Endotoxin downregulates peroxisome proliferator-activated receptor-γ via the increase in TNF-α release

The nuclear receptor peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is anti-inflammatory in a cell-based system and in animal models of endotoxemia. We have shown that PPAR-gamma gene expression is downregulated in macrophages after lipopolysaccharide (LPS) stimulation. However, it remains unknown whether hepatic PPAR-gamma is altered in sepsis and, if so, whether LPS directly downregulates PPAR-gamma. To study this, rats were subjected to sepsis by cecal ligation and puncture (CLP). Hepatic tissues were harvested at 5, 10, and 20 h after CLP. PPAR-gamma gene expression and protein levels were determined by RT-PCR and Western blot analysis, respectively. The results showed that PPAR-gamma gene expression decreased at 10 and 20 h and that its proteins levels were reduced at 20 h after CLP. PPAR-gamma levels were also decreased in animals that were administered LPS. To determine the direct effects of LPS on PPAR-gamma downregulation, LPS binding agent polymyxin B (PMB) was administered intramuscularly after CLP. The administration of PMB significantly reduced plasma levels of endotoxin, but it did not prevent the downregulation of PPAR-gamma expression. We found that circulating levels of TNF-alpha still remained significantly elevated in PMB-treated septic animals. We, therefore, hypothesize that the decrease of PPAR-gamma expression is TNF-alpha dependent. To investigate this, Kupffer cells (KCs) were isolated from normal rats and stimulated with LPS or TNF-alpha. TNF-alpha significantly attenuated PPAR-gamma gene expression in KCs. Although LPS decreased PPAR-gamma in KCs, the downregulatory effect of LPS was blocked by the addition of TNF--alpha neutralizing antibodies. Furthermore, the administration of TNF-alpha-neutralizing antibodies to animals before the onset of sepsis prevented the downregulation of PPAR-gamma in sepsis. We, therefore, conclude that LPS downregulates PPAR-gamma expression during sepsis via an increase in TNF-alpha release.