Succinate Promotes M1 Polarization of Intestinal Macrophages in Mice With Necrotizing Enterocolitis Through the PI3K/AKT Pathway.

Necrotizing enterocolitis (NEC) is a devastating gastrointestinal disorder that frequently affects premature infants, and its pathogenesis is closely related to macrophage polarization. This study investigated the effects of succinate, a metabolite of the intestinal flora, on macrophage polarization in NEC. Succinate aggravated intestinal injury caused by NEC and inhibited the proliferation of damaged mouse monocyte‒macrophage leukemia cells (RAW264.7 cells). It was confirmed by multiple methods that succinate intervention promotes the polarization of intestinal macrophages toward the M1 phenotype in neonatal NEC. This polarization was characterized by a significant upregulation of inducible nitric oxide synthase (iNOS) protein levels and mRNA expression, along with a marked suppression of arginase 1 (ARG1) protein levels and mRNA expression. Moreover, immunofluorescence analysis revealed that in the NEC intestine, the coexpression of the M1 macrophage marker F4/80/CD86 was significantly increased, whereas the coexpression of the M2 macrophage marker F4/80/CD206 was significantly decreased. Mechanistic studies revealed that succinate upregulated the expression levels of phosphorylated protein kinase B (-AKT) and hypoxia-inducible factor 1 alpha (HIF1a) by activating the PI3K/AKT signaling pathway through its specific receptor succinate receptor 1 (SUCNR1). Further experiments revealed that the expression of polarization-related markers in M1-type macrophages was significantly suppressed after treatment with the SUCNR1-neutralizing antibody or the PI3K inhibitor LY294002. These findings suggest that succinate may activate the phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT) signaling pathway via SUCNR1 to promote the polarization of NEC macrophages toward the M1 phenotype, thereby accelerating NEC progression.