Targeting nuclear receptors for the treatment of human breast cancers

Breast cancer is the leading type of cancer in women, accounting for 25.2% of all cases worldwide. Breast cancer is biochemically characterized into three therapeutic groups -- estrogen receptor (ER) positive, human epidermal growth factor receptor 2 (HER2)/ERBB2 amplified and triple negative breast cancer (TNBC). In particular, TNBC lacks the expression of ER, progesterone receptor (PR) and HER2. Due to the lack of molecular targets, chemotherapy is the primary treatment option for patients with TNBC. Identifying additional cellular targets in TNBC will improve the diagnosis and treatment for these patients.;Nuclear receptors (NRs) are ligand-inducible transcriptional factors. They play essential roles in cell differentiation, proliferation, apoptosis and homeostasis. Although ERs are the main targets in breast cancer therapy, emerging evidence suggest that some orphan receptors are expressed in TNBC and could be developed as molecular targets. Photoreceptor cell-specific nuclear receptor (PNR), an orphan nuclear receptor, was reported to regulate ERalpha gene expression. In order to assess the ERalpha-independent role of PNR, PNR was overexpressed in two ERalpha negative breast cancer cell lines, MDA-MB-231 and LM2. Our studies demonstrate that PNR enhances the migration of LM2. Furthermore, PNR stimulates tumor growth and induces lung metastasis in the xenograft mouse model, and the migration and metastasis effects are mediated by IL-13Ralpha2. Further molecular analyses reveal that PNR directly binds to the IL-13Ralpha2 promoter to activate gene transcription. The increase of IL-13Ralpha2 protein level in conjunction with IL-13 treatment activate downstream MAPK-ERK pathway, which leads to breast cancer cell migration and metastasis. This work implies that interfering with this signaling axis may have therapeutic value in ERalpha-negative breast cancer patients.;Seventy percent of breast cancer depends on estrogen for growth. The biological effects of estrogen are transduced by two estrogen receptors (ERs): ERalpha is proliferative while ERbeta is anti-proliferative. Multiple studies have shown that the expression of the two ER subtypes is imbalanced in breast cancers. To restore the balance of the ER protein levels and activities, we screened a small compound library for ERbeta agonists and stabilizers by luciferase reporter assay and western blot using the isogenic Hs578T-ERalphaLuc and Hs578T-ERbetaLuc inducible cell lines. Diptoindonesin G (Dip G) was identified in the initial screen. It reciprocally regulates ER stability and activity by stabilizing ERbeta proteins while destabilizing ERalpha proteins, thus activating ERbeta function and simultaneously suppressing ERalpha activity in breast cancer cells. We further demonstrate that Dip G targets CHIP E3 ligase at different protein domains to differentially modulate ERalpha and ERbeta stabilities. A structure-activity relationship (SAR) analysis was also carried out to comprehensively assess the functional module of this small molecule. Because Dip G can reciprocally regulate ERalpha and ERbeta expression levels and activities, Dip G may be developed as a pro-drug for treatment of breast cancer.