Mechanisms of suppression of macrophage nitric oxide release by transforming growth factor β

Activated mouse peritoneal macrophages produce nitric oxide (NO) via a nitric oxide synthase that is inducible by Interferon γ (IFN-γ): iNOS. We have studied the mechanisms by which transforming growth factor β1 (TGF-β) suppresses IFN-γ-stimulated NO production. TGF-β treatment reduced iNOS specific activity and iNOS protein in both cytosolic and particulate fractions as assessed by Western blot with monospecific anti-iNOS immunoglobuhn G. TGF-β reduced iNOS mRNA without affecting the transcription of iNOS by decreasing iNOS mRNA stability. Even after iNOS was already expressed, TGF-β 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-β 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.