Adipose Tissue Branched Chain Amino Acid (BCAA) Metabolism Modulates Circulating BCAA Levels

Mark A. Herman,Pengxiang She,Odile D. Peroni,Christopher J. Lynch,Barbara B. Kahn

doi:10.1074/jbc.m109.075184

Abstract

Whereas the role of adipose tissue in glucose and lipid homeostasis is widely recognized, its role in systemic protein and amino acid metabolism is less well-appreciated. In vitro and ex vivo experiments suggest that adipose tissue can metabolize substantial amounts of branched chain amino acids (BCAAs). However, the role of adipose tissue in regulating BCAA metabolism in vivo is controversial. Interest in the contribution of adipose tissue to BCAA metabolism has been renewed with recent observations demonstrating down-regulation of BCAA oxidation enzymes in adipose tissue in obese and insulin-resistant humans. Using gene set enrichment analysis, we observe alterations in adipose-tissue BCAA enzyme expression caused by adipose-selective genetic alterations in the GLUT4 glucose-transporter expression. We show that the rate of adipose tissue BCAA oxidation per mg of tissue from normal mice is higher than in skeletal muscle. In mice overexpressing GLUT4 specifically in adipose tissue, we observe coordinate down-regulation of BCAA metabolizing enzymes selectively in adipose tissue. This decreases BCAA oxidation rates in adipose tissue, but not in muscle, in association with increased circulating BCAA levels. To confirm the capacity of adipose tissue to modulate circulating BCAA levels in vivo, we demonstrate that transplantation of normal adipose tissue into mice that are globally defective in peripheral BCAA metabolism reduces circulating BCAA levels by 30% (fasting)-50% (fed state). These results demonstrate for the first time the capacity of adipose tissue to catabolize circulating BCAAs in vivo and that coordinate regulation of adipose-tissue BCAA enzymes may modulate circulating BCAA levels.

Highlights

Regulation of branched chain amino acids (BCAAs) Metabolizing Enzymes in Adipose Tissue— Initially, we sought to gain insight into the mechanisms by which changes in adipose tissue glucose flux affect adipocyte function and systemic fuel metabolism
Gene set enrichment analysis revealed that enzymes of BCAA oxidation are coordinately down-regulated and up-regulated in adipose-specific GLUT4-overexpressing mice (AG4OX) and adipose-specific GLUT4 knock-out mice (AG4KO) adipose tissue, respectively [21]
In AG4OX mice, down-regulation of the BCAA oxidative enzymes caused a significant decrease in BCAA oxidation in adipose tissue explants

Summary

EXPERIMENTAL PROCEDURES

Animal Studies—Generation and initial metabolic characterization of the adipose-specific GLUT4-overexpressing mice (AG4OX) and adipose-specific GLUT4 knock-out mice (AG4KO) were previously described [16, 17]. For assessment of mTOR signaling, rapamycin (10 mg/kg body weight, LC Labs) or vehicle (2% EtOH in phosphate-buffered saline) was injected intraperitoneally in 2-month-old, female AG4OX and wild-type littermates 5 h after food removal. Soleus, or extensor digitorum longus muscle from fed, 7-month-old, female AG4OX and wild-type littermates were excised, weighed, and placed in Erlenmeyer flasks (10 ml) containing 1.5 ml of Krebs-Ringer-phosphate-HEPES buffer (pH 7.4). Kinase activity toward a recombinant GST-S6 peptide (32 final amino acids of ribosomal S6) in washed immunoprecipitates was assayed in a reaction containing 200 mM Tris, pH 7.4, 100 mM MgCl2, 1 mg/ml BSA, 0.3 ␮g/␮l GST-S6 peptide, 50 ␮M ATP unlabeled, 0.1 ␮Ci [␥-32P]ATP/␮l reaction volume for 10 min at 30 °C. Differences between two groups were assessed using unpaired two-tailed Student’s t tests unless otherwise indicated in the text and figure legends

RESULTS

Fed and fasting blood metabolites after fat transplantation

DISCUSSION