Abstract 1969: Overexpression of specific CD44 variants mediate endocrine insensitivity and invasion in breast cancer cells
Authors: Bellerby Rebecca L. , Smith Christopher , Gee Julia , Martin Tracey , Barrett-Lee Peter , Hiscox Stephen 
Published: May 2015

The majority of breast cancers express the oestrogen receptor and are potentially amenable to antihormone therapy; however the clinical effectiveness of these agents is limited by the phenomenon of acquired resistance which is associated with disease relapse and poor prognosis. In vitro evidence suggests that acquired resistance to tamoxifen and fulvestrant results in a significant gain to the migratory and invasive nature of breast cancer cells which may augment their metastatic potential in vivo. We have previously demonstrated that the CD44 receptor is overexpressed in acquired endocrine resistance and contributes to a gain in aggressive phenotype and poor outcome clinically. However, little is known regarding the effects of CD44 splice variants in this context. Here we have explored the role of CD44 variant 3 (v3) and CD44 variant 6 (v6) in endocrine-resistant and endocrine-sensitive breast cancer models.

CD44v3 and CD44v6 isoform expression was assessed in cell models of acquired tamoxifen (Tam-R) and fulvestrant (Fas-R) resistance along with their endocrine-sensitive MCF-7 counterparts by microarray analysis, Western blotting, immunofluorescence and immunohistochemistry. The ability of CD44v3 and CD44v6 to modulate tumor cell growth (cell counting assay), migration (fibronectin motility assay), invasion (Matrigel Boyden chamber assay), adhesion (matrix adhesion assay) and response to endocrine agents were determined using cells in which CD44v3 or CD44v6 expression was modulated by overexpression (MCF-7) or by siRNA (Tam-R, Fas-R). Changes in cellular signalling pathways as a result of CD44 modulation and activation were assessed using Western blotting.

CD44v3 and CD44v6 were overexpressed at gene and protein level in Tam-R and Fas-R cells versus endocrine-sensitive MCF-7 cells. Inhibition of CD44 expression (all isoforms) in resistant cells resulted in a loss of migratory, proliferative and invasive phenotype. Overexpression of CD44v6 in MCF-7 cells resulted in a gain in EGFR signalling and enhanced their intrinsic invasive capacity and promoted a loss of endocrine response. In contrast, overexpression of CD44v3 decreased EGFR pathway activity in MCF-7 cells and did not augment their invasive capacity or alter their endocrine sensitivity.

Our data suggests that upregulation of CD44v6 in acquired resistant breast cancer may contribute to their gain in aggressive phenotype and loss of endocrine response through transactivation of the EGFR pathway. Future therapeutic targetting of CD44v6 may prove to be an effective strategy alongside EGFR targeted agents in delaying/preventing acquired resistance in breast cancer.

Citation Format: Rebecca L. Bellerby, Christopher Smith, Julia Gee, Tracey Martin, Peter Barrett-Lee, Stephen Hiscox. Overexpression of specific CD44 variants mediate endocrine insensitivity and invasion in breast cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1969. doi:10.1158/1538-7445.AM2014-1969