PPARdelta regulates glucose metabolism and insulin sensitivity

Abstract

The metabolic syndrome is a collection of obesity-related disorders. The peroxisome proliferator-activated receptors (PPARs) regulate transcription in response to fatty acids and, as such, are potential therapeutic targets for these diseases. We show that PPARdelta (NR1C2) knockout mice are metabolically less active and glucose-intolerant, whereas receptor activation in db/db mice improves insulin sensitivity. Euglycemic-hyperinsulinemic-clamp experiments further demonstrate that a PPARdelta-specific agonist suppresses hepatic glucose output, increases glucose disposal, and inhibits free fatty acid release from adipocytes. Unexpectedly, gene array and functional analyses suggest that PPARdelta ameliorates hyperglycemia by increasing glucose flux through the pentose phosphate pathway and enhancing fatty acid synthesis. Coupling increased hepatic carbohydrate catabolism with its ability to promote beta-oxidation in muscle allows PPARdelta to regulate metabolic homeostasis and enhance insulin action by complementary effects in distinct tissues. The combined hepatic and peripheral actions of PPARdelta suggest new therapeutic approaches to treat type II diabetes.

Fig. 1.

Depressed metabolism and glucose intolerance in PPARδ null mice. Metabolic parameters of wild-type and PPARδ null mice were measured in metabolic cages (n = 8). PPARδ null mice ate and drank less (A), consumed and produced less O₂ and CO₂ (B), expended less energy (presented as heat) (C), and have lower RERs (RER = VCO₂/VO₂) (D), particularly in the fed state (6 p.m. to 6 a.m.). (E) The glucose-tolerance test (GTT), showing that PPARδ null animals were glucose-intolerant on normal chow. (F) The GTT, showing a receptor-dependent effect of a PPARδ agonist (GW) on improving glucose tolerance. Both wild-type and PPARδ^−/− mice were placed on a high-fat high-carbohydrate diet for 10 weeks, followed by a 2-week ligand treatment (n = 4). Statistical differences were observed only between vehicle- and ligand (GW)-treated wild-type mice. ∗, P < 0.05.

Fig. 2.

Treatment with a PPARδ agonist increases insulin sensitivity in db/db mice. PPARδ ligand treatment increased RER (A) and improved performance in the glucose-tolerance test (B) and insulin-tolerance test (C) in db/db mice (n = 10). In the insulin-tolerance test, glucose levels after insulin injection were presented as the percentage of initial glucose concentrations. (D–G) Euglycemic–hyperinsulinemic clamp in db/db mice, showing improved insulin sensitivity in multiple tissues after ligand treatment. PPARδ ligand-treated mice have a higher glucose infusion rate (GIR) (D), decreased hepatic glucose production (HGP) (F), increased insulin stimulated-glucose disposal rate (IS-GDR) (G), and lower free fatty acid levels after clamp (F). Vehicle (□); GW501516 (■); ∗, P < 0.05

Fig. 3.

PPARδ regulates hepatic glucose and fatty acid homeostasis through direct transcriptional regulation. (A) Q-PCR analyses demonstrating increased ACCβ expression in livers of GW-treated db/db mice (Left). GW had no effects on levels of SREBP-1c (Center) or HMG-CoA reductase (Right). (B) The 5′ regulatory regions of the human ACCβ gene. ACCβ is controlled by two promoters. A putative PPARδ response element (PPRE) was identified in the promoter I (−480 to −468, relative to the transcriptional starting site). Residues that were mutated in the reporter construct [PI (0.58 kb/M)] are indicated. (C) ACCβ is a direct target gene of PPARδ. HepG2 cells were cotransfected with a luciferase reporter driven by the promoter I or II of the ACCβ gene, together with control vectors or expression vectors for PPARδ and the heterodimer partner RXRα, as well as a β-galactosidase internal control. Cells were treated with GW at 0.1 μM for 16 h. PPARδ increases the activity of the promoter I in a PPRE-dependent manner. PI (0.58 kb), the luciferase reporter driven by 0.58-kb 5′ upstream region of the promoter; I, PI (0.58 kb/M); 0.58-kb promoter I with PPRE mutations; PII (1.3 kb), 1.3-kb promoter II.

Fig. 4.

PPARδ promotes glucose flux to the pentose phosphate pathway and fatty acid synthesis in liver. (A) PPARδ ligand activates the pentose phosphate pathway. GW treatment increases the activity of glucose-6-phosphate dehydrogenase, the first enzyme in the pentose phosphate pathway, by 2-fold. The activities were determined by NADPH produced (OD 340 nm) by using liver lysates from vehicle or GW-treated db/db mice (n = 10). (B) PPARδ increases hepatic glucose-to-fatty acid conversion. Primary hepatocytes were isolated from vehicle or GW-treated db/db mice and loaded with [¹⁴C]glucose. The amount of glucose-derived fatty acid was determined by the radioactivity recovered from the organic phase after lipid extraction and was normalized to protein concentration. (C) Increased hepatic TG, but not cholesterol, content in ligand-treated db/db mice. (D) PPARδ increases β-oxidation in muscle. Soleus muscle strips were isolated from vehicle or GW-treated db/db mice and loaded with [³H]palmitic acid. The catabolic rate was determined by measuring the fatty acid β-oxidation product ³H₂O and normalized to tissue weight. Vehicle (□); GW501516 (■); ∗, P < 0.05.