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1 Department of Human Biology and Nutrtitional Sciences, University of Guelph, Guelph, Ontario, Canada
2 Department of Molecular Genetics, Maastricht University, Maastricht, The Netherlands
3 Section of Endocrinology and Metabolism, Wake Forest University School of Medicine & Baptist Medical Center, Winston-Salem, NC, USA
4 Thrombosis Research Laboratory, Otsuka America Pharmaceutical Inc., Rockville, MD, USA
* To whom correspondence should be addressed. E-mail: abonen{at}uoguelph.ca.
Since insulin has been shown to stimulate long chain fatty acid (LCFA) esterification in skeletal muscle and cardiac myocytes, we investigated whether insulin increased the rate of LCFA transport by altering the expression and the subcellular distribution of the fatty acid transporters FAT/CD36 and FABPpm. In cardiac myocytes, insulin very rapidly increased the expression of FAT/CD36 protein in a time- and dose-dependent manner. During a 2 h period, insulin (10 nM) increased cardiac myocyte FAT/CD36 protein by 25% after 60 min, and attained a maximum after 90-120 min (+ 40-50%). There was a dose-dependent relationship between insulin (10-12 to 10-7 M) and FAT/CD36 expression. The half-maximal increase in FAT/CD36 protein occurred at 0.5 x 10-9 M insulin, and the maximal increase occurred at 10-9 M to 10-8 M insulin (+40-50%). There were similar insulin-induced increments in FAT/CD36 protein in cardiac myocytes (+43%) and in Langendorff perfused hearts (+32%). In contrast to FAT/CD36, insulin did not alter the expression of FABPpm protein, either in cardiac myocytes or the perfused heart. By using specific inhibitors of insulin signaling pathways, it was shown that the insulin-induced expression of FAT/CD36 occurred via the PI3K/Akt insulin-signaling pathway. Subcellular fractionation of cardiac myocytes revealed that insulin not only increased the expression of FAT/CD36, but this hormone also targeted some of the FAT/CD36 to the plasma membrane, while concomitantly lowering the intracellular depot of FAT/CD36. At the functional level, the insulin-induced increase in FAT/CD36 protein resulted in an increased rate of palmitate transport into giant vesicles (+34%), which paralleled the increase in plasmalemmal FAT/CD36 (+29%). The present studies have shown that insulin regulates the protein expression of the LCFA transporter FAT/CD36, but not FABPpm, via the PI3K/Akt insulin-signaling pathway.
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