SDF-1 synergistically enhances IL-3-induced activation of the Raf-1/MEK/Erk signaling pathway through activation of Rac and its effector Pak kinases to promote hematopoiesis and chemotaxis

Elucidating the Morphogenic and Signaling Roles of Defined Growth Factors Controlling Human Endothelial Cell Lumen Formation Versus Sprouting Behavior

Five growth factors [ie, insulin, fibroblast growth factor-2 (FGF-2), stem cell factor, IL-3, and stromal-derived factor 1α] in combination are necessary for human endothelial cells (ECs) to undergo tube morphogenesis, a process requiring both lumen formation and sprouting behavior. This study investigated why these factors are required by subdividing the factors into 4 separate groups: insulin-only, insulin and FGF-2, no FGF-2 (all factors but without FGF-2), and all factors. The study found that the insulin-only condition failed to support EC morphogenesis or survival, the insulin and FGF-2 condition supported primarily EC lumen formation, and the no FGF-2 condition supported EC sprouting behavior. By comparison, the all-factors condition more strongly stimulated both EC lumen formation and sprouting behavior, and signaling analysis revealed prolonged stimulation of multiple promorphogenic signals coupled with inhibition of proregressive signals. Pharmacologic inhibition of Jak kinases more selectively blocked EC sprouting behavior, whereas inhibition of Raf, phosphatidylinositol 3-kinase, and Akt kinases showed selective blockade of lumen formation. Inhibition of Src family kinases and Notch led to increased sprouting coupled to decreased lumen formation, whereas inhibition of Pak, Mek, and mammalian target of rapamycin kinases blocked both sprouting and lumen formation. These findings reveal novel downstream biological and signaling activities of defined factors that are required for the assembly of human EC-lined capillary tube networks.

Humanized zebrafish enhance human hematopoietic stem cell survival and promote acute myeloid leukemia clonal diversity

Xenograft models are invaluable tools in establishing the current paradigms of hematopoiesis and leukemogenesis. The zebrafish has emerged as a robust alternative xenograft model but, like mice, lack specific cytokines that mimic the microenvironment found in human patients. To address this critical gap, we generated the first humanized zebrafish that express human hematopoietic-specific cytokines (GM-CSF, SCF, and SDF1α). Termed GSS fish, these zebrafish promote survival, self-renewal and multilineage differentiation of human hematopoietic stem and progenitor cells and result in enhanced proliferation and hematopoietic niche-specific homing of primary human leukemia cells. Using error-corrected RNA sequencing, we determined that patient-derived leukemias transplanted into GSS zebrafish exhibit broader clonal representation compared to transplants into control hosts. GSS zebrafish incorporating error-corrected RNA sequencing establish a new standard for zebrafish xenotransplantation that more accurately recapitulates the human context, providing a more representative cost-effective preclinical model system for evaluating personalized response-based treatment in leukemia and therapies to expand human hematopoietic stem and progenitor cells in the transplant setting.

The molecular basis for immune dysregulation by the hyperactivated E62K mutant of the GTPase RAC2

The RAS-related C3 botulinum toxin substrate 2 (RAC2) is a member of the RHO subclass of RAS superfamily GTPases required for proper immune function. An activating mutation in a key switch II region of RAC2 (RAC2E62K) involved in recognizing modulatory factors and effectors has been identified in patients with common variable immune deficiency. To better understand how the mutation dysregulates RAC2 function, we evaluated the structure and stability, guanine nucleotide exchange factor (GEF) and GTPase-activating protein (GAP) activity, and effector binding of RAC2E62K Our findings indicate the E62K mutation does not alter RAC2 structure or stability. However, it does alter GEF specificity, as RAC2E62K is activated by the DOCK GEF, DOCK2, but not by the Dbl homology GEF, TIAM1, both of which activate the parent protein. Our previous data further showed that the E62K mutation impairs GAP activity for RAC2E62K As this disease mutation is also found in RAS GTPases, we assessed GAP-stimulated GTP hydrolysis for KRAS and observed a similar impairment, suggesting that the mutation plays a conserved role in GAP activation. We also investigated whether the E62K mutation alters effector binding, as activated RAC2 binds effectors to transmit signaling through effector pathways. We find that RAC2E62K retains binding to an NADPH oxidase (NOX2) subunit, p67phox, and to the RAC-binding domain of p21-activated kinase, consistent with our earlier findings. Taken together, our findings indicate that the RAC2E62K mutation promotes immune dysfunction by promoting RAC2 hyperactivation, altering GEF specificity, and impairing GAP function yet retaining key effector interactions.

Relevance of small GTPase Rac1 pathway in drug and radio-resistance mechanisms: Opportunities in cancer therapeutics

Rac1 GTPase signaling pathway has a critical role in the regulation of a plethora of cellular functions governing cancer cell behavior. Recently, it has been shown a critical role of Rac1 in the emergence of resistance mechanisms to cancer therapy. This review describes the current knowledge regarding Rac1 pathway deregulation and its association with chemoresistance, radioresistance, resistance to targeted therapies and immune evasion. This supports the idea that interfering Rac1 signaling pathway could be an interesting approach to tackle cancer resistance.

Molecular mechanisms for enhancement of stromal cell-derived factor 1-induced chemotaxis by platelet endothelial cell adhesion molecule 1 (PECAM-1)

Platelet endothelial cell adhesion molecule 1 (PECAM-1) is a cell adhesion protein involved in the regulation of cell adhesion and migration. Interestingly, several PECAM-1-deficient hematopoietic cells exhibit impaired chemotactic responses to stromal cell-derived factor 1 (SDF-1), a chemokine essential for B lymphopoiesis and bone marrow myelopoiesis. However, whether PECAM-1 is involved in SDF-1-regulated chemotaxis is unknown. We report here that SDF-1 induces tyrosine phosphorylation of PECAM-1 at its immunoreceptor tyrosine-based inhibition motifs in several hematopoietic cell lines via the Src family kinase Lyn, Bruton's tyrosine kinase, and JAK2 and that inhibition of these kinases reduced chemotaxis. Overexpression and knockdown of PECAM-1 enhanced and down-regulated, respectively, SDF-1-induced Gα_i-dependent activation of the PI3K/Akt/mTORC1 pathway and small GTPase Rap1 in hematopoietic 32Dcl3 cells, and these changes in activation correlated with chemotaxis. Furthermore, pharmacological or genetic inhibition of the PI3K/Akt/mTORC1 pathway or Rap1, respectively, revealed that these pathways are independently activated and required for SDF-1-induced chemotaxis. When coexpressed in 293T cells, PECAM-1 physically associated with the SDF-1 receptor CXCR4. Moreover, PECAM-1 overexpression and knockdown reduced and enhanced SDF-1-induced endocytosis of CXCR4, respectively. Furthermore, when expressed in 32Dcl3 cells, an endocytosis-defective CXCR4 mutant, CXCR4-S324A/S325A, could activate the PI3K/Akt/mTORC1 pathway as well as Rap1 and induce chemotaxis in a manner similar to PECAM-1 overexpression. These findings suggest that PECAM-1 enhances SDF-1-induced chemotaxis by augmenting and prolonging activation of the PI3K/Akt/mTORC1 pathway and Rap1 and that PECAM-1, at least partly, exerts its activity by inhibiting SDF-1-induced internalization of CXCR4.

Syndecan-1 increases B-lymphoid cell extravasation in response to HIV-1 Tat via αvβ3/pp60src/pp125FAK pathway

Syndecan-1 is a heparan sulfate proteoglycan (HSPG) commonly upregulated in AIDS-related B lymphoid malignancies. Tat is the main HIV-1 transactivating factor that has a major role in the pathogenesis of AIDS-related lymphomas (ARL) by engaging heparan sulfate proteoglycans (HSPGs), chemokine receptors and integrins at the lymphoid cell (LC) surface. Here B-lymphoid Namalwa cell clones that do not express or overexpress syndecan-1 (EV-Ncs and SYN-Ncs, respectively) were compared for their responsiveness with Tat: in the absence of syndecan-1, Tat induces a limited EV-Nc migration via C-X-C motif chemokine receptor 4 (CXCR4), G-proteins and Rac. Syndecan-1 overexpression increases SYN-Nc responsiveness to Tat and makes this response independent from CXCR4 and G-protein and dependent instead on pp60src phosphorylation. Tat-induced SYN-Nc migration and pp60src phosphorylation require the engagement of α_vβ₃ integrin and consequent pp125FAK phosphorylation. This complex set of Tat-driven activations is orchestrated by the direct interaction of syndecan-1 with pp60src and its simultaneous coupling with α_vβ₃. The Tat/syndecan-1/α_vβ₃ interplay is retained in vivo and is shared also by other syndecan-1⁺ B-LCs, including BJAB cells, whose responsiveness to Tat is inhibited by syndecan-1 knockdown. In conclusion, overexpression of syndecan-1 confers to B-LCs an increased capacity to migrate in response to Tat, owing to a switch from a CXCR4/G-protein/Rac to a syndecan-1/α_vβ₃/pp60src/pp125FAK signal transduction pathway that depends on the formation of a complex in which syndecan-1 interacts with Tat via its HS-chains, with α_vβ₃ via its core protein ectodomain and with pp60src via its intracellular tail. These findings have implications in ARL progression and may help in identifying new therapeutical targets for the treatment of AIDS-associated neoplasia.

p21-activated kinase 2 regulates HSPC cytoskeleton, migration, and homing via CDC42 activation and interaction with β-Pix

Cytoskeletal remodeling of hematopoietic stem and progenitor cells (HSPCs) is essential for homing to the bone marrow (BM). The Ras-related C3 botulinum toxin substrate (Rac)/cell division control protein 42 homolog (CDC42) effector p21-activated kinase (Pak2) has been implicated in HSPC homing and engraftment. However, the molecular pathways mediating Pak2 functions in HSPCs are unknown. Here, we demonstrate that both Pak2 kinase activity and its interaction with the PAK-interacting exchange factor-β (β-Pix) are required to reconstitute defective ITALIC! Pak2 (ITALIC! Δ/Δ)HSPC homing to the BM. Pak2 serine/threonine kinase activity is required for stromal-derived factor-1 (SDF1α) chemokine-induced HSPC directional migration, whereas Pak2 interaction with β-Pix is required to regulate the velocity of HSPC migration and precise F-actin assembly. Lack of SDF1α-induced filopodia and associated abnormal cell protrusions seen in ITALIC! Pak2 (ITALIC! Δ/Δ)HSPCs were rescued by wild-type (WT) Pak2 but not by a Pak2-kinase dead mutant (KD). Expression of a β-Pix interaction-defective mutant of Pak2 rescued filopodia formation but led to abnormal F-actin bundles. Although CDC42 has previously been considered an upstream regulator of Pak2, we found a paradoxical decrease in baseline activation of CDC42 in ITALIC! Pak2 (ITALIC! Δ/Δ)HSPCs, which was rescued by expression of Pak2-WT but not by Pak2-KD; defective homing of ITALIC! Pak2-deleted HSPCs was rescued by constitutive active CDC42. These data demonstrate that both Pak2 kinase activity and its interaction with β-Pix are essential for HSPC filopodia formation, cytoskeletal integrity, and homing via activation of CDC42. Taken together, we provide mechanistic insights into the role of Pak2 in HSPC migration and homing.

Pak2 restrains endomitosis during megakaryopoiesis and alters cytoskeleton organization

Megakaryocyte maturation and polyploidization are critical for platelet production; abnormalities in these processes are associated with myeloproliferative disorders, including thrombocytopenia. Megakaryocyte maturation signals through cascades that involve p21-activated kinase (Pak) function; however, the specific role for Pak kinases in megakaryocyte biology remains elusive. Here, we identify Pak2 as an essential effector of megakaryocyte maturation, polyploidization, and proplatelet formation. Genetic deletion of Pak2 in murine bone marrow is associated with macrothrombocytopenia, altered megakaryocyte ultrastructure, increased bone marrow megakaryocyte precursors, and an elevation of mature CD41(+) megakaryocytes, as well as an increased number of polyploid cells. In Pak2(-/-) mice, platelet clearance rate was increased, as was production of newly synthesized, reticulated platelets. In vitro, Pak2(-/-) megakaryocytes demonstrate increased polyploidization associated with alterations in β1-tubulin expression and organization, decreased proplatelet extensions, and reduced phosphorylation of the endomitosis regulators LIM domain kinase 1, cofilin, and Aurora A/B/C. Together, these data establish a novel role for Pak2 as an important regulator of megakaryopoiesis, polyploidization, and cytoskeletal dynamics in developing megakaryocytes.

Silencing HIF-1α reduces the adhesion and secretion functions of acute leukemia hBMSCs

Hypoxia inducible factor-1α (HIF-1α) is an important transcription factor, which plays a critical role in the formation of solid tumor and its microenvironment. The objective of the present study was to evaluate the expression and function of HIF-1α in human leukemia bone marrow stromal cells (BMSCs) and to identify the downstream targets of HIF-1α. HIF-1α expression was detected at both the RNA and protein levels using real-time PCR and immunohistochemistry, respectively. Vascular endothelial growth factor (VEGF) and stromal cell-derived factor-1α (SDF-1α) were detected in stromal cells by enzyme-linked immunosorbent assay. HIF-1α was blocked by constructing the lentiviral RNAi vector system and infecting the BMSCs. The Jurkat cell/BMSC co-cultured system was constructed by putting the two cells into the same suitable cultured media and conditions. Cell adhesion and secretion functions of stromal cells were evaluated after transfection with the lentiviral RNAi vector of HIF-1α. Increased HIF-1α mRNA and protein was detected in the nucleus of the acute myeloblastic and acute lymphoblastic leukemia compared with normal BMSCs. The lentiviral RANi vector for HIF-1α was successfully constructed and was applied to block the expression of HIF-1α. When HIF-1α of BMSCs was blocked, the expression of VEGF and SDF-1 secreted by stromal cells were decreased. When HIF-1α was blocked, the co-cultured Jurkat cell's adhesion and migration functions were also decreased. Taken together, these results suggest that HIF-1α acts as an important transcription factor and can significantly affect the secretion and adhesion functions of leukemia BMSCs.

Group I PAK inhibitor IPA-3 induces cell death and affects cell adhesivity to fibronectin in human hematopoietic cells

P21-activated kinases (PAKs) are involved in the regulation of multiple processes including cell proliferation, adhesion and migration. However, the current knowledge about their function is mainly based on results obtained in adherent cell types. We investigated the effect of group I PAK inhibition using the compound IPA-3 in a variety of human leukemic cell lines (JURL-MK1, MOLM-7, K562, CML-T1, HL-60, Karpas-299, Jurkat, HEL) as well as in primary blood cells. IPA-3 induced cell death with EC50 ranging from 5 to more than 20 μM. Similar range was found for IPA-3-mediated dephosphorylation of a known PAK downstream effector, cofilin. The cell death was associated with caspase-3 activation, PARP cleavage and apoptotic DNA fragmentation. In parallel, 20 μM IPA-3 treatment induced rapid and marked decrease of the cell adhesivity to fibronectin. Per contra, partial reduction of PAK activity using lower dose IPA-3 or siRNA resulted in a slight increase in the cell adhesivity. The changes in the cell adhesivity were also studied using real-time microimpedance measurement and by interference reflection microscopy. Significant differences in the intracellular IPA-3 level among various cell lines were observed indicating that an active mechanism is involved in IPA-3 transport.

PI3K regulates MEK/ERK signaling in breast cancer via the Rac-GEF, P-Rex1

The PI3K pathway is genetically altered in excess of 70% of breast cancers, largely through PIK3CA mutation and HER2 amplification. Preclinical studies have suggested that these subsets of breast cancers are particularly sensitive to PI3K inhibitors; however, the reasons for this heightened sensitivity are mainly unknown. We investigated the signaling effects of PI3K inhibition in PIK3CA mutant and HER2 amplified breast cancers using PI3K inhibitors currently in clinical trials. Unexpectedly, we found that in PIK3CA mutant and HER2 amplified breast cancers sensitive to PI3K inhibitors, PI3K inhibition led to a rapid suppression of Rac1/p21-activated kinase (PAK)/protein kinase C-RAF (C-RAF)/ protein kinase MEK (MEK)/ERK signaling that did not involve RAS. Furthermore, PI3K inhibition led to an ERK-dependent up-regulation of the proapoptotic protein, BIM, followed by induction of apoptosis. Expression of a constitutively active form of Rac1 in these breast cancer models blocked PI3Ki-induced down-regulation of ERK phosphorylation, apoptosis, and mitigated PI3K inhibitor sensitivity in vivo. In contrast, protein kinase AKT inhibitors failed to block MEK/ERK signaling, did not up-regulate BIM, and failed to induce apoptosis. Finally, we identified phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchanger 1 (P-Rex1) as the PI(3,4,5)P3-dependent guanine exchange factor for Rac1 responsible for regulation of the Rac1/C-RAF/MEK/ERK pathway in these cells. The expression level of P-Rex1 correlates with sensitivity to PI3K inhibitors in these breast cancer cell lines. Thus, PI3K inhibitors have enhanced activity in PIK3CA mutant and HER2 amplified breast cancers in which PI3K inhibition down-regulates both the AKT and Rac1/ERK pathways. In addition, P-Rex1 may serve as a biomarker to predict response to single-agent PI3K inhibitors within this subset of breast cancers.

The Rac GTPase effector p21-activated kinase is essential for hematopoietic stem/progenitor cell migration and engraftment

The p21-activated kinases (Paks) are serine/threonine kinases that are major effectors of the Rho guanosine 5'\x{2011}triphosphatase, Rac, and Cdc42. Rac and Cdc42 are known regulators of hematopoietic stem and progenitor cell (HSPC) function, however, a direct role for Paks in HSPCs has yet to be elucidated. Lin(-)Sca1(+)c-kit(+) (LSK) cells from wild-type mice were transduced with retrovirus expressing Pak inhibitory domain (PID), a well-characterized inhibitor of Pak activation. Defects in marrow homing and in vitro cell migration, assembly of the actin cytoskeleton, proliferation, and survival were associated with engraftment failure of PID-LSK. The PID-LSK demonstrated decreased phosphorylation of extracellular signal-regulated kinase (ERK), whereas constitutive activation of ERK in these cells led to rescue of hematopoietic progenitor cell proliferation in vitro and partial rescue of Pak-deficient HSPC homing and engraftment in vivo. Using conditional knock-out mice, we demonstrate that among group A Paks, Pak2(-/-) HSPC show reduced homing to the bone marrow and altered cell shape similar to PID-LSK cells in vitro and are completely defective in HSPC engraftment. These data demonstrate that Pak proteins are key components of multiple engraftment-associated HSPC functions and play a direct role in activation of ERK in HSPCs, and that Pak2 is specifically essential for HSPC engraftment.

CXCL12-CXCR4 signaling is required for the maintenance of mouse spermatogonial stem cells

Continual spermatogenesis relies on the activities of a tissue-specific stem cell population referred to as spermatogonial stem cells (SSCs). Fate decisions of stem cells are influenced by their niche environments, a major component of which is soluble factors secreted by support cells. At present, the factors that constitute the SSC niche are undefined. We explored the role of chemokine (C-X-C motif) ligand 12 (CXCL12) signaling via its receptor C-X-C chemokine receptor type 4 (CXCR4) in regulation of mouse SSC fate decisions. Immunofluorescent staining for CXCL12 protein in cross sections of testes from both pup and adult mice revealed its localization at the basement membrane of seminiferous tubules. Within the undifferentiated spermatogonial population of mouse testes, a fraction of cells were found to express CXCR4 and possess stem cell capacity. Inhibition of CXCR4 signaling in primary cultures of mouse undifferentiated spermatogonia resulted in SSC loss, in part by reducing proliferation and increasing the transition to a progenitor state primed for differentiation upon stimulation by retinoic acid. In addition, CXCL12-CXCR4 signaling in mouse SSCs was found to be important for colonization of recipient testes following transplantation, possibly by influencing homing to establish stem-cell niches. Furthermore, inhibition of CXCR4 signaling in testes of adult mice impaired SSC maintenance, leading to loss of the germline. Collectively, these findings indicate that CXCL12 is an important component of the growth factor milieu of stem cells in mammalian testes and that it signals via the CXCR4 to regulate maintenance of the SSC pool.

Control of human hematopoietic stem/progenitor cell migration by the extracellular matrix protein Slit3

Patients whose hematopoietic system is compromised by chemo- and/or radiotherapy require transplantation of hematopoietic stem and progenitor cells (HSPCs) to restore hematopoiesis. Successful homing of transplanted HSPCs to the bone marrow (BM) largely depends on their migratory potential, which is critically regulated by the chemokine CXCL12. In this study, we have investigated the expression and function of Slit proteins and their corresponding Roundabout (Robo) receptors in human HSPC migration. Slit proteins are extracellular matrix proteins that can modulate the (chemoattractant-induced) migration of mature leukocytes. We show that mRNAs for all Slits (Slit1-3) are expressed in primary BM stroma and BM-derived endothelial and stromal cell lines, but not in CD34⁺ HSPCs. Human CD34⁺ HSPCs expressed mRNAs for all Robos (Robo1-4), but only the Robo1 protein was detected on their cell surface. Functionally, Slit3 treatment increased the in vivo homing efficiency of CD34⁺ HSPCs to the BM in NOD/SCID mice, whereas Slit3-exposed HSPC migration in vitro was inhibited. These effects do not appear to result from modulated CXCL12 responsiveness as CXCR4 expression, CXCL12-induced actin polymerization or the basal and CXCL12-induced adhesion to fibronectin or BM-derived endothelial cells of CD34⁺ HSPC were not altered by Slit3 exposure. However, we show that Slit3 rapidly reduced the levels of active RhoA in HL60 cells and primary CD34⁺ HSPC, directly affecting a pathway involved in actin cytoskeleton remodeling and HSPC migration. Together, our results support a role for Slit3 in human HSPC migration in vitro and homing in vivo and might contribute to the design of future approaches aimed at improving transplantation efficiency of human CD34⁺ HSPCs.

VEGF and FGF prime vascular tube morphogenesis and sprouting directed by hematopoietic stem cell cytokines

Here, we demonstrate a novel, direct-acting, and synergistic role for 3 hematopoietic stem cell cytokines: stem cell factor, interleukin-3, and stromal derived factor-1α, in controlling human endothelial cell (EC) tube morphogenesis, sprouting, and pericyte-induced tube maturation under defined serum-free conditions in 3-dimensional matrices. Angiogenic cytokines such as vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) alone or VEGF/FGF combinations do not support these responses. In contrast, VEGF and FGF prime EC responses to hematopoietic cytokines via up-regulation of c-Kit, IL-3Rα, and C-X-C chemokine receptor type 4 from either human ECs or embryonic quail vessel explants. In support of these findings, EC Runx1 is demonstrated to be critical in coordinating vascular morphogenic responses by controlling hematopoietic cytokine receptor expression. Combined blockade of hematopoietic cytokines or their receptors in vivo leads to blockade of developmental vascularization in quail embryos manifested by vascular hemorrhage and disrupted vascular remodeling events in multiple tissue beds. This work demonstrates a unique role for hematopoietic stem cell cytokines in vascular tube morphogenesis and sprouting and further demonstrates a novel upstream priming role for VEGF and FGF to facilitate the action of promorphogenic hematopoietic cytokines.

c/EBPbeta is a major regulatory element driving transcriptional activation of the CXCL12 promoter

CXCL12 is considered a constitutively expressed chemokine with homeostatic functions. However, induction of CXCL12 expression and its potential role in several pathologic conditions have been reported, suggesting that CXCL12 gene expression can be induced by different stimuli. To elucidate the molecular mechanisms involved in the regulation of CXCL12 gene expression, we aim to define the molecular factors that operate at the transcriptional level. Basal, constitutive expression of CXCL12 was dependent on basic helix-loop-helix factors. Transcriptional up-regulation of the CXCL12 gene was induced by cellular confluence or inflammatory stimuli such as interleukin-1 and interleukin-6, in a CCAAT/enhancer binding protein beta (c/EBPbeta)-dependent manner. Chromatin immunoprecipitation assays confirmed c/EBPbeta binding to a specific response element located at -1171 of the promoter region of CXCL12. Our data show that c/EBPbeta is a major regulatory element driving transcription of the CXCL12 gene in response to cytokines and cell confluence.

Contribution of insulin resistance to vascular dysfunction

Insulin is a vascular hormone, able to influence vascular cell responses. In this review, we consider the insulin actions on vascular endothelium and on vascular smooth muscle cells (VSMC) both in physiological conditions and in the presence of insulin resistance. In particular, we focus the relationships between activation of insulin signalling pathways of phosphatidylinositol-3 kinase (PI3-K) and mitogen-activated protein kinase (MAPK) and the different vascular actions of insulin, with a particular attention to the insulin ability to activate the pathway nitric oxide (NO)/cyclic GMP/PKG via PI3-K, owing to the peculiar relevance of NO in vascular biology. We also discuss the insulin actions mediated by the MAPK pathway (such as endothelin-1 synthesis and secretion and VSMC proliferation and migration) and by the interactions between the two pathways, both in insulin-sensitive and in insulin-resistant states. Finally, we consider the influence of free fatty acids, cytokines and endothelin on vascular insulin resistance.

SDF-1 and PDGF enhance alphavbeta5-mediated ERK activation and adhesion-independent growth of human pre-B cell lines

CD23 acts through the alphavbeta5 integrin to promote growth of human pre-B cell lines in an adhesion-independent manner. alphavbeta5 is expressed on normal B-cell precursors in the bone marrow. Soluble CD23 (sCD23), short CD23-derived peptides containing the arg-lys-cys (RKC) motif recognized by alphavbeta5 and anti-alphavbeta5 monoclonal antibodies (MAbs) all sustain growth of pre-B cell lines. The chemokine stromal cell-derived factor-1 (SDF-1) regulates key processes during B-cell development. SDF-1 enhanced the growth-sustaining effect driven by ligation of alphavbeta5 with anti-alphavbeta5 MAb 15F-11, sCD23 or CD23-derived RKC-containing peptides. This effect was restricted to B-cell precursors and was specific to SDF-1. The enhancement in growth was associated with the activation of extracellular signal-regulated kinase (ERK) and both these responses were attenuated by the MEK inhibitor U0126. Finally, platelet-derived growth factor also enhanced both alphavbeta5-mediated cell growth and ERK activation. The data suggest that adhesion-independent growth-promoting signals delivered to B-cell precursors through the alphavbeta5 integrin can be modulated by cross-talk with receptors linked to both G-protein and tyrosine kinase-coupled signalling pathways.

Oxidative stress and vascular disease in diabetes: is the dichotomization of insulin signaling still valid?

The current wisdom indicates that insulin's positive effects, normoglycemia, vasodilation, and anti-inflammation, are mediated by the canonical phosphoinositide 3-kinase (PI3K)/Akt pathway whereas the negative effects are mediated by the mitogen-activated protein kinase (MAPK)/extracellular regulated kinase (ERK) pathway. Much of the intracellular oxidant stress is mediated by the MAPK/ERK pathway which is a downstream signal also for other proatherogenic hormones such as angiotensin II. However, recent evidence links MAPK activation to antioxidant activity and vascular protection. We argue against a dichotomization of insulin signaling also in light of the concept that ERK-MAPK represents a critical node in the intracellular insulin network responsible for several positive effects related not only to vascular function but also to life span.

An in vitro method to study the effects of hematopoietic regulators during immune and blood cell development

In adults, hematopoiesis occurs in bone marrow (BM) through a complex process with differentiation of hematopoietic stem cells (HSCs) to immune and blood cells. Human HSCs and their progenitors express CD34. Methods on hematopoietic regulation are presented to show the effects of the chemokine, stromal-derived growth factor (SDF)-1I and the neuropeptide, substance P (SP). SDF-1I production in BM stroma causes interactions with HSCs, thereby retaining the HSCs in regions close to the endosteum, at low oxygen. Small changes in SDF-1I levels stimulate HSC functions through direct and indirect mechanisms. The indirect method occurs by SP production, which stimulates CD34+ cells, supported by ligand-binding studies, long-term culture-initiating cell assays for HSC functions, and clonogenic assays for myeloid progenitors. These methods can be applied to study other hematopoietic regulators.

Phospho-MAPK as a marker of myogenic satellite cell responsiveness to growth factors

To determine if differential response to growth factor stimuli between subpopulations of satellite cells was due to variation in the levels of activated intracellular signaling proteins, the levels of phospho-MAPK (phospho-ERK 1/2) were determined in clonal populations of turkey (Meleagris gallopavo) satellite cells. Relative levels of phospho-ERK 1/2 between clones were determined by Western blotting of extracts from satellite cells exposed to growth factor stimuli. Initial measurements using serum mitogenic stimuli showed differences in phospho-MAPK levels between the clonal subpopulations, but the responses did not correlate with proliferation rates of the individual clones (P>0.05). IGF-I alone did not increase phospho-MAPK levels compared to unstimulated controls (P>0.05), whereas FGF-2 did increase levels (P<0.05). A synergistic response was seen in satellite cells as well as embryonic myoblasts administered both IGF-I and FGF-2. When administered FGF-2 and IGF-I, 2 of the slow growing satellite cell clones exhibited lowest levels of phospho-MAPK (P<0.05). One of the slow growing clones had levels of phospho-MAPK similar to the three fast growing clones (P>0.05). The results suggest that variation in responsiveness to growth factor stimuli among satellite cell populations within muscles may be due to several different reasons. Some differences in cell responsiveness appear to be due to variation in phospho-MAPK generation.

Chemokine stromal cell-derived factor-1 induction by C/EBPβ activation is associated with all-trans-retinoic acid-induced leukemic cell differentiation

Stromal cell-derived factor-1 (SDF-1/CXCL12) is one of the essential chemokines, which mediates hematopoietic differentiations. However, the mechanism by which SDF-1 expression is regulated in granulocyte differentiation is poorly understood. Here, we suggest a novel mechanism by which all-trans-retinoic acid (ATRA) induces the expression of SDF-1 during the differentiation of promyelomonocytic leukemic U937 cells. Moreover, we also demonstrate that activation of transcription factor C/EBPbeta by ATRA regulates SDF-1 expression in U937 cells. In addition, we show that the cyclin-dependent kinase inhibitors p21(WAF1/CIP1) and Pyk2 are up-regulated by SDF-1 and increased markedly by the costimulation of ATRA and SDF-1. Furthermore, ATRA and SDF-1alpha additively induce U937 cell differentiation. Indeed, silencing the expression of SDF-1 inhibits ATRA-induced granulocyte differentiation significantly. Taken together, these results indicate that SDF-1alpha is involved in granulocyte differentiation in response to ATRA, mediated by the activation of the transcription factor C/EBPbeta.

Stromal Derived Growth Factor-1α: Another Mediator in Neural-Emerging Immune System throughTac1Expression in Bone Marrow Stromal Cells

Stromal cell-derived growth factor-1alpha (SDF-1alpha) is a member of the CXC chemokines and interacts with the G protein, seven-transmembrane CXCR4 receptor. SDF-1alpha acts as a chemoattractant for immune and hemopoietic cells. The Tac1 gene encodes peptides belonging to the tachykinin family with substance P being the predominant member. Both SDF-1alpha and Tac1 peptides are relevant hemopoietic regulators. This study investigated the effects of SDF-1alpha on Tac1 expression in the major hemopoietic supporting cells, the bone marrow stroma, and addresses the consequence to hemopoiesis. Reporter gene assays with the 5' flanking region of Tac1 showed a bell-shaped effect of SDF-1alpha on luciferase activity with 20 ng/ml SDF-1alpha acting as stimulator, whereas 50 and 100 ng/ml SDF-1alpha acted as inhibitors. Gel shift assays and transfection with wild-type and mutant IkappaB indicate NF-kappaB as a mediator in the repressive effects at 50 and 100 ng/ml SDF-1alpha. Northern analyses and ELISA showed correlations among reporter gene activities, mRNA (beta-preprotachykinin I), and protein levels for substance P. Of relevance is the novel finding by long-term culture-initiating cell assays that showed an indirect effect of SDF-1alpha on hemopoiesis through substance P production. The results also showed neurokinin 1 and not neurokinin 2 as the relevant receptor. Another crucial finding is that substance P does not regulate the production of SDF-1alpha in stroma. The studies indicate that SDF-1alpha levels above baseline production in bone marrow stroma induce the production of substance P to stimulate hemopoiesis. Substance P, however, does not act as autocrine stimulator to induce the production of SDF-1alpha. This study adds SDF-1alpha as a mediator within the neural-immune-hemopoietic axis.

CrkL plays a role in SDF-1-induced activation of the Raf-1/MEK/Erk pathway through Ras and Rac to mediate chemotactic signaling in hematopoietic cells

Intracellular signaling mechanisms regulating SDF-1-induced chemotaxis of hematopoietic cells have remained elusive. Here we demonstrate that overexpression of the adaptor molecule CrkL enhances SDF-1-induced chemotaxis of hematopoietic BaF3 and 32Dcl3 cells. Overexpression of CrkL also enhanced SDF-1-induced activation of the Raf-1/MEK/Erk signaling pathway as well as that of the small GTPases Ras, Rap1, and Rac, while a dominant negative mutant of Ras or Rac suppressed CrkL-enhanced Erk activation. SDF-1 stimulation induced tyrosine phosphorylation of CrkL, which was inhibited by the Src family kinase inhibitor PP1 or by dominant negative mutants of Lyn, thus indicating that Lyn mediated SDF-1-induced phosphorylation of CrkL. However, inhibition of the Lyn kinase activity failed to affect SDF-1-induced activation of the small GTPases and Erk. On the other hand, SDF-1-induced activation of the Erk signaling pathway as well as chemotaxis was inhibited by overexpression of a CrkL mutant lacking the N-terminal SH3 domain, which mediates interaction with various signaling molecules including guanine nucleotide exchange factors for the Ras and Rho family GTPases. SDF-1-induced chemotaxis was also inhibited by the dominant negative Ras or Rac mutant as well as by the MEK inhibitor PD98059. These results indicate that CrkL mediates SDF-1-induced activation of the Raf-1/MEK/Erk signaling pathway through Ras as well as Rac in hematopoietic cells and, thereby, plays important roles in the induction of chemotactic response.