Fak depletion in both hematopoietic and nonhematopoietic niche cells leads to hematopoietic stem cell expansion

"Caught in the net": the extracellular matrix of the bone marrow in normal hematopoiesis and leukemia

The influence of the bone marrow microenvironment on normal hematopoiesis, but also leukemia, has largely been accepted. However, the focus has been predominantly on the role of various cell types or cytokines maintaining hematopoietic stem cells or protecting leukemia stem cells from different therapies. A frequently overlooked component of the bone marrow microenvironment is the extracellular matrix, which not only provides a mechanical scaffold, but also serves as a source of growth factors. We discuss here how extracellular matrix proteins directly or indirectly modulate hematopoietic stem cell physiology and influence leukemia progression. It is hoped that existing and future studies on this topic may propel forward the possibility of augmenting normal hematopoiesis and improving therapies for leukemia, for instance, by targeting of the extracellular matrix in the bone marrow.

Modeling the Pro-inflammatory Tumor Microenvironment in Acute Lymphoblastic Leukemia Predicts a Breakdown of Hematopoietic-Mesenchymal Communication Networks

Lineage fate decisions of hematopoietic cells depend on intrinsic factors and extrinsic signals provided by the bone marrow microenvironment, where they reside. Abnormalities in composition and function of hematopoietic niches have been proposed as key contributors of acute lymphoblastic leukemia (ALL) progression. Our previous experimental findings strongly suggest that pro-inflammatory cues contribute to mesenchymal niche abnormalities that result in maintenance of ALL precursor cells at the expense of normal hematopoiesis. Here, we propose a molecular regulatory network interconnecting the major communication pathways between hematopoietic stem and progenitor cells (HSPCs) and mesenchymal stromal cells (MSCs) within the BM. Dynamical analysis of the network as a Boolean model reveals two stationary states that can be interpreted as the intercellular contact status. Furthermore, simulations describe the molecular patterns observed during experimental proliferation and activation. Importantly, our model predicts instability in the CXCR4/CXCL12 and VLA4/VCAM1 interactions following microenvironmental perturbation due by temporal signaling from Toll like receptors (TLRs) ligation. Therefore, aberrant expression of NF-κB induced by intrinsic or extrinsic factors may contribute to create a tumor microenvironment where a negative feedback loop inhibiting CXCR4/CXCL12 and VLA4/VCAM1 cellular communication axes allows for the maintenance of malignant cells.

Hematopoietic Stem Cell Activity Is Regulated by Pten Phosphorylation Through a Niche-Dependent Mechanism

The phosphorylated form of Pten (p-Pten) is highly expressed in >70% of acute myeloid leukemia samples. However, the role of p-Pten in normal and abnormal hematopoiesis has not been studied. We found that Pten protein levels are comparable among long-term (LT) hematopoietic stem cells (HSCs), short-term (ST) HSCs, and multipotent progenitors (MPPs); however, the levels of p-Pten are elevated during the HSC-to-MPP transition. To study whether p-Pten is involved in regulating self-renewal and differentiation in HSCs, we compared the effects of overexpression of p-Pten and nonphosphorylated Pten (non-p-Pten) on the hematopoietic reconstitutive capacity (HRC) of HSCs. We found that overexpression of non-p-Pten enhances the LT-HRC of HSCs, whereas overexpression of p-Pten promotes myeloid differentiation and compromises the LT-HRC of HSCs. Such phosphorylation-regulated Pten functioning is mediated by repressing the cell:cell contact-induced activation of Fak/p38 signaling independent of Pten's lipid phosphatase activity because both p-Pten and non-p-Pten have comparable activity in repressing PI3K/Akt signaling. Our studies suggest that, in addition to repressing PI3K/Akt/mTor signaling, non-p-Pten maintains HSCs in bone marrow niches via a cell-contact inhibitory mechanism by inhibiting Fak/p38 signaling-mediated proliferation and differentiation. In contrast, p-Pten promotes the proliferation and differentiation of HSCs by enhancing the cell contact-dependent activation of Src/Fak/p38 signaling. Stem Cells 2016;34:2130-2144.

Hypoxic niche-mediated regeneration of hematopoiesis in the engraftment window is dominantly affected by oxygen tension in the milieu

The bone marrow (BM) microenvironment or the hematopoietic stem cell (HSC) niche is normally hypoxic, which maintains HSC quiescence. Paradoxically, transplanted HSCs rapidly proliferate in this niche. Pretransplant myelosuppression results in a substantial rise in oxygen levels in the marrow microenvironment due to reduced cellularity and consequent low oxygen consumption. Therefore, it may be construed that the rapid proliferation of the engrafted HSCs in the BM niche is facilitated by the transiently elevated oxygen tension in this milieu during the “engraftment window.” To determine whether oxygen tension dominantly affects the regeneration of hematopoiesis in the BM niche, we created an “oxygen-independent hypoxic niche” by treating BM-derived mesenchymal stromal cells (BMSCs) with a hypoxia-mimetic compound, cobalt chloride (CoCl₂) and cocultured them with BM-derived HSC-enriched cells under normoxic conditions (HSCs; CoCl₂-cocultures). Cocultures with untreated BMSCs incubated under normoxia (control- cocultures) or hypoxia (1% O₂; hypoxic-cocultures) were used as comparators. Biochemical analyses showed that though, both CoCl₂ and hypoxia evoked comparable signals in the BMSCs, the regeneration of hematopoiesis in their respective cocultures was radically different. The CoCl₂-BMSCs supported robust hematopoiesis, while the hypoxic-BMSCs exerted strong inhibition. The hematopoiesis-supportive ability of CoCl₂-BMSCs was abrogated if the CoCl₂-cocultures were incubated under hypoxia, demonstrating that the prevalent oxygen tension in the milieu dominantly affects the outcome of the HSC-BM niche interactions. Our data suggest that pharmacologically delaying the reestablishment of hypoxia in the BM may boost post-transplant regeneration of hematopoiesis.

Regulation of Stat5 by FAK and PAK1 in Oncogenic FLT3- and KIT-Driven Leukemogenesis

Oncogenic mutations of FLT3 and KIT receptors are associated with poor survival in patients with acute myeloid leukemia (AML) and myeloproliferative neoplasms (MPNs), and currently available drugs are largely ineffective. Although Stat5 has been implicated in regulating several myeloid and lymphoid malignancies, how precisely Stat5 regulates leukemogenesis, including its nuclear translocation to induce gene transcription, is poorly understood. In leukemic cells, we show constitutive activation of focal adhesion kinase (FAK) whose inhibition represses leukemogenesis. Downstream of FAK, activation of Rac1 is regulated by RacGEF Tiam1, whose inhibition prolongs the survival of leukemic mice. Inhibition of the Rac1 effector PAK1 prolongs the survival of leukemic mice in part by inhibiting the nuclear translocation of Stat5. These results reveal a leukemic pathway involving FAK/Tiam1/Rac1/PAK1 and demonstrate an essential role for these signaling molecules in regulating the nuclear translocation of Stat5 in leukemogenesis.

Haematopoietic focal adhesion kinase deficiency alters haematopoietic homeostasis to drive tumour metastasis

Metastasis is the main cause of cancer-related death and thus understanding the molecular and cellular mechanisms underlying this process is critical. Here, our data demonstrate, contrary to established dogma, that loss of haematopoietic-derived focal adhesion kinase (FAK) is sufficient to enhance tumour metastasis. Using both experimental and spontaneous metastasis models, we show that genetic ablation of haematopoietic FAK does not affect primary tumour growth but enhances the incidence of metastasis significantly. At a molecular level, haematopoietic FAK deletion results in an increase in PU-1 levels and decrease in GATA-1 levels causing a shift of hematopoietic homeostasis towards a myeloid commitment. The subsequent increase in circulating granulocyte number, with an increase in serum CXCL12 and granulocyte CXCR4 levels, was required for augmented metastasis in mice lacking haematopoietic FAK. Overall our findings provide a mechanism by which haematopoietic FAK controls cancer metastasis.

FAK in cancer: mechanistic findings and clinical applications

Focal adhesion kinase (FAK) is a cytoplasmic protein tyrosine kinase that is overexpressed and activated in several advanced-stage solid cancers. FAK promotes tumour progression and metastasis through effects on cancer cells, as well as stromal cells of the tumour microenvironment. The kinase-dependent and kinase-independent functions of FAK control cell movement, invasion, survival, gene expression and cancer stem cell self-renewal. Small molecule FAK inhibitors decrease tumour growth and metastasis in several preclinical models and have initial clinical activity in patients with limited adverse events. In this Review, we discuss FAK signalling effects on both tumour and stromal cell biology that provide rationale and support for future therapeutic opportunities.

Feedback signals in myelodysplastic syndromes: increased self-renewal of the malignant clone suppresses normal hematopoiesis

Myelodysplastic syndromes (MDS) are triggered by an aberrant hematopoietic stem cell (HSC). It is, however, unclear how this clone interferes with physiologic blood formation. In this study, we followed the hypothesis that the MDS clone impinges on feedback signals for self-renewal and differentiation and thereby suppresses normal hematopoiesis. Based on the theory that the MDS clone affects feedback signals for self-renewal and differentiation and hence suppresses normal hematopoiesis, we have developed a mathematical model to simulate different modifications in MDS-initiating cells and systemic feedback signals during disease development. These simulations revealed that the disease initiating cells must have higher self-renewal rates than normal HSCs to outcompete normal hematopoiesis. We assumed that self-renewal is the default pathway of stem and progenitor cells which is down-regulated by an increasing number of primitive cells in the bone marrow niche – including the premature MDS cells. Furthermore, the proliferative signal is up-regulated by cytopenia. Overall, our model is compatible with clinically observed MDS development, even though a single mutation scenario is unlikely for real disease progression which is usually associated with complex clonal hierarchy. For experimental validation of systemic feedback signals, we analyzed the impact of MDS patient derived serum on hematopoietic progenitor cells in vitro: in fact, MDS serum slightly increased proliferation, whereas maintenance of primitive phenotype was reduced. However, MDS serum did not significantly affect colony forming unit (CFU) frequencies indicating that regulation of self-renewal may involve local signals from the niche. Taken together, we suggest that initial mutations in MDS particularly favor aberrant high self-renewal rates. Accumulation of primitive MDS cells in the bone marrow then interferes with feedback signals for normal hematopoiesis – which then results in cytopenia.

Myelodysplastic syndromes are diseases which are characterized by ineffective blood formation. There is accumulating evidence that they are caused by an aberrant hematopoietic stem cell. However, it is yet unclear how this malignant clone suppresses normal hematopoiesis. To this end, we generated mathematical models under the assumption that feedback signals regulate self-renewal and proliferation of normal and diseased stem cells. The simulations demonstrate that the malignant cells must have particularly higher self-renewal rates than normal stem cells – rather than higher proliferation rates. On the other hand, down-regulation of self-renewal by the increasing number of malignant cells in the bone marrow niche can explain impairment of normal blood formation. In fact, we show that serum of patients with myelodysplastic syndrome, as compared to serum of healthy donors, stimulates proliferation and moderately impacts on maintenance of hematopoietic stem and progenitor cells in vitro. Thus, aberrant high self-renewal rates of the malignant clone seem to initiate disease development; suppression of normal blood formation is then caused by a rebound effect of feedback signals which down-regulate self-renewal of normal stem and progenitor cells as well.

Mechanobiology of bone marrow stem cells: from myosin-II forces to compliance of matrix and nucleus in cell forms and fates

Adult stem cells and progenitors are of great interest for their clinical application as well as their potential to reveal deep sensitivities to microenvironmental factors. The bone marrow is a niche for at least two types of stem cells, and the prototype is the hematopoietic stem cell/progenitors (HSC/Ps), which have saved many thousands of patients for several decades now. In bone marrow, HSC/Ps interact functionally with marrow stromal cells that are often referred to as mesenchymal stem cells (MSCs) or derivatives thereof. Myosin and matrix elasticity greatly affect MSC function, and these mechanobiological factors are now being explored with HSC/Ps both in vitro and in vivo. Also emerging is a role for the nucleus as a mechanically sensitive organelle that is semi-permeable to transcription factors which are modified for nuclear entry by cytoplasmic mechanobiological pathways. Since therapies envisioned with induced pluripotent stem cells and embryonic stem cells generally involve in vitro commitment to an adult stem cell or progenitor, a very deep understanding of stem cell mechanobiology is essential to progress with these multi-potent cells.

PTK2 and PTPN11 expression in myelodysplastic syndromes

OBJECTIVE

The aim of this study was to evaluate the expression of protein tyrosine kinase 2 and protein tyrosine phosphatase non-receptor type 11, which respectively encode focal adhesion kinase protein and src homology 2 domain-containing protein-tyrosine phosphatase 2, in hematopoietic cells from patients with myelodysplastic syndromes.

METHODS

Protein tyrosine kinase 2 and tyrosine phosphatase non-receptor type 11 expressions were analyzed by quantitative polymerase chain reaction in bone marrow cells from patients with myelodysplastic syndromes and healthy donors.

RESULTS

Protein tyrosine kinase 2 and tyrosine phosphatase non-receptor type 11 expressions did not significantly differ between normal cells and myelodysplastic cells.

CONCLUSIONS

Our data suggest that despite the relevance of focal adhesion kinase and src homology 2 domain-containing protein-tyrosine phosphatase 2 in hematopoietic disorders, their mRNA expression do not significantly differ between total bone marrow cells from patients with myelodysplastic syndromes and healthy donors.

Erythroid cells generated in the absence of specific β1-integrin heterodimers accumulate reactive oxygen species at homeostasis and are unable to mount effective antioxidant defenses

We have previously reported that β1(Δ/Δ) mice have a markedly impaired response to hemolytic stress, but the mechanisms of this were unclear. In the present study we explored in detail quantitative, phenotypic and functional aspects of erythropoiesis at homeostasis in a large number of animals for each of 3 murine models with specific β1 heterodimer integrin deficiencies. We found that, at homeostasis, β1-deficient mice have a modest uncompensated anemia with ineffective erythropoiesis and decreased red blood cell survival. Mice lacking only α4 integrins (α4β1/α4β7) do not share this phenotype. There is an increased tendency for reactive oxygen species accumulation in β1(Δ/Δ) erythroid cells with decreased anti-oxidant defenses at homeostasis which are exaggerated after stress. Furthermore, expansion of erythroid cells in spleen post-stress is dependent on α5β1, likely through mechanisms activating focal adhesion kinase complexes that are distinct from α4β1-mediated responses. In vivo inhibition of focal adhesion kinase activation partially recapitulates the β1(Δ/Δ) stress response. Mice lacking all α4 and β1 integrins (double knockouts) had, at homeostasis, the most severe phenotype with selective impairment of erythroid responses. The fact that integrins participate in mitigating stress in erythroid cells through redox activation of distinct signaling pathways by specific integrin heterodimers is a link that has not been appreciated until now.

Adhesion in the stem cell niche: biological roles and regulation

Stem cell self-renewal is tightly controlled by the concerted action of stem cell-intrinsic factors and signals within the niche. Niche signals often function within a short range, allowing cells in the niche to self-renew while their daughters outside the niche differentiate. Thus, in order for stem cells to continuously self-renew, they are often anchored in the niche via adhesion molecules. In addition to niche anchoring, however, recent studies have revealed other important roles for adhesion molecules in the regulation of stem cell function, and it is clear that stem cell-niche adhesion is crucial for stem cell self-renewal and is dynamically regulated. Here, we highlight recent progress in understanding adhesion between stem cells and their niche and how this adhesion is regulated.