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

Barbara B. Kahn,Thomas Pulinilkunnil,Dong Kong,Kenji Asakura,Odile D. Peroni,Huamei He,Anna Lee

doi:10.1074/jbc.m111.218719

Abstract

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 α2 protein and activity were reduced by 45%. In β(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 α- or β-adrenergic blockade in WT mice resulted in increased AMPK α Thr(172) phosphorylation and AMPK α1 and α2 activity in BAT. But short term blockade of α-adrenergic signaling in β(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 α-adrenergic blockade and prevented decreases in AMPK activity. In vitro mechanistic studies in BAT explants showed that the effects of α-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 α catalytic subunit protein levels. Furthermore, AMPK α Ser(485/491) phosphorylation may be a novel mechanism to inhibit AMPK activity in vivo and alter its biological effects.

Highlights

Over the last several years it has become evident that AMPK plays a major role in mediating hormonal, nutrient, and stressrelated signals [1, 2]
Metabolic effects of the sympathetic nervous system are mediated through G-protein-coupled receptors that are broadly classified into two main subtypes: ␣1–2, and ␤1–3-adrenergic receptors [13]
This decline in activity was accompanied by a decrease in AMPK ␣ catalytic subunit Thr172 phosphorylation (Fig. 1C), which appeared to be secondary to a 40% decline in total AMPK ␣2 protein level (Fig. 1C)

Summary

Introduction

Over the last several years it has become evident that AMPK plays a major role in mediating hormonal, nutrient, and stressrelated signals [1, 2]. We studied Ser485/491 phosphorylation because the effects of phosphorylation changes at this site on AMPK activity in vivo have not been definitively reported, and this could be a novel mechanism for physiologic regulation of AMPK activity. In vivo studies demonstrated that acute exercise or epinephrine activates AMPK in rat adipose tissue [18] Because these effects were blunted by a ␤-adrenergic receptor blocker, the activation of AMPK with exercise or epinephrine appeared to be mediated by ␤-adrenergic receptors [18]. Adrenergic regulation of Ser485/491 phosphorylation of the AMPK ␣ subunit involves upstream kinases, PKA and AKT These data provide novel mechanistic insights into how ␣- and ␤-adrenergic signals integrate to regulate AMPK activity and expand our understanding of physiologic regulation of AMPK. Because there is currently a large amount of interest in modulating AMPK activity to treat metabolic diseases and potentially cancer, these new findings could have important therapeutic implications

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Conclusion