UVA irradiation induces energy-independent phospholipid-flip in mammalian plasma membrane

Translocation from the outer to the inner membrane leaflet (flip) of phosph olipids after ultraviolet A (UVA) irradiation was investigated in Chinese h amster ovary cells, Fluorescent 1wpalmitoyl-2-[6-[(7-nitro-2-1,3-benzox- ad iazol-4-yl)amino] caproyl]-sn-glycero-3-phosphoserine (NBD-labeled phosphat idylserine [NBD-PS]) was used to assay transbilayer lipid movement. A marke d increase in flip of NBD-PS was observed immediately after low-dose UVA ir radiation which was not lethal and returned to the basal level after 6 h, U VA-induced dip was not attributed to the increase of permeability by UVA ir radiation because cells that were negative for staining with propidium iodi de also showed increased flip of NBD-PS, Furthermore, the enhancement was i ndependent of adenosine 5 ' -triphosphate, demonstrating the lack of involv ement of phospholipid translocase. Marked increases were also observed in d ip of both NBD-phosphatidylethanolamine and NBD-phosphatidylcholine immedia tely after UVA irradiation, showing that the increase was independent on th e head groups of phospholipids. These findings indicated that UVA changes t he flip-flop of phospholipids and that the cell membrane is a molecular and cellular target of UVA.