The Leucine Catabolite and Dietary Supplement β-Hydroxy-β-Methyl Butyrate (HMB) as an Epigenetic Regulator in Muscle Progenitor Cells.

Virve Cavallucci,Giovambattista Pani

doi:10.3390/metabo11080512

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.

Highlights

Muscle wasting occurs in several pathophysiological conditions from prolonged immobilization, aging and malnutrition to chronic inflammatory disorders and cancer, leading to the patient’s loss of independence and increased risk for falls, severe injury and death [1]
Hydroxy-β-Methyl Butyrate (HMB) is structurally related to both Butyrate, a product of bacterial fermentation, and the ketone body and endogenous lipid catabolite βHB (Supplementary Figure S1); these two short-chain fatty acids act as Class I HDAC inhibitor (HDACi) and post-translational histone modifiers via lysine acetylation and β-hydroxybutyrylation [23,25], but a similar activity for HMB
The mechanisms underlying the beneficial effects of HMB on muscle mass and strength in sports training and pathologic states remain poorly understood

Summary

Introduction

Muscle wasting occurs in several pathophysiological conditions from prolonged immobilization, aging and malnutrition to chronic inflammatory disorders and cancer, leading to the patient’s loss of independence and increased risk for falls, severe injury and death [1]. Upregulation of muscle-specific E3 ubiquitin ligases Atrogin-1 and Murf-1 is pivotal to enhanced proteasome-dependent proteolysis and atrophy of muscle cells [7] Autophagy, another nutrient-regulated degradative process, is involved in muscle mass maintenance it would seem to play a double and opposite role according to age (myofiber atrophy in the young and maintenance of muscle mass during aging) [8]. Another nutrient-regulated degradative process, is involved in muscle mass maintenance it would seem to play a double and opposite role according to age (myofiber atrophy in the young and maintenance of muscle mass during aging) [8] Such “final common pathway” for muscle wasting is in turn governed by multiple metabolic (Insulin/IGF, mTOR/Akt) and pro-/anti-inflammatory (TNF alpha/IL-6, Myostatin) factors and signaling pathways, eventually converging on the transcriptional regulators FoxOs and NF-kB [9,10]. Aberrant muscle degradation in sarcopenia appears to result from specific disease-triggered genetic programs [11], whose manipulation may hold the key for successful prevention and treatment of the disease

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