The liver plays a crucial role in mobilizing energy during nutritional deprivation. During the early stages of fasting, hepatic glycogenolysis is a primary energy source. As fasting progresses and glycogen stores are depleted, hepatic gluconeogenesis and ketogenesis become major energy sources. Here, we show that fibroblast growth factor 21 (FGF21), a hormone that is induced in liver by fasting, induces hepatic expression of peroxisome proliferatoractivated receptor ␥ coactivator protein-1␣ (PGC-1␣), a key transcriptional regulator of energy homeostasis, and causes corresponding increases in fatty acid oxidation, tricarboxylic acid cycle flux, and gluconeogenesis without increasing glycogenolysis. Mice lacking FGF21 fail to fully induce PGC-1␣ expression in response to a prolonged fast and have impaired gluconeogenesis and ketogenesis. These results reveal an unexpected relationship between FGF21 and PGC-1␣ and demonstrate an important role for FGF21 in coordinately regulating carbohydrate and fatty acid metabolism during the progression from fasting to starvation.lipid metabolism ͉ liver ͉ gluconeogenesis ͉ glycogenolysis ͉ ketogenesis I n mammals, the liver plays a crucial role in maintaining systemic energy balance during fasting and starvation through coordinate effects on carbohydrate and lipid metabolism. During the early stages of fasting, the liver mobilizes glucose from its glycogen stores. As fasting progresses and glycogen reserves are depleted, the liver oxidizes fat to provide both energy for gluconeogenesis and substrate for ketogenesis. This synchronization of hepatic lipid and carbohydrate metabolism is critical for the normal fasting response; disruption of either one of these pathways has profound effects on the other (1-4).Hormones such as glucagon, catecholamines, and glucocorticoids have important roles in controlling substrate utilization and maintaining energy balance during fasting. Recently, the hormone fibroblast growth factor 21 (FGF21) was shown to be induced in the liver during fasting (5-7). FGF21 is an unusual FGF family member in that it lacks the conventional heparinbinding domain (8) and thus can diffuse away from its tissue of origin and function as a hormone. FGF21 signals through cell-surface receptors composed of classic FGF receptors complexed with ␤-klotho, a membrane-spanning protein (9-14). Induction of FGF21 during fasting occurs through a mechanism that requires peroxisome proliferator-activated receptor ␣ (PPAR␣) (5-7). FGF21 has diverse metabolic actions that include stimulating hepatic fatty acid oxidation and ketogenesis (5,6,15) and blocking the growth hormone signaling pathway (16). FGF21 also sensitizes mice to torpor, a short-term hibernation-like state of regulated hypothermia (6). Pharmacologic administration of FGF21 to insulin-resistant rodents and monkeys improves glucose tolerance and reduces plasma insulin and triglyceride concentrations (15,17).Peroxisome proliferator-activated receptor ␥ coactivator protein-1␣ (PGC-1␣) is a transcriptional coactivator ...

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Ceramides contribute to the lipotoxicity that underlies diabetes, hepatic steatosis, and heart disease. By genetically engineering mice, we deleted the enzyme dihydroceramide desaturase 1 (DES1), which normally inserts a conserved double bond into the backbone of ceramides and other predominant sphingolipids. Ablation of DES1 from whole animals or tissue-specific deletion in the liver and/or adipose tissue resolved hepatic steatosis and insulin resistance in mice caused by leptin deficiency or obesogenic diets. Mechanistic studies revealed ceramide actions that promoted lipid uptake and storage and impaired glucose utilization, none of which could be recapitulated by (dihydro)ceramides that lacked the critical double bond. These studies suggest that inhibition of DES1 may provide a means of treating hepatic steatosis and metabolic disorders.

Summary Regulation of hepatic carbohydrate homeostasis is crucial for maintaining energy balance in the face of fluctuating nutrient availability. Here, we show that the hormone fibroblast growth factor 15/19 (FGF15/19), which is released postprandially from the small intestine, inhibits hepatic gluconeogenesis, like insulin. However, unlike insulin, which peaks in serum 15 minutes after feeding, FGF15/19 expression peaks approximately 45 min later, when bile acid concentrations increase in the small intestine. FGF15/19 blocks the expression of genes involved in gluconeogenesis through a mechanism involving the dephosphorylation and inactivation of the transcription factor cAMP regulatory element binding protein (CREB). This in turn blunts expression of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and other genes involved in hepatic metabolism. Overexpression of PGC-1α blocks the inhibitory effect of FGF15/19 on gluconeogenic gene expression. These results demonstrate that FGF15/19 works subsequent to insulin as a postprandial regulator of hepatic carbohydrate homeostasis.

Two weeks of dietary intervention (≈4.3% weight loss) reduced hepatic triglycerides by ≈42% in subjects with NAFLD; however, reductions were significantly greater with dietary carbohydrate restriction than with calorie restriction. This may have been due, in part, to enhanced hepatic and whole-body oxidation. This trial was registered at clinicaltrials.gov as NCT01262326.

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