Metformin induces PGC-1α expression and selectively affects hepatic PGC-1α functions

The objective of this study was to determine how the AMPK activating antidiabetic drug metformin affects the major activator of hepatic gluconeogenesis, PPARγ coactivator 1α (PGC-1α) and liver functions regulated by PGC-1α.

Mouse and human primary hepatocytes and mice in vivo were treated with metformin. Adenoviral overexpression, siRNA and reporter gene constructs were used for mechanistic studies.

Metformin increased PGC-1α mRNA and protein expression in mouse primary hepatocytes. 5-Aminoimidazole-4-carboxamide ribonucleotide (AICAR) (another AMPK activator) had the opposite effect. Metformin also increased PGC-1α in human primary hepatocytes; this effect of metformin was abolished by AMPK inhibitor compound C and sirtuin 1 siRNA. AMPK overexpression by AMPK-Ad also increased PGC-1α. Whereas metformin increased PGC-1α, it down-regulated gluconeogenic genes phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase). Furthermore, metformin attenuated the increase in PEPCK and G6Pase mRNAs induced by PGC-1α overexpression, but did not affect PGC-1α-mediated induction of mitochondrial genes. Metformin down-regulated several key transcription factors that mediate the effect of PGC-1α on gluconeogenic genes including Krüppel-like factor 15, forkhead box protein O1 and hepatocyte NF 4α, whereas it increased nuclear respiratory factor 1, which is involved in PGC-1α-mediated regulation of mitochondrial proteins.

Down-regulation of PGC-1α is not necessary for suppression of gluconeogenic genes by metformin. Importantly, metformin selectively affects hepatic PGC-1α-mediated gene regulation and prevents activation of gluconeogenesis, but does not influence its regulation of mitochondrial genes. These results identify selective modulation of hepatic PGC-1α functions as a novel mechanism involved in the therapeutic action of metformin.