Fas-induced arachidonic acid release is mediated by Ca²⁺- independent phospholipase A₂ but not cytosolic phospholipase A₂, which undergoes proteolytic inactivation

Fas-mediated apoptosis of human leukemic U937 cells was accompanied by increased arachidonic acid (AA) and oleic acid release from membrane glycerophospholipids, indicating phospholipase A₂ (PLA₂) activation. During apoptosis, type IV cytosolic PLA₂ (cPLA₂), a PLA₂ isozyme with an apparent molecular mass of 110 kDa critical for stimulus-coupled AA release, was converted to a 78-kDa fragment with concomitant loss of catalytic activity. Cleavage of cPLA₂ correlated with increased caspase-3-like protease activity in apoptotic cells and was abrogated by a caspase-3 inhibitor. A mutant cPLA₂ protein in which Asp⁵²² was replaced by Asn, which aligns with the consensus sequence of the caspase-3 cleavage site (DXXD ↓ X), was resistant to apoptosis-associated proteolysis. Moreover, a COOH-terminal deletion mutant of cPLA₂ truncated at Asp⁵²² comigrated with the 78-kDa fragment and exhibited no enzymatic activity. Thus, caspase-3-mediated cPLA₂ cleavage eventually leads to destruction of a catalytic triad essential for cPLA₂ activity, thereby terminating its AA-releasing function. In contrast, the activity of type VI Ca²⁺-independent PLA₂ (iPLA₂), a PLA₂ isozyme implicated in phospholipid remodeling, remained intact during apoptosis. Inhibitors of iPLA₂, but neither cPLA₂ nor secretory PLA₂ inhibitors, suppressed AA release markedly and, importantly, delayed cell death induced by Fas. Therefore, we conclude that iPLA₂-mediated fatty acid release is facilitated in Fas-stimulated cells and plays a modifying although not essential role in the apoptotic cell death process.