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Chebulinic acid, an ellagitannin found in the fruits of Terminalia chebula, has been extensively used in traditional Indian system of medicine. It has shown to have various biological activities including antitumor activity. The present study aims to investigate the cytotoxic potential of chebulinic acid in human myeloid leukemia cells. Interestingly, chebulinic acid caused apoptosis of acute promyelocytic leukemia HL-60 and NB4 cells but not K562 cells. In vitro antitumor effects of chebulinic acid were investigated by using various acute myeloid leukemia cell lines. Chebulinic acid treatment to HL-60 and NB4 cells induced caspase activation, cleavage of poly(ADP-ribose) polymerase, DNA fragmentation, chromatin condensation, and changes in the mitochondrial membrane permeability. Additionally, inhibition of caspase activation drastically reduced the chebulinic acid-induced apoptosis of acute promyelocytic leukemia cells. Our data also demonstrate that chebulinic acid-induced apoptosis in HL-60 and NB4 cells involves activation of extracellular signal-regulated kinases, which, when inhibited with ERK inhibitor PD98059, mitigates the chebulinic acid-induced apoptosis. Taken together, our findings exhibit the selective potentiation of chebulinic acid-induced apoptosis in acute promyelocytic leukemia cells. Copyright © 2017 John Wiley & Sons, Ltd.

Granulocyte colony-stimulating factor receptor (G-CSFR) plays a crucial role in regulating myeloid cell survival, proliferation, and neutrophilic granulocyte precursor cells maturation. Previously, we demonstrated that Fbw7α negatively regulates G-CSFR and its downstream signaling through ubiquitin-proteasome mediated degradation. However, whether additional ubiquitin ligases for G-CSFR exist is not known. Identifying multiple E3 ubiquitin ligases for G-CSFR shall improve our understanding of activation and subsequent attenuation of G-CSFR signaling required for differentiation and proliferation. Here, for the first time we demonstrate that E6 associated protein (E6AP), an E3 ubiquitin ligase physically associates with G-CSFR and targets it for ubiquitin-mediated proteasome degradation and thereby attenuates its functions. We further show that E6AP promoted G-CSFR degradation leads to reduced phosphorylation of signal transducer and activator of transcription 3 (STAT3) which is required for G-CSF dependent granulocytic differentiation. More importantly, our finding shows that E6AP also targets mutant form of G-SCFR (G-CSFR-T718), frequently observed in severe congenital neutropenia (SCN) patients that very often culminate to AML, however, at a quite slower rate than wild type G-CSFR. In addition, our data showed that knockdown of E6AP restores G-CSFR and its signaling thereby promoting granulocytic differentiation. Collectively, our data demonstrates that E6AP facilitates ubiquitination and subsequent degradation of G-CSFR leading to attenuation of its downstream signaling and inhibition of granulocytic differentiation.