Cytoprotective effect of ataxia telangiectasia mutated(ATM) against oxidative stress is mediated by heme oxygenase-1

Abstract

Ataxia telangiectasia (AT) is caused by the mutational inactivation of ataxia telangiectasia mutated (ATM) gene which is involved in DNA repair process. A part of the clinical phenotype of AT is associated with oxidative stress. Compelling evidence shows the increased oxidative stress in human AT cells and neuronal tissues of ATM-deficient mice. However, it is unclear whether ATM itself is directly involved in sensing the increase of reactive oxygen species (ROS) or whether ATM regulates the expression of genes involved in oxidative stress responses. Heme oxygenase-1 (HO-1), whose main function is the degradation of heme, is known to be an important antioxidant enzyme. The role of HO-1 is cytoprotection against oxidant injury. Therefore, it is hypothesized that ATM may induce antioxidant enzyme such as HO-1 against oxidative stress. In the present study, A-T fibroblasts stably transfected with human full-length ATM cDNA (Atm+/+ cells) or empty vector (Atm-/- cells) were used. As a source of reactive oxygen species, hydrogen peroxide was treated to the cells. The cytoprotective effect of ATM against oxidative stress was investigated by determining the activation of nuclear factor-κB (NF-κB) and protein kinase C (PKC δ) as well as HO-1 induction. As a result,transfection of ATM inhibited ROS-induced cell injuries including apoptosis through the induction of HO-1 in Atm-/- cells. Transfection of ATM induces the expression of HO-1 which was mediated by PKC δ and NF-κB in H2O2–treated Atm-/- cells. ZnPP, an HO-1 inhibitor, and transfection of HO-1 siRNA increased ROS levels and apoptosis in H2O2-treated cells, whereas hemin, an HO-1 activator, decreased ROS levels and apoptosis. Transfection of a dominant-negative mutant I-κBa (MAD-3) decreased H2O2-induced HO-1 expression. Rottlerin, a PKC δ inhibitor, inhibited NF-κB activation as well as HO-1 expression.

These results suggest that PKC δ is an upstream signaling for the activation of NF-κB, which induces HO-1 expression in H2O2-treated Atm+/+ cells. In conclusion, cytoprotective effect of ATM against oxidative stress is mediated by induction of antioxidant enzyme HO-1 via activation of PKC δ and NF-κB. The results may suggest the possible mechanisms how AT patients are vulnerable to oxidative stress.