Mesenchymal stromal cell senescence in haematological malignancies.

Acute myeloid leukaemia (AML), chronic lymphocytic leukaemia (CLL), and multiple myeloma (MM) are age-related haematological malignancies with defined precursor states termed myelodysplastic syndrome (MDS), monoclonal B-cell lymphocytosis (MBL), and monoclonal gammopathy of undetermined significance (MGUS), respectively. While the progression from asymptomatic precursor states to malignancy is widely considered to be mediated by the accumulation of genetic mutations in neoplastic haematopoietic cell clones, recent studies suggest that intrinsic genetic changes, alone, may be insufficient to drive the progression to overt malignancy. Notably, studies suggest that extrinsic, microenvironmental changes in the bone marrow (BM) may also promote the transition from these precursor states to active disease. There is now enhanced focus on extrinsic, age-related changes in the BM microenvironment that accompany the development of AML, CLL, and MM. One of the most prominent changes associated with ageing is the accumulation of senescent mesenchymal stromal cells within tissues and organs. In comparison with proliferating cells, senescent cells display an altered profile of secreted factors (secretome), termed the senescence-associated-secretory phenotype (SASP), comprising proteases, inflammatory cytokines, and growth factors that may render the local microenvironment favourable for cancer growth. It is well established that BM mesenchymal stromal cells (BM-MSCs) are key regulators of haematopoietic stem cell maintenance and fate determination. Moreover, there is emerging evidence that BM-MSC senescence may contribute to age-related haematopoietic decline and cancer development. This review explores the association between BM-MSC senescence and the development of haematological malignancies, and the functional role of senescent BM-MSCs in the development of these cancers.

Supporting gut health with medicinal cannabis in people with advanced cancer: potential benefits and challenges.

The side effects of cancer therapy continue to cause significant health and cost burden to the patient, their friends and family, and governments. A major barrier in the way in which these side effects are managed is the highly siloed mentality that results in a fragmented approach to symptom control. Increasingly, it is appreciated that many symptoms are manifestations of common underlying pathobiology, with changes in the gastrointestinal environment a key driver for many symptom sequelae. Breakdown of the mucosal barrier (mucositis) is a common and early side effect of many anti-cancer agents, known to contribute (in part) to a range of highly burdensome symptoms such as diarrhoea, nausea, vomiting, infection, malnutrition, fatigue, depression, and insomnia. Here, we outline a rationale for how, based on its already documented effects on the gastrointestinal microenvironment, medicinal cannabis could be used to control mucositis and prevent the constellation of symptoms with which it is associated. We will provide a brief update on the current state of evidence on medicinal cannabis in cancer care and outline the potential benefits (and challenges) of using medicinal cannabis during active cancer therapy.

More than a small adult brain: Lessons from chemotherapy-induced cognitive impairment for modelling paediatric brain disorders.

Childhood is recognised as a period of immense physical and emotional development, and this, in part, is driven by underlying neurophysiological transformations. These neurodevelopmental processes are unique to the paediatric brain and are facilitated by augmented rates of neuroplasticity and expanded neural stem cell populations within neurogenic niches. However, given the immaturity of the developing central nervous system, innate protective mechanisms such as neuroimmune and antioxidant responses are functionally naïve which results in periods of heightened sensitivity to neurotoxic insult. This is highly relevant in the context of paediatric cancer, and in particular, the neurocognitive symptoms associated with treatment, such as surgery, radio- and chemotherapy. The vulnerability of the developing brain may increase susceptibility to damage and persistent symptomology, aligning with reports of more severe neurocognitive dysfunction in children compared to adults. It is therefore surprising, given this intensified neurocognitive burden, that most of the pre-clinical, mechanistic research focuses exclusively on adult populations and extrapolates findings to paediatric cohorts. Given this dearth of age-specific research, throughout this review we will draw comparisons with neurodevelopmental disorders which share comparable pathways to cancer treatment related side-effects. Furthermore, we will examine the unique nuances of the paediatric brain along with the somatic systems which influence neurological function. In doing so, we will highlight the importance of developing in vitro and in vivo paediatric disease models to produce age-specific discovery and clinically translatable research.

Myeloperoxidase creates a permissive microenvironmental niche for the progression of multiple myeloma.

Expression of myeloperoxidase (MPO), a key inflammatory enzyme restricted to myeloid cells, is negatively associated with the development of solid tumours. Activated myeloid cell populations are increased in multiple myeloma (MM); however, the functional consequences of myeloid-derived MPO within the myeloma microenvironment are unknown. Here, the role of MPO in MM pathogenesis was investigated, and the capacity for pharmacological inhibition of MPO to impede MM progression was evaluated. In the 5TGM1-KaLwRij mouse model of myeloma, the early stages of tumour development were associated with an increase in CD11b+ myeloid cell populations and an increase in Mpo expression within the bone marrow (BM). Interestingly, MM tumour cell homing was increased towards sites of elevated myeloid cell numbers and MPO activity within the BM. Mechanistically, MPO induced the expression of key MM growth factors, resulting in tumour cell proliferation and suppressed cytotoxic T-cell activity. Notably, tumour growth studies in mice treated with a small-molecule irreversible inhibitor of MPO (4-ABAH) demonstrated a significant reduction in overall MM tumour burden. Taken together, our data demonstrate that MPO contributes to MM tumour growth, and that MPO-specific inhibitors may provide a new therapeutic strategy to limit MM disease progression.

Macrophages in multiple myeloma: key roles and therapeutic strategies.

Macrophages are a vital component of the tumour microenvironment and crucial mediators of tumour progression. In the last decade, significant strides have been made in understanding the crucial functional roles played by macrophages in the development of the plasma cell (PC) malignancy, multiple myeloma (MM). Whilst the interaction between MM PC and stromal cells within the bone marrow (BM) microenvironment has been extensively studied, we are only just starting to appreciate the multifaceted roles played by macrophages in disease progression. Accumulating evidence demonstrates that macrophage infiltration is associated with poor overall survival in MM. Indeed, macrophages influence numerous pathways critical for the initiation and progression of MM, including homing of malignant cells to BM, tumour cell growth and survival, drug resistance, angiogenesis and immune suppression. As such, therapeutic strategies aimed at targeting macrophages within the BM niche have promise in the clinical setting. This review will discuss the functions elicited by macrophages throughout different stages of MM and provide a comprehensive evaluation of potential macrophage-targeted therapies.

The Balance of Stromal BMP Signaling Mediated by GREM1 and ISLR Drives Colorectal Carcinogenesis.

BACKGROUND & AIMS: Cancer-associated fibroblasts (CAFs), key constituents of the tumor microenvironment, either promote or restrain tumor growth. Attempts to therapeutically target CAFs have been hampered by our incomplete understanding of these functionally heterogeneous cells. Key growth factors in the intestinal epithelial niche, bone morphogenetic proteins (BMPs), also play a critical role in colorectal cancer (CRC) progression. However, the crucial proteins regulating stromal BMP balance and the potential application of BMP signaling to manage CRC remain largely unexplored. METHODS: Using human CRC RNA expression data, we identified CAF-specific factors involved in BMP signaling, then verified and characterized their expression in the CRC stroma by in situ hybridization. CRC tumoroids and a mouse model of CRC hepatic metastasis were used to test approaches to modify BMP signaling and treat CRC. RESULTS: We identified Grem1 and Islr as CAF-specific genes involved in BMP signaling. Functionally, GREM1 and ISLR acted to inhibit and promote BMP signaling, respectively. Grem1 and Islr marked distinct fibroblast subpopulations and were differentially regulated by transforming growth factor ß and FOXL1, providing an underlying mechanism to explain fibroblast biological dichotomy. In patients with CRC, high GREM1 and ISLR expression levels were associated with poor and favorable survival, respectively. A GREM1-neutralizing antibody or fibroblast Islr overexpression reduced CRC tumoroid growth and promoted Lgr5+ intestinal stem cell differentiation. Finally, adeno-associated virus 8 (AAV8)-mediated delivery of Islr to hepatocytes increased BMP signaling and improved survival in our mouse model of hepatic metastasis. CONCLUSIONS: Stromal BMP signaling predicts and modifies CRC progression and survival, and it can be therapeutically targeted by novel AAV-directed gene delivery to the liver.

Reciprocal signaling between mTORC1 and MNK2 controls cell growth and oncogenesis.

eIF4E plays key roles in protein synthesis and tumorigenesis. It is phosphorylated by the kinases MNK1 and MNK2. Binding of MNKs to eIF4G enhances their ability to phosphorylate eIF4E. Here, we show that mTORC1, a key regulator of mRNA translation and oncogenesis, directly phosphorylates MNK2 on Ser74. This suppresses MNK2 activity and impairs binding of MNK2 to eIF4G. These effects provide a novel mechanism by which mTORC1 signaling impairs the function of MNK2 and thereby decreases eIF4E phosphorylation. MNK2[S74A] knock-in cells show enhanced phosphorylation of eIF4E and S6K1 (i.e., increased mTORC1 signaling), enlarged cell size, and increased invasive and transformative capacities. MNK2[Ser74] phosphorylation was inversely correlated with disease progression in human prostate tumors. MNK inhibition exerted anti-proliferative effects in prostate cancer cells in vitro. These findings define a novel feedback loop whereby mTORC1 represses MNK2 activity and oncogenic signaling through eIF4E phosphorylation, allowing reciprocal regulation of these two oncogenic pathways.

CKLF and IL1B transcript levels at diagnosis are predictive of relapse in children with pre-B-cell acute lymphoblastic leukaemia.

Disease relapse is the greatest cause of treatment failure in paediatric B-cell acute lymphoblastic leukaemia (B-ALL). Current risk stratifications fail to capture all patients at risk of relapse. Herein, we used a machine-learning approach to identify B-ALL blast-secreted factors that are associated with poor survival outcomes. Using this approach, we identified a two-gene expression signature (CKLF and IL1B) that allowed identification of high-risk patients at diagnosis. This two-gene expression signature enhances the predictive value of current at diagnosis or end-of-induction risk stratification suggesting the model can be applied continuously to help guide implementation of risk-adapted therapies.

A niche-dependent myeloid transcriptome signature defines dormant myeloma cells.

The era of targeted therapies has seen significant improvements in depth of response, progression-free survival, and overall survival for patients with multiple myeloma. Despite these improvements in clinical outcome, patients inevitably relapse and require further treatment. Drug-resistant dormant myeloma cells that reside in specific niches within the skeleton are considered a basis of disease relapse but remain elusive and difficult to study. Here, we developed a method to sequence the transcriptome of individual dormant myeloma cells from the bones of tumor-bearing mice. Our analyses show that dormant myeloma cells express a distinct transcriptome signature enriched for immune genes and, unexpectedly, genes associated with myeloid cell differentiation. These genes were switched on by coculture with osteoblastic cells. Targeting AXL, a gene highly expressed by dormant cells, using small-molecule inhibitors released cells from dormancy and promoted their proliferation. Analysis of the expression of AXL and coregulated genes in human cohorts showed that healthy human controls and patients with monoclonal gammopathy of uncertain significance expressed higher levels of the dormancy signature genes than patients with multiple myeloma. Furthermore, in patients with multiple myeloma, the expression of this myeloid transcriptome signature translated into a twofold increase in overall survival, indicating that this dormancy signature may be a marker of disease progression. Thus, engagement of myeloma cells with the osteoblastic niche induces expression of a suite of myeloid genes that predicts disease progression and that comprises potential drug targets to eradicate dormant myeloma cells.

Expression of the chemokine receptor CCR1 promotes the dissemination of multiple myeloma plasma cells in vivo

Multiple myeloma (MM) disease progression is dependent on the ability of MM plasma cells (PCs) to egress from the bone marrow (BM), enter the circulation and disseminate to distal BM sites. Expression of the chemokine CXCL12 by BM stromal cells is crucial for MM PC retention within the BM. However, the mechanisms which overcome CXCL12-mediated retention to enable dissemination are poorly understood. We have previously identified that treatment with the CCR1 ligand CCL3 inhibits the response to CXCL12 in MM cell lines, suggesting that CCL3/CCR1 signalling may enable egress of MM PC from the BM. Here, we demonstrated that CCR1 expression was an independent prognostic indicator in newly diagnosed MM patients. Furthermore, we showed that CCR1 is a crucial driver of dissemination in vivo, with CCR1 expression in the murine MM cell line 5TGM1 being associated with an increased incidence of bone and splenic disseminated tumours in C57BL/KaLwRij mice. Furthermore, we demonstrated that CCR1 knockout in the human myeloma cell line OPM2 resulted in a >95% reduction in circulating MM PC numbers and BM and splenic tumour dissemination following intratibial injection in NSG mice. Therapeutic targeting of CCR1 with the inhibitor CCX9588 significantly reduced OPM2 or RPMI-8226 dissemination in intratibial xenograft models. Collectively, our findings suggest a novel role for CCR1 as a critical driver of BM egress of MM PCs during tumour dissemination. Furthermore, these data suggest that CCR1 may represent a potential therapeutic target for the prevention of MM tumour dissemination.

Tyrosine kinase receptor c-ros-oncogene 1 inhibition alleviates aberrant bone formation of TWIST-1 haploinsufficient calvarial cells from Saethre-Chotzen syndrome patients.

Saethre-Chotzen syndrome (SCS), associated with TWIST-1 mutations, is characterized by premature fusion of cranial sutures. TWIST-1 haploinsufficiency, leads to alterations in suture mesenchyme cellular gene expression patterns, resulting in aberrant osteogenesis and craniosynostosis. We analyzed the expression of the TWIST-1 target, Tyrosine kinase receptor c-ros-oncogene 1 (C-ROS-1) in TWIST-1 haploinsufficient calvarial cells derived from SCS patients and calvaria of Twist-1del/+ mutant mice and found it to be highly expressed when compared to TWIST-1 wild-type controls. Knock-down of C-ROS-1 expression in TWIST-1 haploinsufficient calvarial cells derived from SCS patients was associated with decreased capacity for osteogenic differentiation in vitro. Furthermore, treatment of human SCS calvarial cells with the tyrosine kinase chemical inhibitor, Crizotinib, resulted in reduced C-ROS-1 activity and the osteogenic potential of human SCS calvarial cells with minor effects on cell viability or proliferation. Cultured human SCS calvarial cells treated with Crizotinib exhibited a dose-dependent decrease in alkaline phosphatase activity and mineral deposition, with an associated decrease in expression levels of Runt-related transcription factor 2 and OSTEOPONTIN, with reduced PI3K/Akt signalling in vitro. Furthermore, Crizotinib treatment resulted in reduced BMP-2 mediated bone formation potential of whole Twist-1del/+ mutant mouse calvaria organotypic cultures. Collectively, these results suggest that C-ROS-1 promotes osteogenic differentiation of TWIST-1 haploinsufficient calvarial osteogenic progenitor cells. Furthermore, the aberrant osteogenic potential of these cells is inhibited by the reduction of C-ROS-1. Therefore, targeting C-ROS-1 with a pharmacological agent, such as Crizotinib, may serve as a novel therapeutic strategy to alleviate craniosynostosis associated with aberrant TWIST-1 function.

Osteocalcin-dependent regulation of glucose metabolism and fertility: Skeletal implications for the development of insulin resistance.

The skeleton has recently emerged as a critical insulin target tissue that regulates whole body glucose metabolism and male reproductive function. While our understanding of these new regulatory axes remains in its infancy, the bone-specific protein, osteocalcin, has been shown to be centrally involved. Undercarboxylated osteocalcin acts as a secretagogue in a feed-forward loop to stimulate pancreatic ß-cell proliferation and insulin secretion, improve insulin sensitivity, and promote testosterone production. Importantly, dysregulation of insulin signaling in bone causes a reduction in serum osteocalcin levels that is associated with elevated blood glucose and reduced serum insulin levels, suggesting that the skeleton may play a significant role in the development of diet-induced insulin resistance. Insulin signaling is negatively regulated by the mammalian target of rapamycin complex 1 (mTORC1) which becomes hyper-activated in response to nutrient overload. Loss- and gain-of function models suggest that mTORC1 function in bone is essential for normal skeletal development; however, the role of this complex in the regulation of glucose metabolism remains to be determined. This review highlights our current understanding of the role played by osteocalcin in the skeletal regulation of glucose metabolism and fertility. In particular, it examines data emerging from transgenic mouse models which have revealed a pancreas-bone-testis regulatory axis and discusses recent human studies which seek to corroborate findings from mouse models with clinical observations. Moreover, we review recent studies which suggest dysregulation of insulin signaling in bone leads to the development of insulin resistance and discuss the potential role of mTORC1 signaling in this process.

The Mesenchymal Precursor Cell Marker Antibody STRO-1 Binds to Cell Surface Heat Shock Cognate 70.

Since its discovery more than 25 years ago, the STRO-1 antibody has played a fundamental role in defining the hierarchical nature of mesenchymal precursor cells (MPC) and their progeny. STRO-1 antibody binding remains a hallmark of immature pluripotent MPC. Despite the significance of STRO-1 in the MPC field, the identity of the antigen has remained elusive. Using a combination of two-dimensional gel electrophoresis, coupled with Western blotting and Tandem mass spectroscopy, we have identified the STRO-1 antigen as heat shock cognate 70 (HSC70;HSPA8). STRO-1 binds to immune-precipitated HSC70 and siRNA-mediated knock down of HSPA8 reduced STRO-1 binding. STRO-1 surface binding does not correlate with HSC70 expression and sequestration of cholesterol reduces STRO-1 surface binding, suggesting that the plasma membrane lipid composition may be an important determinant in the presentation of HSC70 on the cell surface. HSC70 is present on the surface of STRO-1+ but not STRO-1- cell lines as assessed by cell surface biotinylation and recombinant HSC70 blocks STRO-1 binding to the cell surface. The STRO-1 epitope on HSC70 was mapped to the ATPase domain using a series of deletion mutants in combination with peptide arrays. Deletion of the first four amino acids of the consensus epitope negated STRO-1 binding. Notably, in addition to HSC70, STRO-1 cross-reacts with heat shock protein 70 (HSP70), however all the clonogenic cell activity is restricted to the STRO-1BRIGHT /HSP70- fraction. These results provide important insight into the properties that define multipotent MPC and provide the impetus to explore the role of cell surface HSC70 in MPC biology. Stem Cells 2017;35:940-951.

Cutting edge genomics reveal new insights into tumour development, disease progression and therapeutic impacts in multiple myeloma.

Multiple Myeloma (MM) is a haematological malignancy characterised by the clonal expansion of plasma cells (PCs) within the bone marrow. Despite advances in therapy, MM remains a largely incurable disease with a median survival of 6 years. In almost all cases, the development of MM is preceded by the benign PC condition Monoclonal Gammopathy of Undetermined Significance (MGUS). Recent studies show that the transformation of MGUS to MM is associated with complex genetic changes. Understanding how these changes contribute to evolution will present targets for clinical intervention. We discuss three models of MM evolution; the linear, the expansionist and the intraclonal heterogeneity models. Of particular interest is the intraclonal heterogeneity model. Here, distinct populations of MM PCs carry differing combinations of genetic mutations. Acquisition of additional mutations can contribute to subclonal lineages where "driver" mutations may influence selective pressure and dominance, and "passenger" mutations are neutral in their effects. Furthermore, studies show that clinical intervention introduces additional selective pressure on tumour cells and can influence subclone survival, leading to therapy resistance. This review discusses how Next Generation Sequencing approaches are revealing critical insights into the genetics of MM development, disease progression and treatment. MM disease progression will illuminate possible mechanisms underlying the tumour.

The cationic small molecule GW4869 is cytotoxic to high phosphatidylserine-expressing myeloma cells.

We have discovered that a small cationic molecule, GW4869, is cytotoxic to a subset of myeloma cell lines and primary myeloma plasma cells. Biochemical analysis revealed that GW4869 binds to anionic phospholipids such as phosphatidylserine - a lipid normally confined to the intracellular side of the cell membrane. However, interestingly, phosphatidylserine was expressed on the surface of all myeloma cell lines tested (n = 12) and 9/15 primary myeloma samples. Notably, the level of phosphatidylserine expression correlated well with sensitivity to GW4869. Inhibition of cell surface phosphatidylserine exposure with brefeldin A resulted in resistance to GW4869. Finally, GW4869 was shown to delay the growth of phosphatidylserine-high myeloma cells in vivo. To the best of our knowledge, this is the first example of using a small molecule to target phosphatidylserine on malignant cells. This study may provide the rationale for the development of phosphatidylserine-targeting small molecules for the treatment of surface phosphatidylserine-expressing cancers.

Twist-1 Enhances Bone Marrow Mesenchymal Stromal Cell Support of Hematopoiesis by Modulating CXCL12 Expression.

Twist-1 encodes a basic helix-loop-helix transcription factor, known to contribute to mesodermal and skeletal tissue development. We have reported previously that Twist-1 maintains multipotent human bone marrow-derived mesenchymal stem/stromal cells (BMSC) in an immature state, enhances their life-span, and influences cell fate determination. In this study, human BMSC engineered to express high levels of Twist-1 were found to express elevated levels of the chemokine, CXCL12. Analysis of the CXCL12 proximal promoter using chromatin immunoprecipitation analysis identified several E-box DNA sites bound by Twist-1. Functional studies using a luciferase reporter construct showed that Twist-1 increased CXCL12 promoter activity in a dose dependent manner. Notably, Twist-1 over-expressing BMSC exhibited an enhanced capacity to maintain human CD34 + hematopoietic stem cells (HSC) in long-term culture-initiating cell (LTC-IC) assays. Moreover, the observed increase in HSC maintenance by Twist-1 over-expressing BMSC was blocked in the presence of the CXCL12 inhibitor, AMD3100. Supportive studies, using Twist-1 deficient heterozygous mice demonstrated a significant decrease in the frequency of stromal progenitors and increased numbers of osteoblasts within the bone. These observations correlated to a decreased incidence in the number of clonogenic stromal progenitors (colony forming unit-fibroblasts) and lower levels of CXCL12 in Twist-1 mutant mice. Furthermore, Twist-1 deficient murine stromal feeder layers, exhibited a significant decrease in CXCL12 levels and lower numbers of hematopoietic colonies in LTC-IC assays, compared with wild type controls. These findings demonstrate that Twist-1, which maintains BMSC at an immature state, endows them with an increased capacity for supporting hematopoiesis via direct activation of CXCL12 gene expression.

A non-canonical role for desmoglein-2 in endothelial cells: implications for neoangiogenesis.

Desmogleins (DSG) are a family of cadherin adhesion proteins that were first identified in desmosomes and provide cardiomyocytes and epithelial cells with the junctional stability to tolerate mechanical stress. However, one member of this family, DSG2, is emerging as a protein with additional biological functions on a broader range of cells. Here we reveal that DSG2 is expressed by non-desmosome-forming human endothelial progenitor cells as well as their mature counterparts [endothelial cells (ECs)] in human tissue from healthy individuals and cancer patients. Analysis of normal blood and bone marrow showed that DSG2 is also expressed by CD34(+)CD45(dim) hematopoietic progenitor cells. An inability to detect other desmosomal components, i.e., DSG1, DSG3 and desmocollin (DSC)2/3, on these cells supports a solitary role for DSG2 outside of desmosomes. Functionally, we show that CD34(+)CD45(dim)DSG2(+) progenitor cells are multi-potent and pro-angiogenic in vitro. Using a 'knockout-first' approach, we generated a Dsg2 loss-of-function strain of mice (Dsg2 (lo/lo)) and observed that, in response to reduced levels of Dsg2: (i) CD31(+) ECs in the pancreas are hypertrophic and exhibit altered morphology, (ii) bone marrow-derived endothelial colony formation is impaired, (iii) ex vivo vascular sprouting from aortic rings is reduced, and (iv) vessel formation in vitro and in vivo is attenuated. Finally, knockdown of DSG2 in a human bone marrow EC line reveals a reduction in an in vitro angiogenesis assay as well as relocalisation of actin and VE-cadherin away from the cell junctions, reduced cell-cell adhesion and increased invasive properties by these cells. In summary, we have identified DSG2 expression in distinct progenitor cell subpopulations and show that, independent from its classical function as a component of desmosomes, this cadherin also plays a critical role in the vasculature.

EphB4 Expressing Stromal Cells Exhibit an Enhanced Capacity for Hematopoietic Stem Cell Maintenance.

The tyrosine kinase receptor, EphB4, mediates cross-talk between stromal and hematopoietic populations during bone remodeling, fracture repair and arthritis, through its interactions with the ligand, ephrin-B2. This study demonstrated that transgenic EphB4 mice (EphB4 Tg), over-expressing EphB4 under the control of collagen type-1 promoter, exhibited higher frequencies of osteogenic cells and hematopoietic stem/progenitor cells (HSC), correlating with a higher frequency of long-term culture-initiating cells (LTC-IC), compared with wild type (WT) mice. EphB4 Tg stromal feeder layers displayed a greater capacity to support LTC-IC in vitro, where blocking EphB4/ephrin-B2 interactions decreased LTC-IC output. Similarly, short hairpin RNA-mediated EphB4 knockdown in human bone marrow stromal cells reduced their ability to support high ephrin-B2 expressing CD34(+) HSC in LTC-IC cultures. Notably, irradiated EphB4 Tg mouse recipients displayed enhanced bone marrow reconstitution capacity and enhanced homing efficiency of transplanted donor hematopoietic stem/progenitor cells relative to WT controls. Studies examining the expression of hematopoietic supportive factors produced by stromal cells indicated that CXCL12, Angiopoietin-1, IL-6, FLT-3 ligand, and osteopontin expression were more highly expressed in EphB4 Tg stromal cells compared with WT controls. These findings indicate that EphB4 facilitates stromal-mediated support of hematopoiesis, and constitute a novel component of the HSC niche.

Brief report: the differential roles of mTORC1 and mTORC2 in mesenchymal stem cell differentiation.

Adipocytes (AdCs) and osteoblasts (OBs) are derived from mesenchymal stem cells (MSCs) and differentiation toward either lineage is both mutually exclusive and transcriptionally controlled. Recent studies implicate the mammalian target of rapamycin (mTOR) pathway as important in determining MSC fate, with inhibition of mTOR promoting OB differentiation and suppressing AdC differentiation. mTOR functions within two distinct multiprotein complexes, mTORC1 and mTORC2, each of which contains the unique adaptor protein, raptor or rictor, respectively. While compounds used to study mTOR signaling, such as rapamycin and related analogs, primarily inhibit mTORC1, prolonged exposure can also disrupt mTORC2 function, confounding interpretation of inhibitor studies. As a result, the relative contribution of mTORC1 and mTORC2 to MSC fate determination remains unclear. In this study, we generated primary mouse MSCs deficient in either Rptor (RapKO) or Rictor (RicKO) using the Cre/loxP system. Cre-mediated deletion of Rptor or Rictor resulted in impaired mTORC1 and mTORC2 signaling, respectively. Under lineage-inductive culture conditions, RapKO MSCs displayed a reduced capacity to form lipid-laden AdCs and an increased capacity to form a mineralized matrix. In contrast, RicKO MSCs displayed reduced osteogenic differentiation capacity and enhanced adipogenic differentiation potential. Taken together, our findings reveal distinct roles for mTORC1 and mTORC2 in MSC lineage commitment.

Protein kinase activity of phosphoinositide 3-kinase regulates cytokine-dependent cell survival.

The dual specificity protein/lipid kinase, phosphoinositide 3-kinase (PI3K), promotes growth factor-mediated cell survival and is frequently deregulated in cancer. However, in contrast to canonical lipid-kinase functions, the role of PI3K protein kinase activity in regulating cell survival is unknown. We have employed a novel approach to purify and pharmacologically profile protein kinases from primary human acute myeloid leukemia (AML) cells that phosphorylate serine residues in the cytoplasmic portion of cytokine receptors to promote hemopoietic cell survival. We have isolated a kinase activity that is able to directly phosphorylate Ser585 in the cytoplasmic domain of the interleukin 3 (IL-3) and granulocyte macrophage colony stimulating factor (GM-CSF) receptors and shown it to be PI3K. Physiological concentrations of cytokine in the picomolar range were sufficient for activating the protein kinase activity of PI3K leading to Ser585 phosphorylation and hemopoietic cell survival but did not activate PI3K lipid kinase signaling or promote proliferation. Blockade of PI3K lipid signaling by expression of the pleckstrin homology of Akt1 had no significant impact on the ability of picomolar concentrations of cytokine to promote hemopoietic cell survival. Furthermore, inducible expression of a mutant form of PI3K that is defective in lipid kinase activity but retains protein kinase activity was able to promote Ser585 phosphorylation and hemopoietic cell survival in the absence of cytokine. Blockade of p110α by RNA interference or multiple independent PI3K inhibitors not only blocked Ser585 phosphorylation in cytokine-dependent cells and primary human AML blasts, but also resulted in a block in survival signaling and cell death. Our findings demonstrate a new role for the protein kinase activity of PI3K in phosphorylating the cytoplasmic tail of the GM-CSF and IL-3 receptors to selectively regulate cell survival highlighting the importance of targeting such pathways in cancer.