The oestrogen receptor (ER) mediates both normal and malignant breast
development. The ER can exert its ligand-dependent gene regulatory functions
directly by association with target genes and subsequent dynamic formation of a
transcription complex through its oestrogen response element. The complex receptorligand signalling network has received much interest as a potential therapeutic target.
Endocrine agents, such as tamoxifen, have been developed to reverse oestrogen (E2)
stimulated gene transcription and tumour growth, however progression to endocrine
resistance provides a major obstacle in breast cancer treatment. This study
investigates changes in endocrine response and ER transcription activation during the
acquisition of endocrine resistance.Breast cancer cell lines were selected that encompassed the range of oestrogen
and anti-oestrogen sensitivities from the E^-dependent and tamoxifen-sensitive MCF7 cell line through the less sensitive LCC-1 and LCC-2 lines to the insensitive LCC-9
and independently derived LY2 line. All lines had been derived from MCF-7 cells.
These models represent different endocrine phenotypes and were designed to reflect
sequential changes in the clinical progression from hormone sensitive to hormone
insensitive and antioestrogen resistant. MDA-MB-231 breast cancer cells were used
as ERα negative controls. Growth assays confirmed these phenotypes and
proliferative behaviour in response to E₂ and tamoxifen.To help evaluate the role of the ERα in the development of endocrine resistance,
quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) and Western
blot analysis was carried out to assess transcriptional and translational receptor status
for each phenotype. All resistant lines possessed greater levels of ERα mRNA than
wild-type MCF-7 cells. E₂ downregulated ERα mRNA and protein. Results suggest
ERa functionality in these cell lines. Analysis of mRNA levels of several ER target
genes (pS2, progesterone receptor (PR), cathepsin D and MYC) indicated
informative differences between lines. pS2 was highly expressed in resistant lines as
was PR in most lines compared to MCF-7 cells. E2 and tamoxifen produced
attenuated or no effects in pS2 expression compared to the marked induction
produced by E2 in MCF-7 cells. Minor changes were also observed in the expression
levels of various coactivators (SRC-1, SRC-2, SRC-3) and corepressors (NCoR,
SMRT and RIP140) investigated by qRT-PCR and Western blot.A subset of three cell lines (MCF-7, LCC-1 and LCC-9) was used to examine
transcription complex assembly at the pS2 promoter in response to E2 utilizing
chromatin immunoprecipitation. This identified a dynamic cycle of increasing H4
acetylation (indicative of active transcription) and ERα as well as cofactor
recruitment upon E₂ addition in MCF-7 cells. A distinctive H4 acetylation pattern
was revealed for LCC-1 and LCC-9 cells. While ERα recruitment was similar to
MCF-7 cells, particularly strong SRC-1 and SRC-3 recruitment was detected in
LCC-1 but most markedly in LCC-9 cells, implying altered pS2 transcription
complex assembly.The data suggest that a functional oestrogen receptor may remain in these
endocrine resistant models. Cell proliferation and E₂ target gene expression in LCC-1
cells proved to be oestrogen and antioestrogen independent but responsive in the
presence of the ligand whereas LCC-9 cells manifest complete endocrine resistance.
In addition, changes have been detected in the assembly of a gene transcription
complex at the pS2 promoter. Taken together, this is evidence for modified ER
mediated transcription activity. These results may help to identify potential
mechanisms of endocrine resistance
