Isolation of Estrogen Receptor Cofactors fron Breast Tumors

Institution: University of California, San Diego
Investigator(s): Thorsten Heinzel, Ph.D. -
Award Cycle: 1995 (Cycle I) Grant #: 1FB-0439 Award: $68,607
Award Type: Postdoctoral Fellowship
Research Priorities
Biology of the Breast Cell>Pathogenesis: understanding the disease



Initial Award Abstract (1995)
The estrogen receptor is a protein which, in the presence of estradiol, binds to DNA and activates genes involved in cell growth. Estrogen receptor is present in approximately two-thirds of all tumors of postmenopausal breast cancer patients. The growth of these tumors is stimulated by estrogen through the action of estrogen receptor. However, the precise mechanism by which estrogen receptor regulates cell growth is still unknown. Antiestrogens, in particular tamoxifen, are widely used as drugs in the treatment of breast cancer because they can inactivate the estrogen receptor. Recently, proteins have been identified which bind to the estrogen receptor in the presence of estradiol but not in the presence of antiestrogens. Experimental data suggest that these proteins play an important role in the activation of genes by the estrogen receptor. They are therefore referred to as coactivators. Mutations affecting these cofactors could potentially cause breast cancer or could be responsible for the development of drug resistance during tamoxifen treatment of breast cancer tumors. The identification and characterization of coactivator proteins that interact with estrogen receptor specifically in breast tumor tissue or cell lines, as proposed for this project, will improve our understanding of the causes of breast cancer. Ultimately a biochemical assay could be used to screen new antiestrogens for maximum effects on estrogen receptor-coactivator interaction. This assay would facilitate the development of new drugs for breast cancer endocrine therapy. Additional drugs would be extremely valuable for this type of therapy because most breast cancer tumors eventually develop resistance after initially responding to tamoxifen.

In this project the approach for the discovery of novel coactivators will be based on a biochemical assay which analyzes protein-protein interactions of the estrogen receptor bound to DNA. Proteins from breast cancer cells or breast tumor tissue will be identified that can bind to the receptor in the presence of estrogen. Following the initial characterization of these potential coactivators the proteins will be isolated in quantities allowing the determination of partial amino acid sequences. DNA amplification techniques can then be used to clone the coactivator genes. The effects of coactivators on target gene regulation by estrogen receptor will be evaluated by introducing these coactivators into cell lines that do not normally produce them. A gene deletion approach can be used to test the relevance of the coactivators for cell growth in cell culture and mutations that either affect coactivator genes directly or alter their regulation can be evaluated for their role in breast cancer.


Final Report (1997)
Hormones, including estrogen, are considered to be a component in development and growth of many human breast cancers. The estrogen receptor is present in approximately two-thirds of all tumors of post-menopausal breast cancer patients, and the growth of these tumors is stimulated by estrogen through the action of the estrogen receptor. In order to advance the development of diagnostic and therapeutic agents, it is critical to understand how the estrogen receptor functions. In the initial year of BCRP support, we discovered that estrogen receptors mediate their actions by binding to specific co-factors that either repress or stimulate the production of other gene products. Further, we identified these co-factors, and started the analysis of the molecular mechanisms underlying their function. In the second year, I completed the analysis of the effect of anti-estrogens on ER/co-repressor interactions, which would permit the development of therapeutic and preventative approaches to fight human breast cancers.

We have identified two classes of factors required for estrogen receptor to function in breast. The first includes several new specific estrogen receptor coactivators, termed SRC/N-CoA proteins, and the CREB-binding protein CBP, which we have shown to coordinate signals from multiple growth pathways. Indeed, different growth factors, such as prolactin, wnt-1 and estrogen, appear to regulate each other by competing for the limiting amounts of CBP. This observation could be exploited in altering transforming events. Further, we have discovered a co-repressor, N-CoR, that can bind to the estrogen receptor in the presence of antagonists and is required to mediate the antagonistic function of these drugs. Consistent with this model, we have found that decreased N-CoR levels correlate with the acquisition of tamoxifen resistance in a nude mouse model. This observation could at least in part explain the development of drug resistance during tamoxifen treatment of breast cancer tumors. We hypothesize that the interaction with coactivators and co-repressors potentially provides a novel index of the therapeutic value of treatments, such as anti-estrogens in breast cancer. Ultimately, a biochemical assay could be used to screen new anti-estrogens for maximum effects on estrogen receptor-cofactor interaction. This assay would facilitate the development of new drugs for breast cancer endocrine therapy. Additional drugs would be extremely valuable for this type of therapy because most breast cancer tumors eventually develop resistance after initially responding to tamoxifen

N-CoR, mSin3, and histone deacetylases are components of a complex mediating transcriptional repression.
Periodical:Nature
Index Medicus: Nature
Authors: Heinzel T, Lavinsky RM, Mullen T-M, et al.
Yr: 1997 Vol: 387 Nbr: Abs: Pg:43-48

Differential effects of nuclear receptor corepressor (N-CoR) expression levels on retinoic acid receptor-mediated repression support the existance of dynamically regulated corepressor complexes.
Periodical:Molecular Endocrinology
Index Medicus: Mol Endocrinol
Authors: Soderstrom M, Vo A, Heinzel T, et al.
Yr: 1997 Vol: 11 Nbr: Abs: Pg:682-692

Ligand-independent repression by the thyroid hormone receptor mediated by a nuclear receptor co-repressor.
Periodical:Nature
Index Medicus: Nature
Authors: Horlein AJ, Naar AM, Heinzel T, Torchia J, Gloss B, Kurokawa R, Ryan A, Kamei Y, et al.
Yr: 1995 Vol: 377 Nbr: 6548 Abs: Pg:397-404

A CBP integrator complex mediates transcriptional activation and AP-1 inhibition by nuclear receptors.
Periodical:Cell
Index Medicus: Cell
Authors: Kamei Y, Xu L, Heinzel T, Torchia J, Kurokawa R, Gloss B, Lin SC, Heyman
Yr: 1996 Vol: 85 Nbr: 3 Abs: Pg:403-414

Diverse signaling pathways modulate nuclear receptor recruitment of N-CoR and SMRT complexes.
Periodical:Proceedings of the National Academy of Sciences of the United States of America
Index Medicus: Proc Nat Acad Sci, U S A
Authors: Lavinsky RM, Jepsen K, Heinzel T, Torchia J, Rose DM et al
Yr: 1998 Vol: 95 Nbr: Abs: Pg:2920-2925

Signal-specific co-activator domain requirements for Pit-1 activation.
Periodical:Nature
Index Medicus: Nature
Authors: Xu L, Lavinsky RM, Dasen JS, Heinzel T, Rosenfeld MG, et al
Yr: 1998 Vol: 395 Nbr: Abs: Pg:301-306