Single amino acid (arginine) deprivation: rapid and selective death of cultured transformed and malignant cells

Aminopeptidase inhibition as a targeted treatment strategy in myeloma

Myeloma cells are highly dependent on the unfolded protein response to assemble folded immunoglobulins correctly. Therefore, targeting protein handling within a myeloma cell by inhibiting the aminopeptidase enzyme system, which catalyses the hydrolysis of amino acids from the proteins NH2 terminus, represents a therapeutic approach. CHR-2797, a novel aminopeptidase inhibitor, is able to inhibit proliferation and induce growth arrest and apoptosis in myeloma cells, including cells resistant to conventional chemotherapeutics. It causes minimal inhibition of bone marrow stromal cell (BMSC) proliferation but is able to overcome the microenvironmental protective effects, inhibiting the proliferation of myeloma cells bound to BMSCs and the increase in vascular endothelial growth factor levels seen when myeloma cells and BMSCs are bound together. Additive and synergistic effects are seen with bortezomib, melphalan, and dexamethasone. Apoptosis occurs via both caspase-dependent and non-caspase-dependent pathways with an increase in Noxa, cleavage of Mcl-1, and activation of the unfolded protein response. Autophagy is also seen. CHR-2797 causes an up-regulation of genes involved in the proteasome/ubiquitin pathway, as well as aminopeptidases, and amino acid deprivation response genes. In conclusion, inhibiting protein turnover using the aminopeptidase inhibitor CHR-2797 results in myeloma cell apoptosis and represents a novel therapeutic approach that warrants further investigation in the clinical setting.

Recombinant human arginase inhibits proliferation of human hepatocellular carcinoma by inducing cell cycle arrest

Human hepatocellular carcinoma (HCC) has an elevated requirement for arginine in vitro, and pegylated recombinant human arginase I (rhArg-PEG), an arginine-depleting enzyme, can inhibit the growth of arginine-dependent tumors. While supplementation of the culture medium with ornithine failed to rescue Hep3B cells from growth inhibition induced by rhArg-PEG, citrulline successfully restored cell growth. The data support the roles previously proposed for ornithine transcarbamylase (OTC) in the arginine auxotrophy and rhArg-PEG sensitivity of HCC cells. Expression profiling of argininosuccinate synthetase (ASS), argininosuccinate lyase (ASL) and OTC in 40 HCC tumor biopsy specimens predicted that 16 of the patients would be rhArg-sensitive, compared with 5 who would be sensitive to arginine deiminase (ADI), another arginine-depleting enzyme with anti-tumor activity. Furthermore, rhArg-PEG-mediated deprivation of arginine from the culture medium of different HCC cell lines produced cell cycle arrests at the G(2)/M or S phase, possibly mediated by transcriptional modulation of cyclins and/or cyclin dependent kinases (CDKs). Based on these results, together with further validation of the in vivo efficacy of rhArg-PEG against HCC, we propose that the application of rhArg-PEG alone or in combination with existing chemotherapeutic drugs may represent a specific and effective therapeutic strategy against HCC.

Stable isotopic labeling in proteomics

Labeling of proteins and peptides with stable heavy isotopes (deuterium, carbon-13, nitrogen-15, and oxygen-18) is widely used in quantitative proteomics. These are either incorporated metabolically in cells and small organisms, or postmetabolically in proteins and peptides by chemical or enzymatic reactions. Only upon measurement with mass spectrometers holding sufficient resolution, light, and heavy labeled peptide ions or reporter peptide fragment ions segregate and their intensity values are subsequently used for quantification. Targeted use of these labels or mass tags further leads to specific monitoring of diverse aspects of dynamic proteomes. In this review article, commonly used isotope labeling strategies are described, both for quantitative differential protein profiling and for targeted analysis of protein modifications.

CHR-2797: an antiproliferative aminopeptidase inhibitor that leads to amino acid deprivation in human leukemic cells

CHR-2797 is a novel metalloenzyme inhibitor that is converted into a pharmacologically active acid product (CHR-79888) inside cells. CHR-79888 is a potent inhibitor of a number of intracellular aminopeptidases, including leucine aminopeptidase. CHR-2797 exerts antiproliferative effects against a range of tumor cell lines in vitro and in vivo and shows selectivity for transformed over nontransformed cells. Its antiproliferative effects are at least 300 times more potent than the prototypical aminopeptidase inhibitor, bestatin. However, the mechanism by which inhibition of these enzymes leads to proliferative changes is not understood. Gene expression microarrays were used to profile changes in mRNA expression levels in the human promyelocytic leukemia cell line HL-60 treated with CHR-2797. This analysis showed that CHR-2797 treatment induced a transcriptional response indicative of amino acid depletion, the amino acid deprivation response, which involves up-regulation of amino acid synthetic genes, transporters, and tRNA synthetases. These changes were confirmed in other leukemic cell lines sensitive to the antiproliferative effects of CHR-2797. Furthermore, CHR-2797 treatment inhibited phosphorylation of mTOR substrates and reduced protein synthesis in HL-60 cells, both also indicative of amino acid depletion. Treatment with CHR-2797 led to an increase in the concentration of intracellular small peptides, the substrates of aminopeptidases. It is suggested that aminopeptidase inhibitors, such as CHR-2797 and bestatin, deplete sensitive tumor cells of amino acids by blocking protein recycling, and this generates an antiproliferative effect. CHR-2797 is orally bioavailable and currently undergoing phase II clinical investigation in the treatment of myeloid leukemia.

Selective amino acid restriction differentially affects the motility and directionality of DU145 and PC3 prostate cancer cells

We previously found that selective restriction of amino acids inhibits invasion of two androgen-independent human prostate cancer cell lines, DU145 and PC3. Here we show that the restriction of tyrosine (Tyr) and phenylalanine (Phe), methionine (Met) or glutamine (Gln) modulates the activity of G proteins and affects the balance between two actin-binding proteins, cofilin and profilin, in these two cell lines. Selective amino acid restriction differentially reduces G protein binding to GTP in DU145 cells. Tyr/Phe deprivation reduces the amount of Rho-GTP and Rac1-GTP. Met deprivation reduces the amount of Ras-GTP and Rho-GTP, and Gln deprivation decreases Ras-GTP, Rac-GTP, and Cdc42-GTP. Restriction of these amino acids increases the amount of profilin, cofilin and phosphorylation of cofilin-Ser(3). Increased PAK1 expression and phosphorylation of PAK1-Thr(423), and Ser(199/204) are consistent with the increased phosphorylation of LIMK1-Thr(508). In PC3 cells, Tyr/Phe or Gln deprivation reduces the amount of Ras-GTP, and all of the examined amino acid restrictions reduce the amount of profilin. PAK1, LIMK1 and cofilin are not significantly altered. These data reveal that specific amino acid deprivation differentially affects actin dynamics in DU145 and PC3. Modulation on Rho, Rac, PAK1, and LIMK1 likely alter the balance between cofilin and profilin in DU145 cells. In contrast, profilin is inhibited in PC3 cells. These effects modulate directionality and motility to inhibit invasion.

Effect of arginase II on L-arginine depletion and cell growth in murine cell lines of renal cell carcinoma

Background

L-arginine is the common substrate for the two isoforms of arginase. Arginase I, highly expressed in the liver and arginase II mainly expressed in the kidney. Arginase I-producing myeloid derived suppressor cells have been shown to inhibit T-cell function by the depletion of L-arginine. On the other hand, arginase II has been detected in patients with cancer and is thought to metabolize L-arginine to L-ornithine needed to sustain rapid tumor growth; however its role in L-arginine depletion is unclear. Thus, in tumor biology, L-arginine metabolism may play a dual role in tumor growth and in the induction of T cell dysfunction. Therefore, we studied in murine renal cell carcinoma (RCC) cell lines, the effect of arginase II on tumor cell proliferation and L-arginine depletion. The effect of arginase inhibitors on cell proliferation was also tested.

Methods

Three murine renal cell carcinoma (mRCC) cell lines were tested for the presence of arginase. nor-NOHA, an arginase inhibitor was used to substantiate the effect of arginase on cell growth and L-arginine depletion. Amino acid levels were tested by HPLC.

Results

Our results show that mRCC cell lines express only arginase II and were able to deplete L-arginine from the medium. Cell growth was independent of the amount of arginase activity expressed by the cells. nor-NOHA significantly (P = 0.01) reduced arginase II activity and suppressed cell growth in cells exhibiting high arginase activity.

The depletion of L-arginine by mRCC induced the decrease expression of CD3ζ a key element for T-cell function.

Conclusion

The results of this study show for the first time that arginase II produced by RCC cell lines depletes L-arginine resulting in decreased expression of CD3ζ. These results indicate that RCC cell lines expressing arginase II can modulate the L-arginine metabolic pathway to regulate both cell growth and T-cell function. Blocking arginase may lead to a decrease in RCC cell growth and aid in restoring immune function by increasing L-arginine availability for T-cell use. Understanding the interplay between arginase II and its interaction with the immune system may provide future therapeutic benefits to treat patients with RCC.

Prevention of amino acid conversion in SILAC experiments with embryonic stem cells

Recent studies using stable isotope labeling with amino acids in culture (SILAC) in quantitative proteomics have made mention of the problematic conversion of isotope-coded arginine to proline in cells. The resulting converted proline peptide divides the heavy peptide ion signal causing inaccuracy when compared with the light peptide ion signal. This is of particular concern as it can effect up to half of all peptides in a proteomic experiment. Strategies to both compensate for and limit the inadvertent conversion have been demonstrated, but none have been shown to prevent it. Additionally, these methods combined with SILAC labeling in general have proven problematic in their large scale application to sensitive cell types including embryonic stem cells (ESCs) from the mouse and human. Here, we show that by providing as little as 200 mg/liter L-proline in SILAC media, the conversion of arginine to proline can be rendered completely undetectable. At the same time, there was no compromise in labeling with isotope-coded arginine, indicating there is no observable back conversion from the proline supplement. As a result, when supplemented with proline, correct interpretation of "light" and "heavy" peptide ratios could be achieved even in the worst cases of conversion. By extending these principles to ESC culture protocols and reagents we were able to routinely SILAC label both mouse and human ESCs in the absence of feeder cells and without compromising the pluripotent phenotype. This study provides the simplest protocol to prevent proline artifacts in SILAC labeling experiments with arginine. Moreover, it presents a robust, feeder cell-free, protocol for performing SILAC experiments on ESCs from both the mouse and the human.

L-arginine reduces cell proliferation and ornithine decarboxylase activity in patients with colorectal adenoma and adenocarcinoma

PURPOSE

Evidence suggests that the majority of colorectal carcinomas arise from adenomas, and L-arginine suppresses colorectal tumorigenesis. We suppose that L-arginine may inhibit the process of carcinogenesis from colorectal adenoma to adenocarcinoma. The aim of this study was to investigate the effects of L-arginine on the formation and development of colorectal tumors.

EXPERIMENTAL DESIGN

We selected 60 patients with colorectal cancer and 60 patients with colorectal adenoma (CRA) and divided them into four groups of 30 patients each. We gave 30 g (120 mL) of L-arginine everyday for 3 days to the test groups, whereas L-arginine was substituted by 5% glucose in the control groups. The expression of the proliferating cell nuclear antigen, survivin, and nitric oxide synthase was examined immunohistochemically, and ornithine decarboxylase (ODC) activity was examined spectrophotometrically. Serum nitric oxide (NO) was detected by the Griess assay.

RESULTS

In patients with CRA, the proliferating cell nuclear antigen and survivin labeling indexes and ODC activity of the tumor and paratumor mucosa in the L-arginine-treated group after L-arginine treatment were significantly lower as compared with the corresponding pretreatment values (P < 0.01). Moreover, inducible nitric oxide synthase expression in the tumor markedly increased after L-arginine treatment (P < 0.05). Serum NO levels in the patients with colorectal cancer were markedly higher than those in the patients with CRA, and L-arginine treatment was responsible for this increase (P < 0.05).

CONCLUSIONS

Our results show that L-arginine can restrain crypt cell hyperproliferation and the expression of survivin, an inhibitor of apoptosis protein. This suggests that L-arginine can block the formation and development of colorectal tumors, and this effect might be related to the increased serum NO concentration and decreased ODC activity.

Arginine deprivation as a targeted therapy for cancer

Certain cancers may be auxotrophic for a particular amino acid, and amino acid deprivation is one method to treat these tumors. Arginine deprivation is a novel approach to target tumors which lack argininosuccinate synthetase (ASS) expression. ASS is a key enzyme which converts citrulline to arginine. Tumors which usually do not express ASS include melanoma, hepatocellular carcinoma, some mesotheliomas and some renal cell cancers. Arginine can be degraded by several enzymes including arginine deiminase (ADI). Although ADI is a microbial enzyme from mycoplasma, it has high affinity to arginine and catalyzes arginine to citrulline and ammonia. Citrulline can be recycled back to arginine in normal cells which express ASS, whereas ASS(-) tumor cells cannot. A pegylated form of ADI (ADI-PEG20) has been formulated and has shown in vitro and in vivo activity against melanoma and hepatocellular carcinoma. ADI-PEG20 induces apoptosis in melanoma cell lines. However, arginine deprivation can also induce ASS expression in certain melanoma cell lines which can lead to in vitro drug resistance. Phase I and II clinical trials with ADI-PEG20 have been conducted in patients with melanoma and hepatocellular carcinoma, and antitumor activity has been demonstrated in both cancers. This article reviews our laboratory and clinical experience as well as that from others with ADI-PEG20 as an antineoplastic agent. Future direction in utilizing this agent is also discussed.

An engineered L-arginine sensor of Chlamydia pneumoniae enables arginine-adjustable transcription control in mammalian cells and mice

For optimal compatibility with biopharmaceutical manufacturing and gene therapy, heterologous transgene control systems must be responsive to side-effect-free physiologic inducer molecules. The arginine-inducible interaction of the ArgR repressor and the ArgR-specific ARG box, which synchronize arginine import and synthesis in the intracellular human pathogen Chlamydia pneumoniae, was engineered for arginine-regulated transgene (ART) expression in mammalian cells. A synthetic arginine-responsive transactivator (ARG), consisting of ArgR fused to the Herpes simplex VP16 transactivation domain, reversibly adjusted transgene transcription of chimeric ARG box-containing mammalian minimal promoters (P_ART) in an arginine-inducible manner. Arginine-controlled transgene expression showed rapid induction kinetics in a variety of mammalian cell lines and was adjustable and reversible at concentrations which were compatible with host cell physiology. ART variants containing different transactivation domains, variable spacing between ARG box and minimal promoter and several tandem ARG boxes showed modified regulation performance tailored for specific expression scenarios and cell types. Mice implanted with microencapsulated cells engineered for ART-inducible expression of the human placental secreted alkaline phosphatase (SEAP) exhibited adjustable serum phosphatase levels after treatment with different arginine doses. Using a physiologic inducer, such as the amino acid l-arginine, to control heterologous transgenes in a seamless manner which is devoid of noticeable metabolic interference will foster novel opportunities for precise expression dosing in future gene therapy scenarios as well as the manufacturing of difficult-to-produce protein pharmaceuticals.

The relationship of arginine deprivation, argininosuccinate synthetase and cell death in melanoma

It has been shown that melanoma cells do not express argininosuccinate synthetase (ASS) and therefore are unable to synthesize arginine from citrulline. Depleting arginine using pegylated arginine deiminase (ADI-PEG20) results in cell death in melanoma but not normal cells. This concept was translated into clinical trial and responses were seen. However, induction of ASS expression does occur which results in resistance to ADI-PEG20. We have used 4 melanoma cell lines to study factors which may govern ASS expression. Although these 4 melanoma cell lines do not express ASS protein or mRNA as detected by both immunoblot and northernblot analysis, ASS protein can be induced after these cells are grown in the presence of ADI-PEG20, but again repressed after replenishing arginine in the media. The levels of induction are different and one cell line could not be induced. Interestingly, a melanoma cell line with the highest level of induction could also be made resistant to ADI-PEG20. This resistant line possesses high levels of ASS mRNA and protein expression which cannot be repressed with arginine. Our study indicates that ASS expression in melanoma cells is complex and governed by biochemical parameters which are different among melanoma cells.

Specific amino acid dependency regulates the cellular behavior of melanoma

Relative specific amino acid dependency is one of the metabolic abnormalities of melanoma cells and metabolic studies of this dependency are in their infancy. Herein, we review the current studies in this area and present new information that adds to the understanding of how tyrosine (Tyr) and phenylalanine (Phe) dependency as well as other amino acids regulate the cell behaviors of melanoma cells. Amino acid dependency of human melanoma cells is multifactorial and restricting Tyr and Phe to melanoma triggers a series of alterations in metabolic and signaling pathways in a time-ordered fashion to alter different cellular behaviors. For example, at early time points, the reduction of Tyr and Phe alters metabolic reactions quantitatively or qualitatively. The alterations include modulation of integrin/focal adhesion kinase (FAK)/G protein pathways and the plasminogen activator (PA)/PA inhibitor pathways to inhibit tumor cell invasion. At later time periods, a further drop in intracellular amino acids induces more metabolic alterations to impact the FAK/Ras/Raf and Bcl-2 pathways leading to apoptosis. The threshold effects and the targeting of multiple pathways by restriction of specific amino acids provide a connection between the metabolic alterations and signaling pathways that modulate the cellular behaviors of melanoma cells. Decoding the metabolic alterations that connect amino acid concentration to the crucial step(s) in signaling is important and an exciting area of cancer research.

Enhanced stability of heterologous proteins by supramolecular self-assembly

Recently, we reported on the dual function of human ferritin heavy chain (hFTN-H) used for the fusion expression and solubility enhancement of various heterologous proteins: (1) high-affinity interaction with HSP70 chaperone DnaK and (2) formation of self-assembled supramolecules with limited and constant sizes. Especially the latter, the self-assembly function of hFTN-H is highly useful in avoiding the undesirable formation of insoluble macroaggregates of heterologous proteins in bacterial cytoplasm. In this study, using enhanced green fluorescent protein (eGFP) and several deletion mutants of Mycoplasma arginine deiminase (ADI(132-410)) as reporter proteins, we confirmed through TEM image analysis that the recombinant fusion proteins (hFTN-H::eGFP and hFTN-H::ADI(132-410)) formed intracellular spherical particles with nanoscale diameter ( approximately 10 nm), i.e., noncovalently cross-linked supramolecules. Surprisingly, the supramolecular eGFP and ADI showed much enhanced stability in bioactivity. That is, the activity level was much more stably maintained for the prolonged period of time even at high temperature, at high concentration of Gdn-HCl, and in wide range of pH. The stability enhancement by supramolecular self-assembly may make it possible to utilize the protein supramolecules as novel means for drug delivery, enzymatic material conversion (biotransformation), protein chip/sensor, etc. where the maintenance of protein/enzyme stability is strictly required.

Risk and risk reduction involving arginine intake and meat consumption in colorectal tumorigenesis and survival

Elevated polyamine and nitric oxide levels (both derived from arginine) promote tumorigenesis, whereas non-steroidal anti-inflammatory drugs (NSAIDs) inhibit colorectal cancer (CRC) incidence in experimental and epidemiologic studies. We investigated dietary arginine-induced intestinal tumorigenesis and NSAID-inhibitory effects in Apc(Min/+) mice differentially expressing nitric oxide synthase-2 (Nos2). We also studied effects of estimated arginine exposures through meat consumption on tumor characteristics and survival in human CRC cases. Dietary arginine increased high-grade colon adenoma incidence in Apc(Min/+)Nos2(+/+) mice, but not in Nos2 knockout mice. Additionally, celecoxib suppressed intestinal steady state ornithine decarboxylase RNA levels (p < 0.001), induced steady state spermidine/spermine N(1)-acetyltransferase RNA levels (p = 0.002), decreased putrescine levels (p = 0.04) and decreased tumor number in the small intestines of Apc(Min/+)Nos2(+/+) mice (p = 0.0003). Five hundred and eleven cases from our NCI-supported CRC gene-environment study were analyzed based on self-reported meat (as a surrogate for arginine) consumption. Familial CRC cases (n = 144) in the highest meat consumption quartile (Q4) had no statistically significant differences in tumor grade compared to cases in Q1-Q3 (p = 0.32); however, they were observed to have decreased overall survival (OS) (10-year OS = 42% vs. 65%; p = 0.017), and increased risk of death in an adjusted analysis (hazards ratio [HR] = 2.24; p = 0.007). No differences in tumor grade, OS or adjusted HR were observed for sporadic CRC cases (n = 367) based on meat consumption. Our results suggest important roles for arginine and meat consumption in CRC pathogenesis, and have implications for CRC prevention.

Pegylated recombinant human arginase (rhArg-peg5,000mw) inhibits the in vitro and in vivo proliferation of human hepatocellular carcinoma through arginine depletion

Hepatocellular carcinoma (HCC) is believed to be auxotrophic for arginine through the lack of expression of argininosuccinate synthetase (ASS). The successful use of the arginine-depleting enzyme arginine deiminase (ADI) to treat ASS-deficient tumors has opened up new possibilities for effective cancer therapy. Nevertheless, many ASS-positive HCC cell lines are found to be resistant to ADI treatment, although most require arginine for proliferation. Thus far, an arginine-depleting enzyme for killing ASS-positive tumors has not been reported. Here, we provide direct evidence that recombinant human arginase (rhArg) inhibits ASS-positive HCCs. All the five human HCC cell lines we used were sensitive to rhArg but ADI had virtually no effect on these cells. They all expressed ASS, but not ornithine transcarbamylase (OTC), the enzyme that converts ornithine, the product of degradation of arginine with rhArg, to citrulline, which is converted back to arginine via ASS. Transfection of HCC cells with OTC resulted in resistance to rhArg. Thus, OTC expression alone may be sufficient to induce rhArg resistance in ASS-positive HCC cells. This surprising correlation between the lack of OTC expression and sensitivity of ASS-positive HCC cells shows that OTC-deficient HCCs are sensitive to rhArg-mediated arginine depletion. Therefore, pretreatment tumor gene expression profiling of ASS and OTC could aid in predicting tumor response to arginine depletion with arginine-depleting enzymes. We have also shown that the rhArg native enzyme and the pegylated rhArg (rhArg-peg(5,000mw)) gave similar anticancer efficacy in vitro. Furthermore, the growth of the OTC-deficient Hep3B tumor cells (ASS-positive and ADI-resistant) in mice was inhibited by treatment with rhArg-peg(5,000mw), which is active alone and is synergistic in combination with 5-fluorouracil. Thus, our data suggest that rhArg-peg(5,000mw) is a novel agent for effective cancer therapy.

A practical recipe for stable isotope labeling by amino acids in cell culture (SILAC)

Stable isotope labeling by amino acids in cell culture (SILAC) is a simple, robust, yet powerful approach in mass spectrometry (MS)-based quantitative proteomics. SILAC labels cellular proteomes through normal metabolic processes, incorporating non-radioactive, stable isotope-containing amino acids in newly synthesized proteins. Growth medium is prepared where natural ("light") amino acids are replaced by "heavy" SILAC amino acids. Cells grown in this medium incorporate the heavy amino acids after five cell doublings and SILAC amino acids have no effect on cell morphology or growth rates. When light and heavy cell populations are mixed, they remain distinguishable by MS, and protein abundances are determined from the relative MS signal intensities. SILAC provides accurate relative quantification without any chemical derivatization or manipulation and enables development of elegant functional assays in proteomics. In this protocol, we describe how to apply SILAC and the use of nano-scale liquid chromatography coupled to electrospray ionization mass spectrometry for protein identification and quantification. This procedure can be completed in 8 days.

In vivo loss of expression of argininosuccinate synthetase in malignant pleural mesothelioma is a biomarker for susceptibility to arginine depletion

PURPOSE

Malignant pleural mesothelioma (MPM) is an increasing health burden on many societies worldwide and, being generally resistant to conventional treatment, has a poor prognosis with a median survival of <1 year. Novel therapies based on the biology of this tumor seek to activate a proapoptotic cellular pathway. In this study, we investigated the expression and biological significance of argininosuccinate synthetase (AS), a rate-limiting enzyme in arginine production.

EXPERIMENTAL DESIGN

Initially, we documented down-regulation of AS mRNA in three of seven MPM cell lines and absence of AS protein in four of seven MPM cell lines. We confirmed that the 9q34 locus, the site of the AS gene, was intact using a 1-Mb comparative genomic hybridization array; however, there was aberrant promoter CpG methylation in cell lines lacking AS expression, consistent with epigenetic regulation of transcription. To investigate the use of AS negativity as a therapeutic target, arginine was removed from the culture medium of the MPM cell lines.

RESULTS

In keeping with the cell line data, 63% (52 of 82) of patients had tumors displaying reduced or absent AS protein, as assessed using a tissue microarray. Cell viability declined markedly in the AS-negative cell lines 2591 and MSTO but not in the AS-positive cell line, 28. This response was apparent by day 4 and maintained by day 9 in vitro. Arginine depletion induced BAX conformation change and mitochondrial inner membrane depolarization selectively in AS-negative MPM cells.

CONCLUSIONS

In summary, we have identified AS negativity as a frequent event in MPM in vivo, leading to susceptibility to cytotoxicity following restriction of arginine. A phase II clinical trial is planned to evaluate the role of arginine depletion in patients with AS-negative MPM.

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.

Mass spectrometry-based proteomics turns quantitative

The field of proteomics is built on technologies to analyze large numbers of proteins--ideally the entire proteome--in the same experiment. Mass spectrometry (MS) has been successfully used to characterize proteins in complex mixtures, but results so far have largely been qualitative. Two recently developed methodologies offer the opportunity to obtain quantitative proteomic information. Comparing the signals from the same peptide under different conditions yields a rough estimate of relative protein abundance between two proteomes. Alternatively, and more accurately, peptides are labeled with stable isotopes, introducing a predictable mass difference between peptides from two experimental conditions. Stable isotope labels can be incorporated 'post-harvest', by chemical approaches or in live cells through metabolic incorporation. This isotopic handle facilitates direct quantification from the mass spectra. Using these quantitative approaches, precise functional information as well as temporal changes in the proteome can be captured by MS.

The role of NO synthases in arginine-dependent small intestinal and colonic carcinogenesis

Arginine is catabolized by NOS2 and other nitric oxide synthases to form nitric oxide. We evaluated the roles of dietary arginine and Nos2 in Apc-dependent intestinal tumorigenesis in Min mice with and without a functional Nos2 gene. NOS2 protein was expressed only in intestinal tissues of Apc(Min/+) Nos2+/+ mice. NOS3 expression was higher in intestinal tissues of mice lacking Nos2, mainly in the small intestine. When diet was supplemented with arginine (0.2% and 2% in drinking water), lack of Nos2 results in decreased tumorigenesis in both small intestine and colon. In Nos2 knockout mice, supplemental arginine (up to 2%) caused a decrease in small intestinal tumor number and size. The arginine-dependent decrease was associated with an increase in nitrotyrosine formation and apoptosis in the region of intestinal stem cells. Mice expressing Nos2 did not show these changes. These mice did, however, show an arginine-dependent increase in colon tumor number and incidence, while no effect on apoptosis was seen. These changes were associated with increased nitrotyrosine formation in epithelial cells. Mice lacking Nos2 did not show changes in tumorigenesis or nitrotyrosine formation, while demonstrating an arginine-dependent increase in apoptosis. These data suggest that Nos2 and dietary arginine have significant effects on intestinal and colonic tumorigenesis in Min mice. In both tissues, loss of Nos2 is associated with decreased tumorigenesis when mice are supplemented with dietary arginine. In the small intestine, Nos2 prevents the arginine-induced decrease in tumor number and size, which is associated with NOS3 expression and increased apoptosis. In the colon, Nos2 is required for the arginine-induced increase in tumor number and incidence.

Integrity and stability of the citrulline-arginine pathway in normal and tumour cell lines

Arginine catabolizing enzymes have been used on cancers for over 60 years. In the last 5 years the ability of arginine catabolizing enzymes, not only to inhibit proliferation, but to kill tumour cells has been reinvestigated. Selectivity of action lies in the inability of many tumours to circumvent arginine deprivation by recycling precursors through the urea cycle. While this offers an immediate window of opportunity to treat, e.g. melanomas and hepatocellular carcinomas (HCC) that have poor citrulline converting ability, it is possible that the deprivation can be applied to many other types of cancer. The problem of deficiency of the urea cycle enzymes in a wider range of normal and malignant cell lines has been addressed, and shown to be variable throughout several different tumour types. We also need to know how fickle recycling enzyme activity can be in both normal and tumour cells, and found to be remarkable stable. Increasing interest is shown in the amino acid (arginine) deprivation protocol because it has already moved into the clinic. Initial findings on a named-patient basis have been encouraging, and the development of a new rational approach to the systemic treatment of melanomas, HCCs and leukemias seems imminent. This is the more attractive because arginine deprivation protocols can also 'stage' tumour cells for combination therapy in cases where they might not be killed outright by deprivation alone.

Arginine deprivation and metabolomics: important aspects of intermediary metabolism in relation to the differential sensitivity of normal and tumour cells

Arginine deprivation causes many types of tumour cells to die, often because they cannot recover or convert urea cycle intermediates into arginine. The powerful homeostatic mechanisms that kicks in to restore arginine levels in vivo are lacking in vitro, where there is no supply of citrulline. Comparison between cells deprived of arginine by direct elimination methods or indirectly via arginine degrading enzymes should show differences depending on their ability to handle alternative intermediates (ornithine, citrulline and argininosuccinate) of the urea cycle. The internal state of cells that can, versus those that cannot, use intermediates will metabolically be quite different. These differences should provide clear indicators regarding the sensitivity (susceptibility) of cells to arginine deprivation, from which we will be in a much better position to judge which tumours to treat, and possibly how to design the best treatment to eliminate them.

Remission of hepatocellular carcinoma with arginine depletion induced by systemic release of endogenous hepatic arginase due to transhepatic arterial embolisation, augmented by high-dose insulin: arginase as a potential drug candidate for hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is auxotrophic for the semi-essential amino acid arginine, depletion of which leads to tumor death. In humans, arginine is not an essential amino acid since many adult somatic cells can re-synthesize it from other sources, such as citrulline. Enzymes capable of depleting arginine in vitro include the urea cycle enzyme arginase, which is found in abundance in human liver. For over three decades, arginase has not been considered as a potential drug candidate because of its low substrate affinity, short circulatory half-life and sub-optimal enzymatic activity at physiological pH, though its in vitro anti-tumor activities in certain tumors have been amply reported. Arginine deiminase, a bacterial enzyme from Mycoplasma hominus has been shown to induce HCC remission through the mechanism of arginine depletion. We report here an innovative treatment approach for the treatment of locally advanced and metastatic HCC with transhepatic arterial embolisation (TAE) of the liver tumor with lipiodol and gel foam as a means of inducing a leakage of hepatic arginase from the liver into the circulation. Hepatic arginase released into the systemic circulation rapidly depleted plasma arginine. High-dose insulin was included to induce a state of hypoaminoacidaemia to augment arginine depletion. With this protocol, we have treated seven patients with locally advanced and/or metastatic HCC. Five patients achieved arginine depletion, ranging from 0 to 20 microM (normal plasma level 100-120 microM); all had varying degrees of tumor remission in their primary tumors and extra-hepatic sites in the lymph nodes, lungs and bones, suggesting systemic anti-cancer effect of arginine depletion. The two non-responders did not show significant reduction in plasma arginine. Based on our findings, we propose that the urea cycle enzyme, arginase, is a good drug candidate for the treatment of HCC.

Controlling cancer by restricting arginine availability--arginine-catabolizing enzymes as anticancer agents

Interest has recently been revived in enzymes that degrade essential amino acids. Arginine-catabolizing enzymes now predominate and are discussed in this review. Apart from reducing tumor load through cell death occurring as a result of deprivation alone, these catabolic enzymes conveniently leave the remaining malignant cells vulnerable to other therapeutic modalities through combinatorial treatments with cycle-dependent drugs, the timing of additional treatment after deprivation being crucial.

Deorphanization of GPRC6A: a promiscuous L-alpha-amino acid receptor with preference for basic amino acids

One of the most important tasks of molecular pharmacology is the deorphanization of the large number of G-protein-coupled receptors with unidentified endogenous agonists. We recently reported the cloning and analysis of expression of a novel human family C G-protein-coupled receptor, termed hGPRC6A. To identify agonists at this orphan receptor, we faced the challenges of achieving surface expression in mammalian cell lines and establishing an appropriate functional assay. Generating a chimeric receptor construct, h6A/5.24, containing the ligand binding amino-terminal domain (ATD) of hGPRC6A with the signal transducing transmembrane and C terminus of the homologous goldfish 5.24 receptor allowed us to overcome these obstacles. Homology modeling of the hGPRC6A ATD based on the crystal structure of the metabotropic glutamate receptor subtype 1 predicted interaction with alpha-amino acids and was employed to rationally select potential ligands. Measurement of Ca2+-dependent chloride currents in Xenopus laevis oocytes facilitated the deorphanization of h6A/5.24 and identification of L-alpha-amino acids as agonists. The most active agonists were basic L-alpha-amino acids, L-Arg, L-Lys, and L-ornithine, suggesting that these may function as endogenous signaling molecules. Measurement of intracellular calcium in tsA cells expressing h6A/5.24 allowed determination of EC50 values, which confirmed the agonist preferences observed in oocytes. Cloning, cell surface expression and deorphanization of the mouse ortholog further reinforces the assignment of the agonist preferences of hGPRC6A. This study demonstrates the utility of a chimeric receptor approach in combination with molecular modeling, for elucidating agonist interaction with GPRC6A, a novel family C G-protein-coupled receptor.

Multiple pathways for cationic amino acid transport in rat thyroid epithelial cell line PC Cl3

Information regarding cationic amino acid transport systems in thyroid is limited to Northern blot detection of y+LAT1 mRNA in the mouse. This study investigated cationic amino acid transport in PC cell line clone 3 (PC Cl3 cells), a thyroid follicular cell line derived from a normal Fisher rat retaining many features of normal differentiated follicular thyroid cells. We provide evidence that in PC Cl3 cells plasmalemmal transport of cationic amino acids is Na+independent and occurs, besides diffusion, with the contribution of high-affinity, carrier-mediated processes. Carrier-mediated transport is via y+, y+L, and b0,+systems, as assessed by l-arginine uptake and kinetics, inhibition of l-arginine transport by N-ethylmaleimide and neutral amino acids, and l-cystine transport studies. y+L and y+systems account for the highest transport rate (with y+L > y+) and b0,+for a residual fraction of the transport. Uptake data correlate to expression of the genes encoding for CAT-1, CAT-2B, 4F2hc, y+LAT1, y+LAT2, rBAT, and b0,+AT, an expression profile that is also shown by the rat thyroid gland. In PC Cl3 cells cationic amino acid uptake is under TSH and/or cAMP control (with transport increasing with increasing TSH concentration), and upregulation of CAT-1, CAT-2B, 4F2hc/y+LAT1, and rBAT/b0,+AT occurs at the mRNA level under TSH stimulation. Our results provide the first description of an expression pattern of cationic amino acid transport systems in thyroid cells. Furthermore, we provide evidence that extracellular l-arginine is a crucial requirement for normal PC Cl3 cell growth and that long-term l-arginine deprivation negatively influences CAT-2B expression, as it correlates to reduction of CAT-2B mRNA levels.

Arginine deiminase enhances dexamethasone-induced cytotoxicity in human T-lymphoblastic leukemia CCRF-CEM cells

Since arginine deiminase (ADI; EC 3.5.3.6) inhibits cell proliferation by arresting cells in the G1 phase, we tested its synergistic effect on cell death induced by dexamethasone (DEX), which also induces apoptosis by G1 cell cycle arrest. ADI inhibited cell proliferation and induced apoptosis in human leukemic CEM cells in a dose-dependent manner. Simultaneous treatment with ADI and DEX showed synergistic effects on DNA fragmentation and LDH release. In addition, ADI exerted its anti-proliferative activity against DEX-resistant CEM cells. ADI suppressed expression of c-myc, a potential key regulator of cell proliferation and apoptosis, and increased expression of p27Kip1 cyclin-dependent kinase inhibitor. These results suggest that ADI efficiently increases the anti-cancer effect of DEX on human leukemic CEM cells through G1 cell cycle arrest involving downregulation of c-myc and upregulation of p27Kip1.

GSTP1 affects chemoresistance against camptothecin in human lung adenocarcinoma cells

Glutathione S-transferase P1 (GSTP1) is known as a xenobiotic enzyme through conjugation of glutathione and also as an inhibitor of Jun N-terminal kinase (JNK). We intended to investigate whether GSTP1 affects chemoresistance against camptothecin in human lung adenocarcinoma cells. Camptothecin induced GSTP1 expression. Downregulation of GSTP1 increased necrosis induced by camptothecin in A549 cells but not in PC-14 and RERF-LC-KJ cells. This phenomenon in A549 cells was hardly changed by JNK inhibitor SP600125 but was almost diminished by l-buthionine-sulfoximine. These results suggest that GSTP1 has protective effects against camptothecin-induced necrosis in subset of human lung adenocarcinoma through glutathione conjugation.

Nutritional deficiency affects cell cycle status and viability in A549 cells: role of p27Kip1

We investigated how nutritional deficiency affects cell cycle and cell viability in A549 lung adenocarcinoma cells. Deprivation of various amino acids or glucose induced cell cycle arrest and cell death in a different manner. Cell death on deprivation of these nutrients was increased by downregulating of p27Kip1 with RNA interference. It was also observed that intrinsic p27Kip1 was segregated in cytoplasm in a glucose-deprived situation. In conclusion, amino acid or glucose deprivation induced cell cycle arrest and cell death, part of which is thought to be rescued by the existence of cytoplasmic p27Kip1.

Glutathione S-transferase P1 has protective effects on cell viability against camptothecin

Glutathione S-transferase P1 (GSTP1) is one of the important xenobiotic-metabolizing enzymes. It was reported that GSTP1 was overexpressed in malignant tissues, and its expression level was associated with resistance to chemotherapeutics. We carried out transfection of GSTP1 sense and antisense vectors to examine effects of GSTP1 on cell cycle arrest and apoptosis induced by camptothecin in HeLa cells. Transfection of GSTP1 antisense vector induced apoptosis. Camptothecin-induced S- or G2/M arrest was intensified by transfection of GSTP1 antisense vector, and subsequent apoptosis was attenuated by transfection of GSTP1 sense vector. These results suggest that GSTP1 has protective effects against camptothecin-induced cytotoxicity.

Incidence and distribution of argininosuccinate synthetase deficiency in human cancers: a method for identifying cancers sensitive to arginine deprivation

BACKGROUND

Argininosuccinate synthetase (ASS) was the first of two enzymes to convert citrulline to arginine. This pathway allowed cells to synthesize arginine from citrulline, making this amino acid nonessential for the growth of most mammalian cells. Previous studies demonstrated that several human tumor cell lines were auxotrophic for arginine due to an inability to express ASS. Selective elimination of arginine from the circulation of animals with these tumors is a potentially effective anticancer treatment. The purpose of these experiments was to determine the frequency of ASS deficiency and arginine auxotrophy in a variety of human malignant tumors.

METHODS

The authors analyzed the expression of ASS by immunohistochemistry with a monoclonal antibody in a variety of human tumor biopsies. They found that the incidence of ASS deficiency varied greatly with the tumor type and tissue of origin.

RESULTS

Melanoma, hepatocellular carcinoma, and prostate carcinoma were most frequently deficient in ASS. Some human cancers were almost always positive for ASS (e.g., lung and colon carcinomas). However, other human cancers, including sarcomas, invasive breast carcinoma, and renal cell carcinoma, also were sometimes ASS deficient.

CONCLUSIONS

These data indicated that immunohistochemical detection of ASS may prove an effective means for determining ASS deficiency in malignant human tumors and for identifying patients most likely to respond to arginine deprivation therapy. Based on these results, human clinical trials using arginine-degrading enzyme therapy to treat patients with advanced melanoma or hepatocellular carcinoma have been initiated.

Arginine deiminase: a potential inhibitor of angiogenesis and tumour growth

Hydrolysis of plasma arginine to citrulline by arginine deiminase (ADI) was recently shown to suppress lipopolysaccharide-induced nitric oxide (NO) synthesis. Since arginine is the precursor of NO, and the latter modulates angiogenesis, we explored whether ADI treatment significantly affected tube-like (capillary) formation of human umbilical vein endothelial cells. Inhibition occurred in a dose-dependent manner, both in the chorioallantoic membrane and the murine Matrigel plug assay. Inhibition of angiogenesis by ADI was reversed when a surplus of exogenous arginine was provided, indicating that its antiangiogenic effect is primarily due to arginine depletion, although other pathways of interference are not entirely excluded. Arginine deiminase is also shown to be as a potent inhibitor of tumour growth in vitro as in vivo, being effective at nanogram quantities per millilitre in CHO and HeLa cells. Thus, it could be highly beneficial in cancer therapy because of its two-pronged attack as both an antiproliferative and an antiangiogenic agent.

Arginine deprivation, growth inhibition and tumour cell death: 3. Deficient utilisation of citrulline by malignant cells

Arginine deprivation causes death of up to 80% of cancer cell lines in vitro, but in the body, citrulline would be available as a convertible source of this amino acid in vivo. Some tumour cell lines, notably the vast majority of melanomas and hepatocellular carcinomas, tend to be deficient in argininosuccinate synthetase (EC 6.5.4.3.), and therefore cannot recycle citrulline to arginine. Argininosuccinate synthetase is present at levels that convert enough citrulline to arginine to allow limited growth in about half of a modest range of malignant cell types analysed in this study. Attempts to rescue cells that are unable to utilise citrulline with the immediate downstream product, argininosuccinate, had very limited success in a few tumour cell lines. Particularly noteworthy is the demonstration that argininosuccinate was totally incapable of rescuing cells that utilise citrulline efficiently, consistent with tight channelling (coupling) of argininosuccinate synthetase and argininosuccinate lyase in the urea cycle. The findings suggest that an excellent opportunity exists for further exploitation of arginine deprivation in the selective killing of tumour cells.

Resistance to the anti-proliferative activity of recombinant arginine deiminase in cell culture correlates with the endogenous enzyme, argininosuccinate synthetase

Recombinant mycoplasma enzyme, arginine deiminase (rADI), has been proposed as a possible cancer treatment via arginine depletion. However, many cell lines are resistant to rADI-treatment, even though most require arginine for proliferation. We compared eight different cell lines for sensitivity in cell proliferation to the effect of either rADI or arginine deprivation. The activity of argininosuccinate synthetase (AS), the rate-limiting enzyme for converting citrulline to arginine, was also measured. Our results indicate that resistance to rADI-treatment may correlate with cellular AS activity, either constitutive or inducible, allowing cell survival by conversion of the product of the rADI reaction, i.e. citrulline to arginine.

Arginine deprivation, growth inhibition and tumour cell death: 2. Enzymatic degradation of arginine in normal and malignant cell cultures

Arginase added to culture medium reduced arginine to negligible levels within approximately 6 h, and enzyme activity persisted relatively undiminished for at least 3 days. Human and bovine arginase proved equally effective. The response of normal cells was to enter G1 (G0) arrest, from which most of the cells could be recovered weeks later. In contrast, malignant cell lines treated with unpegylated or pegylated enzyme resulted in cell death on a massive scale within 3 - 5 days, with a very low to negligible percentage of cells (<0.01%) being recoverable on restoration with arginine. Although pegylation resulted in a 40% drop in specific activity, arginase was considerably more stable and remained active for >>8 days. Arginine decarboxylase caused malignant cell arrest at the same units per millilitre as arginase. Its breakdown product, agmatine, was relatively nontoxic in the presence of arginine, but exacerbated cell death above millimolar concentration in its absence. Although ornithine failed to rescue cells from deprivation, citrulline recovered cells in all cases, although less well in fast-growing tumour cell populations, whereas readdition of arginine failed to work unless a complete medium change was given (because of the persistence of the enzymes in the medium catabolising its destruction). The advantages and disadvantages of these two arginine-catabolising enzymes are discussed, and compared with arginine deiminase.

Changes in the ornithine cycle following ionising radiation cause a cytotoxic conditioning of the culture medium of H35 hepatoma cells

Cultured H35 hepatoma cells release a cytotoxic factor in response to irradiation with X-rays. When the conditioned medium from irradiated cells is given to nonirradiated cells, growth is inhibited and followed by cell death, possibly apoptosis, Analysis of the conditioned medium reveals a dramatic change in the ornithine (urea) cycle components after the irradiation. A strong decrease in medium arginine is accompanied with parallel increases in ornithine, citrulline and ammonia. The high level of ammonia appears to be largely responsible for the observed cytotoxicity. The development of hyperammonia by irradiated cells and the related toxicity depend on the radiation dose and the number of cells seeded thereafter for the medium conditioning. Development of cytotoxicity by irradiated cells is completely prevented with the arginase inhibitor L-norvaline, in arginine-deficient medium or when citrulline replaces arginine. These preventive measures result in subtoxic ammonia levels.

Arginine deprivation and tumour cell death: arginase and its inhibition

Arginase treatment of cell cultures reduced arginine in the medium to approximately micromolar levels within 5-30 min, and proved as effective as arginine-free medium (AFM) prepared by formulation. The enzyme was heat stable and as active at pH 7.2 as at pH 9.9. It persisted in culture for at least 3 days with only a small diminution in its speed of action, and still actively destroyed arginine after 6 days, since arginine supplementation failed to rescue viable cells. Addition of L-norvaline, an inhibitor of arginase, rescued cells from arginase-induced deprivation. Its efficacy at low concentrations was short-lived (probably < 1 day), while at higher concentrations it did not appear to inhibit completely the enzyme. However, L-norvaline at these same levels also slowed the growth of positive non-enzyme treated controls receiving the normal arginine level. Thus the difference in this growth indicated that arginase was more inhibitory than cursory examination of initial kinetic data suggested. It also agreed with the inhibition of arginase in the ornithine assay used to measure biochemically enzyme activity. We conclude that norvaline partially but not completely antagonises arginase activity, which allows cell rescue in a dose-dependent manner between 0.4 and 4 mM, but cannot be used above about 2 mM without exhibiting a general non-specific interference of cell growth of its own, although no evidence of cell toxicity was observed in either AFM or arginine-containing medium. L-ornithine, the product of arginase that inhibits the enzyme by a feedback mechanism, had no inhibitory effect on arginase over a similar concentration range.

Recombinant arginine deiminase as a potential anti-angiogenic agent

Arginine deiminase (ADI), isolated from Mycoplasma cell extracts, has been suggested to inhibit endothelial cell growth in vitro. However, anti-angiogenic activity by ADI has not yet been demonstrated. In this study, we investigated the in vitro effect of recombinant ADI (rADI) on the growth, migration, and tube formation of human umbilical vein endothelial (HUVE) cells. Mycoplasma arginine deiminase was cloned by PCR and the rADI was expressed in Escherichia coli. and purified to near homogeneity. The purified recombinant protein was found to have characteristics similar to those of the native enzyme: molecular weight (48 kDa) and specific enzymatic activity of converting L-arginine into citrulline (32.7 U/mg). This recombinant enzyme also exhibited an inhibitory effect on the growth of HUVE cells. The anti-angiogenic activity was demonstrated by in vitro inhibition of migration into the scratch wounded area in HUVE cell monolayers and the inhibition of microvessel tube-like formation of HUVE cells on Matrigel-coated surfaces. These results suggest that arginine deiminase is a potential inhibitor for angiogenesis, and that arginine concentrations may play an important role in regulating neovascularization.

Arginine catabolism, liver extracts and cancer

Although it is self evident that cells will not grow in amino acid deficient medium, an observation less well appreciated is that malignant cells are particularly vulnerable to such deprivation, which can lead to their rapid demise. Indeed, the more flagrantly malignant the phenotype (anaplastic the tumor), the more susceptible the cells seem to be to deprivation. While some attempts to employ this strategy in cancer treatment have been made, the difference between normal and malignant cells should be more fully exploited as a means of selectively eliminating tumor cell populations. To be successful, information on differences between the normal and the deranged cell cycle engine and checkpoints, especially how these are affected by deprivation, is of crucial importance. Since it is only recently that the controls at restriction points have been elucidated, it is little surprise that earlier attempts to control tumor cell growth by limiting the availability of an essential amino acid have met with limited success. Studies have been sporadic and isolated, often with little more than anecdotal descriptions as far as clinical work was concerned. This review concentrates on what has been accomplished primarily in vitro and since about 1950 with regard to arginine catabolism, while recognising that other essential amino acids have also been the focus of attention by some investigators. Treatments have included medium and plasma manipulation, dietary control, enzymatic degradation, and the use of liver extracts. On some occasions, substitution of amino acid analogues has been explored. It is argued that current knowledge, combined with past experience, calls for a much closer examination of the full potential of amino acid (and specifically arginine) deprivation as a means of controlling tumor growth, with greater attention to protocols that might be used to treat human cancers.

Role of arginine and its methylated derivatives in cancer biology and treatment

Both L-arginine supplementation and deprivation influence cell proliferation. The effect of high doses on tumours is determined by the optical configuration: L-arginine is stimulatory, D-arginine inhibitory. Arginine-rich hexapeptides inhibited tumour growth. Deprivation of L-arginine from cell cultures enhanced apoptosis. The pro-apoptotic action of NO synthase inhibitors, like NG-monomethyl-L-arginine, is manifested through inhibition of the arginase pathway. NG-hydroxymethyl-L-arginines caused apoptosis in cell cultures and inhibited the growth of various transplantable mouse tumours. These diverse biological activities become manifest through formaldehyde (HCHO) because guanidine group of L-arginine in free and bound form can react rapidly with endogenous HCHO, forming NG-hydroxymethylated derivatives. L-arginine is a HCHO capturer, carrier and donor molecule in biological systems. The role of formaldehyde generated during metabolism of NG-methylated and hydroxymethylated arginines in cell proliferation and death can be shown. The supposedly anti-apoptotic homozygous Arg 72-p53 genotype may increase susceptibility of some cancers. The diverse biological effects of L-arginine and its methylated derivatives call for further careful studies on their possible application in chemoprevention and cancer therapy.

Partial purification of a liver-derived tumor cell growth inhibitor that differentially inhibits poorly-liver metastasizing cell lines: identification as an active subunit of arginase

Organ specific tumor metastasis is thought in part to require the ability of metastatic cells to respond to target-organ-associated growth factors or to avoid the effects of target organ associated growth inhibitors. We previously found that murine and rat liver-conditioned media inhibited the growth of the poorly-liver metastasizing murine RAW117-P large-cell lymphoma cells more than their highly liver-metastasizing RAW117-H10 counterparts. Using a six step chromatographic procedure, the major RAW117-P cell proliferation inhibitor from a rat liver extract was purified. The factor displayed a Mr of approximately 35,000 and an isoelectric point > 8.5. This material inhibited the growth of many cells at high concentration; however, in dose-response studies it displayed a higher IC50 for highly-liver metastatic murine RAW117-H10 lymphoma and human KM12SM colon carcinoma cells than for their poorly-liver metastatic counterparts. Attempts to identify the growth inhibitor led to the supplementation of tissue culture inhibitor assays with various components, including excess amino acids, and this was found to completely abrogate the factor's activity. Specifically, the addition of excess arginine resulted in the complete cellular recovery from inhibitor exposure. This tentatively identified the liver growth inhibitor as the enzyme arginase, a Mr approximately 10,000 multisubunit protein. A microtiter plate-based assay for arginase was developed and the purification repeated using human liver as a source of activity and the human KM12C colon carcinoma line as a target. The growth inhibitory and arginase activities were found to co-purify, identifying the factor as arginase. Highly-metastatic cells displayed no ability to preferentially inactivate or inhibit the activity of arginase, but they did they display slightly greater amounts of intracellular arginine. The liver is a major site of arginase localization as the enzyme is required for the functioning of the urea cycle. The results indicate that certain liver-colonizing tumor cells can escape, to a degree, the proliferation-damping effects of arginine depletion.