Meta-Biol

• Pathways

The AMP-activated protein kinase (AMPK) is a highly conserved heterotrimeric kinase complex composed of a catalytic (alpha) subunit and two regulatory (beta and gamma) subunits. AMPK is activated under conditions of energy stress, when intracellular ATP levels decline and intracellular AMP increases, such as during nutrient deprivation or hypoxia (Hardie 2007). Upon energy stress, AMP directly binds to tandem repeats of cystathionine-beta-synthase (CBS) domains in the AMPK gamma subunit. Binding of AMP is thought to prevent dephosphorylation of the critical activation loop threonine in the alpha subunit (Hardie 2007). The phosphorylation of the activation loop threonine is absolutely required for AMPK activation. Biochemical and genetic analyses in worms, flies, and mice have revealed that the serine/threonine kinase STK11 (LKB1) represents the major kinase phosphorylating the AMPK activation loop under conditions of energy stress across metazoans (Sakamoto et al. 2005, Lee et al. 2007). LKB1 phosphorylates AMPK on Thr174 of the alpha 1 subunit (or Thr172 on the alpha 2 subunit) leading to activation of AMPK if the cellular AMP/ATP ratio is sufficiently high (Hawley et al. 2003, Woods et al. 2003, Shaw et al. 2004). Signals leading to this phosphorylation event can be mediated by exercise, leptin and adiponectin, the hypothalamic-sympathetic nervous system (SNS), and alpha adrenergic receptors, as demonstrated in studies of rat and human skeletal muscle (Minoksohi et al. 2002, Kahn et al. 2005).