Abstract-Angiogenesis is a complex process, requiring a finely tuned balance between numerous stimulatory and inhibitory signals. ALK1 (activin receptor like-kinase 1) is an endothelial-specific type 1 receptor of the transforming growth factor-␤ receptor family. Heterozygotes with mutations in the ALK1 gene develop hereditary hemorrhagic telangiectasia type 2 (HHT2). Recently, we reported that bone morphogenetic protein (BMP)9 and BMP10 are specific ligands for ALK1 that potently inhibit microvascular endothelial cell migration and growth. These data lead us to suggest that these factors may play a role in the control of vascular quiescence. To test this hypothesis, we checked their presence in human serum. We found that human serum induced Smad1/5 phosphorylation. To identify the active factor, we tested neutralizing antibodies against BMP members and found that only the anti-BMP9 inhibited serum-induced Smad1/5 phosphorylation. The concentration of circulating BMP9 was found to vary between 2 and 12 ng/mL in sera and plasma from healthy humans, a value well above its EC 50 (50 pg/mL). These data indicated that BMP9 is circulating at a biologically active concentration. We then tested the effects of BMP9 in 2 in vivo angiogenic assays. We found that BMP9 strongly inhibited sprouting angiogenesis in the mouse sponge angiogenesis assay and that BMP9 could inhibit blood circulation in the chicken chorioallantoic membrane assay. Taken together, our results demonstrate that BMP9, circulating under a biologically active form, is a potent antiangiogenic factor that is likely to play a physiological role in the control of adult blood vessel quiescence. Key Words: BMP9 Ⅲ ALK1 Ⅲ HHT Ⅲ angiogenesis B one morphogenetic proteins (BMPs), which belong to the transforming growth factor (TGF)␤ superfamily, were originally identified as inducers of ectopic bone growth and cartilage formation. Since then, there has been substantial progress in our knowledge of the multiple functions of these growth factors. 1 BMPs regulate cell growth, differentiation, and apoptosis of various cell types, and they are critically important in the morphogenesis and differentiation of tissues and organs. BMP9, also known as growth differentiation factor-2, is expressed in the adult liver by nonparenchymal cells (ie, endothelial, stellate, and Kupffer cells) 2 and in the septum and spinal cord of mouse embryos. 3 BMP9 has been described as a hematopoietic, hepatogenic, osteogenic, and chondrogenic factor. It has also been identified as a regulator of glucose metabolism, capable of reducing glycemia in diabetic mice and as a differentiation factor for cholinergic neurons in the central nervous system. 3 More recently, it was shown to induce the expression of hepcidin, a hormone that plays a key role in iron homeostasis. 4 ALK1 (activin receptor like-kinase 1) is an endothelialspecific type I receptor of the TGF␤ receptor family that is implicated in the pathogenesis of hereditary hemorrhagic telangiectasia type 2 (HHT2), also known as the Rendu-Osler...

Ischemic and solid tumor tissues are less well perfused than normal tissue, leading to metabolic changes and chronic hypoxia, which in turn promotes angiogenesis. We identified human angiopoietin-like 4 (angptl4) as a gene with hypoxia-induced expression in endothelial cells. We showed that the levels of both mRNA and protein for ANGPTL4 increased in response to hypoxia. When tested in the chicken chorioallantoic membrane assay, ANGPTL4 induced a strong proangiogenic response, independently of vascular endothelial growth factor. In human pathology, ANGPTL4 mRNA is produced in ischemic tissues, in conditions such as critical leg ischemia. In tumors, ANGPTL4 is produced in the hypoxic areas surrounding necrotic regions. We observed particularly high levels of ANGPTL4 mRNA in tumor cells of conventional renal cell carcinoma. Other benign and malignant renal tumor cells do not produce ANGPTL4 mRNA. This molecule therefore seems to be a marker of conventional renal cell carcinoma. ANGPTL4, originally identified as a peroxisome proliferator-activated receptor alpha and gamma target gene, has potential for use as a new diagnostic tool and a potential therapeutic target, modulating angiogenesis both in tumors and in ischemic tissues. This study also suggests that ANGPTL4 may provide a link between metabolic disorders and hypoxia-induced angiogenesis.