Attenuated Free Cholesterol Loading-induced Apoptosis but Preserved Phospholipid Composition of Peritoneal Macrophages from Mice That Do Not Express Group VIA Phospholipase A₂*

Mouse macrophages undergo ER stress and apoptosis upon free cholesterol loading (FCL). We recently generated iPLA₂β-null mice, and here we demonstrate that iPLA₂β-null macrophages have reduced sensitivity to FCL-induced apoptosis, although they and wild-type (WT) cells exhibit similar increases in the transcriptional regulator CHOP. iPLA₂β-null macrophages are also less sensitive to apoptosis induced by the sarcoplasmic reticulum Ca²⁺-ATPase inhibitor thapsigargin and the scavenger receptor A ligand fucoidan, and restoring iPLA₂βexpression with recombinant adenovirus increases apoptosis toward WT levels. WT and iPLA₂β-null macrophages incorporate [³H]arachidonic acid ([³H]AA]) into glycerophosphocholine lipids equally rapidly and exhibit identical zymosan-induced, cPLA₂α-catalyzed [³H]AA release. In contrast, although WT macrophages exhibit robust [³H]AA release upon FCL, this is attenuated in iPLA₂β-null macrophages and increases toward WT levels upon restoring iPLA₂β expression. Recent reports indicate that iPLA₂β modulates mitochondrial cytochrome c release, and we find that thapsigargin and fucoidan induce mitochondrial phospholipid loss and cytochrome c release into WT macrophage cytosol and that these events are blunted in iPLA₂β-null cells. Immunoblotting studies indicate that iPLA₂β associates with mitochondria in macrophages subjected to ER stress. AA incorporation into glycerophosphocholine lipids is unimpaired in iPLA₂β-null macrophages upon electrospray ionization-tandem mass spectrometry analyses, and their complex lipid composition is similar to WT cells. These findings suggest that iPLA₂β participates in ER stress-induced macrophage apoptosis caused by FCL or thapsigargin but that deletion of iPLA₂β does not impair macrophage arachidonate incorporation or phospholipid composition.