The Notch Delta-4 ligand helps to maintain the quiescence and the short-term reconstitutive potential of haematopoietic progenitor cells through activation of a key gene network

Vascular Notch Signaling in Stress Hematopoiesis

Canonical Notch signaling is one of the most conserved signaling cascades. It regulates cell proliferation, cell differentiation, and cell fate maintenance in a variety of biological systems during development and cancer (Fortini, 2009; Kopan and Ilagan, 2009; Andersson et al., 2011; Ntziachristos et al., 2014). For the hematopoietic system, during embryonic development, Notch1 is essential for the emergence of hematopoietic stem cells (HSCs) at the aorta-gornado-mesonephro regions of the dorsal aorta. At adult stage, Notch receptors and Notch targets are expressed at different levels in diverse hematopoietic cell types and influence lineage choices. For example, Notch specifies T cell lineage over B cells. However, there has been a long-lasting debate on whether Notch signaling is required for the maintenance of adult HSCs, utilizing transgenic animals inactivating different components of the Notch signaling pathway in HSCs or niche cells. The aims of the current mini-review are to summarize the evidence that disapproves or supports such hypothesis and point at imperative questions waiting to be addressed; hence, some of the seemingly contradictory findings could be reconciled. We need to better delineate the Notch signaling events using biochemical assays to identify direct Notch targets within HSCs or niche cells in specific biological context. More importantly, we call for more elaborate studies that pertain to whether niche cell type (vascular endothelial cells or other stromal cell)-specific Notch ligands regulate the differentiation of T cells in solid tumors during the progression of T-lymphoblastic lymphoma (T-ALL) or chronic myelomonocytic leukemia (CMML). We believe that the investigation of vascular endothelial cells' or other stromal cell types' interaction with hematopoietic cells during homeostasis and stress can offer insights toward specific and effective Notch-related therapeutics.

Junctional Adhesion Molecule 2 Represents a Subset of Hematopoietic Stem Cells with Enhanced Potential for T Lymphopoiesis

The distinct lineage potential is a key feature of hematopoietic stem cell (HSC) heterogeneity, but a subset of HSCs specialized for a single lymphoid compartment has not been identified. Here we report that HSCs expressing junctional adhesion molecule 2 (Jam2) at a higher level (Jam2^high HSCs) have a greater T cell reconstitution capacity. Jam2^high HSCs are metabolically dormant but preferentially differentiate toward lymphocytes, especially T cell lineages. Jam2^high HSCs uniquely express T cell-related genes, and the interaction with Jam1 facilitates the Notch/Delta signaling pathway. Frequency of Jam2^high HSCs changes upon T cell depletion in vivo, potentially suggesting that Jam2 expression may reflect scarcity of T cells and requirement of T cell replenishment. Our findings highlight Jam2 as a potential marker for a subfraction of HSCs with an extensive lymphopoietic capacity, mainly in T lymphopoiesis.

POLR1D promotes colorectal cancer progression and predicts poor prognosis of patients

RNA polymerase I subunit D (POLR1D), which is involved in synthesis of ribosomal RNA precursors and small RNAs, has been shown to be overexpressed in several human cancer types. Nevertheless, the role of POLR1D in the progression of colorectal cancer (CRC) remains unknown. The following study aimed to investigate the role and underlying mechanism of POLR1D in CRC progression. In this report, we found that POLR1D was significantly up-regulated in CRC through data mining of oncomine database. Furthermore, the immunohistochemistry (IHC) staining of a tissue microarray (TMA) of 75 human CRC patients showed that the expression level of POLR1D was positively correlated to tumor size and poor survival of CRC patients. Aberrant expression of POLR1D significantly promoted cell proliferation and migration in vitro, as well as tumor growth in vivo. Conversely, POLR1D knockdown displayed the opposite effects. The flow Cytometry assays showed that POLR1D fostered cell cycle progression at G1-S transition and inhibited cell apoptosis. Finally, at the molecular level, we demonstrated that POLR1D-induced the promotion of G1-S cell cycle transition was mediated by activation of wnt-β-catenin signaling and inactivation of p53 signaling. Our results suggested that POLR1D may function as a risk factor for predicting the outcome of CRC patients, as well as a potential therapeutic target for CRC.

PUMILIO/FOXP1 signaling drives expansion of hematopoietic stem/progenitor and leukemia cells

RNA-binding proteins (RBPs) have emerged as important regulators of invertebrate adult stem cells, but their activities remain poorly appreciated in mammals. Using a short hairpin RNA strategy, we demonstrate here that the 2 mammalian RBPs, PUMILIO (PUM)1 and PUM2, members of the PUF family of posttranscriptional regulators, are essential for hematopoietic stem/progenitor cell (HSPC) proliferation and survival in vitro and in vivo upon reconstitution assays. Moreover, we found that PUM1/2 sustain myeloid leukemic cell growth. Through a proteomic approach, we identified the FOXP1 transcription factor as a new target of PUM1/2. Contrary to its canonical repressive activity, PUM1/2 rather promote FOXP1 expression by a direct binding to 2 canonical PUM responsive elements present in the FOXP1-3' untranslated region (UTR). Expression of FOXP1 strongly correlates with PUM1 and PUM2 levels in primary HSPCs and myeloid leukemia cells. We demonstrate that FOXP1 by itself supports HSPC and leukemic cell growth, thus mimicking PUM activities. Mechanistically, FOXP1 represses the expression of the p21^-CIP1 and p27^-KIP1 cell cycle inhibitors. Enforced FOXP1 expression reverses shPUM antiproliferative and proapoptotic activities. Altogether, our results reveal a novel regulatory pathway, underscoring a previously unknown and interconnected key role of PUM1/2 and FOXP1 in regulating normal HSPC and leukemic cell growth.

The role of GATA2 in lethal prostate cancer aggressiveness

Advanced prostate cancer is a classic example of the intractability and consequent lethality that characterizes metastatic carcinomas. Novel treatments have improved the survival of men with prostate cancer; however, advanced prostate cancer invariably becomes resistant to these therapies and ultimately progresses to a lethal metastatic stage. Consequently, detailed knowledge of the molecular mechanisms that control prostate cancer cell survival and progression towards this lethal stage of disease will benefit the development of new therapeutics. The transcription factor endothelial transcription factor GATA-2 (GATA2) has been reported to have a key role in driving prostate cancer aggressiveness. In addition to being a pioneer transcription factor that increases androgen receptor (AR) binding and activity, GATA2 regulates a core subset of clinically relevant genes in an AR-independent manner. Functionally, GATA2 overexpression in prostate cancer increases cellular motility and invasiveness, proliferation, tumorigenicity, and resistance to standard therapies. Thus, GATA2 has a multifaceted function in prostate cancer aggressiveness and is a highly attractive target in the development of novel treatments against lethal prostate cancer.

TNF-alpha and Notch signaling regulates the expression of HOXB4 and GATA3 during early T lymphopoiesis

During the early thymus colonization, Notch signaling activation on hematopoietic progenitor cells (HPCs) drives proliferation and T cell commitment. Although these processes are driven by transcription factors such as HOXB4 and GATA3, there is no evidence that Notch directly regulates their transcription. To evaluate the role of NOTCH and TNF signaling in this process, human CD34⁺ HPCs were cocultured with OP9-DL1 cells, in the presence or absence of TNF. The use of a Notch signaling inhibitor and a protein synthesis inhibitor allowed us to distinguish primary effects, mediated by direct signaling downstream Notch and TNF, from secondary effects, mediated by de novo synthesized proteins. A low and physiologically relevant concentration of TNF promoted T lymphopoiesis in OP9-DL1 cocultures. TNF positively modulated the expression of both transcripts in a Notch-dependent manner; however, GATA3 induction was mediated by a direct mechanism, while HOXB4 induction was indirect. Induction of both transcripts was repressed by a GSK3β inhibitor, indicating that activation of canonical Wnt signaling inhibits rather than induces their expression. Our study provides novel evidences of the mechanisms integrating Notch and TNF-alpha signaling in the transcriptional induction of GATA3 and HOXB4. This mechanism has direct implications in the control of self-renewal, proliferation, commitment, and T cell differentiation.

Stem Cell Ribonomics: RNA-Binding Proteins and Gene Networks in Stem Cell Differentiation

Stem cells are undifferentiated cells with the ability to self-renew and the potential to differentiate into all body cell types. Stem cells follow a developmental genetic program and are able to respond to alterations in the environment through various signaling pathways. The mechanisms that control these processes involve the activation of transcription followed by a series of post-transcriptional events. These post-transcriptional steps are mediated by the interaction of RNA-binding proteins (RBPs) with defined subpopulations of RNAs creating a regulatory gene network. Characterizing these RNA-protein networks is essential to understanding the regulatory mechanisms underlying the control of stem cell fate. Ribonomics is the combination of classical biochemical purification protocols with the high-throughput identification of transcripts applied to the functional characterization of RNA-protein complexes. Here, we describe the different approaches that can be used in a ribonomic approach and how they have contributed to understanding the function of several RBPs with central roles in stem cell biology.