Akt-Dependent Anabolic Activity of Natural and Synthetic Brassinosteroids in Rat Skeletal Muscle Cells

Brassinosteroids are plant-derived polyhydroxylated derivatives of Sa-cholestane, structurally similar to cholesterol-derived animal steroid hormones and insect ecdysteroids. In this study, we synthesized a set of brassinosteroid analogues of a natural brassinosteroid (22S,23S)-homobrassinolide (HB, 1), including (22S,23S)-homocastasterone (2), (22S,23S)-3 alpha-fluoro-homobrasinolide (3), (22S,23S)-3 alpha-fluoro-homocastasterone (4), (22S,23S)-7-aza-homobrassinolide (5), and (22S,23S)-6-aza-homobrassinolide (6) and studied their anabolic efficacy in the L6 rat skeletal muscle cells in comparison to other synthetic and naturally occurring brassinosteroids (22R,23R)-homobrassinolide (7), (22S,23S)-epibrassinolide (8), and (22R,23R)-epibrassinolide (9). Presence of the 6-keto group in the B ring and stereochemistry of 22 alpha,23 alpha-vicinal hydroxyl groups in the side chain were critical for the anabolic activity, possibly due to higher cytotoxicity of the 22 beta,23 beta-hydroxylated brassinosteroids. All anabolic brassinosteroids tested in this study selectively activated PI3K/Akt signaling pathway as evident by increased Akt phosphorylation in vitro. Plant brassinosteroids and their synthetic derivatives may offer a novel therapeutic strategy for promoting growth, repair, and maintenance of skeletal muscles.