Heparin is an activating ligand of the orphan receptor tyrosine kinase ALK

Abstract

Anaplastic lymphoma kinase (ALK) is one of the few remaining "orphan" receptor tyrosine kinases (RTKs) in which the ligands are unknown. Ligand-mediated activation of RTKs is important throughout development. ALK is particularly relevant to the development of the nervous system. Increased activation of RTKs by mutation, genetic amplification, or signals from the stroma contributes to disease progression and acquired drug resistance in cancer. Aberrant activation of ALK occurs in subsets of lung adenocarcinoma, neuroblastoma, and other cancers. We found that heparin is a ligand that binds specifically to the ALK extracellular domain. Whereas heparins with short chain lengths bound to ALK in a monovalent manner and did not activate the receptor, longer heparin chains induced ALK dimerization and activation in cultured neuroblastoma cells. Heparin lacking N- and O-linked sulfate groups or other glycosaminoglycans with sulfation patterns different than heparin failed to activate ALK. Moreover, antibodies that bound to the extracellular domain of ALK interfered with heparin binding and prevented heparin-mediated activation of ALK. Thus, heparin and perhaps related glycosaminoglycans function as ligands for ALK, revealing a potential mechanism for the regulation of ALK activity in vivo and suggesting an approach for developing ALK-targeted therapies for cancer.

Augmentor α and β (FAM150) are ligands of the receptor tyrosine kinases ALK and LTK: Hierarchy and specificity of ligand–receptor interactions

“…We have previously shown that heparin binds to the NTR region of ALK and stimulates the receptor's kinase activity in NB1 cells (15). Therefore, we studied the effect of heparin on ALK in the presence and absence of AUG-α.…”

“…Previously we have shown that heparin can bind to the NTR domain of ALK and can activate ALK expressed in NB1 cells (15). However, the role of the two MAM and LDL-A domains remains unclear.…”

“…However, subsequent studies were unable to reproduce these results (22)(23)(24). We have recently demonstrated that ALK expressed in the NB1 cell line can be activated by heparin (but not by pleiotrophin or midkine), suggesting a proteoglycan may regulate ALK activity and function through binding to its heparin-binding NTR (15). Two small proteins, designated FAM150A and FAM150B, were recently identified in a screening assay for secreted proteins that bind to the recombinant extracellular domain of LTK (25).…”

“…The ECDs of both LTK and ALK contain a unique glycinerich domain and an EGF-like motif (14). In addition, the ECD of ALK also has a heparin-binding N-terminal domain (NTR) and two meprin, A-5 protein, and receptor protein-tyrosine phosphatase mu (MAM) domains separated by a low-density lipoprotein receptor domain class A (LDL-A) domain (15).…”

Adopting ALK and LTK

“…In studies published earlier this year, Murray et al, also in the Schlessinger group, identified heparin as an activating ligand for ALK in NB-1 neuroblastoma cells (14). These investigators found that acidic sulfated glycosaminoglycan polymers bound to the very basic amino terminus of ALK, and strongly potentiated ALK autophosphorylation in NB-1 cells.…”