Control of nitric oxide production by endogenous TGF-β₁ and systemic nitric oxide in retinal pigment epithelial cells and peritoneal macrophages

Both in vivo and in vitro experiments demonstrate that transforming growth factor-β₁ (TGF-β₁) suppresses expression of the inducible form of nitric oxide synthase (iNOS). In this study, we examined the effects of exogenous and endogenous TGF-β₁ on retinal pigment epithelial (RPE) cells and resident peritoneal macrophages ex vivo using cells from TGF-β₁ null (TGF-β₁(-/-)) mice or age-matched wild-type (TGF-β₁(+/+)) or heterozygous (TGF-β₁(+/-)) littermates. RPE cells from both TGF-β₁(-/-) mice and TGF-β1(+/+) littermates produced NO and were immunocytochemically positive for iNOS protein only following treatment with interferon-γ (IFN-γ) and bacterial lipopolysaccharide (LPS); however, RPE cells from TGF-β₁(-/-) mice produced 40% more NO than cells from TGF-β₁(+/+) mice. In contrast, resident peritoneal macrophages from both TGF-β₁(+/+) and TGF-β₁(-/-) mice expressed iNOS protein without stimulation and in the absence of detectable production of NO. The expression of iNOS was increased by treatment with IFN-γ, resulting in detectable levels of NO. Macrophages from TGF-β₁(+/+) mice appeared to produce NO in a manner inversely proportional to the serum content of NO₂^- and NO₃^- of the mice from which the cells were obtained; no such correlation existed in TGF-β₁(+/-) or TGF-β₁(-/-) mice. Treatment of RPE cells or macrophages from both TGF-β₁(+/+) and TGF-β₁(-/-) mice with exogenous TGF-β₁ decreased both iNOS protein and NO production. These findings demonstrate a novel role of endogenous TGF-β₁ in coupling systemic NO production to the production of NO by macrophages, and demonstrate that endogenous and exogenous TGF-β₁ can act differently to suppress NO production.