A Missing link between lipid metabolism, inflammation and apoptosis: Phospholipase A2-activating protein (PLAA)
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Phospholipase A2-Activating Protein (PLAA) is a novel signaling molecule that regulates the production of arachidonic acid (AA), prostaglandin E2 (PGE2) and TNF-¦Á. Literature suggests that PLAA could be involved in inflammatory responses and apoptosis. However, the in situ function of PLAA is elusive. To elucidate PLAA¡¯s role in TNF-¦Á-induced inflammatory responses and cisplatin-induced apoptosis, we manipulated the expression of the plaa gene at cellular level using overexpression and siRNA approaches. We generated HeLa (Tet-off) cells overexpressing plaa (plaa high) and control (plaa low) cells. We compared plaa high and plaa low cells for transcriptional profiling and their responses to TNF-¦Á stimulation. Overexpression of the plaa gene induced the expression of the proinflammatory cytokine IL-32 and reduced the expression of annexin A4 (a PLA2 inhibitor) and clusterin. We demonstrated that extracellular clusterin limited the production of PGE2. We showed that upon TNF-¦Á stimulation, plaa high cells revealed enhanced PLA2 activation, COX-2 expression and PGE2 production. Furthermore, we found that in response to TNF-¦Á, plaa high cells had significantly enhanced activation of NF-¦ÊB and production of IL-6, compared to the TNF-¦Á-stimulated plaa low cells. To understand regulation of plaa gene expression, we used a luciferase reporter system in normal HeLa cells and identified one stimulatory element, with Sp1 transcription factor-binding site, and one inhibitory element, in exon 1 of the plaa gene. To determine the role of PLAA in apoptosis, we compared the apoptotic responses to cisplatin in plaa high and plaa low cells. Cisplatin-stimulated plaa high cells contained significantly higher levels of DNA fragmentation, caspase activities, PLA2 enzyme activity and mitochondrial damage than did the cisplatin-stimulated plaa low cells. siRNA against PLAA (siRNA-PLAA) reverted the above mentioned trend and promoted cell viability. Further, cisplatin-stimulated plaa high cells produced less cytoprotecive clusterin and more pro-apoptotic IL-32 than did the cisplatin- stimulated plaa low cells. siRNA-PLAA promoted clusterin production and inhibited IL-32 expression from both plaa high and plaa low cells. Finally, our proteomic analysis revealed that cisplatin-stimulated plaa high cells contained higher levels of phosphorylated JNK/c-Jun and FasL than did cisplatin-stimulated plaa low cells.