MicroRNA expression in lung tissues of asbestos‐exposed mice: Upregulation of miR‐21 and downregulation of tumor suppressor genes Pdcd4 and Reck

OBJECTIVES: Asbestos causes lung cancer and malignant mesothelioma in humans, but the precise mechanism has not been well understood. MicroRNA (miRNA) is a short non‐coding RNA that suppresses gene expression and participates in human diseases including cancer. In this study, we examined the expression levels of miRNA and potential target genes in lung tissues of asbestos‐exposed mice by microarray analysis. METHODS: We intratracheally administered asbestos (chrysotile and crocidolite, 0.05 or 0.2 mg/instillation) to 6‐week‐old ICR male mice four times weekly. We extracted total RNA from lung tissues and performed microarray analysis for miRNA and gene expression. We also carried out real‐time polymerase chain reaction (PCR), Western blotting, and immunohistochemistry to confirm the results of microarray analysis. RESULTS: Microarray analysis revealed that the expression levels of 14 miRNAs were significantly changed by chrysotile and/or crocidolite (>2‐fold, P < .05). Especially, miR‐21, an oncogenic miRNA, was significantly upregulated by both chrysotile and crocidolite. In database analysis, miR‐21 was predicted to target tumor suppressor genes programmed cell death 4 (Pdcd4) and reversion‐inducing‐cysteine‐rich protein with kazal motifs (Reck). Although real‐time PCR showed that Pdcd4 was not significantly downregulated by asbestos exposure, Western blotting and immunohistochemistry revealed that PDCD4 expression was reduced especially by chrysotile. Reck was significantly downregulated by chrysotile in real‐time PCR and immunohistochemistry. CONCLUSIONS: This is the first study demonstrating that miR‐21 was upregulated and corresponding tumor suppressor genes were downregulated in lung tissues of asbestos‐exposed animals. These molecular events are considered to be an early response to asbestos exposure and may contribute to pulmonary toxicity and carcinogenesis.