Kateřina Cetkovská

Ubiquitin-specific peptidase 48 regulates Mdm2 protein levels independent of its deubiquitinase activity

The overexpression of Mdm2 has been linked to the loss of p53 tumour suppressor activity in several human cancers. Here, we present results suggesting that ubiquitin-specific peptidase 48 (USP48), a deubiquitinase that has been linked in previous reports to the NF-κB signaling pathway, is a novel Mdm2 binding partner that promotes Mdm2 stability and enhances Mdm2-mediated p53 ubiquitination and degradation. In contrast to other deubiquitinating enzymes (DUBs) that have been previously implicated in the regulation of Mdm2 protein stability, USP48 did not induce Mdm2 stabilization by significantly reducing Mdm2 ubiquitination levels. Moreover, two previously characterized USP48 mutants lacking deubiquitinase activity were also capable of efficiently stabilizing Mdm2, indicating that USP48 utilizes a non-canonical, deubiquitination-independent mechanism to promote Mdm2 oncoprotein stability. This study represents, to the best of our knowledge, the first report suggesting DUB-mediated target protein stabilization that is independent of its deubiquitinase activity. In addition, our results suggest that USP48 might represent a new mechanism of crosstalk between the NF-κB and p53 stress response pathways.

A Novel Interaction between TFII-I and Mdm2 with a Negative Effect on TFII-I Transcriptional Activity.

Williams-Beuren syndrome-associated transcription factor TFII-I plays a critical regulatory role in bone and neural tissue development and in immunity, in part by regulating cell proliferation in response to mitogens. Mdm2, a cellular oncogene responsible for the loss of p53 tumor suppressor activity in a significant proportion of human cancers, was identified in this study as a new binding partner for TFII-I and a negative regulator of TFII-I-mediated transcription. These findings suggest a new p53-independent mechanism by which increased Mdm2 levels found in human tumors could influence cancer cells. In addition to that, we present data indicating that TFII-I is an important cellular regulator of transcription from the immediate-early promoter of human cytomegalovirus, a promoter sequence frequently used in mammalian expression vectors, including vectors for gene therapy. Our observation that Mdm2 over-expression can decrease the ability of TFII-I to activate the CMV promoter might have implications for the efficiency of experimental gene therapy based on CMV promoter–derived vectors in cancers with Mdm2 gene amplification.