BRCA1 Modulates Xenobiotic Stress-inducible Gene Expression by Interacting with ARNT in Human Breast Cancer Cells

Abstract

In the present study, we have investigated the effects of BRCA1 on xenobiotic stress-inducible gene expression. In response to aryl hydrocarbon receptor (AhR) ligands, cytoplasmic AhR becomes activated and then translocates to the nucleus where it forms a complex with the aryl hydrocarbon receptor nuclear translocator (ARNT). Subsequently, the AhR⅐ARNT complex binds to the enhancer or promoter of genes containing a xenobiotic stress-responsive element and regulates the expression of multiple target genes including cytochrome P450 subfamily polypeptide 1 (CYP1A1). In this study, we have found that endogenous and overexpressed exogenous wild-type BRCA1 affect xenobiotic stress-induced CYP1A1 gene expression. Using a standard chromatin immunoprecipitation assay, we have demonstrated that BRCA1 is recruited to the promoter regions of CYP1A1 and CYP1B1 along with ARNT and/or AhR following xenobiotic exposure. Our findings suggest that BRCA1 may be physiologically important for mounting a normal response to xenobiotic insults and that it may function as a coactivator for ARNT activity. Using immunoprecipitation, Western blotting, and glutathione S-transferase capture assays, a xenobiotic-independent interaction between BRCA1 and ARNT has been identified, although it is not yet known whether this is a direct or indirect interaction. We have also found that the inducibility of CYP1A1 and CYP1B1 transcripts following xenobiotic stress was significantly attenuated in BRCA1 knockdown cells. This reduced inducibility is associated with an altered stability of ARNT and was almost completely reversed in cells transfected with an ARNT expression vector. Finally, we have found that xenobiotic (TCDD) treatments of breast cancer cells containing reduced levels of BRCA1 cause the transcription factor ARNT to become unstable.Inherited mutations in the breast cancer susceptibility gene BRCA1 confer increased risk of breast and ovarian cancer (1, 2). In addition, because BRCA1 expression is often decreased or even absent in sporadic breast and ovarian cancer, abnormal BRCA1 expression may also have a role(s) in nonhereditary tumors (3, 4). Although these observations indicate that BRCA1 may act as a tumor suppressor in breast cancer, the specific function(s) of BRCA1 that could have this effect is still not completely understood. However, numerous studies have shown that BRCA1 regulates various pivotal cellular processes, such as cell cycle progression, DNA repair, apoptosis, and transcription, and many of the mechanisms involved have been identified (see Ref. 5 for a review). Recent studies show that BRCA1 proteins are also required for maintaining chromosome stability by regulating centrosome duplication and mitotic spindle checkpoints (6 -8).Although BRCA1 regulates transcription, the mechanisms involved are unlike classical transcriptional factors that directly bind DNA sequences. Rather, BRCA1 regulates transcription via protein-protein interactions. The BRCA1 C-terminal activation domain is responsible for transactivation in ye...

Cobaltous chloride and hypoxia inhibit aryl hydrocarbon receptor-mediated responses in breast cancer cells

“…Moreover, knockdown of constitutively-expressed BRCA1 decreases Ah-responsiveness in human breast cancer cells (Kang, Kim, Kim, Barouki, Cho, Khanna, Rosen, and Bae, 2006). Results in Figure 5B show that hypoxia (24 hr) decreases BRCA1 and this is also accompanied by decreased induction of CYP1A1 by TCDD, suggesting that at this later time point, loss of Ah-responsiveness may be due, in part, to loss of BRCA1 expression.…”

“…A recent study showed that BRCA1 interacts with Arnt and, in breast cancer cells transfected with a DRE-luc construct, overexpression of BRCA1 (transfected) enhanced Ahresponsiveness; moreover, knockdown of constitutively expressed BRCA1 breast cancer cells decreased induction of luciferase activity by TCDD (Kang et al, 2006). Moreover, since hypoxia decreases BRCA1 expression (Bindra et al, 2005), we further investigated the effects of normoxia and hypoxia on CYP1A1 and BRCA1 protein expression in ZR-75 cells (Figs.…”

“…Hypoxia conditions could deplete critical coregulatory proteins required for mediating AhR-dependent activity, and there is evidence for cofactor competition resulting from simultaneous activation of more than one ligand-activated receptor (Meyer et al, 1989). BRCA1 has previously been characterized as a coregulator of AhRmediated responses (Kang, Kim, Kim, Barouki, Cho, Khanna, Rosen, and Bae, 2006) and therefore, hypoxia-induced downregulation of BRCA1 after 24 hr (Fig. 5B) contributes to the loss of Ah-responsiveness in ZR-75 cells.…”

Bayesian Weibull tree models for survival analysis of clinico-genomic data

“…This metagene includes multiple probes for gene CYP1B1; the enzyme encoded by this gene is involved in androgen metabolism and the metabolism of various procarcinogens. CYP1B1 has been associated with risk of endometrial cancer [55] and breast and ovarian cancer as a downstream target of BRCA1 expression during xenobiotic stress [36]. Second, the key variables which appear in the CG trees from run 4 appear in Fig.…”

The correlations between BRCA1 defect and environmental factors in the risk of breast cancer

“…Although major function of BRCA1 is known as a classical tumor suppressor gene, we have demonstrated that BRCA1 regulates transcription of phase I and II enzymes upon exposure to various exogenous stresses (Kang et al, 2006(Kang et al, , 2008a(Kang et al, and 2008b. BRCA1 can stimulate antioxidant gene expression and modulate intracellular reactive oxygen species (ROS) levels through enhancing the activity of the antioxidant response transcription factor, NRF2 (Bae et al, 2004;Kang et al, 2012).…”