Background: Exercise provides both a protective and therapeutic approach to target systemic metabolic dysfunction. In part, the health benefits of exercise are mediated by exerkines as endocrine signals released from skeletal muscle during physical activity. Exerkines include micro-RNAs, proteins, lipids, and metabolites. The bioactive metabolite endocrine signals have been termed metabokines. β-aminoisobutyric acid (BAIBA) is a non-protein-beta-amino-acid, that functions as a Pgc1α (a transcriptional co-regulator; peroxisome proliferator-activated receptor-γ co-activator-1α) and exercise-regulated muscle-derived metabokine. BAIBA modulates crosstalk between skeletal muscle, liver, and fat by inducing white adipocyte browning and hepatic fatty acid β-oxidation (FAβ-O). The liver regulates functional processes including homeostasis of systemic lipid and glucose levels through de-novo lipogenesis (DNL), FAβ-O, and gluconeogenesis. Liver exhibits a high degree of metabolic flexibility (the ability to adapt to excess or restricted substrate to maintain homeostasis). The signalling and phenotypic effects of BAIBA on hepatic tissue remain poorly characterized. Here the role of BAIBA in regulating beneficial effects on liver metabolism is investigated. We hypothesise that BAIBA can improve hepatic metabolic and mitochondrial function.
Methods: Eight-week-old male C57BL/6J mice (n=20) were fed chow-diet ad libitum with/without BAIBA-treatment (100mg/kg/day in drinking water, n=10/group) for 6 weeks under UK Home Office project and personal licences. We investigated BAIBA’s effect on 1) hepatic mitochondrial density (citrate synthase {CS} assay) and function (total carnitine palmitoyl transferase {CPT} enzyme activity and O2K-Oxygraph-high-resolution mitochondrial respirometry analysis); 2) expression of genes and proteins associated with FAβ-O, mitochondrial function, DNL, and carbohydrate metabolism using RT-qPCR and immunoblotting, respectively. Shapiro-Wilk test for normality, Levene's test for equality of variances, Independent samples t-test for parametric and Mann-Whitney U test (exact-p values) for non-parametric data were used for statistical analysis via IBM SPSS Statistics 26. Significance is considered when p<0.05 with 95% confidence interval.
Results: Hepatic tissue from BAIBA-treated mice was characterized by significantly greater gene expression of Cpt1a (carnitine palmitoyl transferase-1a; 66% higher, p=0.015), Pparα (peroxisome proliferator-activated receptor-alpha; 97% greater, exact-p=0.035), and a decrease in Acacα expression (acetyl-CoA carboxylase-alpha; –61%, p=0.013) with a trend to 44% increase in Pgc1α. There was a trend towards lower expression of genes for DNL (as fatty acid synthase and stearoyl-CoA desaturase-1) as well as a trend for enhanced glucose-6-phosphatase and phosphoenolpyruvate carboxykinas-1 representing carbohydrates metabolic activity within the liver of BAIBA-treated mice. In treated group, a significant elevation in hepatic CS activity (28.1% more active, exact-p=0.029) (mitochondrial density) was observed. High-resolution respirometry analysis showed significant functional enhancement of both mitochondrial content (mean-difference±SEM=140.1±38.66, p=0.007; n=5/group) and complex-IV respiration (exact-p=0.008; n=5/group), and FAβ-O (mean-difference±SEM=7.446±2.676, p=0.024; n=5/group) in hepatic tissues from BAIBA-treated mice compared to non-treated controls. Total CPT activity was also higher in the livers of BAIBA treated mice (41.8% greater, p=0.0124; n=10-control Vs 9-treated) compared to controls. Western blotting showed significantly higher expression of the metabolic proteins; Cpt1a (by 5.04-fold, exact-p=0.019) and Pparα (by 2.2-fold, p=0.048) in the livers of the BAIBA-treated group compared to controls.
Conclusion: BAIBA treatment simulates exercise-like beneficial metabolic effects on liver tissue through enhancing hepatic FAβ-O, mitochondrial respiration and function as well as decreasing hepatic DNL.