Hydrogen peroxide downregulates IL-1-driven mesangial iNOS activity: Implications for glomerulonephritis

In glomerulonephritides, autacoids such as nitric oxide (NO), reactive oxygen species, and prostanoids are produced in increased amounts in response to cytokines such as interleukin-1 (IL-1). These autacoids influence the expression of glomerular injury by their direct as well as interactive actions. We studied the effect of hydrogen peroxide (H₂O₂) on NO production in rat mesangial cells. We demonstrate that transient exposure of mesangial cells to H₂O₂ prior to sustained exposure to IL-1 decreased extracellular accumulation of NO₂/NO₃ and cellular guanosine 3',5'-cyclic monophosphate (cGMP) content. H₂O₂ markedly impaired inducible nitric oxide synthase (iNOS) activity induced by IL-1 directly measured by the conversion of L- [¹⁴C]arginine to L-[¹⁴C]citrulline. Such impairment in iNOS activity was accompanied by a parallel reduction in iNOS protein abundance but not by a reduced expression of iNOS mRNA. The inhibitory effect of H₂O₂ on NOS activity was further supported by peroxide-induced impairment in IL-1- driven, NO-dependent synthesis of prostaglandin E₂. Our studies thus provide the first direct evidence of a posttranscriptional inhibitory effect of H₂O₂ on iNOS activity. Additionally, our studies uncover the existence of a previously unrecognized effect of H₂O₂ on the production of NO that may exert influence on the severity of glomerular injury during glomerular inflammation.