P-088: Indirubin-3’-monoxime acts as an alternative proteasome inhibitor and confers new regimens for the treatment of Multiple Myeloma

Background Multiple myeloma (MM) is still an incurable malignancy of plasma cells. Despite the therapeutic benefit of the proteasome inhibitor (PI) bortezomib, drug resistance still accounts for majority of tumor relapses and cancer-related death in MM patients. Thus, exploring the mechanism underlying BTZ resistance and investigating a novel treatment strategy are urgent requisites for clinical practise. Indirubin-3’-monoxime (Id-3) is an active ingredient of a traditional Chinese medicine. Method In the present study, we firstly investigated Id-3 effects on myeloma treatment in patient primary samples and myeloma cell lines. We also investigated the molecular mechanism of Id-3 treatment on MM cells via in vivo and in vitro study. Results We first reported that Id-3 is an effective cytotoxicity agentia against MM cells growth by inducing the MM cells apoptosis. Id-3 treatment also effectively induces the cell death in bortezomib-resistant MM cell lines and primary myeloma patient samples. Stringently, we found there is a synergistic anti-MM effect between Id-3 and bortezomib. The treatment with lower dosage combination of Id-3 and bortezomib exhibited a notably inhibition of the growth of myeloma cells both in xenograft mouse model and MM-PDX model. Mechanism study revealed that Id-3 treatment efficiently inhibited the chymotrypsin-like and caspase-like activity of proteasome in MM cells. The level of the proteasome 11s activator, PA200 (PSME4) and PA28y (PSME3) was significantly down-regulated both in mRNA and protein level. GEO datasets analysis showed that the high levels of PSME4 and PSME3 were over-expressed in relapsed patients and MM cell lines which correlated with the drug-resistance of MM cells. Kaplan-Meier analysis showed that patients with high-levels of PSME3 or PSME4 had the shortened survival. Knocking-down the level of PSME4 or PSME3 by shRNA noteworthy inhibited the cell growth and enhanced the sensitivity to bortezomib of MM cells. In vivo study also found that down-regulated the level of PSME3 or PSME4 can suppress the proliferation of MM cells and prolong the survival of MM-bearing mouse. Conclusions Taken together, our study supported that Id-3 is an effective agentia against the survival of MM cells both in bortezomib-sensitive and -resistance ones. We further identified that the 11s proteasome activators, PSME3 and PSME4, were down-regulated effectively by Id-3 treatment. High-levels of PSME3 or PSME4 were correlated with the inferior outcome of MM patients. Down-regulating PSME3 and PSME4 by Id-3 or shRNA could efficiently suppressed the growth of MM cells and improve the survival of MM-bearing mice. Our study provides the rational evidence for the clinical application of Id-3 in the treatment of MM.