Activation of p53 by MDM2 Antagonists Can Protect Proliferating Cells from Mitotic Inhibitors

Abstract

Recent studies have shown that activation of cell cycle checkpoints can protect normal proliferating cells from mitotic inhibitors by preventing their entry into mitosis. These studies have used genotoxic agents that act, at least in part, by activation of the p53 pathway. However, genotoxic drugs are known also to have p53-independent activities and could affect the sensitivity of tumor cells to antimitotic agents. Recently, we have developed the first potent and selective small-molecule inhibitors of the p53-MDM2 interaction, the nutlins, which activate the p53 pathway only in cells with wild-type but not mutant p53. Using these compounds, we show that p53 activation leads to G 1 and G 2 phase arrest and can protect cells from mitotic block and apoptosis caused by paclitaxel. Pretreatment of HCT116 and RKO colon cancer cells (wild-type p53) or primary human fibroblasts (1043SK) with nutlins for 24 hours followed by incubation with paclitaxel for additional 48 hours did not increase significantly their mitotic index and protected the cells from the cytotoxicity of paclitaxel. Cancer cells with mutant p53 (MDA-MB-435) responded to the same treatment with mitotic arrest and massive apoptosis. These results have two major implications for cancer therapy. First, p53-activating therapies may have antagonistic effect when combined with mitotic poisons. Second, pretreatment with MDM2 antagonists before chemotherapy of tumors with mutant p53 may offer a partial protection to proliferating normal tissues. (Cancer Res 2005; 65(5): 1918-24)

Induced G1 cell-cycle arrest controls replication-dependent histone mRNA 3′ end processing through p21, NPAT and CDK9

“…The induction of p53-dependent cell-cycle arrest has an important role in the control of cellular proliferation and may help protect cells against the effects of certain chemotherapeutic agents (Carvajal et al, 2005;Kranz and Dobbelstein, 2006). The induction of polyadenylated mRNAs produced from replication-dependent histone genes following p53-induced G1 cell-cycle arrest may be significant as the polyadenylated histone transcripts have a longer half-life than the normally processed transcripts from the same genes (Kirsh et al, 1989) and are translated in a cell cycle-independent manner.…”

Mouse double minute antagonist Nutlin-3a enhances chemotherapy-induced apoptosis in cancer cells with mutant p53 by activating E2F1

“…In tumor cells with wild-type p53, this results in induction of p53 target genes, cell cycle arrest and apoptosis (Vassilev et al, 2004). In some cases, growth arrest at the G1 and G2 phases of the cell cycle by Nutlin-3a can protect proliferating cells from the cytotoxic effect of mitotic poisons (Carvajal et al, 2005). Previous findings have suggested that Nutlins may offer a new therapeutic option to patients with tumors that express wild-type p53 either as single agent (Vassilev, 2005;Tovar et al, 2005) or as combination therapy (Kojima et al, 2005;Stuhmer et al, 2005).…”

“…Previous findings have suggested that Nutlins may offer a new therapeutic option to patients with tumors that express wild-type p53 either as single agent (Vassilev, 2005;Tovar et al, 2005) or as combination therapy (Kojima et al, 2005;Stuhmer et al, 2005). In contrast, tumor cells with mutant p53 are resistant to Nutlin-3a (Carvajal et al, 2005). This would indicate that therapeutic utility of MDM2 antagonists is limited to the treatment of tumors that are wild type for p53.…”

“…The effects of Nutlin3a in cell lines with wild-type p53 have been well Nutlin-3a enhances E2F1-induced apoptosis G Ambrosini et al characterized (Carvajal et al, 2005). However, there is no mechanistic evidence of Nutlin-3a-mediated effects in p53 mutant cells.…”

“…Cellular viability was evaluated by 3-[4,5]dimethylthiazol-2,5-diphenyltetrazolium bromide (MTT) assay as described (Carvajal et al, 2005). Briefly, eight cell types were seeded in 96-well plates at a density of 5 Â 10 3 cells/well in 100 ml of medium for 24 h. Drug dilutions were made in medium at twofold increments to the final concentrations of 2.5, 5, 10 and 20 mM.…”