RETRACTED: Activation of peroxisome proliferator-activated receptor β/δ (PPARβ/δ) increases the expression of prostaglandin E2 receptor subtype EP4 - The roles of phosphatidylinositol 3-kinase and CCAAT/enhancer-binding protein β (Retracted Article)

The prostaglandin E-2 receptor subtype EP4 has been implicated in the growth and progression of human non-small cell lung carcinoma (NSCLC). However, the factors that control its expression have not been entirely elucidated. Our studies show that NSCLC cells express peroxisome proliferator-activated receptor beta/delta (PPAR beta/delta) protein and that treatment with a selective PPAR beta/delta agonist (GW501516) increases EP4 mRNA and protein levels. GW501516 induced NSCLC cell proliferation, and this effect was prevented by PPAR beta/delta antisense or EP4 short interfering RNA ( siRNA). GW501516 increased the phosphorylation of Akt and decreased PTEN expression. The selective inhibitor of phosphatidylinositol 3-kinase (PI3-K), wortmannin, and PPAR beta/delta antisense, abrogated the effect of GW501516 on EP4 expression, whereas that of the inhibitor of Erk did not. GW501516 also increased EP4 promoter activity through effects on the region between - 1555 and - 992 bp in the EP4 promoter, and mutation of the CCAAT/enhancer-binding protein (C/EBP) site in this region abrogated the effect of GW501516. GW501516 increased not only the binding activity of C/EBP to the NF-IL6 site in the EP4 promoter, which was prevented by the inhibitor of PI3-K, but also increased C/EBP beta protein in a dose- and PPAR beta/ delta-dependent manner. The effect of GW501516 on EP4 protein was eliminated in the presence of C/EBP beta siRNA. Finally, we showed that pretreatment of NSCLC with GW501516 further increased NSCLC cell proliferation in response to exogenous dimethylprostaglandin E-2 (PGE(2)) that was diminished in the presence of PPAR beta/delta antisense and EP4 siRNA. Taken together, these findings suggest that activation of PPAR beta/delta induces PGE(2) receptor subtype EP4 expression through PI3-K signals and increases human lung carcinoma cell proliferation in response to PGE(2). The increase in transcription of the EP4 gene by PPAR beta/delta agonist was associated with increased C/EBP binding activity in the NF-IL6 site of EP4 promoter region and C/EBP beta protein expression that were mediated through both PI3-K/Akt and PPAR beta/delta signaling pathways.