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Abstract:

Osteoporosis is a skeletal disease characterized by compromised bone strength predisposing to an increased risk of fractures. The lifetime risk of osteoporotic fractures is as high as 20-25% in Caucasian men, which accounts for about one-third of the societal and economic burden of the disease. Still, older men have about half the fracture risk of post-menopausal women, in part because men develop wider and stronger bones during puberty which are better preserved with aging. Testosterone, via a dual effect on the androgen receptor (AR) and via aromatization into estrogens and stimulation of estrogen receptor alpha, contributes to male peak bone mass acquisition and subsequent maintenance. According to the free hormone hypothesis, the bioavailable and free concentrations and biological actions of androgens and estrogens are further regulated by sex hormone-binding globulin (SHBG). Androgens are well known for their anabolic actions on muscle, but they also influence osteoblasts and osteocytes as well as the osteogenic response to mechanical stimulation. Indeed, the skeleton is not only an endocrine but also a biomechanical tissue which adapts to loading from e.g. physical exercise or greater body weight in overweight or obese subjects, as well as to unloading e.g. in disuse osteosarcopenia. This thesis focuses on the importance of androgen bioactivity and mechanical loading for skeletal health in aging men. In the first part of this thesis, we established an in vitro AR luciferase reporter bioassay with the aims of investigating the effects of SHBG on androgen bioactivity in vitro. We determined that such assays are sensitive to dilution effects, and therefore adopted a method using undiluted serum. We then showed that AR bioassay results can indeed reflect the inhibitory influence of SHBG (or mutants thereof). AR bioactivity correlated with free and bioavailable testosterone concentrations in adult hypogonadal men and in patients receiving anti-androgens or androgen deprivation therapy. These findings may lead to a more appropriate use of bioassays and a better understanding of normal and suppressed androgen bioactivity in situations of altered SHBG concentrations. In the second part, we examined the physiological effect of SHBG using a transgenic mouse model (since mice and rats normally lack circulating SHBG post-natally). We found that these mice had increased total androgen and estrogen concentrations in serum due to hypothalamic-pituitary feedback regulation of free sex steroid concentrations, as well as due to prolonged ligand half-life. Nevertheless, bioactivity on male and female reproductive organs was inhibited by SHBG in vivo, via an endocrine, ligand-dependent mechanism of SHBG. These results offer a better understanding of the free hormone hypothesis with regards to sex steroids, provide compelling support for the determination of free or bioavailable sex steroid concentrations in medical practice, and clarify important comparative differences between translational mouse models and human endocrinology. In the third part, we investigated whether androgens prevent simultaneous bone and muscle decay in a model of adult orchidectomized mice given a unilateral hindlimb botulinum toxin type A (Botox) injection. The trabecular bone resorption induced by Botox was increased by castration and prevented by androgens in both short- and long-term follow-up. The cortical bone loss following disuse was only prevented by T and not DHT. Androgens prevented body weight loss, but did not influence disuse muscle atrophy. Conditional AR knock-out in mature osteoblasts and osteocytes, or in the muscle satellite cell lineage, increased age-related trabecular bone loss in both hindlimbs but did not alter the trabecular disuse osteopenia. Thus, androgens -particularly testosterone- may be useful to prevent acute disuse osteoporosis via a muscle-independent mechanism. The fourth part is a human epidemiological study concerning the effects of body weight on male bone mass and turnover. We hypothesized that obesity might be stimulatory for bone mass via an effect of mechanical loading, but that adverse hormonal or metabolic factors might also blunt this skeletal mechanoadaptation. Using data from the European Male Ageing Study, we examined cross-sectional associations of body mass, endocrine and metabolic parameters with bone turnover markers, heel bone quantitative ultrasound, hip and spine dual-energy X-ray absorptiometry and radius peripheral quantitative computed tomography results. We found that bone turnover markers were negatively associated with hyperglycemia and hypertriglyceridemia, independent of other metabolic syndrome components and body mass index (BMI). Bone mass at the heel, hip and radius was positively associated with waist circumference and/or BMI. When adjusted for BMI however, metabolic factors related to insulin resistance were negatively associated with bone mass. Decreased physical activity/performance or adverse hormonal parameters did not explain lower bone turnover or relative bone mass deficits in men with metabolic syndrome. We conclude that although greater body mass appears beneficial for bone mass at first sight, obesity is also associated with underlying insulin resistance wich is in turn associated with low bone turnover and a relative bone mass deficit in aging men with metabolic syndrome. These findings may explain why aging obese men are not necessarily protected against fractures. Collectively, these studies provide a better understanding of how androgen bioactivity and mechanical stimuli contribute to musculoskeletal health in aging men.