The effects of peroxisome proliferator-activated receptor-gamma (ppar-γ) agonists on monocytic cell activation and endothelial function in diabetes

Abstract

Peroxisome Proliferator-activated Receptor-y (PPARy) is a ligand-activated transcription factor responsible for controlling genes involved in lipid and glucose metabolism. The thiazolidinediones (TZDs) are a class of synthetic PPARy ligands used as anti-hyperglycaemic agents in the treatment of type 2 diabetes. Recently it has been shown that these agonists have additional anti-inflammatory properties beyond their metabolic functions. This study evaluated the role of TZDs in monocytic cell activation and endothelial function. Human monocytic cell lines and peripheral monocytes were stimulated with various advanced glycation end-products (AGEs) to mimic the chronic inflammatory hyperglycaemic state typical of diabetes. Release of cytokines, such as TNF-α, was determined as indicators of inflammation. Endothelial function was studied using rabbit aortic rings and primary human aortic cells PPARy agonists, GW7845 and rosiglitazone, were observed to significantly reduce AGE-induced TNF-α expression and release. It was demonstrated that this reduction was not completely dependent on PPARy transcriptional activity, as PPARy antagonists did not negate the actions of the agonist. The anti-inflammatory properties of PPARy agonists appeared to be (i). time-dependent, (ii) dose-dependent (iii) cell type specific and (iv) involve two distinct pathways: PPARy-dependent and PPARy-independent. To elucidate the molecular mechanisms controlling the observed inhibition of TNF-α release the effect of PPARy agonists on the activation of ERK and NF-кB was investigated. It was found that rosiglitazone reduced gBSA-induced ERK activity, although basal levels remained unaffected. Surprisingly, rosiglitazone did not appear to modulate glycated BSA-stimulated NF-кB DNA binding activity. Conversely, rosiglitazone increase basal levels of NF-кB DNA binding activity but this did not result in NF-кB-dependent TNF-α gene expression. It was postulated that this up regulation of NF-кB might interfere with signalling processes necessary for inflammatory responses. The effect of PPARy agonist on receptor of AGE (RAGE) expression was studied at both transcriptional and translational level. PPARy agonist did not increase mRNA expression for RAGE, however the protein expression was increased in the presence of the agonist after 24hrs, indicating that the regulation of RAGE by the agonist may be at the post-transcriptional and/or post-translational levels. PPARy is also expressed in endothelium and vascular smooth muscle cells. Therefore this study also investigated the role of agonists on the vascular tone of the vessel wall in rabbit aortic rings. It was determined that GW7845 (>20µM) inhibited both phenylephrine-induced contraction and acetylcholine-induced relaxation, whereas sodium nitroprusside (SNP)-induced relaxation was slightly improved. This enhancement of SNP induced relaxation may indicate an increased sensitivity of VSMCs to the exogenous nitric oxide. GW7845 did not influence the NO release and eNOS expression in human primary endothelial cells. It was postulated that these direct vasomodulatory effects involve cGMP signalling and calcium trafficking in smooth muscle cells. Taken together these findings indicate that PPARy agonists can reduce the inflammatory processes of monocytic cells. However the mechanisms involved appear to be influenced by both PPRE-dependent and PPRE-independent pathways and the net anti-inflammatory and anti-atherogenic clinical benefits of TZD therapy are likely to be governed by the interactions and balance between these two pathways.