[Cancer cell-specific functional relation between Na⁺,K⁺-ATPase and volume-regulated anion channel]

Digitoxin and digoxin are plant-derived cardiac glycosides. They are Na⁺,K⁺-ATPase (sodium pump) inhibitors, and have been used clinically for treatment and prevention of heart failure and various tachycardia. On the other hand, some epidemiological studies showed that digoxin users have a lower cancer risk compared to the non-users, and that cancer patients who had been treated with digoxin face on improvement of their survival. In various in vitro studies, cardiac glycosides at sub-μM concentrations, which have no significant effect on enzymatic and ion-transporting activities of Na⁺,K⁺-ATPase, show anti-cancer effects. Na⁺,K⁺-ATPase is ubiquitously expressed, so it remains unclear why low concentrations of cardiac glycosides have cancer-specific effects. Recently, we found that the receptor-type Na⁺,K⁺-ATPase, which has no pumping activity, is associated with leucine-rich repeat-containing 8 family, member A(LRRC8A), one of the components of volume-regulated anion channel (VRAC), in the membrane microdomains of plasma membrane of cancer cells, and that this crosstalk contributes to the inhibition of the cancer cell growth by sub-μM cardiac glycosides. In this mechanism, cardiac glycosides bind to the receptor-type Na⁺,K⁺-ATPase, and then stimulate the production of reactive oxygen species (ROS) via NADPH oxidase. The ROS activate VRAC within the membrane microdomains, thus eliciting anti-proliferative effects. VRAC is ubiquitously expressed, and it is normally activated by cell swelling. However, VRAC is activated by cardiac glycoside without cell swelling. On the other hand, the cardiac glycosides-induced effects were not observed in non-cancer cells. Our findings can partly explain why cardiac glycosides elicit selective effects in cancer cells.