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Articles by A. Bonen
Total Records ( 3 ) for A. Bonen
  H Alkhateeb , A Chabowski , J. F. C Glatz , B Gurd , J. J. F. P Luiken and A. Bonen

We examined whether AICAR or leptin rapidly rescued skeletal muscle insulin resistance via increased palmitate oxidation, reductions in intramuscular lipids, and/or restoration of insulin-stimulated AS60 phosphorylation. Incubation with palmitate (2 mM, 0–18 h) induced insulin resistance in soleus muscle. From 12–18 h, palmitate was removed or AICAR or leptin was provided while 2 mM palmitate was maintained. Palmitate oxidation, intramuscular triacylglycerol, diacylglycerol, ceramide, AMPK phosphorylation, basal and insulin-stimulated glucose transport, plasmalemmal GLUT4, and Akt and AS160 phosphorylation were examined at 0, 6, 12, and 18 h. Palmitate treatment (12 h) increased intramuscular lipids (triacylglycerol +54%, diacylglycerol +11%, total ceramide +18%, C16:0 ceramide +60%) and AMPK phosphorylation (+118%), whereas it reduced fatty acid oxidation (–60%) and insulin-stimulated glucose transport (–70%), GLUT4 translocation (–50%), and AS160 phosphorylation (–40%). Palmitate removal did not rescue insulin resistance or associated parameters. The AICAR and leptin treatments did not consistently reduce intramuscular lipids, but they did rescue palmitate oxidation and insulin-stimulated glucose transport, GLUT4 translocation, and AS160 phosphorylation. Increased AMPK phosphorylation was associated with these improvements only when AICAR and leptin were present. Hence, across all experiments, AMPK phosphorylation did not correlate with any parameters. In contrast, palmitate oxidation and insulin-stimulated AS160 phosphorylation were highly correlated (r = 0.83). We speculate that AICAR and leptin activate both of these processes concomitantly, involving activation of unknown kinases in addition to AMPK. In conclusion, despite the maintenance of high concentrations of palmitate (2 mM), as well as increased concentrations of intramuscular lipids (triacylglycerol, diacylglycerol, and ceramide), the rapid AICAR- and leptin-mediated rescue of palmitate-induced insulin resistance is attributable to the restoration of insulin-stimulated AS160 phosphorylation and GLUT4 translocation.

  V. A Lira , C. R Benton , Z Yan and A. Bonen

The peroxisome proliferator-activated receptor- (PPAR) coactivator-1 (PGC-1) is a major regulator of exercise-induced phenotypic adaptation and substrate utilization. We provide an overview of 1) the role of PGC-1 in exercise-mediated muscle adaptation and 2) the possible insulin-sensitizing role of PGC-1. To these ends, the following questions are addressed. 1) How is PGC-1 regulated, 2) what adaptations are indeed dependent on PGC-1 action, 3) is PGC-1 altered in insulin resistance, and 4) are PGC-1-knockout and -transgenic mice suitable models for examining therapeutic potential of this coactivator? In skeletal muscle, an orchestrated signaling network, including Ca2+-dependent pathways, reactive oxygen species (ROS), nitric oxide (NO), AMP-dependent protein kinase (AMPK), and p38 MAPK, is involved in the control of contractile protein expression, angiogenesis, mitochondrial biogenesis, and other adaptations. However, the p38 MAPK/PGC-1 regulatory axis has been confirmed to be required for exercise-induced angiogenesis and mitochondrial biogenesis but not for fiber type transformation. With respect to a potential insulin-sensitizing role of PGC-1, human studies on type 2 diabetes suggest that PGC-1 and its target genes are only modestly downregulated (≤34%). However, studies in PGC-1-knockout or PGC-1-transgenic mice have provided unexpected anomalies, which appear to suggest that PGC-1 does not have an insulin-sensitizing role. In contrast, a modest (~25%) upregulation of PGC-1, within physiological limits, does improve mitochondrial biogenesis, fatty acid oxidation, and insulin sensitivity in healthy and insulin-resistant skeletal muscle. Taken altogether, there is substantial evidence that the p38 MAPK-PGC-1 regulatory axis is critical for exercise-induced metabolic adaptations in skeletal muscle, and strategies that upregulate PGC-1, within physiological limits, have revealed its insulin-sensitizing effects.

  J. G Nickerson , H Alkhateeb , C. R Benton , J Lally , J Nickerson , X. X Han , M. H Wilson , S. S Jain , L. A Snook , J. F. C Glatz , A Chabowski , J. J. F. P Luiken and A. Bonen

In selected mammalian tissues, long chain fatty acid transporters (FABPpm, FAT/CD36, FATP1, and FATP4) are co-expressed. There is controversy as to whether they all function as membrane-bound transporters and whether they channel fatty acids to oxidation and/or esterification. Among skeletal muscles, the protein expression of FABPpm, FAT/CD36, and FATP4, but not FATP1, correlated highly with the capacities for oxidative metabolism (r ≥ 0.94), fatty acid oxidation (r ≥ 0.88), and triacylglycerol esterification (r ≥ 0.87). We overexpressed independently FABPpm, FAT/CD36, FATP1, and FATP4, within a normal physiologic range, in rat skeletal muscle, to determine the effects on fatty acid transport and metabolism. Independent overexpression of each fatty acid transporter occurred without altering either the expression or plasmalemmal content of other fatty acid transporters. All transporters increased fatty acid transport, but FAT/CD36 and FATP4 were 2.3- and 1.7-fold more effective than FABPpm and FATP1, respectively. Fatty acid transporters failed to alter the rates of fatty acid esterification into triacylglycerols. In contrast, all transporters increased the rates of long chain fatty acid oxidation, but the effects of FABPpm and FAT/CD36 were 3-fold greater than for FATP1 and FATP4. Thus, fatty acid transporters exhibit different capacities for fatty acid transport and metabolism. In vivo, FAT/CD36 and FATP4 are the most effective fatty acid transporters, whereas FABPpm and FAT/CD36 are key for stimulating fatty acid oxidation.

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