MECHANISMS OF SUPPRESSION OF MACROPHAGE NITRIC-OXIDE RELEASE BY TRANSFORMING GROWTH-FACTOR-BETA

Activated mouse peritoneal macrophages produce nitric oxide (NO) via a nitric oxide synthase that is inducible by interferon gamma (IFN-gamm a): iNOS. We have studied the mechanisms by which transforming growth factor beta1 (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 mR NA without affecting the transcription of iNOS by decreasing iNOS mRNA stability. Even after iNOS was already expressed, TGF-beta reduced th e amount of iNOS protein. This was due to reduction of iNOS mRNA trans lation and increased degradation of iNOS protein. The potency of TGF-b eta as a deactivator of NO production (50% inhibitory concentration, 5 .6 +/- 2 pM) may reflect its ability to suppress iNOS expression by th ree distinct mechanisms: decreased stability and translation of iNOS m RNA, and increased degradation of iNOS protein. This is the first evid ence that iNOS is subject to other than transcriptional regulation.