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...