Doxorubicin (DOX) is a leading anthracycline drug with exceptional efficacy; however, little is known about the molecular mechanisms of its side effects, which include heart muscle damage, noncancerous cell death, and drug resistance. A total of 17,950 human proteins expressed in HEK293 cells were screened and yielded 14 hits. Competitive and binding experiments further verified the binding of DOX to UMP/CMP kinase 1 (CMPK1), and microscale thermophoresis showed that DOX binds to CMPK1 with a K of 1216 nM. In addition, we observed that the binding of DOX to CMPK1 activated the phosphorylation of CMP, dCMP, and UMP. A significant activation was observed at the concentration of 30 µM DOX and reached plateau at the concentration of DOX 30 µM, 150 µM, and 100 µM, respectively. DOX would add up stimulation of CMPK1 by DTT and overcome inhibition of CMPK1 by NaF, EDTA. In summary, we showed that DOX might bind to the nonactive site of CMPK1 and regulate its activity with magnesium.

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Background: Gemcitabine is a deoxycytidine analog, which is used as first-line agent for pancreatic cancer therapy, and its efficacy relied on its intracellular conversion to active triphosphate form. However, administration with gemcitabine still has limited effect on the overall survival of patients with pancreatic cancer. Objective: We aimed to study the combination effect of gemcitabine and doxorubicin to pancreatic cancer cells BxPC3 and PANC1, and unveil the mechanism. Methods: The study was performed in pancreatic cancer cells PANC1 and BxPC3, the contribution of UMP/CMP kinase 1 (CMPK1) to gemcitabine in PANC1 and BxPC3 cells was measured by transfection of CMPK1 plasmid or CMPK1 siRNA treatment to adjust the expression of CMPK1 in the cells; then analyzed the cell vitality and migration after treated with 1% IC 50 of doxorubicin and gemcitabine or only with gemcitabine; the activity of CMPK1 and the effect of doxorubicin to the reaction was measured by HPLC assay in vitro; at last, docking analysis by computer was used to calculate the possible interaction sites of CMPK1 to DOX. Results: The sensitivity of PANC1 and BxPC3 cells to gemcitabine was improved when increasing the expression of CMPK1, and decreased when knockout CMPK1 by CMPK1 siRNA in BxPC3 cells; when combined with doxorubicin, the sensitivity of PANC1 and BxPC3 cells to gemcitabine also increased, and the cells migration reduced; we further found out that by adding 10 μM doxorubicin, the catalyzing activity of CMPK1 elevated about 2 times in vitro; the docking result showed that the association of CMPK1 to DOX was mainly by hydrogen bond and ionic interaction. Conclusion: CMPK1 can catalyze gemcitabine to its active form within the cells so that the sensitivity of the cells to gemcitabine elevated, and doxorubicin may enhance the cytotoxic effect to pancreatic cancer by up-regulate the activity of CMPK1, the combination of these deoxycytidine analogs with DOX might exert better efficacy.