Binding of Autotaxin to Integrins Localizes Lysophosphatidic Acid Production to Platelets and Mammalian Cells

Abstract

Autotaxin (ATX) is a secreted lysophospholipase D that generates the bioactive lipid mediator lysophosphatidic acid (LPA).We and others have reported that ATX binds to integrins, but the function of ATX-integrin interactions is unknown. The recently reported crystal structure of ATX suggests a role for the solvent-exposed surface of the N-terminal tandem somatomedin B-like domains in binding to platelet integrin ␣IIb␤ 3 . The opposite face of the somatomedin B-like domain interacts with the catalytic phosphodiesterase (PDE) domain to form a hydrophobic channel through which lysophospholipid substrates enter and leave the active site. Based on this structure, we hypothesize that integrin-bound ATX can access cell surface substrates and deliver LPA to cell surface receptors. To test this hypothesis, we investigated the integrin selectivity and signaling pathways that promote ATX binding to platelets. We report that both platelet ␤1 and ␤3 integrins interact in an activation-dependent manner with ATX via the SMB2 domain. ATX increases thrombin-stimulated LPA production by washed platelets ϳ10-fold. When incubated under conditions to promote integrin activation, ATX generates LPA from CHO cells primed with bee venom phospholipase A 2 , and ATX-mediated LPA production is enhanced more than 2-fold by CHO cell overexpression of integrin ␤ 3 . The effects of ATX on platelet and cell-associated LPA production, but not hydrolysis of small molecule or detergent-solubilized substrates, are attenuated by point mutations in the SMB2 that impair integrin binding. Integrin binding therefore localizes ATX activity to the cell surface, providing a mechanism to generate LPA in the vicinity of its receptors.

Thematic Review Series: Phospholipases: Central Role in Lipid Signaling and Disease Autotaxin, a lysophospholipase D with pleomorphic effects in oncogenesis and cancer progression

“…Because SMB-like domains are in close proximity to a hydrophobic channel connecting the catalytic site to the external environment, they are believed to act as a bridge between ATX and the extracellular space where enzymatic products can directly interact with their cognate receptors ( 16 ). This view is supported by studies in platelets and mammalian cells showing that both SMBlike domains and integrins are required for the interaction of ATX with platelets, and that integrin activation increases ATX-dependent LPA production ( 39 ). The wealth of information available on both enzymatic dynamics and ATX structure is helping to establish new operative frameworks for inhibitor design which will hopefully lead to the synthesis of potent and selective molecules capable of effectively inhibiting ATX function in the context of cancer and other pathologies where its activity is dysregulated (see Role in Physiology and Cancer Pathophysiology below).…”

Autotaxin: structure-function and signaling

“…The binding site on ATX is unknown, but it is of note that a known integrin-binding motif (LDV) on the surface of the PDE domain is exposed and probably accessible for integrin interaction ( 72 ). In conclusion, the available evidence supports a model in which distinct domains mediate ATX binding to integrins, thus serving to localize ATX to the plasma membrane ( 73 ).…”

“…Both SMB domains are structurally similar, but have differential intra-and inter-molecular interactions. While SMB1 is involved in the formation of the tunnel, the SMB2 surface is in the best position to engage with other binding partners ( 63,73 ). Indeed, ATX binds to activated platelets and other cells through ␤ 1 and ␤ 3 integrins via its N-terminal SMB2 domain ( 73,74 ).…”

Lysophosphatidic acid produced by Autotaxin acts as an allosteric modulator of its catalytic efficiency

“…Second, it has been shown that ATX binds surface integrins through the SMB domains 13,31,38 , whereas the longer isoform of ATX (ATXα) binds to heparan sulfate proteoglycans 32 . While this binding will localize ATX at the cell surface, likely making LPA delivery to surface receptors more efficient, it cannot be excluded that it also affects the kinetics of LPC hydrolysis in our model.…”