METTL3-catalyzed m6A modification of NSUN4 mediates m5C modification of SLC2A3 mRNA to regulate PDGF-BB-induced proliferation, hyaluronan production and glycolysis in human orbital fibroblasts.

Thyroid eye disease (TED) is the most common extra-thyroidal complication of Graves' disease, but its molecular pathogenesis is not fully understood. This study explored the mechanism associated with methyltransferase like 3 (METTL3), NOP2/Sun RNA methyltransferase 4 (NSUN4), and solute carrier family 2 member 3 (SLC2A3) in an in vitro TED model. Human normal orbital fibroblasts were stimulated with platelet-derived growth factor BB (PDGF-BB) to establish in vitro TED model. Differentially expressed genes were screened using online databases. Western blotting was performed for protein detection, and qPCR was used for mRNA quantification. CCK-8 assay and EdU assay were conducted to examine metabolic viability and proliferation, respectively. Hyaluronan (HA) production and glycolysis metabolism were assessed via commercial kits. Methylated RNA immunoprecipitation (MeRIP), RIP, dual-luciferase reporter assay, and RNA-protein pull-down assay were utilized for interaction analysis. Bioinformatics screening obtained 1139 differential genes and 45 glycolytic genes, yielding 2 overlapping genes that were considered as glycolysis-related genes in TED. SLC2A3 was highly expressed in TED and PDGF-BB-stimulated orbital fibroblasts, and silencing SLC2A3 suppressed PDGF-BB-induced cell proliferation, HA production and glycolysis. NSUN4 mediated the m5C methylation modification to increase SLC2A3 expression in an YBX1-dependent manner. NSUN4 inhibition restrained PDGF-BB-induced effects via downregulating SLC2A3. METTL3 mediated m6A methylation modification to reduce NSUN4 expression, with YTHDF2 as a reader protein. The influences of PDGF-BB on human orbital fibroblasts were significantly alleviated by METTL3 overexpression to down-regulate NSUN4. NSUN4 downregulation caused by METTL3-induced m6A modification of NSUN4 mRNA regulated m5C modification of SLC2A3 mRNA, thereby ameliorating PDGF-BB-induced proliferation, HA generation and glycolysis of human orbital fibroblasts in TED. The evidence uncovered the specific molecular mechanism underlying TED pathogenesis.