PS1356 INDIRUBIN‐3’‐MONOXIME INDUCES APOPTOSIS AND OVERCOMES BORTEZOMIB RESISTANCE IN MULTIPLE MYELOMA VIA TARGETING THE UBIQUITIN AND PROTEASOME SYSTEM

Background: Deregulation of the ubiquitin‐proteasome system (UPS) is linked to pathogenesis and drug resistance of Multiple Myeloma (MM). Proteasome inhibitor has provided significant therapeutic advances in the treatment of MM, however resistance and dosage dependent side effects remain a clinical challenge. Our recent research efforts led to the discovery of new agents which can potentially overcome the resistance and toxicity of bortezomib. Aims: To profile the anti‐MM activity and explore the possible mechanisms of indirubin‐3’‐monoxime (Id3) mediating. Methods: Cytotoxicity assays were performed using MM cell lines, primary patient cells, and peripheral blood mononuclear cells from normal healthy donors in the treatment of Id3. Cell viability was assessed using CCK8 assay. RNA‐seq was performed in MM cell lines treated with Id3 on a BGISEQ‐500 platform. The signal transduction pathways were evaluated using immunoblotting. Proteasome activity assay was conducted using proteasome‐Glo™ kit. The treatment of Id3 in xenograft myeloma murine model was performed by subcutaneous injection of ARP1 cells into NOD/SCID mice. Statistical significance of data was determined using a Student's t test. Results: Id3 can induce cell death efficiently in MM cell lines and in patient samples. Id3 treatment promoted the apoptosis of MM cells in a caspase dependent pathway and induced the arrest of cell cycle. Combination therapy of Id3 and bortezomib exhibited a synergistic anti‐MM effect. Id3 treatment alone or combined with bortezomib reduced the growth of myeloma cells in xenograft mouse models as well. Importantly, our data indicated that the Id3 treatment effectively promoted the cell death in those bortezomib‐resistant MM cell lines and the relapsed and refractory MM patient samples. RNA‐seq and GSEA analysis demonstrated that Id3 treatment induced the apoptosis and G2/M phase arrest of MM cells by targeting the ubiquitin and proteasome system. Id3 treatment significantly inhibited the proteasome activity in an ubiquitin independent and dependent ways. The cellular ubiquitin level in MM cells was down‐regulated after the treatment of Id3 which caused the accumulation of mis‐folded proteins in MM cell. Furthermore, Id3 treatment declined the proteasome activity. The protein level of catalytically activity of the proteasome subunit beta (PSMB8, PSMB9) and the proteasome activator complex subunit 4 (PSME4) were significantly decreased after the treatment of Id3. Id3 therapy inhibited bortezomib resistant MM cells growth via targeting the ubiquitin and proteasome system. Summary/Conclusion: Taken together, our studies demonstrate the effective growth inhibition of Id3 both in vitro and in vivo study. Id3 treatment significantly inhibited the proteasome activity in an ubiquitin independent or dependent manner and overcomes bortezomib resistance in MM via targeting the ubiquitin and proteasome system. These data provide a rational possibility for clinical evaluation of Id3 alone or combined with bortezomib as a potential therapeutic agent to improve outcomes in clinic patients with MM.