Phospholipid Absorption and Diffusion through Membranes
Phospholipids are constituents of cell membranes. A main field of investigation is the study of mechanisms directing their synthesis and their selected distribution in different membranes of the cell. However, it is now widely appreciated that, together with a structural function, phospholipids have regulatory activity inside the cell, within the plasma membrane, as well as outside the cell (9). In their natural position, phospholipids modulate the activity of membrane-bound enzymes. Furthermore, the operation of selected synthetic pathways yields phospholipid derivatives behaving as second messengers. Thus, it is found that phosphatidic acid (PA), a phospholipid produced during the activation of signalling mechanisms, from the phosphorylation of diacyl glycerol (DAG) or by the action „f phospholipase D, may activate the superoxide production in neutrophils (2). Also, a lysophosphatidylserine analogue bearing an ether group at the sn-1 position of glycerol, acts as a negative modulator of the interaction between steroids and their intracellular receptors (5). Within the plasma membrane phospholipids serve as a substrate for the receptor-dependent phospholipases involved in the transduction of external stimuli into intracellular signals. Second messengers such as DAG are generated by these reactions. Outside the cells phospholipids behave as auto-pharmacological agents (autacoids, 6). For example, phosphatidylserine exposed to the extracellular environment activates the monocyte-macrophage system (30), exogenous PA is translocated inside the cell in the form of DAG (25), lysophosphatidylserine activates mouse mast cells (4) and phosphatidylserine (PS) vesicles interact with lymphocytes (20). The purpose of this review is to outline unifying aspects of these different actions of phospholipids.
KeywordsMast Cell Nerve Growth Factor Phosphatidic Acid Phosphatidic Acid Mucosal Epithelial Cell
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