Homoharringtonine Deregulates Myc Transcriptional Expression By Directly Binding Nf-Kappa B Repressing Factor

Homoharringtonine (HHT), a known protein synthesis inhibitor, has an anti-myeloid leukemia effect and potentiates the therapeutic efficacy of anthracycline/cytarabine induction regimens for acute myelogenous leukemia (AML) with favorable and intermediate prognoses, especially in the t(8; 21) subtype. Here we provide evidence showing that HHT inhibits the activity of leukemiainitiating cells (Lin(-)/Sca-1(-)/c-kit(+); LICs) in a t(8; 21) murine leukemia model and exerts a down-regulating effect on MYC pathway genes in human t(8; 21) leukemia cells (Kasumi-1). We discovered that NF-kappa B repressing factor (NKRF) is bound directly by HHT via the second double-strand RNA-binding motif (DSRM2) domain, which is the nuclear localization signal of NKRF. A series of deletion and mutagenesis experiments mapped HHT direct binding sites to K479 and C480 amino acids in the DSRM2 domain. HHT treatment shifts NKRF from the nucleus (including nucleoli) to the cytoplasm by occupying the DSRM2 domain, strengthens the p65-NKRF interaction, and interferes with p65-p50 complex formation, thereby attenuating the transactivation activity of p65 on the MYC gene. Moreover, HHT significantly decreases the expression of KIT, a frequently mutated and/or highly expressed gene in t(8; 21) AML, in concert with MYC down-regulation. Our work thus identifies a mechanism of action of HHT that is different from, but acts in concert with, the known mode of action of this compound. These results justify further clinical testing of HHT in AML.