Endocrine Abstracts

Regulation of hepatic gluconeogenesis by insulin and glucagon is central to blood glucose homeostasis. It has been proposed that the members of AMP-activated protein kinase (AMPK)-related kinases, the salt inducible kinase (SIK) isoforms, may play a role as signalling mediator in the control of insulin- and glucagon-regulated hepatic gluconeogenesis. However, the exact regulation and contribution of SIKs in hepatic gluconeogenesis is largely elusive. Here we employed selective pan SIK inhibitors (HG-9-91-01 and KIN112) to investigate the role of SIKs in hepatic gluconeogenesis. Both of these inhibitors showed high selectivity against SIKs among other AMPK-related kinases in vitro. SIK inhibitor treatment of mouse primary hepatocytes showed robust dose- and time-dependent increase in gluconeogenic gene expression (PEPCK, G6Pase) and glucose production. These effects of SIK inhibitors on gluconeogenesis were validated as SIK-specific using multiple approaches including inhibitor-resistant SIK mutants, LKB1-null hepatocytes (cells where SIK activity was already ablated) and also AMPKα1α2-null hepatocytes. In addition, investigation into hormonal-regulation of SIK2 reveale that SIK2 was phosphorylated on multiple residues in response to glucagon and fasting but not following insulin or refeeding in primary hepatocytes and in vivo liver, respectively. Unexpectedly, fasting- or glucagon-stimulated phosphorylation of SIK2 did not modulate SIK2 kinase activity and its subcellular localisation. Thus, these experiments have proposed a novel insight that SIKs function as ‘molecular gatekeeper’ to suppress hepatic gluconeogenesis in an insulin-independent mechanism and thus contribute significantly towards maintenance of glucose homeostasis.