Peroxisome Proliferation-Activated Receptor (PPAR)γ Is Not Necessary for Synthetic PPARγ Agonist Inhibition of Inducible Nitric-Oxide Synthase and Nitric Oxide

Peroxisome proliferation-activated receptor (PPAR)γ agonists inhibit inducible nitric-oxide synthase (iNOS), tumor necrosis factor-α, and interleukin-6. Because of these effects, synthetic PPARγ agonists, including thiazolidinediones, are being studied for their impact on inflammatory disease. The anti-inflammatory concentrations of synthetic PPARγ agonists range from 10 to 50 μM, whereas their binding affinity for PPARγ is in the nanomolar range. The specificity of synthetic PPARγ agonists for PPARγ at the concentrations necessary for anti-inflammatory effects is thus in question. We report that PPARγ is not necessary for the inhibition of iNOS by synthetic PPARγ agonists. RAW 264.7 macrophages possess little PPARγ, yet lipopolysaccharide (LPS)/interferon (IFN)γ-induced iNOS was inhibited by synthetic PPARγ agonists at 20 μM. Endogenous PPARγ was inhibited by the transfection of a dominant-negative PPARγ construct into murine mesangial cells. In the transfected cells, synthetic PPARγ agonists inhibited iNOS production at 10 μM, similar to nontransfected cells. Using cells from PPARγ Cre/lox conditional knockout mice, baseline and LPS/IFNγ-induced nitric oxide levels were higher in macrophages lacking PPARγ versus controls. However, synthetic PPARγ agonists inhibited iNOS at 10 μM in the PPARγ-deficient cells, similar to macrophages from wild-type mice. These results indicate that PPARγ is not necessary for inhibition of iNOS expression by synthetic PPARγ agonists at concentrations over 10 μM. Intrinsic PPARγ function, in the absence of synthetic agonists, however, may play a role in inflammatory modulation.