Pparα and fatty acid oxidation coordinate hepatic transcriptional architecture

Fasting requires tight coordination between the metabolism and transcriptional output of hepatocytes to maintain systemic glucose and lipid homeostasis. Deficits in hepatic fatty acid oxidation result in dramatic fasting-induced hepatocyte lipid accumulation and induction of genes for oxidative metabolism that are largely driven by Pparα. While fatty acid oxidation is required for a rise in acetyl-CoA and subsequent lysine acetylation following a fast, changes in histone acetylation (total, H3K9ac, and H3K27ac) do not require fatty acid oxidation. Active enhancers in fasting mice are enriched for Pparα binding motifs. Genetically-defined inhibition of hepatic fatty acid oxidation results in higher levels of chromatin accessibility as well as elevated enhancer priming and acetylation proximal to Pparα sites largely associated with genes in lipid metabolism. Also, greater number of Pparα-associated H3K27ac signal changes occur at active enhancers compared to promoters, suggesting a mechanism for Pparα to tune target expression levels at pre-primed sites. Overall, these data show the requirement for Pparα activation in maintaining transcriptionally permissive hepatic genomic architecture particularly when fatty acid oxidation is limiting.

CDKN2A/p16INK4a suppresses hepatic fatty acid oxidation through the AMPKα2-SIRT1-PPARα signaling pathway

In addition to their well-known role in the control of cellular proliferation and cancer, cell cycle regulators are increasingly identified as important metabolic modulators. Several GWAS have identified SNPs near CDKN2A, the locus encoding for p16INK4a (p16), associated with elevated risk for cardiovascular diseases and type-2 diabetes development, two pathologies associated with impaired hepatic lipid metabolism. Although p16 was recently shown to control hepatic glucose homeostasis, it is unknown whether p16 also controls hepatic lipid metabolism. Using a combination of in vivo and in vitro approaches, we found that p16 modulates fasting-induced hepatic fatty acid oxidation (FAO) and lipid droplet accumulation. In primary hepatocytes, p16-deficiency was associated with elevated expression of genes involved in fatty acid catabolism. These transcriptional changes led to increased FAO and were associated with enhanced activation of PPARα through a mechanism requiring the catalytic AMPKα2 subunit and SIRT1, two known activators of PPARα. By contrast, p16 overexpression was associated with triglyceride accumulation and increased lipid droplet numbers in vitro, and decreased ketogenesis and hepatic mitochondrial activity in vivo. Finally, gene expression analysis of liver samples from obese patients revealed a negative correlation between CDKN2A expression and PPARA and its target genes. Our findings demonstrate that p16 represses hepatic lipid catabolism during fasting and may thus participate in the preservation of metabolic flexibility.

A dietary anthocyanin cyanidin-3-O-glucoside binds to PPARs to regulate glucose metabolism and insulin sensitivity in mice

We demonstrate the mechanism by which C3G, a major dietary anthocyanin, regulates energy metabolism and insulin sensitivity. Oral administration of C3G reduced hepatic and plasma triglyceride levels, adiposity, and improved glucose tolerance in mice fed high-fat diet. Hepatic metabolomic analysis revealed that C3G shifted metabolite profiles towards fatty acid oxidation and ketogenesis. C3G increased glucose uptake in HepG2 cells and C2C12 myotubes and induced the rate of hepatic fatty acid oxidation. C3G directly interacted with and activated PPARs, with the highest affinity for PPARα. The ability of C3G to reduce plasma and hepatic triglycerides, glucose tolerance, and adiposity and to induce oxygen consumption and energy expenditure was abrogated in PPARα-deficient mice, suggesting that PPARα is the major target for C3G. These findings demonstrate that the dietary anthocyanin C3G activates PPARs, a master regulators of energy metabolism. C3G is an agonistic ligand of PPARs and stimulates fuel preference to fat.

Maternal Supplementation of Clofibrate Stimulates Hepatic Fatty Acid Oxidation in Newborn Suckling Piglets

Objectives To evaluate effects of maternal feeding of clofibrate, a PPARα agonist, on development of hepatic fatty acid metabolism in offspring using pig as a model. Methods Pregnant sows (N = 27) were randomly assigned into three treatment groups. Each group was fed a standard diet (3265 kcal ME/kg) supplemented with either 0, 0.25% or 5% clofibrate (w/w) from d 107 of gestation to d 7 of lactation. Liver tissue was collected from piglets at birth, d1, 7, 14 and 19. Fatty acid oxidation was examined in fresh homogenates using 1 mM [1–14C] oleic acid (9.9 mBq/mmol) as substrate. Oxidation was measured in the absence or presence of in vitro supplemented L-carnitine (1 mM) and/or malonate (5 mM). Results Clofibrate was not detected in piglet liver or sow milk. Interactions between clofibrate and postnatal age (P < 0.001) on the 14C accumulation in 14CO2, acid soluble products (14C-ASP) and esterified products (14C-ESP) were observed. Accumulation in 14CO2 was not altered by piglet age in control sows; however, accumulation in 14C-ASP was higher at d14 and lower at d19 compared to d1. In contrast, maternal clofibrate increased 14CO2 by 100% and 14C-ASP by 80% in pigs at d1, and the increase was higher in pigs from sows given 0.5% versus 0.25% clofibrate. Accumulation in 14C-ESP in pigs from control sows increased from d1 to d14, but there was no difference detected between d14 and 19. Assessment of pigs from sows fed the 0.25% clofibrate dose revealed no impact on 14C-ESP, but the 0.5% dose increased 14C-ESP by 31%. No interaction was observed between clofibrate and the in vitro treatments (carnitine and malonate; P = 0.5). In vitro supplementation of carnitine increased radiolabel accumulation in CO2 by 60% and in ASP by 120%, but reduced 14C-ESP by 39% compared to control incubations. Supplementation of malonate reduced 14CO2 by 95% and 14C-ESP by 44%, but had no impact on 14C-ASP. Conclusions Maternal clofibrate enhances hepatic fatty acid metabolism in offspring, but the effect fades with postpartum age. The availability of carnitine in the milk could be a key element to support fatty acid oxidation in postnatal pigs. Funding Sources USDA National Institute of Food and Agriculture.