The phosphoprotein phosphatase calcineurin controls calcium‐dependent apoptosis in B cell lines

Group I Burkitt's lymphoma cell lines and the B104 lymphoma cell line which expresses a phenotype of immature B cells undergo apoptosis after cross‐linking of their surface immunoglobulin (Ig) receptors or after exposure to a calcium ionophore, while protein kinase C (PKC)‐activating phorbol esters prevent such apoptosis. We show here that blockade of the phosphoprotein phosphatase calcineurin or phosphatase 2B by cyclosporin A (CsA) also protects these B cell lines against Ca2+‐dependent apoptosis but not against apoptosis triggered by the PKC inhibitor chelerythrine or by serum deprivation. Okadaic acid, an inhibitor of phosphatases 1, 2A and 2C was ineffective. Among a series of human cytokines tested, only interferon‐α and tumor necrosis factor‐α were shown to protect against Ca2+‐dependent apoptosis when used alone or in combination with CsA. In contrast to phorbol esters which block the progression into the S/G2 phases of the cell cycle, CsA partially restored the proliferation of cells exposed to the calcium ionophore. Altogether these data provide indirect evidence for the control of B cell apoptosis by the serine/threonine phosphorylation status of yet undefined key cellular substrates.