The BH3-only protein Bim plays a critical role in cell death triggered by concomitant inhibition of the PI3K/Akt and MEK/ERK pathways in human leukemia cells

2332 Mechanisms underlying apoptosis induced by concomitant interruption of the MEK/ERK1/2 and Akt pathways were investigated in human leukemia cells. Apoptosis induction by perifosine, an alkyl-lysophospholipid (ALP) that inhibits PI3K/Akt and activates ERK1/2, was dramatically potentiated by co-administration of the MEK1/2 inhibitors PD184352 or U0126 in multiple malignant hematopoietic cells. These, events were associated with pronounced JNK1/2 activation, Mcl-1 downregulation, Bim upregulation, and marked Bak/Bax conformational change accompanied by Bax membrane translocation. Enforced expression of constitutively active (myristoylated) Akt sharply reduced perifosine/PD184352-mediated lethality. Genetic or pharmacological inhibition of JNK activation by shRNA directed against JNK1/2 or by the JNK inhibitor SP600125 significantly attenuated PD184352/perifosine-induced apoptosis, but not Mcl-1 downregulation. Notably, ectopic Mcl-1 expression potently inhibited perifosine/PD184352 lethality, as did Bak or Bax knockdown.Notably, knockdown of Bim, but not Bad, with shRNA blocked Bak and Bax conformational change, Bax membrane translocation, and apoptosis induced by perifosine/PD184352. Conversely, enforced expression of mutant Bim (ser55,65,100→ala) that is unphosphorylatable by ERK1/2 significantly enhanced apoptosis induced by PI3/Akt kinase inhibitors (i.e. perifosine, LY294002, PI-103). Collectively, these findings suggest that Bim, Mcl-1, and JNK, but not Bad, integrate death signaling triggered by concomitant disruption of the PI3K/Akt and MEK1/2/ERK1/2 pathways in human leukemia cells.