Adrenergic Regulation of AMP-activated Protein Kinase in Brown Adipose Tissue in Vivo

AMP-activated protein kinase (AMPK), an evolutionarily conserved serine-threonine kinase that senses cellular energy status, is activated by stress and neurohumoral stimuli. We investigated the mechanisms by which adrenergic signaling alters AMPK activation in vivo. Brown adipose tissue (BAT) is highly enriched in sympathetic innervation, which is critical for regulation of energy homeostasis. We performed unilateral denervation of BAT in wild type (WT) mice to abolish neural input. Six days post-denervation, UCP-1 protein levels and AMPK alpha 2 protein and activity were reduced by 45%. In beta(1,2,3)-adrenergic receptor knock-out mice, unilateral denervation led to a 25-45% decrease in AMPK activity, protein expression, and Thr(172) phosphorylation. In contrast, acute alpha -or beta-adrenergic blockade in WT mice resulted in increased AMPK alpha Thr(172) phosphorylation and AMPK alpha 1 and alpha 2 activity in BAT. But short term blockade of alpha-adrenergic signaling in beta(1,2,3)-adrenergic receptor knock-out mice resulted in decreased AMPK activity in BAT, which strongly correlated with enhanced phosphorylation of AMPK on Ser(485/491), a site associated with inhibition of AMPK activity. Both PKA and AKT inhibitors attenuated AMPK Ser(485/491) phosphorylation resulting from alpha-adrenergic blockade and prevented decreases in AMPK activity. In vitro mechanistic studies in BAT explants showed that the effects of alpha-adrenergic blockade appeared to be secondary to inhibition of oxygen consumption. In conclusion, adrenergic pathways regulate AMPK activity in vivo acutely via alterations in Thr(172) phosphorylation and chronically through changes in the alpha catalytic subunit protein levels. Furthermore, AMPK alpha Ser(485/491) phosphorylation may be a novel mechanism to inhibit AMPK activity in vivo and alter its biological effects.