The laboratory is investigating the role of the human papillomavirus (HPV) E6 and E7 proteins in cervical cancer, with particular attention to their ability to inhibit cellular senescence. We showed that expression of the bovine papillomavirus E2 transcription factor represses expression of the E6 and E7 genes from the integrated HPV genomes in human cervical cancer cells. This causes transient activation of the endogenous p53 and retinoblastoma tumor suppressor pathways and dramatic growth inhibition (Figure E6/7.1).

Figure E6/7.1 Repression of HPV18 E6/E7 transcription and activation of p53 by expression of E2 transcriptional repressor. Top panel shows HPV 18 E6/E7 northern blot, bottom panels show p53 western blot, at various times after E2 expression or mock treatment.

These results demonstrated that continued expression of the HPV oncogenes is required to maintain the proliferative state of these cancer cells and imply that manipulations that inhibit the activity of the viral oncoproteins and activate cellular tumor suppressor pathways may be an approach to treat cancer. The growth-arrested cells do not die but rather enter an irreversible non-replicating state that resembles replicative senescence, which normally blocks of the unlimited proliferation of cells (Figure E6/7.2).

Figure E6/7.2 Induction of senescence by repression of E6 plus E7 (top panels) or by repression of E7 alone (bottom panels). Senescent cells flatten and stain blue for senescence-associated ß galactosidase activity.

Further experiments showed that repression of the E7 protein is sufficient to induce a robust senescence response that is dependent on the retinoblastoma pathway. We are attempting to identify genes and chemicals that interfere with the action of the HPV oncogenes or the cellular pathways that lead to senescence, and we are conducting experiments to determine the basis for the changes in cellular gene expression that occur in response to E7 repression. We are also interested in exploiting the viral etiology of some human cancers to develop novel methods to treat these tumors. Toward this end, we have developed restriction enzymes that specifically cleave HPV DNA in cancer cells, as well as an HPV16 E7 vaccine that can eliminate cancer cells in animal models.