ABSTRACT: β-Hydroxy-β-Methyl Butyrate (HMB) is a natural catabolite of leucine deemed to play a role in amino acid signaling and the maintenance of lean muscle mass. Accordingly, HMB is used as a dietary supplement by sportsmen and has shown some clinical effectiveness in preventing muscle wasting in cancer and chronic lung disease, as well as in age-dependent sarcopenia. However, the molecular cascades underlying these beneficial effects are largely unknown. HMB bears a significant structural similarity with Butyrate and β-Hydroxybutyrate (βHB), two compounds recognized for important epigenetic and histone-marking activities in multiple cell types including muscle cells. We asked whether similar chromatin-modifying actions could be assigned to HMB as well. Exposure of murine C2C12 myoblasts to millimolar concentrations of HMB led to an increase in global histone acetylation, as monitored by anti-acetylated lysine immunoblotting, while preventing myotube differentiation. In these effects, HMB resembled, although with less potency, the histone deacetylase (HDAC) inhibitor Sodium Butyrate. However, initial studies did not confirm a direct inhibitory effect of HMB on HDACs in vitro. β-Hydroxybutyrate, a ketone body produced by the liver during starvation or intense exercise, has a modest effect on histone acetylation of C2C12 cells or in vitro HDAC inhibitor activities, and, unlike Butyrate and HMB, did not interfere with myotube formation in a myoblast differentiation assay. Instead, βHB dramatically increased lysine β-hydroxybutyrylation (Kbhb) of histone tails, an epigenetic mark associated with fasting responses and muscle catabolic states. However, when C2C12 cells were exposed to βHB in the presence of equimolar HMB this chromatin modification was drastically reduced, pointing to a role for HMB in attenuating ketosis-associated muscle wasting. In conclusion, while their mechanistic underpinnings remain to be clarified, these preliminary observations highlight novel and potentially important activities of HMB as an epigenetic regulator and βHB antagonist in muscle precursor cells, to be further explored in their biomedical implications.
