Mammalian muscles present a wide range of contractile and metabolic properties resulting from the relative proportions of oxidative (type I) and glycolytic (type II) fibres making up a given muscle. Besides these innate properties, muscles are also highly plastic, responding to environmental change. In particular, oxygen availability has a strong influence on muscle metabolism. When oxygen concentration drops (hypoxia), proteins such as the hypoxia inducible factor-1 (HIF-1) are activated to orchestrate a switch to glycolytic (anaerobic) metabolism in an effort to spare oxygen. Over longer periods of hypoxia, HIF-1 also promotes increased vascularisation via angiogenic factors such as the vascular endothelial growth factor (VEGF) to improve oxygen delivery to the hypoxic tissue. The HIF-1 protein is composed of two components; HIF-1α is primarily regulated by oxygen availability, while the other subunit is constitutively expressed. When the oxygen concentration is normal, HIF-1α protein tends to be targeted for degradation, lowering HIF-1 activity. However, under hypoxic conditions HIF-1α is stabilized, allowing the HIF-1 dimer to act on several metabolic pathways to help the cell cope with low oxygen levels. Although the role of HIF-1 under hypoxia has been widely studied, relatively little is known about the function of this protein under normoxic conditions in resting muscle. Furthermore it is unclear whether this factor is differentially regulated in muscles with different metabolic phenotypes.
Remi Mounier, Bente Pedersen and Peter Plomgaard from the University of Copenhagen, Denmark, investigated these two questions by looking at different muscle fibre types in humans. Specifically, the team looked at gene and protein expression profiles in resting muscles, and hypothesized that primarily glycolytic muscles would present higher expression levels of HIF-1 and associated factors than oxidative muscles.
First, Mounier and colleagues selected three skeletal muscles, the oxidative soleus muscle (mostly type I fibres), the mixed vastus lateralis muscle (type I and type II fibres) and the glycolytic triceps brachii (mostly type II fibres) muscles. Using quantitative real-time PCR, the team established that HIF-1α mRNA expression was higher in glycolytic and mixed muscles than in the oxidative soleus muscle, as predicted by their hypothesis. In contrast, the mRNA expression of VEGF, a gene regulated by HIF-1, did not show any fibre type-specific pattern, confirming that HIF-1α mRNA levels do not necessarily reflect HIF-1 activity. So the team measured HIF-1α protein amounts in the muscles to see whether they correlated with the gene expression patterns. Contrary to their expectations, HIF-1α protein levels were threefold lower in glycolytic muscles than in mixed and oxidative muscles. In addition, VEGF protein levels were lowest in glycolytic muscles, intermediate in mixed muscles and highest in oxidative muscles.
The results of this study strongly suggest that different muscle types regulate the HIF pathway at different levels, and further investigation will be necessary to unveil the mechanisms responsible for this tissue-specific regulation. Furthermore, these results also confirm the presence of HIF-1 under normal oxygen conditions, and thus suggest that this factor may also play an important role in regulating skeletal muscle homeostasis even under normoxia.
