Abstract 2686: The CDK inhibitor dinaciclib (SCH727965) inhibits the unfolded protein response (UPR) through a CDK1- and CDK5-dependent mechanism

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA The endonuclease IRE1 (inositol-requiring enzyme-1), responsible for splicing of the transcription factor XBP-1s, is a major mediator of the unfolded protein response (UPR), an adaptive response to endoplasmic reticulum (ER) and other forms of proteotoxic stress. This arm of the UPR is constitutively activated in various malignancies, including multiple myeloma. The chaperone protein Grp78, a target of IRE1/XBP-1s, is also frequently up-regulated in neoplastic cells, confers resistance to diverse chemotherapeutic agents, and has been implicated in leukemia progression. Consequently, XBP-1 and Grp78 represent logical therapeutic targets within the UPR pathway. Currently, inhibitors of IRE1 or Grp78 have not yet been developed for human use, although pre-clinical studies have identified IRE1 inhibitors that are active at relatively high (e.g., μM) concentrations. Here, effects of the cyclin-dependent kinase inhibitor, SCH727965 (dinaciclib), on the IRE1/XBP1 and Grp78 arms of the UPR have been examined in human leukemia and myeloma cells. Exposure of multiple myeloma (J558, 8226, H929, U266) and leukemia (K562, Jurkat, U937, BaF3/T315I and primary AML) cells to extremely low (e.g., nM) concentrations of SCH727965, a potent inhibitor of CDKs 1/2/5/9, sharply attenuated XBP-1s and Grp78 up-regulation by the ER stress-inducers thapsigargin (Tg) and tunicamycin (Tm), while inducing pronounced cell death. SCH727965, in contrast to known IRE1 RNase inhibitors (e.g., STF-083010, MK0186893 and MKC-3946), inhibited the UPR in association with attenuation of XBP-1s nuclear localization and accumulation rather than transcription, translation, or XBP-1 splicing. Notably, in human leukemia cells, CDK1 and CDK5 shRNA knock-down diminished Grp78 and XBP-1s up-regulation while significantly increasing Tg lethality, arguing for a functional role for CDK1/5 in activation of the cytoprotective IRE1/XBP-1s arm of the UPR. In contrast, CDK9 or CDK2 inhibitors or CDK2/9 shRNA knockdown failed to down-regulate XBP-1s or Grp78. Furthermore, IRE1, XBP-1, or Grp78 knockdown significantly increased Tg lethality, as observed with CDK1/5 inhibition/knockdown. Finally, SCH727965 diminished myeloma cell growth in vivo in association with XBP-1s down-regulation. Together, these findings demonstrate, for the first time that SCH727965 acts at extremely low concentrations to attenuate XBP-1s nuclear accumulation and Grp78 up-regulation in response to ER stress inducers. They also highlight a heretofore unrecognized link between specific components of the cell cycle regulatory apparatus (e.g., CDK1/5) and the cytoprotective IRE1/XBP-1s/Grp78 arm of the UPR that potentially may be exploited therapeutically in UPR-driven malignancies e.g., by enhancing the activity of anti-cancer agents that elicit a cytoprotective UPR response. Citation Format: Tri K. Nguyen, Steven Grant. The CDK inhibitor dinaciclib (SCH727965) inhibits the unfolded protein response (UPR) through a CDK1- and CDK5-dependent mechanism. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2686. doi:10.1158/1538-7445.AM2014-2686