β-Hydroxy-β-methylbutyrate (HMB) Counteracts Atrophy and Restores Circadian Rhythms in Myotubes

β-hydroxy-β-methylbutyrate (HMB), a bioactive metabolite of leucine, is widely recognized for its anabolic and anti-catabolic effects in skeletal muscle. However, the molecular mechanisms underlying these effects, particularly in relation to circadian regulation, remain incompletely understood. Here, we investigated the impact of HMB on dexamethasone-induced muscle atrophy in C2C12 myotubes, with a focus on anabolic signaling and circadian clock regulation. C2C12 myotubes were treated with HMB or HMB after dexamethasone-induced atrophy. HMB treatment significantly improved cell viability, surface area and fiber diameter by reducing expression of CBL-B, MuRF1 and Atrogin1, key mediators of muscle proteolysis, and increasing myogenin expression compared with atrophic conditions. While HMB did not activate AKT or mTOR, it robustly increased phosphorylation of P70S6K and S6 through a phospholipase D (PLD)-dependent mechanism. HMB restored disrupted circadian clock gene expression induced by dexamethasone, including normalization of expression patterns. HMB also enhanced circadian rhythmic amplitude and advanced phase timing, indicating improved clock robustness. These findings identify circadian regulation as a novel target of HMB action and demonstrate that HMB preserves muscle homeostasis through coordinated modulation of anabolic signaling and intrinsic circadian machinery. This study provides mechanistic insight into how HMB protects against muscle atrophy and highlights circadian regulation as an important contributor to skeletal muscle health.