The synthetic oleanane triterpenoid CDDO-2P-Im binds GRP78/BiP to induce unfolded protein response-mediated apoptosis in myeloma.

Synthetic oleanane triterpenoids (SOTs) are small molecules with broad anti-cancer properties. A recently developed SOT, 1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]-4(-pyridin-2-yl)-1H-imidazole (CDDO-2P-Im or '2P-Im'), exhibits enhanced activity and improved pharmacokinetics over CDDO-Im, a previous generation SOT. However, the mechanisms leading to these properties are not defined. Here we show synergy of 2P-Im and the proteasome inhibitor ixazomib in human multiple myeloma (MM) cells, and 2P-Im activity in a murine model of plasmacytoma. RNA sequencing and quantitative reverse transcription PCR (qRT-PCR) revealed upregulation of the unfolded protein response (UPR) in MM cells upon 2P-lm treatment, implicating activation of the UPR as a key step in 2P-Im-induced apoptosis. Supporting this hypothesis, deletion of genes encoding either protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) or DNA damage-inducible transcript 3 protein (DDIT3; also known as CHOP) impaired the MM response to 2P-Im, as did treatment with ISRIB, integrated stress response inhibitor, which inhibits UPR signaling downstream of PERK. Finally, both drug affinity responsive target stability (DARTS) and thermal shift assays demonstrated direct binding of 2P-Im to endoplasmic reticulum chaperone BiP (GRP78/BiP), a stress-inducible key signaling molecule of the UPR. These data reveal GRP78/BiP as a novel target of SOTs, and specifically of 2P-Im, and suggest the potential broader utility of this class of small molecules as modulators of the UPR.