PGC‐1α gene expression is down‐regulated by Akt‐mediated phosphorylation and nuclear exclusion of FoxO1 in insulin‐stimulated skeletal muscle
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There are multiple binding domains on the promoter region of the peroxisome proliferator activator receptor gamma coactivator-1 alpha (PGC-1alpha) gene, including a trio of insulin responsive elements that are activated by the forkhead box class-O (FoxO1) winged helix transcription factor, which is known to be regulated by acute transforming retrovirus thymoma (Akt). Here we show that in skeletal muscle biopsy specimens from healthy humans and cultured human skeletal myotubes, insulin phosphorylates Akt (Ser473) and FoxO1 (Thr24, Ser256), leading to reduced nuclear abundance of FoxO1 total protein. This is associated with an insulin-mediated repression of the mRNA expression PGC-1alpha and downstream genes associated with oxidative phosphorylation. In contrast, in muscle taken from insulin resistant humans or in palmitate-treated insulin resistant myotubes, neither Akt nor FoxO1 was phosphorylated by insulin, resulting in a failure for nuclear exclusion of FoxO1 total protein, and an inability for insulin to repress the mRNA expression of PGC-1alpha and down-stream genes. To determine whether the regulation of FoxO1 was Akt dependent, we next treated Akt2 -/- and wild-type mice with or without insulin. Insulin phosphorylated Akt and FoxO1 (Thr24, Ser256) resulting in a reduced nuclear expression of FoxO1 total protein in wild-type but not Akt2 -/- skeletal muscle. We conclude that insulin decreases the expression of genes involved in oxidative metabolism in healthy but not insulin resistant muscle, due to a decrease in FoxO1 phosphorylation and nuclear exclusion secondary to reduced Akt activity.
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