The toxicity of styrene to the nasal epithelium of mice and rats: studies on the mode of action and relevance to humans

Abstract

Inhaled styrene is known to be toxic to the nasal olfactory epithelium of both mice and rats, although mice are markedly more sensitive. In this study, the nasal tissues of mice exposed to 40 and 160 ppm styrene 6 h/day for 3 days had a number of degenerative changes including atrophy of the olfactory mucosa and loss of normal cellular organisation. Pretreatment of mice with 5-phenyl-1-pentyne, an inhibitor of both CYP2F2 and CYP2E1 completely prevented the development of a nasal lesion on exposure to styrene establishing that a metabolite of styrene, probably styrene oxide, is responsible for the observed nasal toxicity. Comparisons of the cytochrome P-450 mediated metabolism of styrene to its oxide, and subsequent metabolism of the oxide by epoxide hydrolases and glutathione S-transferases in nasal tissues in vitro, have provided an explanation for the increased sensitivity of the mouse to styrene. Whereas cytochrome P-450 metabolism of styrene is similar in rats and mice, the rat is able to metabolise styrene oxide at higher rates than the mouse thus rapidly detoxifying this electrophilic metabolite. Metabolism of styrene to its oxide could not be detected in human nasal tissues in vitro, but the same tissues did have epoxide hydrolase and glutathione S-transferase activities, and were able to metabolise styrene oxide efficiently, indicating that styrene is unlikely to be toxic to the human nasal epithelium.

Introduction

Styrene is a major industrial chemical used extensively in the production of plastics, resins and synthetic rubbers. Occupational exposure occurs mostly by inhalation of styrene vapour during various manufacturing processes. It is of concern therefore, that styrene, in common with a number of chemicals, is toxic to the nasal epithelium of rodents when inhaled. Toxicity was seen in the nasal passages of male and female CD-1 mice at all dose levels following exposure by inhalation to 50, 100, 150 or 200 ppm styrene, 6 h/day, for 13 weeks [1]. The effects, which included atrophy and respiratory metaplasia of the olfactory mucosa, atrophy of the olfactory nerves and hypertrophy and hyperplasia of Bowman's glands, increased in severity with increasing dose. Rats (CD-Sprague Dawley) similarly exposed were markedly less sensitive with changes to the olfactory epithelium being seen after exposure to 1000 and 1500 ppm styrene, but not after exposure to 500 ppm. Although toxic to the nasal epithelium, exposure of mice to 20, 40, 80 and 160 ppm styrene 6 h/day, 5 days/week for 104 weeks did not lead to the development of nasal tumours [2], [3]. Consistent with the effects reported in these studies, radioactivity has been shown to localise in the nasal passages following exposure of rats and mice to atmospheres containing C-14 styrene [4], [5].

The primary metabolic pathway for styrene involves oxidation by cytochromes P-450, principally CYP2E1 and CYP2F2 [6], [7], [8], [9], to two enantiomeric forms of styrene oxide (R and S), weak electrophiles which are reactive with biological molecules [10], [11], [12] and may be responsible for the observed toxicities. Styrene oxides are further metabolised by both epoxide hydrolases and by glutathione S-transferases, these two metabolic transformations being detoxification reactions resulting in the deactivation of a potentially reactive epoxide [13]. In vivo, the potential of styrene oxide to react with the tissues in which it is formed will be dependent upon the relative rates of its formation from styrene and the rates of its deactivation by these enzymes. Although the metabolism of styrene and its oxide has been measured in a number of tissues [6], [13], their metabolism has not been determined in nasal tissues.

Rodent nasal tissues are known to have high concentrations of metabolising enzymes, including cytochromes P-450, particularly in the olfactory epithelium [14], [15], [16], [17], [18], which may explain the sensitivity of this tissue to styrene. In humans, the levels of these enzymes appear to be greatly reduced or even absent, suggesting that human nasal tissue will be much less sensitive than rodents to chemicals such as styrene [19]. However, there is a paucity of good metabolic data in humans due to the difficulty in obtaining fresh human nasal tissue, and hence the metabolism of styrene in this tissue has not been studied previously.

The purpose of the present study was to understand the role of styrene metabolism in the development of the nasal toxicities seen in rats and mice exposed to this chemical. Styrene metabolism in human nasal tissues has also been compared in vitro with that in rodents in order to assess the potential toxicity of styrene to the nasal tissues of humans exposed to styrene. A limited amount of hepatic data have also been obtained for comparative purposes.