Persistent infection with oncogenic human papillomaviruses (HPVs) is the most important factor in the induction of
uterine cervical cancer, a leading cause of
cancer mortality in women worldwide. Upon cell transformation, continual expression of the viral oncogenes is required to maintain the transformed phenotype. The viral E6
protein forms a ternary complex with the cellular E6-AP
protein and p53
protein which promotes the rapid degradation of p53. Recent studies have revealed that
lignans from the creosote bush (3'-O-methyl-nordihydroguaiaretic acid) can repress the viral promoter responsible for E6 gene expression. Work reported here shows that the
lignan can subvert viral oncogene function resulting in stabilized p53
protein within treated HPV-containing
tumor cells. The stabilized p53 is transcriptionally active as demonstrated by a
luciferase reporter vector and induction of genes for Bax and PUMA
proteins. Apoptosis is detected by
annexin V binding to treated cells as analyzed by flow cytometry. Programmed cell death is confirmed by the induction of active
caspases and TUNEL assay. Initiator
caspase-9 is activated first, followed later by the effector
caspase-3 enzyme. The stabilization and induced apoptosis are not observed within treated HPV-negative cervical
tumor cells. Quantitative real time RT-PCR analysis of endogenous E6 gene transcription from the integrated HPV 16 promoter shows at least a fivefold repression of expression as compared to untreated cells. These results indicate that the loss of E6
protein in treated cells could be, in part, responsible for the stabilization of p53 within the
lignan treated cells.