Previous studies have shown that administration of fibroblast growth factor-19 (FGF-19) reverses diabetes, hepatic steatosis, hyperlipidemia, and adipose accretion in animal models of obesity. To investigate the mechanism for this effect, we determined whether FGF-19 modulated hepatic fatty acid synthesis, a key process controlling glucose tolerance and triacylglycerol accumulation in liver, blood, and adipose tissue. Metabolic syndrome is a state of metabolic dysregulation that is characterized by obesity, hepatic steatosis, hyperlipidemia, atherosclerosis, and glucose intolerance (1). A key mechanism contributing to the development of metabolic syndrome is an elevation in the rate of hepatic fatty acid synthesis (2, 3). Hepatic fatty acid synthesis drives the synthesis of triacylglycerols that accumulate in the liver, blood, and adipose tissue. An elevation in hepatic fatty acid synthesis also promotes glucose intolerance, as accumulation of fatty acid metabolites in the liver suppresses the ability of insulin activate glycogen synthesis and inhibit gluconeogenesis (3). Accordingly, one approach to treating metabolic syndrome has been to manipulate the activity of signal transduction pathways that modulate hepatic fatty acid synthesis. For example, the beneficial effect of metformin on glucose tolerance in diabetic animals is mediated by a decrease in the rate of hepatic fatty acid synthesis (4). Metformin suppresses fatty acid synthesis by inhibiting the activity of acetyl-CoA carboxylase-␣ (ACC␣) 2 and decreasing the expression of sterol regulatory element-binding protein-1c (SREBP-1c), a key transcriptional activator of lipogenic genes. Metformin also increases the rate of hepatic fatty acid oxidation, an effect that contributes to the improvement in glucose tolerance. Metformin-induced changes in hepatic fatty acid synthesis and fatty acid oxidation are mediated by an activation of AMP-activated protein kinase (AMPK). As metformin administration causes undesirable side effects, the identification of new signaling pathways that modulate hepatic fatty acid metabolism may lead to the development of more effective therapies for treating metabolic syndrome.Fibroblast growth factor-19 (FGF-19) was originally identified as a signal promoting the development of the inner ear in chick embryos (5). Subsequent studies have shown that FGF-19 and its mouse ortholog, FGF-15, also function in adult animals. For example, FGF-19/FGF-15 expressed in the small intestine acts as an enterohepatic hormone, mediating the inhibitory effects of intestinal bile acids on expression of hepatic cholesterol 7␣-hydroxylase (CYP7A1), a key regulatory step in the bile acid synthesis pathway (6, 7). FGF-19 also regulates carbohydrate and lipid metabolism in adult animals. Administration of recombinant human FGF-19 or transgenic expression of the human FGF-19 gene in obese/diabetic mice causes an increase in energy expenditure and a decrease in adipose tissue stores (8, 9). Treatment of obese/diabetic mice with FGF-19 also reduces serum and...

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We previously mapped a type 2 diabetes (T2D) locus on chromosome 16 (Chr 16) in an F2 intercross from the BTBR T (+) tf (BTBR) Lepob/ob and C57BL/6 (B6) Lepob/ob mouse strains. Introgression of BTBR Chr 16 into B6 mice resulted in a consomic mouse with reduced fasting plasma insulin and elevated glucose levels. We derived a panel of sub-congenic mice and narrowed the diabetes susceptibility locus to a 1.6 Mb region. Introgression of this 1.6 Mb fragment of the BTBR Chr 16 into lean B6 mice (B6.16BT36–38) replicated the phenotypes of the consomic mice. Pancreatic islets from the B6.16BT36–38 mice were defective in the second phase of the insulin secretion, suggesting that the 1.6 Mb region encodes a regulator of insulin secretion. Within this region, syntaxin-binding protein 5-like (Stxbp5l) or tomosyn-2 was the only gene with an expression difference and a non-synonymous coding single nucleotide polymorphism (SNP) between the B6 and BTBR alleles. Overexpression of the b-tomosyn-2 isoform in the pancreatic β-cell line, INS1 (832/13), resulted in an inhibition of insulin secretion in response to 3 mM 8-bromo cAMP at 7 mM glucose. In vitro binding experiments showed that tomosyn-2 binds recombinant syntaxin-1A and syntaxin-4, key proteins that are involved in insulin secretion via formation of the SNARE complex. The B6 form of tomosyn-2 is more susceptible to proteasomal degradation than the BTBR form, establishing a functional role for the coding SNP in tomosyn-2. We conclude that tomosyn-2 is the major gene responsible for the T2D Chr 16 quantitative trait locus (QTL) we mapped in our mouse cross. Our findings suggest that tomosyn-2 is a key negative regulator of insulin secretion.

We previously demonstrated that micro-RNAs (miRNAs) 132 and 212 are differentially upregulated in response to obesity in two mouse strains that differ in their susceptibility to obesity-induced diabetes. Here we show the overexpression of miRNAs 132 and 212 enhances insulin secretion (IS) in response to glucose and other secretagogues including nonfuel stimuli. We determined that carnitine acyl-carnitine translocase (CACT; Slc25a20) is a direct target of these miRNAs. CACT is responsible for transporting long-chain acyl-carnitines into the mitochondria for β-oxidation. Small interfering RNA–mediated knockdown of CACT in β-cells led to the accumulation of fatty acyl-carnitines and enhanced IS. The addition of long-chain fatty acyl-carnitines promoted IS from rat insulinoma β-cells (INS-1) as well as primary mouse islets. The effect on INS-1 cells was augmented in response to suppression of CACT. A nonhydrolyzable ether analog of palmitoyl-carnitine stimulated IS, showing that β-oxidation of palmitoyl-carnitine is not required for its stimulation of IS. These studies establish a link between miRNA-dependent regulation of CACT and fatty acyl-carnitine–mediated regulation of IS.

Autophagy, an integral part of the waste recycling process, plays an important role in cellular physiology and pathophysiology. Impaired autophagic flux causes ectopic lipid deposition, which is defined as the accumulation of lipids in non-adipose tissue. Ectopic lipid accumulation is observed in patients with cardiometabolic syndrome, including obesity, diabetes, insulin resistance, and cardiovascular complications. Metformin is the first line of treatment for type 2 diabetes, and one of the underlying mechanisms for the anti-diabetic effect of metformin is mediated by the stimulation of AMP-activated protein kinase (AMPK). Because the activation of AMPK is crucial for the initiation of autophagy, we hypothesize that metformin reduces the accumulation of lipid droplets by increasing autophagic flux in vascular endothelial cells. Incubation of vascular endothelial cells with saturated fatty acid (SFA) increased the accumulation of lipid droplets and impaired autophagic flux. We observed that the accumulation of lipid droplets was reduced, and the autophagic flux was enhanced by treatment with metformin. The knock-down of AMPKα by using siRNA blunted the effect of metformin. Furthermore, treatment with SFA or inhibition of autophagy increased leukocyte adhesion, whereas treatment with metformin decreased the SfA-induced leukocyte adhesion. The results suggest a novel mechanism by which metformin protects vascular endothelium from SFA-induced ectopic lipid accumulation and pro-inflammatory responses. In conclusion, improving autophagic flux may be a therapeutic strategy to protect endothelial function from dyslipidemia and diabetic complications. Abbreviations SFA Saturated fatty acid T2DM Type 2 diabetes mellitus AMPK AMP-activated protein kinase LC3 Microtubule-associated protein 1A/1B-light chain 3 ER Endoplasmic reticulum eNOS Endothelial nitric oxide synthase ULK1 Unc51-like kinase 1 CPT1 Carnitine palmitoyltransferase 1 LD Lipid droplet AICAR 5-Aminoimidazole-4-carboxamide 1-β-d-ribofuranoside, Acadesine, N 1-(β-d-ribofuranosyl)-5aminoimidazole-4-carboxamide