Emerging roles for phospholipase A2 enzymes in cancer

Phospholipase A2 (PLA2) enzymes (EC3.1.4.4) regulate the release of biologically active fatty acids and lysophospholipids from membrane phospholipid pools. These lipids are also substrates for intracellular biochemical pathways that generate potent autocrine and paracrine lipid mediators such as the eicosanoids and platelet activating factor. These factors, in turn, regulate cell proliferation, survival, differentiation, motility, tissue vascularisation, and immune surveillance in virtually all tissues, functions that are subverted by cancer cells for tumour growth and metastasis. Thus the relevance of PLA2-dependent pathways to the genesis and progression of cancer has been of interest since their discovery and with recent technological advances, their role in tumourigenesis has become more tractable experimentally. Limited human genetic studies have not yet identified PLA2 enzymes as classical mutated oncogenes or tumour suppressor genes. However, there is strong evidence that of the 22 identified human PLA2 enzymes, ten of which have been studied in cancer to date, most are aberrantly expressed in a proportion of tumours derived from diverse organs. Correlative and functional studies implicate the expression of some secreted enzymes (sPLA2s), particularly the best studied enzyme Group IIA sPLA2 in either tumour promotion or inhibition, depending on the organ involved and the biochemical microenvironment of tumours. As in immune-mediated inflammatory pathologies, genetic deletion studies in mice, supported by limited studies with human cells and tissues, have identified an important role for Group IVA PLA2 in regulating certain cancers. Pharmacological intervention studies in prostate cancer suggest that hGIIA-dependent tumour growth is dependent on indirect regulation of Group IVA PLA2. Group VI calcium-independent PLA2 enzymes have also been recently implicated in tumourigenesis with in vitro studies suggesting multiple possible roles for these enzymes. Though apparently complex, further characterization of the regulatory relationships amongst PLA2 enzymes, lipid mediator biosynthetic enzymes and the lipid mediators they produce during tumour progression is required to define the biochemical context in which the enzymes modulate cancer growth and development.