Regulation of AMP‐activated protein kinase by LKB1 and CaMKK in adipocytes

AMP‐activated protein kinase (AMPK) is a serine/threonine kinase that regulates cellular and whole body energy homeostasis. In adipose tissue, activation of AMPK has been demonstrated in response to a variety of extracellular stimuli. However, the upstream kinase that activates AMPK in adipocytes remains elusive. Previous studies have identified LKB1 as a major AMPK kinase in muscle, liver, and other tissues. In certain cell types, Ca 2+ /calmodulin‐dependent protein kinase kinase β (CaMKKβ) has been shown to activate AMPK in response to increases of intracellular Ca 2+ levels. Our aim was to investigate if LKB1 and/or CaMKK function as AMPK kinases in adipocytes. We used adipose tissue and isolated adipocytes from mice in which the expression of LKB1 was reduced to 10–20% of that of wild‐type (LKB1 hypomorphic mice). We show that adipocytes from LKB1 hypomorphic mice display a 40% decrease in basal AMPK activity and a decrease of AMPK activity in the presence of the AMPK activator phenformin. We also demonstrate that stimulation of 3T3L1 adipocytes with intracellular [Ca 2+ ]‐raising agents results in an activation of the AMPK pathway. The inhibition of CaMKK isoforms, particularly CaMKKβ, by the inhibitor STO‐609 or by siRNAs, blocked Ca 2+ ‐, but not phenformin‐, AICAR‐, or forskolin‐induced activation of AMPK, indicating that CaMKK activated AMPK in response to Ca 2+ . Collectively, we show that LKB1 is required to maintain normal AMPK‐signaling in non‐stimulated adipocytes and in the presence of phenformin. In addition, we demonstrate the existence of a Ca 2+ /CaMKK signaling pathway that can also regulate the activity of AMPK in adipocytes. J. Cell. Biochem. 112: 1364–1375, 2011. © 2011 Wiley‐Liss, Inc.