Abstract 2104: Targeting stem cell proteostasis pathways sensitizes acute myeloid leukemia to proteasome inhibitors

Abstract Proteasome inhibitors are highly effective for treating multiple myeloma, but exhibit little efficacy in other blood cancers. In this study, we set out to determine why acute myeloid leukemia (AML) is largely refractory to proteasome inhibition and if we could sensitize human AML to this class of drugs. Efficacy of proteasome inhibitors in myeloma partly depends on disruption of protein homeostasis (proteostasis). This is driven by an accumulation of misfolded proteins that activate the unfolded protein response (UPR) and integrated stress response (ISR). Due to their highly secretory nature, myeloma cells exhibit relatively elevated baseline endoplasmic reticulum stress, and are thus sensitive to proteasome inhibition as they readily activate a terminal UPR. However, we found that proteasome inhibition fails to significantly disrupt proteostasis in human AML cells. We recently discovered that hematopoietic stem cells utilize distinct pathways to regulate proteostasis, and we hypothesized that AML cells co-opt stem cell proteostasis pathways to cope with proteasome inhibition and preserve proteostasis. To test this, we first performed RNA-sequencing which revealed that AML cells respond to proteasome inhibition by activating two distinct proteostasis pathways normally utilized by hematopoietic stem cells: autophagy and the heat shock response. Concurrent proteasome and autophagy inhibition significantly increased unfolded protein content in multiple human AML cell lines. This proteostasis disruption was sufficient to induce a terminal ISR marked by activation of the eIF2α-ATF4-CHOP signaling axis that severely attenuated protein synthesis, reduced proliferation, and induced widespread apoptosis in human AML cell lines and primary AML patient samples. In contrast to AML cells, dual proteasome and autophagy inhibition was well tolerated by healthy human CD34+ hematopoietic stem and progenitor cells, suggesting a tractable therapeutic window for treating AML. Indeed, dual proteasome and autophagy inhibition was tolerated in vivo, and decreased leukemia burden in mice xenografted with human AML cell lines. Similar to autophagy inhibition, disabling the heat shock response by deleting Heat shock factor 1 (HSF1) sensitized human AML cell lines to proteasome inhibitors. Treatment of HSF1-deficient AML cells with a proteasome inhibitior induced a significant increase in unfolded protein, activated the ISR, reduced growth and protein synthesis, and led to severe apoptosis. Concurrent HSF1 deletion and proteasome inhibition dramatically reduced AML disease burden in vivo and extended median survival from 40 to 140 days. Overall, this study revealed that AML cells hijack stem cell proteostasis pathways to promote growth, survival, and therapeutic resistance. Targeting the unique configuration of the proteostasis network is uncovering new therapeutic strategies to eliminate AML. Citation Format: Kentson Lam, Yoon Joon Kim, Carlo M. Ong, Fanny J. Zhou, Andrea Z. Liu, Bernadette A. Chua, Jie-Hua Zhou, Edward D. Ball, Robert A. Signer. Targeting stem cell proteostasis pathways sensitizes acute myeloid leukemia to proteasome inhibitors abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 2104.