Mechanism of fat-induced hepatic gluconeogenesis: effect of metformin

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Abstract
High-fat feeding has been shown to cause hepatic insulin resistance. The aims of this study were to investigate the biochemical steps responsible for enhanced gluconeogenesis as a result of increased dietary fat intake and the site or sites at which the antihyperglycemic agent metformin acts to inhibit this process. Male Hooded Wistar rats were fed either a standard chow diet (5% fat by weight) or a high-fat diet (60% fat by weight) for 14 days with or without metformin. Total endogenous glucose production and gluconeogenesis were determined using [6-3H]glucose and [U-14C]alanine, respectively. Gluconeogenic enzyme activity and, where appropriate, protein and mRNA levels were measured in liver tissues. The high-fat diet increased endogenous glucose production (21.9 ± 4.4 vs. 32.2 ± 4.8 μmol · kg−1 · min−1,P < 0.05) and alanine gluconeogenesis (4.5 ± 0.9 vs. 9.6 ± 1.9 μmol · kg−1 · min−1,P < 0.05). Metformin reduced both endogenous glucose production (32.2 ± 4.8 vs. 16.1 ± 2.1 μmol · kg−1 · min−1,P < 0.05) and alanine gluconeogenesis (9.6 ± 1.9 vs. 4.7 ± 0.8 μmol · kg−1 · min−1,P < 0.05) after high-fat feeding. These changes were reflected in liver fructose-1,6-bisphosphatase protein levels (4.5 ± 0.9 vs. 9.6 ± 1.9 arbitrary units, P < 0.05 chow vs. high-fat feeding; 9.5 ± 1.9 vs. 4.7 ± 0.8 arbitrary units, P < 0.05 high fat fed in the absence vs. presence of metformin) but not in changes to the activity of other gluconeogenic enzymes. There was a significant positive correlation between alanine gluconeogenesis and fructose-1,6-bisphosphatase protein levels (r = 0.56, P < 0.05). Therefore, excess supply of dietary fat stimulates alanine gluconeogenesis via an increase in fructose-1,6-bisphosphatase protein levels. Metformin predominantly inhibits alanine gluconeogenesis by preventing the fat-induced changes in fructose-1,6-bisphosphatase levels.