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

Abstract

Peroxisome proliferation-activated receptor (PPAR)␥ agonists inhibit inducible nitric-oxide synthase (iNOS), tumor necrosis factor-␣, and interleukin-6. Because of these effects, synthetic PPAR␥ agonists, including thiazolidinediones, are being studied for their impact on inflammatory disease. The anti-inflammatory concentrations of synthetic PPAR␥ agonists range from 10 to 50 M, whereas their binding affinity for PPAR␥ is in the nanomolar range. The specificity of synthetic PPAR␥ agonists for PPAR␥ at the concentrations necessary for anti-inflammatory effects is thus in question. We report that PPAR␥ is not necessary for the inhibition of iNOS by synthetic PPAR␥ agonists. RAW 264.7 macrophages possess little PPAR␥, yet lipopolysaccharide (LPS)/interferon (IFN)␥-induced iNOS was inhibited by synthetic PPAR␥ agonists at 20 M. Endogenous PPAR␥ was inhibited by the transfection of a dominant-negative PPAR␥ construct into murine mesangial cells. In the transfected cells, synthetic PPAR␥ agonists inhibited iNOS production at 10 M, similar to nontransfected cells. Using cells from PPAR␥ Cre/lox conditional knockout mice, baseline and LPS/ IFN␥-induced nitric oxide levels were higher in macrophages lacking PPAR␥ versus controls. However, synthetic PPAR␥ agonists inhibited iNOS at 10 M in the PPAR␥-deficient cells, similar to macrophages from wild-type mice. These results indicate that PPAR␥ is not necessary for inhibition of iNOS expression by synthetic PPAR␥ agonists at concentrations over 10 M. Intrinsic PPAR␥ function, in the absence of synthetic agonists, however, may play a role in inflammatory modulation.Synthetic agonists of peroxisome proliferation-activated receptor (PPAR)␥, known as thiazolidinediones (TZDs), are currently being used for treatment of type II diabetes. Their clinical efficacy in diabetes coupled with the early reports of their effect on inflammatory mediator reduction in macrophage cell lines has lead to the study of PPAR␥ as an endogenous modulator of inflammation. The intrinsic ligand for PPAR␥ remains unclear. A prostaglandin J 2 metabolite was proposed to be an endogenous PPAR␥ agonist. This metabolite was shown effective at reducing markers of inflammation, such as inducible nitric-oxide synthase (iNOS), at concentrations less than 10 M (Jiang et al., 1998;Ricote et al., 1998;Petrova et al., 1999). However, it was later shown that in addition to binding to PPAR␥, 15-deoxy-⌬ 12,14 -prostaglandin J 2 (15dPGJ 2 ) also stabilized NF-B and prevented its activation of the iNOS promoter (Straus et al., 2000). A reactive metabolite, 15dPGJ 2 is capable of donating an electron to the NF-B. Recently, the lack of physiologic relevance of 15dPGJ 2 as an endogenous PPAR␥ agonist was demonstrated, and toxicity of the compound limits its clinical utility (Bell-Parikh et al., 2003). Thus, it is clear from a number of studies that PPAR␥ is not necessary for the anti-inflammatory properties of 15dPGJ 2 . This same PPAR␥-independent reactivity has not been reported for the synthetic PPAR␥ agonists s...

Deletion of PPARγ in Alveolar Macrophages Is Associated with a Th-1 Pulmonary Inflammatory Response

“…To determine whether the low levels of iNOS and IFN-␥ expression in WT alveolar macrophages were due to the high constitutive expression of PPAR␥, as was shown previously (21), we used GW9662, a known PPAR␥ antagonist (5,23,24). Untreated alveolar macrophages demonstrated no detectable iNOS mRNA expression.…”

“…IFN-␥ is a known stimulator of iNOS (5,23), and so we thus investigated iNOS mRNA and protein in BAL cells (Fig. 4A).…”

Pioglitazone protects the myocardium against ischemia-reperfusion injury in eNOS and iNOS knockout mice

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Augmentation of Myocardial Production of 15-Epi-Lipoxin-A 4 by Pioglitazone and Atorvastatin in the Rat

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