Distinct ICAM-1 forms and expression pathways in synovial microvascular endothelial cells

Human synovial endothelial cell (HSE) intracellular adhesion molecule-1 (IC AM-1) is upregulated maximally by synergy of tumor necrosis factor alpha (T NF alpha) and interferon gamma (TFN gamma). Such synergy is not as pronounc ed in human umbilical vein endothelium (HUVE). ICAM surface staining and EL ISA detection reflected similar levels on HUVE and HSE cells, yet mRNA leve ls were much higher in HSE cells in response to TNF alpha/IFN gamma. To cor relate protein and mRNA levels of ICAM-1, both cell types were permeabilize d and stained with a monoclonal antibody against ICAM-1. HSE cells displaye d a distinct vesicular cytoplasmic staining for ICAM while HUVE cells were devoid of such stained vesicles upon staining with the antibody. ICAM-1 imm unostaining of HSE cytoplasmic vesicles appeared enhanced in cells treated with TNF alpha/IFN gamma and monensin, an endosomal processing inhibitor. M onensin inhibited HSE cell surface expression of ICAM-1 routinely up to 70% , while HUVE cell expression was unaffected. In addition, monensin also inh ibited soluble ICAM-1 release from HSE cells while not effecting HUVE cells . Immunoprecipitation of ICAM-1 followed by gel electrophoresis indicated t hat HUVE and HSE cell ICAMs are expressed in cell-specific forms. These res ults define distinct forms and distinct secretory pathways for ICAM-1 in HS E cells and HUVE cells that indicate functional differences between these h uman endothelia.