Bone morphogenetic protein-9 is a circulating vascular quiescence factor

Bone morphogenetic protein 9 (BMP9) controls lymphatic vessel maturation and valve formation

Lymphatic vessels are critical for the maintenance of tissue fluid homeostasis and their dysfunction contributes to several human diseases. The activin receptor-like kinase 1 (ALK1) is a transforming growth factor-β family type 1 receptor that is expressed on both blood and lymphatic endothelial cells (LECs). Its high-affinity ligand, bone morphogenetic protein 9 (BMP9), has been shown to be critical for retinal angiogenesis. The aim of this work was to investigate whether BMP9 could play a role in lymphatic development. We found that Bmp9 deficiency in mice causes abnormal lymphatic development. Bmp9-knockout (KO) pups presented hyperplastic mesenteric collecting vessels that maintained LYVE-1 expression. In accordance with this result, we found that BMP9 inhibited LYVE-1 expression in LECs in an ALK1-dependent manner. Bmp9-KO pups also presented a significant reduction in the number and in the maturation of mesenteric lymphatic valves at embryonic day 18.5 and at postnatal days 0 and 4. Interestingly, the expression of several genes known to be involved in valve formation (Foxc2, Connexin37, EphrinB2, and Neuropilin1) was upregulated by BMP9 in LECS. Finally, we demonstrated that Bmp9-KO neonates and adult mice had decreased lymphatic draining efficiency. These data identify BMP9 as an important extracellular regulator in the maturation of the lymphatic vascular network affecting valve development and lymphatic vessel function.

The ALK-1/Smad1 pathway in cardiovascular physiopathology. A new target for therapy?

Activin receptor-like kinase-1 or ALK-1 is a type I cell surface receptor for the transforming growth factor-β (TGF-β) family of proteins. The role of ALK-1 in endothelial cells biology and in angiogenesis has been thoroughly studied by many authors. However, it has been recently suggested a possible role of ALK-1 in cardiovascular homeostasis. ALK-1 is not only expressed in endothelial cells but also in smooth muscle cells, myofibroblast, hepatic stellate cells, chondrocytes, monocytes, myoblasts, macrophages or fibroblasts, but its role in these cells have not been deeply analyzed. Due to the function of ALK-1 in these cells, this receptor plays a role in several cardiovascular diseases. Animals with ALK-1 haploinsufficiency and patients with mutations in Acvrl1 (the gene that codifies for ALK-1) develop type-2 Hereditary Hemorrhagic Telangiectasia. Moreover, ALK-1 heterozygous mice develop pulmonary hypertension. Higher levels of ALK-1 have been observed in atherosclerotic plaques, suggesting a possible protector role of this receptor. ALK-1 deficiency is also related to the development of arteriovenous malformations (AVMs). Besides, due to the ability of ALK-1 to regulate cell proliferation and migration, and to modulate extracellular matrix (ECM) protein expression in several cell types, ALK-1 has been now demonstrated to play an important role in cardiovascular remodeling. In this review, we would like to offer a complete vision of the role of ALK-1 in many process related to cardiovascular homeostasis, and the involvement of this protein in the development of cardiovascular diseases, suggesting the possibility of using the ALK-1/smad-1 pathway as a powerful therapeutic target.

The lysosomal inhibitor, chloroquine, increases cell surface BMPR-II levels and restores BMP9 signalling in endothelial cells harbouring BMPR-II mutations

Pulmonary arterial hypertension (PAH) is characterized by dysregulated pulmonary artery endothelial cell (PAEC) proliferation, apoptosis and permeability. Loss-of-function mutations in the bone morphogenetic protein receptor type-II (BMPR-II) are the most common cause of heritable PAH, usually resulting in haploinsufficiency. We previously showed that BMPR-II expression is regulated via a lysosomal degradative pathway. Here, we show that the antimalarial drug, chloroquine, markedly increased cell surface expression of BMPR-II protein independent of transcription in PAECs. Inhibition of protein synthesis experiments revealed a rapid turnover of cell surface BMPR-II, which was inhibited by chloroquine treatment. Chloroquine enhanced PAEC expression of BMPR-II following siRNA knockdown of the BMPR-II transcript. Using blood outgrowth endothelial cells (BOECs), we confirmed that signalling in response to the endothelial BMPR-II ligand, BMP9, is compromised in BOECs from patients harbouring BMPR-II mutations, and in BMPR-II mutant PAECs. Chloroquine significantly increased gene expression of BMP9-BMPR-II signalling targets Id1, miR21 and miR27a in both mutant BMPR-II PAECs and BOECs. These findings provide support for the restoration of cell surface BMPR-II with agents such as chloroquine as a potential therapeutic approach for heritable PAH.

Enhanced responses to angiogenic cues underlie the pathogenesis of hereditary hemorrhagic telangiectasia 2

Hereditary Hemorrhagic Telangiectasia (HHT) is a genetic vascular disease in which arteriovenous malformations (AVMs) manifest in skin and multiple visceral organs. HHT is caused by heterozygous mutations in endoglin (ENG), activin receptor-like kinase 1 (ALK1), or SMAD4. ALK1 regulates angiogenesis, but the precise function of ALK1 in endothelial cells (ECs) remains elusive. Since most blood vessels of HHT patients do not produce pathological vascular lesions, ALK1 heterozygous ECs may be normal unless additional genetic or environmental stresses are imposed. To investigate the cellular and biochemical phenotypes of Alk1-null versus Alk1-heterozygous ECs, we have generated pulmonary EC lines in which a genotype switch from the Alk1-conditional allele (Alk1 (2f)) to the Alk1-null allele (Alk1 (1f)) can be induced by tamoxifen treatment. Alk1-null (1 f/1 f) ECs displayed increased migratory properties in vitro in response to bFGF compared with Alk1-het (2 f/1 f) ECs. The 1 f/1 f-ECs formed a denser and more persistent tubular network as compared with their parental 2 f/1 f-ECs. Interestingly, the response to BMP-9 on SMAD1/5 phosphorylation was impaired in both 2 f/1 f- and 1 f/1 f-ECs at a comparable manner, suggesting that other factors in addition to SMADs may play a crucial role for enhanced angiogenic activity in 1 f/1 f-ECs. We also demonstrated in vivo that Alk1-deficient ECs exhibited high migratory and invasive properties. Taken together, these data suggest that enhanced responses to angiogenic cues in ALK1-deficient ECs underlie the pathogenesis of HHT2.

The dual effect of mesenchymal stem cells on tumour growth and tumour angiogenesis

INTRODUCTION

Understanding the multiple biological functions played by human mesenchymal stem cells (hMSCs) as well as their development as therapeutics in regenerative medicine or in cancer treatment are major fields of research. Indeed, it has been established that hMSCs play a central role in the pathogenesis and progression of tumours, but their impact on tumour growth remains controversial.

METHODS

In this study, we investigated the influence of hMSCs on the growth of pre-established tumours. We engrafted nude mice with luciferase-positive mouse adenocarcinoma cells (TSA-Luc+) to obtain subcutaneous or lung tumours. When tumour presence was confirmed by non-invasive bioluminescence imaging, hMSCs were injected into the periphery of the SC tumours or delivered by systemic intravenous injection in mice bearing either SC tumours or lung metastasis.

RESULTS

Regardless of the tumour model and mode of hMSC injection, hMSC administration was always associated with decreased tumour growth due to an inhibition of tumour cell proliferation, likely resulting from deep modifications of the tumour angiogenesis. Indeed, we established that although hMSCs can induce the formation of new blood vessels in a non-tumoural cellulose sponge model in mice, they do not modify the overall amount of haemoglobin delivered into the SC tumours or lung metastasis. We observed that these tumour vessels were reduced in number but were longer.

CONCLUSIONS

Our results suggest that hMSCs injection decreased solid tumour growth in mice and modified tumour vasculature, which confirms hMSCs could be interesting to use for the treatment of pre-established tumours.

Anti-inflammatory and antiatherogenic role of BMP receptor II in endothelial cells

OBJECTIVE

Atherosclerosis is an inflammatory disease with multiple underlying metabolic and physical risk factors. Bone morphogenic protein 4 (BMP4) expression is increased in endothelium in atherosclerosis-prone regions and is known to induce endothelial inflammation, endothelial dysfunction, and hypertension. BMP actions are mediated by 2 different types of BMP receptors (BMPRI and BMPRII). Here, we show a surprising finding that loss of BMPRII expression causes endothelial inflammation and atherosclerosis.

APPROACH AND RESULTS

Using BMPRII siRNA and BMPRII(+/-) mice, we found that specific knockdown of BMPRII, but not other BMP receptors (Alk1, Alk2, Alk3, Alk6, ActRIIa, and ActRIIb), induced endothelial inflammation in a ligand-independent manner by mechanisms mediated by reactive oxygen species, nuclear factor-KappaB, and reduced nicotinamide adenine dinucleotide phosphate oxidases. Further, BMPRII(+/-)ApoE(-/-) mice developed accelerated atherosclerosis compared with BMPRII(+/+)ApoE(-/-) mice. Interestingly, we found that multiple proatherogenic stimuli, such as hypercholesterolemia, disturbed flow, prohypertensive angiotensin II, and the proinflammatory cytokine (tumor necrosis factor-α), downregulated BMPRII expression in endothelium, whereas antiatherogenic stimuli, such as stable flow and statin treatment, upregulated its expression in vivo and in vitro. Moreover, BMPRII expression was significantly diminished in human coronary advanced atherosclerotic lesions. Also, we were able to rescue the endothelial inflammation induced by BMPRII knockdown by overexpressing the BMPRII wild type, but not by the BMPRII short form lacking the carboxyl-terminal tail region.

CONCLUSIONS

These results suggest that BMPRII is a critical, anti-inflammatory, and antiatherogenic protein that is commonly targeted by multiple pro- and antiatherogenic factors. BMPRII may be used as a novel diagnostic and therapeutic target in atherosclerosis.

Shear induced collateral artery growth modulated by endoglin but not by ALK1

Transforming growth factor-beta (TGF-β) stimulates both ischaemia induced angiogenesis and shear stress induced arteriogenesis by signalling through different receptors. How these receptors are involved in both these processes of blood flow recovery is not entirely clear. In this study the role of TGF-β receptors 1 and endoglin is assessed in neovascularization in mice. Unilateral femoral artery ligation was performed in mice heterozygous for either endoglin or ALK1 and in littermate controls. Compared with littermate controls, blood flow recovery, monitored by laser Doppler perfusion imaging, was significantly hampered by maximal 40% in endoglin heterozygous mice and by maximal 49% in ALK1 heterozygous mice. Collateral artery size was significantly reduced in endoglin heterozygous mice compared with controls but not in ALK1 heterozygous mice. Capillary density in ischaemic calf muscles was unaffected, but capillaries from endoglin and ALK1 heterozygous mice were significantly larger when compared with controls. To provide mechanistic evidence for the differential role of endoglin and ALK1 in shear induced or ischaemia induced neovascularization, murine endothelial cells were exposed to shear stress in vitro. This induced increased levels of endoglin mRNA but not ALK1. In this study it is demonstrated that both endoglin and ALK1 facilitate blood flow recovery. Importantly, endoglin contributes to both shear induced collateral artery growth and to ischaemia induced angiogenesis, whereas ALK1 is only involved in ischaemia induced angiogenesis.

BMP9 signaling in stem cell differentiation and osteogenesis

Bone morphogenetic proteins (BMPs) are members of the TGF-β superfamily and play a critical role in skeletal development, bone formation and stem cell differentiation. Disruptions in BMP signaling result in a variety of skeletal and extraskeletal anomalies. BMP9 is a poorly characterized member of the BMP family and is among the most osteogenic BMPs, promoting osteoblastic differentiation of mesenchymal stem cells (MSCs) both in vitro and in vivo. Recent findings from various in vivo and molecular studies strongly suggest that the mechanisms governing BMP9-mediated osteoinduction differ from other osteogenic BMPs. Many signaling pathways with diverse functions have been found to play a role in BMP9-mediated osteogenesis. Several of these pathways are also critical in the differentiation of other cell lineages, including adipocytes and chondrocytes. While BMP9 is known to be a potent osteogenic factor, it also influences several other pathways including cancer development, angiogenesis and myogenesis. Although BMP9 has been demonstrated as one of the most osteogenic BMPs, relatively little is known about the specific mechanisms responsible for these effects. BMP9 has demonstrated efficacy in promoting spinal fusion and bony non-union repair in animal models, demonstrating great translational promise. This review aims to summarize our current knowledge of BMP9-mediated osteogenesis by presenting recently completed work which may help us to further elucidate these pathways.

Specialized tip/stalk-like and phalanx-like endothelial cells from embryonic stem cells

Endothelial cells (EC) generated in vitro from stem cells are desirable for their potential in a variety of in vitro models and cell-based therapeutic approaches; however, EC can take on a number of functionally and phenotypically distinct specializations. Here, we show the generation of functionally distinct EC subpopulations, including (1) the pro-angiogenic migrating tip-like and proliferative stalk-like EC, and (2) the less migratory cobblestone-shaped phalanx-like EC. Both embryonic stem cell (ESC)-derived EC subpopulations are generated from outgrowths of Flk-1+ vascular progenitor cells with high levels of vascular endothelial growth factor treatment, while the phalanx-like ESC-derived EC (ESC-EC) are subsequently isolated by selecting for cobblestone shape. Compared with the ESC-derived angiogenic endothelial cells (named ESC-AEC) that contain only 14% Flt-1+ and 25% Tie-1+ cells, the selected phalanx-like ESC-EC express higher numbers of cells expressing the phalanx markers Flt-1+ and Tie-1+, 89% and 90%, respectively. The ESC-AEC also contain 35% CXCR4+ tip cells, higher expression levels of stalk marker Notch-1, and lower expression levels of Tie-2 compared with the phalanx-type ESC-EC that do not contain discernible numbers of CXCR4+ tip cells. Perhaps most notably, the ESC-AEC display increased cell migration, proliferation, and 3 times more vessel-like structures after 48 h on Matrigel compared with the phalanx-like ESC-EC. This work analyzes, for the first time, the presence of distinct EC subtypes (tip/stalk, and phalanx) generated in vitro from ESC, and shows that phalanx-like EC can be purified and maintained in culture separate from the tip/stalk-like containing EC.

Endoglin requirement for BMP9 signaling in endothelial cells reveals new mechanism of action for selective anti-endoglin antibodies

Endoglin (ENG), a co-receptor for several TGFβ-family cytokines, is expressed in dividing endothelial cells alongside ALK1, the ACVRL1 gene product. ENG and ACVRL1 are both required for angiogenesis and mutations in either gene are associated with Hereditary Hemorrhagic Telangectasia, a rare genetic vascular disorder. ENG and ALK1 function in the same genetic pathway but the relative contribution of TGFβ and BMP9 to SMAD1/5/8 activation and the requirement of ENG as a co-mediator of SMAD phosphorylation in endothelial cells remain debated. Here, we show that BMP9 and TGFβ1 induce distinct SMAD phosphorylation responses in primary human endothelial cells and that, unlike BMP9, TGFβ only induces SMAD1/5/8 phosphorylation in a subset of immortalized mouse endothelial cell lines, but not in primary human endothelial cells. We also demonstrate, using siRNA depletion of ENG and novel anti-ENG antibodies, that ENG is required for BMP9/pSMAD1 signaling in all human and mouse endothelial cells tested. Finally, anti-ENG antibodies that interfere with BMP9/pSMAD1 signaling, but not with TGFβ1/pSMAD3 signaling, also decrease in vitro HUVEC endothelial tube formation and inhibit BMP9 binding to recombinant ENG in vitro. Our data demonstrate that BMP9 signaling inhibition is a key and previously unreported mechanism of action of TRC105, an anti-angiogenic anti-Endoglin antibody currently evaluated in clinical trials.

H3K9 methylation is a barrier during somatic cell reprogramming into iPSCs

The induction of pluripotent stem cells (iPSCs) by defined factors is poorly understood stepwise. Here, we show that histone H3 lysine 9 (H3K9) methylation is the primary epigenetic determinant for the intermediate pre-iPSC state, and its removal leads to fully reprogrammed iPSCs. We generated a panel of stable pre-iPSCs that exhibit pluripotent properties but do not activate the core pluripotency network, although they remain sensitive to vitamin C for conversion into iPSCs. Bone morphogenetic proteins (BMPs) were subsequently identified in serum as critical signaling molecules in arresting reprogramming at the pre-iPSC state. Mechanistically, we identified H3K9 methyltransferases as downstream targets of BMPs and showed that they function with their corresponding demethylases as the on/off switch for the pre-iPSC fate by regulating H3K9 methylation status at the core pluripotency loci. Our results not only establish pre-iPSCs as an epigenetically stable signpost along the reprogramming road map, but they also provide mechanistic insights into the epigenetic reprogramming of cell fate.

Inhibition of apelin expression by BMP signaling in endothelial cells

The transforming growth factor-β/bone morphogenic protein (BMP) system is a major pathway for angiogenesis and is involved in hereditary vascular diseases. Here we report that the gene encoding the vasoactive and vascular cell growth-regulating peptide apelin is a target of the BMP pathway. We demonstrate that apelin expression is strongly downregulated by BMP in an endothelial cell line as well as in lung endothelial microvascular cells. We show that BMP signals through the BMPR2-SMAD pathway to downregulate apelin expression and that a transcriptional direct and indirect mechanism is required. The BMP-induced downregulation of apelin expression was found to be critical for hypoxia-induced growth of endothelial cells, because the growth inhibitory effect of BMP in this condition is suppressed by enforced expression of apelin. Thus, we describe an important link between a signaling pathway involved in angiogenesis and vascular diseases and a peptide regulating vascular homeostasis.

BMP9-regulated angiogenic signaling plays an important role in the osteogenic differentiation of mesenchymal progenitor cells

Mesenchymal stromal progenitor cells (MSCs) are multipotent progenitors that can be isolated from numerous tissues. MSCs can undergo osteogenic differentiation under proper stimuli. We have recently demonstrated that bone morphogenetic protein 9 (BMP9) is one of the most osteogenic BMPs. As one of the least studied BMPs, BMP9 has been shown to regulate angiogenesis in endothelial cells. However, it is unclear whether BMP9-regulated angiogenic signaling plays any important role in the BMP9-initiated osteogenic pathway in MSCs. Here, we investigate the functional role of hypoxia-inducible factor 1α (HIF1α)-mediated angiogenic signaling in BMP9-regulated osteogenic differentiation of MSCs. We find that BMP9 induces HIF1α expression in MSCs through Smad1/5/8 signaling. Exogenous expression of HIF1α potentiates BMP9-induced osteogenic differentiation of MSCs both in vitro and in vivo. siRNA-mediated silencing of HIF1α or HIF1α inhibitor CAY10585 profoundly blunts BMP9-induced osteogenic signaling in MSCs. HIF1α expression regulated by cobalt-induced hypoxia also recapitulates the synergistic effect between HIF1α and BMP9 in osteogenic differentiation. Mechanistically, HIF1α is shown to exert its synergistic effect with BMP9 by inducing both angiogenic signaling and osteogenic signaling in MSCs. Thus, our findings should not only expand our understanding of the molecular basis behind BMP9-regulated osteoblastic lineage-specific differentiation, but also provide an opportunity to harness the BMP9-induced synergy between osteogenic and angiogenic signaling pathways in regenerative medicine.

Vascular injury triggers Krüppel-like factor 6 mobilization and cooperation with specificity protein 1 to promote endothelial activation through upregulation of the activin receptor-like kinase 1 gene

RATIONALE

Activin receptor-like kinase-1 (ALK1) is an endothelial transforming growth factor β receptor involved in angiogenesis. ALK1 expression is high in the embryo vasculature, becoming less detectable in the quiescent endothelium of adult stages. However, ALK1 expression becomes rapidly increased after angiogenic stimuli such as vascular injury.

OBJECTIVE

To characterize the molecular mechanisms underlying the regulation of ALK1 on vascular injury.

METHODS AND RESULTS

Alk1 becomes strongly upregulated in endothelial (EC) and vascular smooth muscle cells of mouse femoral arteries after wire-induced endothelial denudation. In vitro denudation of monolayers of human umbilical vein ECs also leads to an increase in ALK1. Interestingly, a key factor in tissue remodeling, Krüppel-like factor 6 (KLF6) translocates to the cell nucleus during wound healing, concomitantly with an increase in the ALK1 gene transcriptional rate. KLF6 knock down in human umbilical vein ECs promotes ALK1 mRNA downregulation. Moreover, Klf6(+/-) mice have lower levels of Alk1 in their vasculature compared with their wild-type siblings. Chromatin immunoprecipitation assays show that KLF6 interacts with ALK1 promoter in ECs, and this interaction is enhanced during wound healing. We demonstrate that KLF6 is transactivating ALK1 gene, and this transactivation occurs by a synergistic cooperative mechanism with specificity protein 1. Finally, Alk1 levels in vascular smooth muscle cells are not directly upregulated in response to damage, but in response to soluble factors, such as interleukin 6, released from ECs after injury.

CONCLUSIONS

ALK1 is upregulated in ECs during vascular injury by a synergistic cooperative mechanism between KLF6 and specificity protein 1, and in vascular smooth muscle cells by an EC-vascular smooth muscle cell paracrine communication during vascular remodeling.

BMP9 regulates endoglin-dependent chemokine responses in endothelial cells

BMP9 signaling has been implicated in hereditary hemorrhagic telangiectasia (HHT) and vascular remodeling, acting via the HHT target genes, endoglin and ALK1. This study sought to identify endothelial BMP9-regulated proteins that could affect the HHT phenotype. Gene ontology analysis of cDNA microarray data obtained after BMP9 treatment of primary human endothelial cells indicated regulation of chemokine, adhesion, and inflammation pathways. These responses included the up-regulation of the chemokine CXCL12/SDF1 and down-regulation of its receptor CXCR4. Quantitative mass spectrometry identified additional secreted proteins, including the chemokine CXCL10/IP10. RNA knockdown of endoglin and ALK1 impaired SDF1/CXCR4 regulation by BMP9. Because of the association of SDF1 with ischemia, we analyzed its expression under hypoxia in response to BMP9 in vitro, and during the response to hindlimb ischemia, in endoglin-deficient mice. BMP9 and hypoxia were additive inducers of SDF1 expression. Moreover, the data suggest that endoglin deficiency impaired SDF1 expression in endothelial cells in vivo. Our data implicate BMP9 in regulation of the SDF1/CXCR4 chemokine axis in endothelial cells and point to a role for BMP9 signaling via endoglin in a switch from an SDF1-responsive autocrine phenotype to an SDF1 nonresponsive paracrine state that represses endothelial cell migration and may promote vessel maturation.

SMAD versus non-SMAD signaling is determined by lateral mobility of bone morphogenetic protein (BMP) receptors

Bone (or body) morphogenetic proteins (BMPs) belong to the TGFβ superfamily and are crucial for embryonic patterning and organogenesis as well as for adult tissue homeostasis and repair. Activation of BMP receptors by their ligands leads to induction of several signaling cascades. Using fluorescence recovery after photobleaching, FRET, and single particle tracking microscopy, we demonstrate that BMP receptor type I and II (BMPRI and BMPRII) have distinct lateral mobility properties within the plasma membrane, which is mandatory for their involvement in different signaling pathways. Before ligand binding, BMPRI and a subpopulation of BMPRII exhibit confined motion, reflecting preassembled heteromeric receptor complexes. A second free diffusing BMPRII population only becomes restricted after ligand addition. This paper visualizes time-resolved BMP receptor complex formation and demonstrates that the lateral mobility of BMPRI has a major impact in stabilizing heteromeric BMPRI-BMPRII receptor complexes to differentially stimulate SMAD versus non-SMAD signaling.

Endoglin mediates fibronectin/α5β1 integrin and TGF-β pathway crosstalk in endothelial cells

Both the transforming growth factor β (TGF-β) and integrin signalling pathways have well-established roles in angiogenesis. However, how these pathways integrate to regulate angiogenesis is unknown. Here, we show that the extracellular matrix component, fibronectin, and its cellular receptor, α5β1 integrin, specifically increase TGF-β1- and BMP-9-induced Smad1/5/8 phosphorylation via the TGF-β superfamily receptors endoglin and activin-like kinase-1 (ALK1). Fibronectin and α5β1 integrin increase Smad1/5/8 signalling by promoting endoglin/ALK1 cell surface complex formation. In a reciprocal manner, TGF-β1 activates α5β1 integrin and downstream signalling to focal adhesion kinase (FAK) in an endoglin-dependent manner. α5β1 integrin and endoglin form a complex on the cell surface and co-internalize, with their internalization regulating α5β1 integrin activation and signalling. Functionally, endoglin-mediated fibronectin/α5β1 integrin and TGF-β pathway crosstalk alter the responses of endothelial cells to TGF-β1, switching TGF-β1 from a promoter to a suppressor of migration, inhibiting TGF-β1-mediated apoptosis to promote capillary stability, and partially mediating developmental angiogenesis in vivo. These studies provide a novel mechanism for the regulation of TGF-β superfamily signalling and endothelial function through crosstalk with integrin signalling pathways.

Overactive bone morphogenetic protein signaling in heterotopic ossification and Duchenne muscular dystrophy

Bone morphogenetic proteins (BMPs) are important extracellular cytokines that play critical roles in embryogenesis and tissue homeostasis. BMPs signal via transmembrane type I and type II serine/threonine kinase receptors and intracellular Smad effector proteins. BMP signaling is precisely regulated and perturbation of BMP signaling is connected to multiple diseases, including musculoskeletal diseases. In this review, we will summarize the recent progress in elucidation of BMP signal transduction, how overactive BMP signaling is involved in the pathogenesis of heterotopic ossification and Duchenne muscular dystrophy, and discuss possible therapeutic strategies for treatment of these diseases.

Crossveinless 2 regulates bone morphogenetic protein 9 in human and mouse vascular endothelium

The importance of morphogenetic proteins (BMPs) and their antagonists in vascular development is increasingly being recognized. BMP-4 is essential for angiogenesis and is antagonized by matrix Gla protein (MGP) and crossveinless 2 (CV2), both induced by the activin receptor like-kinase 1 (ALK1) when stimulated by BMP-9. In this study, however, we show that CV2 preferentially binds and inhibits BMP-9 thereby providing strong feedback inhibition for BMP-9/ALK1 signaling rather than for BMP-4/ALK2 signaling. CV2 disrupts complex formation involving ALK2, ALK1, BMP-4, and BMP-9 required for the induction of both BMP antagonists. It also limits VEGF expression, proliferation, and tube formation in ALK1-expressing endothelial cells. In vivo, CV2 deficiency translates into a dysregulation of vascular BMP signaling, resulting in an abnormal endothelium with increased endothelial cellularity and expression of lineage markers for mature endothelial cells. Thus, mutual regulation by BMP-9 and CV2 is essential in regulating the development of the vascular endothelium.

BMP9 induces EphrinB2 expression in endothelial cells through an Alk1-BMPRII/ActRII-ID1/ID3-dependent pathway: implications for hereditary hemorrhagic telangiectasia type II

ALK1 (ACVRL1) is a member of the TGFβ receptor family and is expressed predominantly by arterial endothelial cells (EC). Mutations in ACVRL1 are responsible for hereditary hemorrhagic telangiectasia type 2 (HHT2), a disease manifesting as fragile vessels, capillary overgrowth, and numerous arterio-venous malformations. Arterial EC also express EphrinB2, which has multiple roles in vascular development and angiogenesis and is known to be reduced in ACVRL1 knockout mice. Using an in vitro angiogenesis model we find that the Alk1 ligand BMP9 induces EphrinB2 in EC, and this is entirely dependent on expression of Alk1 and at least one of the co-receptors BMPRII or ActRII. BMP9 induces both ID1 and ID3, and both are necessary for full induction of EphrinB2. Loss of Alk1 or EphrinB2 results in increased arterial-venous anastomosis, while loss of Alk1 but not EphrinB2 results in increased VEGFR2 expression and enhanced capillary sprouting. Conversely, BMP9 blocks EC sprouting and this is dependent on Alk1, BMPRII/ActRII and ID1/ID3. Finally, notch signaling overcomes the loss of Alk1-restoring EphrinB2 expression in EC, and curbing excess sprouting. Thus, in an in vitro model of HHT2, loss of Alk1 blocks BMP9 signaling, resulting in reduced EphrinB2 expression, enhanced VEGFR2 expression, and misregulated EC sprouting and anastomosis.

ALK1 signaling inhibits angiogenesis by cooperating with the Notch pathway

Activin receptor-like kinase 1 (ALK1) is an endothelial-specific member of the TGF-β/BMP receptor family that is inactivated in patients with hereditary hemorrhagic telangiectasia (HHT). How ALK1 signaling regulates angiogenesis remains incompletely understood. Here we show that ALK1 inhibits angiogenesis by cooperating with the Notch pathway. Blocking Alk1 signaling during postnatal development in mice leads to retinal hypervascularization and the appearance of arteriovenous malformations (AVMs). Combined blockade of Alk1 and Notch signaling further exacerbates hypervascularization, whereas activation of Alk1 by its high-affinity ligand BMP9 rescues hypersprouting induced by Notch inhibition. Mechanistically, ALK1-dependent SMAD signaling synergizes with activated Notch in stalk cells to induce expression of the Notch targets HEY1 and HEY2, thereby repressing VEGF signaling, tip cell formation, and endothelial sprouting. Taken together, these results uncover a direct link between ALK1 and Notch signaling during vascular morphogenesis that may be relevant to the pathogenesis of HHT vascular lesions.

BMP9 and BMP10 are critical for postnatal retinal vascular remodeling

ALK1 is a type I receptor of the TGF-β family that is involved in angiogenesis. Circulating BMP9 was identified as a specific ligand for ALK1 inducing vascular quiescence. In this work, we found that blocking BMP9 with a neutralizing antibody in newborn mice significantly increased retinal vascular density. Surprisingly, Bmp9-KO mice did not show any defect in retinal vascularization. However, injection of the extracellular domain of ALK1 impaired retinal vascularization in Bmp9-KO mice, implicating another ligand for ALK1. Interestingly, we detected a high level of circulating BMP10 in WT and Bmp9-KO pups. Further, we found that injection of a neutralizing anti-BMP10 antibody to Bmp9-KO pups reduced retinal vascular expansion and increased vascular density, whereas injection of this antibody to WT pups did not affect the retinal vasculature. These data suggested that BMP9 and BMP10 are important in postnatal vascular remodeling of the retina and that BMP10 can substitute for BMP9. In vitro stimulation of endothelial cells by BMP9 and BMP10 increased the expression of genes involved in the Notch signaling pathway (Jagged1, Dll4, Hey1, Hey2, Hes1) and decreased apelin expression, suggesting a possible cross-talk between these pathways and the BMP pathway.

Anti-human activin receptor-like kinase 1 (ALK1) antibody attenuates bone morphogenetic protein 9 (BMP9)-induced ALK1 signaling and interferes with endothelial cell sprouting

Genetic and molecular studies suggest that activin receptor-like kinase 1 (ALK1), a transforming growth factor β (TGF-β) type I receptor, and endoglin, a TGF-β co-receptor, play an essential role in vascular development and pathological angiogenesis. Several agents that interfere with ALK1 and endoglin function are currently in clinical trials for antiangiogenic activity in cancer therapy. One of these agents, PF-03446962 (anti-hALK1 antibody), shows promising results in the clinic. However, its effects on endothelial cell function and mechanism of action are unclear. Here we demonstrate that anti-hALK1 antibody selectively recognizes human ALK1. The anti-hALK1 antibody interfered with bone morphogenetic protein 9 (BMP9)-induced signaling in endothelial cells. Consistent with this notion, anti-hALK1 antibody was found to compete highly efficiently with the binding of the ALK1 ligand BMP9 and TGF-β to ALK1. Moreover, it prevented BMP9-dependent recruitment of co-receptor endoglin into this angiogenesis-mediating signaling complex. In addition, we demonstrated that anti-hALK1 antibody inhibited endothelial cell sprouting but did not directly interfere with vascular endothelial growth factor (VEGF) signaling, VEGF-induced proliferation, and migration of endothelial cells. Finally, we demonstrated that BMP9 in serum is essential for endothelial sprouting and that anti-hALK1 antibody inhibits this potently. Our data suggest that both the VEGF/VEGF receptor and the BMP9/ALK1 pathways are essential for stimulating angiogenesis, and targeting both pathways simultaneously may be an attractive strategy to overcome resistance to antiangiogenesis therapy.

BMPs and their clinical potentials

Bone morphogenetic protein (BMP) signaling in diseases is the subject of an overwhelming array of studies. BMPs are excellent targets for treatment of various clinical disorders. Several BMPs have already been shown to be clinically beneficial in the treatment of a variety of conditions, including BMP-2 and BMP-7 that have been approved for clinical application in nonunion bone fractures and spinal fusions. With the use of BMPs increasingly accepted in spinal fusion surgeries, other therapeutic approaches targeting BMP signaling are emerging beyond applications to skeletal disorders. These approaches can further utilize next-generation therapeutic tools such as engineered BMPs and ex vivo- conditioned cell therapies. In this review, we focused to provide insights into such clinical potentials of BMPs in metabolic and vascular diseases, and in cancer. [BMB reports 2011; 44(10): 619-634].

BMP signaling is responsible for serum-induced Id2 expression

Ids function as negative regulators of basic helix-loop-helix transcription factors and their expression is rapidly induced by serum stimulation in various cell types. In this study, we investigated the molecular basis of serum-induced expression of the mouse Id2 gene in NIH3T3 cells. A small-molecule inhibitor of bone morphogenetic protein (BMP) type I receptor kinases blocked the serum induction of Id2 mRNA. The chemical compound and several inhibitory proteins specific for BMP signaling suppressed the serum-induced activation of the luciferase construct with the mouse Id2 4.6-kb promoter region. Importantly, serum stimulation evoked rapid phosphorylation of Smad1/5/8 and significant activation of the reporter plasmid containing the recently identified BMP-responsive element (BRE) of the mouse Id2. Mutation analysis demonstrated that the binding sites for Smad proteins in the Id2 BRE were critical for serum response of the 4.6-kb whole construct. Gel shift and chromatin immunoprecipitation (ChIP) assays confirmed the serum-inducible binding of Smad1/5/8 and Smad4 to the Id2 BRE in vitro and in vivo. Finally, a knockdown experiment revealed the functional importance of Smad1 in the serum induction of Id2 expression. Thus, we concluded that BMP signaling is primarily responsible for the serum-induced Id2 expression. Our results also suggest that some of the cellular effects caused by serum are mediated through BMP signaling.

BMP-9 induced endothelial cell tubule formation and inhibition of migration involves Smad1 driven endothelin-1 production

BACKGROUND

Bone morphogenetic proteins (BMPs) and their receptors, such as bone morphogenetic protein receptor (BMPR) II, have been implicated in a wide variety of disorders including pulmonary arterial hypertension (PAH). Similarly, endothelin-1 (ET-1), a mitogen and vasoconstrictor, is upregulated in PAH and endothelin receptor antagonists are used in its treatment. We sought to determine whether there is crosstalk between BMP signalling and the ET-1 axis in human pulmonary artery endothelial cells (HPAECs), possible mechanisms involved in such crosstalk and functional consequences thereof.

METHODOLOGY/PRINCIPAL FINDING

Using western blot, real time RT-PCR, ELISA and small RNA interference methods we provide evidence that in HPAECs BMP-9, but not BMP-2, -4 and -6 significantly stimulated ET-1 release under physiological concentrations. This release is mediated by both Smad1 and p38 MAPK and is independent of the canonical Smad4 pathway. Moreover, knocking down the ALK1 receptor or BMPR II attenuates BMP-9 stimulated ET-1 release, whilst causing a significant increase in prepro ET-1 mRNA transcription and mature peptide release. Finally, BMP-9 induced ET-1 release is involved in both inhibition of endothelial cell migration and promotion of tubule formation.

CONCLUSIONS/SIGNIFICANCE

Although our data does not support an important role for BMP-9 as a source of increased endothelial ET-1 production seen in human PAH, BMP-9 stimulated ET-1 production is likely to be important in angiogenesis and vascular stability. However, increased ET-1 production by endothelial cells as a consequence of BMPR II dysfunction may be clinically relevant in the pathogenesis of PAH.

Mouse and human strategies identify PTPN14 as a modifier of angiogenesis and hereditary haemorrhagic telangiectasia

Hereditary haemorrhagic telangiectasia (HHT) [corrected] is a vascular dysplasia syndrome caused by mutations in transforming growth factor-β/bone morphogenetic protein pathway genes, ENG and ACVRL1. HHT [corrected] shows considerable variation in clinical manifestations, suggesting environmental and/or genetic modifier effects. Strain-specific penetrance of the vascular phenotypes of Eng(+/-) and Tgfb1(-/-) mice provides further support for genetic modification of transforming growth factor-β pathway deficits. We previously identified variant genomic loci, including Tgfbm2, which suppress prenatal vascular lethality of Tgfb1(-/-) mice. Here we show that human polymorphic variants of PTPN14 within the orthologous TGFBM2 locus influence clinical severity of HHT, [corrected] as assessed by development of pulmonary arteriovenous malformation. We also show that PTPN14, ACVRL1 and EFNB2, encoding EphrinB2, show interdependent expression in primary arterial endothelial cells in vitro. This suggests an involvement of PTPN14 in angiogenesis and/or arteriovenous fate, acting via EphrinB2 and ACVRL1/activin receptor-like kinase 1. These findings contribute to a deeper understanding of the molecular pathology of HHT [corrected] in particular and to angiogenesis in general.

Review on Hypothetical Implementing TGF-β Family Members in Glaucoma Therapy

For quite some time, glaucoma has been regarded as more than just intraocular pressure [IOP] elevation. Significant contribution to this conceptual improvement has risen from a better understanding of ocular blood flow, vessel wall integrity and certain advanced ideas in neuroophthalmology, for example neuroprotection. Transforming growth factor-β (TGF-β) molecule, its inhibitors and antagonists have been increasingly researched as possible new anti-glaucoma drugs for its many, pleiotropic, effects. Among those effects, enhancing fibrosis is one of the most apparent, but certain members of this cytokine's superfamily act as anti-fibrotics. Recent scientific efforts strongly support pushing back the frontier of conventional medical treatment. Current medical approaches already use effects on blood flow and neuronal quiescence, with significant systemic side-effects. Endeavours on the ophthalmologic exploitation of selected, favourable effects of pleiotropic TGF-βs could promote TGF-β, its inhibitors or specific antibodies as new, ideal drugs in glaucoma therapy.

BMP9 is produced by hepatocytes and circulates mainly in an active mature form complexed to its prodomain

Bone Morphogenetic Protein 9 (BMP9) has been recently found to be the physiological ligand for the activin receptor-like kinase 1 (ALK1), and to be a major circulating vascular quiescence factor. Moreover, a soluble chimeric ALK1 protein (ALK1-Fc) has recently been developed and showed powerful anti-tumor growth and anti-angiogenic effects. However, not much is known concerning BMP9. This prompted us to investigate the human endogenous sources of this cytokine and to further characterize its circulating form(s) and its function. Analysis of BMP9 expression reveals that BMP9 is produced by hepatocytes and intrahepatic biliary epithelial cells. Gel filtration analysis combined with ELISA and biological assays demonstrate that BMP9 circulates in plasma (1) as an unprocessed inactive form that can be further activated by furin a serine endoprotease, and (2) as a mature and fully active form (composed of the mature form associated with its prodomain). Analysis of BMP9 circulating levels during mouse development demonstrates that BMP9 peaks during the first 3 weeks after birth and then decreases to 2 ng/mL in adulthood. We also show that circulating BMP9 physiologically induces a constitutive Smad1/5/8 phosphorylation in endothelial cells. Taken together, our results argue for the role of BMP9 as a hepatocyte-derived factor, circulating in inactive (40%) and active (60%) forms, the latter constantly activating endothelial cells to maintain them in a resting state.

Few Smad proteins and many Smad-interacting proteins yield multiple functions and action modes in TGFβ/BMP signaling in vivo

Signaling by the many ligands of the TGFβ family strongly converges towards only five receptor-activated, intracellular Smad proteins, which fall into two classes i.e. Smad2/3 and Smad1/5/8, respectively. These Smads bind to a surprisingly high number of Smad-interacting proteins (SIPs), many of which are transcription factors (TFs) that co-operate in Smad-controlled target gene transcription in a cell type and context specific manner. A combination of functional analyses in vivo as well as in cell cultures and biochemical studies has revealed the enormous versatility of the Smad proteins. Smads and their SIPs regulate diverse molecular and cellular processes and are also directly relevant to development and disease. In this survey, we selected appropriate examples on the BMP-Smads, with emphasis on Smad1 and Smad5, and on a number of SIPs, i.e. the CPSF subunit Smicl, Ttrap (Tdp2) and Sip1 (Zeb2, Zfhx1b) from our own research carried out in three different vertebrate models.

Bone Morphogenetic Protein functions as a context-dependent angiogenic cue in vertebrates

Bone Morphogenetic Protein (BMP) signaling has been implicated in diverse biological processes. Although how BMP signaling regulates behaviors of endothelial cells during angiogenesis are not fully understood, increasing evidence indicate functions of BMP signaling components are essential in developmental and pathological angiogenesis. Here we review recent advances in delineating the functions of BMP signaling during angiogenesis. In addition, we discuss downstream pathways that transduce BMP signaling in endothelial cells, and factors that modulate BMP signaling response in endothelial cells. Finally, we provide recent insight on how BMP signaling functions as a context dependent angiogenic cue.

Clinical and genetic analyses of three Korean families with hereditary hemorrhagic telangiectasia

Background

Hereditary hemorrhagic telangiectasia (HHT) is an autosomal-dominant vascular disorder, characterized by recurrent epistaxis, mucocutaneous telangiectases, and arteriovenous malformations (AVMs) in various visceral organs. Endoglin (ENG) and activin receptor-like kinase 1 (ACVRL1; ALK1), receptors for transforming growth factor-β (TGF-β) superfamily, have been identified as the principal HHT-causing genes.

Methods

Three unrelated Korean HHT patients and their asymptomatic as well as symptomatic family members were genetically diagnosed by sequencing whole exons and their flanking regions of ENG and ACVRL1. Functionality of an aberrant translation start codon, which is created by a substitution mutation at the 5'-untranslated region (UTR) of ENG found in a HHT family, was tested by transient in vitro transfection assay. Decay of the mutant transcripts was also assessed by allele-specific expression analysis.

Results

Two ENG and one ACVRL1 mutations were identified: a known ENG mutation (c.360+1G > A; p.Gly74_Tyr120del); a novel ENG mutation (c.1-127C > T); and a novel ACVRL1 mutation (c.252_253insC; p.Val85fsX168). We further validated that the 5'-UTR ENG mutation prevents translation of ENG from the biological translation initiation site of the mutant allele, and leads to degradation of the mutant transcripts.

Conclusions

This is the first experimental demonstration that a 5'-UTR mutation can prevent translation of ENG among HHT patients, and further supports the previous notion that haploinsufficiency is the primary mechanism of HHT1. Our data also underscore the importance of including exons encoding 5' UTR for HHT mutation screening.

Matrix Gla protein deficiency causes arteriovenous malformations in mice

Arteriovenous malformations (AVMs) in organs, such as the lungs, intestine, and brain, are characteristic of hereditary hemorrhagic telangiectasia (HHT), a disease caused by mutations in activin-like kinase receptor 1 (ALK1), which is an essential receptor in angiogenesis, or endoglin. Matrix Gla protein (MGP) is an antagonist of BMPs that is highly expressed in lungs and kidneys and is regulated by ALK1. The objective of this study was to determine the role of MGP in the vasculature of the lungs and kidneys. We found that Mgp gene deletion in mice caused striking AVMs in lungs and kidneys, where overall small organ size contrasted with greatly increased vascularization. Mechanistically, MGP deficiency increased BMP activity in lungs. In cultured lung epithelial cells, BMP-4 induced VEGF expression through induction of ALK1, ALK2, and ALK5. The VEGF secretion induced by BMP-4 in Mgp-/- epithelial cells stimulated proliferation of ECs. However, BMP-4 inhibited proliferation of lung epithelial cells, consistent with the increase in pulmonary vasculature at the expense of lung tissue in the Mgp-null mice. Similarly, BMP signaling and VEGF expression were increased in Mgp-/- mouse kidneys. We therefore conclude that Mgp gene deletion is what we believe to be a previously unidentified cause of AVMs. Because lack of MGP also causes arterial calcification, our findings demonstrate that the same gene defect has drastically different effects on distinct vascular beds.

ChIP-seq reveals cell type-specific binding patterns of BMP-specific Smads and a novel binding motif

Dysregulated bone morphogenetic protein (BMP) signaling in endothelial cells (ECs) and pulmonary arterial smooth muscle cells (PASMCs) are implicated in human genetic disorders. Here, we generated genome-wide maps of Smad1/5 binding sites in ECs and PASMCs. Smad1/5 preferentially bound to the region outside the promoter of known genes, and the binding was associated with target gene upregulation. Cell-selective Smad1/5 binding patterns appear to be determined mostly by cell-specific differences in baseline chromatin accessibility patterns. We identified, for the first time, a Smad1/5 binding motif in mammals, and termed GC-rich Smad binding element (GC-SBE). Several sequences in the identified GC-SBE motif had relatively weak affinity for Smad binding, and were enriched in cell type-specific Smad1/5 binding regions. We also found that both GC-SBE and the canonical SBE affect binding affinity for the Smad complex. Furthermore, we characterized EC-specific Smad1/5 target genes and found that several Notch signaling pathway-related genes were induced by BMP in ECs. Among them, a Notch ligand, JAG1 was regulated directly by Smad1/5, transactivating Notch signaling in the neighboring cells. These results provide insights into the molecular mechanism of BMP signaling and the pathogenesis of vascular lesions of certain genetic disorders, including hereditary hemorrhagic telangiectasia.

MicroRNAs, diet, and cancer: new mechanistic insights on the epigenetic actions of phytochemicals

There is growing interest in the epigenetic mechanisms that impact human health and disease, including the role of microRNAs (miRNAs). These small (18-25 nucleotide), evolutionarily conserved, non-coding RNA molecules regulate gene expression in a post-transcriptional manner. Several well-orchestered regulatory mechanisms involving miRNAs have been identified, with the potential to target multiple signaling pathways dysregulated in cancer. Since the initial discovery of miRNAs, there has been progress towards therapeutic applications, and several natural and synthetic chemopreventive agents also have been evaluated as modulators of miRNA expression in different cancer types. This review summarizes the most up-to-date information related to miRNA biogenesis, and critically evaluates proposed miRNA regulatory mechanisms in relation to cancer signaling pathways, as well as other epigenetic modifications (DNA methylation patterns, histone marks) and their involvement in drug resistance. We also discuss the mechanisms by which dietary factors regulate miRNA expression, in the context of chemoprevention versus therapy.

Soluble endoglin specifically binds bone morphogenetic proteins 9 and 10 via its orphan domain, inhibits blood vessel formation, and suppresses tumor growth

Endoglin (CD105), a transmembrane protein of the transforming growth factor β superfamily, plays a crucial role in angiogenesis. Mutations in endoglin result in the vascular defect known as hereditary hemorrhagic telangiectasia (HHT1). The soluble form of endoglin was suggested to contribute to the pathogenesis of preeclampsia. To obtain further insight into its function, we cloned, expressed, purified, and characterized the extracellular domain (ECD) of mouse and human endoglin fused to an immunoglobulin Fc domain. We found that mouse and human endoglin ECD-Fc bound directly, specifically, and with high affinity to bone morphogenetic proteins 9 and 10 (BMP9 and BMP10) in surface plasmon resonance (Biacore) and cell-based assays. We performed a function mapping analysis of the different domains of endoglin by examining their contributions to the selectivity and biological activity of the protein. The BMP9/BMP10 binding site was localized to the orphan domain of human endoglin composed of the amino acid sequence 26-359. We established that endoglin and type II receptors bind to overlapping sites on BMP9. In the in vivo chick chorioallantoic membrane assay, the mouse and the truncated human endoglin ECD-Fc both significantly reduced VEGF-induced vessel formation. Finally, murine endoglin ECD-Fc acted as an anti-angiogenic factor that decreased blood vessel sprouting in VEGF/FGF-induced angiogenesis in in vivo angioreactors and reduced the tumor burden in the colon-26 mouse tumor model. Together our findings indicate an important role of soluble endoglin ECD in the regulation of angiogenesis and highlight efficacy of endoglin-Fc as a potential anti-angiogenesis therapeutic agent.

Colorectal carcinoma cell production of transforming growth factor beta decreases expression of endothelial cell vascular endothelial growth factor receptor 2

BACKGROUND

Vascular endothelial growth factor (VEGF) signaling is a target for antiangiogenic cancer therapy. The authors have previously observed that up to 40% of vessels in colorectal carcinoma (CRC) tumors are negative for VEGF receptor 2 (VEGFR2) expression. Differential activity of transforming growth factor beta (TGF-β) is a potential contributor to this receptor heterogeneity because TGF-β contributes to both angiogenesis and CRC tumor progression.

METHODS

The authors analyzed VEGFR2 expression by Western blotting, and TGF-β expression in endothelial and CRC cell lines, respectively. In addition, they immunostained endothelial cells in CRC xenografts to find an association between VEGFR2 and TGF-β levels or activity.

RESULTS

In bovine aortic endothelial cells (BAECs), TGF-β1 significantly repressed VEGFR2 protein in a time-dependent and dose-dependent fashion (P < .05). Serum-free conditioned media from various malignant human CRC cell lines (HCT116, 379.2, Dks8, and DLD1) induced down-regulation of VEGFR2 in BAECs. This effect was proportional to the total levels of TGF-β1 and TGF-β2 and was blocked by SB-431542 and SD-208, TGF-β receptor I inhibitors. Immunofluorescence staining of subcutaneous mouse xenografts of HCT116, 379.2, Dks8, and SW480 cells revealed vessels with an inverse relationship between TGF-β activity and VEGFR2 expression. Oxygen and bone morphogenetic protein 9 levels were shown to modulate TGF-β-induced VEGFR2 down-regulation.

CONCLUSIONS

In combination with other factors, TGF-β may contribute to the vascular heterogeneity in human colorectal tumors.

Interaction between alk1 and blood flow in the development of arteriovenous malformations

Arteriovenous malformations (AVMs) are fragile direct connections between arteries and veins that arise during times of active angiogenesis. To understand the etiology of AVMs and the role of blood flow in their development, we analyzed AVM development in zebrafish embryos harboring a mutation in activin receptor-like kinase I (alk1), which encodes a TGFβ family type I receptor implicated in the human vascular disorder hereditary hemorrhagic telangiectasia type 2 (HHT2). Our analyses demonstrate that increases in arterial caliber, which stem in part from increased cell number and in part from decreased cell density, precede AVM development, and that AVMs represent enlargement and stabilization of normally transient arteriovenous connections. Whereas initial increases in endothelial cell number are independent of blood flow, later increases, as well as AVMs, are dependent on flow. Furthermore, we demonstrate that alk1 expression requires blood flow, and despite normal levels of shear stress, some flow-responsive genes are dysregulated in alk1 mutant arterial endothelial cells. Taken together, our results suggest that Alk1 plays a role in transducing hemodynamic forces into a biochemical signal required to limit nascent vessel caliber, and support a novel two-step model for HHT-associated AVM development in which pathological arterial enlargement and consequent altered blood flow precipitate a flow-dependent adaptive response involving retention of normally transient arteriovenous connections, thereby generating AVMs.

Activation of vascular bone morphogenetic protein signaling in diabetes mellitus

RATIONALE

Diabetes mellitus is frequently complicated by cardiovascular disease, such as vascular calcification and endothelial dysfunction, which have been associated with bone morphogenetic proteins (BMPs).

OBJECTIVE

To determine whether hyperglycemia in vitro and diabetes in vivo promote vascular BMP activity and correlate with vascular calcification.

METHODS AND RESULTS

Increased glucose augmented expression of BMP-2 and BMP-4; the BMP inhibitors matrix Gla protein (MGP) and Noggin; activin-like kinase receptor (ALK)1, -2, -3 and -6; the BMP type 2 receptor; and the vascular endothelial growth factor in human aortic endothelial cells (HAECs). Diabetes induced expression of the same factors in the aortic wall of 3 animal models of diabetes, Ins2(Akita/+) mice, db/db mice, and HIP rats (rats transgenic for human islet amyloid polypeptide), representative of types 1 and 2 diabetes. Conditioned media from glucose-treated HAECs increased angiogenesis in bovine aortic endothelial cells, as mediated by BMP-4, and osteogenesis in calcifying vascular cells, as mediated by BMP-2. BMP-4, MGP, ALK1, and ALK2 were predominantly expressed on the endothelial side of the aorta, and small interfering RNA experiments showed that these genes were regulated as a group. Diabetic mice and rats showed a dramatic increase in aortic BMP activity, as demonstrated by SMAD1/5/8 phosphorylation. This was associated with increased osteogenesis and calcium accumulation. These changes were prevented in the Ins2(Akita/+) mice by breeding them with MGP transgenic mice, which increased aortic BMP inhibition.

CONCLUSIONS

Hyperglycemia and diabetes activate vascular BMP activity, which is instrumental in promoting vascular calcification and may be limited by increasing BMP inhibition.

[Rendu-Osler disease: clinical and molecular update]

The Rendu-Osler disease, also called Hereditary Hemorrhagic Telangiectasia (HHT) affects 1 in -5-8000 people. A french epidemiological study pointed out that it was particularly high in the Haut-Jura mountains in France. This pathology is characterized by frequent nosebleeds, mucocutaneous and visceral telangiectasia and hereditary autosomal-dominant trait. The mucocutaneous telangiectasia are hemorrhagic while the visceral telangiectasia, less frequent, lead to arteriovenous fistula in the lungs, the liver and the brain. HHT disease-causing genes (ENG, ACVRL1 and MADH4) encode proteins that modulate TGFβ superfamilly signaling in vascular endothelial cells. The recent discovery that BMP9 acts as the specific ligand of the receptor ALK1 and endoglin as its co-receptor shows that this signaling pathway is involved in the maturation phase of angiogenesis. Mice heterozygous for endoglin or ALK1 defects reproduce the HHT phenotype and further support the involvement of endothelial hyper proliferation in the pathogenesis of the disease. The medical management of patients remains mainly symptomatic, however the angiogenic trait of this disease should allow us to consider in the future new -therapeutic approaches using anti-angiogenic drugs.

[Tumor angiogenesis: recent progress and remaining challenges]

Anti-angiogenic therapies of solid cancers aim at specifically destroying the tumor vasculature in order to "asphyxiate" the tumors. Since few years, they represent a novel therapeutic tool, which allowed to significantly improve the survival of patients suffering from colon, breast, kidney and lung cancers. However, these therapies are limited in their efficacy by the appearance of tumor resistance phenomena. In this review, I describe the molecular and cellular mechanisms of tumor angiogenesis with a special focus on the important roles played by hypoxia, the endothelial growth factor VEGF and the endothelial tip-cells located at the extremity of sprouting neo-vessels. I present the factors that respectively control the activation phase and the maturation phase of angiogenesis, as well as their mechanisms of action. In a second part, the efficacy and the limits of anti-angiogenic therapies presently available on the market are described, and the recent elucidation of some molecular mechanisms of tumor resistance to anti-angiogenic therapies is reviewed.

Bone morphogenetic proteins: a critical review

Bone Morphogenetic Proteins (BMPs) are potent growth factors belonging to the Transforming Growth Factor Beta superfamily. To date over 20 members have been identified in humans with varying functions during processes such as embryogenesis, skeletal formation, hematopoiesis and neurogenesis. Though their functions have been identified, less is known regarding levels of regulation at the extracellular matrix, membrane surface, and receptor activation. Further, current models of activation lack the integration of these regulatory mechanisms. This review focuses on the different levels of regulation, ranging from the release of BMPs into the extracellular components to receptor activation for different BMPs. It also highlights areas in research that is lacking or contradictory.

Insights After 40 Years of the Fontan Operation

Fontan’s visionary operation and its modifications over the ensuing decades have re-established nonturbulent flow and substantially reduced cyanosis for patients with severe hypoplasia of one ventricle. However, a long list of largely unexpected sequelae has emerged over the last 40 years. Although it is not difficult to understand how care providers could become discouraged, a number of myths have arisen, which we will attempt to dispel with real-world counterexamples as well as with lessons learned from other disciplines: evolutionary, developmental, and computational biology. We argue that distinctive biochemical abnormalities pointing to dysfunction in multiple organs, including the largest organ system in the body, the endothelium, occur long before grossly observable changes in cardiac imaging can be recognized. With a rational redesign of both our surveillance scheme and our wellness strategies, we hope that Fontan survivors and their families, as well as physicians, nurses, and therapists, will see why Fontan’s principle remains just as vibrant today as it was in 1971.

Inhibition of bone morphogenetic proteins protects against atherosclerosis and vascular calcification

RATIONALE

The bone morphogenetic proteins (BMPs), a family of morphogens, have been implicated as mediators of calcification and inflammation in the vascular wall.

OBJECTIVE

To investigate the effect of altered expression of matrix Gla protein (MGP), an inhibitor of BMP, on vascular disease.

METHODS AND RESULTS

We used MGP transgenic or MGP-deficient mice bred to apolipoprotein E mice, a model of atherosclerosis. MGP overexpression reduced vascular BMP activity, atherosclerotic lesion size, intimal and medial calcification, and inflammation. It also reduced expression of the activin-like kinase receptor 1 and the vascular endothelial growth factor, part of a BMP-activated pathway that regulates angiogenesis and may enhance lesion formation and calcification. Conversely, MGP deficiency increased BMP activity, which may explain the diffuse calcification of vascular medial cells in MGP deficient aortas and the increase in expression of activin-like kinase receptor 1 and vascular endothelial growth factor. Unexpectedly, atherosclerotic lesion formation was decreased in MGP-deficient mice, which may be explained by a dramatic reduction in expression of endothelial adhesion molecules limiting monocyte infiltration of the artery wall.

CONCLUSIONS

Our results indicate that BMP signaling is a key regulator of vascular disease, requiring careful control to maintain normal vascular homeostasis.