General discussion

A high oxidative capacity is important for the maintenance of health and prevention of disease. Skeletal muscle is highly plastic and a key player for whole body metabolism and its wide dynamic range of the metabolic turnover is well recognized as an important feature of a healthy individual.

This thesis examines the influence of exercise on PGC-1a, a key regulator in the control of oxidative function. Nevertheless, the specific regulation of cell metabolism and the involvement of PGC-1a in aging and metabolic disorders are poorly understood. Exercise has been acknowledged in the same aspects as medicine in the treatment of some diseases, and concepts such as EIM (Exercise Is Medicine) has been established.

The concept that exercise is beneficial for health is nothing new (Berryman 2010). Over 2,500 years ago, a physician called Susruta prescribed exercise as a treatment for his patients.

Documents even show that T2DM was believed to be a curable disease of the urinary tract for which he prescribed a certain diet and exercise. However, according to Susruta it was important that the exercise performed was not vigorous or with high-intensity since this was regarded as a risk factor for developing multiple diseases and potentially leading to death (Tipton 2014). Later, during the Persian empire, military leaders recognized the link between soldiers with good strength and endurance with the success on the battlefield. This lead to rigid training programs which included both strength, endurance and nutritional optimization (Kokkinos n.d.). And as one of the most famous exercise physicians, Hippocrates (460-370 BCE) wrote “eating alone will not keep a man well, he must also take exercise“ (Berryman 2010; Tipton 2014). In this perspective, exercise as prevention and treatment for common diseases such as T2DM is regarded as crucial, and the research field of molecular exercise physiology has evolved tremendously over the last decades. However, there are significant gaps in our understanding of the biology of exercise and its beneficial health effects and skeletal muscle adaptations.

In this thesis, human exercise studies were performed in an attempt to advance the understanding of the regulation of the adaptations that occur following both acute and prolonged exercise training. By having the opportunity to study men with impaired glucose regulation, new insights into the connections between the mitochondria and metabolic control were also achieved. However, when performing human exercise studies, it is important to be

muscle response and adaptations. In addition, differences in genetics and epigenetics, sex hormonal profile, lifestyle choices as well as numerous physiological and psychological factors may all affect the results of the study. This is something that needs to be considered before any general conclusions can be drawn from the data. In study 1, an intra-individual control (non-exercising leg) was used, and in study 3 and 4 a control group was included.

However, in study 2, inter-individual variations and sex differences, might have blunted some of the exercise effects since the number of study subjects was rather small. In study 4, the randomized groups were not fully homogenous, with the control group being more fit and also displaying lower body weight than the intervention groups. This might have influenced some of the comparisons made. And since the control group also was given information about the benefits of exercise it is quite likely that some of the included subjects were more active than before the intervention period.

Human studies provide somewhat limited mechanistic insights, why they sometimes need to be complemented with animal models and cell studies to establish more direct mechanistic relationships, as done in study 1 and 2. The results from animal studies and in vitro experiments are commonly extrapolated to humans and there is not always an agreement between the results from different species (Burkhardt & Zlotnik 2013; Vandamme 2014).

This was the case in study 2, in which the animal model showed sex differences in the magnitude of interaction between BRCA1 and ACC-p, as a result of the exercise stimuli but not in the human biopsy material. The high variability seen in the human samples may be, in part, due to the large degree of genetic variation in BRCA1, which can result in altered protein function and/or expression. The phosphorylated form of ACC interacts with BRCA1 in the C-terminal region, specifically in the BRCT domains. Even though the BRCT domains in BRCA1 are well conserved across species, the BRCT domains in the human BRCA1 gene contain the highest degree of genetic variation (M. S. Lee et al. 2010).

Also, conducting human exercise studies gives access to limited amount of material and usually mRNA analysis is conducted since it is quite straight forward and does not demand high quantities of material. In this thesis, the majority of the studies quantified gene expression changes. Gene expression changes measured as mRNA expression, are not necessarily synonymous with concurrent protein or functional changes. Specific mRNA levels do not always correlate with protein levels, although a high over all correlation has been suggested (Vogel & Marcotte 2012). Usually proteins exhibit a larger dynamic range of concentrations than transcripts do, which might mainly be due to post transcriptional mechanisms as well as a cumulative “stair-case” pattern of a repeatedly activated mRNA and

protein expressoin (Vogel et al. 2010; Perry et al. 2010). And this dynamic range is highly individual within humans regarding its magnitude of response.

In animal models, the whole muscle is used for homogenized instead of a small biopsy as in humans. Therefore, it is also important to remember that the skeletal muscle is not a fully homogenous tissue. Possible differences in fiber type composition, and thereby differences in e.g. oxidative capacity, are present along the muscle length and at various depths (Vogel et al. 2010; Perry et al. 2010). Also, differences in the activation pattern and metabolism during exercise within the skeletal muscle fibers have been observed (Lexell et al. 1985). However, in all the studies conducted in this thesis, the biopsy procedure was highly standardized to minimize these types of errors. Nevertheless, when interpreting results from animal studies in the molecular exercise field, you need to remember that voluntary exercise is not always the case. Some exercise studies in animals apply an exercise stimulus that is more of a “life or death type” of situation along with substantial stress levels, which could highly influence e.g.

the metabolic pathways aimed to be studied. Given these interspecies differences, there are clear advantages with human research. Furthermore, performing exercise studies may lead to increased understanding of skeletal muscle function and metabolism which is useful in an attempt to improve over-all health in humans.