Estrogen Receptor α and β signaling in breast and lung cancers

Proc Amer Assoc Cancer Res, Volume 46, 2005 SY14-4 Tamoxifen, a selective estrogen receptor modulator (SERM), has been one of the most widely used and effective drugs for treatment and prevention of breast cancer. However there are a substantial number of breast cancer patients (30%) who fail to respond to tamoxifen even if they have estrogen receptor (ER+), or who become resistant to the effects of tamoxifen after initial response. Insight into the basis for such resistance to the potential benefits of tamoxifen would be expected to lead to novel strategies or drugs to address the problem. Effective tamoxifen therapy requires a functional ER, yet the predominant clinical use of immunohistochemistry for ER is an imprecise determination of receptor status: the receptor protein may be present but have mutations that inactivate or alter its function or half-life. The p160 coactivator proteins function to amplify transcription via ER at estrogen responsive genes and are a prime target of proven SERM inhibition. SERMs block the function of ER by misfolding the AF-2 transcriptional activation region in the ligand binding domain, altering its association with coactivator proteins. We have made use of a unique approach to detect functional changes that alter the dominant AF-2 activity of ER in selected breast cancers and tissues. Such changes will likely affect SERM efficacy, subsequent coactivator protein interactions and transcription, and will delineate one type of antiestrogen resistance. We have screened and characterized several mutations at the p160 coactivator-binding site of ERα from human breast cancers that alter tamoxifen activity. One mutation actually makes tamoxifen a better agonist than estradiol for that ER. These mutations may alter the ability of tamoxifen to inhibit ER induced growth. In addition to its role in breast cancer, data from lung cancers and precursor lesions in females have suggested that estrogen receptor (ER) expression and signaling may play a significant role in tumor development and/or proliferation in this non-classical target tissue. Presence of ER has proven valuable as a predictor of breast cancer responsiveness to hormone therapy. Additionally, presence and abundance of specific coregulator gene products are proposed to explain tissue-specific ligand differences, e.g. tamoxifen antagonism in breast and partial agonism in endometrium. Therefore expression of functional ER in lung tissues would suggest effective tumor inhibition by SERMs, an unexplored treatment paradigm for lung cancer. We initiated studies to identify relevant estrogen receptors and cofactors in lung cancers and have found that ERβ and GRIP1/TIF2 are most prevalent. We have also performed gene array experiments to understand the pertinent stimulated products of estrogenic signals in lung cells and lines and if SERMs inhibits those genes. We have found several genes that regulate growth control and are differentially regulated by estrogen and SERMs. These studies should further mechanistic understanding of antiestrogen usefulness in lung cancers with a goal to improved therapy. ERα and ERβ are encoded by distinct genes. Although ERα and ERβ display striking sequence homology in their DNA binding domains, they are only 50% identical in their hormone binding domains and 37% identical at the amino terminus. ERα and ERβ show differential tissue distribution, affinity for p160 coactivators, and transcriptional responses. As a result of these differences, the overall response of a cell expressing ERα and/or ERβ to a particular ligand cannot be simply predicted. Receptor subtype selective ligands would be expected to enhance understanding of the relative biological roles of the two ERs. We have characterized a new set of dichloro-triarylcyclopropanes (DTAC) compounds that have antiestrogenic character and may show ER subtype specificity. We have demonstrated that DTAC compounds function as antiestrogens at the level of transcriptional regulation, effectively blocking the stimulatory effects of E2 in the presence of ERα (Cheng et al., 2004, Mol. Pharm., 66:970-7). Because of the potential differences in ERα and ERβ activity noted above, a series of studies was conducted to evaluate the behavior of DTAC compounds in the presence of ERβ. The ability of DTACs to modulate ERβ-mediated transcription from either estrogen response elements (EREs) or AP-1 elements was evaluated and compared to activities with ERα. Several compounds showed a selective profile with ERβ while others showed an ERα preference. Taken together, these data indicate that the structural determinants of DTAC stimulation of ER-mediated transcription are (a) different for ERα and ERβ and (b) different at EREs and AP-1 sites. These studies were supported by NIH grants CA87414, DK59516 and DOD Breast Cancer Program grant (DAMD17-01-1-0378) to MN and NIH P50CA90440, SPORE in Lung Cancer to the University of Pittsburgh.