Menadione Induced Reset of Circadian Superoxide Rhythms in Human Colon Cancer Cells

The effectiveness of some anticancer therapeutics have been reported to be dependent on the circadian timing of their administration. But this strategy is not widely used probably because the underlying rhythms and their alterations by external zeitgebers are not fully understood. Rhythms in the pseudo-steady-state (PSS) levels of particular intracellular reactive species (RS), the crucial effectors of cytotoxicity induced by many anticancer drugs in cancer cells have not been reported so far. In this work, we have shown that superoxide radicals have a basal circadian rhythm in the colon cancer cell line HCT116, which was reset by superoxide generating anti-cancer drug menadione. We have found a p53 dependent reset of superoxide rhythm in menadione treated HCT116 wt human colon cancer cells. The reset in period of oscillation was found to have a linear correlation with the drug concentration, given by the equation, T (in hours) = 22.57-0.96[menadione concentration in µM] and was absent in the p53-/- variant of the cell line. The 2 fold higher sensitivity of the wt cells to menadione with an IC 50 value of 15 µM as compared to 30 µM for the p53-/- variant suggests a significant role for the reset of superoxide rhythms in sensitizing the cells to menadione induced cytotoxicity. Further, to understand the mode of action of the drug menadione, we propose a reaction scheme using a superoxide – ERK – p53 – MnSOD loop for the rhythm reset and construct a representative mathematical model. The model generated results were found to be consistent with experimental observations. Further, the model predicts the changes in the concentration profiles of various species in the system, which are difficult to observe experimentally in real time.