Tetra-arsenic tetra-sulfide (As₄S₄) promotes apoptosis in retinoid acid -resistant human acute promyelocytic leukemic NB4-R1 cells through downregulation of SET protein

Tetra-arsenic tetra-sulfide (As₄S₄) is an arsenic compound with antitumor activity, especially in acute promyelocytic leukemia (APL) that are resistant to retinoic acid (RA). Although recent studies have revealed that the therapeutic action of As₄S₄ is closely associated with the induction of cellular apoptosis, the exact molecular mechanism underlying this action in RA-resistant APL remains to be clarified. In this study, we found that As₄S₄-induced apoptosis was accompanied by reduced mRNA and protein expression of SET gene in RA-resistant NB4-R1 cells. Moreover, RNAi knockdown of SET gene further promoted As₄S₄-induced apoptosis, while SET overexpression recovered the cell viability, suggesting that As₄S₄ induces apoptosis through the reduction of SET protein in NB4-R1 cells. We also observed that the knockdown of SET gene resulted in the upregulation of protein phosphatase 2 (PP2A) expression and the downregulation of promyelocytic leukemia and retinoic acid receptor α fusion gene (PML-RARα) expression, which were enhanced by As₄S₄ treatments. By contrast, overexpression of SET gene resulted in PP2A downregulation and PML-RARα upregulation, which were abolished by As₄S₄ pretreatment. Since PP2A is a proapoptotic factor and PML-RARα is an antiapoptotic factor, our results suggest that As₄S₄-induced apoptosis in RA-resistant NB4-R1 cells is through the downregulation of SET protein expression, which, in turn, increases PP2A and reduces PML-RARα expressions to lead to cell apoptosis.