Cellular control of human papillomavirus gene activity in cervical cancer

Abstract

Specific types of high risk human papillomaviruses(HPV s), particularly the HPV type 16 and HPV 18 are known to cause cervical cancer in women. Constitutive expression of two early genes, E6 and E7 of these HPVs which are responsible for tumorigenic transformation, is dependent mainly on the availability of host-cell transcription factor AP-1 that plays a key role during development of cervical cancer. Different oneoproteins such as, c-jun, jun-B, jun-D, c-fos, fos-B, fra-1 and fra-2 can either homo- or heterodimerize to form a functional AP-1 which binds to cognate DNA sequences within the viral upstream regulatory region. In order to understand the involvement of redox regulatory pathways in the expression of HPVs, HPV positive cells were treated with a potent antioxidant, pyrrolidine dithiocarbamate (PDTC). It selectively suppressed the viral gene expression at the level of initiation of transcription but induced an enhanced binding of AP-l. Molecular dissection of AP-1 complex in electrophoretic mobility supershift assays (EMSAs) using antibodies raised against AP-1 family members showed that the normal composition of AP-l consisting of c-jun homodimer was altered. c-jun became phosphorylated and heterodimerized with Fra-1 instead of its canonical dimerization partner c-fos. To address the question whether alteration in AP-1 composition could be responsible for a negative regulatory pathway controlling HPV transcription, different cell lines, such as HeLa-fibroblast non-tumerigenic hybrid cells 444, their highly tumorigenic segregants CGL3 and parental HeLa cells, all positive for HPV 18, were used as experimental models. AP-1 composition was found to differ considerably between these cell lines: c-fos was found to express in high quantity in tumorigenic cells HeLa and CGL3 while it is completely absent in non-tumerigenic 444 cells indicating crucial role of c-fos in tumorigenic transformation. An enhanced ectopic expression of c-fos gene in nontumorigenic 444 cells caused a change in the Jun/Fra-1 heterodimerization towards jun-fos which is detected in CGL3 and HeLa cells and the cells became tumorigenic. These results unravel a novel role of AP-1 transcription factor as an indispensable component controlling transcription of pathogenic HPV through alteration of AP-l composition. Furthermore, the antioxidant-induced selective suppression of HPV expression provides a clue for a possible development of a novel therapeutic approach to control HPVs by antioxidative drugs.