AMP-activated protein kinase regulation and biological actions in the heart

Abstract

AMP-activated protein kinase (AMPK) is a stress-activated kinase that functions as a cellular fuel gauge and master metabolic regulator. Recent investigation has elucidated novel molecular mechanisms of AMPK regulation and important biological actions of the AMPK pathway that are highly relevant to cardiovascular disease. Activation of the intrinsic AMPK pathway plays an important role in the myocardial response to ischemia, pressure overload, and heart failure. Pharmacological activation of AMPK shows promise as a therapeutic strategy in the treatment of heart disease. The purpose of this review is to assess how recent discoveries have extended and in some cases challenged existing paradigms, providing new insights into the regulation of AMPK, its diverse biological actions, and therapeutic potential in the heart.

Figure 1. Molecular structure of the AMPK complex

The AMPK complex is composed of a catalytic subunit (α, blue) and two regulatory subunits (β, green and γ, red). The α subunit contains a serine-threonine kinase domain (KD), which is highly activated by phosphorylation of the Thr¹⁷² residue in the catalytic cleft by upstream kinases (LKB1, CAMKKβ). AMPK is maintained in an unphosphorylated inactive state by the interaction of kinase domain with an autoinhibitory domain (AID) and with the myristoylated N-terminus of the regulatory β subunit. AMP interaction with the nucleotide binding sites in the γ-subunit induces a conformational change in the heterotrimeric complex via an α hook domain, that relieves the autoinhibition by the AID and promotes phosphorylation of the Thr¹⁷² residue. The catalytic cleft of activated AMPK is in a closed conformation, which protects phosphorylated Thr¹⁷² from being dephosphorylated by protein phosphatases (PPase).

Figure 2. Regulators of AMPK activity

AMPK activity is modulated by several physiologic, pathologic and pharmacologic factors. Factors that are known to regulate heart AMPK are highlighted in bold, while inhibitors of heart AMPK activity are designated by asterisks and interventions that have been shown only in non-cardiac cells are in italics.

Figure 3. AMPK regulation of heart glucose and fatty acid metabolism

AMPK regulates substrate transporters and the concentrations of allosteric regulators of glycolysis and fatty acid oxidation. Molecules that are phosphorylated by AMPK are designated by the symbol “P”. AMPK phosphorylates the Rab GTPase AS160, which induces GLUT4 glucose transporter translocation to the sarcolemma. AMPK phosphorylates phospho-fructokinase 2 (PFK2), leading to the synthesis of fructose-2,6-bisphosphate, an allosteric activator of PFK1 and glycolysis. AMPK promotes lipoprotein lipase (LPL) translocation from the myocyte sarcolemma to the luminal surface of capillary endothelial cells, where it catalyzes the release of long-chain fatty acids (LCFA) from triglyceride-containing lipoproteins for heart substrate metabolism. Activated AMPK also stimulates CD36 translocation to the sarcolemma increasing cardiomyocyte free fatty acid uptake. AMPK also phosphorylates and inactivates acetyl-coenzyme A carboxylase (ACC) decreasing the concentration of the fatty acid oxidation inhibitor malonyl-CoA. Proteins activated by AMPK are highlighted in red and proteins inactivated by AMPK in grey. (Illustration credit: Ben Smith).

Figure 4. Activated AMPK inhibits protein synthesis and increases authophagy

Proteins phosphorylated by AMPK and designated with the symbol “P”. AMPK phosphorylates eukaryotic elongation factor (eEF)-2 kinase, which inhibits eEF-2 activity. AMPK also phosphorylates tuberous sclerosis complex (TSC)-2, which increases the GTP-ase activity of the TSC1-TSC2 complex. AMPK also phosphorylates raptor, which removes it from the mammalian target of rapamycin complex (mTORC) 1 complex. Coordinated TSC2 and raptor phosphorylation result in inactivation of mTOR signaling, which inhibits the function of mTOR in the activation of protein synthesis and inhibition of autophagy. AMPK also phosphorylates Unc-51-like kinase 1 (Ulk1) in the ULK1-Atg13-FIP200 complex to directly promote autophagy. Direct phosphorylation of eEF2 has been shown in the heart, while the other pathways were demonstrated in non-cardiac cells.

Figure 5. Mutations in the human γ2 subunit of AMPK

Schematic diagram of the PRKAG2 gene product γ2 isoform highlights the nucleotide binding domains (Bateman domains) and lists mutations with clinical expression curated in the OMIM database (http://www.omim.org/). All mutations cause cardiac hypertrophy and those that cause Wolff-Parkinson-White syndrome are highlighted in bold.