PU.1 is dispensable to block erythroid differentiation in Friend erythroleukemia cells

Mbd2-CP2c loop drives adult-type globin gene expression and definitive erythropoiesis

During hematopoiesis, red blood cells originate from the hematopoietic stem cell reservoir. Although the regulation of erythropoiesis and globin expression has been intensively investigated, the underlining mechanisms are not fully understood, including the interplay between transcription factors and epigenetic factors. Here, we uncover that the Mbd2-free NuRD chromatin remodeling complex potentiates erythroid differentiation of proerythroblasts via managing functions of the CP2c complexes. We found that both Mbd2 and Mbd3 expression is downregulated during differentiation of MEL cells in vitro and in normal erythropoiesis in mouse bone marrow, and Mbd2 downregulation is crucial for erythropoiesis. In uninduced MEL cells, the Mbd2-NuRD complex is recruited to the promoter via Gata1/Fog1, and, via direct binding through p66α, it acts as a transcriptional inhibitor of the CP2c complexes, preventing their DNA binding and promoting degradation of the CP2c family proteins to suppress globin gene expression. Conversely, during erythropoiesis in vitro and in vivo, the Mbd2-free NuRD does not dissociate from the chromatin and acts as a transcriptional coactivator aiding the recruitment of the CP2c complexes to chromatin, and thereby leading to the induction of the active hemoglobin synthesis and erythroid differentiation. Our study highlights the regulation of erythroid differentiation by the Mbd2-CP2c loop.

CRISPR/Cas9-mediated deletion of the Wiskott-Aldrich syndrome locus causes actin cytoskeleton disorganization in murine erythroleukemia cells

Wiskott-Aldrich syndrome (WAS) is a recessive X-linked inmmunodeficiency caused by loss-of-function mutations in the gene encoding the WAS protein (WASp). WASp plays an important role in the polymerization of the actin cytoskeleton in hematopoietic cells through activation of the Arp2/3 complex. In a previous study, we found that actin cytoskeleton proteins, including WASp, were silenced in murine erythroleukemia cells defective in differentiation. Here, we designed a CRISPR/Cas9 strategy to delete a 9.5-kb genomic region encompassing the Was gene in the X chromosome of murine erythroleukemia (MEL) cells. We show that Was-deficient MEL cells have a poor organization of the actin cytoskeleton that can be recovered by restoring Was expression. We found that whereas the total amount of actin protein was similar between wild-type and Was knockout MEL cells, the latter exhibited an altered ratio of monomeric G-actin to polymeric F-actin. We also demonstrate that Was overexpression can mediate the activation of Bruton’s tyrosine kinase. Overall, these findings support the role of WASp as a key regulator of F-actin in erythroid cells.

Differential gene expression analysis by RNA-seq reveals the importance of actin cytoskeletal proteins in erythroleukemia cells

Development of drug resistance limits the effectiveness of anticancer treatments. Understanding the molecular mechanisms triggering this event in tumor cells may lead to improved therapeutic strategies. Here we used RNA-seq to compare the transcriptomes of a murine erythroleukemia cell line (MEL) and a derived cell line with induced resistance to differentiation (MEL-R). RNA-seq analysis identified a total of 596 genes (Benjamini–Hochberg adjusted p-value < 0.05) that were differentially expressed by more than two-fold, of which 81.5% (486/596) of genes were up-regulated in MEL cells and 110 up-regulated in MEL-R cells. These observations revealed that for some genes the relative expression of mRNA amount in the MEL cell line has decreased as the cells acquired the resistant phenotype. Clustering analysis of a group of genes showing the highest differential expression allowed identification of a sub-group among genes up-regulated in MEL cells. These genes are related to the organization of the actin cytoskeleton network. Moreover, the majority of these genes are preferentially expressed in the hematopoietic lineage and at least three of them, Was (Wiskott Aldrich syndrome), Btk (Bruton’s tyrosine kinase) and Rac2, when mutated in humans, give rise to severe hematopoietic deficiencies. Among the group of genes that were up-regulated in MEL-R cells, 16% of genes code for histone proteins, both canonical and variants. A potential implication of these results on the blockade of differentiation in resistant cells is discussed.

DNA methylation-mediated silencing of PU.1 in leukemia cells resistant to cell differentiation

In mice, the proviral integration of the Friend Spleen Focus Forming Virus (SFFV) within the PU.1 locus of erythroid precursors results in the development of erythroleukemia. SFFV integrates several kilobases upstream of the PU.1 transcription initiation start site leading to the constitutive activation of the gene which in turn results in a block of erythroid differentiation. In this study we have mapped and sequenced the exact location of the retroviral integration site. We have shown that SFFV integrates downstream of a previously described upstream regulatory element (URE), precisely 2,976 bp downstream of the URE-distal element. We have also found that SFFV persists integrated within the same location in resistant cell lines that have lost their differentiation capacity and in which case PU.1 remains silent. We have examined the methylation status of PU.1 and found that in resistant cells the nearby CpG islands remained methylated in contrast to a non-methylated status of the parental cell lines. Treatment with 5-aza-2′-deoxycytidine caused resistant cells to differentiate yet only when combined with HMBA. Altogether these results strongly suggest that methylation plays a crucial role with regard to PU.1 silencing. However, although demethylation is required, it is not sufficient to overcome the differentiation impasse. We have also showed that activation blockage of the Epo/Epo-R pathway remains despite of the absence of PU.1.

Emodin can induce K562 cells to erythroid differentiation and improve the expression of globin genes

In China, the traditional Chinese medicine "YiSui ShenXu Granule" has been used for treating β-thalassemia over 20 years and known to be effective in clinic. Several purified components from "YiSui ShenXu Granule" are tested in K562 cells to reveal its effect on globin expression and erythroid differentiation, and one of the purified components, emodin, was demonstrated to increase the expression of α-, ε-, γ-globin, CD235a, and CD71 in K562 cells. Moreover, the increase of their expression is emodin concentration-dependent. The mRNA and microRNA (miRNA) expression profiles are further analyzed and 417 mRNAs and 35 miRNAs with differential expression between untreated and emodin-treated K562 cells were identified. Among them, two mRNAs that encode known positive regulators of erythropoiesis, ALAS2, and c-KIT respectively, increased during emodin-induced K562 erythroid differentiation, meanwhile, two negative regulators, miR-221 and miR-222, decreased during this process. These results indicate that emodin can improve the expression of globin genes in K562 cells and also induce K562 cells to erythroid differentiation possibly through up-regulating ALAS2 and c-KIT and down-regulating miR-221 and miR-222.

DNA replication fading as proliferating cells advance in their commitment to terminal differentiation

Terminal differentiation is the process by which cycling cells stop proliferating to start new specific functions. It involves dramatic changes in chromatin organization as well as gene expression. In the present report we used cell flow cytometry and genome wide DNA combing to investigate DNA replication during murine erythroleukemia-induced terminal cell differentiation. The results obtained indicated that the rate of replication fork movement slows down and the inter-origin distance becomes shorter during the precommitment and commitment periods before cells stop proliferating and accumulate in G1. We propose this is a general feature caused by the progressive heterochromatinization that characterizes terminal cell differentiation.