The role of CtIP (RBBP8) in tamoxifen resistance and human breast cancer
Abstract
Acquired resistance to the antiestrogen tamoxifen constitutes a major clinical
challenge in breast cancer therapy. However, the mechanisms involved are still poorly
understood. Therefore, the overall goal and focus of this dissertation are to better
understand the phenomenon of tamoxifen resistance. In preliminary studies, we generated
two independently derived isogenic MCF-7 breast cancer cell line variants (TAMR1 and
TAMR2) that were resistant to the inhibitory growth effects of tamoxifen. Using serial
analysis of gene expression (SAGE), we identified CtIP (CtBP-interacting protein), a
BRCA1 (Breast cancer 1)- and CtBP (C-terminal binding protein)-interacting protein, as
one of the most significantly down-regulated transcripts in the aforementioned tamoxifen
resistant cells. We hypothesized that CtIP silencing constitutes a critical event for the
development of tamoxifen resistance in breast cancer. We found that silencing
endogenous CtIP in tamoxifen sensitive cells confers tamoxifen resistance and estrogen
independence. On the other hand, re-expression of CtIP in tamoxifen resistant cells
restores sensitivity to the inhibitory growth effects of tamoxifen. Importantly, poor
clinical response to neo-adjuvant endocrine therapy is associated with CtIP deficiency in
primary breast carcinomas. Meta-analysis of seven publicly available gene expression
microarray data sets shows that CtIP expression is significantly associated with estrogen
receptor (ER), disease free survival and breast cancer metastasis status. Furthermore, we
found CtIP protein expression in a majority of ER positive breast cancer cell lines, but
none or very little CtIP expression in ER negative lines. These findings indicate that CtIP
silencing may be a novel mechanism for the development of tamoxifen resistance in
breast cancer, and suggest that CtIP is likely associated with ER function and that CtIP
gene and protein expression may be useful biomarkers for breast cancer prognosis and
clinical management. Subsequent studies found a BRCA1-CtIP-CtBP complex in
tamoxifen sensitive but not resistant cells, whereas BRCA1 is associated with ER in both
cell lines. We also observed different patterns of occupancy by BRCA1, CtIP and CtBP
on ERE (Estrogen Response Element) region of the pS2 promoter after E2 or tamoxifen
treatment. These results support the potential involvement of a BRCA1-CtIP-CtBP
complex in the development of tamoxifen resistance.
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