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Project

Study of androgen effects on skeletal muscle by inactivating the androgen receptor in muscle-progenitor cells.

Androgens are steroid hormones which are, in men, secreted predominantly by the testes. The major gonadal androgen is testosterone (T). In peripheral tissues, T can be converted by 5α-reductase enzymes into the more potent androgen 5α-dihydrotestosterone (DHT), which, like T, activates the androgen receptor (AR). In addition, the aromatase enzyme complex can convert T into 17β-estradiol (E2), which activates the estrogen receptors α and β (ERα/β). Both AR and ERα/β are ligand-inducible transcription factors that bind to specific DNA sequences to facilitate the transcription of their respective target genes. Androgens are responsible for establishing and maintaining the male phenotype. In addition, they have anabolic effects on several non-reproductive organs as well as metabolic actions. In this thesis, we focused on androgen effects on skeletal muscle and glucose homeostasis.

Muscle frailty is a major cause of disability in elderly that greatly impairs their quality of life. Furthermore, loss of muscle mass and function is also observed in immobilized patients and is a common feature of various chronic illnesses. Androgens could potentially be exploited in these patient groups, as the increase in muscle mass and strength in response to T is well-documented. However, the lack of knowledge regarding the exact mechanism of androgen action in muscle, as well as the many effects on other organ systems, currently limits their use in a routine clinical setting. The first part of this project aimed to unravel the androgen response in skeletal muscle and to characterize the signaling pathways involved in their anabolic effects. To this end, we developed a new model to inactivate the AR in the muscle precursor cells, called satellite cells (satARKO = satellite cell-specific knockout of the AR). Total-limb maximal grip strength was decreased in satARKO mice, with soleus muscles containing more type I fibers and less type IIa fibers than the corresponding control littermates. The weight of the perineal levator ani (LA) muscle was markedly reduced. Thus, muscle AR regulates muscle mass, muscle strength, and fiber type distribution. Orchidectomy further decreased LA weight in satARKO mice, suggesting that at least part of the androgen effect on muscle is mediated indirectly via non-muscle AR pathways. In an attempt to identify target genes mediating androgen anabolic action, a transcriptomic analysis was performed in LA muscle of satARKO and control mice. Surprisingly, myostatin (Mstn), a strong inhibitor of skeletal muscle growth, was one of the most androgen-responsive genes (6-fold reduction in satARKO). We identified two conserved androgen response elements in the promotor and in exon 2 of the Mstn gene, pointing towards a direct transcription activation by the AR. As a consequence, muscle hypertrophy in response to androgens was augmented in Mstn knockout mice, suggesting that androgens induce Mstn signaling to restrain their own anabolic actions. Clinically, our data imply that Mstn inhibitors could potentiate the androgen response during the treatment of muscle wasting.

Concerns about adverse effects of T on the cardiovascular system as well as prostate hyperplasia have stimulated the development of tissue-selective AR-ligands, the so-called selective AR modulators (SARMs), which are proposed to have desired anabolic effects in muscle and bone while avoiding prostatic side effects by their inability to be converted to DHT due to their non-steroidal structure. The processes by which SARMs induce muscle hypertrophy are, however, not fully elucidated. In the second part of this project, we investigated the mechanism of action of SARMs. We focused on GTx-024, a SARM that was reported to increase muscle mass in elderly men and cancer patient with muscle wasting. In orchidectomized satARKO mice, GTx-024 was still able to restore LA muscle weight, suggesting that GTx-024 action on muscle is, at least in part, indirect via non-muscle AR pathways. The transcript levels of Mstn and Amd1, two genes strongly downregulated in satARKO muscle, were normalized to sham levels in castrated control animals treated with GTx-024, indicating a role for muscle AR in mediating GTx-024 effects. Thus, just like androgens, the SARM GTx-024 has a dual mechanism of action in skeletal muscle, with both muscle AR and non-muscle AR pathways contributing to its anabolic effect.

Obesity increases the risk of developing metabolic syndrome and its complications including type 2 diabetes (T2D). Although low T levels are associated with obesity, metabolic syndrome, and T2D, the causality and mechanisms of this association remain unclear. In the third part of this project, we examined the combined effect of androgen deficiency and high fat diet (HFD) on body composition and glucose homeostasis in two different mouse models: orchidectomy and global deletion of the AR. Androgen deficiency led to increased adiposity and serum leptin levels in HFD-fed mice. Fat accumulation was correlated with both increased lipogenesis and decreased lipolysis. Mitochondrial dysfunction in adipocytes may also contribute as reduced mitochondrial DNA content as well as decreased expression of the mitochondrial gene mtNd1 was observed in adipose tissue of castrated HFD-fed animals. Orchidectomy exacerbated HFD-induced glucose intolerance by impairing insulin action in both liver and skeletal muscle, as evidenced by the increased triglyceride and decreased glycogen content in these tissues. Pancreatic insulin secretion was also impaired. The metabolic alterations observed in androgen deficient HFD-fed animals were mainly due to concomitant estrogen deficiency, as T but not DHT supplementation restored the castration effects on body composition, serum leptin levels, glucose tolerance, and pancreatic insulin secretion. Our findings have implications for the prevention and treatment of metabolic disorders in male hypogonadism. Indeed, treatment of hypogonadal men with aromatizable androgens would be preferable to treatment with non-aromatizable androgens, as treatment with DHT or a SARM would not reverse the metabolic adversities due to estrogen deficiency.

Date:1 Oct 2010 →  19 Mar 2015
Keywords:Androgens, Body composition
Disciplines:Biochemistry and metabolism, Medical biochemistry and metabolism, Orthopaedics, Endocrinology and metabolic diseases
Project type:PhD project