Charlotte Breuillard

Citrulline and nitrogen homeostasis: an overview

Citrulline (Cit) is a non-essential amino acid whose metabolic properties were largely ignored until the last decade when it began to emerge as a highly promising nutrient with many regulatory properties, with a key role in nitrogen homeostasis. Because Cit is not taken up by the liver, its synthesis from arginine, glutamine, ornithine and proline in the intestine prevents the hepatic uptake of the two first amino acids which activate the urea cycle and so prevents amino acid catabolism. This sparing effect may have positive spin-off for muscle via increased protein synthesis, protein content and functionality. However, the mechanisms of action of Cit are not fully known, even if preliminary data suggest an implication of mTOR pathway. Further exploration is needed to gain a complete overview of the role of Cit in the control of nitrogen homeostasis.

In vitro anti-inflammatory effects of citrulline on peritoneal macrophages in Zucker diabetic fatty rats

In type 2 diabetes (T2D) macrophage dysfunction increases susceptibility to infection and mortality. This may result from the associated decreased plasma concentration of arginine, an amino acid that plays an important role in immunity. In vitro, increasing arginine availability leads to an improvement in macrophage function; however, arginine supplementation in diabetic obese patients may be detrimental. The aim of the present study was to assess in vitro whether citrulline, an arginine precursor, could replace arginine in the regulation of macrophage function under a condition of diabetes and obesity. Peritoneal macrophages from diabetic obese or lean rats were incubated for 6 h in an arginine-free medium, in the presence of increasing citrulline concentrations (0·1, 0·5, 1 or 2 mmol/l). Cytokine and NO production was determined. Peritoneal macrophages from either lean or diabetic obese rats produced NO, and at higher levels in the cells from lean rats. In diabetic obese rats, TNF-α production decreased with increasing citrulline concentrations, but was higher than that in the cells from lean rats. In contrast, IL-6 production increased with increasing citrulline concentrations. The present experiment shows that citrulline is effectively used for NO production and regulates cytokine production in macrophages from diabetic obese rats. This effect warrants in vivo evaluation in T2D-related inflammation.

Effects of a diabetes-specific enteral nutrition on nutritional and immune status of diabetic, obese, and endotoxemic rats: interest of a graded arginine supply.

Objective: Obese and type 2 diabetic patients present metabolic disturbance–related alterations in nonspecific immunity, to which the decrease in their plasma arginine contributes. Although diabetes-specific formulas have been developed, they have never been tested in the context of an acute infectious situation as can be seen in intensive care unit patients. Our aim was to investigate the effects of a diabetes-specific diet enriched or not with arginine in a model of infectious stress in a diabetes and obesity situation. As a large intake of arginine may be deleterious, this amino acid was given in graded fashion. Design: Randomized, controlled experimental study. Setting: University research laboratory. Subjects: Zucker diabetic fatty rats. Interventions: Gastrostomized Zucker diabetic fatty rats were submitted to intraperitoneal lipopolysaccharide administration and fed for 7 days with either a diabetes-specific enteral nutrition without (G group, n = 7) or with graded arginine supply (1–5 g/kg/day) (GA group, n = 7) or a standard enteral nutrition (HP group, n = 10). Measurements and Main Results: Survival rate was better in G and GA groups than in the HP group. On day 7, plasma insulin to glucose ratio tended to be lower in the same G and GA groups. Macrophage tumor necrosis factor-&agr; (G: 5.0 ± 1.1 ng/2 × 106 cells·hr−1; GA: 3.7 ± 0.8 ng/2 × 106 cells·hr−1; and HP: 1.7 ± 0.6 ng/2 × 106 cells·hr−1; p < .05 G vs. HP) and nitric oxide (G: 4.5 ± 1.1 ng/2 × 106 cells·hr−1; GA: 5.1 ± 1.0 ng/2 × 106 cells·hr−1; and HP: 1.0 ± 0.5 nmol/2 × 106 cells·hr−1; p < .05 G and GA vs. HP) productions were higher in the G and GA groups compared to the HP group. Macrophages from the G and GA groups exhibited increased arginine consumption. Conclusions: In diabetic obese and endotoxemic rats, a diabetes-specific formula leads to a lower mortality, a decreased insulin resistance, and an improvement in peritoneal macrophage function. Arginine supplementation has no additional effect. These data support the use of such disease-specific diets in critically ill diabetic and obese patients.

l-Arginine Metabolism Impairment in Sepsis and Diseases: Causes and Consequences

Sepsis is recognized as a common cause for admission in ICUs. In general, sepsis and many diseases lead to alterations of the metabolism of amino acids (AA), among them arginine (Ventura et al. Amino Acids 39:1417–1426, 2010) more especially. Indeed, in humans, sepsis is characterized by a substantial decrease in arginine pools. This decrease of arginine concentration is usually due to a major increase of its consumption and a decrease of its endogenous production leading to the concept of “arginine deficiency” (Ventura et al. Amino Acids 39:1417–1426, 2010). Hence, in sepsis, it is generally admitted that endogenous synthesis (i.e., arginine is a non-essential amino acid) cannot meet the needs and arginine becomes a conditionally essential AA (Pribis et al. JPEN J Parenter Enteral Nutr 36:53–59, 2012). This may have important consequences. As a matter of fact, arginine is not only a component of proteins but also a molecule that can generate a number of active metabolites (Fig. 12.1): arginine may be the precursor of nitric oxide (NO, which is essential for the immune system), of ornithine (which is recognized as a polyamine precursor), or of agmatine (which is a major regulator of cell functions). Moreover, arginine is an important element in muscle energy: after reacting with glycine and methionine, it allows the formation of creatine. Finally, arginine acts as a secretagogue (such as insulin, glucagon, growth hormones, prolactin, and catecholamines) (Wu. Amino Acids 37:1–17, 2009). This could explain why the impairment of arginine homeostasis in sepsis and several diseases can contribute to pathophysiological alterations.

l-Arginine and TNFα Production in Macrophages: A Focus on Metabolism, Aging, Metabolic Syndrome, and Type 2 Diabetes

Arginine (Arg) has long been known to be a major regulator of immunity via its metabolic and physiological functions. In the 1950s, it was classified as a non-essential amino acid by Rose since Arg could be synthesized at the whole body level, mainly in the kidneys, after the conversion of intestinal citrulline (Cit) via argininosuccinate synthase (ASS) and argininosuccinate lyase (ASL). But things changed when Barbul et al. (Surg Forum 28:101–103, 1977) observed, surprisingly, that Arg was an immunomodulator via a thymic effect. At the same time, they observed that Arg could be essential in several situations, like growth or sepsis. Finally in the 1990s, Albina et al. (J Immunol 147:144–148, 1991) demonstrated that macrophages were able to produce nitric oxide (NO). These pioneering observations opened a new field of research focused on the regulation of macrophage functions by Arg.