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Articles in PresS, published online ahead of print November 19, 2002
Am J Physiol Endocrinol Metab, 10.1152/ajpendo.00318.2002
Submitted on July 16, 2002
Accepted on November 14, 2002
1 Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada
2 Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada
* To whom correspondence should be addressed. E-mail: ddyck{at}uoguelph.ca.
We have previously reported that chronic leptin administration (2 wks) increases fatty acid (FA) oxidation and triacylglycerol (TG) hydrolysis in rodent soleus muscle. Acute stimulation of AMP-activated protein kinase (AMPK) results in a repartitioning of FA towards oxidation and away from esterification in rodent soleus muscle, and has recently been shown to be responsible, at least in part, for the acute stimulatory effect of leptin on FA oxidation. Therefore, we hypothesized that the effects of chronic leptin treatment on muscle FA metabolism are mediated in part through an increased expression and/or activation of AMPK, and a subsequent phosphorylation of acetyl CoA carboxylase and decrease in malonyl CoA content. Female Sprague-Dawley rats were infused for 2 wks with leptin (LEPT; 0.5 mg/kg/d) using subcutaneously implanted mini-osmotic pumps. Control (CONT) and pair-fed (PF) animals received saline-filled implants. Leptin levels were elevated ~4-fold (p<0.001) in treated animals, relative to controls. Chronic leptin treatment resulted in a ~2 to 3-fold greater protein expression of AMPK catalytic (
2) and regulatory (
2) units, as well as a 1.5 to 2-fold increase in Thr172 phosphorylation of AMPK, in both soleus and white gastrocnemius muscles. The increased expression/phosphorylation of AMPK was not due to an altered energy status of the muscle. Correspondingly, there was also a 1.5 to 2-fold increase in acetyl CoA carboxylase (ACC) phosphorylation following leptin treatment in soleus and white gastrocnemius. In spite of the measured increase in ACC phosphorylation following leptin treatment, we were unable to detect a decrease in resting malonyl CoA content in either muscle. However, taken as a whole, our data support recent evidence in rodent muscle that leptin stimulates FA oxidation through stimulation of AMPK, and a subsequent down-regulation of ACC activity.
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