Abstract
Mouse Double Minute 2 Homolog (MDM2) is a key negative regulator of the master tumor suppressor p53. MDM2 regulates p53 on multiple levels, including acting as an ubiquitin ligase for the protein, thereby promoting its degradation by the proteasome. MDM2 is oncogenic and is frequently found to be over-expressed in human tumors, suggesting its dysregulation plays an important role in human cancers. We have recently found that the Ras effector and RASSF (Ras Association Domain Family) family member RASSF5/NORE1A enhances the levels of nuclear p53. We have also found that NORE1A (Novel Ras Effector 1A) binds the substrate recognition component of the SCF-ubiquitin ligase complex β-TrCP. Here, we now show that NORE1A regulates MDM2 protein levels by targeting it for ubiquitination by SCF-β-TrCP. We also show the suppression of NORE1A protein levels enhances MDM2 protein expression. Finally, we show that MDM2 can suppress the potent senescence phenotype induced by NORE1A over-expression. Thus, we identify a mechanism by which Ras/NORE1A can modulate p53 protein levels. As MDM2 has several important targets in addition to p53, this finding has broad implications for cancer biology in tumor cells that have lost expression of NORE1A due to promoter methylation.
Highlights
Mouse Double Minute 2 Homolog (MDM2) is an E3 ubiquitin ligase and serves as a key regulator of the p53 master tumor suppressor [1]
As MDM2 is a substrate of the complex and NORE1A can directly bind MDM2 [18], we measured the ability of
To confirm that effects we had observed were due to proteosomal degradation of MDM2, we took HEK-293 cells again transfected with GFP-MDM2 paired with an empty vector or an HA-tagged NORE1A. 24 h post-transfection, the transfected cells were split into two groups, one treated with DMSO and the other treated with the proteasome inhibitor
Summary
Mouse Double Minute 2 Homolog (MDM2) is an E3 ubiquitin ligase and serves as a key regulator of the p53 master tumor suppressor [1]. MDM2 can negatively regulate p53 protein levels by binding and ubiquitinating p53. This post-translational modification results in the nuclear export of p53 and subsequent degradation by the 26S proteasome [2]. MDM2 is shown to regulate p53 mediated gene transcription by binding and inhibiting its activity as well as suppressing expression of the p53 gene locus [1]. This situation is further complicated by the observation that p53 can stimulate MDM2 transcription. The frequently observed over-expression of MDM2 in primary tumor cells correlates well with the loss of p53 expression [4] and suggests a central physiological role for MDM2 dysregulation in human disease
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