Role of polycyclic aromatic hydrocarbons as a co-factor in human papillomavirus-mediated carcinogenesis

Human papillomavirus (HPV) is an etiological agent of cervical cancer. Yet co-factors are believed to be involved in HPV-mediated carcinogenesis. Polycyclic aromatic hydrocarbons (PAHs) are considered as one of these co-factors. Epidemiologic studies have associated high PAH exposure with increased risk for cancer development. To date, many studies focus on benzo[a]pyrene, however, the role of other PAHs should not be neglected. This study aimed to compare the potential of different PAHs as a co-factor in HPV-mediated carcinogenesis, and to investigate the possible mechanisms involved.

The effect of 17 PAHs on high-risk HPV (HPV16) were examined in this study. HPV16 E7 oncogene was expressed in primary cells extracted from baby rat kidney and treated with PAHs. The co-transforming ability of PAHs were measured by colony formation index according to the number and size of transformed colonies. Effects of PAHs on proliferation of HPV-null (C33A) and -infected (CaSki) were examined using CCK-8 assay. Wound healing assay and matrigel invasion chambers were used to investigate effects of PAHs on cell motility and invasivion of HPV-null (MCF7, C33A) and -infected (SiHa) cells.

Benzo[a]pyrene (BaP), dibenz[a,h]anthracene (DBA) and indeno[1,2,3-cd]pyrene (IDP) showed the greatest co-transforming potential in the baby rat kidney cell system. Short-term exposure to BaP, DBA, IDP and pyrene (PR) did not affect proliferation of C33A or CaSki cells, however, long-term exposure of these four PAHs led to dramatic increase in growth rate of CaSki cells by 120-140%. Besides, exposure of PAHs has an effect on cell motility and invasiveness of C33A and SiHa cells, but not for MCF7 cells. Exposure of BaP and DBA enhanced migration (1.26 to 1.40-fold) and invasion (1.68 to 1.94-fold) capacity of C33A cells. Intriguingly, exposure of all four types of PAHs boosted the migration (1.12 to 1.28-fold) and invasion (1.26 to 1.40-fold) capacity of SiHa cells.

Our results indicate that exposure to PAHs can be a key co-factor in HPV-related cancer development. They could act on all three stages, namely initiation, promotion and progression. Further study is needed to unveil the mechanisms by which PAHs interact with HPV to cause malignancy.