Synergism between arsenite and proteasome inhibitor MG132 over cell death in myeloid leukaemic cells U937 and the induction of low levels of intracellular superoxide anion

Abstract

Increased oxygen species production has often been cited as a mechanism determining synergismon cell deathand growth inhibition effects of arsenic-combined drugs. However the net effect of drug combination may not beeasily anticipated solely from available knowledge of drug-induced death mechanisms. We evaluated the combinedeffect of sodium arsenite with the proteasome inhibitor MG132, and the anti-leukaemic agent CAPE, ongrowth-inhibition and cell death effect in acute myeloid leukaemic cells U937 and Burkitt´s lymphomaderivedRaji cells, by the Chou?Talalay method. In addition we explored the association of cytotoxic effect ofdrugs with changes in intracellular superoxide anion (O2−) levels. Our results showed that combined arsenite+MG132 produced low levels of O2− at 6 h and 24 h after exposure and were synergic on cell death induction inU937 cells over thewhole dose range, although the combinationwas antagonistic on growth inhibition effect. Exposureto a constant non-cytotoxic dose of 80 μM hydrogen peroxide together with arsenite+MG132 changedsynergism on cell death to antagonism at all effect levels while increasing O2− levels. Arsenite+hydrogen peroxidealso resulted in antagonism with increased O2− levels in U937 cells. In Raji cells, arsenite+MG132 also producedlow levels of O2− at 6 h and 24 h but resulted in antagonism on cell death and growth inhibition. Bycontrast, the combination arsenite+CAPE showed high levels of O2− production at 6 h and 24 h post exposurebut resulted in antagonism over cell death and growth inhibition effects in U937 and Raji cells. We concludethat synergism between arsenite and MG132 in U937 cells is negatively associated to O2− levels at early timepoints after exposure.