Protein phosphatase-2A activation is a critical step for enzastaurin activity in chronic lymphoid leukemia cells

Targeting B-cell receptor (BCR) downstream pathways may be of therapeutic importance in eradicating chronic lymphocytic leukemia (CLL) cells. Since protein kinase C-β(II) (PKC-β(II)) is a key element of BCR signaling, we evaluated the impact of enzastaurin on cell survival. Enzastaurin classically activates glycogen synthase kinase-3β through inhibition of PKC-β, Akt and target of rapamycin pathways in cancer cell lines. Here, we show that in primary CLL cells, enzastaurin activates protein phosphatase-2A (PP-2A) to mediate dephosphorylating events in responding patients. In patients' cells, both PP-2A activation and Bcl-2 dephosphorylation are statistically linked to enzastaurin-induced CLL death. Protein phosphatase-2A inhibition, through pharmacological agents or siRNA, significantly hampers cell death induced by the drug. Despite limited activity in in vitro culture, enzastaurin is able to sensitize CLL cells to fludarabine, even in patients refractory to either agent used alone. These results argue for the use of enzastaurin in combination therapy in patients with CLL.