Targeting Metabolism for Cancer Therapy

Metabolic reprogramming contributes to tumor development and introduces metabolic liabilities that can be exploited to treat cancer. Chemotherapies targeting metabolism have been effective cancer treatments for decades, and the success of these therapies demonstrates that a therapeutic window exists to target malignant metabolism. New insights into the differential metabolic dependencies of tumors have provided novel therapeutic strategies to exploit altered metabolism, some of which are being evaluated in preclinical models or clinical trials. Here, we review our current understanding of cancer metabolism and discuss how this might guide treatments targeting the metabolic requirements of tumor cells.

Diversion of aspartate in ASS1-deficient tumours fosters de novo pyrimidine synthesis

Cancer cells hijack and remodel existing metabolic pathways for their benefit. Argininosuccinate synthase (ASS1) is a urea cycle enzyme that is essential in the conversion of nitrogen from ammonia and aspartate to urea. A decrease in nitrogen flux through ASS1 in the liver causes the urea cycle disorder citrullinaemia. In contrast to the well-studied consequences of loss of ASS1 activity on ureagenesis, the purpose of its somatic silencing in multiple cancers is largely unknown. Here we show that decreased activity of ASS1 in cancers supports proliferation by facilitating pyrimidine synthesis via CAD (carbamoyl-phosphate synthase 2, aspartate transcarbamylase, and dihydroorotase complex) activation. Our studies were initiated by delineating the consequences of loss of ASS1 activity in humans with two types of citrullinaemia. We find that in citrullinaemia type I (CTLN I), which is caused by deficiency of ASS1, there is increased pyrimidine synthesis and proliferation compared with citrullinaemia type II (CTLN II), in which there is decreased substrate availability for ASS1 caused by deficiency of the aspartate transporter citrin. Building on these results, we demonstrate that ASS1 deficiency in cancer increases cytosolic aspartate levels, which increases CAD activation by upregulating its substrate availability and by increasing its phosphorylation by S6K1 through the mammalian target of rapamycin (mTOR) pathway. Decreasing CAD activity by blocking citrin, the mTOR signalling, or pyrimidine synthesis decreases proliferation and thus may serve as a therapeutic strategy in multiple cancers where ASS1 is downregulated. Our results demonstrate that ASS1 downregulation is a novel mechanism supporting cancerous proliferation, and they provide a metabolic link between the urea cycle enzymes and pyrimidine synthesis.

Argininosuccinate synthetase from the urea cycle to the citrulline-NO cycle

Argininosuccinate synthetase (ASS, EC 6.3.4.5) catalyses the condensation of citrulline and aspartate to form argininosuccinate, the immediate precursor of arginine. First identified in the liver as the limiting enzyme of the urea cycle, ASS is now recognized as a ubiquitous enzyme in mammalian tissues. Indeed, discovery of the citrulline-NO cycle has increased interest in this enzyme that was found to represent a potential limiting step in NO synthesis. Depending on arginine utilization, location and regulation of ASS are quite different. In the liver, where arginine is hydrolyzed to form urea and ornithine, the ASS gene is highly expressed, and hormones and nutrients constitute the major regulating factors: (a) glucocorticoids, glucagon and insulin, particularly, control the expression of this gene both during development and adult life; (b) dietary protein intake stimulates ASS gene expression, with a particular efficiency of specific amino acids like glutamine. In contrast, in NO-producing cells, where arginine is the direct substrate in the NO synthesis, ASS gene is expressed at a low level and in this way, proinflammatory signals constitute the main factors of regulation of the gene expression. In most cases, regulation of ASS gene expression is exerted at a transcriptional level, but molecular mechanisms are still poorly understood.

Mitochondrial aspartate glutamate carrier (citrin) deficiency as the cause of adult-onset type II citrullinemia (CTLN2) and idiopathic neonatal hepatitis (NICCD)

By using homozygosity mapping and positional cloning, we have shown that adult-onset type II citrullinemia (CTLN2) is caused by mutations of the SLC25A13 gene, which is localized on chromosome 7q21.3 and encodes a mitochondrial solute carrier protein named citrin. So far, we have reported nine mutations, most of which cause loss of citrin, and we have established several methods for DNA diagnosis. These methods have shown that more than 90% of the patients diagnosed as suffering from CTLN2 by enzymatic analysis carry SLC25A13 mutations in both alleles, indicating that CTLN2 is caused by citrin deficiency. Furthermore, by using the same DNA diagnosis methods, we discovered that 70 neonates or infants suffering from a particular type of neonatal hepatitis carry the same SLC25A13 mutations. Since the symptoms of the neonates are different from those of the more severe CTLN2 and usually ameliorate without special treatment, we designated the neonatal disease neonatal intrahepatic cholestasis caused by citrin deficiency (NICCD). We conclude that citrin deficiency causes NICCD in neonates and CTLN2 in adults through the additional effects of genetic or environmental modifiers. Since the function of citrin, together with that of an isoform, aralar, was found to be as a mitochondrial aspartate glutamate carrier, the various symptoms of NICCD and CTLN2 may be understood as caused by defective aspartate export from the mitochondria to the cytosol and defects in the malate aspartate shuttle. It is, however, still difficult to understand the cause of the hepatic deficiency of argininosuccinate synthetase protein in CTLN2.

Arginine deprivation and argininosuccinate synthetase expression in the treatment of cancer

Arginine, a semi-essential amino acid in humans, is critical for the growth of human cancers, particularly those marked by de novo chemoresistance and a poor clinical outcome. In addition to protein synthesis, arginine is involved in diverse aspects of tumour metabolism, including the synthesis of nitric oxide, polyamines, nucleotides, proline and glutamate. Tumoural downregulation of the enzyme argininosuccinate synthetase (ASS1), a recognised rate-limiting step in arginine synthesis, results in an intrinsic dependence on extracellular arginine due to an inability to synthesise arginine for growth. This dependence on extracellular arginine is known as arginine auxotrophy. Several tumours are arginine auxotrophic, due to variable loss of ASS1, including hepatocellular carcinoma, malignant melanoma, malignant pleural mesothelioma, prostate and renal cancer. Importantly, targeting extracellular arginine for degradation in the absence of ASS1 triggers apoptosis in arginine auxotrophs. Several phase I/II clinical trials of the arginine-lowering drug, pegylated arginine deiminase, have shown encouraging evidence of clinical benefit and low toxicity in patients with ASS1-negative tumours. In part, ASS1 loss is due to epigenetic silencing of the ASS1 promoter in various human cancer cell lines and tumours, and it is this silencing that confers arginine auxotrophy. In relapsed ovarian cancer, this is associated with platinum refractoriness. In contrast, several platinum sensitive tumours, including primary ovarian, stomach and colorectal cancer, are characterised by ASS1 overexpression, which is regulated by proinflammatory cytokines. This review examines the prospects for novel approaches in the prevention, diagnosis and treatment of malignant disease based on ASS1 pathophysiology and its rate-limiting product, arginine.

Arginine deficiency in preterm infants: biochemical mechanisms and nutritional implications

Arginine, an amino acid that is nutritionally essential for the fetus and neonate, is crucial for ammonia detoxification and the synthesis of molecules with enormous importance (including creatine, nitric oxide, and polyamines). A significant nutritional problem in preterm infants is a severe deficiency of arginine (hypoargininemia), which results in hyperammonemia, as well as cardiovascular, pulmonary, neurological, and intestinal dysfunction. Arginine deficiency may contribute to the high rate of infant morbidity and mortality associated with premature births. Although hypoargininemia in preterm infants has been recognized for more than 30 years, it continues to occur in neonatal intensive care units in the United States and worldwide. On the basis of recent findings, we propose that intestinal citrulline and arginine synthesis (the major endogenous source of arginine) is limited in preterm neonates owing to the limited expression of the genes for key enzymes (e.g., pyrroline-5-carboxylate synthase, argininosuccinate synthase and lyase), thereby contributing to hypoargininemia. Because premature births in humans occur before the normal perinatal surge of cortisol (an inducer of the expression of key arginine-synthetic enzymes), its administration may be a useful tool to advance the maturation of intestinal arginine synthesis in preterm neonates. Additional benefits of cortisol treatment may include the following: 1) allowing early introduction of enteral feeding to preterm infants, which is critical for intestinal synthesis of citrulline, arginine, and polyamines as well as for intestinal motility, integrity, and growth; and 2) shortening the expensive stay of preterm infants in hospitals as a result of accelerated organ maturation and the restoration of full enteral feeding. Further studies of fetal and neonatal arginine metabolism will continue to advance our understanding of the mechanisms responsible for the survival and growth of preterm infants. This new knowledge will be beneficial for designing the next generation of enteral and parenteral amino acid solutions to optimize nutrition and health in this compromised population.

The genetic organization of arginine biosynthesis in Pseudomonas aeruginosa

Six loci coding for arginine biosynthetic enzymes in Pseudomonas aeruginosa strain PAO were identified by enzyme assay: argA (N-acetylglutamate synthase), argB (N-acetylglutamate 5-phosphotransferase), argC (N-acetylglutamate 5-semialdehyde dehydrogenase), argF (anabolic ornithine carbamoyl-transferase), argG (argininosuccinate synthetase), and argH (argininosuccinase). One-step mutants which had a requirement for arginine and uracil were defective in carbamoylphosphate synthase, specified by a locus designated car. To map these mutations we used the sex factor FP2 in an improved interrupted mating technique as well as the generalized transducing phages F116L and G101. We confirmed earlier studies, and found no clustering of arg and car loci. However, argA, argH, and argB were mapped on a short chromosome segment (approx. 3 min long), and argF and argG were cotransducible, but not contiguous. N-Acetylglutamate synthase, the enzyme which replenishes the cycle of acetylated intermediates in ornithine synthesis of Pseudomonas, appears to be essential for arginine synthesis since argA mutants showed no growth on unsupplemented minimal medium.

Renal cell carcinoma does not express argininosuccinate synthetase and is highly sensitive to arginine deprivation via arginine deiminase

Recently, pegylated arginine deiminase (ADI; EC 3.5.3.6) has been used to treat the patients with hepatocellular carcinoma or melanoma, in which the level of argininosuccinate synthetase (ASS) activity is low or undetectable. The efficacy of its antitumor activity largely depends on the level of intracellular ASS, which enables tumor cells to recycle citrulline to arginine. Thus, we examined the expression levels of ASS in various cancer cells and found that it is low in renal cell carcinoma (RCC) cells, rendering the cells highly sensitive to arginine deprivation by ADI treatment. Immunohistochemical analysis revealed that in biopsy specimens from RCC patients (n = 98), the expression of ASS is highly demonstrated in the epithelium of normal proximal tubule but not seen in tumor cells. Furthermore, RCC cells treated with ADI showed remarkable growth retardation in a dose dependent manner. ADI also exerted in vivo antiproliferative effect on the allografted renal cell carcinoma (RENCA) tumor cells and prolonged the survival of tumor-bearing mice. Histological examination of the tumors revealed that tumor angiogenesis and vascular endothelial growth factor (VEGF) expression were significantly diminished by ADI administration. Therefore, these findings suggest that arginine deprivation by ADI could provide a beneficial strategy for the treatment of RCC in ways of inhibitions of arginine availability and neovascularization.

Glucocorticoids regulate inducible nitric oxide synthase by inhibiting tetrahydrobiopterin synthesis and L-arginine transport

The cytokine-inducible isoform of nitric oxide synthase (iNOS or NOS2) plays an important role in the immune response to some pathogens. Within the heart, increased activity of NOS2 in cardiac microvascular endothelial cells (CMEC) also can diminish the contractile function of adjacent cardiac myocytes. Glucocorticoids, which are known to suppress cytokine induction of NOS2 in many cell types, caused only a moderate (approximately 20%) decline in NOS2 protein content and maximal activity measured in homogenates of cytokine-treated CMEC, but almost completely inhibited synthesis of nitrogen oxides (NOx) by intact cells. To determine whether glucocorticoids were inhibiting cellular NOx production by limiting the availability of NOS co-factors or substrate, the effect of dexamethasone on tetrahydrobiopterin (BH4) and L-arginine availability in cytokine-treated CMEC was examined. Dexamethasone prevented the coordinate induction of GTP cyclohydrolase I with NOS2 after exposure to interleukin-1beta and interferon-gamma and also the increase in intracellular BH4 content in cytokine-treated CMEC. Addition of BH4 overcame dexamethasone-mediated suppression of nitrite production. Dexamethasone also prevented a cytokine-mediated increase in L-arginine uptake into CMEC by suppressing the induction of the high affinity cationic amino acid transporters CAT-1 and CAT-2B and the low affinity CAT-2A transporter. In addition, dexamethasone also inhibited cytokine induction in CMEC of argininosuccinate synthase, the rate-limiting enzyme for the de novo synthesis of arginine from citrulline. Thus, glucocorticoids regulate NOx production following cytokine exposure in cardiac microvascular endothelial cells primarily by limiting BH4 and L-arginine availability.

The Gap1 general amino acid permease acts as an amino acid sensor for activation of protein kinase A targets in the yeast Saccharomyces cerevisiae

Addition of a nitrogen source to yeast (Saccharomyces cerevisiae) cells starved for nitrogen on a glucose-containing medium triggers activation of protein kinase A (PKA) targets through a pathway that requires for sustained activation both a fermentable carbon source and a complete growth medium (fermentable growth medium induced or FGM pathway). Trehalase is activated, trehalose and glycogen content as well as heat resistance drop rapidly, STRE-controlled genes are repressed, and ribosomal protein genes are induced. We show that the rapid effect of amino acids on these targets specifically requires the general amino acid permease Gap1. In the gap1Delta strain, transport of high concentrations of l-citrulline occurs at a high rate but without activation of trehalase. Metabolism of the amino acids is not required. Point mutants in Gap1 with reduced or deficient transport also showed reduced or deficient signalling. However, two mutations, S391A and S397A, were identified with a differential effect on transport and signalling for l-glutamate and l-citrulline. Specific truncations of the C-terminus of Gap1 (e.g. last 14 or 26 amino acids) did not reduce transport activity but caused the same phenotype as in strains with constitutively high PKA activity also during growth with ammonium as sole nitrogen source. The overactive PKA phenotype was abolished by mutations in the Tpk1 or Tpk2 catalytic subunits. We conclude that Gap1 acts as an amino acid sensor for rapid activation of the FGM signalling pathway which controls the PKA targets, that transport through Gap1 is connected to signalling and that specific truncations of the C-terminus result in permanently activating Gap1 alleles.

Neuronal and glial coexpression of argininosuccinate synthetase and inducible nitric oxide synthase in Alzheimer disease

The enzyme argininosuccinate synthetase (ASS) is the rate limiting enzyme in the metabolic pathway leading from L-citrulline to L-arginine, the physiological substrate of all isoforms of nitric oxide synthases (NOS). ASS and inducible NOS (iNOS) expression in neurons and glia was investigated by immunohistochemistry in brains of Alzheimer disease (AD) patients and nondemented, age-matched controls. In 3 areas examined (hippocampus, frontal, and entorhinal cortex), a marked increase in neuronal ASS and iNOS expression was observed in AD brains. GFAP-positive astrocytes expressing ASS were not increased in AD brains versus controls, whereas the number of iNOS expressing GFAP-positive astrocytes was significantly higher in AD brains. Density measurements revealed that ASS expression levels were significantly higher in glial cells of AD brains. Colocalization of ASS and iNOS immunoreactivity was detectable in neurons and glia. Occasionally, both ASS-and iNOS expression was detectable in CD 68-positive activated microglia cells in close proximity to senile plaques. These results suggest that neurons and astrocytes express ASS in human brain constitutively, whereas neuronal and glial ASS expression increases parallel to iNOS expression in AD. Because an adequate supply of L-arginine is indispensable for prolonged NO generation, coinduction of ASS enables cells to sustain NO generation during AD by replenishing necessary supply of L-arginine.

Metabolic engineering against the arginine microenvironment enhances CAR-T cell proliferation and therapeutic activity

Hematological and solid cancers catabolize the semiessential amino acid arginine to drive cell proliferation. However, the resulting low arginine microenvironment also impairs chimeric antigen receptor T cells (CAR-T) cell proliferation, limiting their efficacy in clinical trials against hematological and solid malignancies. T cells are susceptible to the low arginine microenvironment because of the low expression of the arginine resynthesis enzymes argininosuccinate synthase (ASS) and ornithine transcarbamylase (OTC). We demonstrate that T cells can be reengineered to express functional ASS or OTC enzymes, in concert with different chimeric antigen receptors. Enzyme modifications increase CAR-T cell proliferation, with no loss of CAR cytotoxicity or increased exhaustion. In vivo, enzyme-modified CAR-T cells lead to enhanced clearance of leukemia or solid tumor burden, providing the first metabolic modification to enhance CAR-T cell therapies.

Arginine Deprivation Inhibits the Warburg Effect and Upregulates Glutamine Anaplerosis and Serine Biosynthesis in ASS1-Deficient Cancers

Targeting defects in metabolism is an underutilized strategy for the treatment of cancer. Arginine auxotrophy resulting from the silencing of argininosuccinate synthetase 1 (ASS1) is a common metabolic alteration reported in a broad range of aggressive cancers. To assess the metabolic effects that arise from acute and chronic arginine starvation in ASS1-deficient cell lines, we performed metabolite profiling. We found that pharmacologically induced arginine depletion causes increased serine biosynthesis, glutamine anaplerosis, oxidative phosphorylation, and decreased aerobic glycolysis, effectively inhibiting the Warburg effect. The reduction of glycolysis in cells otherwise dependent on aerobic glycolysis is correlated with reduced PKM2 expression and phosphorylation and upregulation of PHGDH. Concurrent arginine deprivation and glutaminase inhibition was found to be synthetic lethal across a spectrum of ASS1-deficient tumor cell lines and is sufficient to cause in vivo tumor regression in mice. These results identify two synthetic lethal therapeutic strategies exploiting metabolic vulnerabilities of ASS1-negative cancers.

Macrophage fumarate hydratase restrains mtRNA-mediated interferon production

Metabolic rewiring underlies the effector functions of macrophages1-3, but the mechanisms involved remain incompletely defined. Here, using unbiased metabolomics and stable isotope-assisted tracing, we show that an inflammatory aspartate-argininosuccinate shunt is induced following lipopolysaccharide stimulation. The shunt, supported by increased argininosuccinate synthase (ASS1) expression, also leads to increased cytosolic fumarate levels and fumarate-mediated protein succination. Pharmacological inhibition and genetic ablation of the tricarboxylic acid cycle enzyme fumarate hydratase (FH) further increases intracellular fumarate levels. Mitochondrial respiration is also suppressed and mitochondrial membrane potential increased. RNA sequencing and proteomics analyses demonstrate that there are strong inflammatory effects resulting from FH inhibition. Notably, acute FH inhibition suppresses interleukin-10 expression, which leads to increased tumour necrosis factor secretion, an effect recapitulated by fumarate esters. Moreover, FH inhibition, but not fumarate esters, increases interferon-β production through mechanisms that are driven by mitochondrial RNA (mtRNA) release and activation of the RNA sensors TLR7, RIG-I and MDA5. This effect is recapitulated endogenously when FH is suppressed following prolonged lipopolysaccharide stimulation. Furthermore, cells from patients with systemic lupus erythematosus also exhibit FH suppression, which indicates a potential pathogenic role for this process in human disease. We therefore identify a protective role for FH in maintaining appropriate macrophage cytokine and interferon responses.

Identification of two novel mutations in the SLC25A13 gene and detection of seven mutations in 102 patients with adult-onset type II citrullinemia

Adult-onset type II citrullinemia (CTLN2) is characterized by a liver-specific deficiency of argininosuccinate synthetase (ASS) protein. We have recently identified the gene responsible for CTLN2, viz., SLC25A13, which encodes a calcium-binding mitochondrial carrier protein, designated citrin, and found five mutations of the SLC25A13 gene in CTLN2 patients. In the present study, we have identified two novel mutations, 1800ins1 and R605X, in SLC25A13 mRNA and the SLC25A13 gene. Diagnostic analysis for the seven mutations in 103 CTLN2 patients diagnosed by biochemical and enzymatic studies has revealed that 102 patients had one or two of the seven mutations and 93 patients were homozygotes or compound heterozygotes. These results indicate that CTLN2 is caused by an abnormality in the SLC25A13 gene, and that our criteria for CTLN2 before DNA diagnosis are correct. Five of 22 patients from consanguineous unions have been shown to be compound heterozygotes, suggesting a high frequency of the mutated genes. The frequency of homozygotes is calculated to be more than 1 in 20,000 from carrier detection (6 in 400 individuals tested) in the Japanese population. We have detected no cross-reactive immune materials in the liver of CTLN2 patients with any of the seven mutations by Western blot analysis with anti-human citrin antibody. From these findings, we hypothesize that CTLN2 is caused by a complete deletion of citrin, although the mechanism of ASS deficiency is still unknown.

S-nitrosylation is emerging as a specific and fundamental posttranslational protein modification: head-to-head comparison with O-phosphorylation

Nitric oxide (NO) is a free-radical product of mammalian cell metabolism that plays diverse and important roles in the regulation of cell function. Biological actions of NO arise as a direct consequence of chemical reactions between NO or NO-derived species and protein targets. Reactions of NO with transition metals in target proteins have garnered the most attention to date as the principal mechanism of NO signaling; nonetheless, S-nitrosylation of protein Cys residues is rapidly moving to center stage in importance. In general, however, there has been a delay in adequate appreciation of the role of S-nitrosylation in biological signaling by NO. This lag is attributed to a poor understanding of the basis for selective targeting of NO to particular thiols, and methodological limitations in accurately quantifying this modification--recent breakthroughs in concepts and methods diminish these barriers. Here, we consider the wheres and whys of protein S-nitrosylation and its basis for specificity. Protein S-nitrosylation potentially represents a ubiquitous and fundamental mechanism for posttranslational control of protein activity on a par with that of O-phosphorylation.

L-arginine uptake, the citrulline-NO cycle and arginase II in the rat brain: an in situ hybridization study

Nitric oxide (NO) is synthesized from a unique precursor, arginine, by nitric oxide synthase (NOS). In brain cells, arginine is supplied by protein breakdown or extracted from the blood through cationic amino acid transporters (CATs). Arginine can also be recycled from the citrulline produced by NOS activity, through argininosuccinate synthetase (AS) and argininosuccinate lyase (AL) activities, and metabolized by arginase. NOS, AS and AL constitute the so-called citrulline-NO cycle. In order to better understand arginine transport, recycling and degradation, we studied the regional distribution of cells expressing CAT1, CAT3, AS, AL, neuronal NOS (nNOS) and arginase II (AII) in the adult rat brain by non-radioisotopic in situ hybridization (ISH). CAT1, AL and AII presented an ubiquitous neuronal and glial expression, whereas CAT3 and AS were confined to neurons. nNOS was restricted to scattered neurons and a few brain nuclei and layers. We demonstrate by this study that cells expressing nNOS all appear to express the entire citrulline-NO cycle, whereas numerous cells expressing AL do not express AS. The differential expression of these genes within the same anatomical structure could indicate that intercellular exchanges of citrulline-NO cycle metabolites are relevant. Thus vicinal interactions should be taken into account to study their regulatory mechanisms.

Control of the flux in the arginine pathway of Neurospora crassa. Modulations of enzyme activity and concentration

The influence of particular enzyme activities on the flux of metabolites in a pathway can be estimated by 'modulating' enzymes (i.e. changing turnover or concentration) and measuring the response in various parts of the system. By controlling the nuclear ration of two genetically different nuclear types in heterokaryons, the enzyme concentrations at four different steps in the arginine pathway were decreased over a range. This range was extended by the use of bradytrophs, mutant strains specifying enzymes with greatly diminished enzyme activities. Strains altered simultaneously at more than one step were also constructed by genetic recombination. By measuring the outputs of the pathway and the steady-state concentrations of intermediate pools, the fluxes in different parts of the pathway were calculated. This allowed the construction of flux/enzyme relationships, the slope of which is a measure of the sensitivity of a flux to the change in enzyme activity at that step. All fluxes were found to be considerably buffered for quite substantial decreases in the activities of all enzymes. Mass action plays an important part in this phenomenon, as do inhibition and repression. Because of the existence of expansion fluxes in growing systems, we find quantitatively different fluxes in different parts of the single pathway. For the same reason some enzyme modulations given decreased fluxes in one part and increased fluxes in another. The understanding of control in the pathway thus involves consideration of many mechanisms operating simultaneously and the estimation of changes in the whole system. The concept of a 'rate-limiting step' is found to be inadequate and is replaced by a quantitative measure, the Sensitivity Coefficient, which takes account of all the interactions. It is shown that control of the flux is shared among all the enzymes of the pathway. The results are discussed in terms of the theory of flux control.

Cardiac-specific overexpression of inducible nitric oxide synthase does not result in severe cardiac dysfunction

Nitric oxide (NO), a potent regulator of myocardial contractility, has been implicated in the development of heart failure; however, no study exists describing the relation between expression of inducible nitric oxide synthase (iNOS), formation of NO in vivo, and cardiac contractility. We have therefore generated transgenic (TG) mice overexpressing iNOS under the cardiospecific alpha-myosin heavy chain (alpha-MHC) promoter. In vitro, iNOS activity in hearts of two transgenic lines was 260- to 400-fold above controls (wild type [WT]), but TG mice were viable and appeared normal. Ventricular mass/body weight ratio did not differ; heart rate and cardiac output as well as mean arterial blood pressure were decreased by 10%. NO(x) levels of hearts and blood of TG mice were 2.5- and 2-fold above WT controls, respectively. In the isolated heart, release of the NO oxidation products nitrate and nitrite, an index of in vivo NOS activity, was 40-fold over WT. However, cardiac hemodynamics and levels of ATP and phosphocreatine were unaltered. The high iNOS activity was associated with reduced cardiac L-arginine in TG hearts to only 15% of the WT, indicating limited substrate availability, whereas L-citrulline was 20-fold elevated. Our findings demonstrate that the heart can tolerate high levels of iNOS activity without detrimental functional consequences. The concept that iNOS-derived NO is the triggering factor in the pathomechanism leading to heart failure therefore needs to be reevaluated.

Regulation of arginine biosynthesis in Saccharomyces cerevisiae: isolation of a cis-dominant, constitutive mutant for ornithine carbamoyltransferase synthesis

A cis-dominant mutation linked to argF, the structural gene specifying ornithine carbamoyltransferase, and affecting the control of the synthesis of this enzyme has been obtained. The level of ornithine carbamoyltransferase in this mutation is depressed and less repressible by addition of L-arginine than it is in the wild-type strain. Of 38 tetrads analyzed, resulting from a cross of a strain harboring this mutation with a strain carrying an argF- mutation, none was a tetratype or a nonparental ditype. This operator mutation helps to define a negative mode of control of the synthesis of the arginine biosynthetic enzymes, as had been suggested earlier upon the isolation of argRI- (arg80), argRII- (arg81), and argRIII- (arg82) specific regulatory mutations.

The utilization of protein and energy during lactation in the rat, with particular regard to the use of fat accumulated in pregnancy

1. The deposition of fat during pregnancy and its subsequent mobilization during lactation was studied in the rat. The utilization of protein during lactation was also investigated. 2. Fat accumulation was rapid and continuous from early pregnancy. Approximately one-third was deposited subcutaneously, the remainder being stored in central depots. 3. The volume of milk produced during lactation was varied by offering protein at two concentrations from day 2 post partum. Optimum lactation was achieved with a high-protein diet. The dams lost no body protein, but 60% of the total body fat was mobilized, despite a marked increase in food consumption. On a low-protein diet, food intake did not rise during lactation. Some body protein was catabolized and, again, 60% of the fat was mobilized although milk output, as judged by the gain in carcass energy of their pups, was reduced to approximately one-third of optimum. 4. Protein was used more efficiently by lactating rats, when compared with virgin controls. 5. We conclude that fat stored during pregnancy makes a major contribution to the energy costs of lactation, and that its mobilization is under hormonal rather than dietary control.

Pegylated arginine deiminase: a novel anticancer enzyme agent

Pegylated arginine deiminase (ADI-PEG20) is a novel anticancer enzyme that produces depletion of arginine, which is a nonessential amino acid in humans. Certain tumours, such as malignant melanoma and hepatocellular carcinoma, are auxotrophic for arginine. These tumours that are sensitive to arginine depletion do not express argininosuccinate synthetase, a key enzyme in the synthesis of arginine from citrulline. ADI-PEG20 inhibits human melanomas and hepatocellular carcinomas in vitro and in vivo. Phase I - II trials in patients with melanoma and hepatocellular carcinomas have shown the drug to have antitumour activity and tolerable side effects. Large Phase II trials and randomised, controlled Phase III trials are needed to determine its overall efficacy in the treatment of these malignancies and others.

Immunohistochemical localization of argininosuccinate synthetase in the rat brain in relation to nitric oxide synthase-containing neurons

The distribution of the urea cycle enzyme, argininosuccinate synthetase, in the rat brain was determined using immunohistochemistry. This enzyme participates in the only known metabolic pathway for citrulline, its condensation with aspartate to form argininosuccinate, which can then be cleaved to fumarate and arginine. It may thus provide a mechanism to recycle citrulline, formed in the nervous system via nitric oxide synthase activity, back to the nitric oxide precursor, L-arginine. Argininosuccinate synthetase immunoreactivity was detected in discrete populations of neurons throughout the brain. Double-staining with nicotinamide adenine dinucleotide phosphate (reduced form)-diaphorase histochemistry for the localization of nitric oxide synthase demonstrated that argininosuccinate synthetase coexists with nitric oxide synthase in some brain regions. However, many neurons were found that contained one of these two enzymes, but not the other. Thus some nitric oxide synthase-containing neurons appear able to recycle citrulline via argininosuccinate, while others do not. Additional roles for argininosuccinate synthetase in the brain are discussed.

Proteomic analysis of cytokine induced proteins in human intestinal epithelial cells: implications for inflammatory bowel diseases

A role for cytokine regulated proteins in epithelial cells has been suggested in the pathogenesis of inflammatory bowel diseases (IBD). The aim of this study was to identify such cytokine regulated targets using a proteomic functional approach. Protein patterns from (35)S-radiolabeled homogenates of cultured colon epithelial cells were compared before and after exposure to interferon-gamma, interleukin-1beta and interleukin-6. Proteins were separated by two-dimensional polyacrylamide gel electrophoresis. Both autoradiographies and silver stained gels were analyzed. Proteins showing differential expression were identified by tryptic in-gel digestion and mass spectrometry. Metabolism related proteins were also investigated by Western blot analysis. Tryptophanyl-tRNA synthetase, indoleamine-2,3-dioxygenase, heterogeneous nuclear ribonucleoprotein JKTBP, interferon-induced 35kDa protein, proteasome subunit LMP2 and arginosuccinate synthetase were identified as cytokine modulated proteins in vitro. Using purified epithelial cells from patients, overexpression of indoleamine-2,3-dioxygenase, an enzyme involved in tryptophan metabolism, was confirmed in Crohn's disease as well as in ulcerative colitis, as compared to normal mucosa. No such difference was found in diverticulitis. Potentially, this observation opens new avenues in the treatment of IBD.