[Effect of metformin on gene expression of phosphoenolpyruvate carboxykinase]

To investigate the effect and mechanism of antihyperglycemic agent metformin on the gene expression of phosphoenolpyruvate carboxykinase (PEPCK)-a key enzyme within the regulation of gluconeogenesis in hepatocytes and to determine whether the effects of metformin on hepatocytes are transmitted throughout the known insulin signaling pathways.

Confluent H4IIE rat heptoma cells were cultured with metformin 0.1 mmol/L for 16 h and then stimulated with various agents: insulin, cyclic adenosine monophosphate (cAMP), dexamethasone, signaling transduction inhibitor wortmannin and UO126. The gene expression of PEPCK was examined by Northern blot analysis.

Metformin significantly decreased basal PEPCK mRNA levels by 75% (P < 0.01). Incubation with metformin alone had no significant effect on cAMP/dexamethasone stimulated PEPCK gene expression. In contrast, insulin 0.1 nmol/L significantly inhibited cAMP/dexamethasone stimulated PEPCK gene expression by 67% (P < 0.01), when insulin 0.1 nmol/L was present together with metformin, cAMP/dexamethasone induced PEPCK gene expression was decreased by 94% (P < 0.01). Both insulin signaling pathway inhibitors, phosphatidyl-inositol-3-kinase (PI3K) inhibitor wortmannin and mitogen-activated protein kinase (MAPK) inhibitor UO126, had no significant effect on inhibited PEPCK mRNA expression by metformin, but wortmannin blocked significantly inhibitory regulation of insulin on PEPCK gene expression (P < 0.01).

Metformin can inhibit the PEPCK gene expression via either an insulin-independent or interaction with insulin manner. The inhibitory effects of insulin on PEPCK gene expression are mediated through PI3K pathway, not MAPK pathway.