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Abstract:
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MDM4, also called MDMX or HDMX in humans, is an important negative regulator of the p53 tumor suppressor. MDM4 is overexpressed in about 17% of all cancers. MDM4 is known to be post-translationally regulated by MDM2-mediated ubiquitination to decrease its protein levels in response to genotoxic stress, resulting in accumulation and activation of p53. However, at the transcriptional level, MDM4 gene regulation has been less clearly understood. Through our work it is seen that MDM4 mRNA is a target of hsa-mir-34a (miR-34a). Micro-RNAs typically bind to their target genes in the 3’ untranslated region. MDM4 mRNA contains a lengthy 3’ untranslated region; however, it is a miR-34a site within the open reading frame (ORF) of the last exon that is responsible for the repression. Overexpression of miR-34a, but not a mutant miR-34a, is sufficient to decrease MDM4 mRNA levels to an extent identical to those of known miR-34a target genes. Likewise, MDM4 protein levels are decreased by miR-34a overexpression, and introduction of a miR-34a inhibitor can induce MDM4. A portion of MDM4 exon 11 containing this miR-34a site fused to a luciferase reporter gene is sufficient to confer responsiveness, being inhibited by additional expression of exogenous mir-34a and activated by inhibition of miR-34a. These data establish a mechanism for the observed negative regulation of MDM4 and potentially provide a means to manipulate MDM4 expression without introducing DNA damage. |
