Ubiquitin E3 Ligase A20 is Required in Degradation of Microbial Superantigens in Vascular Endothelial Cells

The endothelial cells and tight junctions or adherens junctions form the endothelial barrier on the inner surface of the blood vessels. How the endothelial barrier degrades the endocytic microbial products, such as Staphylococcal enterotoxin B (SEB), is not fully understood yet. Ubiquitination is involved in protein degradation. This study aims to investigate the role of ubiquitin E3 ligase A20 (A20) in the degradation of endocytic SEB in endothelial cells. The human microvascular endothelial cell line, Hmvec, was cultured to monolayers in the inserts of transwells. SEB was added to the apical chambers to observe the endocytosis and degradation of SEB in Hmvecs. The fusion of endosome/lysosome was observed by immune staining. After exposed to SEB for 30 min, SEB was detected in Hmvecs. SEB could attach to the surface of Hmvecs and endocytosed into the cytoplasm of Hmvecs. The endocytosed SEB was degraded in the Hmvecs, which was transported to the transwell basal chambers in A20-deficient Hmvec monolayers. The SEB-carrying endosomes fused to the lysosomes in Hmvecs; the fusion of endosome/lysosome was disturbed in A20-deficient Hmvecs. In conclusion, A20 plays an important role in the degradation of the endocytic microbial product, SEB, in cardiac endothelial cells.