ARS | Publication request: Leucine and protein metabolism in obese zucker rats

Submitted to: PLoS One
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 14, 2013
Publication Date: N/A

Interpretive Summary: Branched-chain amino acids (BCAAs) are derived from dietary and body proteins and are considered essential amino acids for humans. Interestingly, blood and urine metabolite profiling studies in the literature have revealed that in the obese, insulin-resistant state and in type 2 diabetes mellitus (T2DM), BCAA concentrations are elevated in the fasted state, indicating that these conditions elicit perturbations in not just blood sugar control but also proteins and amino acids. It may be that under conditions of poor blood sugar control, processes within the body that normally control the release of BCAA from breakdown of tissue proteins and/or those that control the use of BCAA for energy or new protein synthesis are disrupted leading to a net increase in body-wide circulating BCAA concentrations. For instance, under the insulin-resistant state or in T2DM, excessive protein breakdown or reduced tissue utilization may be evident. These questions were addressed in an animal model of obesity and insulin resistance, the Zucker rat that lacks proper leptin signaling. In this model, using radio-labeled BCAA, assessments of BCAA-associated enzymes in tissues, and comprehensive metabolite profiling of acylcarnitines, amino acids, lean body mass breakdown products, and amino acid catabolic products (branched chain alpha-ketoacids, BCKAs), it was discovered that lean mass breakdown is highly-elevated and this seems to drive higher catabolism of the liberated amino acids despite lower enzyme levels in tissues. It is proposed that tissue, blood, and urinary BCKAs, in addition to BCAAs and acylcarnitine metabolites, can be sensitive markers of BCAA oxidation that track metabolic status. Additional studies to compare these rodent results to human obesity are warranted. Overall, the experiments highlight that when considering health issues associated with obesity and diabetes, it is important to consider metabolites such as amino acids in addition to simply measuring blood sugar.

Technical Abstract: Branched-chain amino acids (BCAAs) are circulating nutrient signals for protein accretion, however they increase in obesity and appear to prognosticate diabetes onset. To understand the mechanisms whereby obesity affects BCAAs and protein metabolism, we employed metabolomics and measured rates of [1-14-C]-leucine metabolism, tissue-specific protein synthesis and branched-chain keto-acid (BCKA) dehydrogenase complex (BCKDC) activities. Male obese Zucker rats (11-weeks old) had increased body weight (BW, 53%), liver (107%) and fat masses (~300%), with 21 and 24% declines in plantaris and gastrocnemius weights, respectively. Plasma BCAAs and BCKAs were elevated 45-69% and ~100%, respectively, in obese rats. Processes facilitating these rises appeared to include: dietary intake, increased 23%, leucine (Leu) turnover and proteolysis, increased 35% along with urinary markers of proteolysis, 3-methylhistidine (183%) and 4-hydroxyproline (766%), and BCKDC activity decreased 47-66% in kidney, heart, gastrocnemius and liver. A process disposing circulating BCAAs, protein synthesis, was increased 23-29% in whole-body (corrected for fat free mass, FFM), gastrocnemius and liver from obese rats. In addition, despite the observed decreases in BCKDC activities/g, rates of whole-body Leu oxidation in obese rats were 22% and 59% higher normalized to BW and FFM, respectively. Consistently, urinary concentrations of eight BCAA catabolism-derived acylcarnitines were also elevated. This unexpected increase in BCAA oxidation may be due to a substrate effect in liver. Supporting this idea, BCKAs were elevated more in liver (193-418%) than plasma or muscle, and while hepatic BCKDC activity/g was decreased in obesity, this was completely offset by increased liver mass, in contrast to other tissues. In summary, our results indicate that plasma BCKAs may represent a more sensitive metabolic signature for obesity than BCAAs. Processes supporting elevated BCAA/BCKAs in the obese Zucker rat include increased dietary intake, lower BCKDC activity and increased proteolysis. Elevated BCAAs/BCKAs may contribute to observed elevations in protein synthesis and BCAA oxidation.