2016 年 65 巻 5 号 p. 455-462
The plasticity of skeletal muscle facilitates adaptation to various stimuli. Sex steroid hormones (androgens and estrogens) are involved in a variety of physiological and pathological processes. In skeletal muscle, sex steroid hormones affect growth, strength, metabolism, and antioxidant levels and are associated with exercise-induced skeletal muscular adaptation. Sex steroid hormone levels also decrease with aging and are thought to be a factor in muscle atrophy. Though sex steroid hormones play an important role in skeletal muscular homeostasis, the role of the endocrine system in muscle plasticity is unknown. Sex steroid hormones are synthesized from cholesterol by steroidogenic enzymes, such as 3β-hydroxysteroid dehydrogenase (HSD), and 17β-HSD, with testosterone being irreversibly converted to estrogen by aromatase cytochrome P450 (P450arom). Testosterone is also converted into its bioactive metabolite dihydrotestosterone (DHT) by 5α-reductase. Sex steroid hormones are produced by various peripheral target tissues including the kidney, liver, and brain in addition to endocrine organs such as the testis or ovary in the recent research. For instance, steroidogenic enzymes expressed in skeletal muscle have been reported to locally synthesize sex steroid hormones from circulating dehydroepiandrosterone (DHEA) or testosterone in response to exercise. Thus, local steroidogenesis in skeletal muscle provides further evidence for the presence of an autocrine/paracrine system for sex steroid hormones and their roles in skeletal muscle function and adaptation. This review focuses on the steroidogenesis of skeletal muscle and discusses the physiological significance of the sex steroid hormones network of circulation and skeletal muscle.