Generation of 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene by murine splenic macrophages

Interactions between immune and biotransformation systems in fish: a review

In animals biotransformation and immune system are not totally independent, there are numerous functional interrelationships between these two systems. They are both implicated in the capacity of organisms to resist to a wide variety of environmental components such as viruses, bacteria and xenobiotics. It is known for a long time that the immune system functions as a physiologic system and interacts with all the other components of the organism including nervous or endocrine ones. In the same manner, the biotransformation system (especially the cytochrome P450 monooxygenases) is involved in the regulation of numerous hormone productions. In this way, many studies in mammals have revealed the possible interaction between immune and biotransformation systems. Among these interactions, the capacity of the activation of host defense mechanisms to down-regulate microsomal cytochrome P450 and the role of biotransformation system in the xenobiotic-mediated immunotoxicity have been underlined. Advances in the basic knowledge of fish immune and biotransformation systems should lead to a better understanding of the possible interactions between both systems and should improve fish health monitoring which is a crucial ecotoxicological goal.

Alterations in immune function and CYP450 activity in adult male deer mice (Peromyscus maniculatus) following exposure to benzo[a]pyrene, pyrene, or chrysene

Polycyclic aromatic hydrocarbons (PAHs) are among the most common classes of chemical contaminants found at hazardous waste sites. Deer mice (Peromyscus maniculatus) exhibit a wide geographic distribution throughout North America and have been suggested as a terrestrial biomonitoring species to facilitate comparisons between superfund sites. Chemicals tested were benzo[a]pyrene (BaP; CAS number 50-32-8), pyrene (Pyr; CAS number 129-00-0), and chrysene (Chr; CAS number 218-01-9). Adult male deer mice were exposed via intraperitoneal (i.p.) injection every other day for 11 d to the PAHs (0.3, 1, 3, 10, or 30 mg/kg) or a corn oil carrier control. Both BaP and Chr suppressed the plaque-forming cell (PFC) response at all treatment levels. Pyr exposure (1-30 mg/kg) also resulted in suppression of this response. Macrophage pinocytosis was suppressed only by Chr (3, 10, and 30 mg/kg). Concanavalin A-induced proliferation was stimulated by BaP at all dose levels, by Pyr at 1-30 mg/kg, and by Chr at 30 mg/kg. Chr did not affect pokeweed mitogen (PWM)-induced proliferation; however, BaP (1-30 mg/kg) and Pyr (0.3-30 mg/kg) produced stimulation of this response as compared to respective controls. BaP and Chr stimulated cytochrome P-450 1A1 (CYP1A1) activity (3, 10, or 30 mg/kg) as measured by ethoxyresorufin O-deethylase (EROD) activity, but Pyr did not. These results indicate that immune function endpoints appear to be more sensitive to these PAHs than measured hepatic CYP450 activity.

Benzo[a]pyrene-induced immunotoxicity in Japanese medaka (Oryzias latipes): relationship between lymphoid CYP1A activity and humoral immune suppression

Exposure to the environmental contaminant benzo[a]pyrene (BaP) results in suppression of immune function in both mammalian and fish species. This laboratory has previously demonstrated that a single intraperitoneal (IP) injection of BaP reduced lymphocyte proliferation, phagocyte-mediated superoxide generation, and antibody-forming cell (AFC) numbers in Japanese medaka (Oryzias latipes). The objective of the current study was to determine the role of BaP metabolism in the observed immunosuppression. Results from rodent studies have suggested that BaP elicits its immunotoxic effects via upregulation of cytochrome P4501A1 (CYP1A1) and the subsequent production of immunosuppressive BaP metabolites. In this study, exposure of medaka to 200 microg BaP/g BW significantly induced CYP1A expression or activity within lymphoid tissue 48 h post-IP injection; induction was observed specifically within distinct subpopulations of kidney mononuclear cells. Concurrent injection of fish with BaP and the CYP1A1 inhibitors alpha-naphthoflavone (ANF) or dehydroepiandrosterone (DHEA) resulted in inhibition of renal EROD activity and amelioration of BaP-induced suppression of medaka AFC numbers. Results of this study suggest that (1) BaP-induced suppression of medaka humoral immunity relies upon the CYP1A-catalyzed production of immunotoxic BaP metabolites and (2) BaP metabolites may be created in situ, directly by specific cells within kidney lymphoid tissue. Thus, apparently, mechanisms involved in BaP-induced immunosuppression have been phylogenetically conserved from fish to mammals.

Beta-carotene: a cancer chemopreventive agent or a co-carcinogen?

Evidence from both epidemiological and experimental observations have fueled the belief that the high consumption of fruits and vegetables rich in carotenoids may help prevent cancer and heart disease in humans. Because of its well-documented antioxidant and antigenotoxic properties, the carotenoid beta-carotene (betaCT) gained most of the attention in the early 1980s and became one of the most extensively studied cancer chemopreventive agents in population-based trials supported by the National Cancer Institute. However, the results of three randomized lung cancer chemoprevention trials on betaCT supplementation unexpectedly contradicted the large body of epidemiological evidence relating to the potential benefits of dietary carotenoids. Not only did betaCT show no benefit, it was associated with significant increases in lung cancer incidence, cardiovascular diseases, and total mortality. These findings aroused widespread scientific debate that is still ongoing. It also raised the suspicion that betaCT may even possess co-carcinogenic properties. In this review, we summarize the current data on the co-carcinogenic properties of betaCT that is attributed to its role in the induction of carcinogen metabolizing enzymes and the over-generation of oxidative stress. The data presented provide convincing evidence of the harmful properties of this compound if given alone to smokers, or to individuals exposed to environmental carcinogens, as a micronutrient supplement. This has now been directly verified in a medium-term cancer transformation bioassay. In the context of public health policies, while the benefits of a diet rich in a variety of fruits and vegetables should continue to be emphasized, the data presented here point to the need for consideration of the possible detrimental effects of certain isolated dietary supplements, before mass cancer chemoprevention clinical trials are conducted on human subjects. This is especially important for genetically predisposed individuals who are environmentally or occupationally exposed to mutagens and carcinogens, such as those found in tobacco smoke and in industrial settings.

Inhibitory activity of vitamin E and alpha-naphthoflavone on beta-carotene-enhanced transformation of BALB/c 3T3 cells by benzo(a)pyrene and cigarette-smoke condensate

We previously found that beta-carotene (betaCT) can act as a co-carcinogenic agent enhancing the cell transforming activity of powerful carcinogens such as benzo(a)pyrene (B(a)P) and cigarette-smoke condensate (TAR) in an in vitro medium-term ( approximately 8 weeks) experimental model utilizing BALB/c 3T3 cells (Mutat. Res. 440 (1999) 83-90). Here, we investigated whether vitamin E (VitE) and alpha-naphthoflavone (alphaNF) are able to affect the co-carcinogenic activity of betaCT in terms of inhibiting B(a)P and TAR cell transforming potential. The following experimental schedules were performed: (i) cultures treated for 72 h with chemicals in various experimental combinations (acute treatment); (ii) cultures grown in presence of tester agents for the whole period of the assay (chronic treatment) to more closely mimic human exposure. While the co-carcinogenic potential of betaCT was confirmed on both B(a)P and TAR, the latter being ineffective by itself, we found in repeated experiments that the presence of VitE or alphaNF significantly reduced the betaCT's enhancing effect in the formation of transformation foci by B(a)P and TAR. The mechanism of the inhibition could be explained by the known ability of alphaNF to inhibit cytochrome P450-linked B(a)P-bioactivating monooxygenases, while VitE may contrast the prooxidant activity of betaCT (e.g., oxygen radicals overgeneration). While highlighting the importance of increasing knowledge of the role of single provitamins, vitamins and micronutrients, our findings also underline the potential advantages of combining several dietary supplements in in vitro preventive investigations.

Immunotoxicity of a reconstituted polynuclear aromatic hydrocarbon mixture in B6C3F1 mice

Previous studies on the immunotoxicity of a complex mixture of polynuclear aromatic hydrocarbon (PAH) by-products from a manufactured gas plant indicated possible synergistic interactions which were investigated by determining the immunosuppressive effects of a reconstituted PAH mixture in female B6C3F1 mice challenged with TNP-haptenated sheep red blood cells (SRBCs) (T-cell-dependent) or trinitrophenyl-lipopolysaccharide (TNP-LPS) (T-cell-independent) antigens. The reconstituted PAH mixture contained the following 17 congeners: 2-rings (indan, naphthalene, 1- and 2-methylnaphthalene), 3-rings (acenaphthylene, acenaphthene, dibenzofuran, fluorene, phenanthrene and anthracene), and > or = 4-rings (pyrene, fluoranthene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene and benzo[a]pyrene), and resembled mixtures identified as by-products from manufactured gas plants. The reconstituted mixture and the 2-, 3- and > or = 4-ring PAH fractions all caused a dose-dependent decrease in the splenic plaque-forming cell (PFC) response to SRBCs or TNP-LPS, and their ED50 values for the four treatment groups were 86, 354, 145, and 23 or 163, 439, 637 and 31 mg/kg, respectively. The corresponding ED50 values for decreased serum anti-TNP IgM levels for these same mixtures were (TNP-haptenated SRBCs, T-cell-dependent) 144, 231, 42 and 27 units, respectively, and (TNP-LPS, T-cell-independent) 161, 406, 312 and 69 units, respectively. The suppression of anti-TNP IgM titers was similar to the suppression of the PFC response and shows that antigen-specific immunoglobulin titer can be used as a biomarker of PAH exposure. A direct comparison of the immunotoxic responses of the reconstituted PAH mixture and the corresponding dose of the > or = 4-ring PAHs indicated that the latter fraction was primarily responsible for the activity of the reconstituted mixture.

Immunosuppressive potential of several polycyclic aromatic hydrocarbons (PAHs) found at a Superfund site: new model used to evaluate additive interactions between benzo[a]pyrene and TCDD

Exposure to environmental pollution is rarely limited to a single compound or even a single class of compounds. The Superfund site located in Massena, NY, is contaminated by both halogenated aromatic hydrocarbons (HAHs) and polycyclic aromatic hydrocarbons (PAHs). Since representatives of both HAHs and PAHs are capable of binding to the aromatic hydrocarbon receptor (AhR), two well-documented AhR-mediated effects, immunosuppression and induction of hepatic aryl hydrocarbon hydroxylase (AHH) activity, were used to evaluate the individual and interactive toxicity of these compounds. Fifteen PAHs were first screened for their ability to suppress the antibody response in C57BL/6 (Ah+/+) mice immunized 12 h after a single oral dose of 0.1, 1, 10, or 100 mg/kg. Acenaphthene, anthracene, benzo[g,h,i]perylene, fluoranthene, fluorene, naphthalene, phenanthrene, and pyrene had little or no effect. Seven PAHs caused > 50% suppression at 100 mg/kg. Listed in order of decreasing potency they were benzo[k]fluoranthene, benzo[b]fluoranthene, indeno[1,2,3,c,d]pyrene, benzo[a]pyrene, chrysene, dibenzo[a,h]anthracene, and benz[a]anthracene. Chrysene and benzo[a]pyrene (B[a]P), were further evaluated to determine the dependence of these effects on the Ah phenotype by comparing responses of C57BL/6 and congenic B6.D2 (Ah-/-) mouse strains. Chrysene immunosuppression was maximal at 0.1 mg/kg and was Ah phenotype-independent whereas chrysene AHH induction was Ah phenotype-dependent, but a 100-fold less sensitive indicator of exposure. In contrast, B[a]P immunosuppression and AHH induction were coincident in B6 mice and Ah phenotype-dependent. In the final phase, a new approach was used to evaluate toxic interactions. This approach considers the mechanism of action of each compound and accounts for the fact that the extent of increase in toxic response caused by an incremental change of dose is determined by its position on the dose-response curve rather than on the absolute amount of dose administered. Thus, the immunotoxic effects of combined exposure to B[a]P and the AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a representative HAH, were evaluated by combining the ED20 of B[a]P with the difference between the ED20 and ED40 of TCDD, and vice versa, to produce 40% suppression. The results of the combination were consistent with additivity regardless of the composite arrangement or phenotype although some antagonism could not be excluded with certainty.

Role of alterations in Ca(2+)-associated signaling pathways in the immunotoxicity of polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are an important class of environmental pollutants that are known to be carcinogenic and immunotoxic. The effects of PAHs on the immune system of various animals and models have been studied for at least 30 yr. Despite these efforts, the mechanism or mechanisms by which PAHs exert their effects on the immune system are still largely unknown. During recent years, the molecular events associated with lymphocyte activation and receptor-mediated signaling have become increasingly clear. Substantial progress has been made in understanding the molecular and cellular bases for toxicant-induced immune cell injury. Understanding mechanisms of drug or chemical effects on the immune system is an important area of research in the field of immunotoxicology, and indeed in all fields of toxicology. Mechanistic toxicology plays an important role in risk assessment and extrapolation of potential human health effects. In this review, we have summarized recent evidence that has examined the effects of PAHs on the immune system of animals and humans. In particular, we have focused on the effects of PAHs on cell signaling in lymphoid cells and have examined the hypothesis that PAHs alter lymphocyte activation via calcium-dependent mechanisms. Previously published reports are discussed, and new data obtained with murine B cells and cell lines are presented demonstrating the relationship between alterations in intracellular calcium and immune dysregulation. These data demonstrate a strong association between PAH-induced alterations in B- and T-lymphocyte activation and changes in calcium homeostasis.

Metabolism of benzo[a]pyrene by murine splenic cell types

The objective of the present study was to determine which splenic cell type(s) of B6C3F1 mice was capable of metabolizing B(a)P. Separation of splenocytes based on density by centrifugation through discontinuous Percoll gradients along with immunomagnetic negative selection or antibody-mediated complement lysis was utilized to obtain highly enriched populations of splenocytes for B(a)P metabolism studies. Immunofluorescent cell staining in conjunction with flow cytometry and examination of Giemsa-stained cytospin cell preparations indicated that B- or T-cell populations of greater than 95% purity and an 80-90% pure population of splenic macrophages were routinely attained. Splenic cell populations were incubated with [3H]B(a)P for 24 hr. High-pressure liquid chromatography was used to separate and quantitate the B(a)P metabolites generated by the enriched splenic cell populations. The results of these studies demonstrate that the macrophage is the cell type responsible for the metabolism of B(a)P within the spleen. The major metabolites of B(a)P produced were as follows: an unidentified peak of polar metabolites containing polyhydroxylated metabolites, B(a)P-9,10-dihydroxy-9,10-dihydrodiol, and B(a)P-7,8-dihydroxy-7,8- dihydrodiol. Other splenic cell types examined, including B and T cells, polymorphonuclear cells, or the spleen capsule did not produce amounts of B(a)P metabolites significantly above background levels. Based on these findings, macrophages are the splenic cell types which metabolize B(a)P. As a result, macrophages may be the cell type targeted by B(a)P resulting in suppression of splenic humoral immune responses.

Evaluation of murine splenic cell type metabolism of benzo[a]pyrene and functionality in vitro following repeated in vivo exposure to benzo[a]pyrene

Recent studies have demonstrated that macrophages are the cell types capable of metabolizing benzo[a]pyrene (B(a)P) within the spleens of untreated mice. Since repeated exposure to B(a)P results in immunosuppression and B(a)P is known to induce cytochrome P450 levels, the first objective of this study was to investigate whether exposure of mice to B(a)P could increase the amounts of immunosuppressive B(a)P metabolites generated and/or alter the pattern of B(a)P metabolites formed by several different splenic cell types. Mice were dosed with a daily sc dose of 200 mg/kg B(a)P or vehicle for 4 days. Separation of splenocytes based on density by centrifugation through discontinuous Percoll gradients along with immunomagnetic negative selection or antibody-mediated complement lysis was used to obtain different splenic cell populations. Cells were incubated with [3H]B(a)P for 24 hr. High-pressure liquid chromatography was used to separate and quantitate B(a)P metabolites. Results indicate that splenic macrophages of B(a)P-treated mice produced significantly greater amounts of some metabolites compared to those of vehicle-treated mice. The three major metabolites produced were an unidentified peak of polar metabolites containing polyhydroxylated metabolites, B(a)P-9,10- and B(a)P-7,8-dihydrodiols. Other splenic cell types examined did not produce metabolite amounts significantly above (T-cells, PMNs, or the capsule) or just above (B-cells) background. The second objective was to investigate the splenic cell type(s) targeted by B(a)P resulting in suppression of humoral immunity. Separation-reconstitution studies along with in vitro sensitization techniques with several different antigens (sheep red blood cells (SRBC), dinitrophenyl-Ficoll (DNP-Ficoll), lipopolysaccharide (LPS)) were used to identify splenic target cells following exposure of mice to B(a)P (200 mg/kg/day, sc for 4 days). Findings indicate that in vitro plaque-forming cell (PFC) suppression was due to alterations in the adherent (macrophage) cell population. Exposure also suppressed the PFC response to the T-dependent antigen SRBC and the T-independent antigen DNP-Ficoll, but did not suppress the PFC response to the polyclonal antigen, LPS. These data suggest that B(a)P is targeting macrophages.