Effect of arginase on splenic killer cell activity in patients with gastric cancer

Helicobacter pylori arginase mutant colonizes arginase II knockout mice

AIM: To investigate the role of host and bacterial arginases in the colonization of mice by Helicobacter pylori (H. pylori).

METHODS: H. pylori produces a very powerful urease that hydrolyzes urea to carbon dioxide and ammonium, which neutralizes acid. Urease is absolutely essential to H. pylori pathogenesis; therefore, the urea substrate must be in ample supply for urease to work efficiently. The urea substrate is most likely provided by arginase activity, which hydrolyzes L-arginine to L-ornithine and urea. Previous work has demonstrated that H. pylori arginase is surprisingly not required for colonization of wild-type mice. Hence, another in vivo source of the critical urea substrate must exist. We hypothesized that the urea source was provided by host arginase II, since this enzyme is expressed in the stomach, and H. pylori has previously been shown to induce the expression of murine gastric arginase II. To test this hypothesis, wild-type and arginase (rocF) mutant H. pylori strain SS1 were inoculated into arginase II knockout mice.

RESULTS: Surprisingly, both the wild-type and rocF mutant bacteria still colonized arginase II knockout mice. Moreover, feeding arginase II knockout mice the host arginase inhibitor S-(2-boronoethyl)-L-cysteine (BEC), while inhibiting > 50% of the host arginase I activity in several tissues, did not block the ability of the rocF mutant H. pylori to colonize. In contrast, BEC poorly inhibited H. pylori arginase activity.

CONCLUSION: The in vivo source for the essential urea utilized by H. pylori urease is neither bacterial arginase nor host arginase II; instead, either residual host arginase I or agmatinase is probably responsible.

Regulation of NK cell function by human granulocyte arginase

The arginine-hydrolyzing enzyme arginase is constitutively expressed by human polymorphonuclear granulocytes (PMN). Upon PMN cell death arginase is liberated and depletes arginine in the microenvironment. This amino acid depletion suppresses T cell proliferation and cytokine secretion and emerges as a key mechanism of immunosuppression during chronic inflammation and tumor growth. Here we show that PMN arginase also severely impairs key functions of primary human NK cells as well as IL-2-activated NK cells. In the absence of arginine, NK cell proliferation and IL-12/IL-18-induced secretion of IFN-gamma are severely diminished. In contrast, NK cell viability, granule exocytosis, and cytotoxicity are independent of extracellular arginine. The mechanism of NK cell suppression by arginine depletion is posttranscriptional since mRNA transcript frequency is unaffected upon NK cell activation in the absence of arginine. Finally, we demonstrate that human purulent exudate ex vivo inhibits NK cell functions exclusively due to liberated arginase. Arginase inhibitors are therefore promising pharmacological agents to treat unwanted suppression of the innate (NK cell) as well as the adaptive (T cell) immune system.

Structure and function of arginases

The arginases catalyze the divalent cation dependent hydrolysis of L-arginine to produce L-ornithine and urea. Although traditionally considered in terms of its role as the final enzyme of the urea cycle, the enzyme is found in a variety of nonhepatic tissues. These findings suggest that the enzyme may have other functions in addition to its role in nitrogen metabolism. High-resolution crystal structures have been determined for recombinant rat liver (type I) arginase and for recombinant human kidney (type II) arginase, their variants, and complexes with products and inhibitors. Each identical subunit of the trimeric enzyme contains an active site that lies at the bottom of a 15 A deep cleft. The 2 essential Mn(II) ions are located at the bottom of this cleft, separated by approximately 3.3 A and bridged by oxygens derived from 2 aspartic acid residues and a solvent-derived hydroxide. This metal bridging hydroxide is proposed to be the nucleophile that attacks the guanidinium carbon of substrate arginine. On the basis of this proposed mechanism, boronic acid inhibitors of the enzyme have been synthesized and characterized kinetically and structurally. These inhibitors display slow-onset inhibition at the pH optimum of the enzyme, and are found as tetrahedral species at the active site, as determined by X-ray diffraction. The potent inhibition of arginases I and II by these compounds has not only delineated key enzyme-substrate interactions, but has also led to a greater understanding of the role of arginase in nonhepatic tissues.

rIFN-gamma-mediated growth suppression of platinum-sensitive and -resistant ovarian tumor cell lines not dependent upon arginase inhibition

Background

Arginine metabolism in tumor cell lines can be influenced by various cytokines, including recombinant human interferon-γ (rIFN-γ), a cytokine that shows promising clinical activity in epithelial ovarian cancer (EOC).

Methods

We examined EOC cell lines for the expression of arginase in an enzymatic assay and for transcripts of arginase I and II, inducible nitric oxide synthase (iNOS), and indoleamine 2,3-dioxygenase (IDO) by reverse transcription-polymerase chain reaction. The effects of rIFN-γ on arginase activity and on tumor cell growth inhibition were determined by measuring [³H]thymidine uptake.

Results

Elevated arginase activity was detected in 5 of 8 tumor cell lines, and analysis at the transcriptional level showed that arginase II was involved but arginase I was not. rIFN-γ reduced arginase activity in 3 EOC cell lines but increased activity in the 2008 cell line and its platinum-resistant subline, 2008.C13. iNOS transcripts were not detected in rIFN-γ-treated or untreated cell lines. In contrast, IDO activity was induced or increased by rIFN-γ. Suppression of arginase activity by rIFN-γ in certain cell lines suggested that such inhibition might contribute to its antiproliferative effects. However, supplementation of the medium with polyamine pathway products did not interfere with the growth-inhibitory effects of rIFN-γ EOC cells.

Conclusions

Increased arginase activity, specifically identified with arginase II, is present in most of the tested EOC cell lines. rIFN-γ inhibits or stimulates arginase activity in certain EOC cell lines, though the decrease in arginase activity does not appear to be associated with the in vitro antiproliferative activity of rIFN-γ. Since cells within the stroma of EOC tissues could also contribute to arginine metabolism following treatment with rIFN-γ or rIFN-γ-inducers, it would be helpful to examine these effects in vivo.

Significance of arginase and ornithine in malignant tumors of the human skin

During neoplastic development, several aspects of the regulation of polyamine synthesis undergo profound changes. In extrahepatic mammalian tissues in which the urea cycle is not functioning, arginase is believed to supply the cell with ornithine, a non-protein amino acid that is a precursor for biosynthesis of polyamines. Because the activity of ornithine decarboxylase and polyamine levels have been shown to be elevated during carcinogenesis, we decided to investigate the role of arginase in the development of malignant tumors of the human skin and to examine whether arginase activity and ornithine level can be used as biologic markers for distinguishing patients with squamous cell cancer from patients with basal cell cancer. For this purpose, we studied tissue arginase activity and ornithine level in tumor and adjacent normal tissues in 16 patients (55 +/- 10 years of age) with malignant skin tumors (8 of which were squamous cell cancers and 8 of which were basal cell cancers). The mean arginase activity and ornithine levels in tumor tissues (total) were 17.75 +/- 8.54 U/mg protein and 40.89 +/- 14.88 nmol/mg protein, respectively, versus 3.69 +/- 1.71 U/mg protein and 12.98 +/- 6.21 nmol/mg protein, respectively, for normal tissues. The mean specific arginase activity levels in squamous cell and basal cell cancers of the human skin were 18.49 +/- 10.47 U/mg protein and 16.63 +/- 6.00 U/mg protein, respectively. The mean ornithine levels in squamous cell and basal cell cancers of the human skin were 42.45 +/- 19.10 nmol/mg protein and 39.33 +/- 10.19 nmol/mg protein, respectively. Our results indicated that (1) arginase activity and ornithine levels are elevated in squamous cell and basal cell cancers of the human skin; (2) the increased activity of arginase and hence the elevated levels of ornithine may be important in the development of malignant tumors of the human skin; and (3) although arginase activity and ornithine level may be useful for distinguishing patients with malignant skin tumors from healthy subjects, they cannot be used as biologic markers for distinguishing patients with squamous cell cancer from patients with basal cell cancer.

Immunohistochemical study of arginase in cancer of the stomach

High levels of arginase have been detected in gastric adenocarcinoma. To examine the hypothesis that this is due to macrophage infiltration into the tumour, we localized the cellular distribution of arginase by immunohistochemical staining. We examined gastric adenocarcinomas and their corresponding normal tissues (n = 45), leiomyomas (n = 2), leiomyosarcomas (n = 3), human gastric adenocarcinoma cell lines (n = 3), and benign gastric ulcers (n = 4) by the avidin-biotin-peroxidase complex technique. Macrophages with strong arginase immunoreactivity were observed infiltrating both gastric normal and cancer tissues. No arginase immunoreactivity was observed in normal mucosal gland, muscular and serosal tissues or benign gastric ulcers. The immunoreactivity of arginase was positive but heterogeneous in most specimens of gastric adenocarcinoma (62.2%) and was absent from gastric intestinal metaplasia, leiomyomas and leiomyosarcomas. Among the 28 neoplasms with arginase immunoreactivity, scattered immunoreactivity was also noted in adjacent dysplastic glands in 12 (42.8%) specimens. Arginase immunoreactivity was observed in all three gastric cancer cell lines. Arginase is present in the cytoplasm but not in the nucleus. These data suggest that the high arginase levels in adenocarcinoma cancer tissues originate largely from cancer cells.

Steroid hormone receptors in three human gastric cancer cell lines

Steroid hormone receptors in three human gastric adenocarcinoma cell lines and their transplanted tumors (except nontumorigenic KATO-III) in nude mice were determined by dextran-coated charcoal assay. Progesterone receptors (PgR) were found in all cell lines, transplanted NUGC-3, and AZ 521 tumors. Estrogen receptors (ER) were found in KATO-III cells, transplanted NUGC-3, and AZ 521 tumors, whereas glucocorticoid receptors (GR) were found only in NUGC-3 tumor and no androgen receptor was found in any cell lines or transplanted tumors. Since NUGC-3 cells had ER, PgR, and GR, it was used for the study of the effects of steroid hormones on growth. The results showed the cell cycle phase distributions and growth rate of transplanted tumors were similar in hormone-treated and nontreated groups. The persistent expression of PgR in gastric cancer cell lines and tumors, and the slight increase of tumor volumes in the progesterone-treated group suggests that progesterone and its receptors may be important in the pathogenesis of gastric cancer, but their biological function remains to be elucidated.