Thiasyrbactins Induce Cell Death via Proteasome Inhibition in Multiple Myeloma Cells

Abstract

Background/Aim: Proteasome inhibition is a validated therapeutic strategy for the treatment of refractory and relapsed multiple myeloma (MM) and mantle cell lymphoma. We previously showed that in vitro, thiasyrbactins (NAM compounds) are inhibitors with an affinity for the trypsin-like (T-L, 2) site of the constitutive proteasome, and more profoundly on the T-L site of the immunoproteasome. Materials and Methods: In this study, the biological activity of three NAM compounds was evaluated on four hematologic cell lines of plasma immune cells. Using four MM cell lines (ARD, U266, MM1R, and MM1S) we assessed the effect of three NAM compounds (NAM-93, NAM-95, and NAM-105) on cell viability as well as cell-based proteasomal activities, and determined the EC50 and Ki50 values. Results: MM cells were most sensitive to NAM-93 with EC50 values <0.75 μM after 48 h of treatment. NAM-105 had a similar profile in most of the MM cells with EC50 values ranging between 0.42 and 3.02 μM. The level of inhibition of the proteasome T-L sub-catalytic activity in actively-growing MM cells was similar for NAM-93 and NAM-105. However, in each cell line, NAM-93 was more effective than NAM-105 at inhibiting overall trypsin-like sub-catalytic activity while NAM-105 was typically more effective at inhibiting overall chymotrypsin-like (CT-L, 5) sub-catalytic activity. Conclusion: These results show for the first time the proteasome-targeted biological activity of thiasyrbactins in MM tumor cells.

Syrbactin proteasome inhibitor TIR-199 overcomes bortezomib chemoresistance and inhibits multiple myeloma tumor growth in vivo

“…The cell-based proteasome activity assay to determine the sub-catalytic proteasomal activities in cells was performed as previously described [45]. Cells were seeded in solid white 96-well plates 24h prior to treatment.…”

“…The syrbactins represent the latest (seventh) class of mammalian proteasome inhibitors that irreversibly (covalently) bind to the catalytic Thr1 residue of the proteasome [8,20]. Since the discovery of these microbe-derived natural products, which include the syringolins [20][21][22][23][24][25], glidobactins [23,[26][27][28][29][30], and cepafungins [23,31,32], the total synthesis of SylA and SylB [33][34][35] and a number of syrbactin-inspired structural analogs and their biological activities have been reported [33,[36][37][38][39][40][41][42][43][44][45][46][47]. TIR-199 is the most potent syrbactin analog to date and selectively inhibits the CT-L and T-L activities of both the constitutive proteasome and immunoproteasome in vitro.…”