The Hsp90 inhibitor SNX-2112 induces apoptosis of human hepatocellular carcinoma cells: the role of ER stress.

Heat shock protein 90 (Hsp90) has been predicted to be involved in hepatocellular carcinoma (HCC) therapy; however, the mechanisms of action remain elusive. SNX-2112 is an Hsp90 inhibitor showing broad antitumor activity. Here we aim to determine the role of the endoplasmic reticulum (ER) stress in SNX-2112-induced apoptosis in HCC cells. In general, three HCC cells (i.e., HepG2, Huh7, and SK-Hep1) were used in our experiments. The cell viability was determined by the CCK-8 assay. The apoptosis was analyzed using flow cytometry, laser scanning confocal microscopy (LSM) and Western blotting. The efficacy and mechanisms of action of SNX-2112 were also evaluated in a mouse xenograft model. We found that SNX-2112 showed stronger inhibition on cell growth than 17-AAG, a classical Hsp90 inhibitor. SNX-2112 treatment led to the caspase-dependent apoptosis. Interestingly, SNX-2112 decreased the expression levels of the ER chaperone proteins calnexin and immunoglobulin binding protein (BiP). It also inhibited all three ER stress sensors, namely, inositol-requiring gene 1 (IRE1), PKR-like ER kinase (PERK), and activating transcription factor 6 (ATF-6) in vitro and/or in vivo. However, the ER stress inducer tunicamycin strongly enhanced SNX-2112-induced apoptosis, whereas the IRE1 knockdown did not. Taken together, we for the first time indicated the possible apoptotic pathways of SNX-2112 in HCC cells, raising the possibility that the induction of ER stress might be favorable for SNX-2112-induced apoptosis.