Comparison of responses of human melanoma cell lines to MEK and BRAF inhibitors.

The NRAS and BRAF genes are frequently mutated in melanoma, suggesting that the NRAS-BRAF-MEK-ERK signaling pathway is an important target for therapy. Two classes of drugs, one targeting activated BRAF and one targeting MEK, are currently undergoing clinical evaluation. We have analysed the NRAS and BRAF mutational status of a series of 44 early passage lines developed from New Zealand patients with metastatic melanoma. 41% of the lines analysed had BRAF mutations, 23% had NRAS mutations, and 36% had neither. We then determined IC50 values (drug concentrations for 50% growth inhibition) for CI-1040, a commonly used inhibitor of MEK kinase; trametinib, a clinical agent targeting MEK kinase; and vemurafenib, an inhibitor of mutant BRAF kinase. Cell lines with activating BRAF mutations were significantly more sensitive to vemurafenib than lines with NRAS mutations or lines lacking either mutation (p < 0.001). IC50 values for CI-1040 and trametinib were strongly correlated (r = 0.98) with trametinib showing ~100-fold greater potency. Cell lines sensitive to vemurafenib were also sensitive to CI-1040 and trametinib, but there was no relationship between IC50 values and NRAS mutation status. A small number of lines lacking a BRAF mutation were sensitive to CI-1040 but resistant to vemurafenib. We used western blotting to investigate the effect on ERK phosphorylation of CI-1040 in four lines, of vemurafenib in two lines and of trametinib in two lines. The results support the view that MEK inhibitors might be combined with BRAF inhibitors in the treatment of melanomas with activated BRAF. The high sensitivity to trametinib of some lines with wildtype BRAF status also suggests that MEK inhibitors could have a therapeutic effect against some melanomas as single agents.

Centrosomal dysregulation in human metastatic melanoma cell lines.

Correct partitioning of the replicated genome during mitosis is orchestrated by centrosomes, and chromosomal instability is a commonly reported feature of human cancer. Melanomas are notorious for their genetic instability and rapid clonal evolution that may be manifested as aggressive growth and facile generation of therapy-resistant variants. We characterized the centrosomal status, ploidy, and gene status (TP53, CDKN2A/B, BRAF, and NRAS) of 15 human metastatic melanoma cell lines. Cells were labelled for pericentrin (a centrosomal marker), DNA and α-tubulin, and scored for centrosome morphology, supernumerary centrosomes, and mitotic symmetry. The incidence of supernumerary centrosomes correlated with that of gross centrosomal abnormalities (r = 0.90), mitotic asymmetry (r = 0.90), and, surprisingly, increased content of G/M cells (r = 0.79). Centrosomal numerical dysregulation, observed in all cell lines, was found not to be specifically related to the status of any of the characterized gene mutations that were found in 13/15 cell lines. We conclude that centrosomal dysregulation may arise from multiple mechanisms and may drive the generation of genetic and phenotypic diversity in melanoma.