Date Published: March 6, 2014
Publisher: Public Library of Science
Author(s): Jenny Leitz, Miriam Reuschenbach, Claudia Lohrey, Anja Honegger, Rosita Accardi, Massimo Tommasino, Manuel Llano, Magnus von Knebel Doeberitz, Karin Hoppe-Seyler, Felix Hoppe-Seyler, Paul Francis Lambert.
The expression of the human papillomavirus (HPV) E6/E7 oncogenes is crucial for HPV-induced malignant cell transformation. The identification of cellular targets attacked by the HPV oncogenes is critical for our understanding of the molecular mechanisms of HPV-associated carcinogenesis and may open novel therapeutic opportunities. Here, we identify the Lens Epithelial-Derived Growth Factor (LEDGF) gene as a novel cellular target gene for the HPV oncogenes. Elevated LEDGF expression has been recently linked to human carcinogenesis and can protect tumor cells towards different forms of cellular stress. We show that intracellular LEDGF mRNA and protein levels in HPV-positive cancer cells are critically dependent on the maintenance of viral oncogene expression. Ectopic E6/E7 expression stimulates LEDGF transcription in primary keratinocytes, at least in part via activation of the LEDGF promoter. Repression of endogenous LEDGF expression by RNA interference results in an increased sensitivity of HPV-positive cancer cells towards genotoxic agents. Immunohistochemical analyses of cervical tissue specimens reveal a highly significant increase of LEDGF protein levels in HPV-positive lesions compared to histologically normal cervical epithelium. Taken together, these results indicate that the E6/E7-dependent maintenance of intracellular LEDGF expression is critical for protecting HPV-positive cancer cells against various forms of cellular stress, including DNA damage. This could support tumor cell survival and contribute to the therapeutic resistance of cervical cancers towards genotoxic treatment strategies in the clinic.
Oncogenic types of human papillomaviruses (HPVs), such as HPV16 and HPV18, are major human carcinogens. They cause cervical carcinoma, the second most common cancer in females worldwide and are closely linked to the development of other malignancies, including a subset of additional anogenital (e.g. anal, vulvar and penile) and oropharyngeal (e.g. tonsillar) cancers . Two viral oncogenes, E6 and E7, are crucial for both the induction and the maintenance of the malignant phenotype of HPV-positive cervical cancer cells, indicating that cervical cancer cells display features of a phenomenon termed “oncogene addiction” . On the basis of many mechanistic studies, the picture emerges that the two HPV oncogenes inactivate crucial tumorsuppressive responses of the cell, such as induction of senescence or apoptosis –. Importantly, at least some of these pathways are not irreversibly deregulated by HPVs. Rather, inhibition of viral E6/E7 activities in HPV-positive cancer cells leads to the reactivation of dormant tumor suppressor pathways and can eventually result in efficient growth arrest, senescence, and/or cell death –.
In this study, we identify the cellular LEDGF gene as a novel target for oncogenic HPVs. We show that continuous E6/E7 oncogene expression is required to maintain intracellular LEDGF expression in HPV-positive cancer cells and that HPVs can transcriptionally stimulate the LEDGF gene via LEDGF promoter activation. Further, LEDGF expression is crucial for the resistance of HPV-positive cancer cells towards genotoxic stress. In line with the in vitro data demonstrating a positive correlation between HPV oncogene and LEDGF expression, we found that HPV-positive preneoplastic and neoplastic lesions exhibit significantly enhanced levels of LEDGF. We propose that stimulation of LEDGF expression by the viral E6/E7 oncogenes is a crucial mechanism to protect HPV-positive cancer cells towards different forms of cellular stress, including DNA damage.