The E6 oncoprotein of human papillomavirus 16 as a therapeutic target

C245

The etiology of cervical cancer, the second most frequent malignancy in women, is closely associated with high risk human papillomaviruses (HPVs). Tumor growth is caused by the joint function of the viral E6 and E7 oncoproteins, both of them interfering with key regulators of the cell cycle and apoptosis. Since selective inhibition of E6 expression induces apoptosis and blocks the malignant growth of HPV positive cancer cells, E6 should provide a promising target for therapeutic intervention.
 In a yeast two-hybrid screen of a randomized peptide expression library, we identified a 15mer peptide, pep11, that binds to HPV16 E6. Binding was highly specific, as pep11 did not bind to the E6 protein of other common HPV types (6, 11, 18) or to unrelated control proteins. The pep11 sequence substantially differs from the E6 binding motif within E6AP, a described cellular E6 binding partner. Expression of pep11 led to increased p53 protein levels and reactivation of p53 transcriptional activity, indicating that pep11 can interfere with the E6-induced degradation of p53. In addition, pep11 expression induced apoptosis and growth inhibition selectively in HPV16-positive cell lines.
 Our results show that pep11 displays a novel binding motif which targets a critical region of 16 E6, thereby blocking its anti-apoptotic activity. Thus, pep11 could serve as a useful tool for structural and functional analyses of E6. The pep11/E6 interaction further provides a basis to screen for non-peptide small molecule inhibitors of E6. Screening of a 50,000 compound library in a competitive binding assay led to the identification of 40 compounds with IC50 values below 50µM. Hits will be characterized further in secondary assays for their ability to act as pep11 mimics and may subsequently be refined into lead structures for drug development. This screening approach may eventually result in the development of novel therapeutic strategies against HPV16-positive dysplasias and cancers.