Casein kinase 2β as a novel enhancer of activin‐like receptor‐1 signaling

Abstract

ALK-1 is a transforming growth factor beta (TGF-beta) superfamily receptor that is predominantly expressed in endothelial cells and is essential for angiogenesis, as demonstrated by the embryonic lethal phentoype when targeted for deletion in mice and its mutation in the human disease hereditary hemorrhagic telangiectasia. Although ALK-1 and the endothelial-specific TGF-beta superfamily coreceptor, endoglin, form a heteromeric complex and bind similar TGF-beta superfamily ligands, their signaling mechanisms remain poorly characterized. Here we report the identification of CK2beta, the regulatory subunit of protein kinase CK2, as a novel enhancer of ALK-1 signaling. The cytoplasmic domain of ALK-1 specifically binds to CK2beta in vitro and in vivo. NAAIRS mutagenesis studies define amino acid sequences 181-199 of CK2beta and 207-212 of ALK-1 as the interaction domains, respectively. The ALK-1/CK2beta interaction specifically enhanced Smad1/5/8 phosphorylation and ALK-1-mediated reporter activation in response to TGF-beta1 and BMP-9 treatment. In a reciprocal manner, siRNA-mediated silencing of endogenous CK2beta inhibited TGF-beta1 and BMP-9-stimulated Smad1/5/8 phosphorylation and ALK-1-mediated reporter activation. Functionally, CK2beta enhanced the ability of activated or ligand-stimulated ALK-1 to inhibit endothelial cell migration. Similarly, ALK-1 and CK2beta antagonized endothelial tubule formation in Matrigel. These studies support CK2beta as an important regulator of ALK-1 signaling and ALK-1-mediated functions in endothelial cells.

Endoglin regulates PI3-kinase/Akt trafficking and signaling to alter endothelial capillary stability during angiogenesis

“…Although BMP-9 can also bind ALK1, we observed no ALK1-induced enhancement of capillary stability or Akt activity in endoglin-null systems (Figures 1B and 3C). This finding is consistent with our previous report that increased ALK1 signaling antagonizes endothelial capillary sprouting (Lee et al., 2009). On the basis of these results, we propose a model (Supplemental Figure S3D) in which endoglin binds BMP-9 independent of ALK1 and ALK5 to preferentially promote PI3K/Akt activation after membrane recruitment.…”

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

“…Thus, either shifting the balance of Smad1/5/8 and Smad2 signalling towards Smad1/5/8, or selectively increasing Smad1/5/8 signalling, is predicted to result in decreased endothelial cell migration. These results are consistent with our previous findings in which endoglin/GIPC , constitutively activated ALK1, or expression of the ALK1 activator, CK2b (Lee et al, 2009), increased Smad1/5/8 signalling and inhibited endothelial migration. The mechanisms by which these diverse factors might coordinate to regulate TGF-b superfamily signalling and endothelial cell function are currently being explored.…”

Protein Kinase CK2 and Angiogenesis

“…It was shown that CK2b specifically interacts with Alk-1 and by this interaction enhances Alk-1 signalling. This finally results in an inhibition of endothelial cell migration [53]. These functions of CK2b are consistent with its binding to and regulation of other serine/threonine kinases including Mos [54], A-raf [55] and Chk-1 [56].…”