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Background: This study aimed to assess the effects of postbiotic derived from Bacillus subtilis natto (Szendi2020) on endothelial responses under LPS-induced inflammatory stress. Methods: In human umbilical vein endothelial cells (HUVECs), inflammation was induced with 200 ng/mL LPS. Cell viability, apoptosis, and mitochondrial integrity were assessed using MTT assay, DiIC, and Sytox Green permeability assays. Intracellular ROS levels, heat shock proteins (HSPB1/Hsp27, HSPA1L/Hsp70), adhesion molecules (ICAM-1, VCAM-1), tight junction protein (Occludin), transcription regulators (NF-κB, TNFα), and proinflammatory cytokines (IL-1β, IL-6, IL-8) were quantified using qPCR and ELISA. Results: LPS exposure significantly induced apoptosis in HUVECs, as reflected by decreased metabolic activity, decreased mitochondrial membrane potential, and increased cell death (p < 0.05). Concurrent postbiotic administration completely abolished LPS-induced cytotoxicity in all assay platforms, demonstrating a potent cytoprotective effect. Postbiotic treatment significantly reduced LPS-induced ROS accumulation (p < 0.05). LPS significantly increased Hsp27 and Hsp70 mRNA expression. However, combined LPS and postbiotic exposure mitigated Hsp27 and Hsp70 mRNA expression compared with LPS treatment alone (p < 0.001, p < 0.005). Postbiotic treatment also decreased the upregulation of adhesion molecules induced by LPS. Although this effect decreased after 24 h (p < 0.001). LPS strongly increased NF-κB, IL-1β and TNFα mRNA levels and was suppressed by postbiotics at early time points but not maintained over 24 h. Importantly, postbiotics significantly reduced IL-6, and IL-8 expression at both the mRNA and protein levels, highlighting the attenuation of endothelial inflammatory features (p < 0.05, p < 0.005, p < 0.001). Conclusions: Our results are the first to demonstrate that postbiotics derived from Bacillus subtilis natto (Szendi2020) exert potent cytoprotective and anti-inflammatory effects in LPS-induced endothelial inflammation. By reducing ROS accumulation, preventing apoptosis, stabilizing mitochondrial and barrier integrity, modulating HSP, NF-κB, and cytokine responses. Postbiotics may be promising therapeutic candidates for alleviating endothelial inflammation and the resulting endothelial dysfunction.