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Keywords:

  • cPLA2;
  • PPAR-δ;
  • arachidonic acid;
  • β-catenin;
  • cholangiocarcinoma

Abstract

Cytosolic phospholipase A2α (cPLA2α) is a rate-limiting key enzyme that releases arachidonic acid (AA) from membrane phospholipid for the production of biologically active lipid mediators including prostaglandins, leukotrienes and platelet-activating factor. cPLA2α is translocated to nuclear envelope in response to intracellular calcium increase and the enzyme is also present inside the cell nucleus; however, the biological function of cPLA2α in the nucleus remains unknown. Here we show a novel role of cPLA2α for activation of peroxisome proliferator-activated receptor-δ (PPARδ) and β-catenin in the nuclei. Overexpression of cPLA2α in human cholangiocarcinoma cells induced the binding of PPARδ to β-catenin and increased their association with the TCF/LEF response element. These effects are inhibited by the cPLA2α siRNA and inhibitors as well as by siRNA knockdown of PPARδ. Overexpression of PPARδ or treatment with the selective PPARδ ligand, GW501516, also increased β-catenin binding to TCF/LEF response element and increased its reporter activity. Addition of AA and GW501516 to nuclear extracts induced a comparable degree of β-catenin binding to TCF/LEF response element. Furthermore, cPLA2α protein is present in the PPARδ and β-catenin binding complex. Thus the close proximity between cPLA2α and PPARδ provides a unique advantage for their efficient functional coupling in the nucleus, where AA produced by cPLA2α becomes immediately available for PPARδ binding and subsequent β-catenin activation. These results depict a novel interaction linking cPLA2α, PPARδ and Wnt/β-catenin signaling pathways and provide insight for further understanding the roles of these key molecules in human cells and diseases. J. Cell. Biochem. 105: 534–545, 2008. © 2008 Wiley-Liss, Inc.