Skeletal muscle HIF-1 and exercise

Abstract

Regular physical activity prevents and improves a number of disease conditions and reduces the risk for premature death substantially. From a clinical as well as a basic science point of view it is important to create a more fundamental understanding of the molecular mechanisms that contribute to the improved functional capacity induced by regular physical activity. Skeletal muscle tissue exhibits a remarkable ability to adapt to altered demands. Training adaptations include increased capillarisation, altered glycolytic flux and increased mitochondrial density and occur in response to repeated bouts of exercise. Skeletal muscle hypoxia has been addressed as one possible primary stimulus for adaptation to training. Hypoxia Inducible Factor-1 (HIF-1) has been suggested to be a master regulator of hypoxic transcription. Out of the HIF-1 regulated factors, several are known to be affected by exercise or associated with processes important for exercise adaptation e.g. Vascular Endothelial Growth Factor (VEGF), involved in exercise induced angiogenesis. Therefore, the principal aim of the thesis was to investigate a possible activation of and role for HIF-1 in skeletal muscle adaptation to exercise.

Skeletal muscle biopsies were obtained from m. vastus lateralis in three human experimental set-ups; a single bout of exercise (Papers I and III), four weeks of endurance training (V) and elite athletes (V). In addition, two different mouse models were used, skeletal muscle specific HIF-1α KO mice exposed to six weeks of endurance training (IV) and electrically induced contractions in isolated mouse EDL and Soleus muscle (II).

A single bout of exercise activated HIF-1, including protein stabilization, translocation to the nucleus, increased binding to target gene promoters and increased target gene expression (e.g. VEGF and VEGFR1) in both human and mouse skeletal muscle (I-III). However, the HIF-1α KO mice showed several of the features typically associated with a trained muscle, especially in respect of mitochondrial characteristics. This was suggested to at least in part be due to reduced activity of the HIF-1 inducible inhibitor of pyruvate dehydrogenase, pyruvate dehydrogense kinase 1 (PDK1). Recent studies suggest that the acute activation of HIF-1 target genes may be blunted after a period of exercise training.

In Paper V it was hypothesized that a possible training induced moderation of HIF-1 activity would be reflected in skeletal muscle PDK1 levels. The results showed that PDK1 levels were significantly lower in elite athletes compared to moderately active individuals. Furthermore, a possible mechanism for moderating HIF-1 activity in response to endurance training was introduced. HIF-1 stability is known to be regulated by hydroxylation of critical proline residues by prolyl hydroxylase 1-3 (PHD1-3) with consequent proteasomal degradation. PHD2 had the highest expression of the three prolyl hydroxylases in human skeletal muscle and PHD2 levels were increased by training and higher in the elite trained athletes. Evidence of a regulatory link between PHD activity and PDK1 levels in human skeletal muscle was provided by inhibiting PHD activity in human primary myoblasts, resulting in increased PDK1 levels (V).

From the results of Papers I-V it was proposed that an initial activation of HIF-1 may drive angiogenesis adaptation in the early phase of training, but the seeming attenuation of the HIF-1 response later in a training period may represent a switch toward a higher capacity to activate the oxidative system.