Cell Survival Signalling through PPARδ and Arachidonic Acid Metabolites in Neuroblastoma

Retinoic acid (RA) has paradoxical effects on cancer cells: promoting cell death, differentiation and cell cycle arrest, or cell survival and proliferation. Arachidonic acid (AA) release occurs in response to RA treatment and, therefore, AA and its downstream metabolites may be involved in cell survival signalling. To test this, we inhibited phospholipase A2-mediated AA release, cyclooxygenases and lipoxygenases with small-molecule inhibitors to determine if this would sensitise cells to cell death after RA treatment. The data suggest that, in response to RA, phospholipase A2-mediated release of AA and subsequent metabolism by lipoxygenases is important for cell survival. Evidence from gene expression reporter assays and PPAR delta knockdown suggests that lipoxygenase metabolites activate PPAR delta. The involvement of PPAR delta in cell survival is supported by results of experiments with the PPAR delta inhibitor GSK0660 and siRNA-mediated knockdown. Quantitative reverse transcriptase PCR studies demonstrated that inhibition of 5-lipoxygenase after RA treatment resulted in a strong up-regulation of mRNA for PPAR delta 2, a putative inhibitory PPAR delta isoform. Over-expression of PPAR delta 2 using a tetracycline-inducible system in neuroblastoma cells reduced proliferation and induced cell death. These data provide evidence linking lipoxygenases and PPAR delta in a cell survival-signalling mechanism and suggest new drug-development targets for malignant and hyper-proliferative diseases.