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

Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, SE-171 77 Stockholm, Sweden.
Cancer cell (Impact Factor: 23.89). 04/2009; 15(3):171-83. DOI: 10.1016/j.ccr.2009.01.019
Source: PubMed

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.

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Available from: Galina Selivanova, Aug 26, 2015
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    • "RITA was proposed to induce conformational changes in p53, leading to reduced p53-Hdm2 binding. Although this mechanism was questioned by a later study (Krajewski et al., 2005), a more recent report supports it (Enge et al., 2009). "
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    ABSTRACT: The prognosis of patients with uveal melanoma is poor. Because of the limited efficacy of current treatments, new therapeutic strategies need to be developed. Because p53 mutations are uncommon in uveal melanoma, reactivation of p53 may be used to achieve tumor regression. We investigated the use of combination therapies for intraocular melanoma, based on the p53 activators Nutlin-3 and reactivation of p53 and induction of tumor cell apoptosis (RITA) and the topoisomerase I inhibitor Topotecan. Nutlin-3 treatment induced p53-dependent growth inhibition in human uveal melanoma cell lines. The sensitivity to Nutlin-3 of the investigated cell lines did not correlate with basal Hdm2 or Hdmx levels. Nutlin-3 synergized with RITA and Topotecan to induce apoptosis in uveal melanoma cell lines and short-term cultures. Drug synergy correlated with enhanced induction of p53-Ser46 phosphorylation, which was attenuated by ATM inhibition. Nutlin-3 and Topotecan also significantly delayed tumor growth in vivo in a murine B16F10 model for ocular melanoma. Combination treatment appeared to inhibit tumor growth slightly more efficient than either drug alone. Nutlin-3, RITA and Topotecan lead to comparable p53 activation and growth inhibition under normoxia and hypoxia. Treatment with Nutlin-3 or RITA had no effect on HIF-1α induction by hypoxia, whereas the combination of these two drugs did inhibit hypoxia-induced HIF-1α. Also Topotecan, alone or in combination with Nutlin-3, reduced HIF-1α protein levels, suggesting that a certain level of DNA damage response is required for p53-mediated downregulation of HIF-1α. In conclusion, combination treatments based on small-molecule-induced p53 activation may have clinical potential for uveal melanoma.
    Oncogene 07/2011; 31(9):1105-16. DOI:10.1038/onc.2011.309 · 8.56 Impact Factor
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    • "Our finding that genotoxic stress promotes interaction between Rb and Hdm2 parallels recent findings that a p53 activating compound promotes Mdm2-mediated degradation of p21, both ultimately leading to cell death (Enge et al, 2009). Here, we provide new insights into the relationship between Hdm2, an E3 ligase and oncoprotein, and the tumor suppressors p53 and Rb. "
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    ABSTRACT: The retinoblastoma protein (Rb) inhibits both cell division and apoptosis, but the mechanism by which Rb alternatively regulates these divergent outcomes remains poorly understood. Cyclin-dependent kinases (Cdks) promote cell division by phosphorylating and reversibly inactivating Rb by a hierarchical series of phosphorylation events and sequential conformational changes. The stress-regulated mitogen-activated protein kinase p38 also phosphorylates Rb, but it does so in a cell cycle-independent manner that is associated with apoptosis rather than with cell division. Here, we show that p38 phosphorylates Rb by a novel mechanism that is distinct from that of Cdks. p38 bypasses the cell cycle-associated hierarchical phosphorylation and directly phosphorylates Rb on Ser567, which is not phosphorylated during the normal cell cycle. Phosphorylation by p38, but not Cdks, triggers an interaction between Rb and the human homolog of murine double minute 2 (Hdm2), leading to degradation of Rb, release of E2F1 and cell death. These findings provide a mechanistic explanation as to how Rb regulates cell division and apoptosis through different kinases, and reveal how Hdm2 may functionally link the tumor suppressors Rb and p53.
    Oncogene 02/2011; 30(5):588-99. DOI:10.1038/onc.2010.442 · 8.56 Impact Factor
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    • "Our results also do not support an antiapoptotic role of survivin in p53-dependent apoptosis induced by MDM2 antagonists. A recent study published in the course of this study has reported that p21 downregulation by siRNA enhances the apoptotic response to nutlin-3 (Enge et al., 2009). This conclusion was based on limited experiments with a single cell line, HCT116. "
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    ABSTRACT: p21(Waf1/Cip1) is a p53 transcription target implicated in both major functions of the tumor suppressor--cell cycle arrest and apoptosis. It is a potent inhibitor of the key cyclin-dependent kinases (CDK1-4), and has been thought to be the main mediator of p53-dependent G1 and G2 arrest. However, an increasing body of information suggests that in addition to its cell-cycle inhibitory activity, p21 can affect p53-dependent apoptosis. These data have been obtained from experiments in which p53 is activated primarily by genotoxic stress. In this study, we use the selective MDM2 antagonist, nutlin-3a, as a nongenotoxic p53 activator and show that the cell-cycle arrest function of p21 is dependent on the cellular context. In most cancer cell lines, p53-dependent p21 induction is essential for cell-cycle arrest, but in some, p21 is dispensable. Depletion of p21 did not increase the apoptotic response to nutlin-3a in all seven cancer cell lines tested and p21 overexpression did not protect apoptosis-sensitive lines from death. p21 was found to mediate nutlin-induced p53-dependent downregulation of another antiapoptotic protein, survivin, without significantly affecting the apoptotic outcome. Taken together our results suggest that p21 induction does not affect the apoptotic response to nongenotoxic p53 activation.
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