Lactadherin detects early phosphatidylserine exposure on immortalized leukemia cells undergoing programmed cell death.
BACKGROUND
Phosphatidylserine (PS) appears on the outer membrane leaflet of cells undergoing programmed cell death and marks those cells for clearance by macrophages. Macrophages secrete lactadherin, a PS-binding protein, which tethers apoptotic cells to macrophage integrins.


METHODS
We utilized fluorescein-labeled lactadherin together with the benchmark PS Probe, annexin V, to detect PS exposure by flow cytometry and confocal microscopy. Immortalized leukemia cells were treated with etoposide, and the kinetics and topology of PS exposure were followed over the course of apoptosis.


RESULTS
Costaining etoposide-treated leukemoid cells with lactadherin and annexin V indicated progressive PS exposure with dim, intermediate, and bright staining. Confocal microscopy revealed localized plasma membrane staining, then diffuse dim staining by lactadherin prior to bright generalized staining with both proteins. Annexin V was primarily localized to internal cell bodies at early stages but stained the plasma membrane at the late stage. Calibration studies suggested a PS content less, less than or approximately equal to 2.5%-8% for the membrane domains that stained with lactadherin but not annexin V.


CONCLUSIONS
Macrophages may utilize lactadherin to detect PS exposure prior to exposure of sufficient PS to bind annexin V. The methodology enables detection of PS exposure at earlier stages than established methodology.
Shi
Jialan
J
Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, USA.
Shi
Yinan
Y
Waehrens
Lasse N
LN
Rasmussen
Jan T
JT
Heegaard
Christian W
CW
Gilbert
Gary E
GE
D017304:Annexin A5 / Q000493:pharmacokinetics / Q000494:pharmacology; D000954:Antigens, Surface / Q000494:pharmacology*; D017209:Apoptosis; D005047:Etoposide; D005434:Flow Cytometry / Q000379:methods*; D005456:Fluorescent Dyes / Q000494:pharmacology; D018922:HL-60 Cells; D006801:Humans; D020014:K562 Cells; D004915:Leukemia, Erythroblastic, Acute / Q000378:metabolism*; D015473:Leukemia, Promyelocytic, Acute / Q000378:metabolism*; D018613:Microscopy, Confocal / Q000379:methods*; D008894:Milk Proteins / Q000493:pharmacokinetics / Q000494:pharmacology*; D010718:Phosphatidylserines / Q000378:metabolism*
D017304:Annexin A5; D000954:Antigens, Surface; D005456:Fluorescent Dyes; C098942:MFGE8 protein, human; D008894:Milk Proteins; D010718:Phosphatidylserines; D005047:Etoposide