The role of HPV E6 protein in the induction of autophagy

Non-Melanoma skin cancer is the most common cancer in Caucasians. Accumulating evidence suggest a strong role for β-HPV (mainly HPV5 and HPV8) in the development of NMSC alongside UV, the main etiologic agent. The HPV genome encodes two potent oncoproteins, E6 and E7. HPV E6 protein has been shown to inhibit mitochondrial apoptosis following UVB exposure by specifically targeting BAK, a key apoptotic protein for proteolysis. A cross talk exists between the apoptotic pathway and the cellular survival pathway, autophagy. Where both autophagy and apoptosis can be triggered by common upstream signals. Additionally, cell lacking BAK and BAX respond with autophagy following the receipt of an apoptotic stimulus. The aim of this work was to investigate if HPV E6 may induce autophagy in response to an apoptotic stimulus. Cells expressing HPV5 E6 protein were exposed to UVB and autophagy levels, as well as autophagic flux, were investigated using endogenous LC3 staining, LC3 immunoblots and electron microscopy. Results showed an increase in autophagic flux and autophagy levels in HPV5 E6 expressing cells upon UV irradiation. Furthermore, the modulation of autophagy was demonstrated to be a conserved function amongst high-risk HPV (8, 20, 38 and mucosal HPV 16 and 18). Moreover, inhibition of autophagy resulted in a marked decrease in the viability of cells expressing E6 from HPV types 5 and 8. Furthermore, examining the respiration status of HPV5 E6 expressing cells showed a marked increase in both the glycolytic and the oxidative phosphorylation pathways. These results indicate that HPV E6 may promote autophagy following an apoptotic stimulus allowing the cells to survive in the background of DNA damage and that may contribute towards tumorigenesis.