Adaptation to a long term (4 weeks) arginine- and precursor (glutamate, proline and aspartate)-free diet

Dietary Manipulation of Amino Acids for Cancer Therapy

Cancer cells cannot proliferate and survive unless they obtain sufficient levels of the 20 proteinogenic amino acids (AAs). Unlike normal cells, cancer cells have genetic and metabolic alterations that may limit their capacity to obtain adequate levels of the 20 AAs in challenging metabolic environments. However, since normal diets provide all AAs at relatively constant levels and ratios, these potentially lethal genetic and metabolic defects are eventually harmless to cancer cells. If we temporarily replace the normal diet of cancer patients with artificial diets in which the levels of specific AAs are manipulated, cancer cells may be unable to proliferate and survive. This article reviews in vivo studies that have evaluated the antitumor activity of diets restricted in or supplemented with the 20 proteinogenic AAs, individually and in combination. It also reviews our recent studies that show that manipulating the levels of several AAs simultaneously can lead to marked survival improvements in mice with metastatic cancers.

Dietary Approaches to Cancer Therapy

The concept that dietary changes could improve the response to cancer therapy is extremely attractive to many patients, who are highly motivated to take control of at least some aspect of their treatment. Growing insight into cancer metabolism is highlighting the importance of nutrient supply to tumor development and therapeutic response. Cancers show diverse metabolic requirements, influenced by factors such as tissue of origin, microenvironment, and genetics. Dietary modulation will therefore need to be matched to the specific characteristics of both cancers and treatment, a precision approach requiring a detailed understanding of the mechanisms that determine the metabolic vulnerabilities of each cancer.

Quantitation of the arginine family amino acids in the blood of full-term infants perinatally in relation to their birth weight

Background Arginine family amino acids (AFAAs) include glutamine (Gln) plus glutamate (Glu), ornithine (Orn), proline (Pro), citrulline (Cit) and arginine (Arg). We aimed to quantitate these amino acids in the blood of full-term infants in relation to their birth weight (BW) perinatally. Methods Breastfeeding full-term infants (n = 2000, 1000 males, 1000 females) with a BW of 2000-4000 g were divided into four equal groups: group A, 2000-2500 g; B, 2500-3000 g; C, 3000-3500 g and D, 3500-4000 g. Blood samples as dried blood spots (DBS) were collected on the third day of life and analyzed via a liquid chromatography tandem mass spectrometry (LC-MS/MS) protocol. Results Gln plus Glu mean values were found to be statistically significantly different between males and females in all studied groups. The highest values of these amino acids were detected in both males and females in group D. Orn mean values were found to be statistically significantly different between males and females of the same BW in all groups except the last one. The lower mean value was determined in group A, whereas the highest was determined in group D. Cit and Arg mean values were determined to be almost similar in all studied groups. Conclusions Gln plus Glu and Orn blood concentrations were directly related to infants' BW. Conversely, Cit and Arg did not vary significantly in all groups.

Ablation of Arginase II Spares Arginine and Abolishes the Arginine Requirement for Growth in Male Mice

Background: Arginine is considered a semiessential amino acid in many species, including humans, because under certain conditions its demand exceeds endogenous production. Arginine availability, however, is determined not only by its production but also by its disposal. Manipulation of disposal pathways has the potential to increase availability and thus abolish the requirement for arginine.Objective: The objective of the study was to test the hypothesis that arginase II ablation increases arginine availability for growth.Methods: In a completely randomized design with a factorial arrangement of treatments, postweaning growth was determined for 3 wk in male and female wild-type (WT) mice and arginase II knockout mice (ARGII) on a C57BL/6J background fed arginine-sufficient [Arg(+); 8 g arginine/kg] or arginine-free [Arg(-)] diets. Tracers were used to determine citrulline and arginine kinetics.Results: A sex dimorphism in arginine metabolism was detected; female mice had a greater citrulline flux (∼30%, P < 0.001), which translated to greater de novo synthesis of arginine (∼31%, P < 0.001). Female mice also had greater arginine fluxes (P < 0.015) and plasma arginine concentrations (P < 0.01), but a reduced arginine clearance rate (P < 0.001). Ablation of arginase II increased plasma arginine concentrations in both sexes (∼27%, P < 0.01) but increased arginine flux only in males (P < 0.01). The absence of arginine in the diet limited the growth of male WT mice (P < 0.01), but had no effect on male ARGII mice (P = 0.12). In contrast, WT females on the Arg(-) diet grew at the same rate and achieved final weight similar to that of female WT mice fed the Arg(+) diet (P = 0.47).Conclusion: The ablation of arginase II in male mice spares arginine that can then be used for growth and to meet other metabolic functions, thus abolishing arginine requirements.

New insights into the methodological issues of the indicator amino acid oxidation method in preterm neonates

BACKGROUND

We determined the effect of adaptation to the study diet on oxidation of the indicator amino acid and the required tracer washout time in preterms.

METHODS

Subjects received a study diet for 6 d that entailed a 50% reduction in leucine. Tracer studies using enterally infused [(13)C]bicarbonate and [1-(13)C]phenylalanine were performed on days 1, 2, 4, and 6. Breath samples containing (13)CO2 were collected during steady state and measured by infrared spectrometric analysis, and the fraction of (13)CO2 recovery from (13)C oxidation (F(13)CO2) was calculated.

RESULTS

Preterm infants (n = 11, birth weight 1.9 ± 0.1 kg, gestational age 32.6 ± 1.5 wk) received 166 mg/kg/d of leucine. Baseline enrichment changed significantly at day 1 of the study diet. F(13)CO2 did not change significantly between days 2 and 4 but was significantly lower at day 6. The tracer washout time was determined to be 7.5 h using a biphasic regression analysis.

CONCLUSION

One day of adaptation to a new diet is necessary to adapt to the (13)C enrichment of the study formula before starting infant requirement studies. Adaptation for a period of 5 d results in a protein-sparing response. The minimal time between two studies within the same subject is 7.5 h.

De novo synthesis is the main source of ornithine for citrulline production in neonatal pigs

Citrulline is an amino acid synthesized in the gut and utilized for the synthesis of the conditionally essential amino acid arginine. Recently, the origin of the ornithine utilized for citrulline synthesis has become a matter of discussion. Multiple physiological factors may have contributed to the differences found among different researchers; one of these is the developmental stage of the subjects studied. To test the hypothesis that during the neonatal period de novo synthesis is the main source of ornithine for citrulline synthesis, neonatal piglets were infused intravenously or intragastrically with [U-(13)C(6)]arginine, [U-(13)C(5)]glutamine, or [U-(13)C(5)]proline during the fasted and fed periods. [ureido-(15)N]citrulline and [(2)H(2)]ornithine were infused intravenously for the entire infusion protocol. During fasting, plasma proline (13%) and ornithine (19%) were the main precursors for citrulline synthesis, whereas plasma arginine (62%) was the main precursor for plasma ornithine. During feeding, enteral (27%) and plasma (12%) proline were the main precursors for the ornithine utilized in the synthesis of citrulline, together with plasma ornithine (27%). Enteral proline and glutamine were utilized directly by the gut to produce ornithine utilized for citrulline synthesis. Arginine was not utilized by the gut, which is consistent with the lack of arginase activity in the neonate. Arginine, however, was the main source (47%) of plasma ornithine and in this way contributed to citrulline synthesis. In conclusion, during the neonatal period, the de novo pathway is the predominant source for the ornithine utilized in the synthesis of citrulline, and proline is the preferred precursor.

Arginine de novo and nitric oxide production in disease states

Arginine is derived from dietary protein intake, body protein breakdown, or endogenous de novo arginine production. The latter may be linked to the availability of citrulline, which is the immediate precursor of arginine and limiting factor for de novo arginine production. Arginine metabolism is highly compartmentalized due to the expression of the enzymes involved in arginine metabolism in various organs. A small fraction of arginine enters the NO synthase (NOS) pathway. Tetrahydrobiopterin (BH4) is an essential and rate-limiting cofactor for the production of NO. Depletion of BH4 in oxidative-stressed endothelial cells can result in so-called NOS3 "uncoupling," resulting in production of superoxide instead of NO. Moreover, distribution of arginine between intracellular transporters and arginine-converting enzymes, as well as between the arginine-converting and arginine-synthesizing enzymes, determines the metabolic fate of arginine. Alternatively, NO can be derived from conversion of nitrite. Reduced arginine availability stemming from reduced de novo production and elevated arginase activity have been reported in various conditions of acute and chronic stress, which are often characterized by increased NOS2 and reduced NOS3 activity. Cardiovascular and pulmonary disorders such as atherosclerosis, diabetes, hypercholesterolemia, ischemic heart disease, and hypertension are characterized by NOS3 uncoupling. Therapeutic applications to influence (de novo) arginine and NO metabolism aim at increasing substrate availability or at influencing the metabolic fate of specific pathways related to NO bioavailability and prevention of NOS3 uncoupling. These include supplementation of arginine or citrulline, provision of NO donors including inhaled NO and nitrite (sources), NOS3 modulating agents, or the targeting of endogenous NOS inhibitors like asymmetric dimethylarginine.

Everolimus and sirolimus in combination with cyclosporine have different effects on renal metabolism in the rat

Enhancement of calcineurin inhibitor nephrotoxicity by sirolimus (SRL) is limiting the clinical use of this drug combination. We compared the dose-dependent effects of the structurally related everolimus (EVL) and sirolimus (SRL) alone, and in combination with cyclosporine (CsA), on the rat kidney. Lewis rats were treated by oral gavage for 28 days using a checkerboard dosing format (0, 3.0, 6.0 and 10.0 CsA and 0, 0.5, 1.5 and 3.0 mg/kg/day SRL or EVL, n = 4/dose combination). After 28 days, oxidative stress, energy charge, kidney histologies, glomerular filtration rates, and concentrations of the immunosuppressants were measured along with ¹H-magnetic resonance spectroscopy (MRS) and gas chromatography- mass spectrometry profiles of cellular metabolites in urine. The combination of CsA with SRL led to higher urinary glucose concentrations and decreased levels of urinary Krebs cycle metabolites when compared to controls, suggesting that CsA+SRL negatively impacted proximal tubule metabolism. Unsupervised principal component analysis of MRS spectra distinguished unique urine metabolite patterns of rats treated with CsA+SRL from those treated with CsA+EVL and the controls. SRL, but not EVL blood concentrations were inversely correlated with urine Krebs cycle metabolite concentrations. Interestingly, the higher the EVL concentration, the closer urine metabolite patterns resembled those of controls, while in contrast, the combination of the highest doses of CsA+SRL showed the most significant differences in metabolite patterns. Surprisingly in this rat model, EVL and SRL in combination with CsA had different effects on kidney biochemistry, suggesting that further exploration of EVL in combination with low dose calcineurin inhibitors may be of potential benefit.

Arginine can be synthesized from enteral proline in healthy adult humans

There is considerable controversy recently in identifying dietary precursors for arginine synthesis. We have previously shown in human neonates and piglets that proline is the sole dietary precursor for arginine synthesis. It is unclear in adult humans whether proline is a dietary precursor for arginine. We performed a multi-tracer stable isotope study in adults using (15)N(2)-ureido arginine and (15)N proline to elucidate synthesis of citrulline and arginine and determine whether proline is a precursor for arginine. Primed, intermittent infusions of the labeled amino acids were given enterally to 5 healthy men consuming a standardized milkshake diet. Blood was sampled during plateau enrichment between 1.5 and 3 h. Plasma enrichment occurred for both tracers, giving enteral turnover estimates of 93 μmol · kg(-1) · h(-1) for arginine and 154 μmol · kg(-1) · h(-1) for proline. Appearance of the label from proline in arginine and the intermediaries, ornithine and citrulline, was measured in all participants. The rate of synthesis of arginine from proline was 3.7 μmol · kg(-1) · h(-1), which is estimated to be ~40% of newly synthesized arginine. In this first study in adult humans using an enteral proline tracer, we have demonstrated synthesis of arginine from this dietary amino acid. Therefore, as in newborns, proline must now be considered a dietary precursor for arginine in healthy adults.

Plasma arginine and ornithine are the main citrulline precursors in mice infused with arginine-free diets

Dietary arginine is the main dietary precursor for citrulline synthesis, but it is not known if other precursors can compensate when arginine is absent in the diet. To address this question, the contributions of plasma and dietary precursors were determined by using multitracer protocols in conscious mice infused i.g. either an arginine-sufficient diet [Arg(+)] or an arginine-free diet [Arg(-)]. The plasma entry rate of citrulline and arginine did not differ between the 2 diet groups (156 +/- 6 and 564 +/- 30 micromol kg(-1) h(-1), respectively); however, the entry rate of ornithine was greater in the mice fed the Arg(+) than the Arg(-) diet (332 +/- 33 vs. 180 +/- 16 micromol kg(-1) h(-1)). There was a greater utilization of plasma ornithine for the synthesis of citrulline (49 +/- 4 vs. 36 +/- 3 micromol kg(-1) h(-1), 30 +/- 3% vs. 24 +/- 2% of citrulline entry rate) in the mice fed the Arg(-) diet than the Arg(+) diet. The utilization of plasma arginine did not differ between the 2 diet groups for citrulline synthesis, either through plasma ornithine (approximately 29 +/- 3 micromol kg(-1) h(-1)) or at the site of citrulline synthesis (approximately 12 +/- 3 micromol kg(-1) h(-1)). The contribution of dietary proline to the synthesis of citrulline was mainly at the site of citrulline production (17 +/- 1 micromol kg(-1) h(-1)), rather than through plasma ornithine (5 +/- 0.4 micromol kg(-1) h(-1)). Dietary glutamine was utilized only at the site of citrulline synthesis (4 +/- 0.2 micromol kg(-1) h(-1)). Dietary glutamine and proline made a greater contribution to the synthesis of citrulline in mice fed the Arg(-) diet but remained minor sources for citrulline production. Plasma arginine and ornithine are able to support citrulline synthesis during arginine-free feeding.

Improved nutritional management of phenylketonuria by using a diet containing glycomacropeptide compared with amino acids

BACKGROUND

Phenylketonuria (PKU) requires a lifelong low-phenylalanine diet that provides the majority of protein from a phenylalanine-free amino acid (AA) formula. Glycomacropeptide (GMP), an intact protein formed during cheese production, contains minimal phenylalanine.

OBJECTIVE

The objective was to investigate the effects of substituting GMP food products for the AA formula on acceptability, safety, plasma AA concentrations, and measures of protein utilization in subjects with PKU.

DESIGN

Eleven subjects participated in an inpatient metabolic study with two 4-d treatments: a current AA diet (AA diet) followed by a diet that replaced the AA formula with GMP (GMP diet) supplemented with limiting AAs. Plasma concentrations of AAs, blood chemistries, and insulin were measured and compared in AA (day 4) and GMP diets (day 8).

RESULTS

The GMP diet was preferred to the AA diet in 10 of 11 subjects with PKU, and there were no adverse reactions to GMP. There was no significant difference in phenylalanine concentration in postprandial plasma with the GMP diet compared with the AA diet. When comparing fasting with postprandial plasma, plasma phenalyalanine concentration increased significantly with the AA but not with the GMP diet. Blood urea nitrogen was significantly lower, which suggests decreased ureagenesis, and plasma insulin was higher with the GMP diet than with the AA diet.

CONCLUSIONS

GMP, when supplemented with limiting AAs, is a safe and highly acceptable alternative to synthetic AAs as the primary protein source in the nutritional management of PKU. As an intact protein source, GMP improves protein retention and phenylalanine utilization compared with AAs.