Bovine lactadherin as a calcium-independent imaging agent of phosphatidylserine expressed on the surface of apoptotic HeLa cells.

Bovine lactadherin holds a stereo-specific affinity for phosphatidylserine (PS) membrane domains and binds at PS concentrations lower than the benchmark PS probe, annexin V. Accordingly, lactadherin has recognized PS exposure on preapoptotic immortalized leukemia cells at an earlier time point than has annexin V. In the present study, the cervical cancer cell line HeLa has been employed as a model system to compare the topographic distribution of PS with the two PS binding proteins as adherent cells enter the apoptotic program. HeLa cells were cultured on glass-bottom Petri dishes, and apoptosis was induced by staurosporine. Fluorescence-labeled lactadherin and/or annexin V were used to detect PS exposure by confocal microscopy. Both lactadherin and annexin V staining revealed PS localized to plasma membrane rim and blebs. In addition, lactadherin identified PS exposure on long filopodia-like extensions, whereas annexin V internalized in granule-like structures. All in all, the data further delineate the differences in PS binding patterns of lactadherin and annexin V.