Abstract
Inflammatory cytokines, interleukin 1beta and tumor necrosis factor-alpha, potently stimulate rat mesangial cells to express and secrete group IIA phospholipase A(2) (PLA(2)). Cytokine-induced up-regulation of PLA(2) has been blocked by inhibitors (antioxidants) of the transcription factor, nuclear factor-kappaB (NF-kappaB), suggesting a role for NF-kappaB in the regulation of group IIA PLA(2) expression. Reactive oxygen species such as H(2)O(2), which are elevated in mesangial cells after cytokine activation, can mimic cytokine-induced NF-kappaB activation. However, the source of reactive oxygen species generation in mesangial cells, produced by cytokine stimulation, has yet to be clarified. Recently, tumor necrosis factor-alpha has been demonstrated to increase superoxide radical generation in mesangial cells. Therefore, we hypothesized that a selective NADPH oxidase inhibitor, diphenyleneiodium chloride (DPI), could block cytokine-induced group IIA PLA(2) up-regulation by attenuating NF-kappaB binding. To test this hypothesis, we isolated rat mesangial cells and characterized them by ultrastructural and immunochemical methods. This homogeneous mesangial cell population was responsive to cytokine as evidenced by an increase in steady-state levels of group IIA PLA(2) mRNA and extracellular enzymatic activity over time. DPI (0.02-20 microM), added 90 min before cytokine activation, inhibited both group IIA PLA(2) mRNA and enzymatic activity in a concentration-dependent manner. By electrophoretic mobility shift analysis, cytokine activation also increased specific NF-kappaB binding to one of two NF-kappaB consensus elements in the rat group IIA PLA(2) promoter and also was suppressed by DPI pretreatment. Antibodies to NF-kappaB p65 (Rel A) and p50 (but not normal rabbit IgG) supershifted this retardation signal and verified the type of NF-kappaB species as the classical p50/p65 heterodimer.