Research Grants Awarded
ER-alpha36 and development of ER-negative breast cancer
Tumor Cell Biology I
Background: Mitogenic estrogen signaling is mediated by estrogen receptors (ERs). Currently, two ERs are identified, ER-alpha, and ER-beta, both of which are ligand inducible transcription factors. Recently, we have identified and cloned a 36-kDa novel variant of ER-alpha (ER-alpha36). ER-alpha36 is generated from a promoter located in the first intron of the original 66-kDa ER-alpha gene (ER-alpha66), and differs from the ER-alpha66 by lacking both transcriptional activation domains (AF-1 and AF-2). However, it retains the DNA-binding and partial dimerization and ligand-binding domains. ER-alpha36 mediates membrane-initiated estrogen signaling. ER-alpha36 expression was found in 21 of 35 (60%) human breast cancer specimens examined. Interestingly, ER-alpha36 was also found in 11 of 14 (79%) ER-negative breast cancer cases that lack the ER-alpha66 expression. ER-alpha36 was detected in established ER-negative breast cancer cell lines examined such as MDA-MB-231 cells, and estrogen stimulated cell proliferation of MDA-MB-231 cells. BRCA1 (BReast-CAncer susceptibility gene 1), a gene whose mutations are related mainly to ER-negative mammary tumorigenesis, negatively regulates ER-alpha36 expression. These preliminary results strongly suggested that ER-alpha36 plays an important role in mammary tumorigenesis, especially in initiation and progression of breast cancer previously characterized as ER-negative since they lack ER-alpha66 expression. Objective/Hypothesis: Based on our preliminary studies, we hypothesize that ER-alpha36 play an important role in the initiation and development of ER-negative breast cancer. We plan to investigate the functions of ER-alpha36 in mitogenic estrogen-signaling during ER-negative mammary tumorigenesis and progression of ER-negative breast cancer. Specific Aims/Study Design: Specific Aim 1: We aim to investigate the functions of ER-alpha36 in ER-negative mammary tumorigenesis caused by loss of BRCA1 function. We will specifically knock-down BRCA1 expression in normal mammary epithelial MCF10A cells using the siRNA knock-down approach and study the effects of the upregulated ER-alpha36 expression in these cells using in vitro and in vivo tumorigenesis assays. Specific Aim 2: We aim to investigate the functions of ER-alpha36 in malignant growth of established ER-negative breast cancer cells. We will study the function of ER-alpha36 in estrogen-stimulated malignant growth of established ER-negative breast cancer cells using in vitro growth and soft-agar assays, and in vivo tumor formation assay in nude mice combined with the siRNA knock-down approach. Potential Outcomes and Benefits of the Research: The findings that ER-negative breast cancer still express ER-alpha36 and ER-negative breast cancer cells MDA-MB-231, respond to mitogenic estrogen-signaling are significant and innovative. In addition, the finding that BRCA1 functions as a negative regulator of ER-alpha36 expression is also of biological significance. The proposed experiments will directly test our hypothesis that ER-alpha36-mediated estrogen signaling is involved in the initiation and development of ER-negative breast cancer. These experiments will provide important and unique information about the molecular mechanisms by which ER-negative breast cancer initiates and progresses. These experiments will also generate novel insights into the mechanisms by which dysfunctional BRCA1 mutations contribute to ER-negative mammary tumorigenesis. Although this proposal is focused on the function of ER-alpha36 in ER-negative breast cancer cells, the information generated from these studies will also advance our understanding of molecular and cellular events in estrogen-stimulated ER-positive breast cancer development. The results from these studies will greatly advance our progress in the prevention, detection, diagnosis and treatment of human breast cancer.
The roles of endogenous estrogens in breast cancer etiology have been supported by a number of studies in breast cancer patients and healthy women. It is believed that estrogens increase the rate of mammary cell growth and thus the risk of breast cancer. The growth-promoting function of estrogen signaling in breast cancer cells is mediated by specific structures called estrogen receptors (ERs). Depending on the existence of one of the ERs, human breast cancers are divided into ER-positive or ER–negative breast cancers. ER-positive breast cancer is less malignant and has a better prognosis. ER-negative breast cancer that accounts for about one third of breast cancers diagnosed, or approximately 65,000 a year in the United States, is more malignant and aggressive than ER-positive breast cancer. The cause of ER-negative breast cancer and effective therapies for ER-negative breast cancer are under-investigated areas in breast cancer research. BRCA1 (BReast-CAncer susceptibility gene 1) is a tumor suppressor gene that is mutated in the germ-line of more than half of the families with inherited breast and ovarian cancer. Most BRCA1-related breast cancers are ER-negative at the time of diagnosis, suggesting that the loss of BRCA1 function mainly contributes to tumorigenesis of ER-negative breast cancer. Over the last 10 years, much has been learned about the structure, functions and unique features of BRCA1 gene product, but it is still unknown why BRCA1 mutations only predispose to ER-negative breast cancer. Recently, we have discovered a novel estrogen receptor that functions differently from previously discovered estrogen receptors. More importantly, this novel estrogen receptor is found in 79% of the specimens examined from ER-negative breast cancer patients, suggesting that most ER-negative breast cancers retain responsiveness to growth-promoting signals from estrogens. Furthermore, BRCA1 negatively regulates expression of this novel estrogen receptor. We hypothesize that this novel estrogen receptor plays an important role in estrogen-stimulated development of both ER-positive and-negative breast cancers. In this proposal, we will investigate the function of this novel receptor in mammary tumorigenesis caused by BRCA1 dysfunction. We will also study the function of this novel estrogen receptor in estrogen-stimulated malignant progression of ER-negative breast cancer. The results generated from these studies will open a new avenue into the research area of ER-negative breast cancer. The proposed studies will provide important and unique information about the etiology and tumor biology of ER-negative breast cancer and about therapeutic potentials of antiestrogens to treat ER-negative breast cancer. Our studies will directly and greatly benefit ER-negative breast cancer patients in the aspects of diagnosis, prognosis and treatment. It is known that early detection is a key to the survival of breast cancer patients. Unfortunately, no definitive markers are available to detect ER-negative breast cancer at an early stage. Since the novel estrogen receptor we discovered is expressed in most ER-negative breast cancers examined, its expression levels could be used as a novel detection marker for ER-negative breast cancers and a predictor of how these tumors will respond to antiestrogen therapy. In addition, these experiments will also provide important and novel insights into the molecular mechanisms underlying BRCA1-related mammary tumorigenesis and progression, which will lay a foundation for designing non-invasive intervention for women carrying BRCA1 mutations. Finally, our studies will provide information to design better therapeutic approaches by targeting this novel estrogen receptor to treat human ER-negative breast cancer.