Secretory phospholipases and membrane polishing

Abstract

Although secretory phospholipase AZ (PLAZ) isozymes have been identified in human gestational tissues, their role in homeostasis and pathophysiology during pregnancy has yet to be clearly established. The aims of this brief commentary are: (1) to review recent data concerning the expression of secretory PLAZ isozymes in human gestational tissues; and (2) to present a case for their involvement in regulating the expression of glycerophospholipids in the exoplasmic monolayer of the cell membrane. Three secretory PLA 2 isozymes and a secretory PLAZ cell-surface receptor have been identified in human term gestational tissues. In addition to their potential role in the formation of glycerophospholipid-derived metabolites (such as prostaglandins), these isozymes may function to regulate the expression of aminophospholipids on the cell surface. The exposure of aminophospholipids on the cell surface dramatically affects many aspects of cell function. Secreted PLA 2 isozymes that display a substrate preference for the negatively charged aminophospholipids (e.g. phosphatidylserine or phosphatidylethanolamine) in the exoplasmic membrane may affect cell function and reactivity via a process of ‘membrane polishing’, that is, the preferentially removal of aminophospholipids from the exoplasmic leaflet of the cell membranes. By this process, secreted PLA 2 isozymes may limit unsolicited cell-surface binding of exogenous proteins, membrane fusion events and recognition by cellular surveillance systems.