As a transcription factor HOXA regulates
As a transcription factor, HOXA9 regulates multiple target genes related to cell growth, including BRCA1 , BCL-2 , c-MYB , IGF1 , PIM1  and so on. In addition, HOXA9 is a member of both activating and repressive transcriptional regulatory complexes, along with cofactor and collaborator proteins that provide target specificity and stabilization on the DNA, and epigenetic modifiers and transcriptional machinery , . Previous studies suggested that HOXA9 form triple complexes with members of the PBX or MEIS family, which enhances DNA binding affinity and provides selectivity , . However, the target genes of HOXA9 involved in trophoblast behavior still warrant investigation. In this study, direct evidence for an effect of HOXA9 on the EPHB4 transcription was provided by luciferase reporter and chromatin immunoprecipitation assays, which demonstrated that HOXA9 activated the EPHB4 promoter in HTR-8/SVneo cells. What\'s more, since restoration of EPHB4 expression rescued the HTR-8/SVneo Pertussis Toxin from HOXA9-mediated phenotypes in migration and invasion, our studies indicate that HOXA9 may exert its functions, at least in part, through activating the transcription of EPHB4 in HTR-8/SVneo cells.
Conflict of interest statement
Acknowledgment This work was supported by National Natural Science Foundation of China (No. 81370733).
Introduction Most chronic myeloid leukemia (CML) patients in a chronic phase treated with imatinib (IM) have been well controlled; however, some patients still relapse and/or progress to an accelerated phase or blast crisis . Presently, the new second generation tyrosine kinase inhibitors (such as dasatinib) have been proven effective in halting the oncogenic activity of most BCR-ABL mutants. Dasatinib (DAS), as currently implemented into the first line of therapy, is 300 times more potent than IM at BCR-ABL inhibition and has few side effects , . An abnormal bone marrow (BM) microenvironment, which provides a protective environment for leukemia stem cells  make leukemia cells remit chemotherapy, become resistant to drugs and thereby relapse. This is called bone marrow matrix microenvironment-mediated drug resistance. Mutations in the kinase domain of BCR-ABL are the most prevalent mechanisms of acquired kinase inhibitor resistance in patients with CML. However, some mechanisms (such as matrix microenvironments) of drug-resistance concerning a non-kinase domain of BCR-ABL are concerning. The Rho-associated coiled-coil containing protein kinase (ROCK) serine/threonine kinase is a major downstream effector of Rho GTPases that mediates membrane blebbing, enhances actin–myosin contraction, and activates caspase signaling cascades and cellular apoptosis  in the matrix microenvironment. Meanwhile, as a main regulatory factor promoting the stability of the microenvironment , EphB4 (erythropoietin-producing hepatocyte receptor B4), a member of the largest receptor protein tyrosine kinase family, plays an important role in tumorigenesis and regulates diverse cell functions , which promote tumor-adhesion-mediated drug resistance via interplay with the extracellular matrix and modulation of Rho family members . As a non-kinase resistance mechanism comes from the matrix microenvironment, recent findings have shown that EphB4/EphrinB2 signaling may be involved in the pathology of leukemia , and over expression of EphB4 is associated with IM resistance through the RhoA mechanism in CML , . DAS was proven to be effective in the treatment of IM or/and nilotinib-resistant CML patients, especially in both chronic-phase (CP) and accelerated-phase (AP) cohorts . However, fewer patients with advanced CML can reach stable remission and have to receive allo-SCT. Most patients with IM- or/and nilotinib-resistant CML still gain DAS-resistance in the treatment of DAS. Furthermore, it was not determined whether EphB4/EphrinB2 plays an important role in the survival and resistance to DAS chemotherapy in advanced CML. To determine the role and mechanism of EphB4 in DAS resistance to advanced CML, we investigated the role of EphB4 and explored the EphB4-regulated RhoA molecule signals in human CML and K562 cell lines.