Androgens and the molecular regulation of skeletal muscle mass

Muscle atrophy or the unintended loss of skeletal muscle mass can occur as a consequence of pathological disorders e.g. muscular dystrophies, chronic diseases (cachexia), malnutrition, immobilisation (disuse), as well as from normal ageing (sarcopenia). As skeletal muscles play vital roles in maintaining body posture, controlling movement and regulating whole body protein metabolism, muscle atrophy can have debilitating consequences for the patient, often leading to a reduced quality of life. The prevention or therapy of skeletal muscle atrophy has been a topic of interest for many years and its applications are spreading from sport sciences to ageing and clinical medicine. Physical exercise, whether or not combined with nutritional supplements, is an effective and safe countermeasure to attenuate or prevent muscle wasting. However, this approach is not always feasible in seriously ill patients, elderly people or those who have suffered severe injuries. Therefore, a thorough understanding of the etiology and underlying mechanisms of different muscle wasting conditions, together with the identification of factors determining muscle mass in healthy adults, is important to improve the clinical outcome for people suffering from skeletal muscle atrophy. Androgens are considered to be the main sex steroids regulating body composition, with decades of research highlighting their anabolic effect on muscle mass. However, their clinical application in the treatment of muscle wasting is limited because of severe side effects. Therapeutic agents that could achieve anabolic effects on skeletal muscle without androgenic activities on other pheripheral tissues are of great clinical interest. Enhancing our understanding of the androgenic regulation of skeletal muscle mass and of the molecular factors and signalling pathways modulated by androgens, may help in the identification of novel muscle-specific therapeutic targets to combat the devastating effects of muscle wasting. A first aim of this thesis was to gain more insight into the interindividual variation in skeletal muscle mass. In a population-based cross-sectional study (study 1), we extensively investigated the determinants of muscle mass and function in a cohort of 677 healthy young men (25-45 years). Moreover we tried to indentify genetic variations in the androgen receptor (AR) that are associated with serum testosterone (T) levels and muscle mass and function. Our results mainly confirmed previous findings that skeletal muscle mass and strength are highly heritable and are influenced by age, anthropometrics, body composition, physical activity and sex steroid levels. Next to the number of CAG repeats, we identified two single nucleotide polymorphisms (SNPs) (rs5965433 and rs5919392) in the AR gene that are associated with serum T levels. However, we could not provide evidence that these genetic variations in the AR gene also affect muscle mass or function. In a second part of this thesis, we investigated the effects of T and estradiol (E2) administration on the signalling pathway regulating muscle protein degradation in an “androgen deprivation-induced muscle atrophy mice model” (study 2). The gene and protein expression levels of muscle atrophy-inducing targets including Atrogin-1, MuRF1 and myostatin were measured at 3 different time-points (1, 7, and 30 days) and in 3 different muscle types [extensor digitorum longus (EDL), soleus (SOL) and the levator ani/bulcocavernosus muscles (LA/BC)] following orchidectomy of male mice. Our results showed important differences in atrophy signalling response between the LA/BC and the locomotor muscles. In the LA/BC, androgen deprivation resulted in a rapid and persistent upregulation of Atrogin-1 and MuRF1 mRNA and a downregulation of myostatin mRNA during the 30 day period, effects which were fully reversed by T. In the SOL and EDL muscle, a less pronounced upregulation of both atrogenes was only detectable early after orchidectomy (day 1), while myostatin mRNA levels were upregulated in the EDL only. No changes in the protein levels of Atrogin-1, MuRF1 and myostatin in EDL were found at any time point following orchidectomy, questioning their role in the androgenic regulation of the locomotor muscle mass. Furthermore, our results demonstrated that E2-treatment during androgen deprivation has anabolic effects on the LA/BC, which was associated with a partial suppression of Atrogin-1 and MuRF1 gene expression, indicating that further experiments examining the effects of E2 on skeletal muscle mass are of potential significance. In study 3, the regulation of atrophy and hypertrophy signalling molecules by T were examined in a “disuse atrophy mice model”. Following 1, 5 and 14 days of tail suspension, gene and protein expression levels of IGF1/Akt/p70S6K as well as myostatin, REDD1, Atrogin-1 and MuRF1 were examined in the SOL and EDL muscle. Because previous studies have shown that disuse atrophy is associated with reductions in serum T levels, we hypothesized that the therapeutic use of androgens in this setting would be beneficial. Tail suspension resulted in an increase, followed by a transient drop, in T levels and a decrease in muscle mass. IGF1 mRNA levels were downregulated during 1 and 5 days of tail suspension, and a subsequent reduction in the phosphorylated levels of Akt after 14 days of tail suspension was observed. Atrogin-1, MuRF1, myostatin and REDD1 gene expression levels were rapidly and transiently upregulated as early as 1 day following immobilization, even before muscle atrophy was observed. However, T treatment during tail suspension was not able to ameliorate muscle mass and did not restore the alterations in expression levels of catabolic and anabolic signaling molecules, indicating that the known anabolic effects of T are not sufficient to maintain muscle mass during muscle disuse. In conclusion, this thesis contributed to the better understanding of the molecular regulation of skeletal muscle mass by androgens and estrogens using 3 different models: a cohort of eugonadal men, an androgen deprivation-induced muscle atrophy mice model (orchidectomy), and a disuse atrophy mice model (tail suspension). Our results provided evidence for a complex mechanism by which T regulates muscle mass, with important time- and muscle type dependent differences.