Bone morphogenetic protein (BMP) and activin type II receptors balance BMP9 signals mediated by activin receptor-like kinase-1 in human pulmonary artery endothelial cells

BMP-9 induces proliferation of multiple types of endothelial cells in vitro and in vivo

Members of the bone morphogenetic protein (BMP) family have been implicated in the development and maintenance of vascular systems. Whereas members of the BMP-2/4 and osteogenic protein-1 groups signal via activin receptor-like kinase (ALK)-2, ALK-3 and ALK-6, BMP-9 and BMP-10 have been reported to bind to ALK-1 in endothelial cells. However, the roles of BMP-9–ALK-1 signaling in the regulation of endothelial cells have not yet been fully elucidated. Here, using various systems, we examined the effects of BMP-9 on the proliferation of endothelial cells. Vascular-tube formation from ex vivo allantoic explants of mouse embryos was promoted by BMP-9. BMP-9, as well as BMP-4 and BMP-6, also induced the proliferation of in-vitro-cultured mouse embryonic-stem-cell-derived endothelial cells (MESECs) by inducing the expression of vascular endothelial growth factor receptor 2 and Tie2, a receptor for angiopoietin-1. A decrease in ALK-1 expression or expression of constitutively active ALK-1 in MESECs abrogated and mimicked the effects of BMP-9 on the proliferation of MESECs, respectively, suggesting that BMP-9 promotes the proliferation of these cells via ALK-1. Furthermore, in vivo angiogenesis was promoted by BMP-9 in a Matrigel plug assay and in a BxPC3 xenograft model of human pancreatic cancer. Consistent with these in vivo findings, BMP-9 enhanced the proliferation of in-vitro-cultured normal endothelial cells from dermal tissues of adult mice and of tumor-associated endothelial cells isolated from tumor xenografts in host mice. These findings suggest that BMP-9 signaling activates the endothelium tested in the present study via ALK-1.

Endoglin is involved in BMP-2-induced osteogenic differentiation of periodontal ligament cells through a pathway independent of Smad-1/5/8 phosphorylation

The periodontal ligament (PDL), a connective tissue located between the cementum of teeth and the alveolar bone of mandibula, plays a crucial role in the maintenance and regeneration of periodontal tissues. The PDL contains fibroblastic cells of a heterogeneous cell population, from which we have established several cell lines previously. To analyze characteristics unique for PDL at a molecular level, we performed cDNA microarray analysis of the PDL cells versus MC3T3-E1 osteoblastic cells. The analysis followed by validation by reverse transcription-polymerase chain reaction and immunochemical staining revealed that endoglin, which had been shown to associate with transforming growth factor (TGF)-beta and bone morphogenetic proteins (BMPs) as signaling modulators, was abundantly expressed in PDL cells but absent in osteoblastic cells. The knockdown of endoglin greatly suppressed the BMP-2-induced osteoblastic differentiation of PDL cells and subsequent mineralization. Interestingly, the endoglin knockdown did not alter the level of Smad-1/5/8 phosphorylation induced by BMP-2, while it suppressed the BMP-2-induced expression of Id1, a representative BMP-responsive gene. Therefore, it is conceivable that endoglin regulates the expression of BMP-2-responsive genes in PDL cells at some site downstream of Smad-1/5/8 phosphorylation. Alternatively, we found that Smad-2 as well as Smad-1/5/8 was phosphorylated by BMP-2 in the PDL cells, and that the BMP-2-induced Smad-2 phosphorylation was suppressed by the endoglin knockdown. These results, taken together, raise a possibility that PDL cells respond to BMP-2 via a unique signaling pathway dependent on endoglin, which is involved in the osteoblastic differentiation and mineralization of the cells.

ALK1 signaling regulates early postnatal lymphatic vessel development

In vertebrates, endothelial cells form 2 hierarchical tubular networks, the blood vessels and the lymphatic vessels. Despite the difference in their structure and function and genetic programs that dictate their morphogenesis, common signaling pathways have been recognized that regulate both vascular systems. ALK1 is a member of the transforming growth factor-beta type I family of receptors, and compelling genetic evidence suggests its essential role in regulating blood vascular development. Here we report that ALK1 signaling is intimately involved in lymphatic development. Lymphatic endothelial cells express key components of the ALK1 pathway and respond robustly to ALK1 ligand stimulation in vitro. Blockade of ALK1 signaling results in defective lymphatic development in multiple organs of neonatal mice. We find that ALK1 signaling regulates the differentiation of lymphatic endothelial cells to influence the lymphatic vascular development and remodeling. Furthermore, simultaneous inhibition of ALK1 pathway increases apoptosis in lymphatic vessels caused by blockade of VEGFR3 signaling. Thus, our study reveals a novel aspect of ALK1 signaling in regulating lymphatic development and suggests that targeting ALK1 pathway might provide additional control of lymphangiogenesis in human diseases.

Emerging role of bone morphogenetic proteins in angiogenesis

Bone morphogenetic proteins (BMPs) are multifunctional growth factors belonging to the transforming growth factor beta (TGFbeta) superfamily. Recent observations clearly emphasize the emerging role of BMPs in angiogenesis: (i) two genetic vascular diseases (hereditary hemorrhagic telangiectasia (HHT) and pulmonary arterial hypertension (PAH)) are caused by mutations in genes encoding components of the BMP signalling pathway (endoglin, ALK1 and BMPRII). (ii) BMP9 has been identified as the physiological ligand of the endothelial receptor ALK1 in association with BMPRII. This review will focus on the diverse functions of BMPs in angiogenesis. We will propose a model that distinguishes the BMP2, BMP7 and GDF5 subgroups from the BMP9 subgroup on the basis of their functional implication in the two phases of angiogenesis (activation and maturation).