The role of arginine and arginine-metabolizing enzymes during Giardia – host cell interactions in vitro

Abstract

BackgroundArginine is a conditionally essential amino acid important in growing individuals and under non-homeostatic conditions/disease. Many pathogens interfere with arginine-utilization in host cells, especially nitric oxide (NO) production, by changing the expression of host enzymes involved in arginine metabolism. Here we used human intestinal epithelial cells (IEC) and three different isolates of the protozoan parasite Giardia intestinalis to investigate the role of arginine and arginine-metabolizing enzymes during intestinal protozoan infections.ResultsRNA expression analyses of major arginine-metabolizing enzymes revealed the arginine-utilizing pathways in human IECs (differentiated Caco-2 cells) grown in vitro. Most genes were constant or down-regulated (e.g. arginase 1 and 2) upon interaction with Giardia, whereas inducible NO synthase (iNOS) and ornithine decarboxylase (ODC) were up-regulated within 6 h of infection. Giardia was shown to suppress cytokine-induced iNOS expression, thus the parasite has both iNOS inducing and suppressive activities. Giardial arginine consumption suppresses NO production and the NO-degrading parasite protein flavohemoglobin is up-regulated in response to host NO. In addition, the secreted, arginine-consuming giardial enzyme arginine deiminase (GiADI) actively reduces T-cell proliferation in vitro. Interestingly, the effects on NO production and T cell proliferation could be reversed by addition of external arginine or citrulline.ConclusionsGiardia affects the host’s arginine metabolism on many different levels. Many of the effects can be reversed by addition of arginine or citrulline, which could be a beneficial supplement in oral rehydration therapy.

Macrophages expressing arginase 1 and nitric oxide synthase 2 accumulate in the small intestine during Giardia lamblia infection

“…Recent studies on the influence of G. lamblia on epithelial cell ARG activity report a down regulation of host ARG in response to G. lamblia [13]. However these studies exclusively used in vitro culture systems of non-immune cells.…”

Gene expression changes during Giardia–host cell interactions in serum-free medium

“…Arginine metabolism is a primary source of energy in Giardia [4], which is depleted within 1.5 h of interaction with IECs [16]. Genes encoding three key enzymes in the arginine dihydrolase pathway (arginine deiminase (ADI), ornithine carbamoyltransferase (OCT) and carbamate kinase (CK)) were found down-regulated at 2.5 h of interaction.…”

“…Understanding transcriptional changes in Giardia during cellular interaction is crucial for identification of genes contributing to parasite virulence and survival in the host intestines. We previously used differentiated Caco-2 cells and HCT-8 cells to study changes in gene expression in both parasites and IECs during interaction [4][5][6] and HT-29 cells to identify secreted proteins [7]. Nevertheless, it is well known that differences in cell lines, culture conditions and detection techniques affect identification of gene expression changes [3,8].…”

The Immunological Enigma of Human Giardiasis

“…As other metabolic enzymes, ADI is released by an unknown mechanism upon contact with epithelial cells. ADI-dependent hydrolysis of arginine has been linked to inhibited production of the immune effector NO, to reduced proliferation of epithelial cells and of T lymphocytes [59,[109][110][111]. Arginine depletion by ADI also modulates the phenotype and cytokine release from LPS-activated MoDC which in part involves mTOR signaling, one of the sensing pathways of nutritional stress [112].…”