Unfolded Protein Response in Cancer: IRE1α Inhibition by Selective Kinase Ligands Does Not Impair Tumor Cell Viability

Abstract

The kinase/endonuclease inositol requiring enzyme 1 (IRE1α), one of the sensors of unfolded protein accumulation in the endoplasmic reticulum that triggers the unfolded protein response (UPR), has been investigated as an anticancer target. We identified potent allosteric inhibitors of IRE1α endonuclease activity that bound to the kinase site on the enzyme. Structure-activity relationship (SAR) studies led to 16 and 18, which were selective in kinase screens and were potent against recombinant IRE1α endonuclease as well as cellular IRE1α. The first X-ray crystal structure of a kinase inhibitor (16) bound to hIRE1α was obtained. Screening of native tumor cell lines (>300) against selective IRE1α inhibitors failed to demonstrate any effect on cellular viability. These results suggest that IRE1α activity is not essential for viability in most tumor cell lines, in vitro, and that interfering with the survival functions of the UPR may not be an effective strategy to block tumorigenesis.

Targeting ABL-IRE1α Signaling Spares ER-Stressed Pancreatic β Cells to Reverse Autoimmune Diabetes

“…But recently a mono-selective IRE1α inhibitor–compound 18 (Figure 6A)–that possesses all the properties of a KIRA, and will be referred to as KIRA8 henceforth, was described (Harrington et al, 2015). Specifically, KIRA8 blocks IRE1α* oligomerization, and potently inhibits IRE1α* RNase activity against XBP1 and Ins2 RNAs (Figure 6B–E).…”

“…ESI-MS: m/z = 601.3 [M+H] + (consistent with previously described characterization. Compound #18 in (Harrington et al, 2015)).…”

Long-Range Inhibitor-Induced Conformational Regulation of Human IRE1αEndoribonuclease Activity

“…Thus, it is possible that KIRA6 invokes a similar conformational change when bound to IRE1a. Similarly, another inhibitor recently described by Amgen (Thousand Oaks, CA) potently inhibited IRE1a kinase and RNase activity and was cocrystallized with a dephosphorylated IREa monomer (Harrington et al, 2014). The lack of IRE1a dimer structure here could be due to the compound either preventing dimerization or stabilizing a monomeric form of IRE1a.…”

“…Specifically, the IRE1a/XBP 1 pathway has shown to be overexpressed in a variety of human cancers, and activation of the pathway has been shown to be essential for the survival of highly secretory multiple myeloma cells, where the protein load is high (Feldman et al, 2005;Koong et al, 2006;Carrasco et al, 2007). These key findings, coupled with the possibility to modulate IRE1a activity via targeting two potentially druggable domains, has made IRE1a a very attractive target in drug discovery (Hetz et al, 2013;Harrington et al, 2014;Maly and Papa, 2014;Sanches et al, 2014;Joshi et al, 2015).…”

“…More recently, a few other dephosphorylated human crystal structures were reported. One is of a cocrystal structure containing a kinase domain inhibitor bound to an IRE1a monomer (PDB ID 4U6R; Harrington et al, 2014). Here, no dimers were found with the inhibitor-bound structure, suggesting that the compound may either prevent dimerization or stabilize a monomeric IRE1a.…”