Role of catalase in monocytic differentiation of U937 cells by TPA: hydrogen peroxide as a second messenger

Human promonocytic cell line U937 cells can be induced to differentiate into macrophages by treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA). TPA treatment induced the expression of the monocytic differentiation markers CD11b and CD36, with concomitant morphological changes. Moreover, TPA enhanced reactive oxygen species (ROS) generation in these cells, and phagocytic ability was also stimulated during differentiation. The antioxidant agent N-acetyl-L-cysteine inhibited the TPA-induced differentiation of U937 cells. TPA treatment decreased the expression level of catalase, which catalyzes the decomposition of hydrogen peroxide (H2O2) to H2O and O-2. In contrast, TPA increased the level of manganese superoxide dismutase, which catalyzes the dismutation of superoxide into H2O2 and O-2 without affecting the levels of copper-zinc superoxide dismutase or glutathione peroxidase 1, which removes H2O2 using glutathione as substrate. Treatment of U937 cells with catalase inhibited the enhancement of ROS generation induced by TPA, and blocked the TPA-induced differentiation of U937 cells. Human promyelocytic cell line HL60 cells were also induced to differentiate into macrophages by TPA. However, HP100-1 cells, its variant cell line over-expressing catalase, were resistant to TPA-induced differentiation. Our results suggest that catalase inhibits monocytic differentiation by TPA; the decrease in catalase level and the accumulation of H2O2 are significant events for monocyte/macrophage differentiation by TPA.