Prostaglandin E₂ accelerates invasion by upregulating Snail in hepatocellular carcinoma cells

Our previous studies showed that prostaglandin E₂ (PGE₂) promotes hepatoma cell growth and migration, as well as invasion; however, the precise mechanism remains elusive. Snail and p65 protein levels were detected in human samples with hepatocellular carcinoma (HCC) by immunohistochemistry (IHC) staining. HCC cell lines (Huh-7 and Hep3B) were used for in vitro experiments. PGE₂/Akt/NF-κB pathway was investigated in Huh-7 and Hep3B cells after treatment with PGE₂, EP4 receptor (EP4R) agonist, Akt inhibitor, and NF-κB inhibitor, respectively, by real-time reverse transcription (RT)-PCR, Western blotting, and immunofluorescence (IF) staining. In vitro cell invasion assay was performed to evaluate the effect of PGE₂ on tumor invasiveness. Knockdown of EP4R was carried out in Huh-7 cells through plasmid-based small interfering RNA (siRNA) approach to confirm the regulation of PGE₂ on Snail by EP4R. Dual luciferase reporter assay was performed to assess Snail promoter activity in Huh-7 cell after treatment with EP4R agonist. We found that the protein levels of Snail were higher in HCC tissues than those in control and that PGE₂ and EP4R agonist treatment significantly increased Snail expression in Huh-7 and Hep3B cells. EP4R agonist also profoundly promoted invasiveness of Huh-7 cells. Knockdown of the EP4R by siRNA completely blocked the PGE₂-induced upregulation of Snail expression and reduced invasiveness of Huh-7 cells. We failed to find that EP4R-induced upregulation of Snail was reversed by inhibition of cAMP response element-binding protein (CREB), a canonical downstream target of EP4R. Alternatively, EP4R agonist treatment significantly increased the levels of phosphorylated EGFR and Akt both in Huh-7 and Hep3B cells. AG1478, an EGFR inhibitor, blocked the phosphorylation of Akt. The levels of phosphorylated IκB increased in Huh-7 cells after treatment with EP4R agonist for 30 min. The levels of phosphorylated p65 started to increase in Huh-7 cells treated with EP4R agonist for 4 h, and p65 translocated into the nucleus. In EP4R-agonist-treated Hep3B, the levels of phosphorylated p65 were also increased compared to the control group. The phosphorylation levels of p65 were significantly decreased in Huh-7 and Hep3B cells after treatment with the Akt signaling inhibitor LY294002 and EP4R agonist for 24 h. Treatment with the NF-κB inhibitor pyrrolidine dithiocarbamate (PDTC) at 10 μM for 24 h blocked EP4R-agonist-induced Snail upregulation in Huh-7 and Hep3B cells. Furthermore, we obtained human Snail promoter sequence from TRED-Promoter Database and identified a putative binding site of NF-κB in the sequence through TFSEARCH analysis. Subsequently, we treated Huh-7 cells with EP4R agonist or EP4R agonist and PDTC (NF-κB antagonist) and found significantly increased Snail promoter activity after EP4R agonist treatment for 12 h. The increased Snail promoter activity could be partially abolished by additional PDTC treatment. In addition, p65 protein levels were found increased together with Snail in HCC tissues compared to normal liver tissues. In conclusion, PGE₂ activates Akt/NF-κB signaling and then upregulates Snail via the EP4R/EGFR to promote migration and invasion in hepatoma cells. These findings may help future evaluation of novel chemo-preventive strategies for HCC.