HPVE6-USP46 Mediated Cdt2 Stabilization Reduces Set8 Mediated H4K20-Methylation to Induce Gene Expression Changes

SIMPLE SUMMARY: Cervical cancer is one of the most common cancers in women. More than 95% of the cervical cancers are caused by the human papilloma viruses. Earlier we had identified a protein target, USP46 that can be inhibited to block the cancer growth. We found that the viral E6 and human USP46 complex increases the levels of Cdt2 in cervical cancers. In this paper we compared real cervical tumors with matched normal tissue from same patients and found that the effects of E6 mediated USP46 activation are indeed reflected as increased levels of Cdt2 and decreased levels of Set8 protein in the real tumors. We also found that the HPV protein E6 alone can stimulate the enzymatic activity of human USP46. By doing this it activates several cellular pathways that are required for the growth of cancers. One such pathway is the EGFR pathway that is normally upregulated in cervical cancers. We find that E6-USP46 contributes to the activation of EGFR by inducing epigenetic changes on the DNA by degrading the Set8 protein. These findings illuminate the role of viral E6 protein in inducing cancers and substantiate the candidature of USP46 as a drug target for HPV induced cancers. ABSTRACT: E6 from high-risk strains of HPV is well known to transform cells by deregulating p53. We reported that in HPV transformed cell-lines E6 from high-risk HPV can recruit the USP46 deubiquitinase to substrates such as Cdt2 and stabilize the latter, and that USP46 is important for growth of HPV induced tumors in xenografts. Here we show that in cervical cancer biopsies the stabilization of Cdt2 in the HPV-induced cancers leads to the decrease of a CRL4-Cdt2 substrate, the histone H4K20 mono-methyltransferase Set8, and decrease in H4K20me1 or H4K20me3 that can be detected by immunohistochemistry. In HPV-transformed cancer cell lines in vitro, knockdown of E6 decreases Cdt2 and increases Set8. Co-knockdown of Set8 shows that some of the gene expression changes produced by E6 knockdown is due to the increase of Set8. EGFR and EGFR regulated genes were identified in this set of genes. Turning to the mechanism by which E6 stabilizes Cdt2, we find that a purified E6:USP46 complex has significantly more de-ubiquitinase activity in vitro than USP46 alone, demonstrating that E6 can directly interact with USP46 in the absence of other proteins and that it can substitute for the known activators of USP46, UAF1 and WDR20. Deletion mapping of Cdt2 shows that there are three discrete, but redundant, parts of the substrate that are essential for stabilization by E6: USP46. The helix–loop–helix region or the WD40 repeat driven beta-propeller structure of Cdt2 are dispensable for the stabilization implying that interaction with DDB1 (and the rest of the CRL4 complex) or with the substrate of the CRL4-Cdt2 E3 ligase is not necessary for E6:USP46 to interact with and stabilize Cdt2. The identification of 50 amino acid stretches in the 731 amino acid Cdt2 protein as being important for the stabilization by E6 underlines the specificity of the process. In summary, E6 activates the deubiquitinase activity of USP46, stabilizes Cdt2 utilizing multiple sites on Cdt2, and leads to degradation of Set8 and changes in gene-expression in HPV-transformed cells.