Expression of the human papillomavirus type 16 E7 oncoprotein induces an autophagy-related process and sensitizes normal human keratinocytes to cell death in response to growth factor deprivation

Abstract

Expression of oncogenes, such as the human papillomavirus type 16 (HPV16) E7 oncoprotein, promotes aberrant cell proliferation. In the absence of concurrent mitogenic stimuli, this triggers a cell-intrinsic defense mechanism, the “trophic sentinel response”, which eliminates such aberrant cells. The molecular pathways that elicit this response, however, remain obscure. We set up an experimental system to investigate the trophic sentinel pathway triggered by HPV16 E7 expression in normal human keratinocytes, the natural host cells of HPVs. Keratinocytes expressing HPV16 E7 cultured in E-medium undergo cell death and show increased sub-G1 DNA content when grown to confluence or under conditions of serum deprivation. Moreover, HPV16 E7 expressing human keratinocytes express higher levels of the autophagy marker, LC3-II, which can be abrogated by 3-methylaldenine, an autophagy inhibitor. These findings indicate that even under normal culture conditions, HPV16 E7 expression triggers metabolic stress that may result in autophagy, a pathway implicated in carcinogenesis.