Jessica Norrbom

Human skeletal muscle fibre type variations correlate with PPARα, PPARδ and PGC-1α mRNA

Aims:  Studies from genetically modified animals have been instrumental in highlighting genes and their products involved in the regulation of muscle fibre type and oxidative phenotypes; however, evidence in humans is limited. Our aim was therefore to investigate expression of those genes implicated in the regulation of oxidative fibre phenotypes in humans.

Endurance exercise activates matrix metalloproteinases in human skeletal muscle

In the present study, the effect of exercise training on the expression and activity of matrix metalloproteinases (MMPs) in the human skeletal muscle was investigated. Ten subjects exercised one leg for 45 min with restricted blood flow and then exercised the other leg at the same absolute workload with unrestricted blood flow. The exercises were conducted four times per week for 5 wk. Biopsies were taken from the vastus lateralis muscles of both legs at rest before the training period, after 10 days and 5 wk of training, and 2 h after the first exercise bout for analysis of MMP and tissue inhibitor of metalloproteinase-1 (TIMP-1) mRNA, enzyme activity, and protein expression. Levels of MMP-2, MMP-14, and TIMP-1 mRNA in muscle tissue increased after 10 days of training regardless of blood flow condition. MMP-2 mRNA level in laser-dissected myofibers and MMP-2 activity in whole muscle increased with training. The level of MMP-9 mRNA and activity increased after the first bout of exercise. Although MMP-9 mRNA levels appeared to be very low, the activity of MMP-9 after a single bout of exercise was similar to that of MMP-2 after 10 days of exercise. MMP-2 and MMP-9 protein was both present throughout the extracellular matrix of the muscle, both around fibers and capillaries, but MMP-2 was also present within the skeletal muscle fibers. These results show that MMPs are activated in skeletal muscle in nonpathological conditions such as voluntary exercise. The expression and time pattern indicate differences between the MMPs in regards of production sites as well as in the regulating mechanism.

The truncated splice variants, NT‐PGC‐1α and PGC‐1α4, increase with both endurance and resistance exercise in human skeletal muscle

Recently, a truncated peroxisome proliferator‐activated receptor gamma coactivator‐1 alpha (PGC‐1α) splice variant, PGC‐1α4, that originates from the alternative promoter was shown to be induced by resistance exercise and to elicit muscle hypertrophy without coactivation of “classical” PGC‐1α targets involved in mitochondrial biogenesis and angiogenesis. In order to test if distinct physiological adaptations are characterized by divergent induction of PGC‐1α splice variants, we investigated the expression of truncated and nontruncated PGC‐1α splice variants and PGC‐1α transcripts originating from the alternative and the proximal promoter, in human skeletal muscle in response to endurance and resistance exercise. Both total PGC‐1α and truncated PGC‐1α mRNA expression were increased 2 h after endurance (P < 0.01) and resistance exercise (P < 0.01), with greater increases after endurance exercise (P < 0.05). Expression of nontruncated PGC‐1α increased significantly in both exercise groups (P < 0.01 for both groups) without any significant differences between the groups. Both endurance and resistance exercise induced truncated as well as nontruncated PGC‐1α transcripts from both the alternative and the proximal promoter. Further challenging the hypothesis that induction of distinct PGC‐1α splice variants controls exercise adaptation, both nontruncated and truncated PGC‐1α transcripts were induced in AICAR‐treated human myotubes (P < 0.05). Thus, contrary to our hypothesis, resistance exercise did not specifically induce the truncated forms of PGC‐1α. Induction of truncated PGC‐1α splice variants does not appear to underlie distinct adaptations to resistance versus endurance exercise. Further studies on the existence of numerous splice variants originating from different promoters are needed.

Alternative splice variant PGC-1α-b is strongly induced by exercise in human skeletal muscle.

The present study investigated whether exercise induces the expression of PGC-1α splice variants in human skeletal muscle and the possible influence of metabolic perturbation on this response. The subjects exercised one leg for 45 min with restricted blood flow (R-leg), followed by 45 min of exercise using the other leg at the same absolute workload but with normal blood flow (NR-leg). This ischemic model (R-leg) has been shown previously to induce a greater metabolic perturbation and enhance the expression of PGC-1α beyond that observed in the NR-leg. Cultured human myotubes were used to test suggested exercise-induced regulatory stimuli of PGC-1α. We showed, for the first time, that transcripts from both the canonical promoter (PGC-1α-a) and the proposed upstream-located promoter (PGC-1α-b) are present in human skeletal muscle. Both transcripts were upregulated after exercise in the R-leg, but the fold change increase of PGC-1α-b was much greater than that of PGC-1α-a. No differences were observed between the two conditions regarding the marker for calcineurin activation, MCIP1, or p38 phosphorylation. AMPK phosphorylation increased to a greater extent in the R-leg, and AICAR stimulation of cultured human myotubes induced the expression of PGC-1α-a and PGC-1α-b. AICAR combined with norepinephrine yielded an additive effect on the PGC-1α-b expression only. Our results indicate clearly that exercise can activate an upstream promoter in humans and support AMPK as a major regulator of transcripts from the canonical PGC-1α promoter and the involvement of β-adrenergic stimulation in combination with AMPK in the regulation of PGC-1α-b.

Training response of mitochondrial transcription factors in human skeletal muscle

Aim:  Mitochondrial function is essential for physical performance and health. Aerobic fitness is positively associated with mitochondrial (mt) biogenesis in muscle cells through partly unknown regulatory mechanisms. The present study aimed to investigate the influence of exercise and training status on key mt transcription factors in relation to oxidative capacity in human skeletal muscle.

Truncated splice variant PGC-1α4 is not associated with exercise-induced human muscle hypertrophy.

A truncated PGC‐1α splice variant (PGC‐1α4) has been implicated in the regulation of resistance exercise (RE)‐induced muscle hypertrophy, and basal expression levels said to be augmented in response to concurrent aerobic (AE) and RE training.

Rapidly elevated levels of PGC-1α-b protein in human skeletal muscle after exercise: exploring regulatory factors in a randomized controlled trial

Individuals with high skeletal muscle mitochondrial content have a lower risk to acquire cardiovascular and metabolic disease, obesity, and type II diabetes. Regular endurance training increases mitochondrial density through a complex network of transcriptional regulators that in an accumulated way are affected by each single exercise bout. The aim of the present study was to investigate the effect of a single exercise bout on the levels of PGC-1α and related regulatory factors important for the initial phase of skeletal muscle adaptation. Ten men and ten women were randomized to either an exercise group (60 min cycling at a work load corresponding to 70% of peak oxygen uptake) or a nonexercising control group. Skeletal muscle biopsies were taken before, at 30 min, and at 2, 6, and 24 h after the intervention. Twenty-two mRNA transcripts and five proteins were measured. With exercise, protein levels of PGC-1α-ex1b increased, and this elevation occurred before that of total PGC-1α protein. We also demonstrated the existence and postexercise expression pattern of two LIPIN-1 (LIPIN-1α and LIPIN-1β) and three NCoR1 (NCoR1-1, NCoR1-2, and NCoR1-3) isoforms in human skeletal muscle. The present study contributes new insights into the initial signaling events following a single bout of exercise and emphasizes PGC-1α-ex1b as the most exercise-responsive PGC-1α isoform.

BRCA1 is a Novel Regulator of Metabolic Function in Skeletal Muscle.

Breast cancer type 1 (BRCA1) susceptibility protein is expressed across multiple tissues including skeletal muscle. The overall objective of this investigation was to define a functional role for BRCA1 in skeletal muscle using a translational approach. For the first time in both mice and humans, we identified the presence of multiple isoforms of BRCA1 in skeletal muscle. In response to an acute bout of exercise, we found increases in the interaction between the native forms of BRCA1 and the phosphorylated form of acetyl-CoA carboxylase. Decreasing BRCA1 content using a shRNA approach in cultured primary human myotubes resulted in decreased oxygen consumption by the mitochondria and increased reactive oxygen species production. The decreased BRCA1 content also resulted in increased storage of intracellular lipid and reduced insulin signaling. These results indicate that BRCA1 plays a critical role in the regulation of metabolic function in skeletal muscle. Collectively, these data reveal BRCA1 as a novel target to consider in our understanding of metabolic function and risk for development of metabolic-based diseases.

Modelling in vivo creatine/phosphocreatine in vitro reveals divergent adaptations in human muscle mitochondrial respiratory control by ADP after acute and chronic exercise.

Mitochondrial respiratory sensitivity to ADP is thought to influence muscle fitness and is partly regulated by cytosolic–mitochondrial diffusion of ADP or phosphate shuttling via creatine/phosphocreatine (Cr/PCr) through mitochondrial creatine kinase (mtCK). Previous measurements of respiration in vitro with Cr (saturate mtCK) or without (ADP/ATP diffusion) show mixed responses of ADP sensitivity following acute exercise vs. less sensitivity after chronic exercise. In human muscle, modelling in vivo ‘exercising’ [Cr:PCr] during in vitro assessments revealed novel responses to exercise that differ from detections with or without Cr (±Cr). Acute exercise increased ADP sensitivity when measured without Cr but had no effect ±Cr or with +Cr:PCr, whereas chronic exercise increased sensitivity ±Cr but lowered sensitivity with +Cr:PCr despite increased markers of mitochondrial oxidative capacity. Controlling in vivo conditions during in vitro respiratory assessments reveals responses to exercise that differ from typical ±Cr comparisons and challenges our understanding of how exercise improves metabolic control in human muscle.

Humanin skeletal muscle protein levels increase after resistance training in men with impaired glucose metabolism

Humanin (HN) is a mitochondrially encoded and secreted peptide linked to glucose metabolism and tissue protecting mechanisms. Whether skeletal muscle HN gene or protein expression is influenced by exercise remains unknown. In this intervention study we show, for the first time, that HN protein levels increase in human skeletal muscle following 12 weeks of resistance training in persons with prediabetes. Male subjects (n = 55) with impaired glucose regulation (IGR) were recruited and randomly assigned to resistance training, Nordic walking or a control group. The exercise interventions were performed three times per week for 12 weeks with progressively increased intensity during the intervention period. Biopsies from the vastus lateralis muscle and venous blood samples were taken before and after the intervention. Skeletal muscle and serum protein levels of HN were analyzed as well as skeletal muscle gene expression of the mitochondrially encoded gene MT‐RNR2, containing the open reading frame for HN. To elucidate mitochondrial training adaptation, mtDNA, and nuclear DNA as well as Citrate synthase were measured. Skeletal muscle HN protein levels increased by 35% after 12 weeks of resistance training. No change in humanin protein levels was seen in serum in any of the intervention groups. There was a significant correlation between humanin levels in serum and the improvements in the 2 h glucose loading test in the resistance training group. The increase in HN protein levels in skeletal muscle after regular resistance training in prediabetic males may suggest a role for HN in the regulation of glucose metabolism. Given the preventative effect of exercise on diabetes type 2, the role of HN as a mitochondrially derived peptide and an exercise‐responsive mitokine warrants further investigation.