Alteration of pregnancy related hormones and fetal survival in F-344 rats exposed by inhalation to benzo(a)pyrene
Introduction
Accumulating human and laboratory animal data indicate that the reproductive system and its endocrine and neural controls are susceptible to alteration following occupational and environmental exposure to a variety of chemical agents, one of which is benzo(a)pyrene (BaP; [1]). This chemical compound is a lipophilic polycyclic aromatic hydrocarbon (PAH) that is ubiquitous in the environment. The most efficient route of BaP entry into the body is via inhalation and ingestion of contaminated foods. The release of BaP into the environment during volcanic eruptions, forest fires, burning of coal and wood, expulsion of fumes from industrial plants and automobile exhausts, manufacturing of petroleum and aluminum products, and during smoking, can result in accumulation to toxic levels in the body in a short period of time [1].
The carcinogenicity of BaP is well established [2], and information on the acute and chronic toxic effects of this chemical is included in the works of DeJong et al. [3], Hood et al. [4], [5], Ramesh et al. [6], [7], [8], Inyang et al. [9], Nayyar et al. [10], [11], Knuckles et al. [12], and Saunders et al. [13], [14]. Studies conducted in our laboratory indicate significant levels of BaP in the ovaries of rats subjected to acute doses of BaP oral and inhalation exposures compared with that of controls [15]. PAHs may cause toxicity in the ovary after metabolic activation to electrophilic intermediates capable of covalent binding to cellular macromolecules such as DNA, RNA, or proteins [16]. The first step in metabolic activation occurs via a microsomal P450-dependent monooxygenase, aryl hydrocarbon hydroxylase (AHH). The epoxides formed may either spontaneously rearrange, undergo hydrolysis by epoxide hydrase, or become conjugated by related enzymes. The proximate carcinogen is 7,8-dihydrodiol, 9,10-epoxide, which requires recycling of PAH-diols through the microsomal monooxygenase [17]. Interestingly, Miller et al. [17] postulated that the sensitivity of murine ovaries to BaP doses of 1–5 mg/kg appears to be substantially lower than the doses responsible for induction of microsomal monooxygenases or carcinogenesis that is typically ascribed to PAHs. However, at lower doses, reproductive function can be compromised. Miller et al. [17] demonstrated that female mice dosed with BaP sustained a decrease in ovarian corpora lutea numbers and volume compared with controls. Furthermore, female smokers have reduced urinary estrogen during the luteal phase of the menstrual cycle when compared to female nonsmokers [18]. These observations imply a reduction in the metabolic activities responsible for ovarian steroid synthesis and release by luteal tissue.
For normal pregnancy in the rat to advance to term, decidual luteotropin must prevent the involution of the corpora lutea that follows prolactin withdrawal. This action of decidual luteotropin sustains luteal cell progesterone production [19], [20], [21]. Decidual luteotropin also maintains the luteal cell LH receptor content and LH responsive adenylyl cyclase [22] and sustains the capacity of luteal cells to secrete estradiol when stimulated by LH [23]. It is not known whether exposure of pregnant animals to BaP after establishment of pregnancy will affect fetal survival, ovarian steroids necessary for maintenance of pregnancy, or the decidual luteotropin that maintains luteal function.
The objective of this study was to determine the effect of inhaled BaP-exposure of pregnant rats on post-implantation fetal survival and the hormones necessary for the maintenance of pregnancy to term.
Section snippets
Animals and exposure
Timed-pregnant rats at day 5 of gestation (GD-5) were obtained from Harlan Sprague–Dawley (Indianapolis, IN). Animals were housed in pairs, in polyethylene cages and maintained in an environmentally controlled room with a 14 h light:10 h dark cycle (lights on at 6.00 a.m.), 22 °C, and humidity range of 50–60%. Animals were allowed ad libitum access to commercial rat chow (5001 Lab Meal; Ralston Purina Co., MO, USA) and water. Gestational age of mated female rats was determined at Harlan
Results
Exposure of pregnant rats to BaP 25, 75, and 100 μg/m3 caused decreased fetal survival rates in a dose-dependent manner (Table 1). Mean pup weights were similar among litters in both control groups and the group exposed to BaP 25 μg/m3. Exposure to BaP 75 and 100 μg/m3 resulted in a decrease in mean pup weight (Table 1). There was no difference in pup weights between the two high dose groups. Crown-rump length did not differ among groups (Table 1).
Plasma samples from UNC animals and 75 μg/m3
Discussion
The exposure levels of BaP used in our studies are within the levels found in the general environment. Lioy et al. [26] estimated that BaP intake ranged from 20 to 800 ng per day in people living in the vicinity of hazardous waste sites contaminated by PAHs. BaP levels in mainstream tobacco smoke is reported to be 20–40 ng per cigarette [27] and coke oven workers have been exposed to as much as 42 μg/m3 BaP [28]. Indoor exposure to BaP from cooking oil fumes in Taiwan was reported as 20 μg/m3 [29].
Acknowledgements
This research was supported in part by a grant U50/ATU398948 from the Minority Health Professions Foundation through a cooperative agreement with the Agency for Toxic Substances and Diseases Registry. Also, funding from the SCORE grant no. 2SO6GMO8037-28, the PHS grants NIH G12RRO3032, NIEHS 00287, and the RCMI grant no. 5G12RR03032-17 are gratefully acknowledged.
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