Abstract

Arsenite treatment has been found to induce clinical remission in patients with acute promyelocytic leukemia. Although the potential therapeutic value of arsenite may lie in triggering apoptosis, it has not been established that cytotoxicity is the sole mechanism of action. We have used a myelomonocytic leukemia cell line (U937) to characterize the concentration-dependent effects of arsenite on cell growth, viability, apoptosis, and differentiation. Arsenite has multiple effects on U937 cells. Low concentrations of arsenite (i.e., ≤1 μM) potentiate vitamin-D₃-induced differentiation. Two markers of monocyte differentiation, Mac-1 expression and nitroblue tetrazolium reduction, are increased in arsenite-exposed, D₃-costimulated cells. Concentrations of arsenite >10 μM rapidly induce the death of cells irrespective of cell cycle phase. Intermediate concentrations of arsenite (i.e., 5 to 10 μM) are cytostatic initially. Cell cycle analysis using elutriated, synchronous cell populations revealed that intermediate concentrations of arsenite delay both G₁ and G₂transit. G₂ cells appear to be most sensitive to arsenite, in that transit through G₂/M is more delayed than transit through G₁, and apoptosis is induced in these cells as they emerge from an aberrant G₂/M. Arsenite-induced apoptosis was caspase-3 dependent. Arsenite-mediated cytotoxicity was reduced in the presence of the broad caspase inhibitor Z-Val-Ala-dl-Asp-fluoromethylketone; however, caspase inhibition did not reverse arsenite-induced cytostasis. Thus, arsenite has multiple effects on U937 cells that are dependent on concentration and cell cycle phase. Specifically, cell cycle transit and differentiation are more sensitive to arsenite than is the induction of apoptosis.