Mechanisms of suppression of macrophage nitric oxide release by transforming growth factor beta.

TLDR

Activated mouse peritoneal macrophages produce nitric oxide via inducible nitric oxide synthase stimulated by interferon‑γ. The study investigates how transforming growth factor‑β1 suppresses interferon‑γ‑stimulated nitric oxide production in macrophages. TGF‑β1 suppresses NO production by reducing iNOS mRNA stability and translation and by increasing iNOS protein degradation, with an IC₅₀ of 5.6 pM, showing that iNOS is regulated post‑transcriptionally.

Abstract

Activated mouse peritoneal macrophages produce nitric oxide (NO) via a nitric oxide synthase that is inducible by interferon gamma (IFN-gamma): iNOS. We have studied the mechanisms by which transforming growth factor beta 1 (TGF-beta) suppresses IFN-gamma-stimulated NO production. TGF-beta treatment reduced iNOS specific activity and iNOS protein in both cytosolic and particulate fractions as assessed by Western blot with monospecific anti-iNOS immunoglobulin G. TGF-beta reduced iNOS mRNA without affecting the transcription of iNOS by decreasing iNOS mRNA stability. Even after iNOS was already expressed, TGF-beta reduced the amount of iNOS protein. This was due to reduction of iNOS mRNA translation and increased degradation of iNOS protein. The potency of TGF-beta as a deactivator of NO production (50% inhibitory concentration, 5.6 +/- 2 pM) may reflect its ability to suppress iNOS expression by three distinct mechanisms: decreased stability and translation of iNOS mRNA, and increased degradation of iNOS protein. This is the first evidence that iNOS is subject to other than transcriptional regulation.