At high concentrations, the tubule poison paclitaxel is able to kill cancer cells that express Bcl-2; it inhibits the antiapoptotic activity of Bcl-2 by inducing its phosphorylation. To localize the site on Bcl-2 regulated by phosphorylation, mutant forms of Bcl-2 were constructed. Mutant forms of Bcl-2 with an alteration in serine at amino acid 70 (S70A) or,vith deletion of a 60-aa loop region between the alpha 1 and alpha 2 helices (Delta loop Bcl-2, which also deletes amino acid 70) were unable to be phosphorylated by paclitaxel treatment of MDA-MB-231 cells into which the genes for the mutant proteins were transfected, The Delta loop mutant completely inhibited paclitaxel-induced apoptosis, In cells expressing the S70A mutant, paclitaxel induced about one-third the level of apoptosis seen with wild-type Bcl-2, To evaluate the role of mitogen-activated protein kinases (MAPKs) in Bcl-2 phosphorylation, the activation of c-jun N-terminal kinase (JNK), extracellular signal-regulated kinase (ERK, and p38 was examined. Paclitaxel induced apoptosis was associated with phosphorylation of Bcl-2 and activation of ERK and JNK MAPKs. If JNK activation was blocked by transfections with either a stress-activated protein kinase kinase dominant-negative (K-->R) gene (which prevents the activation of a kinase upstream of JNK) or MAPK phosphatase-l gene (which dephosphorylates and inactivates JNK), Bcl-2 phosphorylation did not occur, and the cells were not killed by paclitaxel. By contrast, neither an ERK inhibitor (PD098059) nor p38 inhibitors (SB203580 and SB202190) had an effect on Bcl-2 phosphorylation, Thus, our data show that the antiapoptotic effects of Bcl-2 can be overcome by phosphorylation of Ser-70; forms of Bcl-2 lacking the loop region are much more effective at preventing apoptosis than wild-type Bcl-2 because they cannot be phosphorylated, JNK, but not ERK or p38 MAPK, appear to be involved in the phosphorylation of Bcl-2 induced by paclitaxel.
