Abstract 3368: MEK inhibitor selumetinib (AZD6244 – ARRY-142886) promotes mesenchymal to epithelial transition in triple-negative breast cancer

Selumetinib is a highly selective allosteric inhibitor of MEK1/2. Multiple clinical trials of Selumetinib are ongoing in patients with different types of cancer. However, the therapeutic role of Selumetinib in breast cancer has not been well defined. We sought to determine the effect of targeted inhibition of the MEK-ERK pathway by Selumetinib in triple-negative breast cancer (TNBC). We studied the effect of Selumetinib on 2 TNBC cell lines, MDA-MB-231 and SUM-149. In MDA-MB-231 cells, in 2-dimensional (2D) culture, Selumetinib at 0.1μM did not reduce cell viability, but in a 3-dimensional (3D) cell culture model, which mimics the human microenvironment, Selumetinib at 0.1 μM and 1 μM inhibited epithelial to mesenchymal transition (EMT). This result was confirmed by western blotting: expression of the mesenchymal markers fibronectin and vimentin was inhibited, and the epithelial marker beta-catenin was diffusely expressed in both the cytoplasm and the nucleus before treatment but was localized at the plasma membrane after treatment. Selumetinib in 3D cell culture also inhibited projections/filopodia formation, suggesting reversal to a more epithelial phenotype. Results were similar in SUM-149 cells: Selumetinib at 0.1 μM had minimal impact on cell viability in 2D culture, but Selumetinib at 0.1 μM and 1 μM inhibited projections/filopodia in 3D culture. Inhibition of ERK phosphorylation by Selumetinib correlated with a slight increase in the epithelial marker E-cadherin and loss of vimentin. These results suggest that treating TNBC with Selumetinib induces mesenchymal to epithelial transition (MET). In addition, in MDA-MB-231 cells, Selumetinib significantly inhibited anchorage-independent growth, an indicator of in vivo tumorigenicity. Previously, the ERK2 isoform was shown to induce EMT in epithelial cells. We therefore examined whether ERK1 and 2 expression levels correlate with Selumetinib9s effect on EMT regulation in TNBC cells. We used shRNA specifically targeting ERK1 and ERK2 (shERK1 and 2). Compared with parental MDA-MB-231 cells, stable clones that constitutively expressed shERK1 or shERK2 showed no difference in growth rate in 2D culture or projection formation in 3D culture. However, in 3D culture, treatment with Selumetinib inhibited spindle-shaped cell morphology and reduced scattering of the parental, vector-transfected, and shERK2 clones but did not inhibit mesenchymal filamentous structures in the shERK1 clones. Our data demonstrate that ERK1 may be necessary for Selumetinib -induced mesenchymal to epithelial transition in TNBC. We are planning in vivo studies to determine if low-dose Selumetinib can inhibit EMT, leading to a reduction of metastasis in a TNBC xenograft model. We will further confirm if ERK1 can serve as a biomarker for MEK inhibitor therapy in TNBC. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3368. doi:10.1158/1538-7445.AM2011-3368