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ABSTRACT: The Murine Double Minute 2 (MDM2) protein is a key regulator of cell proliferation and apoptosis that acts primarily by inhibiting the p53 tumor suppressor. Similarly, the PI3-Kinase (PI3K)/AKT pathway is critical for growth factor-mediated cell survival. Additionally, it has been reported that AKT can directly phosphorylate and activate MDM2. In this study, we show that IGF-1 up-regulates MDM2 protein levels in a PI3K/AKT-dependent manner. Inhibition of mTOR by rapamycin or expression of a dominant negative eukaryotic initiation factor 4E binding protein 1 (4EBP1) mutant protein, as well as ablation of eukaryotic initiation factor 4E (eIF4E), efficiently abolishes IGF-1-mediated up-regulation of MDM2. In addition, we show that rapamycin effectively inhibits MDM2 expression and sensitizes cancer cells to chemotherapy. Taken together, this study reveals a novel mechanism by which IGF-1 activates MDM2 via the mTOR pathway, and that pharmacologic inhibition of mTOR combined with chemotherapy may be more effective in treatment of a subset of cancers harboring increased MDM2 activation.
PLoS ONE 01/2013; 8(4):e63179. · 4.09 Impact Factor
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ABSTRACT: The p53 homologue p63 encodes multiple protein isoforms either with (TA) or without (DeltaN) the N-terminal transactivation domain. Accumulating evidence indicates that TAp63 plays an important role in various biological processes, including cell proliferation, differentiation, and apoptosis. However, how TAp63 is regulated remains largely unclear. In this study, we demonstrate that NF-kappaB induces TAp63 gene expression. The responsible elements for NF-kappaB-mediated TAp63 induction are located within the region from -784 to -296 bp in the TAp63 promoter, which contains two NF-kappaB binding sites. Ectopic expression of RelA stimulates TAp63 promoter-driven reporter activity and increases endogenous TAp63 mRNA levels. Inhibition of NF-kappaB by IkappaBalpha super-repressor or with a chemical inhibitor leads to down regulation of TAp63 mRNA expression and activity. In addition, mutations in the critical NF-kappaB-binding sites significantly abolish the effects of NF-kappaB on TAp63. Activation of NF-kappaB by TNFalpha enhances p50/RelA binding to the NF-kappaB binding sites. Furthermore, we show that an Sp1 site adjacent to the NF-kappaB sites plays a role in NF-kappaB-mediated upregulation of TAp63. Taken together, these data reveal that TAp63 is a transcriptional target of NF-kappaB, which may play a role in cell proliferation, differentiation and survival upon NF-kappaB activation by various stimuli.
Journal of Cellular Biochemistry 03/2010; 109(4):702-10. · 2.87 Impact Factor
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ABSTRACT: The p53 homologue p63 encodes multiple protein isoforms either with (TA) or without (ΔN) the N-terminal transactivation domain. Accumulating evidence indicates that TAp63 plays an important role in various biological processes, including cell proliferation, differentiation, and apoptosis. However, how TAp63 is regulated remains largely unclear. In this study, we demonstrate that NF-κB induces TAp63 gene expression. The responsible elements for NF-κB-mediated TAp63 induction are located within the region from −784 to −296 bp in the TAp63 promoter, which contains two NF-κB binding sites. Ectopic expression of RelA stimulates TAp63 promoter-driven reporter activity and increases endogenous TAp63 mRNA levels. Inhibition of NF-κB by IκBα super-repressor or with a chemical inhibitor leads to down regulation of TAp63 mRNA expression and activity. In addition, mutations in the critical NF-κB-binding sites significantly abolish the effects of NF-κB on TAp63. Activation of NF-κB by TNFα enhances p50/RelA binding to the NF-κB binding sites. Furthermore, we show that an Sp1 site adjacent to the NF-κB sites plays a role in NF-κB-mediated upregulation of TAp63. Taken together, these data reveal that TAp63 is a transcriptional target of NF-κB, which may play a role in cell proliferation, differentiation and survival upon NF-κB activation by various stimuli. J. Cell. Biochem. 109: 702–710, 2010. © 2010 Wiley-Liss, Inc.
Journal of Cellular Biochemistry 01/2010; 109(4):702 - 710. · 2.87 Impact Factor
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ABSTRACT: The retinoblastoma protein (Rb) plays a pivotal role in regulating cell proliferation and apoptosis. Nutlin-3, a small molecule
MDM2 antagonist blocking interaction between MDM2 and p53, activates p53 resulting in cell growth arrest or apoptosis in various
cancer cells. However, the molecular basis for the different cellular responses upon nutlin-3 treatment is not fully understood.
In this study, we show that nutlin-3 activates p53 resulting in a dramatic increase in MDM2 expression and a marked reduction
in total Rb protein levels. Interestingly, nutlin-3 reduces the levels of hypophosphorylated Rb and induces massive apoptosis
in SJSA-1 cells, which can be largely rescued by knockdown of MDM2 or by expression of constitutively active Rb. By contrast,
nutlin-3 treatment of several human cancer cells, including A549, U2-OS, and HCT116, results in an accumulation of hypophosphorylated
Rb and cell cycle arrest but not apoptosis. Furthermore, we show that down-regulation of Rb by nutlin-3 does not lead to E2F1
activation nor does E2F1 play a critical role for nutlin-3-induced apoptosis in SJSA-1 cells. Taken together, these results
suggest that Rb plays a critical role in influencing cellular response to activation of p53 pathway by nutlin-3.
Journal of Biological Chemistry 09/2009; 284(39):26315-26321. · 4.77 Impact Factor
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ABSTRACT: The retinoblastoma protein (Rb) plays a pivotal role in regulating cell proliferation and apoptosis. Nutlin-3, a small molecule MDM2 antagonist blocking interaction between MDM2 and p53, activates p53 resulting in cell growth arrest or apoptosis in various cancer cells. However, the molecular basis for the different cellular responses upon nutlin-3 treatment is not fully understood. In this study, we show that nutlin-3 activates p53 resulting in a dramatic increase in MDM2 expression and a marked reduction in total Rb protein levels. Interestingly, nutlin-3 reduces the levels of hypophosphorylated Rb and induces massive apoptosis in SJSA-1 cells, which can be largely rescued by knockdown of MDM2 or by expression of constitutively active Rb. By contrast, nutlin-3 treatment of several human cancer cells, including A549, U2-OS, and HCT116, results in an accumulation of hypophosphorylated Rb and cell cycle arrest but not apoptosis. Furthermore, we show that down-regulation of Rb by nutlin-3 does not lead to E2F1 activation nor does E2F1 play a critical role for nutlin-3-induced apoptosis in SJSA-1 cells. Taken together, these results suggest that Rb plays a critical role in influencing cellular response to activation of p53 pathway by nutlin-3.
Journal of Biological Chemistry 08/2009; 284(39):26315-21. · 4.77 Impact Factor
