BRCA1 deficiency enhances the aggressiveness of breast cancer cells expressing HPV16 oncoproteins

BACKGROUND INFORMATION

The precise etiology of breast cancer is not completely understood, although women with BRCA1 gene mutations have a significantly increased risk of developing the disease. In addition, sporadic breast cancer is frequently associated with decreased BRCA1 gene expression. Growing evidence of Human papillomaviruses (HPVs) infections in breast tumors has raised the possibility of the involvement of HPVs in the pathogenesis of breast cancer. We investigated whether the effects of HPV oncoproteins E6 and E7 were influenced by the expression levels of BRCA1. HPV16E6E7 (prototype or E6D25E/E7N29S Asian variant type) were stably expressed in MDA-MB231 breast cancer cells, wild type for BRCA1, or with BRCA1 knocked down.

RESULTS

Expression of HPV16E6E7 oncogenes did not affect BRCA1 levels and the abundance of HPV16E6E7 was not altered by BRCA1 knockdown. BRCA1 levels did not alter HPV16E6E7-dependent degradation of G1-S cell cycle proteins p53 and pRb. However, we found that the expression of G2-M cell cycle protein cyclin B1 enhanced by HPV16E6E7 was impacted by BRCA1 levels. Especially, we found the correlation between BRCA1 and cyclin B1 expression and this was also confirmed in breast cancer samples from a Thai cohort. We further demonstrated that the combination of HPV oncoproteins and low levels of BRCA1 protein appears to enhance proliferation and invasion. Transactivation activities of HPV16E6E7 on genes regulating cell proliferation and invasion (TGF-β and vimentin) were significantly increased in BRCA1-deficient cells.

CONCLUSIONS

Our results indicate that a deficiency of BRCA1 promotes the transactivation activity of HPV16E6E7 leading to increase of cell proliferation and invasion.

SIGNIFICANCE

HPV infection appears to have the potential to enhance the aggressiveness of breast cancers, especially those deficient in BRCA1.

An internal class III PDZ binding motif in HPV16 E6* protein is required for Dlg degradation activity

BACKGROUND

A splice product of the E6 oncoprotein, E6*, is found in cells infected with HPV associated with a high-risk for cervical cancer. Both E6* and E6 promote Dlg degradation, considered a contributing factor for the tumorigenic potential of high-risk HPVs. The full-length E6 utilizes a conserved PDZ binding motif (PBM) at the extreme C-terminus to promote Dlg degradation. In contrast, this PBM is absent in E6*.

METHODS

We performed western blot analysis, site-directed mutagenesis and co-immunoprecipitation to identify the key elements required for Dlg degradation activity of high-risk HPVE6*, using HPV16E6* as a model.

RESULTS

Our data indicate that only one of the two internal putative class III PBMs, located between amino acids 24-27 (HDII) of HPV16E6*, was required to facilitate degradation of Dlg protein. Substitution of the two consensus residues in this region (D25 and I27) to glycine greatly diminished activity. Whereas substitution of the two conserved residues in the putative internal class I PBM (amino acids 16-19) or the second putative class III PBM (amino acids 28-31) was without effect. Interestingly, HPV66E6* which does not promote Dlg degradation can be converted into a form capable of facilitating Dlg degradation through the insertion of nine amino acids (20-28) containing the class III PBM from HPV16E6*. HPV16E6*-induced Dlg degradation appeared independent of E6AP.

CONCLUSIONS

The internal class III PBM of HPV16E6*I required for Dlg degradation is identified.

GENERAL SIGNIFICANCE

This study highlights that a novel class III PBM as the domain responsible for Dlg degradation activity in high-risk HPVE6*.