Paclitaxel combined with siRNA targeting HPV16 oncogenes improves cytotoxicity for cervical carcinoma

Cervical cancer is attributable to continuous expression of the E6 and E7 oncoproteins of the high-risk human papillomaviruses. These proteins target p53 and members of the retinoblastoma cellular regulatory protein family respectively for degradation, disrupting cellular control over apoptosis, senescence and the cell cycle. Delivery of short interfering RNAs (siRNAs) targeting mRNA from the HPV16 E6/E7 open reading frame to HPV16-positive cell lines, led to an 80% reduction in full-length transcripts and 60% reduction in total (full-length and spliced) transcripts. Downregulation of E6 mRNA led to increased levels of p53 detectable by western blot and resulted in an eightfold increase in luciferase expression from a p53-responsive reporter plasmid. Downregulation of E7 mRNA reduced the levels of E7 protein and increased the levels of hypophosphorylated pRb. Cellular proliferation was reduced after siRNA delivery; the effect being greater in SiHa cells than CaSki cells and when full-length transcripts encoding E6 and spliced transcripts encoding E7 were both targeted. There was no loss of proliferation in human papillomavirus-negative cell lines. Elevation of p53 in the absence of changes to Rb led to 35% of CaSki cells undergoing apoptosis, whereas in SiHa cells restoration of both p53 and hypophosphorylated Rb had the greatest effect with 25% of cells undergoing apoptosis. The combined use of oncogene-targeting siRNA with either carboplatin, irinotecan, leptomycin B or doxorubicin led to additive toxicity, whereas, with cisplatin, led to sub-additive toxicity. In contrast, siRNA combined with paclitaxel treatment resulted in synergistic toxicity, with intronic siRNA (which mainly targets E6) more effective than exonic siRNA (which targets both E6 and E7). The growth of SiHa xenograft tumors was reduced using paclitaxel combined with intronic and exonic siRNA, compared with exonic siRNA alone, confirming the synergistic relationship between p53 restoration and paclitaxel.