miR-214-3p Mediates Samarium Oxide-Induced Pulmonary Fibrosis by Targeting MAP2K3 via the MAPK Signaling Pathway

<b>Objective:</b> Rare-earth elements are extensively employed across diverse industrial sectors, increasingly raising concerns about their potential health hazards in both occupational and environmental contexts. Samarium oxide (Sm₂O₃), a routinely processed rare-earth product, reproducibly precipitates pulmonary fibrosis in experimental models, yet the molecular circuitry that transduces its fibrogenic signal remains almost entirely unmapped. This study aims to elucidate the role of miR-214-3p in Sm₂O₃-induced pulmonary fibrosis and to investigate its regulatory mechanism at the molecular level. <b>Methods:</b> A murine model of pulmonary fibrosis was established via intratracheal instillation of Sm₂O₃, and histopathological changes were assessed using hematoxylin and eosin (H&E) and Masson's trichrome staining. RNA sequencing was performed on lung tissues to identify differentially expressed mRNAs. Leveraging our previously generated miRNA landscape of Sm₂O₃-exposed lungs, we subjected the dataset to Gene Ontology and KEGG enrichment analyses, which convergently identified miR-214-3p as the top-ranking candidate regulator of the fibrogenic MAPK axis. The direct targeting of MAP2K3 by miR-214-3p was validated using a dual-luciferase reporter assay. Expression levels of fibrotic markers (α-SMA, Collagen I) and key components of the MAPK signaling pathway (MAP2K3, p-MAPK14, MST1) were quantified in both in vivo and in vitro models using qRT-PCR and Western blotting. Gain- and loss-of-function studies, complemented by rescue assays, were performed in human embryonic lung fibroblasts (HELFs) via transient transfection of miR-214-3p mimics, inhibitors, or MAP2K3-overexpression plasmids. Cell proliferation was evaluated using the EdU assay, and TGF-β1 secretion was measured by ELISA. <b>Results:</b> Sm₂O₃ exposure induced significant pulmonary fibrosis in mice, accompanied by marked downregulation of miR-214-3p and upregulation of MAP2K3 in lung tissues. Overexpression of miR-214-3p or silencing of MAP2K3 effectively suppressed Sm₂O₃-induced fibroblast activation, including reduced cell proliferation, decreased expression of α-SMA and Collagen I, and inhibition of p38 MAPK phosphorylation. Notably, ectopic overexpression of MAP2K3 reversed the protective effects conferred by miR-214-3p, confirming a functional rescue. <b>Conclusions:</b> miR-214-3p directly silences MAP2K3, thereby blunting p38 MAPK-driven fibrogenesis after Sm₂O₃ exposure. Our data unveil a miR-214-3p-MAP2K3-p38 MAPK axis that constitutes a readily druggable target for rare-earth-element-induced pulmonary fibrosis.