Abstract
During periods of cellular quiescence, the cell membrane bilayer is characterized by an asymmetric phospholipid distribution. The exoplasmic monolayer is principally composed of neutral, choline-containing phospholipids and presents a relatively inert surface to the environmental milieu. Aminophospholipids (e.g., phosphatidylserine) are almost exclusively confined to the cytoplasmic monolayer. Upon cellular activation and increases in cytosolic calcium concentrations, phospholipid asymmetry is rapidly relaxed. The appearance of aminophospholipids on the cell surface dramatically affect many aspects of cellular function, including: cell surface protein binding; membrane fusion events; terminal differentiation; and recognition by phagocytes. Secreted, extracellularly active PLA 2 s that display a substrate preference for aminophospholipids may regulate their expression in the exoplasmic monolayer of the cell membrane and thereby alter cellular function and reactivity. One such secreted PLA 2 isozyme (Type II) is abundantly expressed in and released from human placenta. Although this isozyme contributes to gestational tissue phospholipid metabolism, its role in regulating the expression of aminophopholipids on the cell surface and cell reactivity has yet to be established.
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