Phospolipase A2 and apoptosis.

Phospolipase A(2) (PLA(2)) is the esterase activity that cleaves the sn-2 ester bond in glycerophospholipids, releasing free fatty acids and lysophospholipids. The PLA(2) activity is found in a variety of enzymes which can be divided in several types based on their Ca(2+) dependence for their activity; Ca(2+)-dependent secretory phosholipases (sPLA(2)s) and cytosolic phospholipases (cPLA(2)s), and Ca(2+)-independent phospholipase A(2)s (iPLA(2)s). These enzymes also show diverse size and substrate specificity (i.e., in the fatty acid chain length and extent of saturation). Among the fatty acids released by PLA(2), arachidonic acid (AA) is of particular biological importance, because it is subsequently converted to prostanoids and leukotrienes by cyclooxygenases (COX) and lipoxygenases (LOX), respectively. Free AA may also stimulate apoptosis through activation of sphingomyelinase. Alternatively, it is suggested that oxidized metabolites generated from AA by LOX induce apoptosis. Although the precise mechanisms remain to be elucidated, changes are observed in glycerolipid metabolism during apoptotic processes. In some cells induced to undergo apoptosis, AA is released concomitant with loss of cell viability, caspase activation and DNA fragmentation. Such AA releases appear to be mediated by activation of cPLA(2) and/or iPLA(2). For example, tumor necrosis factor-alpha (TNF-alpha)-induced cell death is mediated by cPLA(2), whereas Fas-induced apoptosis appears to be mediated by iPLA(2). Some discrepancies among early experimental results were probably caused by differences in the experimental conditions such as the serum concentration, inhibitors used that are not necessarily specific to a single-type enzyme, or differential expression of each PLA(2) in cells employed in the experiments. Recent studies eliminated such problems, by carefully defining the experimental conditions, and using multiple inhibitors that show different specificities. Accordingly, more convincing data are available that demonstrate involvement of some PLA(2)s in the apoptotic processes. In addition to cPLA(2) and iPLA(2), sPLA(2)s were recently found to play roles in apoptosis. Moreover, new proteins that appear to control PLA(2)s are being discovered. Here, the roles of PLA(2)s in apoptosis are discussed by reviewing recent reports.