Yoon SY, Lee Y, Kim JH, Chung AS, Joo JH, Kim CN et al.. Over-expression of human UREB1 in colorectal cancer: HECT domain of human UREB1 inhibits the activity of tumor suppressor p53 protein. Biochem Biophys Res Commun 326: 7-17
Many fundamental processes, including oncogenesis, have implicated HECT domain proteins with ubiquitin ligase activity. The protein human upstream regulatory element binding protein 1 (hUREB1) is a HECT domain protein whose function is not defined yet. Here, we investigate the function of hUREB1 as a ubiquitin-protein ligase in human colorectal cells. Ectopic expression of the HECT domain of hUREB1 reduces the protein level and transcriptional activity of the p53 tumor suppressor, which is abrogated by the deletion in the HECT domain or point mutations in the essential residues of the HECT domain. The ubiquitination and destabilization of p53 is observed in cells treated with the protease inhibitor MG132, implying that the HECT domain of hUREB1 suppresses the transcriptional activity of p53 through a ubiquitin-dependent degradation pathway. Based on the results of Northern blot analysis, RT-PCR, and immunohistochemical analyses, the over-expression of hUREB1 is associated with colorectal carcinoma. Moreover, protein levels of hUREB1 and p53 were inversely correlated. These findings suggest that hUREB1 can function, at least in part, as a negative regulator of p53 during the colorectal carcinoma progression through the ubiquitination pathway mediated by the HECT domain.
"Our findings provide the mechanism of how DNA damage is reported to Mule, which consequently contributes to activation of the p53-dependent DNA damage response, as well as allowing for DNA repair to be accomplished within a required time frame through the regulation of the levels of DNA Pol β and Pol λ. Additionally, it should be noted that Mule has been shown to be overexpressed in a high percentage of tumour cell lines (18,19). As we demonstrated (Figure 4B and C), elevated levels of Mule may attenuate, at least partially, the upregulation of the p53 protein in response to DNA damage and inhibit prompt DNA repair via the base excision repair pathway; therefore, it may possibly be one of the major reasons for genetic instability in these cancer cells. "
[Show abstract][Hide abstract] ABSTRACT: The E3 ubiquitin ligase Mule/ARF-BP1 plays an important role in the cellular DNA damage response by controlling base excision repair and p53 protein levels. However, how the activity of Mule is regulated in response to DNA damage is currently unknown. Here, we report that the Ser18-containing isoform of the USP7 deubiquitylation enzyme (USP7S) controls Mule stability by preventing its self-ubiquitylation and subsequent proteasomal degradation. We find that in response to DNA damage, downregulation of USP7S leads to self-ubiquitylation and proteasomal degradation of Mule, which eventually leads to p53 accumulation. Cells that are unable to downregulate Mule show reduced ability to upregulate p53 levels in response to DNA damage. We also find that, as Mule inactivation is required for stabilization of base excision repair enzymes, the failure of cells to downregulate Mule after DNA damage results in deficient DNA repair. Our data describe a novel mechanism by which Mule is regulated in response to DNA damage and coordinates cellular DNA damage responses and DNA repair.
Nucleic Acids Research 12/2012; 41(3). DOI:10.1093/nar/gks1359 · 9.11 Impact Factor
"The importance of the mule gene in colon cancer development was also confirmed by Yoon et al. as expression of this gene was found to be at a moderate or high level in approximately 40% of colon carcinoma tissues (samples from 70 patients were tested) compared to non-tumor colon. Importantly, Mule and p53 levels were found to inversely correlate in the vast majority of colon cancer tissues tested . Furthermore, Chen and co-authors  were able to show that Mule protein depletion represses proliferation of breast cancer cell lines characterized by mutations in the p53 gene. "
[Show abstract][Hide abstract] ABSTRACT: The ARF (Alternative Reading Frame) protein is encoded in the Ink4a locus of human chromosome 9 that is frequently mutated in cancer cells. It was recently demonstrated that ARF is induced in response to DNA damage and inhibits, by direct interaction, the E3 ubiquitin ligase Mule that regulates p53 protein levels. Mule inhibition leads to p53 accumulation and activates cellular DNA damage responses. Mule has also recently been identified as a major E3 ubiquitin ligase involved in the regulation of DNA base excision repair. In this review, we will summarise the major properties of Mule and ARF and their roles in the coordination of DNA repair and DNA replication.
"We note that most cancer cells also have a disrupted p53 pathway that could blunt the APC-mediated mechanism of Cdc6 degradation. Thus Huwe1, which is also highly expressed in multiple tumor cell lines (Chen et al., 2005; Yoon et al., 2005), may represent the primary means of degrading Cdc6 in p53-null cells and may be particularly important in the response to chemotherapies that damage DNA. Huwe1-depletion in the absence of DNA damage did not result in overt rereplication (data not shown). "
[Show abstract][Hide abstract] ABSTRACT: The Cdc6 protein is an essential component of pre-replication complexes (preRCs), which assemble at origins of DNA replication during the G1 phase of the cell cycle. Previous studies have demonstrated that, in response to ionizing radiation, Cdc6 is ubiquitinated by the anaphase promoting complex (APC(Cdh1)) in a p53-dependent manner. We find, however, that DNA damage caused by UV irradiation or DNA alkylation by methyl methane sulfonate (MMS) induces Cdc6 degradation independently of p53. We further demonstrate that Cdc6 degradation after these forms of DNA damage is also independent of cell cycle phase, Cdc6 phosphorylation of the known Cdk target residues, or the Cul4/DDB1 and APC(Cdh1) ubiquitin E3 ligases. Instead Cdc6 directly binds a HECT-family ubiquitin E3 ligase, Huwe1 (also known as Mule, UreB1, ARF-BP1, Lasu1, and HectH9), and Huwe1 polyubiquitinates Cdc6 in vitro. Degradation of Cdc6 in UV-irradiated cells or in cells treated with MMS requires Huwe1 and is associated with release of Cdc6 from chromatin. Furthermore, yeast cells lacking the Huwe1 ortholog, Tom1, have a similar defect in Cdc6 degradation. Together, these findings demonstrate an important and conserved role for Huwe1 in regulating Cdc6 abundance after DNA damage.
Molecular Biology of the Cell 10/2007; 18(9):3340-50. DOI:10.1091/mbc.E07-02-0173 · 4.47 Impact Factor
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