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    • The AMP Activated Protein Kinase AMPK has evolved

    2024-04-24

    The 5′AMP-Activated Protein Kinase (AMPK) has evolved as an important cellular sensor of reduced energy status that can subsequently phosphorylate its target proteins, slowing the rates of key biosynthetic processes and promoting energy producing pathways; for these reasons AMPK has been proposed as a therapeutic target for metabolic diseases (Day et al., 2017; Garcia and Shaw, 2017; Ross et al., 2016). AMPK was initially identified as an enzymatic activity capable of AMP-activated phosphorylation and inhibition of two rate limiting enzymes in fatty Tolvaptan and cholesterol metabolism: Acetyl-CoA Carboxylase (ACC) and HMG-CoA Reductase (HMGCR) (Carling et al., 1987; Yeh et al., 1980). This remarkable evolutionary solution to incorporate energetic regulation to key metabolic pathways has subsequently been shown to have been applied to many more pathways, an ever expanding catalog of AMPK substrates that sit at key regulatory steps in many important pathways, including the master regulator of lipogenic transcription sterol regulatory element binding proteins (SREBP) and the mTOR regulatory proteins TSC2 and Raptor (Gwinn et al., 2008; Inoki et al., 2003; Li et al., 2011). The understanding of the role AMPK plays in the liver during metabolic disease states has evolved during the last decade of intense research on the kinase. Initial work using indirect pharmacological activators of AMPK (i.e. AICAR and metformin) suggested AMPK was capable of dominantly suppressing gluconeogenesis and glucose production (Vincent et al., 1996; Zhou et al., 2001); however more recent work using genetic models of AMPK deficiency or direct pharmacological activators of AMPK indicated that AMPK is not an important regulator of this pathway (Cokorinos et al., 2017; Foretz et al., 2010; Fullerton et al., 2013; Salatto et al., 2017). In contrast to gluconeogenesis, the role of AMPK in regulating fatty acid metabolism has been established with many studies using genetic models of AMPK deficiency alone and in combination with pharmacological activators of AMPK showing that AMPK is important for inhibiting lipogenesis, effects which appear to be largely mediated through inhibitory phosphorylation of ACC (Dzamko et al., 2010; Fullerton et al., 2013; Woods et al., 2017). Importantly, genetic models have established that these changes in AMPK activity and ACC phosphorylation are also important for regulating whole body glucose homeostasis and insulin sensitivity (Fullerton et al., 2013; Woods et al., 2017). AMPK activation has also been implicated in non-hepatocyte cell types in ways that could benefit the metabolic syndrome, in particular in the setting of NAFLD and NASH. During the transition from simple fatty liver to NASH the non-hepatocyte liver cells drive inflammatory signaling and culminate in damaging fibrosis that results in loss of liver function, with both infiltrating and resident inflammatory cells driving this phenotype (Nati et al., 2016). Importantly, genetic removal of AMPKβ1 in hematopoietic cells in vivo increases liver macrophage infiltration and inflammation in obese mice fed a high-fat diet while pharmacological activation of macrophages in vitro using the direct AMPK β1 specific activator A769662 suppresses inflammation (Galic et al., 2011). Consistent with these findings, multiple reports using indirect activators of AMPK have implicated AMPK as a suppressor of inflammatory and fibrotic processes in pre-clinical NASH models, although most fail to demonstrate clearly the mechanisms for this benefit (Chen et al., 2017; Lee et al., 2016; Li et al., 2014; Lim et al., 2012; Smith et al., 2016). Additionally, AMPK has been implicated in inhibiting adipose tissue lipolysis in some (Bourron et al., 2010; Daval et al., 2005; Kim et al., 2016) but not all (Mottillo et al., 2016; Yin et al., 2003) studies as well as inducing the browning of white fat (Mottillo et al., 2016), which collectively may serve to reduce hepatic lipids in a cell non- autonomous manner. However, much of this work has used indirect activators of AMPK to evaluate both the site and mechanism by which activation of AMPK could be beneficial as a therapeutic intervention. Therefore, more selective chemical tools are necessary for the rational and precise drug discovery in this pathway that would be necessary for the development of a safe and effective therapeutic.