AMP-activated protein kinase (AMPK) regulates cellular energy metabolism by phosphorylation of key enzymes in metabolic pathways, such as acetyl–coenzyme A carboxylase (ACC) (1) or mTOR (mammalian target of rapamycin) (2), by modulating their activities, and by regulating the activity of transcription factors (3) and transcriptional cofactors (4). Its functions have been extensively studied in muscles and liver. AMPK stimulates pathways, which increase energy production (glucose transport, fatty acid oxidation) and switches off pathways, which consume energy (lipogenesis, protein synthesis, gluconeogenesis). Activation of AMPK in adipose tissue can be achieved through situations such as fasting and exercise. Adiponectin and leptin as well as hypoglycaemic drugs are activators of adipose tissue AMPK. This activation probably involves changes in the AMP/ATP ratio, and the upstream kinase LKB1 (5). These results have led to the concept that AMPK has an interesting pharmaceutical potential in situations of insulin resistance (type 2 diabetes) and it is indeed the target of existing drugs, e.g. Metformin and hormones which improve insulin sensitivity.
Mammalian AMPK is a trimeric enzyme comprising a catalytic ? subunit (63 kDa) and non-catalytic ? and ? subunits (6, 7). Multiple isoforms of each mammalian enzyme exist (?1, ?2, ?1, ?2, ?1, ?2, ?3), each encoded by a different gene. The ? subunits have a calculated molecular mass of 30 kDa, but migrate on SDS-PAGE with apparent masses of 38 kDa (?1) and 34 kDa (?2), whereas the three ? isoforms have molecular masses of 37 kDa (?1), 63 kDa (?2) and 55 kDa (?3) (8). While AMPK complexes containing ?2 (the first catalytic isoform to be cloned) (9) predominate in skeletal and cardiac muscle (10), approximately equal levels of ?1 and ?2 complexes are present in the liver (11). In contrast, pancreatic islet ?-cells largely express ?1 complex activity (12, 13).