High-risk types of the human papillomavirus (HPV) are the causative agents of nearly all cases of cervical cancer, as well as a significant number of head, neck, penile, vulvar, and anal cancers. Like many other viruses with small genomes, HPV (∼8 kb) utilizes numerous mechanisms to increase the capacity of its genome to encode the proteins necessary for successful completion of its infectious life cycle, including alternative splicing. Studies over the past few decades have focused intensively on the activities and roles of E6 proteins from high-risk types of HPV during the process of cellular transformation, clearly implicating E6 as a major transforming agent. In contrast, the role of the smaller splice isoform, E6*, in the carcinogenic process (if any) has not yet been established. In the present study, we examined the behavior of the E6* protein during tumor growth in an in vivo nude mouse xenograft model. We created E6*-expressing SiHa (HPV+) and C33A (HPV-) cells, then examined the ability of both the parental and E6*-expressing cells to form tumors in nude mice. The difference in tumor size observed in the presence and absence of E6*, when expressed in both HPV+ (SiHa) and HPV- (C33A) cells, was dramatic and consistent, strongly indicating an anti-oncogenic role for E6* in this context. Interestingly, we found that tumor growth inhibition by E6* was greater in tumors derived from SiHa cells, which are HPV16-positive, than in tumors produced by C33A, which are HPV-negative. This difference implies that E6* acts by interfering with the oncogenic activity of the full-length protein as well as through one or more HPV-independent mechanisms. Consistent with this idea, we found that E6* does indeed bind to the full-sized isoform and inhibits its ability to accelerate degradation of p53 and procaspase 8. These data represent the first demonstration of biologically-relevant E6* activities distinct from those of the full-length isoform in vivo. The significance of these findings in the context of human cancers is in the possibility of mimicking or replicating the anti-oncogenic activity of E6* in such a way as to provide therapeutic benefit.

Citation Format: Whitney Evans, Maria Filippova, Robert Aragon, Valeri Filippov, Mark Reeves, Penelope Duerksen-Hughes. The splice variant of the human papillomavirus 16 E6 protein, E6*, displays anti-tumor properties in vivo. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3175. doi:10.1158/1538-7445.AM2014-3175