S-methylisothiourea, an iNOS inhibitor, attenuates inflammation in thyroid eye disease-derived orbital fibroblasts by suppressing the iNOS/NO/NF-κB signaling pathway.

INTRODUCTION: Orbital fibroblasts (OFs) play a central role in the pathogenesis of thyroid eye disease (TED), an autoimmune disease characterized by chronic inflammation and subsequent fibrosis. Although nitric oxide (NO) is a potent inflammatory signaling molecule, its specific contribution to TED progression remains elusive. We hypothesized that the iNOS/NO/NF-κB signaling axis promotes inflammation in TED. Therefore, this study aimed to investigate the therapeutic potential of S-methylisothiourea (SMT), a selective inducible nitric oxide synthase (iNOS) inhibitor, in TED and to elucidate the underlying molecular mechanisms.

PATIENTS AND METHODS: An in vitro inflammatory model was generated by treating human OFs with IL-1β. To characterize the anti-inflammatory mechanism of SMT, TED-OFs were incubated with the compound. The experimental assessments included measurement of NO release, qPCR analysis of gene expression, and western blotting to probe the induction of the iNOS and NF-κB signaling pathways.

RESULTS: Administration of SMT resulted in pronounced inhibition of iNOS synthesis and a consequent reduction in the upregulation of key inflammatory mediators, including IL-6 and TNF-α, in cells treated with IL-1β. Western blot analysis further showed that the anti-inflammatory action of SMT was mediated through suppression of iNOS/NO/NF-κB pathway activation.

CONCLUSION: This study identified SMT as an effective inhibitor of the iNOS/NO/NF-κB axis, which disrupts a key inflammatory signaling cascade in TED by reducing aberrant NO synthesis. These results support further development of SMT as a novel treatment strategy for Thyroid Eye Disease.