Glucagon regulates ACC activity in adipocytes through the CAMKK β/AMPK pathway

Glucagon is important for regulating lipid metabolism in part through its inhibition of fatty acid synthesis in adipocytes. Acetyl-CoA carboxylase 1 (ACC1) is the rate-limiting enzyme for fatty acid synthesis. Glucagon has been proposed to activate cAMP-dependent protein kinase A (PKA), which phosphorylates ACC1 to attenuate the lipogenic activity of ACC1. Because AMP-activated protein kinase (AMPK) also inhibits fatty acid synthesis by phosphorylation of ACC1, we examined the involvement of AMPK and its upstream kinase in the glucagon-elicited signaling in adipocytes in vitro and in vivo. LC-MS-MS analysis suggested that ACC1 was phosphorylated only at Ser ⁷⁹, an AMPK-specific site, in glucagon-treated adipocytes. Pharmacological inhibitors and siRNA knockdown of AMPK or PKA in adipocytes demonstrate that glucagon regulates ACC1 and ACC2 activity through AMPK but not PKA. By using Ca ²⁺/calmodulin-dependent protein kinase kinase-β knockout (CaMKK β ^-/-) mice and cultured adipocytes, we further show that glucagon activates the CaMKK β / AMPK/ACC cascade. Additionally, fasting increases the phosphorylation of AMPK and ACC in CaMKK β ^+/+ but not CaMKK β ^-/- mice. These results indicate that CaMKKf/AMPK signaling is an important molecular component in regulating lipid metabolism in adipocytes responding to glucagon and could be a therapeutic target for the dysregulation of energy storage.