Immobilized alpha2,6-linked sialic acid suppresses caspase-3 activation during anti-IgM antibody-induced apoptosis in Ramos cells

In Ramos cells, a human Burkitt's lymphoma cell line, stimulation of the B cell antigen receptor with anti-IgM antibody (Ab) induces apoptosis as indicated by a decrease in cell viability and an increase in DNA fragmentation and cell surface exposure of phosphatidylserine. Furthermore, these changes are suppressed by incubating the cells in alpha(1)-acid glycoprotein (AGP)-coated tissue culture plates. Here, we found that, during Anti-IgM Ab-induced apoptosis in Ramos cells, caspase-3 is activated downstream of caspase-8 and the mitochondrial pathway is activated, as indicated by a loss of mitochondrial membrane potential, an increase in the release of cytochrome c to the cytoplasm, and enhanced Bax expression. Anti-IgM Ab-induced apoptosis of neuraminidase-treated Ramos cells was suppressed by incubating the cells on plates coated with AGP, which contains a high concentration of alpha2,6-linked sialic acid. The incubation on plates coated with AGP also suppressed anti-IgM Ab-stimulated caspase-3 activity and increased the level of X-linked inhibitor of apoptosis protein (XIAP), but it did not affect caspase-8 activity, the mitochondrial membrane potential, cytochrome c release, or Bax expression. The results indicate that the interaction of Ramos cells with immobilized alpha2,6-linked sialic acid enhances XIAP expression, directly or indirectly suppressing caspase-3 activity and inhibiting anti-IgM Ab-induced apoptosis.