Letting microRNAs overcome resistance to chemotherapy in acute myeloid leukemia

MicroRNAs (miRs) are a family of small, non-coding RNAs that post-transcriptionally regulate gene expression. Over 700 miRs have been described in the human genome, but only approximately 25% are associated with regulation of genes that encode proteins [1]. miRs are expressed in a number of cell types, including endothelial cells, hepatic cells, macrophages and cancer, for example lung cancer, brain tumors, gastric cancer, chronic lymphoid leukemia and pancreatic cancer. In addition, miRs have been shown to regulate a variety of cell functions, such as cell senescence, angiogenesis and vascular inflammation. miRs can also have opposing activities: some contribute to normal function, while others lead to autoimmunity, myeloproliferation and cancer [1,2].

In normal hematopoiesis, the miR-125 cluster regulates hematopoietic stem cells (HSCs). Four miRs (miR-125b, miR-125a, miR-99a and let-7e) have been studied. Overexpression of miR-125b results in a competitive repopulation advantage compared to control HSCs, while miR-99a and let-7e overexpressing HSCs do not engraft as well [2]. In addition, these miRs affected multiple lineages, suggesting that miRs are essential for HSC maintenance and differentiation. Further studies demonstrated that overexpression of miR-125a/miR-99b/let-7e or miR-125a alone in bone marrow cells resulted in an enhanced ability to repopulate the bone marrow, with increased self-renewal capacity of HSCs but not common lymphoid progenitors (CLPs), common myeloid progenitors (CMPs), granulocyte macrophage progenitors (GMPs) or megakaryocyte erythroid progenitors (MEPs), suggesting that the activity of miRs depends on the developmental stage of cells.

Besides HSCs, miRs can regulate mature myeloid cells and their function. miR-125b overexpression in bone marrow-derived macrophages results in an activated state with “spread” morphology. Macrophages overexpressing miR-125b act more like antigen presenting cells for T cells and have increased ability to kill tumor cells [2]. At the same time, miR-125b not only regulates normal hematopoiesis, but is also up-regulated in leukemia samples, which led to investigations on whether this miR plays a role in tumorigenesis. Several groups found that overexpression of miR-125b in hematopoietic cells resulted in decreased apoptosis, increased proliferation and abnormal HSC self-renewal [3,4].

miRs also play an important role in regulation of the immune response. miR-146a not only regulates pro-inflammatory cytokine production, but also negatively regulates type I interferon production in peritoneal macrophages by targeting TRAF6, IRAK1 and IRAK2, thereby altering monocyte/macrophage function [5]. Knockdown of miR-146a in mouse hematopoietic progenitor cells leads to neutropenia and decreased colony forming activity [6]. Interestingly, miR-145 and miR-146a are located in or in close proximity to the 5q− region identified in myelodysplastic syndromes (MDS), and expression of these two microRNAs is decreased in patients with MDS [6]. Knockdown of miR-145 and -146a in mice resulted in an MDS phenotype, including thrombocytosis, megakaryocytic dysplasia and mild neutropenia. In terms of mechanism, increased nuclear factor-κB NF-κB activation appears to contribute to myeloproliferative neoplasm (MPN) and bone marrow (BM) failure, since lack of the p50 subunit significantly reduced the MDS/MPN phenotype [6].

Regulation of miRs through methylation of promoter regions can also lead to aberrant expression of critical transcription factors in transformation to acute meyloid leukemia (AML). One example is that expression of miR-34b is decreased in AML cells due to hypermethylation of its promoter, resulting in increased expression of one of its target genes, cAMP response element binding protein, or CREB [7]. miR-34b directly binds to the 3’-untranslated region of the CREB gene, thereby inhibiting CREB expression. CREB has been shown to be an important regulator of myelopoiesis and is overexpressed in the majority of AML blast cells [8]. Pigazzi et al. showed that expression of miR-34b decreased with an increase in CREB expression in bone marrow cells from patients with AML compared to patients with MDS [9]. Therefore, there appears to be a direct relationship between myeloid transformation, low miR34b expression and high CREB expression.

In this month's Letter to the Editor in the Leukemia and Lymphoma journal, Dai et al. report that miR-let-7f is down-regulated in 50 patients with refractory AML compared to 60 patients with de novo AML. In addition, the authors demonstrate that miR-let-7f is involved in chemotherapy resistance of adriamycin-resident cells [10]. Their data show that let-7f mRNA is decreased in refractory primary AML cells compared to AML cells at diagnosis or chronic phase/acute phase primary chronid myeloid leukemia (CML) cells. In addition, overexpression of let-7f in K562 cells resulted in decreased viability and increased sensitivity in response to adriamycin [10]. There did not appear to be any correlation of decreased let-7f expression and specific French–American–British (FAB) subtypes shown in Table I (Dai et al. Leuk) Lymph 2013; however, cytogenetics including translocations or molecular markers, e.g. FLT3-ITD, were not discussed. These results suggest that in a limited number of patient samples, let-7f contributes to the sensitivity of AML cells to adriamycin.

In a recent report, CXCR4 down-regulation of let-7a was found to overcome the chemotherapy resistance of AML cells [11]. Microarray analysis of OCI-AML3 cells showed that miR-let-7a was down-regulated by SDF-1α-mediated CXCR4 activation. This down-regulation was reversed by CXCR4 inhibition, and was shown to be mediated by decreased c-Myc and BCL-XL protein expression, with increased chemosensitivity to cytarabine. Primary AML cells, however, express low levels of let-7a. Furthermore, CXCR4 induces chemoresistance through YY1 induced transcriptional activation of MYC and BCL-XL in AML cells [11]. Similarly, c-myc expression was found to be up-regulated in imatinib-resistant K562 cells, which resulted in increased expression of miR-144/451. Rescuing of miR-144/451 in K562 resistant cells sensitized them to imatinib [12]. This study demonstrates that a related family member of miR-let-7f also plays a role in chemotherapy resistance.

In summary, this report suggests that miRs not only modulate chemosensitivity of AML cells, but could also be exploited to enhance the effects of chemotherapy in patients with AML in the future. Since let-7a and let-7f are related miRs and may have overlapping yet distinct targets, further studies are needed to determine the role of these two miRs and the mechanisms of chemotherapy sensitivity and resistance of AML cells. Finally, since let-7f may act as a tumor suppressor in not only AML but also other types of cancer, future work on let-7f could provide new insights into the role of miRs in transformation.

Potential conflict of interest

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