Metformin is considered to be one of the most effective therapeutics for the treatment of type 2 diabetes (T2D) since it specifically reduces hepatic gluconeogenesis without increasing insulin secretion, inducing weight gain, or posing a risk of hypoglycemia1,2. For over half a century, this agent has been prescribed to T2D patients worldwide, yet the underlying mechanism by which metformin inhibits hepatic gluconeogenesis remains unknown. Here we show that metformin non-competitively inhibits the redox shuttle enzyme mitochondrial glycerophosphate dehydrogenase (mGPD), resulting in an altered hepatocellular redox state, reduced conversion of lactate and glycerol to glucose, and decreased hepatic gluconeogenesis. Acute and chronic low-dose metformin treatment effectively reduced endogenous glucose production (EGP), while increasing cytosolic redox and decreasing mitochondrial redox states. Antisense oligonucleotide (ASO) knockdown of hepatic mGPD in rats resulted in a phenotype akin to chronic metformin treatment, and abrogated metformin-mediated increases in cytosolic redox state, decrease in plasma glucose concentrations and inhibition of EGP. These findings were replicated in whole-body mGPD knockout mice. These results have significant implications for understanding the mechanism of metformin’s blood glucose lowering effects and provide a novel therapeutic target for T2D.

BACKGROUND-Nonalcoholic fatty liver disease is associated with hepatic insulin resistance and type 2 diabetes mellitus. Whether this association has a genetic basis is unknown.

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Nonalcoholic fatty liver disease (NAFLD) is a major contributing factor to hepatic insulin resistance in type 2 diabetes. Diacylglycerol acyltransferase (Dgat), of which there are two isoforms (Dgat1 and Dgat2), catalyzes the final step in triglyceride synthesis. We evaluated the metabolic impact of pharmacological reduction of DGAT1 and -2 expression in liver and fat using antisense oligonucleotides (ASOs) in rats with diet-induced NAFLD. Dgat1 and Dgat2 ASO treatment selectively reduced DGAT1 and DGAT2 mRNA levels in liver and fat, but only Dgat2 ASO treatment significantly reduced hepatic lipids (diacylglycerol and triglyceride but not long chain acyl CoAs) and improved hepatic insulin sensitivity. Because Dgat catalyzes triglyceride synthesis from diacylglycerol, and because we have hypothesized that diacylglycerol accumulation triggers fat-induced hepatic insulin resistance through protein kinase C⑀ activation, we next sought to understand the paradoxical reduction in diacylglycerol in Dgat2 ASO-treated rats. Within 3 days of starting Dgat2 ASO therapy in high fat-fed rats, plasma fatty acids increased, whereas hepatic lysophosphatidic acid and diacylglycerol levels were similar to those of control rats. These changes were associated with reduced expression of lipogenic genes (SREBP1c, ACC1, SCD1, and mtGPAT) and increased expression of oxidative/thermogenic genes (CPT1 and UCP2). Taken together, these data suggest that knocking down Dgat2 protects against fat-induced hepatic insulin resistance by paradoxically lowering hepatic diacylglycerol content and protein kinase C⑀ activation through decreased SREBP1c-mediated lipogenesis and increased hepatic fatty acid oxidation. Nonalcoholic fatty liver disease (NAFLD)5 is the most frequent cause of abnormal liver function tests in the United States (estimated prevalence of 14 -20%) (1, 2). It is caused by triglyceride (TG) accumulation within the liver and is strongly associated with insulin resistance, type 2 diabetes mellitus (T2DM), and the metabolic syndrome (3, 4). Accumulating evidence suggests that hepatic lipid accumulation causes hepatic insulin resistance. For example, increasing hepatic lipid stores in mice by overexpressing lipoprotein lipase in the liver (5) and in rats by short term high fat feeding (6) results in liver-specific fat accumulation and hepatic insulin resistance. Several strategies have been employed to reduce hepatic steatosis in rodents; these include treatment with a mitochondrial uncoupling agent (2,4-dinitrophenol) (6), antisense oligonucleotide inhibition of acetyl-coenzyme A carboxylase I and II (7), adenoviral overexpression of malonyl-CoA decarboxylase (8), and transgenic overexpression of uncoupling protein 1 (9), all of which successfully ameliorated hepatic insulin resistance. We have also shown that moderate weight loss in patients with T2DM lowers liver triglycerides and specifically improves hepatic insulin sensitivity (10). Although the above data strongly suggest that hepatic lipid accumulation causes hepatic insulin r...

Nonalcoholic fatty liver disease (NAFLD) is a major factor in the pathogenesis of type 2 diabetes (T2D) and nonalcoholic steatohepatitis (NASH). The mitochondrial protonophore 2,4 dinitrophenol (DNP) has beneficial effects on NAFLD, insulin resistance, and obesity in preclinical models but is too toxic for clinical use. We developed a controlled-release oral formulation of DNP, called CRMP (controlled-release mitochondrial protonophore), that produces mild hepatic mitochondrial uncoupling. In rat models, CRMP reduced hypertriglyceridemia, insulin resistance, hepatic steatosis, and diabetes. It also normalized plasma transaminase concentrations, ameliorated liver fibrosis, and improved hepatic protein synthetic function in a methionine/choline–deficient rat model of NASH. Chronic treatment with CRMP was not associated with any systemic toxicity. These data offer proof of concept that mild hepatic mitochondrial uncoupling may be a safe and effective therapy for the related epidemics of metabolic syndrome, T2D, and NASH.

Summary Leptin reverses hyperglycemia in T1D though reductions in HPA-mediated lipolysis resulting in reduced hepatic gluconeogenesis through both substrate delivery and allosteric mechanisms.

VT, Shulman GI. A high-fat, ketogenic diet causes hepatic insulin resistance in mice, despite increasing energy expenditure and preventing weight gain. Am J Physiol Endocrinol Metab 299: E808 -E815, 2010. First published August 31, 2010; doi:10.1152/ajpendo.00361.2010.-Low-carbohydrate, high-fat ketogenic diets (KD) have been suggested to be more effective in promoting weight loss than conventional caloric restriction, whereas their effect on hepatic glucose and lipid metabolism and the mechanisms by which they may promote weight loss remain controversial. The aim of this study was to explore the role of KD on liver and muscle insulin sensitivity, hepatic lipid metabolism, energy expenditure, and food intake. Using hyperinsulinemic-euglycemic clamps, we studied insulin action in mice fed a KD or regular chow (RC). Body composition was assessed by 1 H magnetic resonance spectroscopy. Despite being 15% lighter (P Ͻ 0.001) than RC-fed mice because of a 17% increase in energy expenditure (P Ͻ 0.001), KD-fed mice manifested severe hepatic insulin resistance, as reflected by decreased suppression (0% vs. 100% in RC-fed mice, P Ͻ 0.01) of endogenous glucose production during the clamp. Hepatic insulin resistance could be attributed to a 350% increase in hepatic diacylglycerol content (P Ͻ 0.001), resulting in increased activation of PKCε (P Ͻ 0.05) and decreased insulin receptor substrate-2 tyrosine phosphorylation (P Ͻ 0.01). Food intake was 56% (P Ͻ 0.001) lower in KD-fed mice, despite similar caloric intake, and could partly be attributed to a more than threefold increase (P Ͻ 0.05) in plasma N-acylphosphatidylethanolamine concentrations. In conclusion, despite preventing weight gain in mice, KD induces hepatic insulin resistance secondary to increased hepatic diacylglycerol content. Given the key role of nonalcoholic fatty liver disease in the development of type 2 diabetes and the widespread use of KD for the treatment of obesity, these results may have potentially important clinical implications. nonalcoholic fatty liver disease; weight loss; diacylglycerol; ceramide; protein kinase Cε; FGF21 OBESITY IS RISING WORLDWIDE at an alarming rate (27). The clinical consequences of obesity are numerous and include nonalcoholic fatty liver disease (NAFLD) and type 2 diabetes. Although caloric restriction promotes weight loss, long-term compliance with most diets is poor (7). In this regard, ketogenic diets (KD) have been suggested to be more efficacious than conventional caloric restriction by promoting a reduction in hunger (1).Although some human studies have found KD to be effective in short-term weight loss (16,43,45), the effect of KD on glucose metabolism remains controversial (5,8,23,30,37). In most of these studies, insulin sensitivity was assessed by nonspecific indexes, such as the homeostasis model assessment of insulin resistance (HOMA-IR) (40) and the quantitative insulin sensitivity check index (QUICKI) (24), both of which rely on fasting plasma glucose and insulin concentrations. Recent studies in mice have...

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