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Inflammatory bowel disease (IBD) is a well-known risk factor for the development of colorectal cancer. Prior studies have demonstrated that microbial histamine can ameliorate intestinal inflammation in mice. We tested the hypothesis whether microbe-derived luminal histamine suppresses inflammation-associated colon cancer in Apcmin/+ mice. Mice were colonized with the human-derived Lactobacillus reuteri. Chronic inflammation was induced by repeated cycles of low-dose dextran sulfate sodium (DSS). Mice that were given histamine-producing L. reuteri via oral gavage developed fewer colonic tumors, despite the presence of a complex mouse gut microbiome. We further demonstrated that administration of a histamine H1-receptor (H1R) antagonist suppressed tumorigenesis, while administration of histamine H2-receptor (H2R) antagonist significantly increased both tumor number and size. The bimodal functions of histamine include protumorigenic effects through H1R and antitumorigenic effects via H2R, and these results were supported by gene expression profiling studies on tumor specimens of patients with colorectal cancer. Greater ratios of gene expression of H2R ( HRH2) vs. H1R ( HRH1) were correlated with improved overall survival outcomes in patients with colorectal cancer. Additionally, activation of H2R suppressed phosphorylation of mitogen-activated protein kinases (MAPKs) and inhibited chemokine gene expression induced by H1R activation in colorectal cancer cells. Moreover, the combination of a H1R antagonist and a H2R agonist yielded potent suppression of lipopolysaccharide-induced MAPK signaling in macrophages. Given the impact on intestinal epithelial and immune cells, simultaneous modulation of H1R and H2R signaling pathways may be a promising therapeutic target for the prevention and treatment of inflammation-associated colorectal cancer. NEW & NOTEWORTHY Histamine-producing Lactobacillus reuteri can suppress development of inflammation-associated colon cancer in an established mouse model. The net effects of histamine may depend on the relative activity of H1R and H2R signaling pathways in the intestinal mucosa. Our findings suggest that treatment with H1R or H2R antagonists could yield opposite effects. However, by harnessing the ability to block H1R signaling while stimulating H2R signaling, novel strategies for suppression of intestinal inflammation and colorectal neoplasia could be developed.

Microbiome-mediated suppression of carcinogenesis may open new avenues for identification of therapeutic targets and prevention strategies in oncology. Histidine decarboxylase (HDC) deficiency has been shown to promote inflammation-associated colorectal cancer by accumulation of CD11b + Gr-1 + immature myeloid cells, indicating a potential antitumorigenic effect of histamine. Here, we demonstrate that administration of hdc + Lactobacillus reuteri in the gut resulted in luminal hdc gene expression and histamine production in the intestines of Hdc −/− mice. This histamine-producing probiotic decreased the number and size of colon tumors and colonic uptake of [ 18 F]-fluorodeoxyglucose by positron emission tomography in Hdc −/− mice. Administration of L. reuteri suppressed keratinocyte chemoattractant ( KC ), Il22 , Il6 , Tnf , and IL1α gene expression in the colonic mucosa and reduced the amounts of proinflammatory, cancer-associated cytokines, keratinocyte chemoattractant, IL-22, and IL-6, in plasma. Histamine-generating L. reuteri also decreased the relative numbers of splenic CD11b + Gr-1 + immature myeloid cells. Furthermore, an isogenic HDC-deficient L. reuteri mutant that was unable to generate histamine did not suppress carcinogenesis, indicating a significant role of the cometabolite, histamine, in suppression of chronic intestinal inflammation and colorectal tumorigenesis. These findings link luminal conversion of amino acids to biogenic amines by gut microbes and probiotic-mediated suppression of colorectal neoplasia.

Bacterial‐derived compounds from the intestinal microbiome modulate host mucosal immunity. Identification and mechanistic studies of these compounds provide insights into host–microbial mutualism. Specific Lactobacillus reuteri strains suppress production of the proinflammatory cytokine, tumor necrosis factor (TNF), and are protective in a mouse model of colitis. Human‐derived L. reuteri strain ATCC PTA 6475 suppresses intestinal inflammation and produces 5,10‐methenyltetrahydrofolic acid polyglutamates. Insertional mutagenesis identified the bifunctional dihydrofolate synthase/folylpolyglutamate synthase type 2 (folC2) gene as essential for 5,10‐methenyltetrahydrofolic acid polyglutamate biosynthesis, as well as for suppression of TNF production by activated human monocytes, and for the anti‐inflammatory effect of L. reuteri 6475 in a trinitrobenzene sulfonic acid‐induced mouse model of acute colitis. In contrast, folC encodes the enzyme responsible for folate polyglutamylation but does not impact TNF suppression by L. reuteri. Comparative transcriptomics between wild‐type and mutant L. reuteri strains revealed additional genes involved in immunomodulation, including previously identified hdc genes involved in histidine to histamine conversion. The folC2 mutant yielded diminished hdc gene cluster expression and diminished histamine production, suggesting a link between folate and histadine/histamine metabolism. The identification of genes and gene networks regulating production of bacterial‐derived immunoregulatory molecules may lead to improved anti‐inflammatory strategies for digestive diseases.

Probiotics and commensal intestinal microbes suppress mammalian cytokine production and intestinal inflammation in various experimental model systems. Limited information exists regarding potential mechanisms of probiotic-mediated immunomodulation in vivo. In this report, we demonstrate that specific probiotic strains of Lactobacillus reuteri suppress intestinal inflammation in a trinitrobenzene sulfonic acid (TNBS)-induced mouse colitis model. Only strains that possess the hdc gene cluster, including the histidine decarboxylase and histidine-histamine antiporter genes, can suppress colitis and mucosal cytokine (interleukin-6 [IL-6] and IL-1β in the colon) gene expression. Suppression of acute colitis in mice was documented by diminished weight loss, colonic injury, serum amyloid A (SAA) protein concentrations, and reduced uptake of [18F]fluorodeoxyglucose ([18F]FDG) in the colon by positron emission tomography (PET). The ability of probiotic L. reuteri to suppress colitis depends on the presence of a bacterial histidine decarboxylase gene(s) in the intestinal microbiome, consumption of a histidine-containing diet, and signaling via the histamine H2 receptor (H2R). Collectively, luminal conversion of l-histidine to histamine by hdc+ L. reuteri activates H2R, and H2R signaling results in suppression of acute inflammation within the mouse colon., IMPORTANCE Probiotics are microorganisms that when administered in adequate amounts confer beneficial effects on the host. Supplementation with probiotic strains was shown to suppress intestinal inflammation in patients with inflammatory bowel disease and in rodent colitis models. However, the mechanisms of probiosis are not clear. Our current studies suggest that supplementation with hdc+ L. reuteri, which can convert l-histidine to histamine in the gut, resulted in suppression of colonic inflammation. These findings link luminal conversion of dietary components (amino acid metabolism) by gut microbes and probiotic-mediated suppression of colonic inflammation. The effective combination of diet, gut bacteria, and host receptor-mediated signaling may result in opportunities for therapeutic microbiology and provide clues for discovery and development of next-generation probiotics.

Human microbiome-derived strains of Lactobacillus reuteri potently suppress proinflammatory cytokines like human tumor necrosis factor (TNF) by converting the amino acid l -histidine to the biogenic amine histamine. Histamine suppresses mitogen-activated protein (MAP) kinase activation and cytokine production by signaling via histamine receptor type 2 (H 2 ) on myeloid cells. Investigations of the gene expression profiles of immunomodulatory L. reuteri ATCC PTA 6475 highlighted numerous genes that were highly expressed during the stationary phase of growth, when TNF suppression is most potent. One such gene was found to be a regulator of genes involved in histidine-histamine metabolism by this probiotic species. During the course of these studies, this gene was renamed the Lactobacillus reuteri -specific immunoregulatory ( rsiR ) gene. The rsiR gene is essential for human TNF suppression by L. reuteri and expression of the histidine decarboxylase ( hdc ) gene cluster on the L. reuteri chromosome. Inactivation of rsiR resulted in diminished TNF suppression in vitro and reduced anti-inflammatory effects in vivo in a trinitrobenzene sulfonic acid (TNBS)-induced mouse model of acute colitis. A L. reuteri strain lacking an intact rsiR gene was unable to suppress colitis and resulted in greater concentrations of serum amyloid A (SAA) in the bloodstream of affected animals. The P hdcAB promoter region targeted by rsiR was defined by reporter gene experiments. These studies support the presence of a regulatory gene, rsiR , which modulates the expression of a gene cluster known to mediate immunoregulation by probiotics at the transcriptional level. These findings may point the way toward new strategies for controlling gene expression in probiotics by dietary interventions or microbiome manipulation.