It has been hypothesized that insulin resistance is mediated by a deficiency of mitochondria in skeletal muscle. In keeping with this hypothesis, high-fat diets that cause insulin resistance have been reported to result in a decrease in muscle mitochondria. In contrast, we found that feeding rats high-fat diets that cause muscle insulin resistance results in a concomitant gradual increase in muscle mitochondria. This adaptation appears to be mediated by activation of peroxisome proliferator-activated receptor (PPAR)δ by fatty acids, which results in a gradual, posttranscriptionally regulated increase in PPAR γ coactivator 1α (PGC-1α) protein expression. Similarly, overexpression of PPARδ results in a large increase in PGC-1α protein in the absence of any increase in PGC-1α mRNA. We interpret our findings as evidence that raising free fatty acids results in an increase in mitochondria by activating PPARδ, which mediates a posttranscriptional increase in PGC-1α. Our findings argue against the concept that insulin resistance is mediated by a deficiency of muscle mitochondria.

The transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) has been identified as an inducible regulator of mitochondrial function. Skeletal muscle PGC-1alpha expression is induced post-exercise. Therefore, we sought to determine its role in the regulation of muscle fuel metabolism. Studies were performed using conditional, muscle-specific, PGC-1alpha gain-of-function and constitutive, generalized, loss-of-function mice. Forced expression of PGC-1alpha increased muscle glucose uptake concomitant with augmentation of glycogen stores, a metabolic response similar to post-exercise recovery. Induction of muscle PGC-1alpha expression prevented muscle glycogen depletion during exercise. Conversely, PGC-1alpha-deficient animals exhibited reduced rates of muscle glycogen repletion post-exercise. PGC-1alpha was shown to increase muscle glycogen stores via several mechanisms including stimulation of glucose import, suppression of glycolytic flux, and by down-regulation of the expression of glycogen phosphorylase and its activating kinase, phosphorylase kinase alpha. These findings identify PGC-1alpha as a critical regulator of skeletal muscle fuel stores.

A number of studies have reported that a high-fat diet induces increases in mitochondrial fatty acid oxidation enzymes in muscle. In contrast, in two recent studies raising plasma free fatty acids (FFA) resulted in a decrease in mitochondria. In this work, we reevaluated the effects of raising FFA on muscle mitochondrial biogenesis and capacity for fat oxidation. Rats were fed a high-fat diet and given daily injections of heparin to raise FFA. This treatment induced an increase in mitochondrial biogenesis in muscle, as evidenced by increases in mitochondrial enzymes of the fatty acid oxidation pathway, citrate cycle, and respiratory chain, with an increase in the capacity to oxidize fat, as well as an increase in mitochondrial DNA copy number. Raising FFA also resulted in an increase in binding of peroxisome proliferator-activated receptor (PPAR) δ to the PPAR response element on the carnitine palmitoyltransferase 1 promoter. We interpret our results as evidence that raising FFA induces an increase in mitochondrial biogenesis in muscle by activating PPARδ.

The heat stress response is associated with several beneficial adaptations that promote cell health and survival. Specifically, in vitro and animal investigations suggest that repeated exposures to a mild heat stress (~40°C) elicit positive mitochondrial adaptations in skeletal muscle comparable to those observed with exercise. To assess whether such adaptations translate to human skeletal muscle, we produced local, deep tissue heating of the vastus lateralis via pulsed shortwave diathermy in 20 men and women ( n = 10 men; n = 10 women). Diathermy increased muscle temperature by 3.9°C within 30 min of application. Immediately following a single 2-h heating session, we observed increased phosphorylation of AMP-activated protein kinase and ERK1/2 but not of p38 MAPK or JNK. Following repeated heat exposures (2 h daily for 6 consecutive days), we observed a significant cellular heat stress response, as heat shock protein 70 and 90 increased 45% and 38%, respectively. In addition, peroxisome proliferator-activated receptor gamma, coactivator-1 alpha and mitochondrial electron transport protein complexes I and V expression were increased after heating. These increases were accompanied by augmentation of maximal coupled and uncoupled respiratory capacity, measured via high-resolution respirometry. Our data provide the first evidence that mitochondrial adaptation can be elicited in human skeletal muscle in response to repeated exposures to mild heat stress.NEW & NOTEWORTHY Heat stress has been shown to elicit mitochondrial adaptations in cell culture and animal research. We used pulsed shortwave diathermy to produce deep tissue heating and explore whether beneficial mitochondrial adaptations would translate to human skeletal muscle in vivo. We report, for the first time, positive mitochondrial adaptations in human skeletal muscle following recurrent heat stress. The results of this study have clinical implications for many conditions characterized by diminished skeletal muscle mitochondrial function.

It has been reported that 30% calorie restriction (CR) for 3 mo results in large increases in mitochondrial biogenesis in heart, brain, liver, and adipose tissue, with concomitant increases in respiration and ATP synthesis. We found these results surprising, and performed this study to determine whether 30% CR does induce an increase in mitochondria in heart, brain, liver, adipose tissue, and/or skeletal muscle. To this end, we measured the levels of a range of mitochondrial proteins, and mRNAs. With the exception of long‐chain acyl‐CoA dehydrogenase protein level, which was increased ~60% in adipose tissue, none of the mitochondrial proteins or mRNAs that we measured were increased in rats subjected to 30% CR for 14 wk. There was also no increase in citrate synthase activity. Because it is not possible to have an increase in mitochondria without any increase in key mitochondrial proteins, we conclude that 30% CR does not induce an increase in mitochondria in heart, brain, liver, adipose tissue, or skeletal muscle in laboratory rodents.—Hancock, C. R., Han, D.‐H., Higashida, K., Kim, S. H., Holloszy, J. O. Does calorie restriction induce mito‐chondrial biogenesis? A reevaluation. FASEB J. 25, 785–791 (2011). www.fasebj.org

Skeletal muscle immobilization leads to atrophy, decreased metabolic health and substantial losses in function. Animal models suggest that heat stress can provide protection against atrophy in skeletal muscle. This study investigated the effects of daily heat therapy on human skeletal muscle subjected to immobilization. Muscle biopsies were collected, and MRI images were analyzed from the vastus lateralis of 23 healthy volunteers (11 females, 12 males) before and after either 10 days of immobilization with a daily sham treatment (Imm), or with a targeted daily 2-hr heat treatment using pulsed shortwave diathermy (Imm + H). Diathermy increased intramuscular temperature 4.2 ± 0.29°C (p < 0.0001). As a result, HSP70 and HSP90 increased (p < 0.05) following Imm + H (25 ± 6.6 and 20 ± 7.4%, respectively). Heat treatment prevented the immobilization-induced loss of coupled (-27 ± 5.2% vs. -8 ± 6.0%, p = 0.0041), and uncoupled (-25 ± 7.0% vs. -10 ± 3.9%, p = 0.0302) myofiber respiratory capacity. Likewise, heat treatment prevented the immobilation-induced loss of proteins associated with all 5 mitochondrial respiratory complexes (p < 0.05). Furthermore, decreases in muscle cross-sectional area following Imm were greater than Imm + H at both the level of the whole-muscle (-7.6 ± 0.96% vs. -4.5 ± 1.09%, p = 0.0374) and myofiber (-10.8 ± 1.52% vs. -5.8 ± 1.49%, p = 0.0322). Our findings demonstrate that daily heat treatments, applied during 10 days of immobilization, prevent the loss of mitochondrial function and attenuate atrophy in human skeletal muscle.