Alteration of macrophage differentiation into accessory and effector cells from exposure to dimethylnitrosamine (DMN) in vivo

Carcinogenesis: Failure of resolution of inflammation?

Inflammation in the tumor microenvironment is a hallmark of cancer and is recognized as a key characteristic of carcinogens. However, the failure of resolution of inflammation in cancer is only recently being understood. Products of arachidonic acid and related fatty acid metabolism called eicosanoids, including prostaglandins, leukotrienes, lipoxins, and epoxyeicosanoids, critically regulate inflammation, as well as its resolution. The resolution of inflammation is now appreciated to be an active biochemical process regulated by endogenous specialized pro-resolving lipid autacoid mediators which combat infections and stimulate tissue repair/regeneration. Environmental and chemical human carcinogens, including aflatoxins, asbestos, nitrosamines, alcohol, and tobacco, induce tumor-promoting inflammation and can disrupt the resolution of inflammation contributing to a devastating global cancer burden. While mechanisms of carcinogenesis have focused on genotoxic activity to induce mutations, nongenotoxic mechanisms such as inflammation and oxidative stress promote genotoxicity, proliferation, and mutations. Moreover, carcinogens initiate oxidative stress to synergize with inflammation and DNA damage to fuel a vicious feedback loop of cell death, tissue damage, and carcinogenesis. In contrast, stimulation of resolution of inflammation may prevent carcinogenesis by clearance of cellular debris via macrophage phagocytosis and inhibition of an eicosanoid/cytokine storm of pro-inflammatory mediators. Controlling the host inflammatory response and its resolution in carcinogen-induced cancers will be critical to reducing carcinogen-induced morbidity and mortality. Here we review the recent evidence that stimulation of resolution of inflammation, including pro-resolution lipid mediators and soluble epoxide hydrolase inhibitors, may be a new chemopreventive approach to prevent carcinogen-induced cancer that should be evaluated in humans.

Dimethylnitrosamine (DMN)-induced IL-1 beta, TNF-alpha, and IL-6 inflammatory cytokine expression

Altered immune functions have been demonstrated in mice following exposure to dimethylnitrosamine (DMN). In particular, changes in cell-mediated immune responses resulted from chronic DMN exposure in vivo. Since cytokines are potent immunoregulatory peptides, experiments were performed to determine whether DMN exposure results in the induction of serum-borne inflammatory cytokines. Animals were exposed to either vehicle (PBS) or DMN (5.0 mg/kg) every 24 hr for 14 days. Serum and liver samples were obtained from individual mice at 0, 1, 2, 3, 6, 12, and 24 hr following the first exposure, with additional samples collected every 24 hr preceding the daily DMN exposure. Sera were then analyzed for IL-1 beta, IL-3, IL-6, CSF-1, GM-CSF, and TNF-alpha activities using either biological or immunological assays. In addition, liver total cellular RNA was probed for the induction of IL-1 beta transcripts using the solution hybridization/RNase protection assay. IL-1 beta, IL-6, and TNF-alpha serum activities were observed within 2 hr of DMN exposure and returned to vehicle control levels by 3 days even though DMN exposure was maintained. Chronic expression of cytokine activity (after 72 hr) was only observed for GM-CSF. A rapid induction of IL-1 beta transcripts (within 1 hr) in both vehicle and DMN-treated animals was observed by solution hybridization. However, by 3 hr postexposure, transcript levels decreased in the vehicle-treated animals while remaining elevated in the DMN-treated animals for 6 hr. These results demonstrated that DMN exposure in vivo induced: (1) the expression of serum-borne cytokine activities, and (2) IL-1 beta transcription in liver tissue.

Transcriptional changes in macrophage TNF-alpha expression following dimethylnitrosamine exposure in vivo

DMN exposure has been shown to increase macrophage cytotoxic activity against tumor targets both in vitro and in vivo. Since the production and expression of the macrophage-derived cytokine tumor necrosis factor-alpha (TNF-alpha) is associated with such anti-tumor activity, studies were performed to determine whether changes in TNF-alpha gene transcription and biosynthesis resulted following DMN exposure. Thioglycollate-elicited macrophages obtained from DMN-exposed animals displayed enhanced levels of constitutively expressed TNF-alpha transcripts compared to vehicle controls. Northern blot analysis of the time course expression of TNF-alpha following endotoxin (1 microgram/ml) stimulation in vitro showed a significantly greater induction of TNF-alpha transcripts in macrophages from DMN-exposed than control animals, with peak levels detected between 30 and 120 min. Maximum endotoxin-induced TNF-alpha secretion occurred later than the accumulation of the transcripts, with greater secretion observed between 120 and 360 min. In contrast to endotoxin, stimulation with IFN-gamma (100 U/ml) produced no changes in the level of TNF-alpha transcripts. However, stimulation of macrophages with IFN-gamma did greatly enhance the surface expression of membrane-bound TNF-alpha in cells from the DMN-treated animals. Supernatants from media and endotoxin stimulated macrophage were tested for TNF-alpha activity against WEHI-164 cells. In media alone, a five-fold increase in TNF-alpha activity was observed at 6 h in supernatants from macrophage obtained from DMN-exposed animals compared to the vehicle group. Treatment of supernatants with either superoxide dismutase (SOD) or catalase to remove reactive oxygen products did not alter their lytic capacity. However, addition of a neutralizing murine-anti-TNF-alpha antibody reduced the lytic capacity of the supernatants by 90% in both treatment groups. Accumulation of IL-1 beta transcripts gradually increased over the 6 h with a concomitant increase in secreted IL-1 beta that was identical in both DMN and vehicle groups. These results demonstrate that DMN exposure: (1) enhances the expression of TNF-alpha in peripheral macrophages by transcriptional regulatory mechanism(s) and, (2) does not alter the expression or secretion of IL-1 beta.

Enhanced macrophage anti-microbial activity following dimethylnitrosamine exposure in vivo is related to augmented production of reactive oxygen metabolites

Previous results demonstrated that mice exposed in vivo to DMN were more resistant to both bacterial and tumor challenges. Furthermore, macrophages (M phi) isolated from these animals demonstrated increased functional properties. As reactive oxygen intermediates (ROI) represent a key mechanism of anti-microbial action, it was important to determine whether ROI levels in M phi were related to augmented anti-microbial action in animals exposed to DMN in vivo. Peritoneal exudate M phi elicited with either thioglycollate (TG), Con A or C. parvum (CP) were examined for the production of ROIs. TG-M phi, Con A-M phi and CP-M phi obtained from animals exposed to DMN showed increased superoxide anion (O2-) production in vitro following stimulation with either phorbol myristate acetate (PMA) or opsonized zymosan (Op-zym) when compared to vehicle M phi. ROI production by bone marrow-derived macrophages (BMDM) produced by either GM-CSF or CSF-1 was also determined. BMDM from DMN-exposed animals obtained using either growth factor, had increased ROI production at 3, 5, 7 and 9 d of culture compared to vehicle BMDM. There was no shift in the kinetics of ROI production during differentiation of these BMDM. Analysis of extracellular anti-listericidal activity of TG- and CA-elicited M phi demonstrated that only TG-M phi obtained from DMN-exposed animals had enhanced killing capacity. There were no differences in intracellular anti-microbial activity in TG- and CA-elicited M phi obtained from either vehicle or DMN-exposed animals. TG-elicited M phi from either vehicle or DMN-exposed animals were examined for anti-microbial activity and H2O2 production following in vitro exposure to PMA. M phi from both vehicle and DMN treatment groups had enhanced killing and H2O2 production following PMA treatment, while PMA-stimulated TG-M phi from DMN-exposed animals demonstrated significantly higher levels of H2O2 production and cell killing as compared to all other treatment groups. These results suggest that previously observed increases in anti-microbial action by M phi from DMN-exposed animals are due in-part to enhanced ROI production.

Role of metabolism in dimethylnitrosamine-induced immunosuppression: a review

Dimethylnitrosamine (DMN) has been characterized as a potent hepatotoxin, carcinogen and mutagen. As described below, immunotoxicity should be added to its profile of activity. Although a broad spectrum of immune parameters is affected by DMN, humoral immunity is particularly sensitive. In order for DMN to produce its traditional profile of toxicity it requires metabolic activation to reactive intermediates which alkylate macromolecules. Similarly, DMN also must be metabolized to produce its immunological effects. However, as this review suggests, the metabolism of DMN to an intermediate capable of suppressing the humoral immune response may be qualitatively and/or quantitatively different from that which mediates hepatotoxicity and genotoxicity.

Induction of serum colony-stimulating activity (CSA) following dimethylnitrosamine (DMN) exposure: effects on macrophage differentiation

Dimethylnitrosamine (DMN) exposure in vivo affects hematopoiesis at the level of CFU-GM and CFU-M precursor cells. This effect on hematopoiesis has been shown to be an indirect consequence of DMN exposure; therefore, we examined serum from animals exposed to DMN in vivo for the presence of hematopoietic growth factor activity. Serum obtained from animals exposed to DMN in vivo supported colony formation in normal bone marrow stem cells whereas serum obtained from untreated or vehicle-exposed animals failed to support colony formation. Differential staining of the cells which arose following the in vitro culture of normal bone marrow cells with serum from DMN-exposed animals demonstrated the presence of cells of the monocytic and granulocytic lineages. Pre-treatment of serum from DMN-exposed animals with anti-CSF-1 antibodies prior to in vitro culture had no affect on either cell number, cell phenotype or colony-stimulating activity, suggesting the presence of GM-CSF. Administration of serum from DMN-exposed animals to naive recipient animals resulted in increased percentages of both blood-borne monocytes and neutrophils, mimicking the profile observed in DMN-exposed animals. Studies using oligonucleotide-directed PCR demonstrated the presence of GM-CSF transcripts in the livers obtained following DMN exposure. These results demonstrate the presence of a serum-borne activity(ies) following DMN exposure, most likely GM-CSF, which is capable not only of enhancing macrophage and granulocyte hematopoiesis but also of increasing the numbers of these two cell types in the blood.

Augmented macrophage PGE2 production following exposure to dimethylnitrosamine in vivo: relevance to suppressed T cell responses

Previous efforts from our laboratory have investigated the mechanisms responsible for dimethylnitrosamine (DMN)-induced suppression of T cell responses. These studies suggested that such changes in T cell activity were most likely to be due to alterations in the down-regulatory signals controlling T cell activation. Accordingly, the production of PGE2, a potent inhibitor of T cell activation, was examined in macrophages obtained from animals exposed to either DMN or vehicle in vivo. The production of PGE2 was determined in macrophages representing various stages of activation (responsive, primed and fully activated) and various stages of differentiation (CSF-1-derived or GM-CSF-derived macrophages). All peritoneal macrophages obtained from DMN-exposed animals demonstrated enhanced production of PGE2 following stimulation with either endotoxin or IFN-gamma as compared to macrophages obtained from vehicle-exposed macrophages. Moreover, the enhanced levels of PGE2 were due to increased PGE2 production rather than to shifts in the kinetics of PGE2 production and utilization. CSF-1- and GM-CSF-induced bone-marrow-derived macrophages (BMDM) produced minimal levels of PGE2, regardless of the in vitro stimulation of cells obtained from either vehicle or DMN treatment groups. Spleen cells obtained from DMN-exposed animals produced significantly higher amounts of PGE2 following endotoxin stimulation compared to control splenocytes. Splenocytes from DMN-exposed animals also demonstrated a suppressed proliferative response to the mitogen Con A. However, when splenocytes from DMN-exposed animals were co-cultured with indomethacin they demonstrated Con A-stimulated proliferative responses similar to the responses of vehicle control splenocytes. These results demonstrate that DMN exposure results in increased PGE2 production by macrophages and that this increase in PGE2 production may be responsible for suppressed T cell responses observed in DMN-exposed animals.

Immunological aspects of toxicology: premises not promises

The immune system is gaining increasing attention as a target of toxicant action. Many compounds, including metals, drugs, and pesticides, are able to alter immune functions. Additionally, the ability of toxicants to experimentally alter resistance to a variety of bacterial, viral, and tumor challenges is suggestive for the possible involvement of toxicants in morbidity and mortality. However, the implications of immunotoxicology as a subject of study may not be fully realized in some laboratories due to unfamiliarities into the workings of immunity by toxicologists. In an attempt to bridge this toxicology-immunology gap, this paper presents the major concepts of the immune system by reviewing specific examples of immune responses and their environmental interactions. Data from selected toxins are then used to illustrate how these responses may be altered.

Alteration of macrophage anti-tumor activity and transferrin receptor expression by exposure to dimethylnitrosamine in vivo

Dimethylnitrosamine (DMN), a potent immunomodulatory agent, produces its effects on cell-mediated immune reactions through alterations in macrophage production and/or macrophage maturation/differentiation from bone marrow stem cells. Macrophages obtained by the in vitro culture of bone marrow from animals exposed to DMN in vivo (bone marrow-derived macrophages, BMDM) demonstrated enhanced cytotoxicity against L929 target cells. The enhanced cytotoxicity was also present in concanavalin A- and Corynebacterium parvum-elicited peritoneal exudate cells (PEC) obtained from DMN-exposed animals while thioglycollate-elicited PEC from DMN-exposed animals displayed no increase in their cytotoxic activity as compared to vehicle-exposed animals. However, treatment of thioglycollate-elicited PEC with interferon-gamma induced cytotoxic activity in PEC obtained from DMN-exposed animals but not in PEC obtained from control animals. BMDM obtained from DMN-exposed mice also demonstrated an alteration in the kinetics of the expression of the membrane-associated transferrin-binding receptor (mTFR), a marker of the activational status of macrophages. BMDM from animals exposed to DMN in vivo exhibited maximal expression of mTFR on day 7 of culture in vitro as compared to day 5 for BMDM from vehicle-exposed animals. C. parvum- and concanavalin A-elicited PEC from DMN-exposed animals showed dose-related decreases in their expression of mTFR which were associated with their expected enhanced cytotoxicity. Likewise, thioglycollate-elicited PEC from DMN-exposed mice had dose-related decreases in mTFR expression and total transferrin-binding activity, suggesting a change in their state of activation. No alterations in mTFR expression were observed in splenic macrophages. BMDM cultured with T cell-derived lymphokines known to affect mTFR expression demonstrated enhanced expression of mTFR independent of changes in the cell cycle profiles. Furthermore, while lymphokines enhanced mTFR expression, there was no alteration in the kinetics of mTFR expression by BMDM obtained from DMN- or vehicle-exposed animals. These results support the hypothesis that DMN-induced alterations in macrophage hematopoietic differentiation/maturation are manifested in changes in macrophage function.

Modification of macrophage differentiation: dimethylnitrosamine induced alteration in the responses towards the regulatory signals controlling myelopoiesis

Results from our laboratory have demonstrated that the alteration in cellular immunity (CMI) resulting from exposure to dimethylnitrosamine (DMN) in vivo is due to changes in myelopoiesis. Bone marrow stem cells showed no alterations in their capacity to generate CFU-S (pleuripotent stem cells) nor were there any changes in the number of CFU-Mix colonies (IL-3 responsive stem cells) arising from the bone marrow of DMN exposed mice. However, the generation of G/M-CSF and CSF-1 responsive colonies (CFU-G/M and CFU-M) were altered, resulting in an increase in the number of colonies. G/M-CSF colonies generated from the bone marrow stem cells obtained from DMN exposed mice also had increased numbers of cells produced by each colony (total cells/CFU). Indirect immunofluorescence studies demonstrated no changes in the granulocyte/macrophage subsets following G/M-CSF stimulation of bone marrow stem cells obtained from DMN exposed mice. However, there was no change in the total number of cells generated by CSF-1 from the marrows of DMN exposed mice as compared to vehicle treated mice. Marrow cells from DMN exposed mice cultured in vitro with G/M-CSF showed both a shift in their peak proliferative response from 48-72 h to 30-60 h and an increased proliferative response. These same marrow cells showed no shift in their kinetics but a decrease in their proliferative response to CSF-1. Examination of the sera from DMN exposed mice for alterations in the regulatory factors controlling myelopoiesis demonstrated a net decrease of CSF-1 activity but no changes in the concentrations of two inhibitory factors, transferrin and lactoferrin.(ABSTRACT TRUNCATED AT 250 WORDS)