Leptin promotes fatty acid oxidation in skeletal muscle by activating α2 AMP-kinase (AMPK), directly at the muscle level and via the hypothalamic-sympathetic nerve. We examined the role of AMPK upstream kinase such as LKB1, Atm, and Ca2+/calmodulin-dependent protein kinase kinase β (CaMKKβ) in leptin-induced AMPK activation in C2C12 muscle cell line that expresses leptin receptor Ob-Rb. Leptin produced a biphasic activation of α2 but not α1 AMPK in C2C12 cells with an early peak by 1 hr and a late peak by 12-24 hr. Antisense RNA of Atm abolished the 1st peak, while RNAi of CaMKKβ suppressed the late peak. RNAi of LKB1 decreased neither the 1st nor 2nd peak. Consistently, leptin induced expression of CaMKKβ and transcription factor Nur77 from 6 to 24 hr. Nur77 increased CaMKKβ expression via unique NGF1-B (Nur77)-response element. Leptin stimulated Nur77 expression by PI3-kinase (PI3K)/PKCζ pathway, promoting JAK2/IRS1/PI3K complex. Furthermore, leptin increased intracellular Ca2+ from 6 to 24 hr through PI3K/PLCγ- and Ca2+-channel-activation. Nur77 and CaMKKβ expression decreased in skeletal muscle of leptin-deficient mice (ob/ob). In sum, leptin activates α2 AMPK in C2C12 muscle cells through two distinct mechanisms: the early activation is mediated by Atm and the late activation is due to PI3K/Nur77/CaMKKβ. Significant decrease of Nur77 and CaMKKβ expression in ob/ob mice suggests that the signaling pathway as well as Atm is important for leptin-induced AMPK activation in skeletal muscle. [J Physiol Sci. 2006;56 Suppl:S215]
