Heparin binds directly to the N-terminal region of the ALK extracellular domain.
Major finding: Heparin binds directly to the N-terminal region of the ALK extracellular domain.
Concept: Heparin-induced ALK activation is dependent on heparin chain length and sulfation pattern.
Impact: Targeting the N-terminal region to block glycosaminoglycan binding may be an effective way to inhibit ALK.
ALK is considered an orphan receptor tyrosine kinase (RTK) because it has no known ligand. The heparin-binding molecules pleiotropin (PTN) and midkine (MK) were initially reported to be ALK ligands, but these findings have not been confirmed. Murray and colleagues observed that although mixtures of heparin and PTN or MK stimulated ALK autophosphorylation, heparin alone was sufficient to activate ALK, suggesting that heparin, which can bind and activate (when acting together with a growth factor) other RTKs such as FGFRs, is an ALK ligand. Indeed, heparin bound with high affinity and specificity to the full-length ALK extracellular domain (ECD) in vitro but not to an ALK truncation mutant lacking the N-terminal region (NTR) of the ECD. Heparin chain length correlated with affinity for ALK and the degree of ALK oligomerization in vitro, and stimulation of neuroblastoma cells with heparin with various chain lengths revealed that only heparin molecules with more than 15 disaccharide units were capable of inducing ALK autophosphorylation. Given that heparin is an experimental substitute for physiologic glycosaminoglycan ligands distinguished by various sulfation patterns that can affect receptor binding and activation, the authors evaluated the effect of different heparin and glycosaminoglycan sulfation patterns on ALK activation and found that only heparin with N- and O-linked sulfate groups or glycosaminoglycans with sulfation patterns similar to heparin could activate ALK. Consistent with these results, an antibody that competitively blocked heparin binding to the ALK NTR potently inhibited heparin-induced ALK autophosphorylation and downstream activation of AKT and ERK in neuroblastoma cells. The identification of heparin as an activating ALK ligand or co-ligand thus not only provides insight into ALK biology but also suggests that targeting glycosaminoglycan binding by the ALK NTR may be an effective strategy to inhibit ALK signaling.
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