We have previously demonstrated constitutive activation of MAPK signaling in 70% of primary AML samples (

Millela et al, JCI 108:851–859, 2001
), suggesting that upstream kinases (Raf and MEK) may play a role in the leukemic transformation of myeloid cells. BAY 43-9006 is a small molecule Raf kinase inhibitor that has demonstrated potent anti-tumor activity against solid human tumors in xenograft models. In this study, we tested the hypothesis that BAY 43-9006 inhibits leukemia cell growth and/or induces apoptosis by suppressing the activity of the MAPK pathway. In the in vitro kinase assay, BAY 43-9006 inhibited both Raf-1 and B-Raf-mediated MEK1 phosphorylation in a dose-dependent manner, with Raf-1 kinase being more sensitive to the inhibitory effects of BAY 43-9006 (IC50Raf-1, 1.37 μM vs. IC50B-Raf, 4.64 μM). BAY 43-9006 suppressed MEK1/2 and ERK phosphorylation in the AML cell lines OCI-AML3, HL-60, U937 and KG-1 in a dose-dependent manner after 24 hr treatment. Unexpectedly, BAY 43-9006 also inhibited AKT phosphorylation on Ser473 (after 4.5 hrs). BAY 43-9006 inhibited growth of AML cells in a dose- and time-dependent manner. The 50% inhibitory concentration (IC50) of BAY 43-9006 was 0.39, 1.14, 2.86 and 2.80 μM, respectively in OCI-AML3, HL-60, U937 and KG-1 cells after 72 hrs. This growth-inhibitory effect was mediated by a dose-dependent induction of cell cycle arrest in G1 mediated by the down-regulation of the cell cycle-related proteins cyclin E, cdk2 and cdc2, followed by induction of apoptosis after 72 hrs. In primary AML patient samples, BAY 43-9006 not only inhibited cell growth and induced apoptosis after 48–72 hrs in vitro, but also preferentially inhibited colony formation of AML progenitor cells compared to normal bone marrow cells [IC50: 2.33 μM vs. 9.34μM (CFU-GM), 5.69 μM (Erythroid) and 3.75 μM (Mixed), respectively]. Time-course analyses demonstrated that BAY 43-9006 suppressed phosphorylation of the pro-apoptotic protein Bim (at 4.5 hrs), caused loss of the mitochondrial membrane potential and cytochrome c release (at 6 hrs) followed by cleavage of caspases-3 and -9 but not of caspase-8, suggesting primary involvement of the intrinsic mitochondrial pathway. Furthermore, the pro-apoptotic proteins Bim and Bax were up-regulated after 48 hrs of BAY 43-9006 treatment, and the level of the inhibitor-of-apoptosis protein Survivin was down-regulated after 48 hrs. In summary, our data demonstrates that BAY 43-9006 inhibits Raf-MEK-ERK signaling and induces apoptosis in AML via Bim de-phosphorylation and activation of the intrinsic apoptotic pathway. The potential of BAY 43-9006 in the therapy of AML patients will be tested in a Phase I clinical trial.

Topics:
apoptosis, bone marrow cells, caspase-8, caspases, cdc2 protein kinase, cdk2 protein, human, cell cycle arrest, cell growth, cell lines, cyclin e

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2005, The American Society of Hematology
2005
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