Regulation of adipose branched-chain amino acid catabolism enzyme expression and cross-adipose amino acid flux in human obesity

Abstract

Elevated blood branched-chain amino acids (BCAA) are often associated with insulin resistance and type 2 diabetes, which might result from a reduced cellular utilization and/or incomplete BCAA oxidation. White adipose tissue (WAT) has become appreciated as a potential player in whole body BCAA metabolism. We tested if expression of the mitochondrial BCAA oxidation checkpoint, branched-chain ␣-ketoacid dehydrogenase (BCKD) complex, is reduced in obese WAT and regulated by metabolic signals. WAT BCKD protein (E1␣ subunit) was significantly reduced by 35-50% in various obesity models (fa/fa rats, db/db mice, diet-induced obese mice), and BCKD component transcripts significantly lower in subcutaneous (SC) adipocytes from obese vs. lean Pima Indians. Treatment of 3T3-L1 adipocytes or mice with peroxisome proliferatoractivated receptor-␥ agonists increased WAT BCAA catabolism enzyme mRNAs, whereas the nonmetabolizable glucose analog 2-deoxy-D-glucose had the opposite effect. The results support the hypothesis that suboptimal insulin action and/or perturbed metabolic signals in WAT, as would be seen with insulin resistance/type 2 diabetes, could impair WAT BCAA utilization. However, cross-tissue flux studies comparing lean vs. insulin-sensitive or insulin-resistant obese subjects revealed an unexpected negligible uptake of BCAA from human abdominal SC WAT. This suggests that SC WAT may not be an important contributor to blood BCAA phenotypes associated with insulin resistance in the overnight-fasted state. mRNA abundances for BCAA catabolic enzymes were markedly reduced in omental (but not SC) WAT of obese persons with metabolic syndrome compared with weight-matched healthy obese subjects, raising the possibility that visceral WAT contributes to the BCAA metabolic phenotype of metabolically compromised individuals. bariatric; diabetes; hyperinsulinemia; mammalian target of rapamycin; protein IN THE SEARCH FOR BIOMARKERS that associate with or predict type 2 diabetes mellitus (T2DM), it has become appreciated that circulating concentrations of the branched-chain amino acids (BCAA; valine, leucine, isoleucine) are often increased in obese, insulin-resistant states and in T2DM. Higher fasting plasma BCAA concentrations were initially reported in obese subjects by Adibi and by Felig et al. (2,12). Recent metabolomic studies found that plasma concentrations of BCAAs and large neutral amino acids are negatively correlated with insulin sensitivity in overweight and obese subjects (24), whereas the principal component that differentiated lean and obese individuals contained BCAA, methionine, phenylalanine, and tyrosine, with a linear relationship between plasma BCAA and homeostasis model assessment of insulin resistance (HOMA-IR) (36). Plasma concentrations of leucine and valine, as well as the leucine metabolite ␣-ketoisocaproate, were increased in obese female African-American T2DM subjects compared with age-and body mass index (BMI)-matched nondiabetic subjects, and plasma leucine significantly correlated with hemoglobin A ...

Roux-en-Y Gastric Bypass Surgery, but Not Calorie Restriction, Reduces Plasma Branched-Chain Amino Acids in Obese Women Independent of Weight Loss or the Presence of Type 2 Diabetes

“…Thus, our and previous studies suggest that adipose tissue may play an important role in the increased circulating BCAA levels in T2DM. Interestingly, in our study, the downregulation of BCAA degradation genes was more pronounced in omental adipose tissue compared with subcutaneous adipose tissue (18) (Supplementary Table 7), which is in agreement with a more pronounced role for omental adipose tissue in the control of metabolic health (9). Surprisingly, we did not find correlation between expression levels of genes involved in BCAA metabolism and BCAA serum levels (data not shown).…”

“…Increased protein consumption containing these essential amino acids may raise their plasma levels, but some data suggest that protein intake and circulating BCAA levels are not necessarily correlated (16,17). Alternatively, downregulation of catabolic enzymes in adipose or other tissues might be involved (8,9). In a parallel study, we analyzed the transcriptome in visceral and subcutaneous adipose tissue samples by RNA sequencing in the cohort of the current study (18).…”

“…Why levels of circulating BCAAs are elevated in obesity is unclear. Evidence has been provided for a role of white adipose tissue BCAA metabolism in the modulation of circulating BCAA levels (8,9).…”

Genetic evidence of a causal effect of insulin resistance on branched-chain amino acid levels

“…Loss-of-function mutations in PPM1K in humans [21], and disruption of key BCAA metabolism in obese mice and Zucker rats, exhibit a reduced expression of the mitochondrial isoform of branched chain-amino-acid transaminase (BCAT2; which catalyses the first and reversible step in BCAA catabolism), and mitochondrial branched chain α-keto acid dehydrogenase (BCKD) complex E1-α (which catalyses the rate-controlling and the first irreversible step), leading to increased plasma BCAA levels [22,23]. A decreased BCAA metabolism in fat tissue may contribute to higher BCAA levels in individuals with insulin-resistant obesity [2,[22][23][24]. Intriguingly, a recent study also showed that the gut microbiota can contribute to elevated BCAA levels in insulin-resistant states [25].…”

α-Hydroxybutyric Acid Is a Selective Metabolite Biomarker of Impaired Glucose Tolerance

“…PD and BCKD are closely related enzyme complexes (38) present in a number of metabolically active tissues (e.g., muscle, liver, and adipose); they are regulated by a number of common factors, including insulin. It is possible that they both play a role in affecting plasma a-HB and a-KB levels, although their relative activities and tissue specificities may well differ in iIFG and iIGT.…”