Pharmacokinetic studies in Tg.AC and FVB mice administered [14C] benzene either by oral gavage or intradermal injection

A ten fold reduction of nicotine yield in tobacco smoke does not spare the central cholinergic system in adolescent mice

The tobacco industry has gradually decreased nicotine content in cigarette smoke but the impact of this reduction on health is still controversial. Since the central cholinergic system is the primary site of action of nicotine, here, we investigated the effects of exposure of adolescent mice to tobacco smoke containing either high or low levels of nicotine on the central cholinergic system and the effects associated with cessation of exposure. From postnatal day (PN) 30 to 45, male and female Swiss mice were exposed to tobacco smoke (whole body exposure, 8h/day, 7 days/week) generated from 2R1F (HighNic group: 1.74mg nicotine/cigarette) or 4A1 (LowNic group: 0.14mg nicotine/cigarette) research cigarettes, whereas control mice were exposed to ambient air. Cholinergic biomarkers were assessed in the cerebral cortex and midbrain by the end of exposure (PN45), at short- (PN50) and long-term (PN75) deprivation. In the cortex, nicotinic cholinergic receptor upregulation was observed with either type of cigarette. In the midbrain, upregulation was detected only in HighNic mice and remained significant in females at short-term deprivation. The high-affinity choline transporter was reduced in the cortex: of HighNic mice by the end of exposure; of both HighNic and LowNic females at short-term deprivation; of LowNic mice at long-term deprivation. These decrements were separable from effects on choline acetyltransferase and acetylcholinesterase activities, suggesting cholinergic synaptic impairment. Here, we demonstrated central cholinergic alterations in an animal model of tobacco smoke exposure during adolescence. This system was sensitive even to tobacco smoke with very low nicotine content.

Tobacco smoke containing high or low levels of nicotine during adolescence: effects on novelty-seeking and anxiety-like behaviors in mice

RATIONALE

Thousands of adolescents start smoking daily but information on the effects of tobacco exposure on this age group is scarce. Moreover, the available animal models rely on the effects of nicotine, neglecting other neuroactive components of tobacco.

OBJECTIVES

We investigated the effects of exposure of adolescent mice to tobacco smoke generated from cigarettes containing either high or low levels of nicotine on novelty seeking and anxiety-like behaviors.

METHODS

From postnatal day (PN) 30 to 45, male and female Swiss mice were exposed to tobacco smoke (whole body exposure, 8 h/day, 7 days/week) generated from 2R1F (HighNic group: 1.74 mg nicotine/cigarette) or 4A1 (LowNic group: 0.14 mg nicotine/cigarette) research cigarettes, whereas control mice were exposed to ambient air. By the end (PN44-45), shortly (PN49-50), or long after (PN74-75) exposure, mice were tested on the elevated plus maze and on the hole board.

RESULTS

While HighNic mice presented an increased number of head-dips (increased novelty-seeking) and decreased grooming (increased anxiety-like behavior) by the end of adolescent exposure, only the latter effect persisted shortly after its end. Distinctively, LowNic mice presented reduced head-dips both by the end and shortly after exposure as well as decreased grooming shortly and long after the end of exposure. Interestingly, only HighNic mice presented detectable cotinine (nicotine metabolite) serum levels (109.1 ± 24.0 ng/ml).

CONCLUSION

Our results demonstrate that even adolescent exposure to tobacco smoke with very low nicotine content can have significant short- and long-term behavioral effects, supporting the hypothesis that adolescents can be particularly vulnerable to the effects of cigarette consumption.

Hydroquinone and its analogues in dermatology - a potential health risk

Hydroquinone has been used for decades as a skin lightening agent. Since January 1, 2001, its use in cosmetics has been banned. This ban is as a result of mid-term effects such as leukoderma-en-confetti/occupational vitiligo and exogenous ochronosis. However, a recent literature search on hydroquinone as a skin lightening agent suggests that possible long-term effects such as carcinogenesis may be expected as well. Metabolites of hydroquinone formed in the liver, e.g., p-benzoquinone and glutathione conjugates of hydroquinone, are mainly responsible for this. In the bone marrow, hydroquinone is oxidized into p-benzoquinone because of the high myeloperoxidase activity. Topically applied hydroquinone-containing creams may give rise to accumulation of these compounds, which can cause DNA damage and mutations. They also have the capability to disrupt protective mechanisms, whereby they facilitate further development of cancer. In the bone marrow, long-term effects such as aplastic anemia and acute myeloid leukemias may occur. Most of the evidence stems from research on benzene toxicity, which appears to arise via its metabolite hydroquinone. There is no report yet demonstrating carcinogenesis resulting from the application of hydroquinone-containing creams. However doctors should be aware of these potential health risks which were up until now disregarded.

The role of c-MYB in benzene-initiated toxicity

Chronic exposure to benzene has been correlated with increased oxidative stress and leukemia. Oncogene activation, including c-Myb activation, is one of the earliest steps leading to the formation of leukemic cells, however the molecular mechanisms involved in these events are poorly understood. Given that oxidative stress can alter the activity and fate of cell signaling pathways we hypothesize that the bioactivation of benzene leads to the formation of reactive oxygen species (ROS), which if not detoxified can alter the c-Myb signaling pathway. Using chicken erythroblast HD3 cells we have shown that exposure to the benzene metabolites catechol, benzoquinone, and hydroquinone leads to increased c-Myb activity, increased phosphorylation of c-Myb and increased production of ROS supporting our hypothesis. Activation of the aryl hydrocarbon receptor (AhR) by environmental contaminants has also been associated with carcinogenesis and mice lacking this receptor are resistant to benzene-initiated hematotoxicity. Using wild type and AhR deficient cells we are investigating the role of this receptor in benzene-initiated alterations in the c-Myb signaling pathway. We have found that both wild type and AhR deficient cells are sensitive to catechol and hydroquinone-initiated increases in c-Myb activity while both cell types are resistant to benzene-initiated alterations leaving the role of the AhR still undetermined. Interestingly, protein expression of c-Myb is increased after catechol exposure in AhR deficient cells while decreased in wild-type cells. Further studies on the role of the AhR in benzene-initiated alterations on the c-Myb signaling pathway are on going.

Animal models for acquired bone marrow failure syndromes

Bone marrow failure is a disease characterized by a drastic decline in the marrow's functional ability to produce mature blood cells. Aplastic anemia, a disease in which patients have essentially empty bone marrow accompanied by severe anemia, neutropenia, and thrombocytopenia, presents a paradigm for bone marrow failure. Damage to the marrow may first result from exposure to toxic chemicals, drug overdose, radiation, and viral infection; however, it is the extended immune-mediated reaction that causes massive destruction of hematopoietic cells and leads to marrow hypoplasia and peripheral pancytopenia. In recent years, animal models of acquired bone marrow failure syndromes have helped to strengthen our understanding of the mechanisms causing bone marrow failure. In this review, animal models for bone marrow failure are summarized by two groups: 1) bone marrow failure induced by toxic chemicals and drugs such as benzene, busulfan, and chloramphenicol, and radiation, and 2) models developed by an immune-related mechanism such as viral infection or foreign lymphocyte infusion.

Xenobiotic Metabolism and the Mechanism(s) of Benzene Toxicity

The investigation of the mechanism(s) of benzene toxicity/leukemogenesis over the past 50 years has been contemporaneous with developments in the study of xenobiotic metabolism. Research on the cytochrome P450 (CYP) enzyme system, and related systems in vivo and in vitro, which culminated in the isolation and reconstitution of the many CYPs, established pathways for the study of xenobiotic metabolism and its relationship to the biological activity of many chemicals. The essential role for metabolism of benzene as a precursor to the demonstration of benzene toxicity led to extensive studies of benzene metabolism, many of which will be reviewed here. Benzene toxicity/leukemogenesis, however, is a function of the bone marrow, a site remote from the liver where most benzene metabolism occurs. Studies of benzene metabolism have delineated the array of metabolites which appear to play a role in bone marrow damage, but further studies, both in vivo and in vitro, using appropriate animal models, will be needed to fully understand the impact of benzene and its metabolites on bone marrow function.

Exposure of Tg.AC transgenic mice to benzene suppresses hematopoietic progenitor cells and alters gene expression in critical signaling pathways

The effects of acute benzene (BZ) exposure on hematopoietic progenitor cells (HPCs) derived from bone marrow cells were studied using homozygous male v-Ha-ras Tg.AC mice at 8-10 weeks of age. The mice were given 0.02% BZ in their drinking water for 28 days with the dose rate estimated to be 34 mg benzene/kg BW/day. Analysis of cultured HPCs indicated that BZ suppressed the proliferation of the multilineage colony forming unit-granulocyte, erythrocyte, macrophage, megakaryocyte (CFU-GEMM); colony forming unit-granulocyte, macrophage (CFU-GM); and blast forming unit erythrocyte/colony forming unit erythrocyte (BFUE/CFUE). A gene expression profile was generated using nylon arrays spotted with 23 cDNAs involved in selected signal pathways involved in cell distress, inflammation, DNA damage, cell cycle arrest, and apoptosis. Of the 23 marker genes, 6 (bax, c-fos, E124, hsf1, ikBa, and p57) were significantly (Mann-Whitney U tests, P < 0.05) overexpressed in BZ-exposed mice. Two genes (c-myc and IL-2) approached significance (at P = 0.053). The pattern of gene expression was consistent with BZ toxicity and the suppression of HPCs.