Vitamin K-Dependent Protein S Localizing Complement Regulator C4b-Binding Protein to the Surface of Apoptotic Cells

Abstract

Apoptosis is characterized by a lack of inflammatory reaction in surrounding tissues, suggesting local control of complement activation. During the initial stage of apoptosis, cells expose negatively charged phospholipid phosphatidylserine on their surfaces. The vitamin K-dependent protein S has a high affinity for this type of phospholipid. In human plasma, 60–70% of protein S circulates in complex with C4b-binding protein (C4BP). The reason why protein S and C4BP form a high-affinity complex in plasma is not known. However, C4BP is an important regulator of the classical pathway of the complement system where it acts as a cofactor in degradation of complement protein C4b. Using Jurkat cells as a model system for apoptosis, we now show protein S to bind to apoptotic cells. We further demonstrate protein S-mediated binding of C4BP to apoptotic cells. Binding of the C4BP-protein S complex to apoptotic cells was calcium-dependent and could be blocked with Abs directed against the phospholipid-binding domain in protein S. Annexin V, which binds to exposed phosphatidylserine on the apoptotic cell surface, could inhibit the binding of protein S. The C4BP that was bound via protein S to the apoptotic cells was able to interact with the complement protein C4b, supporting a physiological role of the C4BP/protein S complex in regulation of complement on the surface of apoptotic cells.

Leishmaniadisease development depends on the presence of apoptotic promastigotes in the virulent inoculum

“…Protein S binding to promastigotes was Ca 2ϩ -dependent and could be blocked with antibodies directed against the phospholipid-binding domain of protein S (data not shown; ref. 14). Even though our data strongly suggest that a subpopulation of virulent promastigotes cultures expresses PS, we cannot exclude the possibility that AnxA5 and protein S bind another anionic phospholipid.…”

“…Labeling was visualized by using an anti-mouse-IgG mAb conjugated to Alexa-488 (Molecular Probes, Leiden, The Netherlands). Moreover, PS was detected by labeling parasites with 1 g͞ml protein S (Kordia Life Sciences, Leiden, The Netherlands) in the presence of 5 mM Ca 2ϩ Cl 2 (14). As a negative control, protein S was added to the parasites in the absence Ca 2ϩ .…”

“…A recent study demonstrated that, in addition to AnxA5, serum protein S specifically binds PS. Protein S acts by binding with its glutamic acid-rich positively charged N terminus to the negatively charged PS in a Ca 2ϩ -dependent manner (13,14). Moreover, a mAb was developed recognizing PS (15).…”

Annexin A2 and A5 Serve as New Ligands for C1q on Apoptotic Cells

“…The binding of C4BP, which circulates mainly in complex with protein S, to dying cells is mediated by interaction of protein S with PS (14) and to a much lesser extent via an interaction of the C4BP ␣-chains with DNA (16). In comparison, we showed recently that FH binds to annexin A2, DNA, and histones on the surface of apoptotic cells (17).…”

Vitamin K-dependent proteins in Ciona intestinalis , a basal chordate lacking a blood coagulation cascade

“…However, the majority of circulating protein S is bound to the complement regulator C4b-binding protein (7). This protein complex lacks the anticoagulant activity of free protein S but negatively regulates complement on the surface of apoptotic cells, the binding to which depends upon the Gla domain of protein S (8,9). In addition, both protein S and its homolog Gas6 activate certain members of the Axl subfamily of receptor tyrosine kinases to transduce antiapoptotic and proliferative signals (10).…”