IntroductionThe introduction of antiangiogenic therapies into oncologic practice has been highly anticipated because of the success of extensive preclinical testing. However, the initial clinical experience with this new class of anticancer drugs has been sobering, with a measurable therapeutic benefit of a few months observed and very little effect on overall patient survival. To fully realize the potential of therapies inhibiting neoangiogenesis, it is likely that drugs impinging on multiple regulatory pathways must be combined, and thus we must learn more about the biology of various signaling factors affecting endothelial cell (EC) growth and function. Activin receptor-like kinase 1 (ALK1) is an EC-restricted receptor of the large TGF-␤ family. 1 Herein, we will review our current understanding of ALK1 signaling and the potential of ALK1 to serve as a drug target for antiangiogenic therapy for cancer. Signaling by ALK1 in ECsThe large family of TGF-␤ extracellular ligands consists of Ͼ 30 cytokines that exert influence on several cellular compartments, notably epithelial cells, fibroblasts, immune cells, and endothelial and perivascular cells. TGF-␤, the prototypical member of the family, elicits a diverse set of cellular responses, such as growth arrest, immune suppression, differentiation, apoptosis, and specification of developmental cell fate during embryogenesis and pathogenesis, in species ranging from flies and worms to mammals. 2,3 On secretion and subsequent activation, the mature TGF-␤ ligand initiates signaling by inducing specific serine/threonine kinase type I and type II receptor heterotetrameric complexes. 4 Ligand binding results in signal propagation inside the cell by phosphorylation of specific effector proteins, so-called Smads, which translocate to the nucleus and activate transcription of target genes. 2,5 In ECs, TGF-␤ has been shown to signal via both the ubiquitously expressed type I receptor ALK5 and through the predominantly EC restricted receptor ALK1 (Figure 1). Depending on which type I receptor is recruited, different Smad signaling cascades are activated; ALK1 activation induces phosphorylation of Smad1/5/8, whereas ALK5 leads to Smad 2/3 activation. [6][7][8][9] Consequent to engagement of either Smad pathway, the receptoractivated Smads further form a heteromeric complex with a common and related partner molecule, Smad4, which translocates the complexes into the nucleus, where cell type-specific transcriptional modulators collaborate to activate or repress transcription of specific target genes in the angiogenic response. 10,11 In addition to the canonical signaling through Smad activation, TGF-␤ stimulation may lead to Smad-independent regulation of cellular outcomes, such as apoptosis and cell-cycle progression, through the direct modulation of prototypical signaling mediators, including MAP kinases and p21. 12 Furthermore, a third type of TGF-␤ receptor, the type III receptors, is represented by betaglycan and endoglin. Endoglin, primarily a vascular marker, is an auxilia...

Genetic deficiency for endoglin leads to increased metastatic capability by weakening the endothelial cell barrier.

Members of the transforming growth factor β (TGF-β) family have been genetically linked to vascular formation during embryogenesis. However, contradictory studies about the role of TGF-β and other family members with reported vascular functions, such as bone morphogenetic protein (BMP) 9, in physiological and pathological angiogenesis make the need for mechanistic studies apparent. We demonstrate, by genetic and pharmacological means, that the TGF-β and BMP9 receptor activin receptor-like kinase (ALK) 1 represents a new therapeutic target for tumor angiogenesis. Diminution of ALK1 gene dosage or systemic treatment with the ALK1-Fc fusion protein RAP-041 retarded tumor growth and progression by inhibition of angiogenesis in a transgenic mouse model of multistep tumorigenesis. Furthermore, RAP-041 significantly impaired the in vitro and in vivo angiogenic response toward vascular endothelial growth factor A and basic fibroblast growth factor. In seeking the mechanism for the observed effects, we uncovered an unexpected signaling synergy between TGF-β and BMP9, through which the combined action of the two factors augmented the endothelial cell response to angiogenic stimuli. We delineate a decisive role for signaling by TGF-β family members in tumor angiogenesis and offer mechanistic insight for the forthcoming clinical development of drugs blocking ALK1 in oncology.

Lymphatic vessels (LVs) play critical roles in the maintenance of fluid homeostasis and in pathological conditions, including cancer metastasis. Although mutations in ALK1, a member of the transforming growth factor (TGF)-β/bone morphogenetic protein (BMP) receptor family, have been linked to hereditary hemorrhagic telangiectasia, a human vascular disease, the roles of activin receptor-like kinase 1 (ALK-1) signals in LV formation largely remain to be elucidated. We show that ALK-1 signals inhibit LV formation, and LVs were enlarged in multiple organs in Alk1-depleted mice. These inhibitory effects of ALK-1 signaling were mediated by BMP-9, which decreased the number of cultured lymphatic endothelial cells. Bmp9-deficient mouse embryos consistently exhibited enlarged dermal LVs. BMP-9 also inhibited LV formation during inflammation and tumorigenesis. BMP-9 downregulated the expression of the transcription factor prospero-related homeobox 1, which is necessary to maintain lymphatic endothelial cell identity. Furthermore, silencing prospero-related homeobox 1 expression inhibited lymphatic endothelial cell proliferation. Our findings reveal a unique molecular basis for the physiological and pathological roles of BMP-9/ALK-1 signals in LV formation.lymphangiogenesis | angiogenesis | blood vascular endothelial cells

Correct organization of the vascular tree requires the balanced activities of several signaling pathways that regulate tubulogenesis and vascular branching, elongation, and pruning. When this balance is lost, the vessels can be malformed and fragile, and they can lose arteriovenous differentiation. In this review, we concentrate on the transforming growth factor (TGF)-β/bone morphogenetic protein (BMP) pathway, which is one of the most important and complex signaling systems in vascular development. Inactivation of these pathways can lead to altered vascular organization in the embryo. In addition, many vascular malformations are related to deregulation of TGF-β/BMP signaling. Here, we focus on two of the most studied vascular malformations that are induced by deregulation of TGF-β/BMP signaling: hereditary hemorrhagic telangiectasia (HHT) and cerebral cavernous malformation (CCM). The first of these is related to loss-of-function mutation of the TGF-β/BMP receptor complex and the second to increased signaling sensitivity to TGF-β/BMP. In this review, we discuss the potential therapeutic targets against these vascular malformations identified so far, as well as their basis in general mechanisms of vascular development and stability.