The anticancer drug Camptothecin (CPT) specifically targets topoisomerase I (topoI). While this class of drug is used to treat various solid tumors, CPT is only effective in 13-30 percent of the patient population. Although the mechanism of the CPT resistance pathway is not fully understood, we found cancer cells degrade topoisomerase I (topoI) in response to CPT. The observed relationship between higher basal DNA-PKcs mediated phosphorylation of topoI, rapid degradation of topoI, and resistance to CPT suggested that DNA-PKcs is a critical regulator of the phosphorylation status of topoI and the rate of topoI degradation in response to CPT. The data shows CTDSP1 (a dual phosphatase) acts as a negative regulator of DNA-PKcs. Because CTDSP1 functionality plays a role in maintaining higher basal phosphorylation levels of topoI, CTDSP1 impairment contributes to greater CPT resistance. This renders the CPT chemotherapy treatment ineffective. We hypothesized inducing the catalytically inactive form of CTDPS1 via both knockdown and inhibition experiments would increase cancer cell resistance to CPT as a result of an overactive topoI degradation pathway. The function of CTDSP1 was impaired in the two colon cancer cell lines, HCT15 and HCT116, by silencing with siRNA and using Rabeprazole (a pharmacological agent known to inhibit CTDSP1). Inhibition of CTDSP1 phosphatase activity resulted in greater phosphorylation of DNA-PKcs and increased the rate of topoI degradation in the cells in response to CPT. Cellular resistance was also more notable in the sensitive HCT116 cell lines. HCT15 cells degrade topoI rapidly and are resistant to CPT. Thus, the effect of CPT is not as pronounced in this cell line. CTDSP1 knock down cells showed the greatest DNA-PKcs phosphorylation and topoI degradation when treated with CPT. In addition, the present study includes a clonogenic assay that shows a larger cell survival fraction in Rabeprazole treated HCT116 cells in comparison to controls after exposure to CPT. A higher survival fraction after CPT exposure is reflective of greater CPT resistance. This suggests that cell viability is enhanced during CPT chemotherapy treatment in CTDSP1 null colorectal cancer cells. Lastly, An EGFP read out experiment of topoI tagged to EGFP demonstrated CTDSP1 inhibition results in reduced topoI levels in colorectal cancer cells. The present study showed the potential link between lower topoI levels and greater resistance to CPT by showing that both effects are outcomes of silencing CTDSP1. / 2018-07-11T00:00:00Z