Increased EP4 Receptor Expression in Colorectal Cancer Progression Promotes Cell Growth and Anchorage Independence

Abstract

Cyclooxygenase-2 and prostaglandin E₂ (PGE₂) levels are increased in colorectal cancers and a subset of adenomas. PGE₂ signaling through the EP4 receptor has previously been associated with colorectal tumorigenesis. However, changes in EP4 expression during adenoma to carcinoma progression have not been investigated, neither has whether levels of EP4 influence important markers of malignant potential, such as anchorage-independent growth or the tumors growth response to PGE₂. We report using immunohistochemistry that in vivo EP4 receptor protein expression was increased in colorectal cancers (100%) as well as adenomas (36%) when compared with normal colonic epithelium. EP4 expression was also higher in colorectal carcinoma compared with adenoma cell lines and increased with in vitro models of tumor progression. Adenoma (PC/AA/C1 and RG/C2) and carcinoma cell lines (HT29) were growth stimulated by PGE₂ up to 0.5 μmol/L. However, although carcinoma and transformed adenoma (PC/AA/C1SB10C, a transformed derivative of PC/AA/C1) cells remain stimulated by higher doses of PGE₂ (10 μmol/L), the adenoma cell lines were inhibited. Interestingly, enforced expression of EP4 in the adenoma cell line, RG/C2, resulted in stimulation of growth by 10 μmol/L PGE₂ and promoted anchorage-independent growth. Both in vivo and in vitro data from this study suggest that increased EP4 receptor expression is important during colorectal carcinogenesis. We propose that high levels of PGE₂ in a tumor microenvironment would select for cells with increased EP4 expression, and that the EP4 receptor may therefore represent an important target for colorectal cancer prevention and treatment. (Cancer Res 2006; 66(6): 3106-13)