Increased Plasma Citrulline in Mice Marks Diet-Induced Obesity and May Predict the Development of the Metabolic Syndrome

Manuela Sailer,Pieter Giesbertz,Michael Müller,Mark V Boekschoten,Isabel Rubio-Aliaga,Mena K Eidens,Nicole De Wit,Hannelore Daniel,Christoph Dahlhoff,Mihai Covasa

doi:10.1371/journal.pone.0063950

Manuela Sailer, Pieter Giesbertz + Show 8 more

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https://doi.org/10.1371/journal.pone.0063950

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Abstract

In humans, plasma amino acid concentrations of branched-chain amino acids (BCAA) and aromatic amino acids (AAA) increase in states of obesity, insulin resistance and diabetes. We here assessed whether these putative biomarkers can also be identified in two different obesity and diabetic mouse models. C57BL/6 mice with diet-induced obesity (DIO) mimic the metabolic impairments of obesity in humans characterized by hyperglycemia, hyperinsulinemia and hepatic triglyceride accumulation. Mice treated with streptozotocin (STZ) to induce insulin deficiency were used as a type 1 diabetes model. Plasma amino acid profiling of two high fat (HF) feeding trials revealed that citrulline and ornithine concentrations are elevated in obese mice, while systemic arginine bioavailability (ratio of plasma arginine to ornithine + citrulline) is reduced. In skeletal muscle, HF feeding induced a reduction of arginine levels while citrulline levels were elevated. However, arginine or citrulline remained unchanged in their key metabolic organs, intestine and kidney. Moreover, the intestinal conversion of labeled arginine to ornithine and citrulline in vitro remained unaffected by HF feeding excluding the intestine as prime site of these alterations. In liver, citrulline is mainly derived from ornithine in the urea cycle and DIO mice displayed reduced hepatic ornithine levels. Since both amino acids share an antiport mechanism for mitochondrial import and export, elevated plasma citrulline may indicate impaired hepatic amino acid handling in DIO mice. In the insulin deficient mice, plasma citrulline and ornithine levels also increased and additionally these animals displayed elevated BCAA and AAA levels like insulin resistant and diabetic patients. Therefore, type 1 diabetic mice but not DIO mice show the “diabetic fingerprint” of plasma amino acid changes observed in humans. Additionally, citrulline may serve as an early indicator of the obesity-dependent metabolic impairments.

Highlights

The metabolic syndrome is characterized by central obesity, hyperglycemia, dyslipidemia and insulin resistance (IR) associated with an increased risk for the development of cardiovascular disease and type 2 diabetes mellitus (T2DM)
We found that mice with diet-induced obesity (DIO) possess increased plasma citrulline and ornithine levels compared to control (C) animals
We here describe that similar changes in plasma amino acids can be observed in a type 1 diabetic mouse model, but not in DIO models that are used to simulate the metabolic syndrome associated with hyperglycemia, hyperinsulinemia and non-alcoholic fatty liver disease

Summary

Introduction

The metabolic syndrome is characterized by central obesity, hyperglycemia, dyslipidemia and insulin resistance (IR) associated with an increased risk for the development of cardiovascular disease and type 2 diabetes mellitus (T2DM). Insulin regulates protein biosynthesis in a tissue-specific manner [1] and activates amino acid transporters in skeletal muscle [2,3,4]. It is not surprising that recent metabolomics studies in obese and diabetic subjects report significant alterations in plasma amino acid levels, elevations of BCAA and AAA [14,15]. These data confirm essentially the pioneering work of Felig et al who showed increased plasma levels of valine, leucine, isoleucine, tyrosine, and phenylalanine in obese subjects [16]. Similar changes in plasma BCAA are found in Zucker diabetic fatty rats [17], in leptin-deficient ob/ob mice [18], in AKITA mice as a model for type 1 diabetes [19], and in type 1 diabetic patients [20]

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