The deregulation of proliferation in cancer cells often results from activation of the Ras-Raf-MEK-MAP kinase pathway. Mutational activation of the Ras-GTP binding protein is a common event in malignancy and activating mutations of the B-Raf kinase occur in the majority of melanomas and in other tumors as well. Moreover, activation of MAP kinase occurs via wild type Ras in tumors in which transmembrane kinases such as HER2 are overexpressed or stimulated. We used a potent and selective inhibitor of the MEK kinase, CI-1040, to determine the role played by the Ras-MAP kinase pathway in tumor proliferation and survival. CI-1040 inhibited MEK and MAP kinase activity at 100-500 nM in all cells examined but the consequences of MAP kinase inhibition was dependent upon both the lineage and the mutational status of the tumor. All tumor cell lines with B-Raf mutation, regardless of lineage, were exquisitely sensitive to this compound. In cells with B-Raf mutation, inhibition of MEK leads to downregulation of D-cyclins, hypophosphorylation of RB and G1 growth arrest. In some but not all cell lines with B-Raf mutation, G1 arrest is followed by caspase 3 activation and apoptosis. A majority of melanomas with mutant N-Ras and those wild type for both Ras and Raf are also dependent upon MEK activation for expression of cyclin D. The proliferation of these cell lines is inhibited by CI-1040 albeit typically at higher doses than those required to inhibit cells with activating B-Raf mutations. In contrast, in tumor lineages in which Ras and Raf mutations rarely occur (breast/prostate), tumor growth in cells wild type for Ras and Raf is refractory to inhibition of MAPK. In these lineages, B-Raf mutation is uncommon but when present, confers a dependence of the tumor on MEK for cyclin D1 expression, proliferation and survival. In breast cancer cells with elevated HER2 expression, D-cyclin expression is independent of MEK/MAPK and instead dependent upon PI3 kinase, Akt activation. These data suggest that MAP kinase activation is required for the proliferation of tumor cells in which B-Raf is mutated and not in breast cancer cells wild type for Ras and Raf. The data provide rationale for testing MEK inhibitors in melanoma patients with both mutant and wild type B-Raf and N-Ras. The data also suggest that clinical trials of MEK inhibitors in tumor types with a low incidence of B-Raf and Ras mutations (breast/prostate) be enriched with patients whose tumors harbor activating mutations within this pathway.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]