Pancreatic β-cells are susceptible to reactive oxygen species (ROS), which are known to be generated by high or low glucose (LG), hypoxic, or cytokine-producing conditions. When we cultured mouse β-cell-derived MIN6 cells in a LG condition, we detected a significant generation of ROS, including hydrogen peroxide, which was comparable to the ROS production in hypoxic or cytokine-treated conditions. ROS accumulation induced by the LG culture led to cell death, which was prevented by the ROS scavengers N-acetylcysteine and manganese(III)tetrakis(4-benzoic acid) porphyrin. We next investigated the mechanism of stress-activated protein kinases (SAPKs), c-jun N-terminal kinase (JNK) and p38, in ROS-induced MIN6 cell death. Activation of p38 occurred immediately after the LG culture, whereas JNK activation increased slowly 8 h later. Adenoviral p38 expression decreased MIN6 cell death, whereas the JNK expression increased it. Consistently, blocking p38 activation by inhibitors increased β-cell death, whereas JNK inhibitors decreased it. We then examined the role of MAPK phosphatases (MKPs) specific for stress-activated protein kinases in β-cell death. We found that MKP-1 presented an increase in its oxidized product after the LG culture. ROS scavengers prevented the appearance of this oxidized product and JNK activation. Thus, ROS-induced MKP inactivation causes sustained activation of JNK, which contributes to β-cell death. Adenoviral overexpression of MKP-1 and MKP-7 prevented the phosphorylation of JNK at 36 h after the LG culture, and decreased MIN6 β-cell death. We suggest that β-cell death is regulated by interactions between JNK and its specific MKPs.
Reactive oxygen species (ROS) and response of antioxidants as ROS-scavengers during environmental stress in plants