Mitochondrial iPLA2 Activity Modulates the Release of Cytochrome c from Mitochondria and Influences the Permeability Transition

Abstract

The mitochondrial Ca 2؉ -independent phospholipase A 2 is activated during energy-dependent Ca 2؉ accumulation under conditions where there is a sustained depression of the membrane potential. This activation is not dependent on induction of the mitochondrial permeability transition. Bromoenol lactone, which inhibits the phospholipase, is effective as an inhibitor of the transition, and this action can be overcome by low levels of exogenous free fatty acids. Apparently, activation of the Ca 2؉ -independent phospholipase is a factor in the mechanisms by which depolarization and Ca 2؉ accumulation promote opening of the permeability transition pore. Sustained activity of the Ca 2؉ -independent phospholipase A 2 promotes rupture of the outer mitochondrial membrane and spontaneous release of cytochrome c on a time scale similar to that of apoptosis occurring in cells. However, more swelling of the matrix space must occur to provoke release of a given cytochrome c fraction when the enzyme is active, compared with when it is inhibited. Through its effects on the permeability transition and release of intermembrane space proteins, the mitochondrial Ca 2؉ -independent phospholipase A 2 may be an important factor governing cell death caused by necrosis or apoptosis.Mitochondria from rat liver and rabbit heart have been shown to contain a Ca 2ϩ -independent phospholipase A 2 (iPLA 2 ) 3 that has a molecular mass of ϳ80 kDa (1, 2). Like phospholipases of this type from other sources (3, 4), the mitochondrial enzyme is inactivated by bromoenol lactone (BEL), which acts through an activity-dependent mechanism, leading to a covalent modification within the active site (5). No physiological function of the iPLA 2 in mitochondria has been established, but it has been shown that pretreatment with BEL attenuates the loss of phospholipids that accompanies ischemia/reperfusion injury and reduces the size of infarcts by ϳ50% (2).The relationship between mitochondrial energetic status and iPLA 2 activity is an important factor to consider when contemplating potential physiological and pathophysiological roles of the enzyme. More specifically, activity is not seen in mitochondria that are respiring under state 4 conditions but is manifest upon the addition of uncoupler and is fully manifest following the development of inner membrane pores (1). The former property suggests that transient periods of deenergization might cause a transient activation of the iPLA 2 in vivo, with a resulting accumulation of free fatty acids in mitochondria. Such an accumulation could be of interest in many regards, including opening of the permeability transition pore, which is favored by low levels of these compounds (6 -8). Occurrence of the permeability transition leads to apoptosis in many cell types (9 -11), so scenarios arise in which the iPLA 2 contributes to the control of apoptosis by influencing the permeability transition and wherein the facilitative effects of depolarization on the transition (12, 13) might occur through activity of this enzyme...

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

“…Isolated mitochondria from cardiomyocytes, brain, and rat liver all contain immunoreactive, catalytically competent iPLA 2 ␤ (78 -81). Ischemia reperfusion-induced cardiac myocyte death is associated with iPLA 2 ␤ subcellular redistribution, mitochondrial phospholipid hydrolysis, and mitochondrial dysfunction (81), and iPLA 2 ␤ inhibition reduces mitochondrial phospholipid loss and cell death (81,100).…”

Calcium-independent Phospholipase A2γ Enhances Activation of the ATF6 Transcription Factor during Endoplasmic Reticulum Stress

“…Conversely, inhibition of microsomal iPLA 2 ␥ in rabbit renal proximal tubular cells during cisplatin-induced apoptosis reduced apoptosis markers, indicating that iPLA 2 ␥ contributes to apoptosis (7). Mitochondrial iPLA 2 ␥ has been also shown to actively participate in the permeability transition pore opening of the mitochondria and the release of the proapoptotic cytochrome c (8). iPLA 2 ␥ knock-out mice demonstrate multiple bioenergetic dysfunctional phenotypes (9), although they are resistant to obesity and show less insulin resistance (10).…”

Genetic Ablation of Calcium-independent Phospholipase A2γ Leads to Alterations in Hippocampal Cardiolipin Content and Molecular Species Distribution, Mitochondrial Degeneration, Autophagy, and Cognitive Dysfunction

“…The prominent roles of mitochondrial phospholipases in regulating neuronal homeostasis are exemplified by their pleiotropic effects on mitochondrial bioenergetics and signaling (18,19). Through modulating the structure, composition, and organization of mitochondrial membrane constituents, phospholipases participate in the generation and maintenance of highly specialized membrane scaffolds that are necessary for efficient mitochondrial bioenergetic and signaling functions (7, 20 -23).…”