Lopinavir Up-Regulates Expression of the Antiviral Protein Ribonuclease L in Human Papillomavirus-Positive Cervical Carcinoma Cells

Background We have previously shown that the HIV protease inhibitor lopinavir has selective toxicity against human papillomavirus (HPV)-positive cervical carcinoma cells via an unknown mechanism. Methods SiHa cervical carcinoma cells were stably transfected with the proteasome sensor vector pZsPro-Sensor-1 to confirm lopinavir inhibits the proteasome in these cells. The Panorama Xpress profiler 725 antibody array was then used to analyse specific changes in protein expression in lopinavir-treated versus control untreated SiHa cells followed by PCR and western blotting. Colorimetric growth assays of lopinavir-treated E6/E7 immortalised versus control human keratino-cytes were performed. Targeted small interfering RNA gene silencing followed by growth assay comparison of lopinavir- treated/untreated SiHa cells was also used. Results Lopinavir induced an increase in the fluorescence of pZsProSensor-1 transfected SiHa cells, indicative of proteasomal inhibition. Ribonuclease L (RNASEL) protein was shown to be up-regulated in lopinavir-treated SiHa cells, which was confirmed by PCR and western blot. Targeted silencing of RNASEL reduced the sensitivity of SiHa cells to lopinavir. Selective toxicity against E6/E7 immortalised keratinocytes versus control cells was also seen with lopinavir and was associated with up-regulated RNASEL expression. Conclusions These data are consistent with the toxicity of lopinavir against HPV-positive cervical carcinoma cells being related to its ability to block viral proteasome activation and induce an up-regulation of the antiviral protein RNASEL. This is supported by the drug's selective toxicity and up-regulation of RNASEL in E6/E7 immortalised keratinocytes combined with the increased resistance to lopinavir observed in SiHa cells following silencing of RNASEL gene expression.