Abstract

Chronic use of nonsteroidal anti-inflammatory drugs results in a significant reduction of risk and mortality from colorectal cancer in humans. All of the mechanism(s) by which nonsteroidal anti-inflammatory drugs exert their protective effects are not completely understood, but they are known to inhibit cyclooxygenase activity. The cyclooxygenase enzymes catalyze a key reaction in the conversion of arachidonic acid to prostaglandins, such as prostaglandin E 2 (PGE 2 ). Here we demonstrate that PGE 2 treatment of LS-174 human colorectal carcinoma cells leads to increased motility and changes in cell shape. The prostaglandin EP 4 receptor signaling pathway appears to play a role in transducing signals which regulate these effects. PGE 2 treatment results in an activation of phosphatidylinositol 3-kinase/ protein kinase B pathway that is required for the PGE 2 -induced changes in carcinoma cell motility and colony morphology. Our results suggest that PGE 2 might enhance the invasive potential of colorectal carcinoma cells via activation of major intracellular signal transduction pathways not previously reported to be regulated by prostaglandins.There is a 40 -50% reduction in the relative risk of colorectal cancer and colorectal cancer-associated mortality in individuals taking nonsteroidal anti-inflammatory drugs (NSAIDs)

Cyclooxygenase-2 (COX-2) is not normally expressed in the human large intestine, but its levels are increased in the majority of human colorectal carcinomas. Here we investigate the regulation of constitutive COX-2 expression and prostaglandin production in human colorectal carcinoma cells. Both COX-2 mRNA and protein were expressed in well differentiated HCA-7, Moser, LS-174, and HT-29 cells, albeit at different levels. COX-2 expression was not detected in several poorly differentiated colon cancer cell lines including DLD-1. Transcriptional regulation played a key role for the expression of COX-2 in human colon carcinoma cells, and both the nuclear factor for interleukin-6 regulatory element and the cAMP-response element were responsible for regulation of COX-2 transcription. COX-2 mRNA was more stable in HCA-7 cells than in the other cell lines tested. Both transcriptional and post-transcriptional regulation of COX-2 involved the MAP kinase pathway. Modulation of the Akt/protein kinase B or Rho B signaling pathways altered the levels of COX-2 expression. Furthermore, COX-2 protein is degraded through ubiquitin proteolysis, and its half-life was approximately 3.5-8 h. HCA-7 cells produced significant quantities of prostaglandin E(2) and other prostaglandins. Moser and LS-174 cells also generated prostaglandins, but levels were significantly lower than that observed in HCA-7 cells.

Cyclooxygenase and its derived prostaglandin E 2 (PGE 2 ) have been shown to stimulate the growth of cancer cells and promote tumor angiogenesis. Here, we show that PGE 2 activated the ␤-catenin/T cell factordependent transcription in colon cancer cells through the cAMP/protein kinase A pathway. The expression of cyclin D1 and vascular endothelial growth factor was induced by PGE 2 in LS-174T cells. Moreover, PGE 2 and mutated ␤-catenin stimulated the transcription of cyclin D1 and vascular endothelial growth factor in a synergistic fashion. Mechanistically, PGE 2 increased the phosphorylation of glycogen synthase kinase-3 and consequently accumulated ␤-catenin. In addition, PGE 2 induced the expression of T cell factor-4 transcription factor, which formed transcriptionally active complex with ␤-catenin. In animal experiments, administration of 16,16-dimethyl PGE 2 strongly increased the expression of cyclin D1 and vascular endothelial growth factor in APC min/؉ mouse polyps. Thus, our results provide a novel mechanism, suggesting that cyclooxygenase-2/PGE 2 may exert pro-oncogenic actions through stimulating the ␤-catenin/T cell factor-mediated transcription, which plays critical roles in colorectal carcinogenesis.

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