Differential toxicity in an alveolar epithelial cell line of fine particulate matter from brakewear, roadwear, and diesel exhaust

Abstract

Introduction: Airborne fine particulate matter (diameter <2.5μm; PM2.5) is a leading risk factor for lung disease, but differential effects of PM2.5 from different sources are poorly understood. Improvements to vehicle exhaust technology have neglected poorly characterised, unregulated “non-exhaust” PM, predominantly brakewear (BWPM). This study aimed to characterise the toxicology of PM2.5 from 4 brake pad types compared to roadwear and diesel exhaust (DEP).

Methods: An alveolar type-II epithelial cell line (ATII^ER:KRASv12) was exposed to 12.5-100 µg/ml PM2.5 from 4 brake pad types: low-metallic, semi-metallic, non-asbestos organic (NAO), ceramic, roadwear, or DEP for 2-24h. PM composition was determined by ICP-MS. Cytotoxicity was measured by LDH and MTT assays, cytokine release (IL-6/IL-8) by ELISA, and gene expression by RT-qPCR and RNA-Seq.

Results: ICP-MS showed clear compositional variation between BWPM. NAO and ceramic BWPM exerted the greatest effects on cytotoxicity, cytokine release (IL-6: NAO 26 fold, ceramic 20 fold), and upregulation of genes related to oxidative stress (HMOX1 – NAO 28-fold, ceramic 26-fold) and metal-binding (MT1G: NAO 197-fold, ceramic 183-fold). BWPM was consistently more potent than DEP. RNA-Seq indicated considerable heterogeneity in BWPM transcriptomic effects, with NAO and ceramic BWPM leading to most differentially expressed genes vs control, with 395 of these unique to NAO, 304 to ceramic, and 972 in common.

Conclusions: Epithelial cell responses varied by PM type, with NAO and ceramic BWPM most potent. Future work will further characterise the cellular effects of these PM and composition-effect relationships.