Osteopontin inhibits induction of nitric oxide synthase gene expression by inflammatory mediators in mouse kidney epithelial cells

We report that osteopontin (OPN), a secreted, Arg-Gly-Asp-containing phosphoprotein expressed at high levels in the kidney, suppresses nitric oxide (NO) synthesis induced by the inflammatory mediators γ-interferon and lipopolysaccharide in primary mouse kidney proximal tubule epithelial cells. Northern blot and immunofluorescence analyses of inducible nitric oxide synthase (iNOS) expression revealed that the inflammatory mediators increased iNOS mRNA and protein levels. Recombinant human OPN (purified from both mammalian cells and from Escherichia coli) inhibited this response by a process that was blocked by anti-OPN antiserum and by the peptide GRGDS, but not GRGES. The data suggest that inhibition of NO synthesis by OPN in these kidney cells is mediated by an integrin, possibly the α(v)β₃ integrin, which is known to be an OPN receptor. NO is believed to control blood flow through the glomerulus, regulating salt and water balance, and to be important as a defense against tumor cells and infecting microorganisms. The ability of OPN to inhibit the induction of iNOS suggests that OPN may be an important regulator of the NO signaling pathway and NO-mediated cytotoxic processes.