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1Department of Nutritional Sciences, University of Wisconsin-Madison, Madison, Wisconsin; 2Department of Nutrition, Dietetics and Food Science, Brigham Young University, Provo, Utah; and 3Division of Kinesiology, University of Michigan, Ann Arbor, Michigan
Submitted 30 November 2004 ; accepted in final form 20 December 2004
Calorie restriction [CR; 60% of ad libitum (AL) intake] improves insulin-stimulated glucose transport, concomitant with enhanced phosphorylation of Akt. The mechanism(s) for the CR-induced increase in Akt phosphorylation of insulin-stimulated muscle is unknown. The purpose of this study was to determine whether CR increased the ratio of catalytic to regulatory subunits favoring enhanced phosphatidylinositol (PI) 3-kinase signaling, which may contribute to increases in Akt phosphorylation and glucose transport in insulin-stimulated muscles. We measured the PI 3-kinase regulatory (p85
/
, p50, and p55) and catalytic (p110) subunits abundance in skeletal muscle from male F344B/N rats after 8 wk of AL or CR treatment. In CR compared with AL muscles, regulatory isoforms, p50 and p55 abundance were 
40% lower (P < 0.01) with unchanged p85/ levels. There was no diet-related change in catalytic subunit abundance. Despite lower IRS-1 levels (35%) for CR vs. AL, IRS-1-p110 association in insulin-stimulated muscles was significantly (P < 0.05) enhanced by 50%. Downstream of PI 3-kinase, CR compared with AL significantly enhanced Akt serine phosphorylation by 1.5-fold higher (P = 0.01) and 3-O-methylglucose transport by 20% in muscles incubated with insulin. The increased ratio of PI 3-kinase catalytic to regulatory subunits favors enhanced insulin signaling, which likely contributes to greater Akt phosphorylation and improved insulin sensitivity associated with CR in skeletal muscle.
insulin sensitivity; p85 subunits; p55; p50; p110
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