HUMAN PAPILLOMAVIRUS E6 AND E7 ONCOPROTEINS ALTER CELL-CYCLE PROGRESSION BUT NOT RADIOSENSITIVITY OF CARCINOMA-CELLS TREATED WITH LOW-DOSE-RATE RADIATION

Purpose: Low-dose-rate radiation therapy has been widely used in the t reatment of urogenital malignancies, When continuously exposed to low- dose-rate ionizing radiation, target cancer cells typically exhibit ab normalities in replicative cell-cycle progression, Cancer cells that a rrest in the G2 phase of the cell cycle when irradiated may become exq uisitely sensitive to killing by further low-dose-rate radiation treat ment, Oncogenic human papillomaviruses (HPVs), which play a major role in the pathogenesis of uterine cervix cancers and other urogenital ca ncers, encode E6 and E7 transforming proteins known to abrogate a p53- dependent G1 cell-cycle checkpoint activated by conventional acute-dos e radiation exposure, This study examined whether expression of BPV E6 and E7 oncoproteins by cancer cells alters the cell-cycle redistribut ion patterns accompanying low-dose-rate radiation treatment, and wheth er such alterations in cell-cycle redistribution affect cancer cell ki lling. Methods and Materials: RKO carcinoma cells, which contain wild- type P53 alleles, and RKO cell sublines genetically engineered to expr ess HPV E6 and E7 oncoproteins, were treated with low-dose-rate (0.25- Gy/h) radiation and then assessed for p53 and p21WAF1/CIP1 polypeptide induction by immunoblot analysis, for cell-cycle redistribution by fl ow cytometry, and for cytotoxicity by clonogenic survival assay. Resul ts: Low-dose-rate radiation of RKO carcinoma cells triggered p53 polyp eptide elevations, p21WAF1/CIP1 induction, and arrest in the G1 and G2 phases of the cell cycle, In contrast, RKO cells expressing E6 and E7 transforming proteins from high-risk oncogenic HPVs (HPV 16) arrested in G2, but failed to arrest in G1, when treated with low-dose-rate io nizing radiation, Abrogation of the G1 cell-cycle checkpoint activated by low-dose-rate radiation exposure appeared to be a characteristic f eature of transforming proteins from high-risk oncogenic HPVs: RKO cel ls expressing E6 from a low-risk nononcogenic HPV (HPV 11) exposed to low-dose-rate radiation arrested in both G1 and G2, Surprisingly, desp ite differences in cell-cycle redistribution accompanying low-dose-rat e radiation treatment associated with high-risk HPV transforming prote in expression, no consistent differences in clonogenic survival follow ing low-dose-rate radiation treatment were found for RKO cell sublines expressing high-risk HPV oncoproteins and arresting only in G2 during low-dose-rate radiation exposure vs, RKO cell sublines exhibiting bot h G1 and G2 cell-cycle arrest when irradiated. Conclusion: The results of this study demonstrate that neither HPV oncoprotein expression nor loss of the radiation-activated G1 cell-cycle checkpoint alter the se nsitivity of RKO carcinoma cell lines to low-dose-rate radiation expos ure in vitro, Perhaps for urogenital malignanices associated with onco genic HPVs in vivo, HPV oncoprotein-mediated abrogation of the G1 cell -cycle checkpoint may not limit the potential efficacy of low-dose-rat e radiation therapy. Copyright (C) 1997 Elsevier Science Inc.