Evidence for a Cross-Talk Between Cytosolic 5'-Nucleotidases and AMP-Activated Protein Kinase

The maintenance of the correct balance of nucleotide pools is essential for many vital functions (Bester et al., 2011; Garcia-Gil et al., 2018; Camici et al., 2019). The control of several enzyme activities required for nucleotide metabolism contributes to this homeostasis. Among the involved enzymes, cytosolic 5′-nucleotidases (NT5Cs) play a central role in the regulation of the purine nucleotide pool (Figure 1). The major NT5Cs acting on purine nucleotides are cytosolic 5′- nucleotidase I (NT5C1), which exerts its action mainly in skeletal muscle, and cytosolic 5′- nucleotidase II (NT5C2), which is ubiquitously expressed. The preferred substrate for NT5C1 is AMP, with a KM in the millimolar range (Hunsucker et al., 2001; Tkacz-Stachowska et al., 2005). Although preferring IMP and GMP as substrates (KM in the micromolar range) (Tozzi et al., 2013), NT5C2 catalyses also the hydrolysis of the phosphoester bond of AMP (with a KM in the millimolar range) (Tozzi et al., 2013). The rate of the IMP-GMP cycle (Figure 1) which regulates the intracellular purine nucleotide concentrations, depends on NT5C2 activity (Barsotti et al., 2003). In fact, in the presence of high energy charge, NT5C2 catalyses the catabolism of excess IMP, synthesized by de novo or salvage pathways, while allowing for IMP and AMP accumulation in case of low energy charge (Pesi et al., 1994; Allegrini et al., 2004; Wallden and Nordlund, 2011; Camici et al., 2018). For the regulation of the AMP cycle, both NT5C1 and NT5C2 activities are involved (Figure 1). In the last decades growing evidence indicates the central “energy sensing” role played by the AMP-activated protein kinase (AMPK) (Hardie et al., 2012; Garcia and Shaw, 2017). AMPK is a heterotrimer composed of the catalytic α (α1 or α2), the regulatory β (β1 or β2) and the γ subunits (γ1, γ2 or γ3). Alterations in the AMP:ATP ratio are perceived by the γ subunit of AMPK which contains three AMP binding sites, two of which exchangeable with ATP (Xiao et al., 2007). The binding of AMP further increases the kinase activity of AMPK both allosterically and inhibiting its dephosphorylation (Sanders et al., 2007). The major upstream kinases that activate AMPK by phosphorylation of Thr172 (Hawley et al., 1996), are the tumour suppressor kinase LKB1 (Woods et al., 2003) and the Ca2+/calmodulin-dependent kinase kinase β (Hawley et al., 2005). AMPK is activated when the cellular energy charge is low and, acting on several protein targets, this protein kinase switches off the anabolic pathways that require ATP and switches on the catabolic pathways that produce ATP (Figure 1). AMPK activation brings about an increase in muscular glucose uptake and fatty acid oxidation, making AMPK activators useful tools for the treatment of type 2 diabetes (Coughlan et al., 2014). In addition, AMPK activation may be responsible for some of the tumour suppression functions of LKB1 (Hardie and Alessi, 2013). Since NT5Cs are the major responsible for the regulation of the AMP level (Kulkarni et al., 2011), it is conceivable that alterations in their activities may affect the numerous signaling pathways triggered by AMPK activation, and thus the regulation of biological processes including muscle contraction, functioning of the nervous system, and control of body weight.