Chromatin-independent binding of serum amyloid P component to apoptotic cells

Human serum amyloid P component (SAP) is a glycoprotein structurally belong ing to the pentraxin family of proteins, which has a characteristic pentame ric organization. Mice with a targeted deletion of the SAP gene develop ant inuclear Abs, which was interpreted as evidence for a role of SAP in contro lling the degradation of chromatin. However, in vitro SAP also can bind to phosphatidylethanolamine, a phospholipid which in normal cells is located m ainly in the inner leaflet of the cell membrane, to be translocated to the outer leaflet of the cell membrane during a membrane flip-Hop. We hypothesi zed that SAP, because of its specificity for phosphatidylethanolamine, may bind to apoptotic cells independent of its nuclear binding. Calcium-depende nt binding of SAP to early, nonpermeable apoptotic Jurkat, SKW, and Raji ce lls was indeed observed. Experiments with flip-flopped erythrocytes confirm ed that SAP bound to early apoptotic cells via exposed phosphatidylethanola mine. Binding of SAP was stronger to late, permeable apoptotic cells. Exper iments with enucleated neutrophils, with DNase/RNase treatment of late apop totic Jurkat cells, and competition experiments with histones suggested tha t binding of SAP to late apoptotic cells was largely independent of chromat in. Confocal laser microscopic studies indeed suggested that SAP bound to t hese apoptotic cells mainly via the blebs. Thus, this study shows that SAP binds to apoptotic cells already at an early stage, which raises the possib ility that SAP is involved in dealing with apoptotic cells in vivo.