Regulation of IRE1α by the small molecule inhibitor 4μ8c in hepatoma cells.

The unfolded protein response (UPR) is activated in response to impairments of the folding environment in the endoplasmic reticulum (ER). The most conserved arm of the UPR, inositol-requiring ER-to-nucleus signaling protein (IRE1α), has been linked to the regulation of a diverse array of cellular processes including ER-associated degradation, inflammatory signaling, cell proliferation and membrane biogenesis. Recent studies have utilized the selective, small molecule inhibitor, 4μ8c, to examine the role of IRE1α endoribonuclease (RNase) activity in various cell types including multiple myeloma, mouse embryonic fibroblasts and pancreatic beta cells [1-5]. In the present study we utilized this inhibitor to examine the role of IRE1α RNase activity in hepatoma cells (H4IIE), specifically concentrating on cell proliferation and the identification of potential off target effects under both unstressed and stressed conditions. Experiments were performed in H4IIE hepatoma cells in the absence (control conditions (LG)) or presence (LG + Thapsigargin (Thap)) of ER stress. The presence of 4μ8c decreased IRE1α RNase activity, based on reduced splicing of X-box binding protein-1 (XBP1s) and regulated IRE1α-dependent decay of mRNA in both treatments and at concentrations ranging from 10-90 μM. Cell proliferation was significantly reduced at higher concentrations (> 60 μM 4μ8c) in unstressed cells and displayed a dose-response relationship with 4μ8c in both treatments. The presence of 4μ8c did not promote cytoxicity in either of the treatment conditions but higher concentrations of the inhibitor (60 μM) were associated with apparent off-target or compensatory responses that were not observed at 10 μM. Overall, the small-molecule inhibitor, 4μ8c is an effective inhibitor of IRE1α RNase activity in H4IIE cells. Potential off-target effects associated with this inhibitor require the use of multiple inhibitor concentrations in all experiments.