Fibrates and glitazones are two classes of drugs currently used in the treatment of dyslipidemia and insulin resistance (IR), respectively. Whereas glitazones are insulin sensitizers acting via activation of the peroxisome proliferator-activated receptor (PPAR) ␥ subtype, fibrates exert their lipid-lowering activity via PPAR␣. To determine whether PPAR␣ activators also improve insulin sensitivity, we measured the capacity of three PPAR␣-selective agonists, fenofibrate, ciprofibrate, and the new compound GW9578, in two rodent models of high fat diet-induced (C57BL/6 mice) or genetic (obese Zucker rats) IR. At doses yielding serum concentrations shown to activate selectively PPAR␣, these compounds markedly lowered hyperinsulinemia and, when present, hyperglycemia in both animal models. This effect relied on the improvement of insulin action on glucose utilization, as indicated by a lower insulin peak in response to intraperitoneal glucose in ciprofibrate-treated IR obese Zucker rats. In addition, fenofibrate treatment prevented high fat diet-induced increase of body weight and adipose tissue mass without influencing caloric intake. The specificity for PPAR␣ activation in vivo was demonstrated by marked alterations in the expression of PPAR␣ target genes, whereas PPAR␥ target gene mRNA levels did not change in treated animals. These results indicate that compounds with a selective PPAR␣ activation profile reduce insulin resistance without having adverse effects on body weight and adipose tissue mass in animal models of IR. MS,1 which develops as a result of IR (1), is characterized by glucose intolerance, hyperinsulinemia, dyslipidemia, and hypertension. These metabolic abnormalities are frequently associated with visceral obesity (2). The clustering of multiple cardiovascular risk factors in MS results in increased risk for atherosclerotic vascular disease, the major cause of mortality and morbidity in type 2 diabetic patients (3). Pharmacological treatment of MS should therefore aim at ameliorating IR and reducing cardiovascular risk factors.

Chronic inflammation is a hallmark of degenerative diseases such as atherosclerosis. Peroxisome proliferator-activated receptors (PPARs) are transcription factors belonging to the nuclear receptor superfamily, which are expressed in the cells of the atherosclerosic lesion. PPAR␣ ligands have been reported to exert antiinflammatory activities in different cell types by antagonizing the transcriptional activity of NF-B. In the present study, the influence of PPAR␣ activators on the NF-B signaling pathway was investigated. Our results show that fibrates, synthetic PPAR␣ activators, induced the expression of the inhibitory protein IB␣ in human aortic smooth muscle cells as well as in primary human hepatocytes, whereas neither IB-kinase activity nor the degradation rate of IB␣ were affected. Using PPAR␣-null mice, we demonstrated that fibrates induced IB␣ in liver in vivo and that this action required PPAR␣. Furthermore, fibrate treatment induced IB␣ protein expression in the cytoplasm and also enhanced IL-1␤-induced accumulation of IB␣ protein in the nucleus. These actions of fibrates on IB␣ expression were accompanied by a decrease in NF-B DNA binding activity as demonstrated by electrophoretic mobility shift assays. Taken together, these data provide an additional molecular mechanism for the anti-inflammatory activity of PPAR␣ agonists and reinforce their potential use in the treatment of inflammatory diseases.

Fibrates are widely used hypolipidemic drugs which activate the nuclear peroxisome proliferator-activated receptor (PPAR) ␣ and thereby alter the transcription of genes controlling lipoprotein metabolism. Fibrates influence plasma high density lipoprotein and its major protein, apolipoprotein (apo) A-I, in an opposite manner in man (increase) versus rodents (decrease). In the present study we studied the molecular mechanisms of this species-specific regulation of apoA-I expression by fibrates. In primary rat and human hepatocytes fenofibric acid, respectively, decreased and increased apoA-I mRNA levels. The absence of induction of rat apoA-I gene expression by fibrates is due to 3 nucleotide differences between the rat and the human apoA-I promoter A site, rendering a positive PPAR-response element in the human apoA-I promoter nonfunctional in rats. In contrast, rat, but not human, apoA-I transcription is repressed by the nuclear receptor Rev-erb␣, which binds to a negative response element adjacent to the TATA box of the rat apoA-I promoter. In rats fibrates increase liver Rev-erb␣ mRNA levels >10-fold. In conclusion, the opposite regulation of rat and human apoA-I gene expression by fibrates is linked to differences in cis-elements in their respective promoters leading to repression by Rev-erb␣ of rat apoA-I and activation by PPAR␣ of human apoA-I. Finally, Rev-erb␣ is identified as a novel fibrate target gene, suggesting a role for this nuclear receptor in lipid and lipoprotein metabolism.

Rev-Erb␣ (NR1D1) is an orphan nuclear receptor encoded on the opposite strand of the thyroid receptor ␣ gene. Rev-Erb␣ mRNA is induced during adipocyte differentiation of 3T3-L1 cells, and its expression is abundant in rat adipose tissue. Peroxisome proliferator-activated receptor ␥ (PPAR␥) (NR1C3) is a nuclear receptor controlling adipocyte differentiation and insulin sensitivity. Here we show that Rev-Erb␣ expression is induced by PPAR␥ activation with rosiglitazone in rat epididymal and perirenal adipose tissues in vivo as well as in 3T3-L1 adipocytes in vitro. Adipocyte differentiation is a complex biological process, which is reflected at the molecular level by the transcriptional activation of a number of adipocyte-specific genes and by the acquisition of the ability to accumulate cytoplasmic lipid droplets (1-3). The nuclear receptor peroxisome proliferator-activated receptor ␥ (PPAR␥, 1 NR1C3) (4, 5) and members of the CCAAT enhancer-binding protein (C/EBP) family (6 -12) play key roles in this adipogenic process. In addition, the adipocyte differentiation and determination factor-1 (SREBP-1/ADD1) appears to promote adipocyte differentiation by activating the expression of PPAR␥ and increasing the synthesis of endogenous PPAR␥ ligands (13-15). Members of the PPAR family bind as heterodimers with the retinoid X receptors (RXR) to specific response elements termed peroxisome proliferator response elements (PPRE) (for review see Ref. 16). These PPREs usually consist of a direct repeat of the PuGGTCA motif spaced by one nucleotide (DR1). The transcriptional activity of the PPARs is activated by a number of different fatty acid metabolites, most notably products of the cycloxygenase and lipoxygenase pathways. In addition, a large number of synthetic compounds are known to be potent and subtype specific PPAR ligands. For example, thiazolidinedione compounds used as insulin sensitizers in the treatment of type II diabetes are high affinity PPAR␥ ligands (17).Rev-Erb␣ (NR1D1) is another nuclear receptor, the expression of which is induced during adipocyte differentiation (18). Rev-Erb␣ is highly expressed in adipose tissue but also in skeletal muscle, liver and brain (18 -21). Since no ligand has been identified so far, Rev-Erb␣ is considered as an orphan member of the nuclear receptor superfamily. Rev-Erb␣ has been shown to act as a negative regulator of transcription (22) binding either as monomer on nuclear receptor half-site motifs flanked 5Ј by an A/T rich sequence (A/T PuGGTCA), or as a homodimer to a direct repeat of the PuGGTCA motif spaced by two nucleotides (DR2).We have previously shown that PPAR␣ activates the expression of Rev-Erb␣ through an atypical PPRE, a DR-2 element, in the Rev-Erb␣ promoter (23). Transcriptional activation by PPAR␥ through a DR-2 element has so far not been reported. However, since Rev-Erb␣ is induced during the course of adipocyte differentiation, we decided to investigate whether PPAR␥ could be involved in transcriptional induction of RevErb␣ expression in adipocytes. Furth...