Signal transducer and activator of transcription 3 (STAT3) is a transcription factor constitutively activated in many cancer types, and has been identified as a major mediator of cancer stemness (Li, PNAS 2015). The Nrf2 (nuclear factor erythroid 2 [NF-E2]-related factor 2) is the reactive oxygen species (ROS) sensor and a master regulator of antioxidant gene expression. NRF2 has recently been implicated in contributing to cancer stem cell phenotypes, chemoresistance, and is associated with poor clinical prognosis. Hypoxia is considered a major feature of the tumor microenvironment as it can promote tumor progression, increased stemness characteristic, and resistance to conventional therapeutic intervention. Hypoxia can regulate a series of stress-sensor transcription factors, notably, the reactive oxygen species (ROS) sensor and response modulator NRF2.

We studied NRF2 transcript and protein levels in breast cancer cell lines MCF7 and AU565 cells under both hypoxic and normoxic conditions. Our data shows that in response to STAT3 pathway activators, NRF2 is upregulated at both mRNA and protein levels under hypoxia and this upregulation is STAT3 dependent. NRF2 ChIP-Seq experiments demonstrate that both hypoxia and STAT3 stimulation can modulate NRF2 binding in MCF-7 breast cancer cells. Surprisingly, under hypoxic conditions, STAT3 activation dramatically enhances (10 fold) NRF2 binding sites across the genome. Downstream analysis of NRF2 target genes revealed a significant change from stress response to activation of gene signaling pathways involved in stemness-high cancer cells and metastatic cells under both hypoxic and STAT3 stimulation. Finally, we investigated whether the stemness inhibitor BBI-608 (Napabucasin) could reduce chemotherapy-induced NRF2 upregulation. Our results indicate that STAT3 modulates NRF2 levels and transcriptional activity, and that stemness inhibitor BBI-608 can prevent this upregulation. Our data revealed STAT3-NRF2-Hypoxia as a novel reinforcing regulatory mechanism for promoting cancer stemness and chemoresistance.

Citation Format: Luz Elisa Tavera, Karen Simon, Juying Li, Katherine Geromini, Zhuo Zhang, Sarah Keates, Harry A. Rogoff, Chiang J. Li. Identification of STAT3-NRF2-hypoxia as a novel reinforcing mechanism for promoting cancer stemness [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-143. doi:10.1158/1538-7445.AM2017-LB-143