Modification of m⁶ A in human embryonic lung fibroblasts induced by neodymium oxide exposure and its mechanism

Background  Occupational and environmental particulate matter may cause fibrosis, accompanied by RNA m⁶A modification changes. Neodymium oxide (Nd₂O₃) can cause mouse lung fibrosis, which contains a large number of fibroblasts.

Objective  To investigate m⁶A modification of tumor necrosis factor receptor-associated protein 6/nuclear factor-κB (TRAF6/NF-κB) signaling pathway in fibrosis of human embryonic lung fibroblasts induced by Nd₂O₃, and identify the key m⁶A modification sites of TRAF6.

Methods  Designed concentrations of Nd₂O₃ (0, 1.563, 3.125, 6.25, 12.5, 25, 50, 100, and 200 mg∙L⁻¹) were infected with HELF cells for 24 and 48 h, and cell viability was detected to determine exposure time and dose. Measurements included indicators of fibrosis hydroxyproline (HYP) and transforming growth factor-β1 (TGF-β1), m⁶A methylation level, methyltransferases (METTL3 and METTL14), demethylases (FTO and ALKBH5), reading proteins (YTHDC2 and YTHDF2), fibrosis-associated genes (collagen-І, vimentin, and α-SMA), and proteins related to signaling pathway (TRAF6, NFKB1, P65, and P-P65). The enrichment of m⁶A in TRAF6 mRNA was measured by methylated RNA immunoprecipitation-quantitative real-time PCR (MeRIP-qPCR).

Results  The results of cell viability indicated that 6.25, 12.5, 25 mg∙L⁻¹ Nd₂O₃ and 48 h exposure time were used for subsequent experiments. After 48 h exposure, compared with the control group, the HYP level in the 25 mg∙L⁻¹ Nd₂O₃ group was increased, and the levels of TGF-β1 in the 6.25, 12.5, and 25 mg∙L⁻¹ Nd₂O₃ groups were increased (P<0.05); the overall m⁶A methylation levels of HELF cells in the 12.5 and 25 mg∙L⁻¹ Nd₂O₃ groups were increased (P<0.05). At mRNA level, compared with the control group, the mRNA expression levels of methyltransferasesMETTL3 and METTL14 (6.25, 12.5, and 25 mg∙L⁻¹ Nd₂O₃) were increased (P<0.05); the mRNA expression level of reading proteinYTHDF2 (6.25, 12.5, and 25 mg∙L⁻¹ Nd₂O₃) was increased (P<0.05), while the mRNA expression level ofYTHDC2 (25 mg∙L⁻¹ Nd₂O₃) was decreased (P<0.05); the mRNA expression levels of demethylasesFTO (12.5 and 25 mg∙L⁻¹ Nd₂O₃) and ALKBH5 (25 mg∙L⁻¹ Nd₂O₃) were decreased (P<0.05); the mRNA expression levels of fibrosis-related genesvimentin, α-SMA, and collagen-Ⅰ (6.25, 12.5, and 25 mg∙L⁻¹ Nd₂O₃) were increased (P<0.05); the mRNA expression levels of pathway-related genesTRAF6 (25 mg∙L⁻¹ Nd₂O₃) and NFKB1 (12.5 and 25 mg∙L⁻¹ Nd₂O₃) were increased (P<0.05). At protein level, compared with the control group, the expression levels of methyltransferases METTL3 (25 mg∙L⁻¹ Nd₂O₃) and METTL14 (12.5 and 25 mg∙L⁻¹ Nd₂O₃) were increased (P<0.05); the expression level of reading protein YTHDF2 (12.5 and 25 mg∙L⁻¹ Nd₂O₃) was increased, while the expression level of YTHDC2 (25 mg∙L⁻¹ Nd₂O₃) was decreased (P<0.05); the expression level of demethylase FTO (25 mg∙L⁻¹ Nd₂O₃) was decreased (P<0.05); the expression level of fibrosis-associated protein vimentin was increased at 25 mg∙L⁻¹ Nd₂O₃, and the expression levels of α-SMA and collagen-Ⅰ were increased at 12.5 and 25 mg∙L⁻¹ Nd₂O₃ (P<0.05); the expression levels of TRAF6 and P-P65 were increased at 25 mg∙L⁻¹ Nd₂O₃ (P<0.05). The MeRIP-qPCR results showed that compared with the control group, the concentrations of m⁶A in all Nd₂O₃ groups were significantly increased (P<0.05).

Conclusions  Upon exposure of HELF cells to Nd₂O₃, the alterations in fibrosis-related indexes increase the expression of some m⁶A methylases and decrease the expression of demethylases, thereby increasing the m⁶A methylase level, and may promote the progression of fibrosis by activating the TRAF6/NF-κB signaling pathway.