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p300 Modulates the BRCA1 inhibition of estrogen receptor activity
Saijun Fan1, Yong Xian Ma, Chenguang Wang
1Department of Radiation Oncology, Long Island Jewish Medical Center, New Hyde Park, New York 11040, USA.
Abstract:
We previously reported that expression of the breast cancer susceptibility gene BRCA1 strongly inhibits the transcriptional activity of the estrogen receptor (ER-alpha) in human breast and prostate cancer cell lines but only weakly inhibits ER-alpha activity in cervical cancer cells (S. Fan et al., Science (Wash. DC), 284: 1354-1356, 1999). We now report that the ability of BRCA1 to repress ER-alpha activity correlates with its ability to induce down-regulation of the cellular levels of the transcriptional coactivator p300 in breast and prostate, but not in cervical cancer cells. On the other hand, BRCA1 failed to alter the expression of the CREB binding protein (CBP), the structural and functional homologue of p300, in any of these cell types. Ectopic expression of either p300 or CBP "rescued" (i.e., reversed) the BRCA1 inhibition of ER-alpha activity, whereas two other nuclear receptor coactivators, the p300/CBP-associated factor (PCAF) and the glucocorticoid receptor-interacting protein-1 (GRIP1), failed to rescue the ER-alpha activity. The rescue function mapped to the cysteine-histidine rich domain CH3, a region of p300/CBP that we found to interact directly with the conserved COOH-terminal activation domain (AF-2) of ER-alpha. p300 and ER-alpha were also found to interact in vivo and to colocalize within the nucleus in breast cancer cells. These findings suggest that the cofactors p300 and CBP modulate the ability of the BRCA1 protein to inhibit ER-alpha signaling. They further suggest that the BRCA1 inhibition of ER-alpha activity may be attributable, at least in part, to the down-regulation of p300.
Insights
The breast cancer gene BRCA1 inhibits estrogen receptor activity by reducing levels of the coactivator p300 in breast and prostate cells. This interaction suggests p300 and CBP cofactors modulate BRCA1
Area of Science:
- Molecular Biology
- Cancer Research
- Cell Biology
Background:
- BRCA1 expression inhibits estrogen receptor-alpha (ER-alpha) transcriptional activity in breast and prostate cancer cells, but not cervical cancer cells.
- The mechanism by which BRCA1 represses ER-alpha activity remains incompletely understood.
Purpose of the Study:
- To investigate the role of transcriptional coactivators p300 and CREB binding protein (CBP) in BRCA1-mediated repression of ER-alpha activity.
- To determine if BRCA1 affects the cellular levels of p300 and CBP.
- To identify the specific domains of p300/CBP involved in the interaction with ER-alpha and the rescue of BRCA1 inhibition.
Main Methods:
- Assessing the effect of BRCA1 expression on p300 and CBP levels in various cancer cell lines.
- Performing rescue experiments by ectopically expressing p300, CBP, PCAF, or GRIP1 to reverse BRCA1 inhibition of ER-alpha.
- Mapping the functional domains of p300/CBP responsible for interacting with ER-alpha's AF-2 domain.
- Investigating in vivo interaction and nuclear colocalization of p300 and ER-alpha in breast cancer cells.
Main Results:
- BRCA1 expression down-regulates p300 levels in breast and prostate cancer cells, correlating with ER-alpha activity inhibition, but not in cervical cancer cells.
- BRCA1 does not affect CBP levels in any of the tested cell types.
- Ectopic expression of p300 or CBP, but not PCAF or GRIP1, rescued BRCA1-mediated ER-alpha inhibition.
- The CH3 domain of p300/CBP was identified as crucial for interaction with ER-alpha's AF-2 domain.
- p300 and ER-alpha were found to interact in vivo and colocalize in the nucleus of breast cancer cells.
Conclusions:
- The cofactors p300 and CBP play a significant role in modulating BRCA1's ability to inhibit ER-alpha signaling.
- BRCA1-mediated inhibition of ER-alpha activity is, at least partly, due to the down-regulation of p300 levels.
- The interaction between p300 and ER-alpha, particularly involving the CH3 domain of p300, is critical for this regulatory mechanism.