Osteopontin is a negative feedback regulator of nitric oxide synthesis in murine macrophages

In a system of endotoxin (LPS)-mediated NO production in ANA-1 murine macro phages, suppression subtractive hybridization was used to identify genes up -regulated by NO. Osteopontin (OPN), a secreted acidic phosphoprotein that binds to a cell surface RGD integrin-binding motif, was found to be differe ntially expressed in the presence of NO. OPN has been demonstrated to inhib it NO production in a variety of cell types. Northern blot and nuclear run- on analyses demonstrated that OPN mRNA levels and gene transcription were s ignificantly increased in the presence of LPS-induced NO synthesis. Transie nt transfection of an OPN promoter-luciferase reporter plasmid construct sh owed that promoter activity is increased in the presence of LPS and NO. Imm unoblot analysis showed that OPN protein is secreted into the extracellular fluid. Similar results were noted with an alternative cell system, RAW 264 .7 macrophages, and alternative inducers of NO synthesis, IFN-gamma and IL- 1 beta. In the presence of GRGDSP, a hexapeptide that blocks binding of RGD -containing proteins to cell surface integrins, NO production is significan tly increased in the presence of LPS stimulation. These data suggest a uniq ue trans-regulatory mechanism in which LPS-induced NO synthesis feedback re gulates itself through up-regulation of OPN promoter activity and gene tran scription.