Abstract P5-07-05: Regulation of system Xc- by signal transducer and activator of transcription 3 and 5 in human breast cancer cells
Authors: Linher-Melville Katja , Fazzari Jennifer , Gunning Patrick , Singh Gurmit 
Published: May 2015

In order to survive and proliferate, cancer cells adapt to high levels of oxidative stress by countering the accumulation of reactive oxygen species (ROS) with an increased production of intracellular antioxidant molecules such as glutathione. The cell surface transport system Xc- is a cystine/glutamate antiporter that exports glutamate while importing cystine, thereby mediating levels of cysteine required for glutathione synthesis and the maintenance of cellular redox balance. Transcription factors that regulate key antioxidant defense mechanisms, including system Xc-, may therefore be of therapeutic interest. Recently, signal transducer and activator of transcription (STAT) proteins have emerged as potential targets for the development of novel anti-cancer therapies. In particular, inhibitors of STAT3 and STAT5 may become clinically relevant as anti-cancer agents for breast and brain cancer, as well as leukemia. Interestingly, suppression of STAT3 has been linked with increases in ROS and the induction of apoptosis. Upon activation by phosphorylation, dimerization, and nuclear translocation, STAT proteins transcriptionally regulate diverse target genes by binding to promoter regions containing gamma-activated site (GAS) motifs. The human xCT (SLC7A11) gene encodes the functional subunit of system Xc-. We provide evidence that expression of xCT is regulated by STAT3, and potentially also STAT5, affecting antiporter function in both MCF-7 and MDA-MB-231 human breast cancer cells. Computational analysis of the xCT promoter region revealed the presence of a distal GAS site. Its truncation significantly increased luciferase activity in a reporter assay, with similar increases obtained after treating cells transfected with the full-length xCT promoter construct with various STAT3/5 pharmacologic inhibitors. Knock-down of STAT3 or STAT5A using specific siRNAs produced similar results, suggesting that these STAT proteins act in a transcriptionally repressive manner. We also demonstrated binding of STAT3 and STAT5A to the xCT promoter in MDA-MB-231 cells, which was disrupted by preincubating the cells with specific inhibitors. xCT mRNA and protein levels increased significantly following treatment with STAT3/5 inhibitors. Pharmacologically suppressing STAT3/5 activation also significantly increased glutamate release and total levels of intracellular glutathione. We hypothesize that blocking STAT3/5-mediated signaling induces an adaptive, compensatory mechanism that protects breast cancer cells from ROS by up-regulating Xc- antiporter expression and function. Our findings suggest that targeting system Xc- may synergize with STAT3/5 inhibitors, heightening their therapeutic anti-cancer effects, particularly in conjunction with traditional chemotherapy treatments.

This work is supported by the Canadian Breast Cancer Foundation (CBCF).

Citation Format: Katja Linher-Melville, Jennifer Fazzari, Patrick Gunning, Gurmit Singh. Regulation of system Xc- by signal transducer and activator of transcription 3 and 5 in human breast cancer cells [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P5-07-05.