Conditioning for hematopoietic transplantation activates the complement cascade and induces a proteolytic environment in bone marrow: a novel role for bioactive lipids and soluble C5b-C9 as homing factors

Molecular Deconvolution of Bone Marrow Adipose Tissue Interactions with Malignant Hematopoiesis: Potential for New Therapy Development

PURPOSE OF REVIEW

Along with a strong impact on skeletal integrity, bone marrow adipose tissue (BMAT) is an important modulator of the adult hematopoietic system. This review will summarize the current knowledge on the causal relationship between bone marrow (BM) adipogenesis and the development and progression of hematologic malignancies.

RECENT FINDINGS

BM adipocytes (BMAds) support a number of processes promoting oncogenesis, including the evolution of clonal hematopoiesis, malignant cell survival, proliferation, angiogenesis, and chemoresistance. In addition, leukemic cells manipulate surrounding BMAds by promoting lipolysis and release of free fatty acids, which are then utilized by leukemic cells via β-oxidation. Therefore, limiting BM adipogenesis, blocking BMAd-derived adipokines, or lipid metabolism obstruction have been considered as potential treatment options for hematological malignancies. Leukemic stem cells rely heavily on BMAds within the structural BM microenvironment for necessary signals which foster disease progression. Further development of 3D constructs resembling BMAT at different skeletal regions are critical to better understand these relationships in geometric space and may provide essential insight into the development of hematologic malignancies within the BM niche. In turn, these mechanisms provide promising potential as novel approaches to targeting the microenvironment with new therapeutic strategies.

Sphingolipid Signaling and Complement Activation in Glioblastoma: A Promising Avenue for Therapeutic Intervention

Glioblastoma is the most common and aggressive type of malignant brain tumor with a poor prognosis due to the lack of effective treatment options. Therefore, new treatment options are required. Sphingolipids are essential components of the cell membrane, while complement components are integral to innate immunity, and both play a critical role in regulating glioblastoma survival signaling. This review focuses on recent studies investigating the functional roles of sphingolipid metabolism and complement activation signaling in glioblastoma. It also discusses how targeting these two systems together may emerge as a novel therapeutic approach.

Hematopoiesis Revolves Around the Primordial Evolutional Rhythm of Purinergic Signaling and Innate Immunity - A Journey to the Developmental Roots

A cell's most significant existential task is to survive by ensuring proper metabolism, avoiding harmful stimuli, and adapting to changing environments. It explains why early evolutionary primordial signals and pathways remained active and regulate cell and tissue integrity. This requires energy supply and a balanced redox state. To meet these requirements, the universal intracellular energy transporter purine nucleotide-adenosine triphosphate (ATP) became an important signaling molecule and precursor of purinergic signaling after being released into extracellular space. Similarly, ancient proteins involved in intracellular metabolism gave rise to the third protein component (C3) of the complement cascade (ComC), a soluble arm of innate immunity. These pathways induce cytosol reactive oxygen (ROS) and reactive nitrogen species (RNS) that regulate the redox state of the cells. While low levels of ROS and RNS promote cell growth and differentiation, supra-physiological concentrations can lead to cell damage by pyroptosis. This balance explains the impact of purinergic signaling and innate immunity on cell metabolism, organogenesis, and tissue development. Subsequently, along with evolution, new regulatory cues emerge in the form of growth factors, cytokines, chemokines, and bioactive lipids. However, their expression is still modulated by both primordial signaling pathways. This review will focus on the data that purinergic signaling and innate immunity carry on their ancient developmental task in hematopoiesis and specification of hematopoietic stem/progenitor cells (HSPCs). Moreover, recent evidence shows both these regulatory pathways operate in a paracrine manner and inside HSPCs at the autocrine level.

The synthetic phospholipid C8-C1P determines pro-angiogenic and pro-reparative features in human macrophages restraining the proinflammatory M1-like phenotype

Monocytes (Mo) are highly plastic myeloid cells that differentiate into macrophages after extravasation, playing a pivotal role in the resolution of inflammation and regeneration of injured tissues. Wound-infiltrated monocytes/macrophages are more pro-inflammatory at early time points, while showing anti-inflammatory/pro-reparative phenotypes at later phases, with highly dynamic switching depending on the wound environment. Chronic wounds are often arrested in the inflammatory phase with hampered inflammatory/repair phenotype transition. Promoting the tissue repair program switching represents a promising strategy to revert chronic inflammatory wounds, one of the major public health loads. We found that the synthetic lipid C8-C1P primes human CD14⁺ monocytes, restraining the inflammatory activation markers (HLA-DR, CD44, and CD80) and IL-6 when challenged with LPS, and preventing apoptosis by inducing BCL-2. We also observed increased pseudo-tubule formation of human endothelial-colony-forming cells (ECFCs) when stimulated with the C1P-macrophages secretome. Moreover, C8-C1P-primed monocytes skew differentiation toward pro-resolutive-like macrophages, even in the presence of inflammatory PAMPs and DAMPs by increasing anti-inflammatory and pro-angiogenic gene expression patterns. All these results indicate that C8-C1P could restrain M1 skewing and promote the program of tissue repair and pro-angiogenic macrophage.

Hematopoietic stem cells on the crossroad between purinergic signaling and innate immunity

Hematopoiesis is regulated by several mediators such as peptide-based growth factors, cytokines, and chemokines, whose biological effects have been studied for many years. However, several other mediators have been identified recently that affect the fate of hematopoietic stem/progenitor cells (HSPC) as well as non-hematopoietic cells in the bone marrow microenvironment. These new mediators comprise members of purinergic signaling pathways and are active mediators of the soluble arm of innate immunity, the complement cascade (ComC). In this review, we will discuss the coordinated effects of these pathways in regulating the biology of HSPC. Importantly, both purinergic signaling and the ComC are activated in stress situations and interact with specific receptors expressed on HSPC. Evidence has accumulated indicating that some of the purinergic as well as ComC receptors could also be activated intracellularly by intrinsically expressed ligands. To support this recent evidence, our work indicates that the major mediator of purinergic signaling, adenosine triphosphate, and the cleavage product of the fifth component of the ComC (C5), C5a anaphylatoxin, can activate their corresponding receptors expressed on the outer mitochondrial membrane in an autocrine manner. We will also discuss recent evidence that these responses, mediated by purinergic signaling and the ComC network, are coordinated by activation of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 - reactive oxygen species - NLR family pyrin domain containing 3 (NLRP3) inflammasome (Nox2-ROS-NLRP3) axis.

Crosstalk between pro-survival sphingolipid metabolism and complement signaling induces inflammasome-mediated tumor metastasis

Crosstalk between metabolic and signaling events that induce tumor metastasis remains elusive. Here, we determine how oncogenic sphingosine 1-phosphate (S1P) metabolism induces intracellular C3 complement activation to enhance migration/metastasis. We demonstrate that increased S1P metabolism activates C3 complement processing through S1P receptor 1 (S1PR1). S1P/S1PR1-activated intracellular C3b-α'2 is associated with PPIL1 through glutamic acid 156 (E156) and aspartic acid 111 (D111) residues, resulting in NLRP3/inflammasome induction. Inactivation mutations of S1PR1 to prevent S1P signaling or mutations of C3b-α'2 to prevent its association with PPIL1 attenuate inflammasome activation and reduce lung colonization/metastasis in mice. Also, activation of the S1PR1/C3/PPIL1/NLRP3 axis is highly associated with human metastatic melanoma tissues and patient-derived xenografts. Moreover, targeting S1PR1/C3/PPIL1/NLRP3 signaling using molecular, genetic, and pharmacologic tools prevents lung colonization/metastasis of various murine cancer cell lines using WT and C3a-receptor1 knockout (C3aR1-/-) mice. These data provide strategies for treating high-grade/metastatic tumors by targeting the S1PR1/C3/inflammasome axis.

Myeloablative Conditioning for Transplantation Induces State of Sterile Inflammation in the Bone Marrow: Implications for Optimizing Homing and Engraftment of Hematopoietic Stem Cells

Significance: The success rate of hematopoietic stem cell transplantation depends mainly on the number of transplanted hematopoietic stem/progenitor cells (HSPCs) followed by the speed of their engraftment in the myeloablated transplant recipient. Therefore, clinical outcomes will significantly benefit from accelerating the homing and engraftment of these cells. This is, in particular, important when the number of cells available for the transplantation of HSPCs is limited. Recent Advances: We postulated that myeloablative conditioning for hematopoietic transplantation by radio- or chemotherapy induces a state of sterile inflammation in transplant recipient peripheral blood (PB) and bone marrow (BM). This state is mediated by activation of the BM stromal and innate immunity cells that survive myeloablative conditioning and respond to danger-associated molecular patterns released from the cells damaged by myeloablative conditioning. As a result of this, several factors are released that promote proper navigation of HSPCs infused into PB of transplant recipient and prime recipient BM to receive transplanted cells. Critical Issues: We will present data that cellular innate immunity arm and soluble arm comprised complement cascade proteins, promoting the induction of the BM sterile inflammation state that facilitates the navigation, homing, and engraftment of HSPCs. Future Directions: Deciphering these mechanisms would allow us to better understand the mechanisms that govern hematopoietic recovery after transplantation and, in parallel, provide important information on how to optimize this process in the clinic by employing small molecular modifiers of innate immunity and purinergic signaling. Antioxid. Redox Signal. 37, 1254-1265.

Advancements on the Multifaceted Roles of Sphingolipids in Hematological Malignancies

Dysregulation of sphingolipid metabolism plays a complex role in hematological malignancies, beginning with the first historical link between sphingolipids and apoptosis discovered in HL-60 leukemic cells. Numerous manuscripts have reviewed the field including the early discoveries that jumpstarted the studies. Many studies discussed here support a role for sphingolipids, such as ceramide, in combinatorial therapeutic regimens to enhance anti-leukemic effects and reduce resistance to standard therapies. Additionally, inhibitors of specific nodes of the sphingolipid pathway, such as sphingosine kinase inhibitors, significantly reduce leukemic cell survival in various types of leukemias. Acid ceramidase inhibitors have also shown promising results in acute myeloid leukemia. As the field moves rapidly, here we aim to expand the body of literature discussed in previously published reviews by focusing on advances reported in the latter part of the last decade.

Sphingosine-1-phosphate and ceramide-1-phosphate promote migration, pro-inflammatory and pro-fibrotic responses in retinal pigment epithelium cells

Retinal pigment epithelium (RPE) cells, essential for preserving retina homeostasis, also contribute to the development of retina proliferative diseases, through their exacerbated migration, epithelial to mesenchymal transition (EMT) and inflammatory response. Uncovering the mechanisms inducing these changes is crucial for designing effective treatments for these pathologies. Sphingosine-1-phosphate (S1P) and ceramide-1-phosphate (C1P) are bioactive sphingolipids that promote migration and inflammation in several cell types; we recently established that they stimulate the migration of retina Müller glial cells (Simón et al., 2015; Vera et al., 2021). We here analyzed whether S1P and C1P regulate migration, inflammation and EMT in RPE cells. We cultured two human RPE cell lines, ARPE-19 and D407 cells, and supplemented them with either 5 μM S1P or 10 μM C1P, or their vehicles, for 24 h. Analysis of cell migration by the scratch wound assay showed that S1P addition significantly enhanced migration in both cell lines. Pre-treatment with W146 and BML-241, antagonists for S1P receptor 1 (S1P1) and 3 (S1P3), respectively, blocked exogenous S1P-induced migration. Inhibiting sphingosine kinase 1 (SphK1), the enzyme involved in S1P synthesis, significantly reduced cell migration and exogenous S1P only partially restored it. Addition of C1P markedly stimulated cell migration. Whereas inhibiting C1P synthesis did not affect C1P-induced migration, inhibiting S1P synthesis strikingly decreased it; noteworthy, addition of C1P promoted the transcription of SphK1. These results suggest that S1P and C1P stimulate RPE cell migration and their effect requires S1P endogenous synthesis. Both S1P and C1P increase the transcription of pro-inflammatory cytokines IL-6 and IL-8, and of EMT marker α-smooth muscle actin (α-SMA) in ARPE-19 cells. Collectively, our results suggest new roles for S1P and C1P in the regulation of RPE cell migration and inflammation; since the deregulation of sphingolipid metabolism is involved in several proliferative retinopathies, targeting their metabolism might provide new tools for treating these pathologies.

Role of Sphingolipids in Multiple Myeloma Progression, Drug Resistance, and Their Potential as Therapeutic Targets

Multiple myeloma (MM) is an incapacitating hematological malignancy characterized by accumulation of cancerous plasma cells in the bone marrow (BM) and production of an abnormal monoclonal protein (M-protein). The BM microenvironment has a key role in myeloma development by facilitating the growth of the aberrant plasma cells, which eventually interfere with the homeostasis of the bone cells, exacerbating osteolysis and inhibiting osteoblast differentiation. Recent recognition that metabolic reprograming has a major role in tumor growth and adaptation to specific changes in the microenvironmental niche have led to consideration of the role of sphingolipids and the enzymes that control their biosynthesis and degradation as critical mediators of cancer since these bioactive lipids have been directly linked to the control of cell growth, proliferation, and apoptosis, among other cellular functions. In this review, we present the recent progress of the research investigating the biological implications of sphingolipid metabolism alterations in the regulation of myeloma development and its progression from the pre-malignant stage and discuss the roles of sphingolipids in in MM migration and adhesion, survival and proliferation, as well as angiogenesis and invasion. We introduce the current knowledge regarding the role of sphingolipids as mediators of the immune response and drug-resistance in MM and tackle the new developments suggesting the manipulation of the sphingolipid network as a novel therapeutic direction for MM.

Novel Evidence That Alternative Pathway of Complement Cascade Activation is Required for Optimal Homing and Engraftment of Hematopoietic Stem/progenitor Cells

We reported in the past that activation of the third (C3) and fifth element (C5) of complement cascade (ComC) is required for a proper homing and engraftment of transplanted hematopoietic stem/progenitor cells (HSPCs). Since myeloablative conditioning for transplantation triggers in recipient bone marrow (BM) state of sterile inflammation, we have become interested in the role of complement in this process and the potential involvement of alternative pathway of ComC activation. We noticed that factor B deficient mice (FB-KO) that do not activate properly alternative pathway, engraft poorly with BM cells from normal wild type (WT) mice. We observed defects both in homing and engraftment of transplanted HSPCs. To shed more light on these phenomena, we found that myeloablative lethal irradiation conditioning for transplantation activates purinergic signaling, ComC, and Nlrp3 inflammasome in WT mice, which is significantly impaired in FB-KO animals. Our proteomics analysis revealed that conditioned for transplantation lethally irradiated FB-KO compared to normal control animals have lower expression of several proteins involved in positive regulation of cell migration, trans-endothelial migration, immune system, cellular signaling protein, and metabolic pathways. Overall, our recent study further supports the role of innate immunity in homing and engraftment of HSPCs.

Hematopoiesis and innate immunity: an inseparable couple for good and bad times, bound together by an hormetic relationship

Hematopoietic and immune cells originate from a common hematopoietic/lymphopoietic stem cell what explains that these different cell types often share the same receptors and respond to similar factors. Moreover, the common goal of both lineages is to ensure tissue homeostasis under steady-state conditions, fight invading pathogens, and promote tissue repair. We will highlight accumulating evidence that innate and adaptive immunity modulate several aspects of hematopoiesis within the hormetic zone in which the biological response to low exposure to potential stressors generally is favorable and benefits hematopoietic stem/progenitor cells (HSPCs). Innate immunity impact on hematopoiesis is pleiotropic and involves both the cellular arm, comprised of innate immunity cells, and the soluble arm, whose major component is the complement cascade (ComC). In addition, several mediators released by innate immunity cells, including inflammatory cytokines and small antimicrobial cationic peptides, affect hematopoiesis. There are intriguing observations that HSPCs and immune cells share several cell-surface pattern-recognition receptors (PRRs), such as Toll-like receptors (TLRs) and cytosol-expressed NOD, NOD-like, and RIG-I-like receptors and thus can be considered "pathogen sensors". In addition, not only lymphocytes but also HSPCs express functional intracellular complement proteins, defined as complosome which poses challenging questions for further investigation of the intracellular ComC-mediated intracrine regulation of hematopoiesis.

The P2X4 purinergic receptor has emerged as a potent regulator of hematopoietic stem/progenitor cell mobilization and homing-a novel view of P2X4 and P2X7 receptor interaction in orchestrating stem cell trafficking

Recent evidence indicates that extracellular adenosine triphosphate (eATP), as a major mediator of purinergic signaling, plays an important role in regulating the mobilization and homing of hematopoietic stem progenitor cells (HSPCs). In our previous work we demonstrated that eATP activates the P2X7 ion channel receptor in HSPCs and that its deficiency impairs stem cell trafficking. To learn more about the role of the P2X purinergic receptor family in hematopoiesis, we phenotyped murine and human HSPCs with respect to the seven P2X receptors and observed that, these cells also highly express P2X4 receptors, which shows ~50% sequence similarity to P2X7 subtypes, but that P2X4 cells are more sensitive to eATP and signal much more rapidly. Using the selective P2X4 receptor antagonist PSB12054 as well as P2X4-KO mice, we found that the P2X4 receptor, similar to P2X7 receptor, promotes trafficking of HSPCs in that its deficiency leads to impaired chemotaxis of HSPCs in response to a stromal-derived factor 1 (SDF-1) gradient, less effective pharmacological mobilization, and defective homing and engraftment of HSPCs after transplantation into myeloablated hosts. This correlated with a decrease in SDF-1 expression in the BM microenvironment. Overall, our results confirm the proposed cooperative dependence of both receptors in response to eATP signaling. In G-CSF-induced mobilization, a lack of one receptor is not compensated by the presence of the other one, which supports their mutual dependence in regulating HSPC trafficking.

The Nlrp3 inflammasome - the evolving story of its positive and negative effects on hematopoiesis

PURPOSE OF REVIEW

Hematopoiesis is co-regulated by innate immunity, which is an ancient evolutionary defense mechanism also involved in the development and regeneration of damaged tissues. This review seeks to shed more light on the workings of the Nlrp3 inflammasome, which is an intracellular innate immunity pattern recognition receptor and sensor of changes in the hematopoietic microenvironment, and focus on its role in hematopoieisis.

RECENT FINDINGS

Hematopoietic stem progenitor cells (HSPCs) are exposed to several external mediators of innate immunity. Moreover, since hemato/lymphopoietic cells develop from a common stem cell, their behavior and fate are coregulated by intracellular innate immunity pathways. Therefore, the Nlrp3 inflammasome is functional both in immune cells and in HSPCs and affects hematopoiesis in either a positive or negative way, depending on its activity level. Specifically, while a physiological level of activation regulates the trafficking of HSPCs and most likely maintains their pool in the bone marrow, hyperactivation may lead to irreversible cell damage by pyroptosis and HSPC senescence and contribute to the origination of myelodysplasia and hematopoietic malignancies.

SUMMARY

Modulation of the level of Nrp3 inflammasome activation will enable improvements in HSPC mobilization, homing, and engraftment strategies. It may also control pathological activation of this protein complex during HSPC senescence, graft-versus-host disease, the induction of cytokine storms, and the development of hematopoietic malignancies.

The P2X7 Receptor in the Maintenance of Cancer Stem Cells, Chemoresistance and Metastasis

Metastasis is the worst prognosis predictor in the clinical course of cancer development. Features of metastatic cancer cells include migratory ability, low degree of differentiation, self-renewal and proliferation potentials, as well as resistance to therapies. Metastatic cells do not present all of the necessary characteristics at once. Indeed, they have a unique phenotypic plasticity, allowing the acquisition of features that make them successful in all steps of metastasis. Cancer stem cells (CSC), the most undifferentiated cells in the tumor mass, display highest metastatic potential and resistance to radio- and chemotherapy. Growing tumors exhibit marked upregulation of P2X7 receptor expression and secrete ATP. Since the P2X7 receptor plays an important role in the maintenance of undifferentiated state of pluripotent cells, its importance on cell fate regulation in the tumor mass is suggested. Considering the extensive crosstalk between CSCs, epithelial-mesenchymal transition, drug resistance and metastasis, current knowledge implicating P2X7 receptor function in these phenomena and new avenues for therapeutic strategies to control metastasis are reviewed.

Nlrp3 Inflammasome Signaling Regulates the Homing and Engraftment of Hematopoietic Stem Cells (HSPCs) by Enhancing Incorporation of CXCR4 Receptor into Membrane Lipid Rafts

Fast and efficient homing and engraftment of hematopoietic stem progenitor cells (HSPCs) is crucial for positive clinical outcomes from transplantation. We found that this process depends on activation of the Nlrp3 inflammasome, both in the HSPCs to be transplanted and in the cells in the recipient bone marrow (BM) microenvironment. For the first time we provide evidence that functional deficiency in the Nlrp3 inflammasome in transplanted cells or in the host microenvironment leads to defective homing and engraftment. At the molecular level, functional deficiency of the Nlrp3 inflammasome in HSPCs leads to their defective migration in response to the major BM homing chemoattractant stromal-derived factor 1 (SDF-1) and to other supportive chemoattractants, including sphingosine-1-phosphate (S1P) and extracellular adenosine triphosphate (eATP). We report that activation of the Nlrp3 inflammasome increases autocrine release of eATP, which promotes incorporation of the CXCR4 receptor into membrane lipid rafts at the leading surface of migrating cells. On the other hand, a lack of Nlrp3 inflammasome expression in BM conditioned for transplantation leads to a decrease in expression of SDF-1 and danger-associated molecular pattern molecules (DAMPs), which are responsible for activation of the complement cascade (ComC), which in turn facilitates the homing and engraftment of HSPCs.

Heme Oxygenase 1 (HO-1) as an Inhibitor of Trafficking of Normal and Malignant Hematopoietic Stem Cells - Clinical and Translational Implications

Evidence indicates that bone marrow (BM)-residing hematopoietic stem/progenitor cells (HSPCs) are released into peripheral blood (PB) after administration of pro-mobilizing drugs, which induce a state of sterile inflammation in the BM microenvironment. In the reverse process, as seen after hematopoietic transplantation, intravenously injected HSPCs home and engraft into BM niches. Here again, conditioning for transplantation by myeloablative chemo- or radiotherapy induces a state of sterile inflammation that promotes HSPC seeding to BM stem cell niches. Therefore, the trafficking of HSPCs and their progeny, including granulocytes and monocytes/macrophages, is regulated by a response to pro-inflammatory stimuli. This responsiveness to inflammatory cues is also preserved after malignant transformation of hematopoietic cells. Results from our laboratory indicate that the responsiveness of hematopoietic cells to pro-inflammatory stimuli is orchestrated by Nlrp3 inflammasome. As reported, HO-1 effectively attenuates intracellular activation of Nlrp3 inflammasome as well as the pro-inflammatory effects of several humoral mediators, including complement cascade (ComC) cleavage fragments that promote migration of hematopoietic cells. Based on this finding, inhibition of HO-1 activity may become a practical strategy to enhance the mobilization and homing of normal HSPCs, and, alternatively, its activation may prevent unwanted spread and in vivo expansion of leukemic cells. Graphical Abstract.

CD11c regulates hematopoietic stem and progenitor cells under stress

β2 integrins are well-known leukocyte adhesion molecules consisting of 4 members: CD11a-d. Their known biological functions range widely from leukocyte recruitment, phagocytosis, to immunological synapse formation, but the studies have been primarily focused on CD11a and CD11b. CD11c is 1 of the 4 members and is extremely homologous to CD11b. It has been well known as a dendritic cell marker, but the characterization of its function has been limited. We found that CD11c was expressed on the short-term hematopoietic stem cells and multipotent progenitor cells. The lack of CD11c did not affect the number of hematopoietic stem and progenitor cells (HSPCs) in healthy CD11c knockout mice. Different from other β2 integrin members, however, CD11c deficiency was associated with increased apoptosis and significant loss of HSPCs in sepsis and bone marrow transplantation. Although integrins are generally known for their overlapping and redundant roles, we showed that CD11c had a distinct role of regulating the expansion of HSPCs under stress. This study shows that CD11c, a well-known dendritic cell marker, is expressed on HSPCs and serves as their functional regulator. CD11c deficiency leads to the loss of HSPCs via apoptosis in sepsis and bone marrow transplantation.

New insights into renal lipid dysmetabolism in diabetic kidney disease

Lipid dysmetabolism is one of the main features of diabetes mellitus and manifests by dyslipidemia as well as the ectopic accumulation of lipids in various tissues and organs, including the kidney. Research suggests that impaired cholesterol metabolism, increased lipid uptake or synthesis, increased fatty acid oxidation, lipid droplet accumulation and an imbalance in biologically active sphingolipids (such as ceramide, ceramide-1-phosphate and sphingosine-1-phosphate) contribute to the development of diabetic kidney disease (DKD). Currently, the literature suggests that both quality and quantity of lipids are associated with DKD and contribute to increased reactive oxygen species production, oxidative stress, inflammation, or cell death. Therefore, control of renal lipid dysmetabolism is a very important therapeutic goal, which needs to be archived. This article will review some of the recent advances leading to a better understanding of the mechanisms of dyslipidemia and the role of particular lipids and sphingolipids in DKD.

Regulation of cell growth, survival and migration by ceramide 1-phosphate - implications in lung cancer progression and inflammation

Ceramide 1-phosphate (C1P) is a bioactive sphingolipid that is implicated in the regulation of vital cellular functions and plays key roles in a number of inflammation-associated pathologies. C1P was first described as mitogenic for fibroblasts and macrophages and was later found to promote cell survival in different cell types. The mechanisms involved in the mitogenic actions of C1P include activation of MEK/ERK1-2, PI3K/Akt/mTOR, or PKC-α, whereas promotion of cell survival required a substantial reduction of ceramide levels through inhibition of serine palmitoyl transferase or sphingomyelinase activities. C1P and ceramide kinase (CerK), the enzyme responsible for its biosynthesis in mammalian cells, play key roles in tumor promotion and dissemination. CerK-derived C1P can be secreted to the extracellular milieu by different cell types and is also present in extracellular vesicles. In this context, whilst cell proliferation is regulated by intracellularly generated C1P, stimulation of cell migration/invasion requires the intervention of exogenous C1P. Regarding inflammation, C1P was first described as pro-inflammatory in a variety of cell types. However, cigarette smoke- or lipopolysaccharide-induced lung inflammation in mouse or human cells was overcome by pretreatment with natural or synthetic C1P analogs. Both acute and chronic lung inflammation, and the development of lung emphysema were substantially reduced by exogenous C1P applications, pointing to an anti-inflammatory action of C1P in the lungs. The molecular mechanisms involved in the regulation of cell growth, survival and migration with especial emphasis in the control of lung cancer biology are discussed.

Role of bioactive sphingolipids in physiology and pathology

Sphingolipids are a class of complex lipids containing a backbone of sphingoid bases, namely the organic aliphatic amino alcohol sphingosine (Sph), that are essential constituents of eukaryotic cells. They were first described as major components of cell membrane architecture, but it is now well established that some sphingolipids are bioactive and can regulate key biological functions. These include cell growth and survival, cell differentiation, angiogenesis, autophagy, cell migration, or organogenesis. Furthermore, some bioactive sphingolipids are implicated in pathological processes including inflammation-associated illnesses such as atherosclerosis, rheumatoid arthritis, inflammatory bowel disease (namely Crohn's disease and ulcerative colitis), type II diabetes, obesity, and cancer. A major sphingolipid metabolite is ceramide, which is the core of sphingolipid metabolism and can act as second messenger, especially when it is produced at the plasma membrane of cells. Ceramides promote cell cycle arrest and apoptosis. However, ceramide 1-phosphate (C1P), the product of ceramide kinase (CerK), and Sph 1-phosphate (S1P), which is generated by the action of Sph kinases (SphK), stimulate cell proliferation and inhibit apoptosis. Recently, C1P has been implicated in the spontaneous migration of cells from some types of cancer, and can enhance cell migration/invasion of malignant cells through interaction with a Gi protein-coupled receptor. In addition, CerK and SphK are implicated in inflammatory responses, some of which are associated with cancer progression and metastasis. Hence, targeting these sphingolipid kinases to inhibit C1P or S1P production, or blockade of their receptors might contribute to the development of novel therapeutic strategies to reduce metabolic alterations and disease.

Extracellular Adenosine Triphosphate (eATP) and Its Metabolite, Extracellular Adenosine (eAdo), as Opposing "Yin-Yang" Regulators of Nlrp3 Inflammasome in the Trafficking of Hematopoietic Stem/Progenitor Cells

Nlrp3 inflammasome plays a pleiotropic role in hematopoietic cells. On the one hand, physiological activation of this intracellular protein complex is crucial to maintaining normal hematopoiesis and the trafficking of hematopoietic stem progenitor cells (HSPCs). On the other hand, its hyperactivation may lead to cell death by pyroptosis, and prolonged activity is associated with sterile inflammation of the BM and, as a consequence, with the HSPCs aging and origination of myelodysplasia and leukemia. Thus, we need to understand better this protein complex’s actions to define the boundaries of its safety window and study the transition from being beneficial to being detrimental. As demonstrated, the Nlrp3 inflammasome is expressed and active both in HSPCs and in the non-hematopoietic cells that are constituents of the bone marrow (BM) microenvironment. Importantly, the Nlrp3 inflammasome responds to mediators of purinergic signaling, and while extracellular adenosine triphosphate (eATP) activates this protein complex, its metabolite extracellular adenosine (eAdo) has the opposite effect. In this review, we will discuss and focus on the physiological consequences of the balance between eATP and eAdo in regulating the trafficking of HSPCs in an Nlrp3 inflammasome-dependent manner, as seen during pharmacological mobilization from BM into peripheral blood (PB) and in the reverse mechanism of homing from PB to BM and engraftment. We propose that both mediators of purinergic signaling and the Nlrp3 inflammasome itself may become important therapeutic targets in optimizing the trafficking of HSPCs in clinical settings.

Role of Sphingolipid Signaling in Glomerular Diseases: Focus on DKD and FSGS

Sphingolipids are well-recognized as major players in the pathogenesis of many human diseases, including chronic kidney disease. The kidney is a very sensitive organ to alterations in sphingolipid metabolism. The critical issues to be addressed in this review relate to the role of sphingolipids and enzymes involved in sphingolipid metabolism in the pathogenesis of glomerular diseases with a special focus on podocytes, a key cellular component of the glomerular filtration barrier. Among several sphingolipids, we will highlight the role of ceramide, sphingosine, sphingosine-1-phosphate and ceramide-1-phosphate. Additionally, we will summarize the current knowledge with regard to the use of sphingolipids as therapeutic agents for the treatment of podocyte injury in kidney disease.

Adipocytes in hematopoiesis and acute leukemia: friends, enemies, or innocent bystanders?

The bone marrow is home to well-balanced normal hematopoiesis, but also the stage of leukemia's crime. Marrow adipose tissue (MAT) is a unique and versatile component of the bone marrow niche. While the importance of MAT for bone health has long been recognized, its complex role in hematopoiesis has only recently gained attention. In this review article we summarize recent conceptual advances in the field of MAT research and how these developments impact our understanding of MAT regulation of hematopoiesis. Elucidating routes of interaction and regulation between MAT and cells of the hematopoietic system are essential to pinpoint vulnerable processes resulting in malignant transformation. The concept of white adipose tissue contributing to cancer development and progression on the cellular, metabolic, and systemic level is generally accepted. The role of MAT in malignant hematopoiesis, however, is controversial. MAT is very sensitive to changes in the patient's metabolic status hampering a clear definition of its role in different clinical situations. Here, we discuss future directions for leukemia research in the context of metabolism-induced modifications of MAT and other adipose tissues and how this might impact on leukemia cell survival, proliferation, and antileukemic therapy.

An Overview of Novel Unconventional Mechanisms of Hematopoietic Development and Regulators of Hematopoiesis - a Roadmap for Future Investigations

Hematopoietic stem cells (HSCs) are the best-characterized stem cells in adult tissues. Nevertheless, as of today, many open questions remain. First, what is the phenotype of the most primitive "pre-HSC" able to undergo asymmetric divisions during ex vivo expansion that gives rise to HSC for all hemato-lymphopoietic lineages. Next, most routine in vitro assays designed to study HSC specification into hematopoietic progenitor cells (HPCs) for major hematopoietic lineages are based on a limited number of peptide-based growth factors and cytokines, neglecting the involvement of several other regulators that are endowed with hematopoietic activity. Examples include many hormones, such as pituitary gonadotropins, gonadal sex hormones, IGF-1, and thyroid hormones, as well as bioactive phosphosphingolipids and extracellular nucleotides (EXNs). Moreover, in addition to regulation by stromal-derived factor 1 (SDF-1), trafficking of these cells during mobilization or homing after transplantation is also regulated by bioactive phosphosphingolipids, EXNs, and three ancient proteolytic cascades, the complement cascade (ComC), the coagulation cascade (CoA), and the fibrinolytic cascade (FibC). Finally, it has emerged that bone marrow responds by "sterile inflammation" to signals sent from damaged organs and tissues, systemic stress, strenuous exercise, gut microbiota, and the administration of certain drugs. This review will address the involvement of these unconventional regulators and present a broader picture of hematopoiesis.

A Novel Evidence That Mannan Binding Lectin (MBL) Pathway of Complement Cascade Activation is Involved in Homing and Engraftment of Hematopoietic Stem Progenitor Cells (HSPCs)

Delayed homing and engraftment of hematopoietic stem progenitor cells (HSPCs) or even failure to engraft at all is significant clinical problem after hematopoietic transplant. Therefore, in order to develop more efficient homing and engraftment facilitating strategies it is important to learn more about this process. Our team has postulated that myeloablative conditioning for transplantation induces in bone marrow (BM) microenvironment a state of sterile inflammation in which elements of innate immunity activated by radio- or chemotherapy conditioning for transplant play an important role. In frame with this claim we reported that a significant role in this process plays activation of complement cascade (ComC). Accordingly, mice that that lack a fifth component (C5) of ComC turned out to engraft poorly with normal syngeneic BM cells as compared to normal control animals. In extension of our previous studies we provide for first time evidence that mannan binding lectin (MBL) pathway is involved in activation of ComC in myeloablated transplant recipient BM and thus plays an important role in homing and engraftment of HSPCs. To support this MBL-KO mice show significant defect in hematopoietic reconstitution after hematopoietic transplantation. This correlates with a decrease in expression of stromal derived factor-1 (SDF-1) and impaired activation of Nlrp3 inflammasome in irradiated BM of these mice.

Bioactive Phospholipids Enhance Migration and Adhesion of Human Leukemic Cells by Inhibiting Heme Oxygenase 1 (HO-1) and Inducible Nitric Oxygenase Synthase (iNOS) in a p38 MAPK-Dependent Manner

Bioactive phospholipids, including sphingosine-1-phosphate (S1P), ceramide-1-phosphate (C1P), lysophosphatidylcholine (LPC), and its derivative lysophosphatidic acid (LPA), have emerged as important mediators regulating the trafficking of normal and cancer cells. While the role of S1P in regulating migration of hematopoietic cells is well established, in this work we compared its biological effects to the effects of C1P, LPC, and LPA. We employed 10 human myeloid and lymphoid cell lines as well as blasts from AML patients. We observed that human leukemic cells express functional receptors for phospholipids and respond to stimulation by phosphorylation of p42/44 MAPK and AKT. We also found that bioactive phospholipids enhanced cell migration and adhesion of leukemic cells by downregulating expression of HO-1 and iNOS in a p38 MAPK-dependent manner but did not affect cell proliferation. By contrast, downregulation of p38 MAPK by SB203580 enhanced expression of HO-1 and iNOS and decreased migration of leukemic cells in vitro and their seeding efficiency to vital organs in vivo after injection into immunodeficient mice. Based on these findings, we demonstrate that, besides S1P, human leukemic cells also respond to C1P, LPC, and LPA. Since the prometastatic effects of bioactive phospholipids in vivo were mediated, at least in part, by downregulating HO-1 and iNOS expression in a p38 MAPK-dependent manner, we propose that inhibitors of p38 MAPK or stimulators of HO-1 activity will find application in inhibiting the spread of leukemic cells in response to bioactive phospholipids.

Stem cell homing: From physiology to therapeutics

Stem cell homing is a multistep endogenous physiologic process that is also used by exogenously administered hematopoietic stem and progenitor cells (HSPCs). This multistep process involves cell migration and is essential for hematopoietic stem cell transplantation. The process can be manipulated to enhance ultimate engraftment potential, and understanding stem cell homing is also important to the understanding of stem cell mobilization. Homing is also of potential importance in the recruitment of marrow mesenchymal stem and stromal cells (MSCs) to sites of injury and regeneration. This process is less understood but assumes importance when these cells are used for repair purposes. In this review, the process of HSPC and MSC homing is examined, as are methods to enhance this process.

Innate immunity orchestrates the mobilization and homing of hematopoietic stem/progenitor cells by engaging purinergic signaling-an update

Bone marrow (BM) as an active hematopoietic organ is highly sensitive to changes in body microenvironments and responds to external physical stimuli from the surrounding environment. In particular, BM tissue responds to several cues related to infections, strenuous exercise, tissue/organ damage, circadian rhythms, and physical challenges such as irradiation. These multiple stimuli affect BM cells to a large degree through a coordinated response of the innate immunity network as an important guardian for maintaining homeostasis of the body. In this review, we will foc++us on the role of purinergic signaling and innate immunity in the trafficking of hematopoietic stem/progenitor cells (HSPCs) during their egression from the BM into peripheral blood (PB), as seen along pharmacological mobilization, and in the process of homing and subsequent engraftment into BM after hematopoietic transplantation. Innate immunity mediates these processes by engaging, in addition to certain peptide-based factors, other important non-peptide mediators, including bioactive phosphosphingolipids and extracellular nucleotides, as the main topic of this review. Elucidation of these mechanisms will allow development of more efficient stem cell mobilization protocols to harvest the required number of HSPCs for transplantation and to accelerate hematopoietic reconstitution in transplanted patients.

Novel signaling aspects of ceramide 1-phosphate

The bioactive sphingolipid ceramide 1-phosphate (C1P) regulates key physiologic cell functions and is implicated in a number of metabolic alterations and pathological processes. Initial studies using different types of fibroblasts and monocytes/macrophages revealed that C1P was mitogenic and that it promoted cell survival through inhibition of apoptosis. Subsequent studies implicated C1P in inflammatory responses with a specific role as pro-inflammatory agent. Specifically, C1P potently stimulated cytosolic phospholipase A₂ (cPLA₂) resulting in elevation of arachidonic acid and pro-inflammatory eicosanoid levels. However, increasing experimental evidence suggests that C1P can also exert anti-inflammatory actions in some cell types and tissues. Specifically, it has been demonstrated that C1P inhibits the release of pro-inflammatory cytokines and blocks activation of the pro-inflammatory transcription factor NF-κB in some cell types. Moreover, C1P was shown to increase the release of anti-inflammatory interleukin-10 in macrophages, and to overcome airway inflammation and reduce lung emphysema in vivo. Noteworthy, C1P stimulated cell migration, an action that is associated with diverse physiological cell functions, as well as with inflammatory responses and tumor dissemination. More recently, ceramide kinase (CerK), the enzyme that produces C1P in mammalian cells, has been shown to be upregulated during differentiation of pre-adipocytes into mature adipocytes, and that exogenous C1P, acting through a putative Gi protein-coupled receptor, negatively regulates adipogenesis. Although the latter actions seem to be contradictory, it is plausible that exogenous C1P may balance the adipogenic effects of intracellularly generated (CerK-derived) C1P in adipose tissue. The present review highlights novel signaling aspects of C1P and its impact in the regulation of cell growth and survival, inflammation and tumor dissemination.

Crosstalk Between Lipids and Mitochondria in Diabetic Kidney Disease

PURPOSE OF REVIEW

The goal of this review is to review the role that renal parenchymal lipid accumulation plays in contributing to diabetic kidney disease (DKD), specifically contributing to the mitochondrial dysfunction observed in glomerular renal cells in the context of DKD development and progression.

RECENT FINDINGS

Mitochondrial dysfunction has been observed in experimental and clinical DKD. Recently, Ayanga et al. demonstrate that podocyte-specific deletion of a protein involved in mitochondrial dynamics protects from DKD progression. Furthermore, our group has recently shown that ATP-binding cassette A1 (a protein involved in cholesterol and phospholipid efflux) is significantly reduced in clinical and experimental DKD and that genetic or pharmacological induction of ABCA1 is sufficient to protect from DKD. ABCA1 deficiency in podocytes leads to mitochondrial dysfunction observed with alterations of mitochondrial lipids, in particular, cardiolipin (a mitochondrial-specific phospholipid). However, through pharmacological reduction of cardiolipin peroxidation DKD progression is reverted. Lipid metabolism is significantly altered in the diabetic kidney and renders cellular components, such as the podocyte, susceptible to injury leading to worsened DKD progression. Dysfunction of the lipid metabolism pathway can also lead to mitochondrial dysfunction and mitochondrial lipid alteration. Future research aimed at targeting mitochondrial lipids content and function could prove to be beneficial for the treatment of DKD.

Lipid mediators of insulin signaling in diabetic kidney disease

Diabetic kidney disease (DKD) affects ∼40% of patients with diabetes and is associated with high mortality rates. Among different cellular targets in DKD, podocytes, highly specialized epithelial cells of the glomerular filtration barrier, are injured in the early stages of DKD. Both clinical and experimental data support the role of preserved insulin signaling as a major contributor to podocyte function and survival. However, little is known about the key modulators of podocyte insulin signaling. This review summarizes the novel knowledge that intracellular lipids such as cholesterol and sphingolipids are major determinants of podocyte insulin signaling. In particular, the implications of these lipids on DKD development, progression, and treatment will be addressed.

Ceramide 1-Phosphate Protects Endothelial Colony–Forming Cells From Apoptosis and Increases Vasculogenesis In Vitro and In Vivo

Objective:

Ceramide 1-phosphate (C1P) is a bioactive sphingolipid highly augmented in damaged tissues. Because of its abilities to stimulate migration of murine bone marrow–derived progenitor cells, it has been suggested that C1P might be involved in tissue regeneration. In the present study, we aimed to investigate whether C1P regulates survival and angiogenic activity of human progenitor cells with great therapeutic potential in regenerative medicine such as endothelial colony–orming cells (ECFCs).

Approach and Results:

C1P protected ECFC from TNFα (tumor necrosis factor-α)-induced and monosodium urate crystal–induced death and acted as a potent chemoattractant factor through the activation of ERK1/2 (extracellular signal-regulated kinases 1 and 2) and AKT pathways. C1P treatment enhanced ECFC adhesion to collagen type I, an effect that was prevented by β1 integrin blockade, and to mature endothelial cells, which was mediated by the E-selectin/CD44 axis. ECFC proliferation and cord-like structure formation were also increased by C1P, as well as vascularization of gel plug implants loaded or not with ECFC. In a murine model of hindlimb ischemia, local administration of C1P alone promoted blood perfusion and reduced necrosis in the ischemic muscle. Additionally, the beneficial effects of ECFC infusion after ischemia were amplified by C1P pretreatment, resulting in a further and significant enhancement of leg reperfusion and muscle repair.

Conclusions:

Our findings suggest that C1P may have therapeutic relevance in ischemic disorders, improving tissue repair by itself, or priming ECFC angiogenic responses such as chemotaxis, adhesion, proliferation, and tubule formation, which result in a better outcome of ECFC-based therapy.

SMPDL3b modulates insulin receptor signaling in diabetic kidney disease

Sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b) is a lipid raft enzyme that regulates plasma membrane (PM) fluidity. Here we report that SMPDL3b excess, as observed in podocytes in diabetic kidney disease (DKD), impairs insulin receptor isoform B-dependent pro-survival insulin signaling by interfering with insulin receptor isoforms binding to caveolin-1 in the PM. SMPDL3b excess affects the production of active sphingolipids resulting in decreased ceramide-1-phosphate (C1P) content as observed in human podocytes in vitro and in kidney cortexes of diabetic db/db mice in vivo. Podocyte-specific Smpdl3b deficiency in db/db mice is sufficient to restore kidney cortex C1P content and to protect from DKD. Exogenous administration of C1P restores IR signaling in vitro and prevents established DKD progression in vivo. Taken together, we identify SMPDL3b as a modulator of insulin signaling and demonstrate that supplementation with exogenous C1P may represent a lipid therapeutic strategy to treat diabetic complications such as DKD.

Sphingolipids as Emerging Mediators in Retina Degeneration

The sphingolipids ceramide (Cer), sphingosine-1-phosphate (S1P), sphingosine (Sph), and ceramide-1-phosphate (C1P) are key signaling molecules that regulate major cellular functions. Their roles in the retina have gained increasing attention during the last decade since they emerge as mediators of proliferation, survival, migration, neovascularization, inflammation and death in retina cells. As exacerbation of these processes is central to retina degenerative diseases, they appear as crucial players in their progression. This review analyzes the functions of these sphingolipids in retina cell types and their possible pathological roles. Cer appears as a key arbitrator in diverse retinal pathologies; it promotes inflammation in endothelial and retina pigment epithelium (RPE) cells and its increase is a common feature in photoreceptor death in vitro and in animal models of retina degeneration; noteworthy, inhibiting Cer synthesis preserves photoreceptor viability and functionality. In turn, S1P acts as a double edge sword in the retina. It is essential for retina development, promoting the survival of photoreceptors and ganglion cells and regulating proliferation and differentiation of photoreceptor progenitors. However, S1P has also deleterious effects, stimulating migration of Müller glial cells, angiogenesis and fibrosis, contributing to the inflammatory scenario of proliferative retinopathies and age related macular degeneration (AMD). C1P, as S1P, promotes photoreceptor survival and differentiation. Collectively, the expanding role for these sphingolipids in the regulation of critical processes in retina cell types and in their dysregulation in retina degenerations makes them attractive targets for treating these diseases.

NLRP3 inflammasome couples purinergic signaling with activation of the complement cascade for the optimal release of cells from bone marrow

The mechanisms that regulate egress of hematopoietic stem/progenitor cells (HSPCs) into peripheral blood (PB) in response to stress, inflammation, tissue/organ injury, or administration of mobilization-inducing drugs are still not well understood, and because of the importance of stem cell trafficking in maintaining organism homeostasis, several complementary pathways are believed to be involved. Our group proposes that mobilization of HSPCs is mainly a result of sterile inflammation in the bone marrow (BM) microenvironment in response to pro-mobilizing stimuli and that during the initiation phase of the mobilization process BM-residing cells belonging to the innate immunity system, including granulocytes and monocytes, release danger-associated molecular pattern molecules (DAMPs, also known as alarmins), reactive oxygen species (ROS), as well as proteolytic and lipolytic enzymes. These factors together orchestrate the release of HSPCs into PB. One of the most important DAMPs released in the initiation phase of mobilization is extracellular adenosine triphosphate, a potent activator of the inflammasome. As a result of its activation, IL-1β and IL-18 as well as other pro-mobilizing mediators, including DAMPs such as high molecular group box 1 (Hmgb1) and S100 calcium-binding protein A9 (S100a9), are released. These DAMPs are important activators of the complement cascade (ComC) in the mannan-binding lectin (MBL)-dependent pathway. Specifically, Hmgb1 and S100a9 bind to MBL, which leads to activation of MBL-associated proteases, which activate the ComC and in parallel also trigger activation of the coagulation cascade (CoaC). In this review, we will highlight the novel role of the innate immunity cell-expressed NLRP3 inflammasome, which, during the initiation phase of HSPC mobilization, couples purinergic signaling with the MBL-dependent pathway of the ComC and, in parallel, the CoaC for optimal release of HSPCs. These data are important to optimize the pharmacological mobilization of HSPCs.

Suppression of chemotherapy-induced cytokine/lipid mediator surge and ovarian cancer by a dual COX-2/sEH inhibitor

Although chemotherapy is a conventional cancer treatment, it may induce a protumorigenic microenvironment by triggering the release of proinflammatory mediators. In this study, we demonstrate that ovarian tumor cell debris generated by first-line platinum- and taxane-based chemotherapy accelerates tumor progression by stimulating a macrophage-derived "surge" of proinflammatory cytokines and bioactive lipids. Thus, targeting a single inflammatory mediator or pathway is unlikely to prevent therapy-induced tumor progression. Here, we show that combined pharmacological abrogation of the cyclooxygenase-2 (COX-2) and soluble epoxide hydrolase (sEH) pathways prevented the debris-induced surge of both cytokines and lipid mediators by macrophages. In animal models, the dual COX-2/sEH inhibitor PTUPB delayed the onset of debris-stimulated ovarian tumor growth and ascites leading to sustained survival over 120 days postinjection. Therefore, dual inhibition of COX-2/sEH may be an approach to suppress debris-stimulated ovarian tumor growth by preventing the therapy-induced surge of cytokines and lipid mediators.

Novel Lipid Signaling Mediators for Mesenchymal Stem Cell Mobilization during Bone Repair

Introduction

Mesenchymal stem and progenitor cells (MSCs), which normally reside in the bone marrow, are critical to bone health and can be recruited to sites of traumatic bone injury, contributing to new bone formation. The ability to control the trafficking of MSCs provides therapeutic potential for improving traumatic bone healing and therapy for genetic bone diseases such as hypophosphatasia.

Methods

In this study, we explored the sphingosine-1-phosphate (S1P) signaling axis as a means to control the mobilization of MSCs into blood and possibly to recruit MSCs enhancing bone growth.

Results

Loss of S1P receptor 3 (S1PR3) leads to an increase in circulating CD45-/CD29+/CD90+/Sca1 putative mesenchymal progenitor cells, suggesting that blocking S1PR3 may stimulate MSCs to leave the bone marrow. Antagonism of S1PR3 with the small molecule VPC01091 stimulated acute migration of CD45-/CD29+/CD90+/Sca1+ MSCs into the blood as early as 1.5 hours after treatment. VPC01091 administration also increased ectopic bone formation induced by BMP-2 and significantly increased new bone formation in critically sized rat cranial defects, suggesting that mobilized MSCs may home to injuries to contribute to healing. We also explored the possibility of combining S1P manipulation of endogenous host cell occupancy with exogenous MSC transplantation for potential use in combination therapies. Importantly, reducing niche occupancy of host MSCs with VPC01091 does not impede engraftment of exogenous MSCs.

Conclusions

Our studies suggest that MSC mobilization through S1PR3 antagonism is a promising strategy for endogenous tissue engineering and improving MSC delivery to treat bone diseases.

The Role of Ceramide 1-Phosphate in Tumor Cell Survival and Dissemination

Ceramide 1-phosphate (C1P) is a pleiotropic bioactive sphingolipid metabolite capable of regulating key physiologic cell functions and promoting pathologic processes. Concerning pathology, C1P or ceramide kinase (CerK), the enzyme responsible for its biosynthesis in mammalian cells, has been implicated in cancer cell growth, survival, and dissemination and is involved in inflammatory responses associated with different types of cancer cells. The mechanisms or signaling pathways mediating these C1P actions have only been partially described. This chapter reviews recent progress in identifying signal transduction pathways involved in the promotion of cancer cell growth, survival, and dissemination by CerK and C1P.

Complement in stem cells and development

From its discovery in the late nineteenth century, as a 'complement' to the cellular immune response, the complement system has been widely affirmed as a powerful controller of innate and adaptive immune responses. In recent decades however, new roles for complement have been discovered, with multiple complement proteins now known to function in a broad array of non-immune systems. This includes during development, where complement exerts control over stem cell populations from fertilization and implantation throughout embryogenesis and beyond post-natal development. It is involved in processes as diverse as cell localisation, tissue morphogenesis, and the growth and refinement of the brain. Such physiological actions of complement have also been described in adult stem cell populations, with roles in proliferation, differentiation, survival, and regeneration. With such a broad range of complement functions now described, it is likely that current research only describes a fraction of the full reach of complement proteins. Here, we review how complement control of physiological cell processes has been harnessed in stem cell populations throughout both development and in adult physiology.

Evidence that vitronectin is a potent migration-enhancing factor for cancer cells chaperoned by fibrinogen: a novel view of the metastasis of cancer cells to low-fibrinogen lymphatics and body cavities

Diluted (1%) plasma induces migration of malignant cell lines much more strongly than potent pro-metastatic factors. To characterize the factor(s) present in diluted plasma responsible for this phenomenon we performed i) heat inactivation, ii) dialysis, iii) proteinase K treatment, and iv) molecular size filtration studies. We found that this remarkable pro-migratory activity of diluted normal plasma is associated with a ~50–100-kD protein that interacts with G_αI protein-coupled receptors and activates p42/44 MAPK and AKT signaling in target cells. Since this pro-migratory activity of 1% plasma decreases at higher plasma concentrations (> 20%), but is retained in serum, we hypothesized that fibrinogen may be involved as a chaperone of the protein(s). To identify the pro-migratory protein(s) present in diluted plasma and fibrinogen-depleted serum, we performed gel filtration and hydrophobic interaction chromatography followed by mass spectrometry analysis. We identified several putative protein candidates that were further tested in in vitro experiments. We found that this pro-migratory factor chaperoned by fibrinogen is vitronectin, which activates uPAR, and that this effect can be inhibited by fibrinogen. These results provide a novel mechanism for the metastasis of cancer cells to lymphatics and body cavities, in which the concentration of fibrinogen is low, and thus suggests that free vitronectin stimulates migration of tumor cells.

Inducible Nitric Oxide Synthase (iNOS) Is a Novel Negative Regulator of Hematopoietic Stem/Progenitor Cell Trafficking

Nitric oxide (NO) is a gaseous free radical molecule involved in several biological processes related to inflammation, tissue damage, and infections. Based on reports that NO inhibits migration of granulocytes and monocytes, we became interested in the role of inducible NO synthetase (iNOS) in pharmacological mobilization of hematopoietic stem/progenitor cells (HSPCs) from bone marrow (BM) into peripheral blood (PB). To address the role of NO in HSPC trafficking, we upregulated or downregulated iNOS expression in hematopoietic cell lines. Next, we performed mobilization studies in iNOS^-/- mice and evaluated engraftment of iNOS^-/- HSPCs in wild type (control) animals. Our results indicate that iNOS is a novel negative regulator of hematopoietic cell migration and prevents egress of HSPCs into PB during mobilization. At the molecular level, downregulation of iNOS resulted in downregulation of heme oxygenase 1 (HO-1), and, conversely, upregulation of iNOS enhanced HO-1 activity. Since HO-1 is a negative regulator of cell migration, the inhibitory effects of iNOS identified by us can be at least partially explained by its enhancing the HO-1 level in BM cells.

Vascular endothelial growth factor mediates ceramide 1-phosphate-stimulated macrophage proliferation

The bioactive sphingolipid ceramide 1-phosphate (C1P) regulates cell division in a variety of cell types including macrophages. However, the mechanisms involved in this action are not completely understood. In the present work we show that C1P stimulates the release of vascular endothelial growth factor (VEGF) in RAW264.7 macrophages, and that this growth factor is essential for stimulation of cell proliferation by C1P. The stimulation of VEGF release was dependent upon activation of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB-1 also known as Akt-1), and mitogen-activated protein kinase-kinase (MEK)/extracellularly regulated kinase-2 (ERK-2) pathways, as inhibition of these kinases with selective pharmacological inhibitors or with specific gene silencing siRNA, abrogated VEGF release. A key observation was that sequestration of VEGF with a neutralizing antibody, or treatment with VEGF siRNA abolished C1P-stimulated macrophage growth. Also, inhibition of the pathways involved in C1P-stimulated VEGF release inhibited the stimulation of macrophage growth by C1P. Moreover, blockade of VEGF receptor-2 (VEGFR-2), which is the primary receptor for VEGF, with the pharmacological inhibitor DMH4, or with specific VEGFR-2 siRNA, substantially inhibited C1P-stimulated cell growth. It can be concluded that stimulation of VEGF release is a key factor in the promotion of macrophage proliferation by C1P.

Hematopoietic stem cells under pressure

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) and progenitors are tasked with maintaining hematopoietic homeostasis in the face of numerous insults and challenges, including infection, inflammation, and exsanguination. HSCs possess the remarkable ability to reconstitute the entire hematopoietic system of an organism whose own hematopoietic system has been ablated. This ability is exploited routinely in the clinic via HSC transplantation (HSCT). Here, we focus on the physiological and molecular bottlenecks overcome by HSCs during transplantation.

RECENT FINDINGS

During transplantation, HSCs encounter a damaged bone marrow niche, characterized molecularly by increases in oxygen concentrations and an altered cytokine milieu. New mechanisms and pathways have been recently implicated during HSCT, including transplanted HSC-dependent secretion of conditioning molecules that facilitate engraftment and pathways that protect HSCs from perturbed organelle homeostasis.

SUMMARY

Better understanding the molecular processes HSCs employ to withstand the stress of transplant will illuminate novel targets for further improving conditioning regimens and engraftment during HSCT.

Complement C5-inhibiting therapy for the thrombotic microangiopathies: accumulating evidence, but not a panacea

Thrombotic microangiopathy (TMA), characterized by organ injury occurring consequent to severe endothelial damage, can manifest in a diverse range of diseases. In complement-mediated atypical haemolytic uraemic syndrome (aHUS) a primary defect in complement, such as a mutation or autoantibody leading to over activation of the alternative pathway, predisposes to the development of disease, usually following exposure to an environmental trigger. The elucidation of the pathogenesis of aHUS resulted in the successful introduction of the complement inhibitor eculizumab into clinical practice. In other TMAs, although complement activation may be seen, its role in the pathogenesis remains to be confirmed by an interventional trial. Although many case reports in TMAs other than complement-mediated aHUS hint at efficacy, publication bias, concurrent therapies and in some cases the self-limiting nature of disease make broader interpretation difficult. In this article, we will review the evidence for the role of complement inhibition in complement-mediated aHUS and other TMAs.

Sphingosine 1-Phosphate Signaling and Its Pharmacological Modulation in Allogeneic Hematopoietic Stem Cell Transplantation

Allogeneic haemopoietic stem cell transplantation (HSCT) is increasingly used to treat haematological malignant diseases via the graft-versus-leukaemia (GvL) or graft-versus-tumour effects. Although improvements in infectious disease prophylaxis, immunosuppressive treatments, supportive care, and molecular based tissue typing have contributed to enhanced outcomes, acute graft-versus-host disease and other transplant related complications still contribute to high mortality and significantly limit the more widespread use of HSCT. Sphingosine 1-phosphate (S1P) is a zwitterionic lysophospholipid that has been implicated as a crucial signaling regulator in many physiological and pathophysiological processes including multiple cell types such as macrophages, dendritic cells, T cells, T regulatory cells and endothelial cells. Recent data suggested important roles for S1P signaling in engraftment, graft-versus-host disease (GvHD), GvL and other processes that occur during and after HSCT. Based on such data, pharmacological intervention via S1P modulation may have the potential to improve patient outcome by regulating GvHD and enhancing engraftment while permitting effective GvL.

Mobilization studies in mice deficient in sphingosine kinase 2 support a crucial role of the plasma level of sphingosine-1-phosphate in the egress of hematopoietic stem progenitor cells

Sphingosine-1-phosphate (S1P) is a bioactive lipid involved in cell signaling and, if released from cells, also plays a crucial role in regulating the trafficking of lympho-hematopoietic cells, including primitive hematopoietic stem/progenitor cells (HSPCs). It has been demonstrated that S1P chemoattracts HSPCs, and its level in peripheral blood creates a gradient directing egress of these cells during mobilization. In this paper we analyzed hematopoiesis in mice deficient in sphingosine kinase 2 (Sphk2-KO mice) and studied the effect of this mutation on plasma S1P levels. We found that Sphk2-KO mice have normal hematopoiesis, and, in contrast to Sphk1-KO mice, the circulating S1P level is highly elevated in these animals and correlates with the fact that HSPCs in Sphk2-KO animals, also in contrast to Sphk1-KO animals, show enhanced mobilization. These results were recapitulated in wild type (WT) animals employing an Sphk2 inhibitor. We also administered an inhibitor of the S1P-degrading enzyme S1P lyase, known as tetrahydroxybutylimidazole (THI), to WT mice and observed that this resulted in an increase in S1P level in PB and enhanced mobilization of HSPCs. In sum, our results support a crucial role for S1P gradients in blood plasma in the mobilization process and indicate that small-molecule inhibitors of Sphk2 and Sgpl1 could be employed as mobilization-facilitating compounds. At the same time, further studies are needed to explain the unexpected effect of Sphk2 inhibition on increasing S1P levels in plasma.

Innate Immunity and Mobilization of Hematopoietic Stem Cells

PURPOSE OF REVIEW

Several mechanisms have been postulated to orchestrate mobilization of hematopoietic stem/progenitor cells (HSPCs), and still more work is needed to better understand this process and to gain better mechanistic insight.

RECENT FINDINGS

Evidence accumulated that mobilization of HSPCs is a part of innate immunity response to tissue organ injury, stress, and infection. This evolutionary ancient process is orchestrated by granulocytes and monocytes that trigger activation of complement cascade and the coagulation cascade.

SUMMARY

We will present data from our laboratory that initiation of complement cascade activation and subsequently activation of the coagulation cascade during mobilization process are dependent on mannan-binding lectin (MBL). The mannan-binding pathway activates MBL-associated serine proteases (MASP-1 and MASP-2) that cleave the third complement component C3 and prothrombin. Cleavage of C3 leads to formation of classical C5 convertase and cleavage of prothrombin generates thrombin, which has "C5-like convertase" activity. Finally, both C5 convertase and thrombin cleave the fifth complement component C5, and activate distal part of the complement cascade that is crucial for egress of HSCPs from bone marrow niches into peripheral blood.