The SMYD3-MTHFD1L-formate metabolic regulatory axis mediates mitophagy to inhibit M1 polarization in macrophages

Background: SMYD3 (protein 3 containing SET and MYND structural domains) belongs to the SMYD methyl -esterase family and is a histone lysine methyltransferase that promotes gene transcription mainly by catalysing the trimethylation of lysine at position 4 of histone subunit 3 (H3K4me3). Studies have shown that SMYD3 plays a key role in tumour cell proliferation and differentiation; however, its role in macrophage polarization is unclear.Methods: We screened the M1-and M2-polarized macrophage differential histone modifying enzyme using bioinformatics analysis. The SMYD3 overexpression plasmid was transfected into M1 macrophages, and the SMYD3-regulated target gene was analysed by RNA-seq and ChIP-Seq. The effect of knocking down MTHFD1L on M1 polarization and the change of the intracellular metabolite formic acid content were investigated. M1 macrophages were stimulated with different concentrations of formic acid (2, 10 and 40 mM) to detect the expression of M1-related genes, ROS production, and changes in the expression of the mitophagy-related proteins LC3, PINK1 and p-Parkin.Results: Here, we used bioinformatics to analyse SMYD3, a histone methyltransferase associated with M1 po-larization; overexpression of SMYD3 significantly suppressed the LPS/IFN-gamma-induced M1 phenotype in macro-phages. RNA-seq analysis demonstrated that SMYD3 significantly activated the one-carbon folate metabolic pathway in M1 macrophages. In addition, we used ChIP-seq analysis to identify methylenetetrahydrofolate de-hydrogenase 1-like (MTHFD1L) as the target gene of the transcriptional activation by SMYD3 through H3K4me3 histone modification. Activation of MTHFD1L causes the accumulation of the intracellular metabolite formate. Exogenous stimulation with different concentrations of formate increased the expression of key genes involved in the mitochondrial respiratory chain complex, ROS production, and the expression of autophagy-related proteins LC3, PINK1, and p-Parkin and suppressed the expression of M1-related genes.Conclusions: Our study demonstrates that SMYD3 regulates the activity of the mitochondrial metabolic enzyme MTHFD1L through H3K4me3 histone methylation modification, promotes formate synthesis and induces mitophagy, which inhibits M1 polarization in macrophages.