Inactivation of the RB1 and PTPN14 tumor suppressors cooperatively enables the carcinogenic activity of the human papillomavirus E7 oncoprotein.

High-risk human papillomavirus (HPV) E7 proteins bind and inactivate host cellular tumor suppressors and are essential for the immortalization of primary human keratinocytes. E7 proteins from high- and low-risk HPV genotypes bind directly to at least two tumor suppressors, RB1 and PTPN14, and inactivate both. We previously characterized mutations in high-risk HPV E7 proteins that selectively abrogate the ability of E7 to bind either RB1 or PTPN14. Here, we established a genetic complementation system using the E7 mutants defective for binding to RB1 or PTPN14. Neither mutant alone could extend the lifespan of primary keratinocytes. When expressed together, the mutants could, like wild-type high-risk HPV E7, extend keratinocyte lifespan. Both high- and low-risk E7 reduced PTPN14 protein levels and reduced expression of keratinocyte differentiation genes, whereas only high-risk E7 reduced steady-state RB1 levels and induced E2F-dependent genes. Depletion of either RB1 or PTPN14 could cooperate with low-risk HPV6 E7 to extend keratinocyte lifespan, prompting the observation that PTPN14 depletion and RB1 inactivation by HPV E7 acted synergistically to induce certain cell cycle regulatory genes. Our findings advance the model that inactivation of at least two tumor suppressors is required for the carcinogenic activity of high-risk HPV E7. Although RB1 and PTPN14 regulate distinct signaling pathways, their combined inactivation may also contribute to the biological activity of HPV E7.