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1 Division of Nutritional Sciences, University of Illinois, Urbana, Illinois, USA
2 Department of Animal Sciences, University of Illinois, Urbana, Illinois, USA
3 Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Evansville, Indiana, USA
4 Division of Nutritional Sciences, University of Illinois, Urbana, Illinois, USA; Department of Animal Sciences, University of Illinois, Urbana, Illinois, USA; Department of Pathology, University of Illinois, Urbana, Illinois, USA
5 Division of Nutritional Sciences, University of Illinois, Urbana, Illinois, USA; Department of Food Science and Human Nutrition, University of Illinois, Urbana, Illinois, USA
* To whom correspondence should be addressed. E-mail: dlayman{at}uiuc.edu.
Leucine is known to stimulate translation initiation of protein synthesis at mTOR in the insulin signaling pathway. However, potential feedback from mTOR to upstream aspects of the insulin signaling pathway remains controversial. This study evaluates the impact of a physiological oral dose of leucine and/or carbohydrate (CHO) on upstream elements of the insulin signaling pathway using PI3-kinase activity and glucose uptake as markers for insulin sensitivity and glucose homeostasis. Rats (~200g) were fasted 12 h and administered oral doses of CHO (1.31 g glucose, 1.31 g sucrose), leucine (leu, 270 mg), or CHO plus leu. Animals were sacrificed at 15, 30, 60 and 90 min after treatment. Plasma and gastrocnemius muscles were collected for analyses. Treatments were designed to produce elevated blood glucose and insulin with basal levels of leucine (CHO); elevated leucine with basal levels of glucose and insulin (leu); or a combined increase of glucose, insulin and leucine (CHO + leu). The CHO treatment stimulated PI3-kinase activity and glucose uptake with no effect on the downstream translation initiation factor eIF4E. Leu alone stimulated the release of the translation initiation factor eIF4E from 4E-BP1 with no effects on PI3-kinase activity or glucose uptake. The CHO + leu treatment reduced the magnitude and duration of the PI3-kinase response but maintained glucose uptake similar to the CHO treatment and eIF4E levels similar to the leu treatment. These findings demonstrate that leucine reduces insulin-stimulated PI3- kinase activity while increasing downstream translation initiation and with no effect on net glucose transport into skeletal muscle.
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