Human Oncoprotein MDM2 Up-regulates Expression of NF- B2 Precursor p100 Conferring a Survival Advantage to Lung Cells

Abstract

The current model predicts that MDM2 is primarily overexpressed in cancers with wild-type (WT) p53 and contributes to oncogenesis by degrading p53. Following a correlated expression of MDM2 and NF-κB2 transcripts in human lung tumors, we have identified a novel transactivation function of MDM2. Here, we report that in human lung tumors, overexpression of MDM2 was found in approximately 30% of cases irrespective of their p53 status, and expression of MDM2 and NF-κB2 transcripts showed a highly significant statistical correlation in tumors with WT p53. We investigated the significance of this correlated expression in terms of mechanism and biological function. Increase in MDM2 expression from its own promoter in transgenic mice remarkably enhanced expression of NF-κB2 compared with its non-transgenic littermates. Knockdown or elimination of endogenous MDM2 expression in cultured non-transformed or lung tumor cells drastically reduced expression of NF-κB2 transcripts, suggesting a normal physiological role of MDM2 in regulating NF-κB2 transcription. MDM2 could up-regulate expression of NF-κB2 transcripts when its p53-interaction domain was blocked with Nutlin-3, indicating that the MDM2-p53 interaction is dispensable for up-regulation of NF-κB2 expression. Consistently, analysis of functional domains of MDM2 indicated that although the p53-interaction domain of MDM2 contributes to the up-regulation of the NFκB2 promoter, MDM2 does not require direct interactions with p53 for this function. Accordingly, MDM2 overexpression in non-transformed or lung cancer cells devoid of p53 also generated a significant increase in the expression of NF-κB2 transcript and its targets CXCL-1 and CXCL-10, whereas elimination of MDM2 expression had the opposite effects. MDM2-mediated increase in p100/NF-κB2 expression reduced cell death mediated by paclitaxel. Furthermore, knockdown of NF-κB2 expression retarded cell proliferation. Based on these data, we propose that MDM2-mediated NF-κB2 up-regulation is a combined effect of p53-dependent and independent mechanisms and that it confers a survival advantage to lung cancer cells.

DNA replication in progenitor cells and epithelial regeneration after lung injury requires the oncoprotein MDM2

“…Cultured lung cells generated from these mice showed an approximately 7-fold increase in the MDM2 protein level after 24-hour Dox treatment ( Figure 1D). Therefore, the levels of Dox-induced MDM2 expression in the lungs of tightMDM2 mice are comparable with reported MDM2 expression in human lung tumors (2). IHC analysis of formalin fixed paraffin embedded (FFPE) lung tissue sections from Dox-treated (+Dox) tightMDM2 mice confirmed Dox-induced MDM2 expression in the CCSP-expressing cells of respiratory bronchioles.…”

“…Increased frequency of proliferating lung progenitor cells by MDM2 could be a result of its ability to inactivate WT p53 (1). Since lung cancers with p53 mutation often show MDM2 overexpression (2,21,44), the ability of MDM2 to induce DNA replication, expansion of CCSP/SPC-dual-positive progenitor cells, and reepithelialization was determined in the presence of mutant p53. tightMDM2, lox-stop lox (LSL) R172H (knock-in) mice were generated.…”

“…Amplification or overexpression of the human homologue of the mdm2 gene has been implicated in human cancers with or without p53 mutation (1)(2)(3)(4). Moreover, a single nucleotide polymorphism (snp) at bp 309 of the MDM2 promoter leads to MDM2 overexpression (5,6).…”

“…Although MDM2 is frequently overexpressed in human lung cancers with WT or mutant p53 (2,21,22), the consequence of MDM2 overexpression in normal adult lung has not been investigated, and there is no existing mouse model to determine the cell-proliferative effects of MDM2 in adult lung. Lung is a highly quiescent organ with regenerative potential.…”

Human oncoprotein MDM2 activates the Akt signaling pathway through an interaction with the repressor element-1 silencing transcription factor conferring a survival advantage to cancer cells

“…For this purpose, we used a small-molecule inhibitor of MDM2, Nutlin-3, which binds to the p53-binding region of MDM2, which in turn stabilizes p53 and elevates MDM2 expression. 6,20 A549 lung cancer cells, which harbor WT p53, 21 were treated with Nutlin-3 or vehicle (dimethyl sulfoxide (DMSO)). Western blot analysis of the cell extracts revealed that treatment of cells with Nutlin-3 upregulated p53 and MDM2 expression as expected and also upregulated p-Akt levels (Figure 1f).…”