An Approach to Antiestrogen Resistance in Breast Cancer

Institution: University of California, San Diego
Investigator(s): Oksana Tyurina, Ph.D. -
Award Cycle: 2005 (Cycle 11) Grant #: 11FB-0081 Award: $132,860
Award Type: Postdoctoral Fellowship
Research Priorities
Detection, Prognosis and Treatment>Innovative Treatment Modalities: search for a cure

Initial Award Abstract (2005)
Since estrogen is important for breast cancer development and progression, hormone therapy has been a major direction in cancer treatment for decades. These approaches include anti-estrogens (tamoxifen), aromatase inhibitors (anastrozole and letrozole), and luteinizing hormone-releasing hormone agonist (goserelin). Up to 50-70% of invasive breast cancers are positive for estrogen receptor (ER), but only 50-60% of these patients responded to tamoxifen treatment and most of them developed resistance to the drug. The nature of this resistance is not clear and most likely includes multiple factors such as alterations in structure and function of ER, activation of growth factor-related pathways as well as changes of levels and activity of multiple ER cofactors.

In this project, we are investigating a novel hormonal signaling pathway that involves the interaction of ER, its cofactors and N-CoR transcriptional machinery in order to mediate the inhibitory effects of estrogen receptor (ER) responsive genes (SERMs). We plan uncover the role of specific factor recruited into the inhibiting the co-repressor complex for maintaining SERM inhibitory function and attempt to devise a strategy to prevent resistance from developing in response to antiestrogen treatment. The techniques to be used include “ChIP-on-chip” assays and single cell nuclear microinjection. Chromatin immunoprecipitation, or ChIP, refers to a procedure used to determine whether a given protein binds to a specific DNA sequence in vivo. Our specific hypothesis is that inflammatory cytokines, such as IL-1? causes the specific export of a N-CoR/TAB2 co-repressor complex. The net effect is that under certain circumstances in breast cancer, IL-1? will act to convert ER antagonists, such as tamoxifen, into agonists.

We anticipate that this project will shed new light on the various ER and SERM exchange factors and why anti-ER therapies often either fail to work or patients develop resistance. The elucidation of the regulation of novel ER antagonist actions, and a specific strategy that permits conversion of ER antagonist to an agonist function provides an additional approach in designing therapies that should be more effective and applicable to individuals exhibiting resistance.

Final Report (2008)
In recent decades the studies of finding of a cure of breast cancer produced several successful drugs, including tamoxifen that blocks estrogen receptor (ER) signaling. The drawback was that substantial numbers of patients develop resistance to the drug. Our studies have been directed toward understanding the mechanism of this resistance. We identified major players involved, namely, N-CoR/TAB2 repressor complex occupying the promoter of the ER-regulated pS2 gene in presence of ER antagonist, 4-OHT, as well as MEKK1 protein kinase acting on TAB2. Upon pro-inflammatory cytokine IL-1beta stimulation of cells, MEKK1 phosphorylates TAB2 and potentially causes a removal of the repressor complex from pS2 promoter and promotes its expression activation.

Similar events are happening at the ER-negatively regulated promoters, like BMP7 (bone morphogenetic protein 7). The repressed BMP7 by E2 (estradiol) stimulation gets activated with exposure to IL-lbeta. We have shown that an H/LXXAXXXXLL motif (LX7LL helix) in the N-terminus of ER is important in binding of ER to the TAB2 protein.

Next, we took a new direction in studying of ER signaling mechanisms. The new goal is to identify the mechanisms of ER regulation on epigenetic level. Methylation/demethylation of histone DNA among other DNA modifications has been shown to be important in gene activation. During last year we have screened known and novel genes containing Jumonji (Jmj) C domain as potential histone demethylases for their role in ER target gene activation. By comparing the siRNA screen in MCF7 cells revealed a few novel genes that could be involved in ER signaling.

Currently we are validating our findings by Chromatin Immuno Precipitation (ChIP) assay. In the future I plan to continue research on JmjC containing enzymes regulating E2 response in MCF7 cells. I will use biochemical and molecular approaches to find which histone lysine/argentine positions these enzymes de-methylate and the mechanisms that regulate their functions. Also I will use model systems, including zebrafish and/or mouse, to find biological roles of these proteins.