Notably, the DNA viruses strongly up-regulate glycolysis including kinases such as pyruvate kinase. It can be hypothesized that phosphorylation of CDV and other ANPs might be selectively activated in this productive or transformed environment compared to more quiescent normal cells. Accordingly, to explain the selectivity of CDV for HPV-positive cells,

Johnson and Gangemi (1999) claimed that CDV could be mTOR inhibitor differentially metabolized in HPV-positive cells and normal keratinocytes. Following 8 and 16 h incubation, CDV was found to predominantly accumulate in the form of CDVp-choline (considered the intracellular depot form of CDV) in human primary keratinocytes (PHKs) while in HPV16-transformed keratinocytes, CDVpp was the most abundant anabolic product with little CDVp-choline having formed. Recently, we reported that following 72 h incubation with CDV, CDVp-choline appeared to be the most abundant metabolite while the monophosphate form was the least abundant one in PHKs as well as in HPV-positive and HPV-negative tumor cells (De Schutter et al., 2013c). Importantly, no significant differences in the levels of the active metabolite CDVpp, CDVp-choline or CDV were observed between PHKs and HPV-positive tumor cells. However, lower CDVp levels were measured in PHKs compared to HPV-positive cells

following 72 h incubation. Notably, lower concentrations of CDV and of all metabolites were observed in the spontaneously transformed keratinocyte cell line HaCaT that lack HPV sequences, compared to either HPV-positive cells or PHKs, suggesting that

HaCaT cells have a different uptake and/or Tau-protein kinase efflux of CDV, rather than differences in drug metabolism. To reveal Tyrosine Kinase Inhibitor high throughput screening the molecular mechanisms underlying the selectivity of CDV for tumor cells, in particular for HPV-positive carcinoma cells, our research team evaluated gene expression changes following CDV treatment of different cell types [including two HPV-positive cervical carcinoma cell lines (SiHa, HPV16+ and HeLa HPV18+), an HPV-immortalized keratinocyte cell line (HaCaT), and PHKs (De Schutter et al., 2013c). In addition, drug incorporation into genomic DNA was analysed in the four cell types. An exhaustive and thorough analysis of the microarray data highlighted distinct responses to CDV exposure in PHKs compared to HPV-positive cervical carcinoma cells, on the one hand, and to HPV-immortalized keratinocytes, on the other hand. Our data indicated that the selectivity of CDV for HPV-transformed cells is based on differences in response to DNA damage, replication rate and CDV incorporation into cellular DNA between immortalized cells and normal cells, rather than on a specific effect of CDV on expression of the viral oncoproteins (De Schutter et al., 2013c). Normal cells possess an arsenal of repair pathways and cell cycle checkpoints to detect and repair DNA damage unlike transformed cells that have a significantly reduced set of DNA repair pathways for survival (Fig. 12A).