The role of lysophosphatides in cell injury

ATTENUATION OF ISCHEMIA AND REPERFUSION INJURY OF CANINE LIVERS BY INHIBITION OF TYPE II PHOSPHOLIPASE A2 WITH LY3297221

BACKGROUND

Membrane phospholipid breakdown, caused by ischemia and reperfusion (I/R) of the liver, releases free fatty acids including arachidonic acids and lysophospholipids, which serve as precursors of various inflammatory lipid derivatives. Phospholipase A2 (PLA2) is a key enzyme that initiates this reaction. In this study, we tested our hypothesis that a type II PLA2 inhibitor, LY329722, could attenuate hepatic I/R injury caused by a 2-hr total hepatic vascular exclusion (THVE) in dogs.

METHODS

Eighteen beagle dogs, subjected to a 2-hr THVE, were divided into three groups. Group 1 (n=6) was untreated and served as a control group. LY329722 was administered to animals in group 2 (n=6) intravenously (0.2 mg x kg(-1) x hr(-1)) for 60 min before ischemia, and to animals in group 3 (n=6) for 60 min starting 15 min before reperfusion (0.2 mg x kg(-1) x hr(-1)). Animal survival, systemic and splanchnic hemodynamics, hepatic tissue blood flow, liver functions, energy metabolism, hepatic venous thromboxane B2 and endothelin-1 levels, phospholipid levels and tumor necrosis factor-a mRNA expression in liver tissue, and histopathologic findings were evaluated.

RESULTS

Two-week animal survival was 33% (two of six) in group 1, and 100% (six of six) in groups 2 and 3. LY329722 improved systemic and splanchnic hemodynamics, hepatic tissue blood flow, and energy metabolism, reduced liver enzyme, thromboxane B2, and endothelin-1 release, prevented hepatic phospholipid degradation and tumor necrosis factor-alpha mRNA expression, and lessened histopathologic damage and the number of neutrophil infiltrating into the liver tissue.

CONCLUSION

The present study demonstrated that a type II PLA2 inhibitor, LY329722, attenuated hepatic I/R injury caused by a 2-hr THVE model in dogs.

Group II phospholipases A2 are indirectly cytolytic in the presence of exogenous phospholipid

Systemic inflammatory response syndromes including septic shock and salicylate poisoning are associated with high circulating levels of secretory phospholipase A2 (sPLA2). In septic shock, sPLA2 has been implicated in the pathogenesis of multisystem organ failure, presumably by a direct cytotoxic effect on cells. The cytotoxicity of recombinant human sPLA2 and a venom PLA2 were examined on human erythrocytes, erythroleukemia cells and U937 cells. Neither the human nor venom PLA2's were directly injurious to target cells. However, in the presence of liposomal phospholipids, both PLA2's induced irreversible cell injury. Whereas venom PLA2 was cytolytic in the presence of either phosphatidylcholine or phosphatidylethanolamine (PE), rh-sPLA2 caused cell death only in the presence of PE. These data show that normal unperturbed cells are resistant to injury from PLA2, and that additional cofactors such as PE are required to induce cell injury.

Enhanced phospholipase A2 activity in rat plasma, liver, and intestinal mucosa following endotoxin treatment: a possible explanation for the protective effect of indomethacin in endotoxic shock

Endotoxin administration in rats produced a significant increase in plasma, hepatic, and intestinal phospholipase A2 activity within three minutes after injection. The elevated phospholipase A2 activity seen in these tissues returned to normal levels six minutes after injection. The changes in phospholipase A2 activity were dose-dependent within the 0, 10, and 20 mg/kg range of treatment with Escherichia coli endotoxin. This increase in plasma, hepatic, and intestinal phospholipase A2 was abolished by prior treatment of the rats with 3 mg/kg indomethacin, a drug known to improve survival in endotoxic shock. The fact that the change in phospholipase A2 occurs soon after endotoxin administration and that the change in phospholipase is blocked by protective doses of indomethacin suggests that phospholipase A2 activation may be an important initial event in the lethal action of endotoxin, and that the protective effects of indomethacin may be directly related to inhibition of phospholipase A2 activity. Further, in vitro studies of the effects of indomethacin on hepatic phospholipase A2 activity showed that indomethacin significantly inhibited this enzyme. Indomethacin (25 mumol/L) produced 56% inhibition in phospholipase A2 activity and the apparent Ki for indomethacin was 9.2 mumol/L. Kinetic analysis using the Lineweaver-Burk method showed that the indomethacin inhibition was of the noncompetitive type.

Effects of hypothermic perfusion of kidneys on tissue and mitochondrial phospholipids

The changes in the level of phospholipids in kidney tissue and isolated mitochondria from dog kidneys perfused hypothermically (6-8 degrees C) for 1, 3, and 5 days were compared. Following 1 day of perfusion there was no change in total tissue phosphatidylserine (PS), a 25% decrease in the level of phosphatidylethanolamine (PE), and a 16% decrease in phosphatidylcholine (PC). No further decrease was observed with longer perfusion times. In fact, an increase in the level of PE occurred between the third and fifth days. Mitochondria isolated from perfused kidneys also showed a slight decrease in PE and PC following 1 day, no further change at 3 days, and an increase at Day 5. The loss of tissue phospholipids does not appear related to the viability of perfused kidneys. The major loss occurs within 1 day of perfusion and kidneys perfused up to 3 days are fully viable. Five-day perfused kidneys are nonviable, but show no greater loss of phospholipids than the viable 1- or 3-day perfused kidneys.

Changes in mitochondrial lipids of rat kidney during ischemia

Lipid changes in mitochondria isolated from rat kidney after various periods of ischemia were analysed by thin-layer chromatography and gas-liquid chromatography. Free fatty acids were increased at 30 min and more so thereafter. Total phospholipid fatty acids decreased steadily. The proportion of diphosphatidylglycerol (cardiolipin) in the total phospholipid fraction decreased at 30 min, but the proportion of phosphatidylcholine and phosphatidylethanolamine in the total phospholipid fraction did not change until the irreversible phase of ischemic injury. We have shown that decrease of cardiolipin in mitochondrial membrane occurs early during ischemia, and only during the irreversible phase of ischemia are phosphatidylethanolamine and phosphatidylcholine broken down. It is postulated that these phenomena are due to activation of phospholipase in the mitochondrial membrane.