High-Glucose-Induced Metabolic and Epithelial Stress in Grass Carp Intestinal Epithelial Cells Associated with Methylation-Related Transcriptional Responses

High-glucose exposure impairs intestinal metabolic homeostasis and barrier integrity in fish, but the transcriptional responses associated with high-glucose adaptation in fish intestinal epithelial cells remain incompletely understood. This study investigated whether exogenous 5-methylcytosine (5MC) alleviates high-glucose-induced metabolic and epithelial stress in grass carp (Ctenopharyngodon Idella) intestinal epithelial cells and whether these responses are associated with changes in DNA methyltransferase 3 beta (dnmt3b) expression and Caudal type homeobox 1b (cdx1b)/Sodium-glucose cotransporter 1 (sglt1)-related transcriptional responses. As exploratory in silico information, molecular docking predicted candidate complex conformations of DNMT3B with CDX1B and SGLT1, with binding energies of −37.2 and −25.9 kcal/mol, respectively. Functionally, dnmt3b knockdown significantly reduced dnmt3b, Interleukin 6 (il6), and Nuclear factor kappa B (nfκb) expression, while increasing cdx1b, sglt1, Solute carrier family 2 member 3a (slc2a3a), 6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4a (pfkfb4a), and Amine oxidase copper containing 1 (aoc1) expression (p < 0.05). CDX2/CDX1B-like immunoreactive protein and SGLT1 protein levels were also increased after dnmt3b knockdown (p < 0.05). Under high-glucose stress, exogenous 5MC exerted concentration-dependent effects. Specifically, 6 mM 5MC significantly reduced residual extracellular glucose, lactate dehydrogenase and diamine oxidase activities, and malondialdehyde content, while increasing glutathione content, cell viability, and cell migration (p < 0.05). These effects remained detectable after replacement with high-glucose medium for an additional 12 h. By contrast, 24 mM 5MC markedly increased lactate dehydrogenase activity and reduced cell viability, suggesting potential cytotoxicity (p < 0.05). S-adenosylmethionine (SAM) levels were significantly lower in the NC and 6 mM groups than in the HG, 12 mM, and 24 mM groups, suggesting changes in SAM-related one-carbon metabolic status rather than direct evidence of altered DNA methylation (p < 0.05). Exogenous 5MC, particularly at 6 mM, alleviated high-glucose-induced metabolic and epithelial stress in grass carp intestinal epithelial cells. These effects were accompanied by changes in several glucose metabolism- and inflammation-related genes. However, the cellular uptake, metabolic fate, DNA incorporation, methylation consequences, and causal roles of these gene-expression changes remain to be further verified.