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1 Graduate School of Comprehensive Human Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan
2 Tsukuba, Ibaraki, Japan; Center for Tsukuba Advanced Research Alliance, University of Tsukuba, Tsukuba, Ibaraki, Japan
3 Center for Tsukuba Advanced Research Alliance, University of Tsukuba, Tsukuba, Ibaraki, Japan
4 Appalachian State Univesity, Department of Biology, Boone, North Carolina, United States
* To whom correspondence should be addressed. E-mail: aizawa{at}taiiku.tsukuba.ac.jp.
The functional importance of sex steroid hormones (testosterone and estrogens), derived from extra-gonadal tissues, has recently gained significant appreciation. Circulating dehydroepiandrosterone (DHEA) is peripherally taken up and converted to testosterone by 3
-hydroxysteroid dehydrogenase (HSD) and 17-HSD, and testosterone, in turn, is irreversibly converted to estrogens by aromatase cytochrome P450 (P450arom). Although sex steroid hormones have been implicated in skeletal muscle regulation and adaptation, it is unclear whether skeletal muscles have a local steroidogenic enzymatic machinery capable of metabolizing circulating DHEA. Thus, here, we investigate whether the three key steroidogenic enzymes (3-HSD, 17-HSD and P450arom) are present in the skeletal muscle, and are capable of generating sex steroid hormones. Consistent with our hypothesis, the present study demonstrates mRNA and protein expression of these enzymes in the skeletal muscle cells of rats, both in vivo and in culture (in vitro). Importantly, we also show an intracellular formation of testosterone and estradiol from DHEA or testosterone in cultured muscle cell, in a dose-dependent manner. These findings are novel and important in that they provide the first evidence showing that skeletal muscles are capable of locally synthesizing sex steroid hormones from circulating DHEA or testosterone.
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