Research

Triple-negative breast cancers (TNBCs) are highly aggressive diseases that are more likely to metastasize to lungs and brain than other types of breast cancers. Various stresses, including glucose deprivation, hypoxia, and redox perturbation, cause accumulation of unfolded and misfolded proteins in the endoplasmic reticulum (ER), leading to ER stress. In response to ER stress, the cell activates the unfolded protein response (UPR) to restore protein folding homeostasis or induce apoptosis with prolonged ER stress. In mammalian cells, the UPR is divided into 3 primary signaling branches: (i) inositol requiring kinase 1 alpha (IRE1a) that activates XBP1, (ii) eukaryotic translation initiation factor 2-alpha kinase 3 (PERK) that activates activating transcription factor 4 (ATF4), and (iii) activating transcription factor 6 (ATF6) as both the sensor and the transcription factor. At the molecular level, the cytosolic domain of IRE1a has dual enzymatic activities, consisting of a serine/threonine kinase domain and a C-terminal ribonuclease (RNase) domain. When unfolded/misfolded proteins accumulate in the ER, IRE1a auto-phosphorylates and activates the RNase domain. The activated RNase domain excises a 26-bp intron from the XBP1 mRNA in metazoans and causes a translational frame-shift that results in the production of the spliced/activated form of XBP1 protein (XBP1s), an active transcription factor responsible for the induction of a specific set of target genes. In a recent study, the level of activated XBP1 was shown to be higher in TNBC cell lines and primary TNBC patient samples than in luminal breast cancer cell lines and ER⁺/PR⁺ patient samples, respectively. Furthermore, knockdown of XBP1 expression in TNBC cells attenuated colony-forming ability in soft agar and in vitro invasiveness, as well as in vivo xenograft tumor growth and metastasis to lung. The same investigators reported that a 70-gene signature was directly upregulated by XBP1 and highly correlated with a hypoxia-derived gene signature in TNBC. We found that, TNBC patients with higher expression of the XBP1 or HIF-1a gene signature had worse relapse-free survival (RFS) but this correlation is missing in non-TNBC patients. This correlation is unique to the IRE1a/XBP1 branch of the UPR, as an ATF4/CHOP gene signature is not associated with patient survival at all. These studies demonstrate a critical role for XBP1 in TNBC tumorigenesis, progression and relapse after treatment, suggesting that pharmacological inhibition of this pathway would be a sound therapeutic strategy for TNBC patients.

Several groups, including our laboratory, identified small molecule inhibitors that selectively block IRE1a-mediated XBP1 activation. Their inhibitory activity against XBP1 splicing and efficacy in suppressing tumor cell growth were tested extensively in vitro and in vivo. In general, the XBP1 splicing activity of IRE1a can be inhibited through targeting (i) the catalytic core of the RNase domain and in some cases (ii) the ATP binding site of the kinase domain.