BKS-2 is an immature B cell
lymphoma that undergoes apoptotic cell death when signaled via its surface
IgM receptor. To study the signaling components of surface
IgM mediated apoptosis in B
lymphoma cells, we generated mutants of BKS-2 that were resistant to
anti-IgM induced apoptosis. One mutant cell line, 1.B5, did not undergo apoptotic cell death upon treatment with
anti-IgM antibodies and also did not exhibit elevation of intracellular Ca2+ in response to cross-linking of surface
IgM. This appeared to be due to a defect in
protein tyrosine kinase (PTK) activity since fewer
proteins were
tyrosine phosphorylated in the mutant cells stimulated with
anti-IgM when compared to wild type BKS-2. Subsequently, we showed that
protein tyrosine kinases lyn and blk were inducibly
tyrosine phosphorylated in the wild type BKS-2 but not in 1.B5 mutant cells in response to
anti-IgM. Also the
kinase activity of lyn was elevated in the wild type but not in mutant cells upon triggering through surface
IgM. Furthermore,
tyrosine phosphorylation of CD19, a known substrate of lyn, was inducible in
anti-IgM stimulated BKS-2 cells but severely reduced in 1.B5 cells. In contrast,
kinase activity of another
src kinase, blk, was increased on
anti-IgM stimulation in both wild type and mutant cells. Surprisingly, syk, a non-src
protein tyrosine kinase important for surface
IgM mediated signaling, was
tyrosine phosphorylated in the lyn deficient mutant cells as well as in the wild type BKS-2 cells. Furthermore,
anti-IgM induced increase in
kinase activity of syk was similar in the mutant and wild type cells. Thus, in contrast to other studies that propose syk to be a downstream target of
src family kinases, syk may act upstream of lyn in immature B cells. Consistent with a functional syk, its target,
phospholipase gamma2 (PLC-gamma2) was normally
tyrosine phosphorylated in mutant cells.