Inhibition of anti-IgM-induced translocation of protein kinase C beta I inhibits ERK2 activation and increases apoptosis.

Expression of the COOH-terminal residues 179-330 of the LSP1 protein in the LSP1(+) B-cell line W10 increases anti-IgM- or ionomycin-induced apoptosis, suggesting that expression of this LSP1 truncate (B-LSP1) interferes with a Ca(2+)-dependent step in anti-IgM signaling. Here we show that inhibition of Ca(2+)-dependent conventional protein kinase C (cPKC) isoforms with Gö6976 increases anti-IgM-induced apoptosis of W10 cells and that expression of B-LSP1 inhibits translocation of PKCbetaI but not of PKCbetaII or PKCalpha to the plasma membrane. The increased anti-IgM-induced apoptosis is partially reversed by overexpression of PKCbetaI. This shows that the B-LSP1-mediated inhibition of PKCbetaI leads to increased anti-IgM-induced apoptosis. Expression of constitutively active PKCbetaI protein in W10 cells activates the mitogen-activated protein kinase ERK2, whereas expression of B-LSP1 inhibits anti-IgM-induced activation of ERK2, suggesting that anti-IgM-activated PKCbetaI is involved in the activation of ERK2 and that inhibition of ERK2 activation contributes to the increased anti-IgM-induced apoptosis. Pull-down assays show that LSP1 interacts with PKCbetaI but not with PKCbetaII or PKCalpha in W10 cell lysates, while in vitro LSP1 and B-LSP1 bind directly to PKCbetaI. Thus, B-LSP1 is a unique reagent that binds PKCbetaI and inhibits anti-IgM-induced PKCbetaI translocation, leading to inhibition of ERK2 activation and increased apoptosis.