The role of phospholipases, cyclooxygenases, and lipoxygenases in cerebral ischemic/traumatic injuries.

Abstract

Free fatty acids (FFAs) are elevated in the brain following both ischemic and traumatic injury. Phospholipase activation, with the subsequent release of FFAs from membrane phospholipids, is the likely mechanism. In addition to phospholipases A1, B, C, and D, there are at least 19 groups of PLA2, including multiple cytosolic, calcium independent, and secretory isoforms. Phospholipase activity can be regulated by calcium, by phosphorylation, and by agonists binding to G-protein-coupled receptors. These enzymes normally function in the physiological remodeling of cellular membranes, whereby FFAs are removed by phospholipase activity and then reacylated with a different FFA. However, reductions in the cell's ability to maintain normal metabolic function and the resultant fall in ATP levels can cause the failure of reacylation of membrane phospholipids. Alterations to membrane phospholipids would be expected to compromise many cellular functions, including the ability to accumulate excitotoxic amino acids. This review presents evidence for a central role of phospholipases and their products in the etiology of damage following injury to the brain. Phospholipase expression and activity is increased in animal models of cerebral ischemia and trauma. FFA release from the in vivo rat brain is reduced following the application of selective phospholipase inhibitors, and this inhibition also decreases the severity of cortical damage following forebrain ischemia, focal (middle cerebral artery occlusion) ischemia, and cerebral trauma. Mice with knockouts of PLA2 have decreased infarct volumes. Human data demonstrate a correlation between the elevation of CSF FFAs and worsened outcome following stroke, traumatic brain injury, and subarachnoid hemorrhage. The released FFAs, especially arachidonic and docosahexaenoic acids, together with the production of lysophospholipids, can initiate a chain of events which may be responsible for the development of neuronal damage. Inhibitors of both cyclooxygenase and lipoxygenase pathways have been shown to reduce cerebral deficits following ischemia and trauma. These results suggest therapeutic strategies to reduce morbidity following cerebral injury using selective inhibitors of phospholipases, cyclooxygenases, and lipoxygenases, underlining the need for further investigation of their role in the development of cerebral damage.