AMPK-Mediated Regulation of Alpha-Arrestins and Protein Trafficking

Abstract

The adenosine monophosphate-activated protein kinase (AMPK) plays a central role in the regulation of cellular metabolism. Recent studies reveal a novel role for AMPK in the regulation of glucose and other carbohydrates flux by controlling the endocytosis of transporters. The first step in glucose metabolism is glucose uptake, a process mediated by members of the GLUT/SLC2A (glucose transporters) or HXT (hexose transporters) family of twelve-transmembrane domain glucose transporters in mammals and yeast, respectively. These proteins are conserved from yeast to humans, and multiple transporters-each with distinct kinetic properties-compete for plasma membrane occupancy in order to enhance or limit the rate of glucose uptake. During growth in the presence of alternative carbon sources, glucose transporters are removed and replaced with the appropriate transporter to help support growth in response to this environment. New insights into the regulated protein trafficking of these transporters reveal the requirement for specific α-arrestins, a little-studied class of protein trafficking adaptor. A defining feature of the α-arrestins is that each contains PY-motifs, which can bind to the ubiquitin ligases from the NEDD4/Rsp5 (Neural precursor cell Expressed, Developmentally Down-regulated 4 and Reverses Spt- Phenotype 5, respectively) family. Specific association of α-arrestins with glucose and carbohydrate transporters is thought to bring the ubiquitin ligase in close proximity to its membrane substrate, and thereby allows the membrane cargo to become ubiquitinated. This ubiquitination in turn serves as a mark to stimulate endocytosis. Recent results show that AMPK phosphorylation of the α-arrestins impacts their abundance and/or ability to stimulate carbohydrate transporter endocytosis. Indeed, AMPK or glucose limitation also controls α-arrestin gene expression, adding an additional layer of complexity to this regulation. Here, we review the recent studies that have expanded the role of AMPK in cellular metabolism to include regulation of α-arrestin-mediated trafficking of transporters and show that this mechanism of regulation is conserved over the ~150 million years of evolution that separate yeast from man.

Keywords: 2-deoxyglucose; AMP-activated protein kinase; NEDD4; Saccharomyces cerevisiae; Snf1 kinase; arrestin-domain containing proteins; glucose transporters; ubiquitination; α-arrestins.

Figure 1

Snf1/AMPK-mediated regulation of membrane transporter trafficking in yeast and humans. (a) Hyper-phosphorylation of yeast α-arrestin Rod1 by Snf1 kinase inhibits the ability of Rod1 to promote ubiquitination, endocytosis and degradation of hexose transporters 1 and 3 (Hxt1 and Hxt3) [22]. Hyper-phosphorylation of Rod1 may result in 14-3-3 binding and/or Rod1 degradation. (b) Hyper-phosphorylation of yeast α-arrestin Rod1 by Snf1 kinase sequesters Rod1 in a complex with 14-3-3 proteins and inhibits the ability of Rod1 to promote ubiquitination, endocytosis and degradation of lactate transporter Jen1 [76]. (c) Phosphorylation of human α-arrestin TXNIP promotes its degradation, thereby inhibiting its ability to promote endocytosis and degradation of glucose transporter GLUT1 [15]. The ubiquitin ligase involved in this process has yet to be defined. Grey arrows indicate pathway connections and red dashed arrows indicated protein trafficking events.

Figure 2

Snf1/AMPK-mediated regulation of gene expression in yeast and human. (a) Snf1 kinase-mediated phosphorylation of the yeast transcriptional repressors Mig1 and Mig2 promotes their translocation out of the nucleus (denoted by solid black arrow) leading to derepression of α-arrestin Csr2 [24]. (b) AMPK-mediated phosphorylation of human transcriptional activator ChREBP blocks its ability to induce expression of α-arrestin TXNIP [123]. Upon dephosphorylation, ChREBP-Mlx can translocate into the nucleus to activate expression of TXNIP (denoted by dashed black line).