The present study demonstrates that PPARγ is reduced in immunomodulatory and parenchymal cells during polymicrobial sepsis. Furthermore, the downregulation of PPARγ involves its phosphorylation by ERK1/2, and inhibition of ERK1/2 causes a decrease in phosphorylated PPARγ and restores PPARγ. This restoration of PPARγ correlates with an increase in plasma levels of the antiinflammatory adipokine adiponectin ().
Figure 8 Mechanisms of activation of PPARγ through inhibition of MEK1. MEK1 phosphorylates ERK1/2, leading to phosphorylation and inactivation of PPARγ. Inhibition of MEK1 by PD98059 retains PPARγ in the non-phosphorylated active form. (more ...)
In the present study, we observed that PPARγ is altered in PBMCs and lung parenchymal cells during polymicrobial sepsis. Our findings are in agreement with previous studies that demonstrate that the expression, production and activity of PPARγ are affected in other inflammatory conditions. In adipose tissue, PPARγ mRNA and protein expression decreased after mice were challenged in vivo
with endotoxin (5
). Previous results in our laboratory support the idea that PPARγ is important in controlling inflammation and correlates with clinical outcomes. In the cardiovascular hypodynamic phase of septic shock, PPARγ expression was downregulated in the lung and in thoracic aortas in rats (1
). Furthermore, the sepsis-induced reduction in PPARγ expression was reversed by in vivo
treatment with PPARγ ligands. In an experimental model of polymicrobial sepsis, Zhou et al.
) demonstrate that hepatic PPARγ protein and gene expression is downregulated in the late stages of sepsis. Although many studies confirm the effects of sepsis on decreasing PPARγ expression in tissue, contradictory results exist in immune cells. In porcine white blood cells, PPARγ expression increased in the first 6 hours after in vivo
lipopolysaccharide (LPS )challenge (17
). However, PPARγ expression normalized to control levels by 8 hours post-LPS. Similarly, the expression of PPARγ was increased in peripheral blood mononuclear cells and T-cells from patients with septic shock and sepsis (15
Our results provide a mechanism through which a decrease in PPARγ in sepsis may be partially explained and are consistent with previous results demonstrated in endotoxic shock (19
). We hypothesized that posttranslational modifications, including phosphorylation of PPARγ by ERK1/2, may alter PPARγ in a model of polymicrobial sepsis. Our current data demonstrate that nuclear content of p-ERK1/2 increases in PBMCs after CLP and correlates with a decrease in PPARγ. In the lung, an increase of p-ERK1/2 also correlated with phosphorylation of PPARγ. Posttranslational modifications are mechanisms that regulate the function of PPARγ (7
). The AF-1 domain of PPARγ contains a consensus MAPK site, and phosphorylation at serine residue 82 (or 112 for PPARγ2) leads to inhibition of PPARγ transactivation (8
). This phosphorylated- induced repression is due to conformational changes that can lead to altered affinity for ligands and cofactors (8
). Additionally, phosphorylation of PPARγ promotes its degradation through the ubiquitin-proteasome system (10
). In MCF-7 breast cancer cells, inhibition of p-ERK1/2 with α-eleostearic acid correlated with decreased PPARγ in a time-dependent manner (21
). An alternative mechanism resulting in decreased nuclear PPARγ expression and activity could occur through direct interaction of nuclear PPARγ with MEKs, resulting in the nuclear export of PPARγ, thereby preventing its nuclear activation (22
). Thus, it is possible that during the inflammatory process, alteration of protein conformation by posttranslational mechanisms may affect the expression of the receptor (19
Adiponectin is an adipocyte-derived protein that is secreted into plasma. Adiponectin has beneficial effects including an antiatherosclerotic action, it improves insulin sensitivity and it activates glucose uptake in skeletal muscle cells (23
). Additionally, adiponectin is an antiinflammatory cytokine that inhibits nuclear factor (NF)-κB activation in endothelial cells and macrophages (24
). Furthermore, adiponectin is induced by PPARγ agonists via direct binding to the peroxisome proliferator response element in the adiponectin promoter (14
). Tsuchihashi et al.
) previously demonstrated that plasma adiponectin levels were decreased in rats subjected to polymicrobial sepsis. We demonstrate that adiponectin levels were significantly increased in animals who received the ERK1/2 inhibitor PD98059 during polymicrobial sepsis. This increase in adiponectin reflects the changes in the kinetics of PPARγ after CLP. This finding is in agreement with previous studies that demonstrate that alteration of PPARγ through treatment with PPARγ ligands alters adiponectin expression (27
). Combs et al.
) demonstrate that adiponectin levels were significantly increased in healthy male subjects treated with the PPARγ ligand rosiglitazone. Moreover, in patients with the dominant-negative PPARγ mutation, adiponectin levels were lower compared with patients with severe insulin resistance with no mutation.
The inflammatory effects of polymicrobial sepsis cause changes in PPARγ expression and activation in PBMCs and lung tissue in rats. These changes in PPARγ are, in part, due to the phosphorylation of PPARγ by ERK1/2 and can be reversed by ERK1/2 inhibition. Furthermore, adipokines are altered during polymicrobial sepsis and adiponectin plasma levels correlate with PPARγ expression and can be augmented with ERK1/2 inhibition. More studies are necessary to investigate the molecular link between adipokines and the inflammatory response in sepsis.