Thyroid Hormone Activates Adenosine 5′-Monophosphate-Activated Protein Kinase via Intracellular Calcium Mobilization and Activation of Calcium/Calmodulin-Dependent Protein Kinase Kinase-β

AMP-activated protein kinase (AMPK) is a key regulator of glucose and fatty acid homeostasis. In muscle cells, AMPK stimulates mitochondrial fatty acid oxidation and ATP production. The thyroid hormone T3 increases cellular oxygen consumption and is considered to be a major regulator of mitochondrial activities. In this study, we examined the possible involvement of AMPK in the stimulatory action of T3 on mitochondria. Treatment of C2C12 myoblasts with T3 rapidly led to phosphorylation of AMPK. Acetyl-coenzyme A carboxylase, a direct target of AMPK, was also phosphorylated after T3 treatment. Similar results were obtained with 3T3-L1, FRTL-5, and HeLa cells. Stable expression of T3 receptor (TR)-α or TRβ in Neuro2a cells enhanced this effect of T3, indicating the involvement of TRs. Because HeLa cells express only Ca2+/calmodulin-dependent protein kinase kinase-β (CaMKKβ), one of two known AMPK kinases, it was suggested that the effect of T3 is mediated by CaMKKβ. Indeed, experiments using a CaMKK inhibit...