In vitro and in vivo growth suppression of HPV-related cancer cells by siRNA targeting E6 oncogene

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Human papillomavirus (HPV) types 16 and 18, causative agents of cervical cancers, encode the E6 and E7 oncogenes, whose simultaneous expression is pivotal for the malignant transformation and maintenance of malignant phenotypes. In the hope of developing a gene-specific therapy for HPV-related cancer, we examined the effects of E6 short-interfering RNA (siRNA) on the expression of these oncogenes and cell growth of HPV-related cervical cancer cells. Using SiHa cervical cancer cells, we demonstrated that HPV16 E6 siRNA decreased the levels of mRNA coding E6 as well as that coding E7 protein, and also induced nuclear accumulation of p53, the most important target of E6. E6 siRNA suppressed monolayer and anchorage-independent growth of SiHa cells, which was associated with p21^<CIP1/WAF1> induction and hypophosphorylation of retinoblastoma protein (Rb). Further, SiHa cells treated with E6 siRNA formed tumors in NOD/SCID mice that were significantly smaller than in those treated with control siRNA.we also found 2 types of siRNAs targeting HPV18 E6 which exerted a negative growth effect on HPV18-positive cervical cancer cells (HeLa and SW756), in part, inducing cell death. One siRNA (Ex-18E6) knocked down both E6 and E7 expression. The other (Sp-18E6) suppressed E6 to a similar level as Ex-18E6, however, less efficiently inhibited E7 as compared to Ex-18E6. Although both siRNAs induced cell death, Sp-18E6 siRNA induced more prominent cell death than Ex-18E6, suggesting that selective knock-down of E6 might be superior to simultaneous knock-down of E6 and E7 for the siRNA therapy of cervical cancer.Our results show HPV E6 siRNA as a candidate for gene-specific therapy for HPV-related cervical cancer.