Apoptosis profoundly alters the
carbohydrate layer coating the membrane of eukaryotic cells. Previously we showed that apoptotic cells became reactive with the α2,6-sialyl-specific
lectin from Sambucus nigra
agglutinin (SNA), regardless of their histological origin and the nature of the apoptotic stimulus. Here we reveal the basis of the phenomenon by showing that in apoptotic
cancer cell lines SNA reactivity was mainly associated with a 67 kDa
glycoprotein which we identified by MALDI-TOF/TOF and immunoblot analysis as bovine
vitronectin (bVN). bVN was neither present in non-apoptotic cells, nor in cells induced to apoptosis in serum-free medium, indicating that its uptake from the cell culture serum occurred only during apoptosis. The bVN molecules associated with apoptotic
cancer cell lines represented minor
isoforms, lacking the carboxyterminal sequence and paradoxically containing a few α2,6-linked
sialic acid residues. Despite their poor α2,6-sialylation, these bVN molecules were sufficient to turn apoptotic cells to SNA reactivity, which is a late apoptotic event occurring in cells positive to both
annexin-V and
propidium iodide. Unlike in
cancer cell lines, the major bVN form taken up by apoptotic neutrophils and mononuclear cells was a 80 kDa form. In apoptotic SW948 cells we also detected the α2,6-sialylated forms of the stress-70 mitochondrial precursor (
mortalin) and of tubulin-β2C. These data indicate that the acquisition of
vitronectin isoforms from the environment is a general, although cell specific phenomenon, potentially playing an important role in post-apoptotic events and that the α2,6-sialylation of intracellular
proteins is a new kind of posttranslational modification associated with apoptosis.