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Am J Physiol Endocrinol Metab (December 21, 2004). doi:10.1152/ajpendo.00566.2004
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Submitted on November 30, 2004
Accepted on December 20, 2004

Calorie Restriction Increases the Ratio of Phosphatidylinositol 3-Kinase Catalytic to Regulatory Subunits in Rat Skeletal Muscle

Carrie E. McCurdy1, Robert T. Davidson2, and Gregory D. Cartee3*

1 Department of Nutritional Sciences, University of Wisconsin-Madison, Madison, WI, USA
2 Department of Nutrition, Dietetics and Food Science, Brigham Young University, Provo, UT, USA
3 Division of Kinesiology, University of Michigan, Ann Arbor, MI, USA

* To whom correspondence should be addressed. E-mail: gcartee{at}umich.edu.

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{alpha}/{beta}, p50{alpha} and p55{alpha}) and catalytic (p110) subunits abundance in skeletal muscle from male F344B/N rats after 8 wk of AL or CR treatment. In CR compared to AL muscles, regulatory isoforms, p50{alpha} and p55{alpha} abundance was ~40% lower (P<0.01) with unchanged p85{alpha}/{beta} levels. There was no diet-related change in catalytic subunit abundance. Despite lower IRS1 levels (~35%) for CR vs. AL, IRS1-p110 association in insulin-stimulated muscles was significantly (P<0.05) enhanced by ~1.5-fold. Downstream of PI 3- kinase, CR compared to 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.




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