Methyltransferase SMYD5 Exaggerates IBD by Downregulating Mitochondrial Functions via Post-translational Control of PGC-1α Stability

Background and Aims The expression and role of methyltransferase SET and MYND domain-containing protein 5 (SMYD5) in inflammatory bowel diseases (IBD) is completely unknown. Here, we investigated the role and the underlying mechanism of epithelial SMYD5 in IBD pathogenesis and progression. Methods The expression and subcellular localization of SMYD5 and peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) were examined by Western blot analysis, immunofluorescence staining, and immunohistochemistry in intestinal epithelial cells (IECs) and in colon tissues from human IBD patients and mice with experimental colitis. Mice with Smyd5 conditional knockout in IECs and littermate controls were subjected to DSS-induced experimental colitis and the disease severity and inflammation were assessed. SMYD5-regulated mitochondrial biogenesis was examined by RT-qPCR and transmission electron microscopy and mitochondrial oxygen consumption rate was measured in a Seahorse Analyzer system. The interaction between SMYD5 and PGC-1α was determined by co-immunoprecipitation assay. PGC-1α degradation and turnover (half-life) were analyzed by cycloheximide chase assay. SMYD5-mediated PGC-1α methylation was measured via in vitro methylation followed by mass spectrometry to identify the specific lysine residues that were methylated. Results Up-regulated SMYD5 and down-regulated PGC-1α were observed in IECs from IBD patients and mice with experimental colitis. However, Smyd5 depletion in IECs protected mice from DSS-induced colitis. SMYD5 was critically involved in regulating mitochondrial biology such as mitochondrial biogenesis, respiration, and apoptosis. Mechanistically, SMYD5 regulated mitochondrial functions in a PGC-1α dependent manner. Further, SMYD5 mediated lysine methylation of PGC-1α and facilitated its ubiquitination and proteasomal degradation. Conclusion SMYD5 attenuates mitochondrial functions in IECs and promotes IBD progression by enhancing the proteasome-mediated degradation of PGC-1α protein in methylation-dependent manner. Strategies to decrease SMYD5 expression and/or increase PGC-1α expression in IECs might be a promising therapeutic approach to treat patients with IBD. ### Competing Interest Statement The authors have declared no competing interest. * CD : Crohn’s disease CHX : cycloheximide COX I/II : cytochrome c oxidase I/II DAI : disease activity index DSS : dextran sulfate sodium FCCP : trifluoromethoxy carbonyl cyanide phenylhydrazone FITC : fluorescein isothiocyanate H4K20 : histone 4 lysine 20 HA : hemagglutinin HCT116 : human colorectal carcinoma cell line IBD : inflammatory bowel disease IEC : intestinal epithelial cell K223 : lysine 223 K223R : lysine 223 to arginine KO : knockout L3MBTL1/3 : lethal (3) malignant brain tumor-like protein 1/3 LPS : lipopolysaccharide LSD1 : lysine-specific demethylase 1 mtDNA : mitochondrial DNA OCR : oxygen consumption rate OE : overexpression PHF20L1 : plant homeodomain finger protein 20-like 1 PGC-1α : peroxisome proliferator-activated receptor gamma coactivator 1 alpha SMYD5 : SET and MYND domain- containing protein 5 TEM : transmission electron microscopy Tfam/TFAM : mitochondrial transcription factor A UC : ulcerative colitis