Coadministration of ornithine and alpha-ketoglutarate is no more effective than ornithine alone as an arginine precursor in piglets enterally fed an arginine-deficient diet

Comprehensive profiling of amino acid response uncovers unique methionine-deprived response dependent on intact creatine biosynthesis

Besides being building blocks for protein synthesis, amino acids serve a wide variety of cellular functions, including acting as metabolic intermediates for ATP generation and for redox homeostasis. Upon amino acid deprivation, free uncharged tRNAs trigger GCN2-ATF4 to mediate the well-characterized transcriptional amino acid response (AAR). However, it is not clear whether the deprivation of different individual amino acids triggers identical or distinct AARs. Here, we characterized the global transcriptional response upon deprivation of one amino acid at a time. With the exception of glycine, which was not required for the proliferation of MCF7 cells, we found that the deprivation of most amino acids triggered a shared transcriptional response that included the activation of ATF4, p53 and TXNIP. However, there was also significant heterogeneity among different individual AARs. The most dramatic transcriptional response was triggered by methionine deprivation, which activated an extensive and unique response in different cell types. We uncovered that the specific methionine-deprived transcriptional response required creatine biosynthesis. This dependency on creatine biosynthesis was caused by the consumption of S-Adenosyl-L-methionine (SAM) during creatine biosynthesis that helps to deplete SAM under methionine deprivation and reduces histone methylations. As such, the simultaneous deprivation of methionine and sources of creatine biosynthesis (either arginine or glycine) abolished the reduction of histone methylation and the methionine-specific transcriptional response. Arginine-derived ornithine was also required for the complete induction of the methionine-deprived specific gene response. Collectively, our data identify a previously unknown set of heterogeneous amino acid responses and reveal a distinct methionine-deprived transcriptional response that results from the crosstalk of arginine, glycine and methionine metabolism via arginine/glycine-dependent creatine biosynthesis.

In order for mammalian cells to live and function, amino acids are required for protein synthesis and the generation of metabolic intermediates. An imbalance or deficiency of amino acids often triggers an “amino acid response” (AAR) to allow cells to adapt to their environment. However, it remains unclear whether the deprivation of any single amino acid leads to similar or different changes compared to the global AAR response or to other single amino acid deficiencies. To answer this question, we removed each or all of the 15 amino acids found in media from cells and comprehensively profiled the resulting changes in their RNA expression. Strikingly, we found a unique and dramatic gene expression program that occurred only when cells were deprived of methionine, but not any other amino acid. We also found that these methionine-specific changes depended on changes in histone modifications and an intact creatine biosynthesis pathway. Methionine deprivation reduced the degree to which histone proteins were indirectly modified by methionine (histone methylation). Creatine biosynthesis consumed methionine’s derivate S-Adenosyl-L-methionine (SAM), contributing to the reduction of histone methylation and an increase in ornithine-mediated signaling. Since methionine restriction may have anti-aging and other medical uses, our findings provide insights that will lead toward a better understanding of the underlying effects of methionine restriction and eventually improve human health.

Ornithine: the overlooked molecule in the regulation of polyamine metabolism

We overexpressed a mouse ornithine decarboxylase gene under the control of a constitutive and an estradiol-inducible promoter in Arabidopsis thaliana to increase our understanding of the regulation of polyamine metabolism. Of particular interest was the role of the substrate ornithine not only in the regulation of polyamine biosynthesis, but also in the accumulation of related amino acids in response to short-term induction of this enzyme. We hypothesized that the inducible expression of the transgene would mimic the natural responses of plants to changing conditions, e.g. under stress conditions and during rapid growth. Our results reveal that ornithine, even though present in relatively small quantities (compared with other amino acids of the glutamate-arginine-proline pathway), may not only be the key regulator of polyamine biosynthesis in Arabidopsis, but it may also regulate the entire subset of pathways for glutamate to arginine and to proline. Indirectly, it could also regulate putrescine catabolism, therefore contributing to the γ-aminobutyric acid content of the cells. Furthermore, the induction of mouse ornithine decarboxylase resulted in up- and down-regulation of several amino acids in the transgenic plants. It was learned that the turnover of putrescine in both the wild type and the transgenic plants occurs rapidly, with a half-life of 6-8 h.

Discovery of common urinary biomarkers for hepatotoxicity induced by carbon tetrachloride, acetaminophen and methotrexate by mass spectrometry-based metabolomics

Liver toxicity represents an important healthcare issue because it causes significant morbidity and mortality and can be difficult to predict before symptoms appear owing to drug therapy or exposure to toxicants. Using metabolomic techniques, we discovered common biomarkers for the prediction of hepatotoxicity in rat urine using mass spectrometry. For this purpose, liver toxicity was induced by 5 days of oral administration of carbon tetrachloride (1 ml kg(-1) per day), acetaminophen (1000 mg kg(-1) per day) and methotrexate (50 mg kg(-1) per day). Serum levels of alkaline phosphatase aspartate aminotransferase, alanine aminotransferase and histopathology in liver tissue were then checked to demonstrate liver toxicity. Global metabolic profiling with UPLC-TOF-MS (ultraperformance liquid chromatography-mass spectrometry), multivariate analysis (partial least square-discriminant analysis, hierarchical analysis) and database searching were performed to discover common biomarkers for liver toxicity induced by these three compounds. Urinary concentrations of the newly discovered biomarkers were then quantified to confirm them as biomarkers of hepatotoxicity with targeted metabolic profiling using GC (gas chromatography)-MS and CE (capillary electrophoresis)-MS. In the results, steroids, amino acids and bile acids were metabolically changed between the control and drug-treated groups in global metabolic profiling; 11β-hydroxyandrosterone, epiandrosterone, estrone, 11-dehydrocorticosterone, glycine, alanine, valine, leucine, dl-ornithine, 3-methylhistidine, cholic acid and lithocholic acid were selected as liver toxicity biomarkers after performing targeted metabolic profiling. In conclusion, we discovered metabolite biomarkers belonging to three different metabolic pathways to check for liver toxicity with mass spectrometry from a metabolomics study that could be used to evaluate hepatotoxicity induced by drugs or other toxic compounds.

Arginine is synthesized from proline, not glutamate, in enterally fed human preterm neonates

In neonatal mammals, arginine is synthesized in the enterocyte, with either proline or glutamate as the dietary precursor. We have shown several times in piglets that proline is the only precursor to arginine, although in vitro evidence supports glutamate in this role. Because of this uncertainty, we performed a multitracer stable isotope study to determine whether proline, glutamate, or both are dietary precursors for arginine in enterally fed human neonates. Labeled arginine (M + 2), proline (M + 1), and glutamate (M + 3) were given enterally to 15 stable, growing preterm infants (GA at birth 30-35 wk) at 1-3 wk postnatal age. Enrichment in urine of the tracer amino acids and the M + 1 and M + 3 isotopomers of arginine were measured by LC-tandem mass spectrometry to determine the contribution of proline and glutamate to arginine synthesis. Plateau enrichments of arginine and proline tracers were measurable in urine. Urinary glutamate enrichment was not detected. Conversion of proline to arginine was detected. However, the M + 3 isotopomer of arginine, which would have been synthesized from glutamate, was not detected. We conclude that, in contrast to the current consensus in the literature based on in vitro studies, proline is the major contributor to arginine synthesis in human preterm infants.

A multitracer stable isotope quantification of the effects of arginine intake on whole body arginine metabolism in neonatal piglets

We have previously shown that deficient arginine intake increased the rate of endogenous arginine synthesis from proline. In this paper, we report in vivo quantification of the effects of arginine intake on total endogenous arginine synthesis, on the rates of conversion between arginine, citrulline, ornithine, and proline, and on nitric oxide synthesis. Male piglets, with gastric catheters for diet and isotope infusion and femoral vein catheters for blood sampling, received a complete diet for 2 days and then either a generous (+Arg; 1.80 g x kg(-1) x day(-1); n = 5) or deficient (-Arg; 0.20 g.kg(-1).day(-1); n = 5) arginine diet for 5 days. On day 7, piglets received a primed, constant infusion of [guanido-(15)N(2)]arginine, [ureido-(13)C;5,5-(2)H(2)]citrulline, [U-(13)C(5)]ornithine, and [(15)N;U-(13)C(5)]proline in an integrated study of the metabolism of arginine and its precursors. Arginine synthesis (micromol x kg(-1) x h(-1)) from both proline (+Arg: 42, -Arg: 74, pooled SE: 5) and citrulline (+Arg: 67, -Arg: 120; pooled SE: 15) were higher in piglets receiving the -Arg diet (P < 0.05); and for both diets proline accounted for approximately 60% of total endogenous arginine synthesis. The conversion of proline to citrulline (+Arg: 39, -Arg: 67, pooled SE: 6) was similar to the proline-to-arginine conversion, confirming that citrulline formation limits arginine synthesis from proline in piglets. Nitric oxide synthesis (micromol x kg(-1) x h(-1)), measured by the rate conversion of [guanido-(15)N(2)]arginine to [ureido-(15)N]citrulline, was greater in piglets receiving the +Arg diet (105) than in those receiving the -Arg diet (46, pooled SE: 10; P < 0.05). This multi-isotope method successfully allowed many aspects of arginine metabolism to be quantified simultaneously in vivo.