Calorie restriction increases the ratio of phosphatidylinositol 3-kinase catalytic to regulatory subunits in rat skeletal muscle

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 with AL muscles, regulatory isoforms, p50[alpha] and p55[alpha] abundance were ~40% lower (P < 0.01) with unchanged p85[alpha]/[beta] 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[alpha] p50[alpha]; p110