Citrulline reduces glyceroneogenesis and induces fatty acid release in visceral adipose tissue from overweight rats

Anti-obesity and anti-diabetic effects of L-citrulline are sex-dependent

AIMS

Anti-diabetic and anti-obesity effects of L-citrulline (Cit) have been reported in male rats. This study determined sex differences in response to Cit in Wistar rats.

MAIN METHODS

Type 2 diabetes (T2D) was induced using a high-fat diet followed by low-dose of streptozotocin (30 mg/kg) injection. Male and female Wistar rats were divided into 4 groups (n = 6/group): Control, control+Cit, T2D, and T2D + Cit. Cit (4 g/L in drinking water) was administered for 8 weeks. Obesity indices were recorded, serum fasting glucose and lipid profile were measured, and glucose and pyruvate tolerance tests were performed during the Cit intervention. White (WAT) and brown (BAT) adipose tissues were weighted, and the adiposity index was calculated at the end of the study.

KEY FINDINGS

Cit was more effective in decreasing fasting glucose (18 % vs. 11 %, P = 0.0100), triglyceride (20 % vs. 14 %, P = 0.0173), and total cholesterol (16 % vs. 11 %, P = 0.0200) as well as decreasing gluconeogenesis and improving glucose tolerance, in females compared to male rats with T2D. Following Cit administration, decreases in WAT weight (16 % vs. 14 % for gonadal, 21 % vs. 16 % for inguinal, and 18 % vs. 13 % for retroperitoneal weight, all P < 0.0001) and increases in BAT weight (58 % vs. 19 %, for interscapular and 10 % vs. 7 % for axillary, all P < 0.0001) were higher in females than male rats with T2D. The decrease in adiposity index was also higher (11 % vs. 9 %, P = 0.0007) in females.

SIGNIFICANCE

The anti-obesity and anti-diabetic effects of Cit in rats are sex-dependent, with Cit being more effective in female than male rats.

Metabolic effects of L-citrulline in type 2 diabetes

The prevalence of type 2 diabetes (T2D) is increasing worldwide. Decreased nitric oxide (NO) bioavailability is involved in the pathophysiology of T2D and its complications. L-citrulline (Cit), a precursor of NO production, has been suggested as a novel therapeutic agent for T2D. Available data from human and animal studies indicate that Cit supplementation in T2D increases circulating levels of Cit and L-arginine while decreasing circulating glucose and free fatty acids and improving dyslipidemia. The underlying mechanisms for these beneficial effects of Cit include increased insulin secretion from the pancreatic β cells, increased glucose uptake by the skeletal muscle, as well as increased lipolysis and β-oxidation, and decreased glyceroneogenesis in the adipose tissue. Thus, Cit has antihyperglycemic, antidyslipidemic, and antioxidant effects and has the potential to be used as a new therapeutic agent in the management of T2D. This review summarizes available literature from human and animal studies to explore the effects of Cit on metabolic parameters in T2D. It also discusses the possible mechanisms underlying Cit-induced improved metabolic parameters in T2D.

The impact of L-citrulline supplementation on glucose homeostasis, lipid profile, and some inflammatory factors in overweight and obese patients with type 2 diabetes: A double-blind randomized placebo-controlled trial

This study investigated the impact of L-citrulline on glucose homeostasis, lipid profile, and inflammatory factors in overweight and obese patients with type 2 diabetes (T2D). In total, 54 participants with T2D were assigned to L-citrulline (3 g/day L-citrulline) or placebo groups and tested for 8 weeks. Serum levels of insulin, fasting glucose, hemoglobin A1c (HbA1c), lipid profile, tumor necrosis factor-α (TNF-α), high-sensitivity C-reactive protein (hs-CRP), and L-citrulline were measured pre- and post-intervention. Totally, 45 patients were enrolled in the research. L-citrulline supplementation decreased serum levels of insulin (p = .025), glucose (p = .032), HbA1c (p = .001), HOMA-IR (p = .037), TNF-α (p = .036), and hs-CRP (p = .027) significantly. At the end of the study, despite the significant decrease in serum levels of triglyceride (p = .027) and the increase in high-density lipoprotein cholesterol levels (p < .001) in the L-citrulline group, no significant differences were found for these parameters between the groups. Moreover, no significant inter- and intra-group changes were observed for dietary intakes, anthropometric indices, total and low-density lipoprotein cholesterol levels (p > .05). In conclusion, L-citrulline supplementation might improve glucose homeostasis, some lipid factors and inflammatory markers in overweight and obese patients with T2D.

Correlation of plasma metabolites with glucose and lipid fluxes in human insulin resistance

OBJECTIVE

Insulin resistance develops prior to the onset of overt type 2 diabetes, making its early detection vital. Direct accurate evaluation is currently only possible with complex examinations like the stable isotope-based hyperinsulinemic euglycemic clamp (HIEC). Metabolomic profiling enables the detection of thousands of plasma metabolites, providing a tool to identify novel biomarkers in human obesity.

DESIGN

Liquid chromatography mass spectrometry-based untargeted plasma metabolomics was applied in 60 participants with obesity with a large range of peripheral insulin sensitivity as determined via a two-step HIEC with stable isotopes [6,6-2H2]glucose and [1,1,2,3,3-2H5]glycerol. This additionally enabled measuring insulin-regulated lipolysis, which combined with metabolomics, to the knowledge of this research group, has not been reported on before.

RESULTS

Several plasma metabolites were identified that significantly correlated with glucose and lipid fluxes, led by plasma (gamma-glutamyl)citrulline, followed by betaine, beta-cryptoxanthin, fructosyllysine, octanylcarnitine, sphingomyelin (d18:0/18:0, d19:0/17:0) and thyroxine. Subsequent machine learning analysis showed that a panel of these metabolites derived from a number of metabolic pathways may be used to predict insulin resistance, dominated by non-essential amino acid citrulline and its metabolite gamma-glutamylcitrulline.

CONCLUSION

This approach revealed a number of plasma metabolites that correlated reasonably well with glycemic and lipolytic flux parameters, measured using gold standard techniques. These metabolites may be used to predict the rate of glucose disposal in humans with obesity to a similar extend as HOMA, thus providing potential novel biomarkers for insulin resistance.

Potential roles of Citrulline and watermelon extract on metabolic and inflammatory variables in diabetes mellitus, current evidence and future directions: A systematic review

OBJECTIVE

Diabetes mellitus is a prevalent endocrine disorder worldwide. Citrulline is an α-amino acid, which is abundant in watermelon, and a precursor of arginine and nitric oxide. Decreased bioavailability of nitric oxide is associated with insulin resistance. The present systematic review focused on the existing evidence of citrulline and watermelon extract effects on metabolic and inflammatory parameters in diabetes mellitus.

METHODS

A systematic search of the databases PubMed, Scopus, EMBASE, ProQuest and Google Scholar was conducted for relevant papers published from inception until October 2018. All clinical trials, animal and in vitro studies published in the English language that assessed the role of citrulline and watermelon extract on diabetes mellitus, were eligible. Studies providing inadequate information were excluded.

RESULTS

Out of 1262 articles we found, only eight articles met the inclusion criteria for analysis. In three studies an increase in the synthesis of nitric oxide was reported with citrulline and watermelon extract supplementation. Four studies showed a significant reduction in blood glucose after supplementation with watermelon extract, and two studies reported a decrease in a number of inflammatory biomarkers following citrulline supplementation. Although citrulline intake caused a significant reduction in HOMA-IR in one study, inconsistent results were revealed on the effects of citrulline and watermelon extract on insulin levels and lipid profile.

CONCLUSION

Citrulline and watermelon extract could improve nitric oxide synthesis, glycaemic status and inflammation in diabetes mellitus. However, further studies are required to shed light on the underlying mechanisms.

Citrulline stimulates muscle protein synthesis, by reallocating ATP consumption to muscle protein synthesis

BACKGROUND

Animal studies and clinical data support the interest of citrulline as a promising therapeutic for sarcopenia. Citrulline is known to stimulate muscle protein synthesis, but how it affects energy metabolism to support the highly energy-dependent protein synthesis machinery is poorly understood.

METHODS

Here, we used myotubes derived from primary culture of mouse myoblasts to study the effect of citrulline on both energy metabolism and protein synthesis under different limiting conditions.

RESULTS

When serum/amino acid deficiency or energy stress (mild uncoupling) were applied, citrulline stimulated muscle protein synthesis by +22% and +11%, respectively. Importantly, this increase was not associated with enhanced energy status (ATP/ADP ratio) or mitochondrial respiration. We further analysed the share of mitochondrial respiration and thus of generated ATP allocated to different metabolic pathways by using specific inhibitors. Our results indicate that addition of citrulline allocated an increased share of mitochondrially generated ATP to the protein synthesis machinery under conditions of both serum/amino acid deficiency (+28%) and energy stress (+21%). This reallocation was not because of reduced ATP supply to DNA synthesis or activities of sodium and calcium cycling ion pumps.

CONCLUSIONS

Under certain stress conditions, citrulline increases muscle protein synthesis by specifically reallocating mitochondrial fuel to the protein synthesis machinery. Because ATP/ADP ratios and thus Gibbs free energy of ATP hydrolysis remained globally constant, this reallocation may be linked to decreased activation energies of one or several ATP (and GTP)-consuming reactions involved in muscle protein synthesis.

l-Citrulline Supplementation: Impact on Cardiometabolic Health

Diminished bioavailability of nitric oxide (NO), the gaseous signaling molecule involved in the regulation of numerous vital biological functions, contributes to the development and progression of multiple age- and lifestyle-related diseases. While l-arginine is the precursor for the synthesis of NO by endothelial-nitric oxide synthase (eNOS), oral l-arginine supplementation is largely ineffective at increasing NO synthesis and/or bioavailability for a variety of reasons. l-citrulline, found in high concentrations in watermelon, is a neutral alpha-amino acid formed by enzymes in the mitochondria that also serves as a substrate for recycling l-arginine. Unlike l-arginine, l-citrulline is not quantitatively extracted from the gastrointestinal tract (i.e., enterocytes) or liver and its supplementation is therefore more effective at increasing l-arginine levels and NO synthesis. Supplementation with l-citrulline has shown promise as a blood pressure lowering intervention (both resting and stress-induced) in adults with pre-/hypertension, with pre-clinical (animal) evidence for atherogenic-endothelial protection. Preliminary evidence is also available for l-citrulline-induced benefits to muscle and metabolic health (via vascular and non-vascular pathways) in susceptible/older populations. In this review, we examine the impact of supplementing this important urea cycle intermediate on cardiovascular and metabolic health outcomes and identify future directions for investigating its therapeutic impact on cardiometabolic health.

Hepatic steatosis: a role for citrulline

PURPOSE OF REVIEW

The high worldwide prevalence of nonalcoholic fatty liver disease (NAFLD) makes it a major public health issue. Amino acids offer a promising approach for its prevention, and several experimental studies highlight the nutritional importance of citrulline in this setting. The purpose of this review is to discuss the potential interest of citrulline in the prevention and treatment of NAFLD.

RECENT FINDINGS

Current findings shed light on the role of the gut-liver, adipose tissue-liver, and muscle-liver axes in NAFLD progression. Recent experimental studies have produced evidence for a role of citrulline in controlling the pathophysiological mechanisms involved in NAFLD through its action on these three axes. Data are needed to distinguish between direct and indirect effects of citrulline on the liver and between a specific effect and a nitrogen supply-related effect.

SUMMARY

Good level of experimental evidence suggests that citrulline supply may be associated with an attenuation of NAFLD development, but further human studies are now needed to support these findings. This review may help define novel strategies to control fatty liver diseases.

Preventive effects of citrulline on Western diet-induced non-alcoholic fatty liver disease in rats

A Western diet induces insulin resistance, liver steatosis (non-alcoholic fatty liver disease (NAFLD)) and intestinal dysbiosis, leading to increased gut permeability and bacterial translocation, thus contributing to the progression of NAFLD to non-alcoholic steatohepatitis. In the present study, we sought, in a model of Western diet-induced NAFLD, to determine whether citrulline (Cit), an amino acid that regulates protein and energy metabolism, could decrease Western diet-induced liver injuries, as well as the mechanisms involved. Sprague-Dawley rats were fed a high-fat diet (45 %) and fructose (30 %) in drinking water or a control diet associated with water (group C) for 8 weeks. The high-fat, high-fructose diet (Western diet) was fed either alone (group WD) or with Cit (1 g/kg per d) (group WDC) or an isonitrogenous amount of non-essential amino acids (group WDA). We evaluated nutritional and metabolic status, liver function, intestinal barrier function, gut microbiota and splanchnic inflammatory status. Cit led to a lower level of hepatic TAG restricted to microvesicular lipid droplets and to a lower mRNA expression of CCAAT-enhancer-binding protein homologous protein, a marker of endoplasmic reticulum stress, of pro-inflammatory cytokines Il6 (P<0·05) and Tnfα, and of toll-like receptor 4 (Tlr4) (P<0·05). Cit also improved plasma TAG and insulin levels. In the colon, it decreased inflammation (Tnfα and Tlr4 expressions) and increased claudin-1 protein expression. This was associated with higher levels of Bacteroides/Prevotella compared with rats fed the Western diet alone. Cit improves Western diet-induced liver injuries via decreased lipid deposition, increased insulin sensitivity, lower inflammatory process and preserved antioxidant status. This may be related in part to its protective effects at the gut level.

What induces watts in WAT?

Excess calories stored in white adipose tissue (WAT) could be reduced either through the activation of brown adipose tissue (BAT) or the development of brown-like cells ("beige" or "brite") in WAT, a process named "browning." Calorie dissipation in brown and beige adipocytes might rely on the induction of uncoupling protein 1 (UCP1), which is absent in white fat cells. Any increase in UCP1 is commonly considered as the trademark of energy expenditure. The intracellular events involved in the recruitment process of beige precursors were extensively studied lately, as were the effectors, hormones, cytokines, nutrients and drugs able to modulate the route of browning and theoretically affect fat mass in rodents and in humans. The aim of this review is to update the characterization of the extracellular effectors that induce UCP1 in WAT and potentially provoke calorie dissipation. The potential influence of metabolic cycling in energy expenditure is also questioned.

Combining citrulline with atorvastatin preserves glucose homeostasis in a murine model of diet-induced obesity

BACKGROUND AND PURPOSE

NO is a crucial regulator of energy and lipid metabolism, whose homeostasis is compromised during obesity. Combination of citrulline and atorvastatin potentiated NO production in vitro. Here we have assessed the effects of this combination in mice with diet-induced obesity (DIO).

EXPERIMENTAL APPROACH

C57BL/6J male mice were given a standard diet (control) or a high fat-high sucrose diet (DIO) for 8 weeks. DIO mice were then treated with DIO alone, DIO with citrulline, DIO with atorvastatin or DIO with citrulline and atorvastatin (DIOcit-stat) for 3 weeks. Thereafter, body composition, glucose tolerance, insulin sensitivity and liver fat metabolism were measured.

KEY RESULTS

DIOcit-stat mice showed lower body weight, fat mass and epididymal fat depots compared with other DIO groups. Unlike other DIO groups, glucose tolerance and insulin sensitivity of DIOcit-stat, along with blood glucose and insulin concentrations in response to feeding, were restored to control values. Refeeding-induced changes in liver lipogenic activity were also reduced in DIOcit-stat mice compared with those of DIO animals. This was associated with decreased gene expression of the transcription factor SREBP-1, liver X receptor α, ChREBP and of target lipogenic enzymes in the liver of DIOcit-stat mice compared with those of other DIO groups.

CONCLUSIONS AND IMPLICATIONS

The citrulline-atorvastatin combination prevented fat mass accumulation and maintained glucose homeostasis in DIO mice. Furthermore, it potentiated inhibition of hepatic de novo lipogenesis activity. This combination has potential for preservation of glucose homeostasis in patients receiving statin therapy.

Citrulline counteracts overweight- and aging-related effects on adiponectin and leptin gene expression in rat white adipose tissue

We recently demonstrated that citrulline (CIT) reduced the expression of inflammatory genes in cultured explants from retroperitoneal (RET) white adipose tissue (WAT) from young (2–4 months) but not old (25 months) rats. Here we show that in RET WAT from old rats and high-fat-diet-fed (HFD) young rats, the basal expression of the leptin gene was increased (275–345%) whereas that of the adiponectin gene was decreased (48–60%), when compared to those from control-diet-fed (CD) young rats. We show also that in RET WAT from old rats, a diet supplemented with CIT for 3 months reduced macrophage (F4/80, CD68) and inflammation (interleukin-6, tumor necrosis factor-α) marker genes 23–97%. CIT supplementation lowered leptin mRNA 62% and increased adiponectin mRNA 232%. In cultured explants of RET WAT from 4 month-old CD, 4 month-old HFD and 25-month-old CD rats, the exposure to 2.5 mmol/L CIT for 24 h up-regulated adiponectin gene expression 151%, 362% and 216% respectively. In contrast, leptin gene expression was down-regulated 66% selectively in CIT-treated explants from 25-month-old CD rats. These results further support the proposed beneficial effect of CIT to counteract the deleterious effects of aging and overweight on the metabolic, inflammatory and endocrine functions of WAT.

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HFD and aging increase leptin mRNA and decrease adiponectin mRNA in rat adipose tissue.

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In old rats a CIT diet reduces leptin mRNA and augments adiponectin mRNA.

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In adipose tissue explants, CIT induces adiponectin mRNA whatever age and diet.

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CIT exposure of adipose tissue explants reduces leptin mRNA selectively in old rats.

Acute induction of uncoupling protein 1 by citrulline in cultured explants of white adipose tissue from lean and high-fat-diet-fed rats

A diet enriched with citrulline (CIT) reduces white adipose tissue (WAT) mass. We recently showed that CIT stimulated β-oxidation in rat WAT explants from young (2-4 months) but not old (25 months) rats. Here we show that both in old rats and high-fat-diet-fed young rats, uncoupling protein one (UCP1) mRNA and protein expressions were weaker than those in young control rats. Selectively in WAT from young rats, a 24h CIT treatment up-regulated expressions of UCP1, peroxisome proliferator-activated receptor-α (PPARα), PPARγ-coactivator-1-α and mitochondrial-transcription-factor-A whereas it down-regulated PPARγ2 gene expression, whatever the diet. These results suggest that CIT induces a new metabolic status in WAT, with increased β-oxidation and uncoupling of respiratory chain, resulting in energy expenditure that favors fat mass reduction.