The SCL/TAL1 Transcription Factor Represses the Stress Protein DDiT4/REDD1 in Human Hematopoietic Stem/Progenitor Cells.

Hematopoietic stem/progenitor cells (HSPCs) are regulated through numerous molecular mechanisms that have not been interconnected. The transcription factor stem cell leukemia/T-cell acute leukemia 1 (TAL1) controls human HSPC but its mechanism of action is not clarified. In this study, we show that knockdown (KD) or short-term conditional over-expression (OE) of TAL1 in human HSPC ex vivo, respectively, blocks and maintains hematopoietic potentials, affecting proliferation of human HSPC. Comparative gene expression analyses of TAL1/KD and TAL1/OE human HSPC revealed modifications of cell cycle regulators as well as previously described TAL1 target genes. Interestingly an inverse correlation between TAL1 and DNA damage-induced transcript 4 (DDiT4/REDD1), an inhibitor of the mammalian target of rapamycin (mTOR) pathway, is uncovered. Low phosphorylation levels of mTOR target proteins in TAL1/KD HSPC confirmed an interplay between mTOR pathway and TAL1 in correlation with TAL1-mediated effects of HSPC proliferation. Finally chromatin immunoprecipitation experiments performed in human HSPC showed that DDiT4 is a direct TAL1 target gene. Functional analyses showed that TAL1 represses DDiT4 expression in HSPCs. These results pinpoint DDiT4/REDD1 as a novel target gene regulated by TAL1 in human HSPC and establish for the first time a link between TAL1 and the mTOR pathway in human early hematopoietic cells. Stem Cells 2015;33:2268-2279.

The microRNA miR-181 targets the homeobox protein Hox-A11 during mammalian myoblast differentiation.

Deciphering the mechanisms underlying skeletal muscle-cell differentiation in mammals is an important challenge. Cell differentiation involves complex pathways regulated at both transcriptional and post-transcriptional levels. Recent observations have revealed the importance of small (20-25 base pair) non-coding RNAs (microRNAs or miRNAs) that are expressed in both lower organisms and in mammals. miRNAs modulate gene expression by affecting mRNA translation or stability. In lower organisms, miRNAs are essential for cell differentiation during development; some miRNAs are involved in maintenance of the differentiated state. Here, we show that miR-181, a microRNA that is strongly upregulated during differentiation, participates in establishing the muscle phenotype. Moreover, our results suggest that miR-181 downregulates the homeobox protein Hox-A11 (a repressor of the differentiation process), thus establishing a functional link between miR-181 and the complex process of mammalian skeletal-muscle differentiation. Therefore, miRNAs can be involved in the establishment of a differentiated phenotype - even when they are not expressed in the corresponding fully differentiated tissue.

Hypoxia favors chemoresistance in T-ALL through an HIF1α-mediated mTORC1 inhibition loop.

Resistance to chemotherapy, a major therapeutic challenge in the treatment of T-cell acute lymphoblastic leukemia (T-ALL), can be driven by interactions between leukemic cells and the microenvironment that promote survival of leukemic cells. The bone marrow, an important leukemia niche, has low oxygen partial pressures that highly participate in the regulation of normal hematopoiesis. Here we show that hypoxia inhibits T-ALL cell growth by slowing down cell cycle progression, decreasing mitochondria activity, and increasing glycolysis, making them less sensitive to antileukemic drugs and preserving their ability to initiate leukemia after treatment. Activation of the mammalian target of rapamycin (mTOR) was diminished in hypoxic leukemic cells, and treatment of T-ALL with the mTOR inhibitor rapamycin in normoxia mimicked the hypoxia effects, namely decreased cell growth and increased quiescence and drug resistance. Knocking down (KD) hypoxia-induced factor 1α (HIF-1α), a key regulator of the cellular response to hypoxia, antagonized the effects observed in hypoxic T-ALL and restored chemosensitivity. HIF-1α KD also restored mTOR activation in low O2 concentrations, and inhibiting mTOR in HIF1α KD T-ALL protected leukemic cells from chemotherapy. Thus, hypoxic niches play a protective role of T-ALL during treatments. Inhibition of HIF-1α and activation of the mTORC1 pathway may help suppress the drug resistance of T-ALL in hypoxic niches.

Hypoxia Regulates Lymphoid Development of Human Hematopoietic Progenitors.

Hypoxia plays a major role in the physiology of hematopoietic and immune niches. Important clues from works in mouse have paved the way to investigate the role of low O2 levels in hematopoiesis. However, whether hypoxia impacts the initial steps of human lymphopoiesis remains unexplored. Here, we show that hypoxia regulates cellular and metabolic profiles of umbilical cord blood (UCB)-derived hematopoietic progenitor cells. Hypoxia more specifically enhances in vitro lymphoid differentiation potentials of lymphoid-primed multipotent progenitors (LMPPs) and pro-T/natural killer (NK) cells and in vivo B cell potential of LMPPs. In accordance, hypoxia exacerbates the lymphoid gene expression profile through hypoxia-inducible factor (HIF)-1α (for LMPPs) and HIF-2α (for pro-T/NK). Moreover, loss of HIF-1/2α expression seriously impedes NK and B cell production from LMPPs and pro-T/NK. Our study describes how hypoxia contributes to the lymphoid development of human progenitors and reveals the implication of the HIF pathway in LMPPs and pro-T/NK-cell lymphoid identities.

The retinoblastoma protein binds the promoter of the survival gene bcl-2 and regulates its transcription in epithelial cells through transcription factor AP-2.

The retinoblastoma (RB) gene product has been shown to restrict cell proliferation, promote cell differentiation, and inhibit apoptosis. Loss of RB function can induce both p53-dependent apoptosis and p53-independent apoptosis; little is known about the mechanisms of RB-regulated p53-independent apoptosis. Here we show that RB specifically activates transcription of the survival gene bcl-2 in epithelial cells but not in NIH 3T3 mesenchymal cells. This transcriptional activity is mediated by the transcription factor AP-2. By monitoring protein-DNA interactions in living cells using formaldehyde cross-linking and chromatin immunoprecipitation, we show that endogenous RB and AP-2 both bind to the same bcl-2 promoter sequence. In addition, we demonstrate that RB and AP-2 also bind to the E-cadherin gene promoter in vivo, consistent with regulation of this promoter by both AP-2 and RB in epithelial cells. This study provides evidence that RB activates bcl-2 and E-cadherin by binding directly to the respective promoter sequences and not indirectly by repressing an inhibitor. This recruitment is mediated by a transcription factor, in this case AP-2. For the first time, our results suggest a direct molecular mechanism by which RB might inhibit apoptosis independently of p53. The results are discussed in a context where RB and Bcl-2 contribute under nonpathological conditions to the maintenance of cell viability in association with a differentiated phenotype, contributing to the tumor suppressor function of RB and playing important roles in normal development.

Homeobox protein TLX3 activates miR-125b expression to promote T-cell acute lymphoblastic leukemia.

The oncogenic mechanisms driven by aberrantly expressed transcription factors in T-cell acute leukemia (T-ALL) are still elusive. MicroRNAs (miRNAs) play an important role in normal development and pathologies. Here, we examined the expression of 738 miRNA species in 41 newly diagnosed pediatric T-ALLs and in human thymus-derived cells. We found that expression of 2 clustered miRNAs, miR-125b/99a, peaks in primitive T cells and is upregulated in the T leukemia homeobox 3 (TLX3)-positive subtype of T-ALL. Using loss- and gain-of-function approaches, we established functional relationships between TLX3 and miR-125b. Both TLX3 and miR-125b support in vitro cell growth and in vivo invasiveness of T-ALL. Besides, ectopic expression of TLX3 or miR-125b in human hematopoietic progenitor cells enhances production of T-cell progenitors and favors their accumulation at immature stages of T-cell development resembling the differentiation arrest observed in TLX3 T-ALL. Ectopic miR-125b also remarkably accelerated leukemia in a xenograft model, suggesting that miR125b is an important mediator of the TLX3-mediated transformation program that takes place in immature T-cell progenitors. Mechanistically, TLX3-mediated activation of miR-125b may impact T-cell differentiation in part via repression of Ets1 and CBFß genes, 2 regulators of T-lineage. Finally, we established that TLX3 directly regulates miR-125b production through binding and transactivation of LINC00478, a long noncoding RNA gene, which is the host of miR-99a/Let-7c/miR-125b. Altogether, our results reveal an original functional link between TLX3 and oncogenic miR-125b in T-ALL development.

Age-Associated Decrease of the Histone Methyltransferase SUV39H1 in HSC Perturbs Heterochromatin and B Lymphoid Differentiation.

The capacity of hematopoietic stem cells (HSC) to generate B lymphocytes declines with age, contributing to impaired immune function in the elderly. Here we show that the histone methyltransferase SUV39H1 plays an important role in human B lymphoid differentiation and that expression of SUV39H1 decreases with age in both human and mouse HSC, leading to a global reduction in H3K9 trimethylation and perturbed heterochromatin function. Further, we demonstrate that SUV39H1 is a target of microRNA miR-125b, a known regulator of HSC function, and that expression of miR-125b increases with age in human HSC. Overexpression of miR-125b and inhibition of SUV39H1 in young HSC induced loss of B cell potential. Conversely, both inhibition of miR-125 and enforced expression of SUV39H1 improved the capacity of HSC from elderly individuals to generate B cells. Our findings highlight the importance of heterochromatin regulation in HSC aging and B lymphopoiesis.

Lin-28 binds IGF-2 mRNA and participates in skeletal myogenesis by increasing translation efficiency.

Lin-28 is a highly conserved, RNA-binding, microRNA-regulated protein that is involved in regulation of developmental timing in Caenorhabditis elegans. In mammals, Lin-28 is stage-specifically expressed in embryonic muscle, neurons, and epithelia, as well as in embryonic carcinoma cells, but is suppressed in most adult tissues, with the notable exception of skeletal and cardiac muscle. The specific function and mechanism of action of Lin-28 are not well understood. Here we used loss-of-function and gain-of-function assays in cultured myoblasts to show that expression of Lin-28 is essential for skeletal muscle differentiation in mice. In order to elucidate the specific function of Lin-28, we used a combination of biochemical and functional assays, which revealed that, in differentiating myoblasts, Lin-28 binds to the polysomes and increases the efficiency of protein synthesis. An important target of Lin-28 is IGF-2, a crucial growth and differentiation factor for muscle tissue. Interaction of Lin-28 with translation initiation complexes in skeletal myoblasts and in the embryonic carcinoma cell line P19 was confirmed by localization of Lin-28 to the stress granules, temporary structures that contain stalled mRNA-protein translation complexes. Our results unravel novel mechanisms of translational regulation in skeletal muscle and suggest that Lin-28 performs the role of "translational enhancer" in embryonic and adult cells and tissues.

[Micro-RNAs and muscle differentiation]. / Micro-ARNs et différenciation musculaire.

Deciphering the mechanisms underlying skeletal muscle differentiation in mammals is an important challenge. Cell differentiation involves complex pathways regulated at both transcriptional and post-transcriptional levels. Recent observations have revealed the importance of small (20-25 base pairs) non-coding RNAs (microRNAs or miRNAs) that are expressed in both lower organisms and in mammals. miRNAs modulate gene expression by affecting mRNA translation or stability. In lower organisms, miRNAs are essential for cell differentiation during development; some miRNAs are involved in maintenance of the differentiated state. We have shown that miR-181, a microRNA that is strongly upregulated during differentiation, participates in establishing the muscle phenotype. Moreover, our results suggest that miR-181 downregulates the homeobox protein Hox-A11 (a repressor of the differentiation process), thus establishing a functional link between miR-181 and the complex process of mammalian skeletal muscle differentiation. Therefore, miRNAs can be involved in the establishment of a differentiated phenotype - even when they are not expressed in the corresponding fully differentiated tissue.

Conditional gene knock-down by CRE-dependent short interfering RNAs.

Short interfering RNAs (siRNAs) are short (21-23 nt) double-stranded RNAs that direct the sequence-specific degradation of corresponding mRNAs, resulting in suppression of gene activity. siRNAs are powerful tools for gene functional analysis in mammals. Chemically synthesized siRNAs permit transient gene repression but preclude inhibition of stable gene products as well as long-term phenotypic analyses. Permanent gene suppression can be achieved by transcribing siRNAs as stem-loop precursors from Pol III promoters. This approach, however, has a major limitation: inhibition cannot be controlled in a time- or tissue-specific manner. Thus, the approach cannot be applied to genes essential for cell survival or cell proliferation. To overcome these limitations, we have designed a CRE-lox-based strategy that allows one to repress gene activity in a time-dependent manner in cells, and in a time- or tissue-dependent manner in animals. Our approach promises to improve dramatically the procedures for functional genetics in mammals.

Synthetic small inhibiting RNAs: efficient tools to inactivate oncogenic mutations and restore p53 pathways.

Single base pair mutations that alter the function of tumor suppressor genes and oncogenes occur frequently during oncogenesis. The guardian of the genome, p53, is inactivated by point mutation in more than 50% of human cancers. Synthetic small inhibiting RNAs (siRNAs) can suppress gene expression in mammalian cells, although their degree of selectivity might be compromised by an amplification mechanism. Here, we demonstrate that a single base difference in siRNAs discriminates between mutant and WT p53 in cells expressing both forms, resulting in the restoration of WT protein function. Therefore, siRNAs may be used to suppress expression of point-mutated genes and provide the basis for selective and personalized antitumor therapy.

A Suv39h-dependent mechanism for silencing S-phase genes in differentiating but not in cycling cells.

The Rb/E2F complex represses S-phase genes both in cycling cells and in cells that have permanently exited from the cell cycle and entered a terminal differentiation pathway. Here we show that S-phase gene repression, which involves histone-modifying enzymes, occurs through distinct mechanisms in these two situations. We used chromatin immunoprecipitation to show that methylation of histone H3 lysine 9 (H3K9) occurs at several Rb/E2F target promoters in differentiating cells but not in cycling cells. Furthermore, phenotypic knock-down experiments using siRNAs showed that the histone methyltransferase Suv39h is required for histone H3K9 methylation and subsequent repression of S-phase gene promoters in differentiating cells, but not in cycling cells. These results indicate that the E2F target gene permanent silencing mechanism that is triggered upon terminal differentiation is distinct from the transient repression mechanism in cycling cells. Finally, Suv39h-depleted myoblasts were unable to express early or late muscle differentiation markers. Thus, appropriately timed H3K9 methylation by Suv39h seems to be part of the control switch for exiting the cell cycle and entering differentiation.