A comparison between the activation of benzo[a]pyrene in organ cultures and microsomes from the tracheal epithelium of rats and hamsters

Approaches to carcinogenic risk assessment for polycyclic aromatic hydrocarbons: a UK perspective

This paper reviews the approaches to carcinogenic risk assessment of polycyclic aromatic hydrocarbons (PAHs) in air pollution with emphasis on high potency PAHs such as dibenzo[a,l]pyrene (DB[a,l]P). The potency of DB[a,l]P may be 100-fold greater than benzo[a]pyrene (B[a]P); thus the B[a]P surrogate approach currently used to monitor for compliance with UK air pollution standards may not be appropriate. It is suggested that an approach based on potency equivalency factors (PEFs) could be developed to include highly potent PAHs provided an appropriate reference data set for relevant PAHs using a route acceptable for inhalation risk assessment is selected. Available data suggest that intratracheal administration of low doses of PAHs to rats is likely to simulate the kinetics of inhalation exposure to PAHs in a feasible manner. The use of a measure of total DNA adducts as an endpoint, which correlates well with lung tumourigenicity, would provide surrogate data for setting PEFs without the need for long-term bioassays in rodents. Further, dose-response studies using intratracheal administration of a range of PAHs singly and in combination to assess additivity are required to develop a PEF system for inhalation PEFs derived from DNA adduct measurements.

Penetration of benzo[a]pyrene through human buccal and vaginal mucosa

OBJECTIVE

The aim of this study was to compare buccal and vaginal mucosa with respect to their permeability to a potent carcinogen, benzo[a]pyrene.

STUDY DESIGN

Six clinically healthy vaginal mucosa specimens (mean patient age +/- standard deviation, 52+/-13.4 years; age range, 37-69 years) and 6 buccal mucosa specimens (from 5 male patients and 1 female patient: mean patient age +/- standard deviation, 32+/-5.2 years; age range, 24-39 years) were obtained during surgery. In vitro flux rates of benzo[a]pyrene across specimens were determined through use of a flow-through diffusion apparatus. Analysis of variance, a Duncan multiple range test, and an unpaired t test were used to determine steady state kinetics and flux differences over time intervals.

RESULTS

No statistically significant differences were observed between the overall mean flux values of benzo[a]pyrene across the 2 kinds of mucosa.

CONCLUSIONS

The findings further support the hypothesis that human vaginal mucosa can be used as a model for buccal mucosa in studies of permeability to various chemical compounds.

An ex vivo model of the rat trachea to study the effect of inhalable toxic compounds

Different cell culture and organ systems are used to evaluate the physiological responses of the airways to the effects of carcinogenic [e.g., benzo(a)pyrene] and anticarcinogenic (e.g., retinoids) compounds on cellular growth and differentiation. However, in contrast to in vivo conditions dissociated epithelial cells or tracheal ring cultures are covered with medium. Therefore, we developed an ex vivo perfusion model enabling evaluation of morphology and metabolism of different compounds under near-physiological conditions. The trachea was surrounded with culture medium and perfused with air by means of a small animal respirator. To test the viability of the system under various experimental conditions tracheal probes were incubated with either retinoids (retinol 10(-5) mol/l; retinyl palmitate 10(-5) mol/l) or benzo(a)pyrene (10(-7) mol/l) for up to 7 days. At the end of the incubation period metabolites in the trachea and in the medium were measured by means of high-performance liquid chromatography. Samples were examined by light microscopy, and by scanning and transmission electron microscopy for cell morphology. Glycoconjugate expression was assessed by lectin histochemistry. Specimens incubated in a retinoid-supplemented medium revealed no alterations in the distribution of cell types and characteristics of the epithelial layer compared with tracheal biopsies assessed immediately after removal from the animals. Glycoconjugate patterns especially remained intact. Histological changes after incubation with benzo(a)pyrene resembled in vivo morphology of vitamin A-deficient rats. An important advantage of this in vitro model compared with common cell or organ cultures is the preservation of the original phenotype and environment of the tracheobronchial surface. In addition, carcinogenic substances, such as benzo(a)pyrene, can easily be applied by airway or through the medium.

DNA adducts in hamster and rat tracheas exposed to benzo(a)pyrene in vitro

Syrian golden hamsters are much more susceptible than Wistar rats to the induction of tracheal tumors by benzo(a)pyrene (BP). In order to investigate whether this difference is reflected in the pattern of DNA-adduct induction and removal, tracheas from either species were isolated and exposed to BP (5 micrograms/ml) in organ culture. At various time-points BP-DNA adducts in the epithelial cells were quantified by 32P-postlabeling; unscheduled DNA synthesis (UDS) was determined by [3H]thymidine incorporation. In an induction-repair experiment tracheas were exposed to BP for 2 days, and cultured for another 4 days without BP. After 2 days of exposure total BP-DNA adduct levels were 10 times higher in hamster compared to rat tracheas. In hamster tracheas one major adduct was formed (95%), vs. the adduct between (+)-anti-BP-diolepoxide and deoxyguanosine (BPDE-N2dG). In rat tracheas BPDE-N2dG comprised about 60% of the total adduct level. During exposure to BP the adduct level in hamster trachea increased to 36 +/- 19 adducts/10(6) nucleotides (add/10(6) n) on day 2. Two days after removal of BP the BP-DNA adduct level had decreased to 60% of that on day 2; there was no further decrease in the BP-DNA adduct level. UDS increased during exposure to BP and decreased after removal of BP. In rats, removal of BP did not lead to a decrease in the BP-DNA adduct level, which agreed with the observed absence of UDS. In a second experiment tracheas were exposed to BP continuously for 15 days. In hamster tracheas the total BP-DNA adduct level increased from 11 +/- 0.7 add/10(6) n after 1 day of exposure to 105 +/- 2 add/10(6) n after 15 days; also UDS increased with increasing exposure until day 11. In rat tracheas no progressive increase in the BP-DNA adduct level was seen. It was concluded that the difference in trachea tumor susceptibility between hamsters and rats exposed to BP correlates with the difference between the 2 species in BP-DNA adduct kinetics in the trachea epithelial cells.

Cytotoxicity, genotoxicity and transforming activity of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in rat tracheal epithelial cells

The cytotoxicity, genotoxicity and transforming activity of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) were studied by the assays of colony-forming efficiency (CFE), micronucleus formation (MN), and cell transformation in rat tracheal epithelial (RTE) cells both in vitro and in vivo. Liver S9, primary hepatocytes and RTE cells from normal and Aroclor-1254 induced rats were compared for bioactivation of NNK using Salmonella mutagenesis as the endpoint. Results from the in vitro experiments indicated that low concentrations of NNK (0.01-25 micrograms/ml) caused from 15% to greater than 100% increases in CFE of RTE cells. At high concentrations (100-200 micrograms/ml), NNK was significantly toxic to RTE cells. NNK treatment in vitro (50-200 micrograms/ml) increased MN frequency as much as 3-fold above background and significantly increased the transformation frequency (TF) in 4/5 (50 micrograms/ml) and 6/8 (100 micrograms/ml) experiments. The in vivo exposure of rats to NNK (150-450 mg/kg, given i.p.) resulted in a 60-85% reduction in CFE and a 3-5-fold increase in MN formation in RTE cells. In vivo treatment with cumulative doses of 150 and 300 mg/kg of NNK produced significant increases in TF of tracheal cells from 3/3 and 2/3 rats, respectively. Without activation, NNK was not mutagenic in Salmonella TA1535. The bioactivation of NNK to a mutagenic metabolite was achieved by incubation of NNK with liver S9 fraction from Aroclor-1254 induced rats or primary hepatocytes from both untreated and Aroclor-1254 pretreated rats. RTE cells did not produce sufficient quantities of mutagenic NNK metabolites to be detected by the Salmonella assay.

In vitro studies of mechanisms of lung injury in the rodent

In order to better define the responses of lung cells to potentially pathogenic insults, primary cell cultures of dissociated respiratory epithelial cells have been established. These epithelial cells have been obtained from various areas of the respiratory tract ranging from the trachea to the alveolus and the cultures have been demonstrated to mimic the differnetiated state of these cell types as observed in situ. Several procedures which enhance the differentiated state have been evaluated, which include maintenance on more physiologically-relevant substrata, such as collagen gels, use of defined serum-free medium and use of air/liquid interface systems. These approaches have allowed intracellular responses of respiratory epithelium to toxic insult to be better defined.

Evidence for bidirectional transverse diffusion of spin-labeled phospholipids in the plasma membrane of guinea pig blood cells

The distribution and transverse diffusion kinetics of four spin-labeled phospholipid analogues (two with choline heads: phosphatidylcholine (PC) and sphingomyelin (SM); two with amino heads: phosphatidylserine (PS) and phosphatidylethanolamine (PE) were studied in the plasma membrane of guinea pig blood cells: erythrocytes, reticulocytes, and leukemic lymphocytes. Nitroxide reduction by the internal content of the cells was used as an indicator to determine the phospholipids that penetrated the cells. The reduction rates were in the order, PS greater than PE greater than PC greater than SM in all cells. Reoxidation of phospholipids extracted by serum albumin revealed the distribution of the phospholipids at a given time. In all cells, the distribution equilibrium was reached in less than 2 h and the amounts left in the external leaflet were in the following proportional order: PS less than PE less than PC less than SM. In the erythrocytes and especially in the reticulocytes, the shape change induced by adding phospholipids relaxed partially or completely at a lower speed but kept the same proportional order as at equilibrium. All the results were analyzed quantitatively with a simple kinetic model including the rates of transverse diffusion (flip and flop), the exchange between plasma membrane and internal membranes, and the reduction rate of free radicals (determined in either the internal or external membrane leaflet). The calculated rate constants of transverse diffusion varied from 2 x 10(-3) to 1.2 x 10(-1) min-1 for the flip and from 4 x 10(-3) to 1.2 x 10(-1) for the flop, depending on the polar head and the cell type. Possible interpretations of the external phospholipid reduction mechanism and cell deformation are discussed.

Influence of long term treatment with 3-methylcholanthrene or carbaryl on mixed-function oxidase activity in 3T3 fibroblasts: studies by microspectrofluorimetry on single cells

Using benzo(a)pyrene (BaP) as a probe for aryl hydrocarbon hydroxylase (AHH) activity, differences in mixed-function oxidase (MFO) activity were observed using microspectrofluorimetry in single living cells during long term treatment with 3-methylcholanthrene (3-MC) or carbaryl. Although these two compounds differ in chemical structure, similar effects were observed in 3T3 cell populations. The results suggest that the two compounds activate the same enzymatic system and that individual cells of a supposed homogeneous cell population are not equally sensitive to xenobiotics, i.e. subpopulations were observed which have differences in AHH activity.

In vitro models of lung toxicity

In vitro assays that emphasize cellular components critical to the host defense system have been developed to evaluate pulmonary toxicity and define deleterious changes in parenchymal cell populations. Assays that employ pulmonary alveolar macrophages (PAM) have demonstrated good correlation between macrophage toxicity and pulmonary fibrogenicity for many inorganic compounds. The PAM assays provide simple and inexpensive screens of potential respiratory tract toxicity. Many investigators screen chemicals for their ability to alter the mucosal epithelial cell conducting airways by performing tracheal organ cultures. The tracheal assays have also provided useful screens for Vitamin A analogues required for epithelial cell differentiation. Most recently, in vitro respiratory tract models have been extended to include whole-lung explants, an approach that allows for development of fibrosis and epithelial cell toxicity after in vitro exposure to inorganic and organic fibrogens. The whole-lung explant system appears to duplicate the in vivo response to a variety of lung toxins, including bleomycin, silica, and crocidolite asbestos. Together, these assays provide a description of potential toxicity to key components of the lung, emphasizing the pulmonary macrophage, conducting airways, and alveolar septae. It is expected that continued research in these models will enhance their predictive abilities and utility in risk assessment.

Characterization of hepatic and pulmonary cytochromes P-450 in 3-methylcholanthrene-treated hamsters

Two major forms of hepatic cytochrome P-450 (hepatic P-450MCI and P-450MCII) were purified approximately 5-fold from liver microsomes in Syrian golden hamsters treated with 3-methylcholanthrene (MC). The purified preparations of hepatic P-450MCI and P-450MCII contained 9.6 and 8.3 nmol cytochrome P-450 (P-450) per mg protein, respectively, and were essentially free from NADPH-cytochrome c (P-450) reductase (fpT), NADH-cytochrome b5 reductase and cytochrome b5. By sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE), the molecular weights of hepatic P-450MCI and P-450MCII were estimated to be 56,000 and 53,500. Further, a major form of pulmonary P-450 (P-450MC) were purified from lung microsomes of MC-treated hamster, and contained 14.2 nmol P-450 per mg protein, and estimated to be 56,000 in monomeric molecular weight, indicating the similar molecular weight to hepatic P-450MCI in the hamster. From the absorption spectra the oxidized forms of hepatic P-450MCI and P-450MCII were high- and low-spin ferric hemoproteins, respectively, and pulmonary P-450MC was similar to hepatic P-450MCII in their hemoprotein spin state. No difference, however, was observed in the CO-reduced forms among hepatic P-450MCI, P-450MCII and pulmonary P-450MC, all exhibiting 446.5 nm Soret bands. In a reconstituted system containing fpT and dilauroylphosphatidylcholine (DLPC), pulmonary P-450MC efficiently catalyzed benzo[a]pyrene (BP) hydroxylation at a rate of 11.4 mol formed per min per mol P-450, but hepatic P-450MCI and P-450MCII both exhibited lower levels, e.g., 0.49 and 0.54, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)