A NOVEL SOLUBLE FORM OF MOUSE VCAM-1 IS GENERATED FROM A GLYCOLIPID-ANCHORED SPLICING VARIANT

VCAM-1 is a cytokine-induced endothelial adhesion molecule which belongs to the immunoglobulin (Ig) superfamily and mediates the binding of various leukocytes. In addition to the 110-kDa form of VCAM-1, we have found four additional glycoproteins on mouse brain-derived endothelioma cells after stimulation with tumor necrosis factor-alpha (TNF-alpha), which are recognized by several monoclonal antibodies against VCAM-1. Biochemical analysis revealed that the two smaller proteins (35 kDa and 37 kDa) are intracellular precursors of the two larger forms (44 kDa and 35 kDa), that the 44 kDa and 45 kDa proteins are glycolipid-anchored at the cell surface and that they differ in their N-glycosylation. Most likely they are identical to the:recently identified glycolipid-anchored splice variant of VCAM-1, since they are recognized by the M3 antiserum which we raised against a peptide from the unique protein domain of this splicing variant. With the help of this antiserum we could show by immunohistology that the corresponding VCAM-1 protein variant is induced in vivo by lipopolysaccharide (LPS) on endothelium of the mouse. In addition, we found a 42-kDa soluble form of VCAM-1 in the serum of LPS-stimulated mice, which was recognized by the M3 antiserum. This soluble form was undetectable in the serum of unstimulated mice in contrast to the soluble 100-kDa form of VCAM-1 which was clearly detected in serum of unstimulated mice and only increased 2-3-fold upon stimulation with LPS. Thus, only the expression of the 42-kDa shedded form and not of the 100-kDa soluble form of VCAM-1 is strictly dependent on stimulation by LPS.