Estrogen receptor-interacting proteins in breast cancer

Institution: University of Southern California
Investigator(s): Michael Stallcup, Ph.D. -
Award Cycle: 1996 (Cycle II) Grant #: 2RB-0174 Award: $492,719
Award Type: Research Project Awards
Research Priorities
Biology of the Breast Cell>Pathogenesis: understanding the disease

Initial Award Abstract (1996)
The normal development and function of the breast is regulated by a number of hormones including estrogen. Among other effects, estrogen stimulates many cells of the breast tissue to proliferate, i.e. to replicate themselves. Many breast cancer cells also require estrogen to stimulate their growth, and for this reason, drugs that block the action of estrogen are often used effectively in therapy of breast cancers. However, some breast cancers do not respond to these anti-estrogen therapies and, in addition, many breast cancers that initially respond to anti-estrogen therapy eventually become resistant. For these reasons, it is important to continue seeking new methods of treatment. The ability to design new treatments will depend upon our ability to understand the nature of cancer cells and how their growth is stimulated and regulated. Because estrogen remains a central factor in stimulating breast cancer cell growth, we are endeavoring to gain a more detailed understanding of the mechanism of estrogen action, since this knowledge will be essential for designing new therapeutic strategies to block estrogen stimulation of breast cancer cell growth.

Estrogen stimulates breast cancer cell growth by activating specific genes in breast cells that are responsible for cell growth and division. Much is known about this mechanism already. The hormone estrogen enters the cell and binds tightly to a specific protein, the estrogen receptor. The binding of the hormone activates the receptor, which then binds to specific genes in the cell nucleus and activates those genes. Some of these activated genes are responsible for stimulating cell growth. Why and how the binding of the estrogen receptor to specific genes activates those genes is still not known, and a better understanding of this phenomenon is the major goal of this proposed investigation. In order for the estrogen receptor to activate the genes that it binds to, it is believed to require assistance from other, as yet unknown, proteins in the cell. In our preliminary work, we have identified several novel proteins that interact with the estrogen receptor in a very specific manner: these novel proteins bind to the receptor only after the receptor has been activated by hormone; furthermore, they appear to interact with a specific portion of the receptor that is known to be important for the receptorís ability to activate the target genes. It is our hypothesis that these proteins play important roles in the action of the estrogen receptor. We therefore propose to identify these proteins and to determine the nature of their roles in assisting the estrogen receptor to activate specific genes in breast cancer cells.

These novel proteins may represent novel targets for breast cancer therapy, since blocking their action may prevent the growth of some tumors that do not respond to antiestrogen therapy. Furthermore, combination therapy, involving anti-estrogens and agents that block the action of these novel proteins, may prevent the recurrence of some breast tumors. More basic knowledge of how estrogen activates genes is essential before such novel therapies can be designed, and such knowledge is the goal of this proposal.

Final Report (1999)
Transcriptional coactivators for steroid hormone receptors are a newly discovered class of proteins that mediate the action of the estrogen receptor (ER). Our broad goals were to identify transcriptional coactivators for ER, identify their interacting surfaces, and study the mechanism by which coactivators mediate the function of ER. AIM I was to characterize new ER-interacting proteins named GRIP2 and GRIP3. This includes isolation of the complete gene (cDNA) and determining which 'part's of GRIP2 and GRIP3 interact with ER. AIM II was to determine whether GRIP2 and GRIP3 are necessary for ER action and if so to study the mechanism of their action. AIM III was based upon our previous discovery of a coactivator called GRlP1, which mediates the action of ER and other steroid receptors. The goal is to identify fragments of GRIP1 that specifically block ER action, preferably without affecting the action of other steroid receptors.

We determined that GRIP2 and GRIP3 are not natural genes, but cDNA fragments that fortuitously code for peptides that do not represent real cellular proteins. We therefore abandoned AIMS I and II, to focus on Aim III. We identified a specific small peptide from GRIPI that preferentially blocks binding of coactivators to the steroid receptor, ER better than to other steroid receptors. We also determined that multiple regions on the ER protein, one in the ERHBD and another in the N-terminal AF- 1 domain, participate in binding to GRIP1. Finally, we discovered a novel ER-related protein, estrogen receptor related protein 3 or ERR3, that can activate at least some of the same genes as ER, but does not require hormone. ERR3 may be present in large amounts in breast cancer cells and could theoretically cause estrogen independent growth of these cells.

The interaction between the ER and its coactivators potentially represent a new point of intervention to block ER-dependent growth of breast cancer cells. Exogenously supplied peptides or coactivator fragments that mimic the parts of these coactivators where ER binds may be able to block ER binding to the endogenous intact coactivators and thus block ER action. Inappropriate expression of ERR3 may represent a previously unknown mechanism whereby breast cancer cells become hormone independent.

Enhancement of Estrogen Receptor Transcriptional Activity by the coactivator GRIP1 highlinghts the role of activation function 2 in determining estrogen receptor pharmacology
Periodical:Journal of Biological Chemistry
Index Medicus: J Biol Chem
Authors: Norris JD, Fan D, Stallcup MR, and McDonnell DP
Yr: 1998 Vol: 273 Nbr: 12 Abs: Pg:6679-88

Estrogen recptor activation function 1 works by binding p160 coactivator proteins
Periodical:Molecular Endocrinology
Index Medicus: Mol Endocrinol
Authors: Webb P, Nguyen P, Shinsako J, Anderson C, Feng W-J, and Stallcup MR, et al
Yr: 1998 Vol: 12 Nbr: 10 Abs: Pg:1605-1618

GRIP1, a transcriptional coactivator for the AF-2 transactivation domain of steroid, thyroid, retinoid, and vitamin D receptors
Periodical:Molecular and Cellular Biology
Index Medicus: Mol Cell Biol
Authors: Hong H, Kohli K, Garabden MJ, and Stallcup MR
Yr: 1997 Vol: 5 Nbr: 17 Abs: Pg:2735-2744

Hormone independent transactional activation and coactivator binding by novel orphan nuclear receptor ERR3
Periodical:Journal of Biological Chemistry
Index Medicus: J Biol Chem
Authors: Hong H., Yang L., Stallcup MR
Yr: 1999 Vol: 274 Nbr: Abs: Pg:22618-26