Molecular Mechanisms Underlying the Synergism of Erib and Hht in Treating t(8;21) AML with C-KIT mutations

AML1-ETO oncoprotein (AE) resulting from t(8;21)(q22;q22) translocation is known to play a pivotal role in leukemogenesis of t(8;21) AML, yet the activating mutations or over-expression of C-KIT occur in half of the AML patients with t(8;21). Animal models have proven that activating C-KIT mutations cooperate with AE in causing overt AML. The aberrant C-KIT expression was thus thought to be the major “second hit” in stepwise leukemogenesis of t(8;21) AML.

Previous studies have demonstrated that Eriocalyxin B (EriB), a diterpenoid extracted from Isodon eriocalyx, induced apoptosis in t(8;21) leukemia cells through the NF-κB and MAPK signaling pathways and triggered degradation of AE dependent on caspase-3. By targeting AML1-ETO oncoprotein and activating apoptosis pathways simultaneously, EriB has potential to treat the t(8;21) leukemia.

In this study, we observed the synergistic effect of EriB and the chemotherapy drug Homoharringtonine (HHT) on Kasumi-1, an AML cell line harboring t(8;21) and C-KIT mutations. Compared with using EriB or HHT alone, the combination of EriB and HHT (E+H) exhibited a higher efficacy to induce apoptosis and inhibit proliferation of Kasumi-1 cells. Data further showed that HHT enhanced the AE degradation caused by EriB in a dose/time-dependent manner. In addition, HHT significantly decreased the expression of C-KIT in Kasumi-1 cells, and the combination with EriB markedly enhanced this effect. Mechanism study revealed that E+H combination down-regulated the expression of transcriptional factor Sp1 through up-regulating miR-29b, separated Sp1 protein from Sp1/NF-κB complex, and released the Sp1/NF-κB complex from the C-KIT promoter. These data point to a mechanism involving Sp1/NF-κB/miR-29b regulatory network whereby E+H combination down-regulate C-KIT mRNA expression. In addition, HHT induced a proteasome-mediated degradation of C-KIT protein, which was further enhanced by EriB.

From these observations, we conclude that the combined use of EriB and HHT might offer a potential therapeutic avenue for t(8;21) AML with C-KIT mutations.