Role of the transfer of metabolites from hepatocytes to splenocytes in the suppression of in vitro antibody response by dimethylnitrosamine

Whey-hydrolyzed peptide-enriched immunomodulating diet prevents progression of liver cirrhosis in rats

OBJECTIVE

Liver fibrosis and subsequent cirrhosis is a major cause of death worldwide, but few effective antifibrotic therapies are reported. Whey-hydrolyzed peptide (WHP), a major peptide component of bovine milk, exerts anti-inflammatory effects in experimental models. A WHP-enriched diet is widely used for immunomodulating diets (IMD) in clinical fields. However, the effects of WHP on liver fibrosis remain unknown. The aim of this study was to investigate the antifibrotic effects of WHP in a rat cirrhosis model.

METHODS

Progressive liver fibrosis was induced by repeated intraperitoneal administration of dimethylnitrosamine (DMN) for 3 wk. Rats were fed either a WHP-enriched IMD (WHP group) or a control enteral diet (control group). The degree of liver fibrosis was compared between groups. Hepatocyte-protective effects were examined using hepatocytes isolated from rats fed a WHP diet. Reactive oxygen species and glutathione in liver tissue were investigated in the DMN cirrhosis model.

RESULTS

Macroscopic and microscopic progression of liver fibrosis was remarkably suppressed in the WHP group. Elevated serum levels of liver enzymes and hyaluronic acid, and liver tissue hydroxyproline content were significantly attenuated in the WHP group. Necrotic hepatocyte rates with DMN challenge, isolated from rats fed a WHP-enriched IMD, were significantly lower. In the DMN cirrhosis model, reactive oxygen species were significantly lower, and glutathione was significantly higher in the WHP group's whole liver tissue.

CONCLUSION

A WHP-enriched IMD effectively prevented progression of DMN-induced liver fibrosis in rats via a direct hepatocyte-protective effect and an antioxidant effect through glutathione synthesis.

The effect of butylated hydroxytoluene on selected immune surveillance parameters in rats bearing enzyme-altered hepatic preneoplastic lesions

Selected immune function parameters were examined in male Fischer 344 rats following (a) induction of enzyme-altered preneoplastic liver foci (EAF), and (b) growth modulation of EAF by 30-day feeding with the food antioxidant butylated hydroxytoluene (BHT). Glutathione S-transferase-P (GSTP)-positive EAF were observed in livers of rats receiving diethylnitrosamine (DEN), 2-acetylaminofluorene (2-AAF) and partial hepatectomy (PH) (Solt-Farber procedure), with or without BHT treatment. The induction of EAF and/or 0.5% BHT treatment resulted in a significant reduction in the natural killer (NK) cell activity of splenocytes. PH did not affect NK activity significantly compared with control (no PH) rats. The concanavalin A-induced lymphoproliferative activity of splenocytes was increased in rats with PH compared with those without. A lag in time needed to attain maximum calcium release was observed only in the rats with PH compared with those without PH. None of the treatments affected the phagocytic activity of resident peritoneal macrophages. Only EAF-bearing rats without BHT treatment had increased granulocyte and monocyte levels, while the leucocyte and lymphocyte levels were reduced by the initiator DEN. but not by BHT treatment. Further investigations are necessary to determine whether the observed suppression of NK cell activity during EAF induction and growth modulation by BHT is a contributing factor in enhancement of rodent liver neoplasia by this non-genotoxic food antioxidant.

Effects of ethyl carbamate and its metabolites on the antibody response in splenocyte cultures from female Balb/C mice

To investigate a possible role by cytochrome P450 (P450) in ethyl carbamate-induced immunosuppression, an attempt to assess the ability of ethyl carbamate, its metabolites produced by P450 (i.e., ethyl N-hydroxycarbamate and vinyl carbamate), and methyl carbamate to suppress the polyclonal antibody response induced by bacterial lipopolysaccharide was made in splenocyte cultures isolated from female Balb/C mice. The results showed that vinyl carbamate and ethyl N-hydroxycarbamate were more immunosuppressive compared to ethyl carbamate. A structurally related analogue, methyl carbamate, did not suppress the antibody response. These results indicate that metabolism of ethyl carbamate by P450 may produce more immunosuppressive metabolites as in ethyl carbamate-induced carcinogenicity. A pre-incubation study with phenobarbital-induced liver microsomes in the presence of NADPH-generating system showed that the antibody response was suppressed by ethyl carbamate when splenocytes were pre-incubated with ethyl carbamate and microsomes simultaneously. Moreover, the suppression was completely recovered by the addition of a P450 inhibitor, aminoacetonitrile, in the pre-incubation. Taken together, the present results indicate that metabolism of ethyl carbamate by P450 enzyme(s) may be an important pathway to cause immunosuppression.

Role of hydrocortisone in dimethylnitrosamine-induced suppression of antibody response in the mixed culture of murine hepatocytes and splenocytes

We have previously reported that the hormone-supplemented culture condition for primary hepatocytes is required in dimethylnitrosamine (DMN)-induced suppression of antibody response to sheep erythrocytes in the mixed cultures of murine hepatocytes and splenocytes. In the present investigation, the components of the hormone supplement were screened to identify the component(s) responsible for the increased ability of hepatocytes to activate DMN to its immunosuppressive form. The presence of hydrocortisone in the hepatocyte culture media had the primary role in DMN activation in the co-culture system. Other components of the hormone supplement showed slight or no effects. The effects of hydrocortisone were clearly confirmed through the dose-response study of both DMN and hydrocortisone. To characterize whether the effect of hydrocortisone is glucocorticoid-dependent we tested another potent glucocorticoid, dexamethasone (DEX), and determined if the activity by hydrocortisone could be reversed by RU 486. It was found that hepatocytes cultured in DEX-containing media could also activate DMN to its immunosuppressive form. However, the activity by hydrocortisone to increase DMN-induced immunosuppression was not reversed by RU 486. Furthermore, a possible direct interaction between DMN and hydrocortisone was ruled out. Finally, we transferred DMN-pre-treated culture supernatant from hepatocytes to spleen cell cultures, and found that the metabolite of DMN was very unstable, and that DMN-induced suppression of T-dependent antibody response was hepatocyte-dependent. The present results suggest that glucocorticoids, including hydrocortisone and DEX, in hepatocyte culture media can affect DMN-induced immunosuppression in the hepatocyte/splenocyte co-culture system via a pathway which does not appear to be related to the glucocorticoid receptor.

Recovery of dimethylnitrosamine-induced immunosuppression by pargyline in the mixed cultures of murine hepatocytes and splenocytes

To investigate the role of monoamine oxidase (MAO) in dimethylnitrosamine (DMN)-induced suppression of the antibody response to sheep erythrocytes, the effect of an MAO inhibitor, pargyline, was studied in mixed cultures of murine hepatocytes and splenocytes. When pargyline was added simultaneously with DMN during the coculture, DMN-induced immunosuppression was clearly recovered dose-dependently. Cyclophosphamide was used as a comparative control in these studies. Surprisingly, pargyline also reversed cyclophosphamide-induced suppression of the antibody response in the coculture system. The results with cyclophosphamide were not consistent with a role by MAO, and suggested that pargyline may not be selective for MAO. To confirm our hypothesis, the ability of pargyline to inhibit three cytochrome P-450 (P-450) isozyme-specific monooxygenase activities in vitro was studied using mouse liver microsomes. Pargyline, under the same concentration ranges that we used in the coculture studies, clearly inhibited the P-450IIE1-specific p-nitrophenol hydroxylase activity and P-450IIB1-specific pentoxyresorufin O-dealkylase activity. Taken together, our present results indicate that pargyline inhibits P-450 activity and is not selective for MAO. Although further studies are required to confirm a possible role by MAO in DMN-induced immunosuppression, our results suggest that pargyline may recover DMN-induced immunosuppression by primarily inhibiting the ability of P-450 enzymes in hepatocytes to activate DMN to its immunosuppressive metabolite(s).

Immunosuppression by chronic exposure to N-nitrosodimethylamine (NDMA) in mice

Immunosuppression of humoral and cellular responses following chronic oral exposure to 1, 5, 10, and 20 ppm N-nitrosodimethylamine (NDMA) was examined in CD-1 mice. Monitoring of cumulative mortality and the incidence of peritoneal ascites in animals showed an NDMA dose-related mortality and hepatotoxicity. No visible changes in immunological parameters were noted at the 1 ppm NDMA dose. Immunosuppression of immunoglobulin M (IgM) antibody response by NDMA to sheep red blood cells (SRBC) was time-related, dose-related, and could be reversed within 30 d by removal of the chemical from the drinking water. Cellular immune response, monitored by allogeneic stimulation of cells in mixed lymphocyte reaction (MLR), was markedly suppressed by 10 and 20 ppm NDMA. Thus, chronic exposure to NDMA, except for the low-hepatotoxic doses of nitrosamine, resulted in a marked and persistent immunosuppression of cellular and humoral responses in CD-1 mice. In conclusion, chronic exposure to the hepatotoxic (ascite-inducing) doses of NDMA suppressed humoral and cellular immunity. The persistent immunosuppression could be reversed after the removal of NDMA from the drinking water. Although no direct NDMA-related cancer was reported in humans, our data point to a potential epigenetic carcinogenicity of nitrosamines due to chronic immunosuppression.

Importance of hepatocyte culture conditions in dimethylnitrosamine-induced suppression of antibody response in the mixed cultures of murine hepatocytes and splenocytes

The role of the hepatocyte culture media in dimethylnitrosamine (DMN)-induced suppression of antibody responses by splenocytes against sheep erythrocytes (SRBCs) was investigated using the mixed culture system of murine hepatocytes and murine splenocytes. It was observed that hormone-supplemented complete media was required for hepatocyte cultures to optimally activate DMN to its immunosuppressive form(s). In the absence of the hormone supplement, the concentration of DMN required to produce a 50% suppression (IC50) was increased by over 10-fold (i.e., compare the IC50 in complete media of less than 0.5 mM to the IC50 in basal media of almost 10.0 mM). In contrast, the activation of cyclophosphamide (cytoxan, CTX), which was used in these studies as a comparative control, was not affected by the absence of the hormone supplement. These results indicate that the observed effect on the activation of DMN was not due to a generalized loss of metabolic capability of hepatocytes cultured without hormones. To examine the role of drug metabolizing capabilities of hepatocytes in the differential activation of DMN, we compared phase I and phase II enzyme activities of hepatocytes cultured for 24 h in either basal media or hormone-supplemented complete media. Our results indicated that there was a significant decrease of phase I monooxygenase activities of cultured hepatocytes when compared to freshly isolated hepatocytes. However, our results failed to show any difference in the activities of hepatocytes cultured in the two media. Most notably, there was no difference in the activity of either high- or low-affinity DMN demethylase, as measured by the generation of formaldehyde. We observed a similar profile with phase II conjugative capabilities, specifically sulfotransferase and glucuronyltransferase. These results indicate that the activation of DMN to its immunosuppressive form(s) can be modulated in the co-culture system by culturing hepatocytes under different conditions. Because we failed to show any differences in the metabolic capabilities of hepatocytes cultured under the two media conditions, the results suggest that the modulation of immunosuppressive activity may not be related to a change in the generation of the immunosuppressive metabolite(s).

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.

Production of DNA single-strand breaks in unstimulated splenocytes by dimethylnitrosamine

In an attempt to elucidate the mechanism whereby primary hepatocytes, but not liver S9 homogenates, generate immunosupprssive metabolites of dimethylnitrosamine (DMN), the production of DNA single-strand breaks (SSB) in unstimulated splenocytes was investigated with alkaline-elution analysis. Both hepatocytes and S9 homogenates induced SSB in cultured splenocytes by DMN - minimum detectable doses with the two metabolic activation systems (MAS) were 1 microM and 5 mM, respectively. DNA elution profiles were linear in splenocytes co-cultured with DMN and hepatocytes and convex in splenocytes incubated with DMN and S9 homogenates. Aminoacetonitrile (AAN; 10 mM), a DMN demethylase inhibitor, reversed SSB in splenocytes when incubated with either MAS. Addition of exogenous calf-thymus DNA to the hepatocyte co-culture medium did not affect the production of SSB. Rocking the hepatocyte-splenocyte cultures changed the elution profile from linear to convex. All of these treatments have been previously shown to block the immunosuppression by DMN in the hepatocyte co-culture system. These results indicate that the immunosuppression by DMN is not related to DNA damage, as measured by the production of SSB, and suggest that the metabolism of DMN to intermediates capable of producing genotoxicity and immunotoxicity may be qualitatively and/or quantitatively different.