We have previously identified the p53-reactivating compound RITA in a cell-based screen. Here, using microarray analysis, we show that the global transcriptional response of tumor cells to RITA is p53 dependent. Pathway analysis revealed induction of the p53 apoptosis pathway, consistent with apoptosis being the major response to RITA in cancer cells. We uncovered that MDM2 released from p53 by RITA promotes degradation of p21 and the p53 cofactor hnRNP K, required for p21 transcription. Functional studies revealed MDM2-dependent inhibition of p21 as a key switch regulating cell fate decisions upon p53 reactivation. Our results emphasize the utility of targeting wild-type p53 protein itself as a promising approach for anticancer therapy.

Pharmacological targeting of p53 through RITA is an effective antitumoral strategy for malignant pleural mesothelioma

“…Conversely, RITA reduced p21 expression, consistent with what was previously reported in other cell lines 19 ( Fig. 2B).…”

“…39,40 Treatment with nutlin-3, instead, caused a cell cycle arrest in mesothelioma cell lines carrying wt p53, whereas no arrest was achieved in cell lines carrying mutant p53. As described for other tumor types, 19 nutlin-3-induced arrest seems to rely on the upregulation of the p53 target p21. Consistently, we found that nutlin-3 was able to cause cell cycle arrest only in p53 wt cells concomitantly to p21 induction, whereas IST-MES 2 and NCI-H2452, owing to a mutated p53, did not express p21 following nutlin-3 treatment and did not show cell cycle arrest.…”

“…This sub-G 1 cell accumulation is suggestive of apoptosis and in accordance with the observation that p21 downregulation is a crucial event to promote apoptosis. 19,28 As both RITA and nutlin-3 interfere with the p53-MDM2 interaction, we also evaluated MDM2 protein levels in mesothelioma cell lines 24 h following treatment with these agents. Consistently to what previously reported, nutlin-3 strongly induces MDM2 levels in a dosedependent manner, which seems to reflect its p53-mediated transcriptional regulation, except in IST-MES 2 and NCI-H2452 carrying mutant TP53 (Fig.…”

“…15 More recently, another molecule, RITA (reactivation of p53 and induction of tumor cell apoptosis) was found to prevent p53-MDM2 interaction and induce p53-mediated apoptosis in tumor cell lines expressing both wt p53 16 or some mutant forms. 17,18 RITA, in particular, has the appealing property of inducing apoptosis rather than growth arrest in various cancer cell lines, either by promoting the proteasome-mediated degradation of the p53 transcriptional cofactor hnRNPK, which is required for the induction of growth arrest genes such as p21, 19 or through other yet unidentified mechanisms. 20,21 On these bases, the pharmacological targeting of p53 in mesothelioma holds the potential to be a new promising antitumor approach.…”

Inhibition of Glycolytic Enzymes Mediated by Pharmacologically Activated p53

“…Our group has identified p53-reactivating compound RITA (reactivation of p53 and induction of tumor cell apoptosis) (23). RITA binds the p53 N-terminal domain and disrupts the interaction with its negative regulator MDM2, which results in p53 activation and induction of apoptosis (23,24). 5 Notably, we showed that RITA activates p53 in cells expressing oncogenes, whereas the effect in non-transformed cells is almost negligible (23,25).…”