Objectives: The regulation system for oxidative stress in systemic sclerosis (SSc) remains unclear. This study aimed to clarify the possible involvement of ataxia telangiectasia mutated (ATM), which plays a key role in DNA repair and redox balance, in the pathogenesis of SSc. Methods: Thirty patients with SSc and 15 healthy controls were enrolled. Expression of ATM and phosphorylated ATM (pATM), an activated form of ATM, in phagocytes in whole blood samples was analysed by FACS. Correlations between expression levels of ATM/pATM and clinical parameters of SSc patients were statistically analysed. Peripheral monocytes were cultured with an ATM-specific inhibitor (KU55933), and reactive oxygen species production in the cells was measured. Results: Expression level of pATM in peripheral monocytes and neutrophils from SSc patients was significantly lower than those in healthy controls (P = 0.04 and P < 0.001, respectively), while no significant difference in total ATM expression was observed between SSc and healthy controls. In addition, pATM expression in monocytes of SSc patients with interstitial lung disease or digital pitting scar was remarkably lower than in the patients without these clinical features (P = 0.02 and P = 0.03), respectively. Moreover, pATM expression in monocytes positively correlated with forced vital capacity and negatively correlated with the serum Krebs von den Lungen-6 level. Notably, KU55933, an ATM-specific inhibitor, enhanced reactive oxygen species production by monocytes under oxidative stress. Conclusion: Our data revealed that decreased ATM activation in monocytes was associated with SSc-interstitial lung disease and that impaired ATM activation in monocytes may contribute to the disease process of SSc via uncontrolled reactive oxygen species production.
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