Survival function of ERK1/2 as IL-3-activated, staurosporine-resistant Bcl2 kinases

Bcl2 phosphorylation at Ser-70 may be required for the full and potent supp ression of apoptosis in IL-3-dependent myeloid cells and can result from ag onist activation of mitochondrial protein kinase C (PKC). Paradoxically, ex pression of exogenous Bcl2 can protect parental cells from apoptosis induce d by the potent PKC inhibitor, staurosporine (stauro). High concentrations of stauro of up to 1 mu M only partially inhibit IL-3-stimulated Bcl2 phosp horylation but completely block PKC-mediated Bcl2 phosphorylation in vitro. These data indicate a role for a stauro-resistant Bcl2 kinase (SRK). We sh ow that aurintricarboxylic acid (ATA), a nonpeptide activator of cellular M EK/mitogen-activated protein kinase (MAPK) kinase, can induce Ser-70 phosph orylation of Bcl2 and support survival of cells expressing wild-type but no t the phosphorylation-incompetent S70A mutant Bcl2. A role for a MEK/MAPK a s a responsible SRK was implicated because the highly specific MEK/MAPK inh ibitor, PD98059, also can only partially inhibit IL-3-induced Bcl2 phosphor ylation, whereas the combination of PD98059 and stauro completely blocks ph osphorylation and synergistically enhances apoptosis. p44MAPK/extracellular signal-regulated kinase 1 (ERK1) and p42 MAPK/ERK2 are activated by IL-3, colocalize with mitochondrial Bcl2, and can directly phosphorylate Bcl2 on Ser-70 in a stauro-resistant manner both in vitro and in vivo. These findin gs suggest a role for the ERK1/2 kinases as SRKs, Thus, the SRKs can serve to functionally link the IL-3-stimulated proliferative and survival signali ng pathways and, in a novel capacity, may explain how Bcl2 can suppress sta uro-induced apoptosis. In addition, although the mechanism of regulation of Bcl2 by phosphorylation is not yet clear, our results indicate that phosph orylation may functionally stabilize the Bcl2-Bax heterodimerization.