Abstract 2926: Selective PERK kinase inhibition triggers a biphasic concentration-dependent induction of ER stress in multiple myeloma and B-cell lymphoma

Plasma cell malignancies, such as multiple myeloma, are characterized by an extensively developed endoplasmic reticulum (ER) to accommodate the high secretion rate of immunoglobulins. An adaptive stress response mechanism, termed the unfolded protein response (UPR), is markedly elevated in multiple myeloma cells to ensure a homeostatic balance between ER burden and ER capacity. One branch of the UPR consists of the activation of PERK under chronic ER stress: PERK phosphorylates and impairs the function of the translation initiation factor, eIF2α, thereby downregulating global protein synthesis, and reducing the secretory burden on the ER. To test the therapeutic potential of PERK inhibitors in multiple myeloma, we identified two chemically diverse PERK inhibitors: both are sub-nM inhibitors of PERK, and have a >100-fold window against other kinases (including other eIF2α kinases). These compounds inhibit phosphorylation of eIF2α at 10-20nM (IC50) in HEK293 cells, incubated with the ER stressor tunicamycin. Both PERK inhibitors were selectively anti-proliferative in an ER-stressed epithelial cancer model (A549 cells with tunicamycin) at nM concentrations, but not in the absence of ER stress. Furthermore, in the absence of an exogenous ER stressor, both PERK inhibitors induced ER stress (eg, as evidenced by induction of the pro-apoptotic CHOP gene) selectively in multiple myeloma cell lines (and certain B-cell lymphoma lines) at low nM concentrations but not in normal or malignant epithelial cells (induction of CHOP at 10 μM). The magnitude of this induction was comparable to well-established ER stressors, such as bortezomib or tunicamycin, and correlated closely with reduced proliferation in malignant B-cell lines. The potent induction of CHOP occurred both in vitro and upon oral dosing of mice, with a xenografted multiple myeloma tumour (JIM-1); however, in both instances, the induction of ER stress was maximal at a dose corresponding to approximately 50-75% inhibition of PERK, whereas, remarkably, at dose levels corresponding to more complete PERK inhibition (as evidenced by inhibition of P-eIF2α), CHOP induction (and ER-stress induction in general as determined by a genome-wide expression profile) was reduced, eventually down to baseline levels. Consistently, both PERK inhibitors resulted in a biphasic concentration-dependent reduction of proliferation of certain myeloma and B-cell lymphoma lines. Hence, PERK inhibition triggers a biphasic induction of ER stress and inhibition of cell proliferation, selectively in B-cell malignancies. The possible underlying mechanism as well as the therapeutic implications of these findings will be discussed. Citation Format: Matthias Versele, Tamara Geerts, Jeroen Van De Ven, Ilse Van den Wyngaert, Peter Vermeulen, Inge Beerden, Danielle Peeters, Johnny Liebregts, Kurt Van Baelen, Cedric Simillion, Boudewijn Janssen, Tinne Verhulst, Norbert Esser, Christophe Meyer, Ian Stansfield, James Bischoff. Selective PERK kinase inhibition triggers a biphasic concentration-dependent induction of ER stress in multiple myeloma and B-cell lymphoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2926. doi:10.1158/1538-7445.AM2013-2926