Abstract B91: RNAi knockdown of Mek2, but not Mek1, increases proliferation of malignant and non‐malignant human breast cells

We are using shRNAs against Mek1 and/or Mek2 to determine the individual contributions of Mek1 and Mek2 to proliferation, migration and Mek inhibitor sensitivity in malignant and non‐malignant human breast epithelial cell lines. We found in both malignant (MCF7 and MDAMB231) and non‐malignant immortalized (184vTERT and MCF10A) breast cell lines that, despite significant shRNA‐mediated reductions (>95%) in the expression of Mek1, Mek2, or both kinases, the cells maintained long‐term growth. In fact, the shMek2 lines grew significantly faster and were more motile than controls, suggesting that in some breast cancers, drugs that specifically target Mek1 might be more effective than those which target both Mek1 and 2. In MCF7 cells, levels of phosphorylated (P)‐Erk1 and 2 were significantly decreased or undetectable in the presence of Mek shRNAs, indicating that Erk phosphorylation may be dispensable for cell proliferation in these cells. In contrast, in 184vTERT and MCF10A, phosphorylated Erk levels remained constant despite the knockdown of both Mek1 and Mek2. This latter result indicated that, in some breast cells, another kinase is likely to phosphorylate Erk1/2 in the absence of Mek1 and Mek2. The Mek5 kinase is genetically most similar to Mek1 and Mek2, making it an attractive candidate for further investigation. Mek5 is expressed in 184vTERT, and its total protein levels are not altered when Mek1 and 2 are knocked down. A preliminary experiment indicates that siRNA induced knockdown of Mek5 expression leads to decreased ERK1/2 phosphorylation in 184v‐TERT cells independently of Mek1/2 expression. We are currently investigating whether this occurs in other breast cell lines. Collectively, our data indicate that in at least some breast cells, both Mek1 and Mek2 are dispensable for proliferation and survival, and that parallel pathways are sufficient for relaying signals from cell surface receptors, independently of Mek1/2 and/or P‐Erk1/2. This data may explain why Mek inhibitors, which target both Mek1 and Mek2, have not done well clinically. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B91.