Intrinsic resistance to the MEK1/2 inhibitor AZD6244 (ARRY-142886) is associated with weak ERK1/2 signalling and/or strong PI3K signalling in colorectal cancer cell lines.

Mutations in KRAS or BRAF frequently manifest in constitutive activation of the MEK1/2-ERK1/2 signalling pathway. The MEK1/2-selective inhibitor, AZD6244 (ARRY-142886), blocks ERK1/2 activation and is currently undergoing clinical evaluation. Tumour cells can vary markedly in their response to MAPK or ERK kinase (MEK) inhibitors, and the presence of a BRAF mutation is thought to predict sensitivity, with the RAS mutations being associated with intrinsic resistance. We analysed cell proliferation in a panel of 19 colorectal cancer cell lines and found no simple correlation between BRAF or KRAS mutation and sensitivity to AZD6244, though cells that harbour neither mutation tended to be resistant. Cells that were sensitive arrested in G(1) and/or underwent apoptosis and the presence of BRAF or KRAS mutation was not sufficient to predict either fate. Cell lines that were resistant to AZD6244 exhibited low or no ERK1/2 activation or exhibited coincident activation of ERK1/2 and protein kinase B (PKB), the latter indicative of activation of the PI3K pathway. In cell lines with coincident ERK1/2 and PKB activation, sensitivity to AZD6244 could be re-imposed by any of the 3 distinct PI3K/mTOR inhibitors. We conclude that AZD6244 is effective in colorectal cancer cell lines with BRAF or KRAS mutations. Sensitivity to MEK1/2 inhibition correlates with a biochemical signature; those cells with high ERK1/2 activity (whether mutant for BRAF or KRAS) evolve a dependency upon that pathway and tend to be sensitive to AZD6244 but this can be offset by high PI3K-dependent signalling. This may have implications for the use of MEK inhibitors in combination with PI3K inhibitors.

Increased EP4 receptor expression in colorectal cancer progression promotes cell growth and anchorage independence.

Cyclooxygenase-2 and prostaglandin E(2) (PGE(2)) levels are increased in colorectal cancers and a subset of adenomas. PGE(2) 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 PGE(2). 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 PGE(2) up to 0.5 micromol/L. However, although carcinoma and transformed adenoma (PC/AA/C1SB10C, a transformed derivative of PC/AA/C1) cells remain stimulated by higher doses of PGE(2) (10 micromol/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 micromol/L PGE(2) 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 PGE(2) 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.