LB53
Dissociation between oestrogen receptor alpha (ER) expression and stem cell activity in human breast cancer provides a novel mechanism of endocrine resistance
Ciara O'Brien1, Sacha Howell2, Anne Lykkesfeldt3, Julia Gee4, Robert Nicholson4, Robert Clarke1
1Paterson Institute of Cancer Research, University of Manchester, UK; 2Christie Hospital NHS Trust, Manchester, UK; 3Danish Cancer Society, Copenhagen, Denmark; 4Tenovus Centre for Cancer Research, University of Cardiff, UK
Background
Resistance to endocrine therapy reduces survival rates of women with ER+
breast cancer. In the normal breast, multipotent stem cells are ER-. We
hypothesised that breast cancer stem-like cells (CSCs) are ER- and therefore
represent a novel mechanism of resistance to endocrine therapy.
Method
Three ER+ cell lines, MCF-7, T47D and BT474 were assessed for ER expression
by immunocytochemistry (ICC) in monolayer and after CSC enrichment. Putative
CSC number was assessed by the non-adherent mammosphere (MS) assay and by
Fluorescence Activated Cell Sorting (FACS) for the markers CD44+/CD24-/low/epithelial
specific antigen (ESA)+, in the presence and absence of tamoxifen.
The effect of acquired Tamoxifen resistance on the CSC population of two
independent Tamoxifen-resistant (TAM-R) MCF-7 variants, from Cardiff (TAM-RCARD)
and Copenhagen (TAM-RCOP) was compared to parental, tamoxifen
sensitive (TAM-S) controls.
Results
A minor sub-population of ER_ cells were demonstrated in
MCF7 (6.1 1.4%), T47D (6.8 0.5) and BT474 (3.71.3) cells by ICC. However,
the FACS-enriched population of breast CSCs in MCF7 cells were mostly ER_
(73.2 4.6 % p= 0.0007). There was a significant increase in the proportion of
ER- cells in TAM-R cell lines (TAM-RCARD 11.6 1.4% p=0.01 and
TAM-RCOP 22 4.8% p=0.02 respectively) compared to the parental
TAM-S controls. Both TAM-R cell lines demonstrated increased mammosphere
forming efficiency (MFE) compared to TAM-S cells (TAM-RCARD 4% vs.
2.5% p=0.03, and TAM-RCOP 8% vs. 3% p=0.007). Tamoxifen treatment
yielded significantly smaller mammospheres but did not abrogate mammosphere
formation.
Conclusion
The majority of the putative CSC enriched populations in ER+ breast cancer
are ER_. There is a proportional increase of ER- CSCs in
TAM-R MCF7 cell lines. The reduction in MS size suggests that Tamoxifen targets
transit amplifying cells in ER+ breast cancers but not CSCs. These findings
demonstrate a novel mechanism of endocrine resistance.
