Reactive oxygen species H2O2 and •OH, but not O2•- promote oridonin-induced phagocytosis of apoptotic cells by human histocytic lymphoma U937 cells

We reported previously that phagocytosis of apoptotic cells by U937 cells was enhanced by the treatment with oridonin that showed high activity to induce the generation of reactive oxygen species (ROS) in many cells. ROS, important signaling molecules, are involved in the immune defenses, cell repair and proliferation. In this study, oridonin caused modest amount of ROS generation in U937 cells, with hydrogen peroxide (H2O2) and hydroxyl free radical (center dot OH) as the major types. Meanwhile, H2O2 and center dot OH were positive regulators involved in oridonin-enhanced engulfment of apoptotic cells through down-regulating mitochondrial membrane potential (MMP) and inducing autophagy. The ROS-mediated phagocytosis was independent of cellular adenosine triphosphate (ATP) levels. H2O2 and center dot OH generation also activated phosphatidylinositol 3-kinases-Akt (PI3K-Akt) and phospholipase C gamma-protein kinase C(PLC gamma)-Ras-Raf-ERK signaling pathways, which were essential for oridonin-induced engulfment of apoptotic cells. Phagocytosis, the loss of MMP, autophagy and the activated signaling pathways were all suppressed by ROS scavenger N-acetyl-cysteine (NAC), H2O2 scavenger catalase or center dot OH scavenger glutathione (GSH). However, superoxide anion (O-2 center dot-) and its scavenger superoxide dismutase (SOD) did not significantly affect these oridonin-induced biological processes. (C) 2013 Elsevier B.V. All rights reserved.