Human papillomavirus type 16 E7 protein sensitizes cervical keratinocytes to apoptosis and release of interleukin-1α

Abstract

Interleukin-1α (IL-1α) is a multifunctional cytokine that promotes inflammation, tissue remodeling and epithelial hyperplasia. Keratinocytes produce and sequester large amounts of biologically active IL-1α which can be released after injury or infection. We show that high level expression of human papillomavirus (HPV) type 16 E6 and E7 oncoproteins enhanced release of IL-1α from cultures of normal cervical keratinocytes (relative effectiveness E7>E6/E7≫E6> control). The amount of IL-1α released was directly related to the ability of E7 or E6/E7 to stimulate apoptosis. E7 proteins that bound the retinoblastoma protein (Rb) strongly (HPV-16 and -18) induced more IL-1α release than those that bound poorly (HPV-6 and an HPV-16 E7 24gly mutant). Furthermore, overexpression of the E2F-1 transcription factor, a downstream target of Rb, induced extensive apoptosis and IL-1α release. Apoptosis and IL-1α release in response to growth factor removal occurred in part through a p53-independent pathway as coexpression of E6 and downregulation of p53 did not prevent either response. Immunohistochemical analyses showed that IL-1α was expressed by keratinocytes in normal cervical epithelia, low and high grade dysplasias, and cervical carcinomas. However, HPV-16 E6/E7 RNA expression and apoptosis increased in parallel in proliferating keratinocytes in severe dysplasias and carcinomas suggesting that IL-1α release is associated with progression to high grade disease. Thus, high level expression of the HPV-16 E7 protein sensitizes keratinocytes to apoptosis which results in release of IL-1α.