Submitted on July 27, 2004
Accepted on July 13, 2005
Muscle contractions, AICAR, and insulin cause phosphorylation of an AMPK-related kinase
Jonathan S Fisher1*, Jeong-Sun Ju1, Peter J Oppelt1, Jill L Smith1, Atsushi Suzuki1, and Hiroyasu Esumi1
1 Department of Biology, Saint Louis University, St. Louis, MO, USA
* To whom correspondence should be addressed. E-mail: fisherjs{at}slu.edu.
We hypothesized that ARK5/NUAK1, an AMPK-related kinase that has been found to be stimulated by Akt, would be expressed in rat skeletal muscle and activated by electrically-elicited contractions, AICAR, or insulin. We verified expression of ARK5 in muscle through RT-PCR and western blot. Cross-reactivity of ARK5 immunoprecipitates with antibodies against phospho-AMPK (P-AMPK) was increased by ~30% by muscle contractions and ~60% by incubation of muscle with AICAR. AMPK was not detected in the ARK5 immunoprecipitates. Despite the apparent increase in phosphorylation of ARK5 at a site essential to its activation, neither contractions nor AICAR increased ARK5 activity. For muscles from animals injected with saline or insulin, we probed non-immunoprecipitated samples in sequence for phosphotyrosine (P-Tyr), ARK5, and phosphorylated substrates of Akt (P-AS) and found that the ARK5 band could be precisely superimposed on phosphoprotein bands from the P-Tyr and P-AS blots. In the band corresponding to ARK5, insulin increased P-Tyr content by ~45% and cross-reactivity with the antibody against P-AS by ~3-fold. We also detected ARK5 in phosphotyrosine immunoprecipitates. Our data suggest that increased phosphorylation of ARK5 by muscle contractions or exposure to AICAR is insufficient to activate ARK5 in skeletal muscle, suggesting that some other modification (e.g. phosphorylation on tyrosine or by Akt) may be necessary to its activity in muscle.
