br STAR Methods br AMPK A

STAR★Methods
AMPK: A Therapeutic Target in the β Cell? Loss of pancreatic β cell function is a hallmark of the transition to a diagnosis of T2DM (see Glossary) 1, 2, 3, 4. AMPK activation has gained attention for the treatment of hyperglycemia in prediabetes as an insulin-sensitizing agent because it promotes glucose uptake in muscle and, during feeding, shuts down glucose production in the liver, helping to restore euglycemia [5]. Given that AMPK is also a central intrinsic mediator of the cellular response to reduced nutrient availability, consideration of secondary effects in other tissues of this approach is warranted. We have previously discussed the controversial role of AMPK in glucose-stimulated insulin secretion (GSIS) [6]. Here, we provide an update on the role of the LKB1-AMPK pathway in GSIS and β cell mass regulation, highlighting the expanding role for other AMPK family members in the β cell. For a detailed discussion of AMPK biology in other tissues and as a potential drug target for treating T2DM, we refer the reader to excellent recent reviews 5, 7. Due to its discovery as a tumor suppressor in Peutz–Jeghers syndrome, LKB1, an essential upstream AMPK activator, and, in turn, AMPK, have garnered attention as targets for cancer treatment, and we direct the reader to exciting advances in this area 8, 9, 10.
The αβγs of AMPK: Emerging Roles for the α, β, and γ Subunits AMPK is a heterotrimeric enzyme comprising α (catalytic) and β/γ (regulatory) polypeptides, each of which has multiple isoforms, affording considerable potential complexity to trimer composition and intrinsic biological activity [11]. β Nivolumab express multiple isoforms of the α, β, and γ AMPK heterotrimer subunits, including both AMPKα1 and α2 proteins 12, 13, and their differential expression and localization may contribute to functional differences in AMPKα activity in islets 6, 12, 13, 14. Both AMPKβ1 and β2 are expressed in β cells, with β2 65–70% of the total amount of β cells [15]. AMPKβ carries a carbohydrate-binding module (CBM), which mediates recruitment to glycogen, a comparatively understudied area of AMPK biology. A novel AMPKβ2 inhibitor that allosterically alters the CBM of AMPKβ2 in both AMPKα1 and AMPKα2 complexes also enhanced insulin secretion in mouse islets [15], indicating that AMPK complexes containing β2 serve to inhibit GSIS. A recent study reported the protein interactomes of AMPKα1 and AMPKβ2 generated using extracts from rat INS1 cells [16], and we look forward to the validation of these potential candidates as AMPK targets and/or their regulatory function on AMPK in the β cell. For its part, the AMPKγ subunit contains the adenine nucleotide-binding pocket, thus conferring the AMP, ADP, and ATP-sensitive component of the AMPK trimer. AMPKγ1–3 were recently shown to form a precipitable complex with the proline isomerase PIN1, which shuts down AMPK activity in mouse muscle by reducing AMP-mediated amplification of AMPKα phosphorylation. This association is enhanced in the refed state following a fast, and prevents AMPKα phosphorylation following 2-deoxyglucose treatment, supporting a role for a PIN1–AMPKγ interaction in AMPK sensitivity to nutrient status [17]. PIN1 also interacts with the AMPK family kinase salt-inducible kinase 2 (SIK2), and loss of PIN1 in mouse β cells exacerbates glucose intolerance during obesity as a consequence of reduced β cell mass and impaired Ca2+ signaling [17]. To our knowledge, these observations have not been confirmed in human islets, which is an ongoing challenge for the field.
Regulation of AMPK Activity in β Cells The molecular network of AMPK regulation and function has been studied extensively in metabolic tissues, including adipose tissue, skeletal, muscle, and liver, as comprehensively reviewed elsewhere 7, 18. Briefly, AMPK integrates signals of the availability of energy (nutrients, glucose, and amino acids), hormones, and cellular stress to regulate cell metabolism (anabolism and autophagy), survival (apoptosis), and function. Under high-energy conditions, abundantly available ATP occupies allosteric regulatory sites on the AMPKγ subunit. In this state, AMPK is minimally active. Under conditions of reduced energy availability, two interrelated inputs combine to determine the extent of AMPK activation: allosteric binding of adenine nucleotides at two sites on the AMPKγ subunit, and phosphorylation of the activation loop Thr172 of AMPKα by upstream kinases coupled with impaired dephosphorylation by a cognate phosphatase (see below) 19, 20.