Suppression of mesenchymal phenotype in inflammatory breast cancer by EGFR tyrosine kinase inhibitor erlotinib: a novel approach to suppressing metastasis.

Abstract

#2042 Background: Inflammatory breast cancer (IBC) is a rare subtype well known for its propensity for rapid metastasis. The cause of this rapid metastasis in IBC is unknown. An immunohistochemical analysis of 44 cases of IBC showed HER2 overexpression in 48% of patients and epidermal growth factor receptor (EGFR) overexpression in 30% of patients. EGFR overexpression was the only poor prognostic factor: the 5-year overall survival rate was significantly lower for women with EGFR-positive disease than for women with EGFR-negative disease (P=0.01). HER2 overexpression was not a prognostic factor. The association between EGFR overexpression and increased risk of death indicated that EGFR may represent a potential therapeutic target in IBC. We hypothesized that suppression of the EGFR pathway inhibits proliferation and metastasis of IBC.
 Methods: SUM149 IBC cells, which express low levels of HER2 and high levels of EGFR, were treated with siRNA against EGFR and with the EGFR tyrosine kinase inhibitor erlotinib.
 Results: EGFR siRNA knockdown (but not non-targeting siRNA control) inhibited the proliferation of SUM149 cells. SUM149 cells were sensitive to EGFR tyrosine kinase inhibitor erlotinib in a 2-dimensional (2-D) culture system (median inhibitory concentration [IC50] = 0.90 µM). When we activated ERK by transfecting constitutively active MEK1 in SUM149 cells, the cells showed more resistance to erlotinib. Moreover, ERK siRNA knockdown sensitized SUM149 cells to erlotinib. Further, when we cultured the SUM149 cells in matrigel by using the 3-D culture system (100% matrigel in the bottom layer and cultured medium with 5% FBS and 2% matrigel in the top layer), erlotinib treatment changed the molecular phenotype of SUM149 cells from mesenchymal (a phenotype characterized by low beta-catenin expression and high vimentin and fibronectin expression) to epithelial (recovery of beta-catenin to the sites of cell-cell contacts; downregulation of fibronectin). This reversal of the mesenchymal phenotype, a hallmark of inhibition of epithelial-to-mesenchymal transition (EMT), was ERK dependent. Interestingly, the erlotinib concentration that inhibited the mesenchymal phenotype (0.1 mcM) was one log lower than the concentration that inhibited proliferation (1 mcM).
 Conclusion: Inhibition of tumor growth and EMT in SUM149 IBC cell lines is dependent on the ERK pathway through the EGFR pathway. The erlotinib dose needed to produce an anti-mesenchymal effect is much lower than the cytotoxic dose. Thus, we speculate ERK pathway to be important in inhibiting metastasis in IBC. We are currently investigating the effects of erlotinib in a xenograft model of IBC. Our study provides a rationale for developing novel treatment strategies targeting the EGFR and ERK pathways to inhibit the growth and metastasis of IBC. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 2042.