IL-2Rα KO mice exhibit maternal microchimerism and reveal nuclear localization of IL-2Rα in lymphoid and non-lymphoid cells.

Introduction: IL-2Rα knock out (KO) mice have been instrumental to discovering the immunoregulatory properties of IL-2Rα. While initially thought of only as a stimulatory cytokine, IL-2 and IL-2Rα KO mice revealed that this cytokine-receptor system controls immune responses through restimulation-induced cell death and by promoting the survival of T regulatory cells. Although described mostly in the context of lymphocytes, recent studies by our laboratory showed that IL-2R is expressed in smooth muscle cells. Given this finding, we sought to use IL-2Rα KO to determine the function of this receptor in vascular smooth muscle cells. Surprisingly, we found that IL-2Rα KO vascular smooth muscle cells had detectable IL-2Rα. Methods: We used multiple gene and protein-based methods to determine why IL-2Rα KO vascular smooth muscle cells exhibited IL-2Rα protein. These methods included: genomic sequencing, assessing cells and tissues for evidence of maternal microchimerism, and determining the half-life of IL-2Rα protein. Results: Our studies demonstrated the following: (1) in addition to the cell surface, IL-2Rα is localized to the nucleus; (2) the genetic deletion of IL-2Rα is intact in IL-2Rα KO mice; (3) both IL-2Rα KO and WT tissues show evidence of maternal microchimerism, the likely source of IL-2Rα (4) IL-2Rα is transmitted between cells; (5) IL-2Rα has a long half-life; and (6) nuclear IL-2Rα contributes to the regulation of cell proliferation and size. Conclusion: Our findings suggest that the phenotype of complete IL-2Rα loss is more severe than demonstrated by IL-2Rα KO mice, and that IL-2Rα plays a here-to-fore unrecognized role in regulating cell proliferation in non-lymphoid cells.

IL-2RαKO mice exhibit maternal microchimerism and reveal nuclear localization of IL-2Rα in lymphoid and non-lymphoid cells.

IL-2Rα KO mice have been instrumental to discovering the immunoregulatory properties of IL-2Rα. While initially thought of only as a stimulatory cytokine, IL-2 and IL-2Rα knock out (KO) mice revealed that this cytokine-receptor system controls immune responses through restimulation-induced cell death and by promoting the survival of T regulatory cells. Although described mostly in the context of lymphocytes, recent studies by our laboratory showed that IL-2R is expressed in smooth muscle cells. Given this finding, we sought to use IL-2Rα knock mice to determine the function of this receptor in vascular smooth muscle cells. Surprisingly, we found that IL-2Rα knock out vascular smooth muscle cells had detectable IL-2Rα. Further studies suggested that the source of IL-2Rα protein was likely maternal heterozygous cells present in KO offspring due to maternal microchimerism. Because the KO was generated by using a neomycin resistance gene insert, we treated cells with G418 and were able to eliminate the majority of IL-2Rα expressing cells. This elimination revealed that IL-2Rα KO vascular smooth muscle cells exhibited increased proliferation, decreased size, and hypodiploid DNA content when compared to wildtype cells. Our findings suggest that the phenotype of complete IL-2Rα loss is more severe than demonstrated by IL-2Rα KO mice, and that IL-2Rα plays a here-to-fore unrecognized role in regulating cell proliferation in non-lymphoid cells.

Nasal uptake of inhaled acrolein in rats.

An improved understanding of the relationship between inspired concentration of the potent nasal toxicant acrolein and delivered dose is needed to support quantitative risk assessments. The uptake efficiency (UE) of 0.6, 1.8, or 3.6 ppm acrolein was measured in the isolated upper respiratory tract (URT) of anesthetized naive rats under constant-velocity unidirectional inspiratory flow rates of 100 or 300 ml/min for up to 80 min. An additional group of animals was exposed to 0.6 or 1.8 ppm acrolein, 6 h/day, 5 days/wk, for 14 days prior to performing nasal uptake studies (with 1.8 or 3.6 ppm acrolein) at a 100 ml/min airflow rate. Olfactory and respiratory glutathione (GSH) concentrations were also evaluated in naive and acrolein-preexposed rats. Acrolein UE in naive animals was dependent on the concentration of inspired acrolein, airflow rate, and duration of exposure, with increased UE occurring with lower acrolein exposure concentrations. A statistically significant decline in UE occurred during the exposures. Exposure to acrolein vapor resulted in reduced respiratory epithelial GSH concentrations. In acrolein-preexposed animals, URT acrolein UE was also dependent on the acrolein concentration used prior to the uptake exposure, with preexposed rats having higher UE than their naive counterparts. Despite having increased acrolein UE, GSH concentrations in the respiratory epithelium of acrolein preexposed rats were higher at the end of the 80 min acrolein uptake experiment than their in naive rat counterparts, suggesting that an adaptive response in GSH metabolism occurred following acrolein preexposure.

p53 heterozygosity alters the mRNA expression of p53 target genes in the bone marrow in response to inhaled benzene.

C57BL/6 Trp53 heterozygous (N5) mice (p53+/- mice) show an increased sensitivity to tumorigenesis following exposure to genotoxic compounds and are being used as an alternate animal model for carcinogenicity testing. However, there is relatively little data regarding the effect of p53 heterozygosity on the genomic and cellular responses of target tissues in these mice to toxic insult, especially under chronic exposure conditions used in carcinogenicity bioassays. We hypothesized that heterozygosity at the p53 locus in p53+/- mice alters the expression of bone marrow p53-regulated genes involved in cell cycle control and apoptosis during chronic genotoxic stress. We used real-time quantitative reverse transcription polymerase chain reaction (RT-PCR) to examine gene expression alterations in bone marrow cells from C57BL/6 p53+/+ and isogenic p53+/- mice chronically exposed for 15 weeks to genotoxic and carcinogenic levels (100 ppm) of inhaled benzene. Examination of mRNA levels of p53-regulated genes involved in cell cycle control (p21, gadd45, and cyclin G) or apoptosis (bax and bcl-2) showed that during chronic genotoxic stress, bone marrow cells from p53+/+ mice expressed significantly higher levels of a majority of these genes compared to p53+/- bone marrow cells. Our results indicate that p53 heterozygosity results in a haploinsufficient phenotype in p53+/- bone marrow cells as evident by significantly altered mRNA levels of key genes involved in the p53-regulated DNA damage response pathway.

Variations in Prkdc and susceptibility to benzene-induced toxicity in mice.

Benzene, a carcinogen that induces chromosomal breaks, is strongly associated with leukemias in humans. Possible genetic determinants of benzene susceptibility include proteins involved in repair of benzene-induced DNA damage. The catalytic subunit of DNA-dependent protein kinase (DNA-PKcs), encoded by Prkdc, is one such protein. DNA-PKcs is involved in the nonhomologous end-joining (NHEJ) pathway of DNA double-strand break (DSB) repair. Here we compared the toxic effects of benzene on mice (C57BL/6 and 129/Sv) homozygous for the wild-type Prkdc allele and mice (129/SvJ) homozygous for a Prkdc functional polymorphism that leads to diminished DNA-PK activity and enhanced apoptosis in response to radiation-induced damage. Male and female mice were exposed to 0, 10, 50, or 100 ppm benzene for 6 h/d, 5 d/week for 2 weeks. Male mice were more susceptible to benzene toxicity compared with females. Hematotoxicity was evident in all male mice but was not seen in female mice. We observed similar, large increases in both micronucleated erythrocyte populations in all male mice. Female mice had smaller but significant increases in micronucleated cells. The p53-dependent response was induced in all strains and genders of mice following benzene exposure, as indicated by an increase in p21 mRNA levels in bone marrow that frequently corresponded with cell cycle arrest in G2/M. Prkdc does not appear to be a significant genetic susceptibility factor for acute benzene toxicity. Moreover, the role of NHEJ, mediated by DNA-PK, in restoring genomic integrity following benzene-induced DSB remains equivocal.

Male mice deficient in microsomal epoxide hydrolase are not susceptible to benzene-induced toxicity.

Enzymes involved in benzene metabolism are likely genetic determinants of benzene-induced toxicity. Polymorphisms in human microsomal epoxide hydrolase (mEH) are associated with an increased risk of developing leukemia, specifically those associated with benzene. This study was designed to investigate the importance of mEH in benzene-induced toxicity. Male and female mEH-deficient (mEH-/-) mice and background mice (129/Sv) were exposed to inhaled benzene (0, 10, 50, or 100 ppm) 5 days/week, 6 h/day, for a two-week duration. Total white blood cell counts and bone marrow cell counts were used to assess hematotoxicity and myelotoxicity. Micronucleated peripheral blood cells were counted to assess genotoxicity, and the p21 mRNA level in bone marrow cells was used as a determinant of the p53-regulated DNA damage response. Male mEH-/- mice did not have any significant hematotoxicity or myelotoxicity at the highest benzene exposure compared to the male 129/Sv mice. Significant hematotoxicity or myelotoxicity did not occur in the female mEH-/- or 129/Sv mice. Male mEH-/- mice were also unresponsive to benzene-induced genotoxicity compared to a significant induction in the male 129/Sv mice. The female mEH-/- and 129/Sv mice were virtually unresponsive to benzene-induced genotoxicity. While p21 mRNA expression was highly induced in male 129/Sv mice after exposure to 100-ppm benzene, no significant alteration was observed in male mEH-/- mice. Likewise, p21 mRNA expression in female mEH-/- mice was not significantly induced upon benzene exposure whereas a significant induction was observed in female 129/Sv mice. Thus mEH appears to be critical in benzene-induced toxicity in male, but not female, mice.

Gene expression profile in bone marrow and hematopoietic stem cells in mice exposed to inhaled benzene.

Acute myeloid leukemia and chronic lymphocytic leukemia are associated with benzene exposure. In mice, benzene induces chromosomal breaks as a primary mode of genotoxicity in the bone marrow (BM). Benzene-induced DNA lesions can lead to changes in hematopoietic stem cells (HSC) that give rise to leukemic clones. To gain insight into the mechanism of benzene-induced leukemia, we investigated the DNA damage repair and response pathways in total bone marrow and bone marrow fractions enriched for HSC from male 129/SvJ mice exposed to benzene by inhalation. Mice exposed to 100 ppm benzene for 6h per day, 5 days per week for 2 week showed significant hematotoxicity and genotoxicity compared to air-exposed control mice. Benzene exposure did not alter the level of apoptosis in BM or the percentage of HSC in BM. RNA isolated from total BM cells and the enriched HSC fractions from benzene-exposed and air-exposed mice was used for microarray analysis and quantitative real-time RT-PCR. Interestingly, mRNA levels of DNA repair genes representing distinct repair pathways were largely unaffected by benzene exposure, whereas altered mRNA expression of various apoptosis, cell cycle, and growth control genes was observed in samples from benzene-exposed mice. Differences in gene expression profiles were observed between total BM and HSC. Notably, p21 mRNA was highly induced in BM but was not altered in HSC following benzene exposure. The gene expression pattern suggests that HSC isolated immediately following a 2 weeks exposure to 100 ppm benzene were not actively proliferating. Understanding the toxicogenomic profile of the specific target cell population involved in the development of benzene-associated diseases may lead to a better understanding of the mechanism of benzene-induced leukemia and may identify important interindividual and tissue susceptibility factors.

Induction of micronuclei in wild-type and p53(+/-) transgenic mice by inhaled bromodichloromethane.

Bromodichloromethane (BDCM) is commonly present in trace amounts in drinking water as a disinfection by-product. BDCM has been shown to be carcinogenic in mice and rats when given by gavage at relatively high doses. Genotoxic activity as well as induced regenerative cell proliferation may contribute to the carcinogenic potential of BDCM. The purpose of the current studies was to evaluate the ability of BDCM to induce micronuclei (MN) in bone marrow and blood of wild-type and p53(+/-) mice on the C57BL/6 and FVB/N genetic backgrounds using the inhalation route of exposure. Toxicity studies were being conducted in this laboratory with inhaled BDCM to select doses for longer-term cancer bioassays using wild-type and p53(+/-) transgenic mice on different genetic backgrounds. Bone marrow samples from these experiments were evaluated for the induction of MN after 1 and 3 weeks of exposure. Accumulation of MN in the peripheral blood was also evaluated at the 13-week time point of a cancer study with the p53(+/-) mice. For the 1-week time point, male C57BL/6 wild-type and p53(+/-) mice and FVB/N wild-type and p53(+/-) mice were exposed daily for 6h per day for 7 consecutive days to atmospheric BDCM concentrations of 0, 1, 10, 30, 100, or 150 ppm. In a second experiment, mice were exposed daily for 6h per day for 3 weeks to atmospheric BDCM concentrations of 0, 0.5, 1, 3, 10, or 30 ppm. Resulting levels of polychromatic erythrocytes (PCE) containing MN were assessed in the bone marrow. For all of the 1- and 3-week exposure groups, the only statistically significant increase in the percentage of bone marrow PCE cells containing MN was in the 1-week 100 ppm BDCM exposure group in the FVB/N wild-type mice (control 0.26% versus exposed 1.16%). C57BL/6 p53(+/-) mice and FVB/N p53(+/-) mice were exposed daily for 6 h per day for 13 weeks to atmospheric BDCM concentrations of 0, 0.5, 3, 10, or 15 ppm. MN were quantified in samples of peripheral blood. Statistically significant increases in the percentage of peripheral blood NCE cells containing MN were seen at the highest BDCM exposure group of 15 ppm in both the C57BL/6 p53(+/-) strain (control 0.36% versus exposed 0.67%) and the FVB/N p53(+/-) strain (control 0.36% versus exposed 0.86%). These data indicate weak induction of MN by BDCM, but only at high atmospheric concentrations relative to normal environmental exposures and with extended periods of exposure. Although comparisons are difficult because responses were negative or marginal, the p53 genotype or the genetic background did not appear to substantially alter susceptibility to the genotoxic effects of BDCM.

Exposure of hematopoietic stem cells to benzene or 1,4-benzoquinone induces gender-specific gene expression.

Chronic exposure to benzene results in progressive decline of hematopoietic function and may lead to the onset of various disorders, including aplastic anemia, myelodysplastic syndrome, and leukemia. Damage to macromolecules resulting from benzene metabolites and misrepair of DNA lesions may lead to changes in hematopoietic stem cells (HSCs) that give rise to leukemic clones. We have shown previously that male mice exposed to benzene by inhalation were significantly more susceptible to benzene-induced toxicities than females. Because HSCs are targets for benzene-induced cytotoxicity and genotoxicity, we investigated DNA damage responses in HSC from both genders of 129/SvJ mice after exposure to 1,4-benzoquinone (BQ) in vitro or benzene in vivo. 1,4-BQ is a highly reactive metabolite of benzene that can cause cellular damage by forming protein and DNA adducts and producing reactive oxygen species. HSCs cultured in the presence of 1,4-BQ for 24 hours showed a gender-independent, dose-dependent cytotoxic response. RNA isolated from 1,4-BQ-treated HSCs and HSCs from mice exposed to 100 ppm benzene by inhalation showed altered expression of apoptosis, DNA repair, cell cycle, and growth control genes compared with unexposed HSCs. Rad51, xpc, and mdm-2 transcript levels were increased in male but not female HSCs exposed to 1,4-BQ. Males exposed to benzene exhibited higher mRNA levels for xpc, ku80, ccng, and wig1. These gene expression differences may partially explain the gender disparity in benzene susceptibility. HSC culture systems such as the one used here will be useful for testing the hematotoxicity of various substances, including other benzene metabolites.