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Background: K-Ras mutations are common early events in colorectal cancers that correlate with elevated levels of prostaglandin E2 (PGE2). PGE2 synthesis is largely controlled by induced expression of cyclooxygenase-2 (COX-2) and membrane-associated prostaglandin E synthase-1 (mPGES-1). PGE2 is degraded by 15-hydroxyprostaglandin dehydrogenase (PGDH). Decreased PGDH expression has been observed in colorectal cancers suggesting a decrease in PGE2 catalysis in transformed intestinal epithelial cells.

Aim: To determine whether over-expression of PGDH can attenuate PGE2 levels and decrease the tumorigenecity of Ras-transformed intestinal epithelial cells.

Methods: Transduced rat ileum epithelial cell line (IEC-18) with the lentivirus containing GFP (18.GFP), K-RasG12V (18.Kras) or PGDH/K-Ras G12V (18.Kras.PGDH) were injected subcutaneously in the right flank of nude male mice (Nu/Nu). Tumor development was continuously monitored over several weeks. Tumors were excised and analyzed by immunohistochemistry. Prostanoid levels were measured by EIA, mRNA expression by RT-PCR, protein expression by Western blotting and PGDH activity by [15-3H]-PGE2 catabolism.

Results: 18.GFP cells had low basal levels of PGE2 and COX-2 expression. Mice injected with 18.GFP cells did not produce tumors. Mice injected with 18.Kras cells developed tumors within 2 weeks and maintained elevated levels of PGE2, increased expression of COX-2 and mPGES-1. PGDH expression levels were decreased in 18.Kras cells compared to 18.GFP cells whereas PGDH activity were decreased compared to 18.Kras.PGDH cells. Mice injected with 18.Kras.PGDH cells showed delayed tumor formation (within 8 weeks). Antibody staining for the proliferative marker, Ki-67, showed peripheral localization in 18.Kras tumors whereas Ki-67 expression was dispersed in 18.Kras.PGDH tumors. Apoptotic index analysis by in situ DNA strand break detection (TUNEL) showed a decrease in 18.Kras tumors in comparison to 18.Kras.PGDH tumors. COX-2 and mPGES-1 expression in 18.Kras tumors were elevated while PGDH expression was not detected. Conversely, 18.Kras.PGDH tumors showed elevated PGDH expression as well as the presence of COX-2 and mPGES-1 expression.

Conclusion: Over-expression of PGDH in K-RasG12V expressing intestinal epithelial cells decreased PGE2 levels that correlated with delayed tumor formation, suggesting that PGE2 is critical in regulating tumor growth.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA