CXCR4/YinYang1/Hsa-let-7a axis functions as tumor suppressor in acute myeloid leukemia

Surface expression of C-X-C chemokine receptor type 4 (CXCR4) in acute myeloid leukemia (AML) has been reported to be an independent prognostic factor for disease relapse and survival. We previously reported that targeting the stromal-derived factor 1α (SDF-1α)/CXCR4 axis could overcome resistance of AML cells to chemotherapy both in vitro and in vivo. To further explore the mechanism of targeting CXCR4, in the current study we focused on the regulation of microRNA. Microarray analysis revealed that the hsa-let-7a microRNA was down-regulated in OCI-AML3 cells by SDF-1α treatment and increased after CXCR4 inhibition. To further investigate the role of hsa-let-7a in leukemia biology, we overexpressed it in AML cell lines, which resulted in decreased Bcl-xL protein expression and consequently enhanced cell sensitivity to the chemotherapeutic agent cytarabine, both in vitro and in vivo. We also identified the transcription factor Yin Yang 1 (YY1) as a mediator that links the SDF-1α/CXCR4 axis with hsa-let-7a. Western blotting and immunocytochemistry demonstrated a correlation between YY1 and CXCR4 activation. ChIP assay confirmed the binding of YY1 to pri-let-7a DNA fragments. In primary AML samples (n=50), high CXCR4 surface expression was associated with low hsa-let-7a levels (r2=0.53). Improved effects of cytarabine treatment associated with greatly extended survival of human AML carrying mice was observed in primary human AML overexpressing hsa-let-7a. On the basis of these results, we propose that CXCR4 regulation of hsa-let-7a microRNA through YY1 and transcriptional silencing of the Bcl-xL protein together identifies a novel mechanism by which CXCR4 functions to induce chemoresistance in AML cells. MicroRNA expression profiling of OCI-AML3 cell lines treated with CXCR4 antagonist or SDF-1α. OCI-AML3 cells were treated with 100 ng/mL SDF-1α or 250 nM POL6326 (a CXCR4 antagonist, Allschwil, Switzerland), total RNA was extracted at specific time points (1, 2, 4, and 8 h), and miRNA expression profiling was performed