TOXICOLOGICAL HIGHLIGHT |
Site-Specific, Dose-Dependent Transitional Analysis of Toxicologic Mechanisms: The Interplay of Local Metabolic and Physicochemical Saturation
CIIT Centers for Health, Biology Division, Research Triangle Park, North Carolina 27709-2137
1 To whom correspondence should be addressed. E-mail: moss{at}ciit.org.
Received April 7, 2006; accepted April 10, 2006
When animal exposures are used to assess human risk to inhaled chemicals, the animals generally receive higher dose rates over shorter periods of time. This difference in dose rate and duration of exposure, when applied to risk extrapolation, creates a key challenge: the challenge of accounting for the impact of dose and dose rate on possible transitions in mechanisms of toxicity (Slikker et al., 2004a
,b). One such transition and the related microdosimetry are the subject of this issue's highlighted article Increasing Exposure Levels Cause an Abrupt Change in the Absorption and Metabolism of Acutely Inhaled Benzo(a)pyrene in the Isolated, Ventilated, and Perfused Lung of the Rat by Ewing et al. (2006).
This latest article is the culmination, for these investigators, of over a decade of mechanistic research investigating the inhalation of highly lipophilic carcinogens such as polycyclic aromatic hydrocarbons (PAHs) and tobacco-specific nitrosamines (Gerde and Scott, 2001; Gerde et al., 1991, 1993a,b,c, 1997, 1998a,b, 2001, 2004). When inhaled, these highly lipophilic compounds show carcinogenetic activity primarily at the cellular site of entry into the body. However, at cumulative doses even much higher than those required to induce lung tumors in humans, inhalation exposures in laboratory animals generally fail to induce respiratory tract tumors (Coggins, 2001). At the epithelial cell, the initial site of entry, the dose, and the dose rate of PAHs and nitrosamines appear to change the mechanism of toxicity. The epithelial cell may experience two types of saturation: metabolic saturation and physicochemical saturation (Gerde et al., 1991). The physicochemical saturation, in the thicker epithelia of the conducting airways, is proposed to be related to a slow passage of the highly lipophilic PAHs; the metabolic saturation is proposed to be specific to epithelial cell metabolism.
In order to measure the interplay of metabolic saturation and physicochemical saturation, the authors use a three-part experimental approach: (1) nanoporous carrier particles are used to control, in peripheral bronchi and bronchioles, epithelial cell microdosimetry of tritium-labeled, benzo(a)pyrene (BaP); (2) the isolated perfused lung (IPL) of the rat is used to sample lung-related metabolites of BaP; and (3) for inhalation by the IPL, a dry powder generator is used to reproducibly generate a high-concentration bolus of airborne carrier particles. The carrier particles are nanoporous spheres of silica (pore diameter = 80 nm, sphere diameter = 3000 nm); the porosity is sufficient to allow a 1000x increase in mass loading of BaP (3.4–3500 fg per particle) without significant change in the lung deposition site (as defined by aerodynamic particle size). The isolated perfused rat lung was suspended in a humidified, artificial thoracic chamber, ventilated at 75 breaths/min (85 ml/min), and perfused at 19.8 ml/min with Krebs–Ringer bicarbonate buffer (Gerde et al., 2004). By exposing an IPL to an aerosol of carrier particles, generated with a DustGun aerosol generator (Gerde et al., 2004), reproducible mass loadings of BaP were achieved for groups of three lungs at low-, medium-, and high-dose levels: 2.2, 36, and 84,000 ng per lung. For each lung, the values measured were lung volume, carrier particle count per lung, and, in lung tissue and perfusate, the concentration of radioactive BaP and metabolites.
The key contributions of these measurements are twofold: a demonstration of the application of microdosimetry and a demonstration that, at the epithelial cells closest to each carrier particle, not the metabolism but the absorption of BaP saturates at the medium-dose level. The carrier particle deposition allowed, in adjacent epithelial cells, realistic estimates of the concentration of BaP in and above the cell compared to the whole-body dose; for the medium dose, the microdose at the epithelial cell level was 2,300,000nM compared to the whole-body average dose of 0.44nM. For epithelial cells below, or near to, a low-dose carrier particle, the cells absorbed BaP at a rate proportional to the local concentration of BaP (a first-order process). At the medium- or high-exposure levels, the cell or cells absorbed BaP at a constant rate, independent of the local concentration (a zero-order process). At high exposures, the slow (20 min) attainment of steady-state concentration in the perfusate through the lung was estimated to be due to an increase in the area of BaP dispersion around the carrier particles. This increase in surface area is estimated to be due to lateral movement of BaP from the carrier particles in two possible directions: directly into the surfactant lipids of the mucociliary escalator or, secondarily, by lateral diffusion from the initially exposed cells to neighboring cells within the epithelium. In either case the concentration of BaP was less than that for cells directly in contact with the carrier particle. For the medium- and high-exposure levels, the product of BaP concentration x time, as delivered by the carrier particle, did not reflect the actual product of concentration x time within the epithelial cell; in the cell, the value was less.
This highlighted article by Ewing et al. impacts the risk assessment of PAHs: The study presents a plausible explanation why animal studies underestimate the risk of inhaled PAHs. The inhalation dosing regimens commonly used for 1- to 2-year animal studies are at sufficiently high exposure levels to cause the microdosimetry (the PAH concentration at the epithelial cell site of entry) to be at or above PAH supersaturation for physical chemical absorption, whereas for inhalation dosing of humans exposed over decades to low dose rates of PAHs, the PAH microdosimetry is subsaturation for physical chemical absorption. The authors demonstrate an elegant example of site-specific, dose-dependent transition in mechanism of toxicity.
ACKNOWLEDGMENTS
This review was funded at CIIT Centers for Health Research through investigator discretionary funds. To the best of the author's knowledge, there are no perceived conflicts of interest related to the subject matter of this review.
REFERENCES
Coggins, C. R. (2001). A review of chronic inhalation studies with mainstream cigarette smoke, in hamsters, dogs, and nonhuman primates. Toxicol. Pathol. 29, 550–557.[CrossRef][Web of Science][Medline]
Ewing, P., Blomgren, B., Ryrfeldt, Å., Gerde, P. (2006). Increasing exposure levels cause an abrupt change in the absorption and metabolism of acutely inhaled benzo(a)pyrene in the isolated, ventilated, and perfused lung of the rat. Toxicol. Sci. doi:10.1093/toxsci/kfj104.
Gerde, P., Ewing, P., Låstbom, L., Waher, J., Lidén, G., and Ryrfeldt, Å. (2004). A novel method to aerosolize powder for short inhalation exposures at high concentrations: Isolated rat lungs exposed to respirable diesel soot. Inhal. Toxicol. 16, 45–52.[CrossRef][Web of Science][Medline]
Gerde, P., Medinsky, M. A., and Bond, J. A. (1991). The retention of polycyclic aromatic hydrocarbons in the bronchial airways and in the alveolar region—A theoretical comparison. Toxicol. Appl. Pharmacol. 107, 239–252.[CrossRef][Web of Science][Medline]
Gerde, P., Muggenburg, B. A., and Henderson, R. F. (1993a). Disposition of polycyclic aromatic hydrocarbons in the respiratory tract of the Beagle dog: III. Mechanisms of the dosimetry. Toxicol. Appl. Pharmacol. 121, 328–334.[CrossRef][Web of Science][Medline]
Gerde, P., Muggenburg, B. A., Hoover, M. D., and Henderson, R. F. (1993b). Disposition of polycyclic aromatic hydrocarbons in the respiratory tract of the Beagle dog: I. The alveolar region. Toxicol. Appl. Pharmacol. 121, 313–318.[CrossRef][Web of Science][Medline]
Gerde, P., Muggenburg, B. A., Lundblad, M., and Dahl, A. R. (2001). The rapid alveolar absorption of diesel-soot adsorbed benzo(a)pyrene—Bioavailability, metabolism, and dosimetry of an inhaled carcinogen. Carcinogenesis 22, 741–749.
Gerde, P., Muggenburg, B. A., Sabourin, P. J., Harkema, J. R., Hotchkiss, J. A., Hoover, M. D., and Henderson, R. F. (1993c). Disposition of polycyclic aromatic hydrocarbons in the respiratory tract of the Beagle dog: II. The conducting airways. Toxicol. Appl. Pharmacol. 121, 319–327.[CrossRef][Web of Science][Medline]
Gerde, P., Muggenburg, B. A., Stephens, T., Lewis, J. L., Pyon, K. H., and Dahl, A. R. (1998a). A relevant dose of 4-(methyl-nitrosamino)-1-(3-pyridyl)-1-butanone is extensively metabolized and rapidly absorbed in the canine tracheal mucosa. Cancer Res. 58, 1417–1422.
Gerde, P., Muggenburg, B. A., Thornton-Manning, J. R., Lewis, J. L., Pyon, K. H., and Dahl, A. R. (1997). Benzo(a)pyrene at an environmentally relevant dose is slowly absorbed by, and extensively metabolized in, tracheal epithelium. Carcinogenesis 18, 1825–1832.
Gerde, P., and Scott, B. R. (2001). A model for absorption of low-volatile toxicants by the airway mucosa. Inhal. Toxicol. 13, 903–929.[CrossRef][Web of Science][Medline]
Gerde, P., Stephens, T., and Dahl, A. R. (1998b). A rapid combustion method for the accurate determination of low levels of tritium in biological samples. Toxicol. Methods 8, 37–44.
Slikker, W., Jr, Andersen, M. E., Bogdanffy, M. S., Bus, J. S., Cohen, S. D., Conolly, R. B., David, R. M., Doerrer, N. G., Dorman, D. C., Gaylor, D. W., et al. (2004a). Dose-dependent transitions in mechanisms of toxicity. Toxicol. Appl. Pharmacol. 201, 203–225.[CrossRef][Web of Science][Medline]
Slikker, W., Jr, Andersen, M. E., Bogdanffy, M. S., Bus, J. S., Cohen, S. D., Conolly, R. B., David, R. M., Doerrer, N. G., Dorman, D. C., Gaylor, D. W., et al. (2004b). Dose-dependent transitions in mechanisms of toxicity: Case studies. Toxicol. Appl. Pharmacol. 201, 226–294.[CrossRef][Web of Science][Medline]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||