Antiproliferative action of estrogen receptor beta and Hes-1 in breast cancer

<p>Breast cancer is the most common malignancy among Swedish women. Although the mechanism behind the tumorigenesis remains unclear, estrogen receptor a (ERa) plays an important role in the progression of breast cancer and is regarded as a target for endocrine therapy. In this thesis, focus is on the second estrogen receptor, ERbeta and its function in breast cancer. In addition, the significance of the transcription factors Hes-1 and Hes-6 in breast cancer and their relation to ERa has been studied.</p><p>By using T47D breast cancer cells with inducible ERbeta expression, the role of ERbeta has been characterised with respect to proliferation and cell-cycle regulation. In contrast to ERa, expression of ERbeta inhibited the proliferation of 17beta-estradiol (E2) treated breast cancer cells and caused significantly changed levels of cell-cycle regulators. In response to ERbeta expression, the levels of the Cdk2-activating phosphatase Cdc25A as well as cyclin E and E2F1 were reduced with a subsequent decrease of the Cdk2-activity. Moreover, expression of ERbeta reduced the number of tumor associated blood vessels as well as tumor volume in a mouse xenograft model. In addition to the anti-tumorigenic effects, expression of ERbeta reduced the levels of secreted growth factors in vitro as well as in vivo.</p><p>The transcriptional repressor Hes-1 has been described as an essential factor during embryonic development. However, Hes-1 is also important in breast cancer cells where it inhibits proliferation. Downregulation of Hes-1 by ERa is a crucial step in E2 stimulated proliferation of breast cancer cells. We expressed Hes-1 in breast cancer cells to study the mechanism behind its antiproliferative properties upon E2 treatment. Hes-1 expression induced a G1 cell-cycle phase arrest and a concomitant reduction of the E2F1-level. By real-time quantitative PCR and electrophilic mobility shift assay, we can conclude that Hes-1 inhibits E2F1 at the promoter level.</p><p>Hes-6 is an inhibitor of Hes-1 and has been associated with tumorigenesis and metastasis. When Hes-6 was expressed in T47D breast cancer cells, proliferation was increased as well as tumor growth in immunodeficient mice. Furthermore, E2F1 was identified as an important target gene, induced by Hes-6 in breast cancer cells.</p><p>In conclusion, these studies have significantly contributed to the knowledge of estrogen receptor function in breast cancer as well as to elucidate important roles of Hes-1 and Hes-6 in estrogen signalling.</p><h3>List of scientific papers</h3><p>I. Ström A, Hartman J, Foster JS, Kietz S, Wimalasena J, Gustafsson JA (2004). Estrogen receptor beta inhibits 17beta-estradiol-stimulated proliferation of the breast cancer cell line T47D. Proc Natl Acad Sci U S A. 101(6): 1566-71. Epub 2004 Jan 26 <br><a href="https://pubmed.ncbi.nlm.nih.gov/14745018">https://pubmed.ncbi.nlm.nih.gov/14745018</a><br><br></p><p>II. Hartman J, Lindberg K, Morani A, Inzunza J, Ström A, Gustafsson JA (2006). Estrogen receptor beta inhibits angiogenesis and growth of T47D breast cancer xenografts. Cancer Res. 66(23): 11207-13. <br><a href="https://pubmed.ncbi.nlm.nih.gov/17145865">https://pubmed.ncbi.nlm.nih.gov/17145865</a><br><br></p><p>III. Hartman J, Müller P, Foster JS, Wimalasena J, Gustafsson JA, Ström A (2004). HES-1 inhibits 17beta-estradiol and heregulin-beta1-mediated upregulation of E2F-1. Oncogene. 23(54): 8826-33. <br><a href="https://pubmed.ncbi.nlm.nih.gov/15467735">https://pubmed.ncbi.nlm.nih.gov/15467735</a><br><br></p><p>IV. Hartman J, Lam EWF, Gustafsson JÅ, Ström A (2008). Hes-6, an inhibitor of Hes-1, is regulated by 17beta-estradiol and increases the malignant behaviour of breast cancer cells. [Submitted]</p>