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Annexin A2: biology and relevance to the antiphospholipid syndrome

Divisions of Pediatric, Case Western Reserve University School of Medicine, Cleveland, Ohio, USA

RG Espinola

Division of Adult Hematology-Oncology, Case Western Reserve School of Medicine, Cleveland, Ohio, USA

KR McCrae

Division of Adult Hematology-Oncology, Case Western Reserve School of Medicine, Cleveland, Ohio, USA keith.mccrae{at}case.edu

Antiphospholipid antibodies (aPL), the majority of which are directed against β₂-glycoprotein I (β₂GPI), are associated with an increased incidence of venous and arterial thrombosis. The pathogenesis of antiphospholipid/anti-β₂GPI-associated thrombosis has not been defined, and is likely multifactorial. However, accumulating evidence suggests an important role for endothelial cell activation with the acquisition of a procoagulant phenotype by the activated endothelial cell. Previous work demonstrated that endothelial activation by antiphospholipid/anti-β₂GPI antibodies is β₂GPI-dependent. We extended these observations by defining annexin A2 as an endothelial β₂GPI binding site. We also observed that annexin A2 plays a critical role in endothelial cell activation induced by anti-β₂GPI antibodies, and others have described direct endothelial activation by anti-annexin A2 antibodies in patients with aPL . Similar findings have been reported using human monocytes, which also express annexin A2. Because annexin A2 is not a transmembrane protein, how binding of β₂GPI/anti-β₂GPI antibodies, or anti-annexin A2 antibodies, to endothelial annexin A2 causes cellular activation is unknown. Recent studies, however, suggest an important role for the Toll-like receptor family, particularly TLR4. In this article, we review the role of these interactions in the activation of endothelial cells by aPL . The influence of these antibodies on the ability of annexin A2 to enhance t-PA-mediated plasminogen activation is also discussed.

Key Words: annexin • antiphospholipid • β2-glycoprotein I • endothelial • thrombosis

Lupus, Vol. 17, No. 10, 944-952 (2008)
DOI: 10.1177/0961203308095329


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