Annexin A5 Down-regulates Surface Expression of Tissue Factor: A NOVEL MECHANISM OF REGULATING THE MEMBRANE RECEPTOR REPERTOIR

Susana Ravassa,Abdelkader Bennaghmouch,Heidi Kenis,Theo Lindhout,Tilman Hackeng,Jagat Narula,Leo Hofstra,Chris Reutelingsperger

doi:10.1074/jbc.m411710200

Abstract

Phosphatidylserine (PtdSer) is exposed on the external leaflet of the plasma membrane during apoptosis. The protein annexin A5 (anxA5) shows high affinity for PtdSer. When anxA5 binds to the PtdSer-expressing membranes during apoptosis, it crystallizes as an extended two-dimensional network and activates thereby a novel portal of cell entry that results in the internalization of the PtdSer-expressing membrane patches. This novel pathway of cell entry is potentially involved in the regulation of the surface expression of membrane receptors. In this study we report the regulation of surface expression of the initiator of blood coagulation tissue factor (TF) by this novel pathway of cell entry. AnxA5 induces the internalization of tissue factor expressed on the surface of apoptotic THP-1 macrophages. This down-regulation depends on the abilities of anxA5 to bind to PtdSer and to form a two-dimensional crystal at the membrane. We furthermore show that THP-1 cells produce and externalize anxA5 that cause the internalization of TF in an autocrine type of mechanism. We extended our in vitro work to the in vivo situation and show in a mouse model that anxA5 causes the down-regulation of TF expression by smooth muscle cells of the media of the carotid artery that was mechanically injured. In conclusion, anxA5 down-regulates surface-expressed TF by activating the novel portal of cell entry. This mechanism may be part of a more general autocrine function of anxA5 to regulate the plasma membrane receptor repertoir under stress conditions associated with the surface expression of PtdSer.

Highlights

Phosphatidylserine (PtdSer) is exposed on the external leaflet of the plasma membrane during apoptosis
When annexin A5 (anxA5) binds to the PtdSer-expressing membranes during apoptosis, it crystallizes as an extended two-dimensional network and activates thereby a novel portal of cell entry that results in the internalization of the PtdSer-expressing membrane patches
It has been shown that apoptotic cells show an increased tissue factor (TF) activity, which can be inhibited by anxA5 (29 –31). This could be explained by the fact that the crystallization of anxA5 when bound to PtdSer, provokes shielding of the procoagulant surface reducing the availability for anionic phospholipid-dependent coagulation reactions, such as the activation of factor X and IX by the TF-FVIIa complex [32,33,34,35]

Summary

A NOVEL MECHANISM OF REGULATING THE MEMBRANE RECEPTOR REPERTOIR*

AnxA5 induces the internalization of tissue factor expressed on the surface of apoptotic THP-1 macrophages This down-regulation depends on the abilities of anxA5 to bind to PtdSer and to form a two-dimensional crystal at the membrane. It has been shown that apoptotic cells show an increased TF activity, which can be inhibited by anxA5 (29 –31) This could be explained by the fact that the crystallization of anxA5 when bound to PtdSer, provokes shielding of the procoagulant surface reducing the availability for anionic phospholipid-dependent coagulation reactions, such as the activation of factor X and IX by the TF-FVIIa complex [32,33,34,35]. We have conducted in vitro and in vivo experiments showing that by modifying plasma membrane dynamics, anxA5 down-regulates the expression of the transmembrane protein TF through internalization in PtdSer-containing patches

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION