Effect of tobacco smoke exposure cessation on lung response to crystalline silica exposure in rats

Our previous studies have shown that tobacco smoke exposure exacerbated the lung response to crystalline silica exposure in rats. The objective of the present study, a follow-up to our previous study, was to determine the effect of tobacco smoke exposure cessation on the lung response to crystalline silica exposure in the rats. Rats were exposed to air, crystalline silica (1 week followed by a 1 year progression/recovery period with no exposure), tobacco smoke (6 months of exposure followed by 6 months of recovery with no exposure), or crystalline silica (1 week) plus tobacco smoke (6 months of exposure followed by 6 months of recovery with no exposure). Lung toxicity was determined at the end of the 1-year progression/recovery period in all 4 groups of the rats. Silica exposure resulted in significant lung toxicity which was further exacerbated by tobacco smoke exposure in the rats. Cessation of cigarette smoke exposure did not result in reversal of the silica-induced lung toxicity despite exacerbation of the toxicity by tobacco smoke. Overall design: Approximately 3 months old, healthy male Fischer rats (Charles River Laboratories, Wilmington, MA) were used in this study. The study was conducted in an Association for Assessment and Accreditation of Laboratory Animal Care International approved animal facility (NIOSH, Morgantown, WV) following a protocol approved by the Animal Care and Use Committee. The rats were housed in groups of 3 rats/cage and maintained on a 12-hour light-dark cycle in a temperature (68-72oF) and humidity (30-70%) controlled room with free access to filtered tap water and Teklad rodent diet (Envigo, Indianapolis, IN). The rats were divided into 4 groups (n=6) and exposed to air (control) tobacco smoke (80 mg/m3, twice weekly, 6 months), crystalline silica (15 mg/m3, 6 hours/day, 5 days) or crystalline silica (15 mg/m3, 6 hours/day, 5 days) followed by tobacco smoke (80 mg/m3, twice weekly, 6 months) by whole-body inhalation (exposure period). Following the 6 months exposure period, all four groups of rats were maintained approximately for an additional 6 months without any further exposure (recovery period). At the end of the recovery period, the rats from all four groups were euthanized and lung gene expression profiles determined by RNA sequencing. Next generation sequencing libraries prepared using TruSeq Stranded mRNA Library Prep Kit (Illumina, Inc, San Diego, CA) were used in the sequencing.