Hypoxia induces changes in phospholipase A2 in rat proximal tubules: evidence for multiple forms

Increased free fatty release during hypoxia is believed to contribute to cell injury. This phenomenon is likely to be mediated through activation of specific isoforms of phospholipase A2 (PLA2). In this study, PLA2 enzymatic activity was measured in cell-free extracts prepared from rat renal proximal tubules. Both soluble and membrane-associated PLA2 activity were detected. All PLA2 activity detected during normoxia was Ca2+ dependent. Fractionation of cytosolic extracts by gel filtration revealed three peaks of PLA2 activity. Exposure of tubules to hypoxia resulted in stable activation of soluble PLA2 activity, which correlated with disappearance of the highest molecular mass form (> 100 kDa) and appearance of a low-molecular-mass form (approximately 15 kDa) of PLA2. Hypoxia also resulted in release of a low-molecular-mass form of PLA2 into the extracellular medium. Pretreatment of tubules with glycine before hypoxia blocked this release of PLA2 but not activation of soluble PLA2 activity. These studies provide direct evidence for PLA2 activation during hypoxia and suggest that multiple mechanisms regulate free fatty acid release associated with hypoxia injury.