Abstract
The metabolic property of skeletal muscle adapts in response to an increased physiological demand by altering substrate utilization and gene expression. The calcium-regulated serine/threonine protein phosphatase calcineurin has been implicated in the transduction of motor neuron signals to alter gene expression programs in skeletal muscle. We utilized transgenic mice that overexpress an activated form of calcineurin in skeletal muscle (MCK-CnA*) to investigate the impact of calcineurin activation on metabolic properties of skeletal muscle. Activation of calcineurin increased glucose incorporation into glycogen and lipid oxidation in skeletal muscle. Activated calcineurin suppressed skeletal muscle glucose oxidation and increased lactate release. The enhancement in lipid oxidation was supported by increased expression of genes for lipid metabolism and mitochondrial oxidative phosphorylation. In a reciprocal fashion, several genes of glycolysis were down-regulated, whereas pyruvate dehydrogenase kinase 4 was markedly induced. This expression pattern was associated with decreased glucose utilization and enhanced glycogen storage. The peroxisome proliferator-activated receptors (PPARs) and PPARgamma coactivator 1alpha (PGC1alpha) are transcription regulators for the expression of metabolic and mitochondrial genes. Consistent with changes in the gene-regulatory program, calcineurin promoted the expression of PPARalpha, PPARdelta, and PPARgamma coactivator 1alpha in skeletal muscle. These results provide evidence that calcineurin-mediated skeletal muscle reprogramming induces the expression of several transcription regulators that coordinate changes in the expression of genes for lipid and glucose metabolism, which in turn alters energy substrate utilization in skeletal muscle.
Highlights
Activity: fast glycolytic fibers that derive energy primarily from glycolysis for sudden rapid movement and slow oxidative fibers that rely mainly on oxidation of energy substrates for sustained recurring activity
Overexpression of peroxisome proliferator-activated receptor ␣ (PPAR␣)3 in skeletal muscle increases the expression of regulatory genes involved in lipid metabolism and mitochondrial oxidative phosphorylation [14]
We investigated the impact of calcineurinmediated skeletal muscle reprogramming on the energy substrate utilization and determined whether calcineurin induces metabolic adaptations via coordinated changes in gene expression
Summary
Transgenic Mice—A line of transgenic mice expressing a constitutively active form of calcineurin [18] driven by skeletal muscle creatine kinase promoter/enhancer was established at the Karolinska Institutet, using MCK-CnA* mice After 60 min of incubation with or without insulin, medium was collected, and the lactate concentration was measured by using a colorimetric lactate assay kit (Biomedical Research Service Center, University at Buffalo) according to the manufacturer’s instructions. Oleate Oxidation—Muscles were trimmed of excess tendon and weighed before preincubation for 40 min in Krebs-Henseleit buffer supplemented with 5 mM HEPES, 3.5% fatty acid-free bovine serum albumin, 5 mM glucose, and 12 nM insulin. Muscles were transferred to vials containing 1 ml of identical medium with the addition of 0.3 mM [1-14C]oleate (0.2 Ci/ml) and incubated for 60 min. Released CO2 was collected for 60 min, and center wells were transferred to vials for liquid scintillation counting after the addition of 47 l of 5 M HCl. Results are expressed as nmol of oxidized oleate/g of wet weight/h.
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