Deguelin, Selective Silencing of the NPM1 Mutant Protein, Induces Differentiation and Potentiates Apoptosis in Acute Myeloid Leukemia Cells Carrying NPM1 Mutation.

Abstract 2435

Acute myeloid leukemia (AML) carrying nucleophosmin (NPM1) gene mutations, leading to aberrant cytoplasmic expression of the nucleolar protein NPM1, accounts for about one-third of adult AML and shows distinctive biological and clinical features. In spite of the relatively good prognosis of NPM1-mutated AML, there are still cases that show poorer outcome, especially those associated with FLT3-ITD mutation and elderly patient population. Therefore new therapeutic strategies need to be explored.

Deguelin, a rotenoid isolated from several plant species including Mondulea sericea (Leguminosae), exhibits significant inhibitory effects and induce apoptosis in a variety of cancer cell lines in vitro and in vivo. Researches have been revealed that NPM1 wild type (NPM-wt) and NPM1 mutant protein are potential therapeutic targets for AML cells harboring NPM1 mutations, however, isotype-specific inhibitors remain to be developed. Our previous study found deguelin induced apoptosis on Jurkat cells by disrupting NPM1 expression. Thus, in the present report we investigate the effects and molecular mechanisms of deguelin on AML NPMc+ cells, with focus on the NPM1 protein.

Deguelin exerted dose-dependent dural effects on cell line OCI-AML3 carrying NPM1 mutation but not in the OCIM2 cell line (not harboring NPM1 gene mutation). Deguelin strongly induced OCI-AML3 cell apoptosis as shown by annexin V-fluoroisothyocyanate analysis (from 6.8% to 65.9%, 48 h at 32 μM), while apoptosis was minimal in OCIM2 cells (from 2.2% to 11.7%, 48h at 32 μM), which was further confirmed by a western blot assay to evaluate the activation of caspase-3 and the cleavage of PARP. Because of the pivotal role of NPM1 in AML cell survival and proliferation, we explored whether the above effects of deguelin were mediated by interfering with NPM1. Western blot analysis using specific antibodies showed marked downregulation of the leukemic NPM1 mutant protein upon deguelin treatment (0, 4 and 8μM, 48h), while NPM1-wt protein levels reminded unchanged. In addition, western blot and caspase activity assay revealed that the pro-apoptosis effect of deguelin was associated with caspase-6 and −8 activations, which might be caused by NPM mutation protein downregulation. In nontoxic concentration, morphologic features of granulocytic/monocytic differentiation were evident after 8 days treatment with dose of deguelin of 2μM in OCI-AML3 cells whereas no changes were found in OCIM2 cells. A more precise evaluation of cell differentiation by detecting CD11b (from 11.5% to 43.0%, P<0.01) and CD14 (from 1.2% to 11.1%, P<0.05) indicated that the proportion of the differentiation antigens increased in deguelin-treated cells. In accordance with the above results, NPM1 mutant protein was significant downregulated in OCI-AML3 cells even in nontoxic concentration of deguelin (from 0 to 2 μM, at 8 days), accompanied by p53, p21 and the 30kD band of C/EBPα decreasing, while there is no change in the protein level of NPM-wt, p53, p21 and C/EBPα in OCIM2 cells. Moreover, si-NPM-mut was used to further confirm the above results. Si-NPM-mut inhibited OCI-AML3 cell proliferation (inhibition rate 30% vs. control), induced differentiation (CD11b, from 4.2% to 19.2%) and caused significant decrease in the expression of pro-caspase-8, p21, p53 and the 30kD bands of C/EBPα compared with si-NC group, in consistent with the deguelin treatment in nontoxic concentration with long time duration, while it did not affect the levels of pro-caspase-6. Taken together, our results suggest that deguelin is a potent in vitro inhibitor of NPM1 mutation protein which provides the molecular basis for its anti-leukemia activities in NPMc+ AML cells, including suppression of proliferation, induction of apoptosis and differentiation.