A Key Issue In ROS-based Cancer Therapy:Dose-response Relationship Between Cellular Ros Level And Its Cytotoxicity

Targeting cancer via ROS-based mechanism has been proposed as a radical therapeutic approach. Cancer cells exhibit higher endogenous oxidative stress than normal cells and pharmacological ROS insults via either enhancing ROS production or inhibiting ROS-scavenging activity can selectively kill cancer cells. Dose-response relationship is a basic therapeutic principle. However, dose-response relationship between cellular ROS level and its cytotoxicity had never been reported until the study.In this study, we randomly chose4cancer cell lines and primary colon or rectal cancer cells from4patients to test the hypothesis and obtained following paradoxical results:while piperlongumin (PL) and b-phenylethyl isothiocyanate (PEITC),2well-defined ROS-based anticancer agents, induced an increase of cellular ROS and killed effectively the tested cells, lactic acidosis (LA), a common tumor environmental factor that plays multifaceted roles in promoting cancer progression, induced a much higher ROS level in the tested cancer cells than PL and PEITC, but spared them; L-buthionine sulfoximine (L-BSO,20mM) depleted cellular GSH more effectively and increased higher ROS level than PL or PEITC but permitted progressive growth of the tested cancer cells. Taken together, cell growth inhibition or cell death was not associated with the potency of agents to augment ROS, and ROS level was not inversely proportional to GSH level. In addition, antioxidants such as NAC and SOD-CAT mimetics was used to verify if cell death is associated with ROS. The results pose a paradox. Although4mM NAC could completely reverse PEITC-, Dox-or ATO-induced ROS to basal level, it did not block cell death. Interestingly, NAC at4 mM exhibited a weak inhibition on PL-induced ROS, but abolished the hazardous effect of PL. Although EUK8or EUK134could fully reverse PL-or PEITC-induced ROS, they did not rescue cell death. Finally, the results of primary human colon and rectal cancer cells are virtually the same as those of cancer cell lines:while the much stronger ROS inducer LA does not kill the tested primary cancer cells, the much weaker ROS inducer PEITC, PL, or ATO effectively killed them.No evident dose-response relationship between cellular ROS level and cytotoxicity was observed in this study. If ROS is the effecter, it should obey the fundamental therapeutic principle-the dose-response relationship. This is a major concern.