Inflammatory response and genotoxicity of seven wood dusts in the human epithelial cell line A549

Abstract

Exposure to wood dust is common in many workplaces. Epidemiological studies indicate that occupational exposure to hardwood dusts is more harmful than to softwood dusts. In this study, human epithelial cell line A549 was incubated with well-characterized dusts from six commonly used wood species and from medium density fibreboard (MDF), at concentrations between 10 and 300 μg/ml. After 3 and 6 h of incubation, genotoxicity was assessed by measurement of DNA damage with the single-cell gel electrophoresis (comet) assay and inflammation was measured by the expression of IL-6 and IL-8 mRNA and by the amount of IL-8 protein. There was a 1.2–1.4-fold increase in DNA strand breaks after incubation with beech, teak, pine and MDF dusts compared with the levels in untreated cells, but after 6 h only the increase induced by the MDF dust remained. Increased expression of cellular IL-6 and IL-8 mRNA was induced by all of the wood dusts at both times. Similar to IL-8 mRNA expression, the amounts of secreted IL-8 protein were elevated, except after incubation with oak dust, where a marginal reduction was seen. On the basis of the effects on IL-8 mRNA expression, the wood dusts could be divided into three groups, with teak dust being the most potent, MDF, birch, spruce and pine being intermediate, and beech and oak being the least potent. The induction of DNA strand breaks did not correlate well with the interleukin response.

In conclusion, all wood dusts induced cytokine responses, and some dusts induced detectable DNA damage. The inflammatory potency seemed intermediate for dusts from the typical softwoods spruce and pine, whereas the dusts from species linked to cancer, beech and oak, were the least inflammatory. The variation of the effects induced by different wood dusts over time indicates that the DNA damage was not secondary to the cytokine response. Although hardwoods are often considered more harmful than softwoods by regulatory agencies, the current experiments do not provide evidence for a clear-cut distinction between toxicities of hardwood and softwood dust.

Introduction

Wood dust is one of the most common occupational dust exposures. Worldwide, approximately 13 million people were occupied in the forestry sector and about 3.5 million people in the furniture industry in year 2000. This means that about 0.4% of the total labour force were occupied in the forestry sector [1]. Recently, Kauppinen et al. [2] reported that 3.6 million workers, i.e. 2.0% of the workforce, were occupationally exposed to wood dusts in the European Union.

Many countries differentiate the regulation between exposures to hardwoods and softwoods (see, e.g. the European Union Directive 1999/38). Hardwoods are usually defined as woods of broad-leaved trees, whereas softwoods are conifers. This categorization is based on the botanical classification system rather than on the physical and chemical properties of the wood species.

Cancer in the sino-nasal cavities is rare, with a worldwide annual standardized incidence rate of 1.05/100,000 persons for men and 0.45 for women [3]. Cancer at this site has been strongly associated with different occupational exposures [4], including exposure to wood dust. Several independent studies have found excess risks among employees working in wood-related industries [5], [6], [7]. When classifying the cancers by histology, adeno-carcinomas are even more strongly associated with wood dust exposure. In a meta-analysis of 12 case–control studies, an odds ratio of 45.5 (28.3–72.9) was calculated for male workers in jobs with high levels of wood dust exposure [5].

In 1995, the International Agency for Research of Cancer [8] evaluated wood dust as being carcinogenic to humans (Group 1). The conclusion was based on a substantial amount of epidemiological evidence, despite the sparse knowledge about the underlying mechanisms and pathways. The studies showed the strongest correlation between nasal cancer and exposure to dust from hardwood, especially oak and beech [9]. However, sino-nasal cancer among employees who worked with other wood types and species has also been reported [8].

In addition to cancer, wood dust exposure is also associated with non-malignant health effects like impaired lung function, occupational asthma, rhinitis and allergic contact dermatitis [10], [11], [12], [13], [14], [15]. We therefore tested whether an indirect pathway involving inflammation and oxidative stress was related to DNA strand break formation, which is a potential indicator of a carcinogenic mechanism [16].

This study was aimed at investigating whether the inflammatory response and the genotoxicity of dusts from common woods differ. Animal models for studying the carcinogenic effects of wood dusts are lacking. Therefore, in vitro data may be an important tool for suggesting preventive measures, such as distinguishing between the toxicity of certain wood types. Dusts made from six wood species and dust from medium density fibreboard (MDF) was tested to encompass common wood materials (i.e. hard, soft, exotic and artificial wood). Beech, birch and oak were selected because of their widespread use in the furniture industry. Teak is a popular exotic hardwood in furniture making. Pine and spruce represent softwoods, and MDF is a manufactured wood material that is commonly used in furniture and for construction.