Phospholipase A2-induced cytoprotection of proximal tubules: potential determinants and specificity for ATP depletion-mediated injury

Abstract

Addition of phospholipase A2 (PLA2) to isolated proximal tubular segments (PTS) has previously been shown to decrease hypoxic cell death without altering ATP concentrations. The study presented here was undertaken to identify determinant(s) of this protection, and to define the spectrum of injuries against which it can operate. PTS were extracted from mouse kidneys and subjected to diverse forms of injury (hypoxia/reoxygenation, antimycin A, Ca2+ ionophore, amphotericin B, FeSO4, and myohemoglobin). In subtoxic doses, addition of PLA2 significantly reduced hypoxic- and antimycin A-induced injury (percentage of lactate dehydrogenase release); however, a dose-dependent exacerbation of all other forms of injury resulted. The ability of PLA2 to mitigate hypoxic injury remained intact despite the inhibition of Na,K-ATPase (ouabain) or the inducement of cytoskeletal disruption (cytochalasin D). However, it was negated by minimally toxic amphotericin B or Ca2+ ionophore doses, indicating its dependence on preserved ionic gradients. Nevertheless, neither lowering/removing buffer Ca2+ or NaCl concentrations, nor hypertonic mannitol addition reproduced the cytoprotective effect of PLA2. PLA2 induced synergistic deacylation in hypoxic tubules, suggesting that unsaturated fatty-acid accumulation might mediate its cytoprotective effect. The fact that the addition of exogenous arachidonate, but not palmitate, to tubules protected against hypoxia, but worsened nonhypoxic forms of injury, supported this hypothesis. Since arachidonate might induce "feedback" inhibition of intracellular PLA2, the ability of an intracellular phospholipase inhibitor (ONO-RS-082; Biomol, Plymouth, PA) to blunt hypoxic damage was tested. This agent fully reproduced the cytoprotective effect of PLA2. It was concluded that: (1) PLA2-induced cytoprotection is relatively specific for ATP depletion injury; (2) it is dependent on, but not explained by, maintenance of NaCl and Ca2+ gradients; (3) it does not require Na,K-ATPase activity or cytoskeletal integrity for its expression; and (4) extracellular PLA2, via arachidonate release, may cause feedback inhibition of intracellular PLA2, thereby protecting critical intracellular targets from attack.