Nitric oxide inhibits endothelial IL-1β-induced ICAM-1 gene expression at the transcriptional level decreasing Sp1 and AP-1 activity

Background: Nitric oxide (NO) has frequently been shown to inhibit leukocyte adherence to activated endothelium thus displaying anti-adhesive and immunosuppressive activities. A molecular mechanism contributing to this effect is described. Materials and Methods: Primary murine aortic endothelial cells were activated with interleukin (IL)-1beta to express intercellular adhesion molecule-1 (ICAM-1) mRNA in the presence or absence of the physiological spontaneous NO-donor S-nitrosocysteine. Subsequently, semiquantitative RT-PCR and gel shift assays with nuclear extracts were performed to analyse the effects of NO on ICAM-1 mRNA expression and on the activity Of transcription factors involved in ICAM-1 transcription. In addition, luciferase reporter gene activity of cytokine-activated cells transiently transfected with an ICAM-1 promoter-luciferase construct and cultured in the presence of the slow-releasing NO-donor DETA/NO was determined. Results: NO at subtoxic concentrations decreases IL-1beta-induced endothelial ICAM-1 mRNA expression. This inhibition occurs at the transcriptional level, as NO affects IL-1beta-induced ICAM-1 promoter activity in transiently transfected cells. Using gel-shift assays and double-stranded oligonucleotide consensus sequences of the known transcription factor binding sites of the ICAM-1 promoter, Sp1 and AP-1 were identified as transcriptional activators of IL1beta-driven ICAM-1 expression. The DNA binding of both of these transcription factors to specific binding sites of the ICAM-1 promoter was decreased in MAEC exposed to NO. Conclusions: Our studies indicate that the anti-adhesive effect of NO concentrations equivalent to high-output NO synthesis is mediated, at least in part, by inhibition of ICAM-I expression via a concerted action of No on the redox-sensitive transcriptional activators Sp1 and AP-1. This molecular mechanism may contribute to the anti-inflammatory actions of NO synthesized by the inducible NO synthase.