Metalloprotease-dependent activation of EGFR modulates CD44/CD24 populations in triple negative breast cancer cells through the MEK/ERK pathway.

PURPOSE

The CD44/CD24 cell phenotype is enriched in triple negative breast cancers, is associated with tumor invasive properties, and serves as a cell surface marker profile of breast cancer stem-like cells. Activation of Epidermal Growth Factor Receptor (EGFR) promotes the CD44/CD24 phenotype, but the specific signaling pathway downstream of EGFR responsible for this effect is not clear. The purpose of this study was to determine the role of the MEK/ERK pathway in the expansion of CD44/CD24 populations in TNBC cells in response to EGFR activation.

METHODS

Representative TNBC cell lines SUM159PT (claudin-low) and SUM149PT (basal) were used to evaluate cell surface expression of CD44 and CD24 by flow cytometry in response to EGFR and MEK inhibition or activation. EGFR and ERK phosphorylation levels were analyzed by Western blotting. The relationship between EGFR phosphorylation and MEK activation score in basal and claudin-low tumors from the TCGA database was examined.

RESULTS

Inhibition of ERK activation with selumetinib, a MEK1/2 inhibitor, blocked EGF-induced expansion of CD44/CD24 populations. Sustained activation of ERK by overexpression of constitutively active MEK1 was sufficient to expand CD44/CD24 populations in cells in which EGFR activity was blocked by either erlotinib, an EGFR kinase inhibitor, or BB-94, a metalloprotease inhibitor that prevents generation of soluble EGFR ligands. In basal and claudin-low tumors from the TCGA database, there was a positive correlation between EGFR_pY1068 and MEK activation score in tumors without genomic loss of DUSP4, a negative regulator of ERK, but not in tumors harboring DUSP4 deletion.

CONCLUSION

Our results demonstrate that ERK activation is a key event in EGFR-dependent regulation of CD44/CD24 populations. Furthermore, our findings highlight the role of ligand-mediated EGFR signaling in the control of MEK/ERK pathway output in TNBC tumors without DUSP4 loss.