Induction of ER Stress in Acute Lymphoblastic Leukemia Cells by the Deubiquitinase Inhibitor VLX1570

Abstract

<jats:p>The proteasome is a validated target of cancer therapeutics. Inhibition of proteasome activity results in the activation of the unfolded protein response (UPR) characterized by phosphorylation of eukaryotic initiation factor 2α (eIF2α), global translational arrest, and increased expression of the proapoptotic CHOP (C/EBP homologous protein) protein. Defects in the UPR response has been reported to result in altered sensitivity of tumor cells to proteasome inhibitors. Here, we characterized the effects of the deubiquitinase (DUB) inhibitor VLX1570 on protein homeostasis, both at the level of the UPR and on protein translation, in acute lymphoblastic leukemia (ALL). Similar to the 20S inhibitor bortezomib, VLX1570 induced accumulation of polyubiquitinated proteins and increased expression of the chaperone Grp78/Bip in ALL cells. Both compounds induced cleavage of PARP (Poly (ADP-ribose) polymerase) in ALL cells, consistent with induction of apoptosis. However, and in contrast to bortezomib, VLX1570 treatment resulted in limited induction of the proapoptotic CHOP protein. Translational inhibition was observed by both bortezomib and VLX1570. We report that in distinction to bortezomib, suppression of translation by VXL1570 occurred at the level of elongation. Increased levels of Hsc70/Hsp70 proteins were observed on polysomes following exposure to VLX1570, possibly suggesting defects in nascent protein folding. Our findings demonstrate apoptosis induction in ALL cells that appears to be uncoupled from CHOP induction, and show that VLX1570 suppresses protein translation by a mechanism distinct from that of bortezomib.</jats:p>

Sensitivity of Acute Myelocytic Leukemia Cells to the Dienone Compound VLX1570 Is Associated with Inhibition of the Ubiquitin-Proteasome System

“…We found that HMOX1 was induced in MOLM-14 cells at 32 nM VLX1570 and at 64 nM VLX1570 in KG1a cells ( Figure 4 ). HMOX1 was not, however, induced at high concentrations of VLX1570, consistent with previous observations of inhibition of elongation of translation at high concentrations of this compound [ 39 ]. We conclude that the Nrf2 target HMOX1 is induced at lower concentrations of VLX1570 in the more sensitive MOLM-14 cell line.…”

“…Consistent with a recent report [ 16 ], we found that VLX1570 induces the accumulation of polyubiquitinated proteins and of Hsp70 in AML cells at the concentration range where the loss of cell viability is observed. Similar results have been reported using other types of tumor cells [ 39 , 48 ]. We also found a dose- and time-dependent induction of the ER chaperone Grp78/BiP in MOLM-14 and KG1a AML cells, indicating the induction of ER stress.…”

Dienone Compounds: Targets and Pharmacological Responses

“…VLX1570 was indeed shown to inhibit translation by another mechanism, at the level of translational elongation, in a recent study. 114 …”

VLX1570 induces apoptosis through the generation of ROS and induction of ER stress on leukemia cell lines

“… 17 , 18 The antiproliferative effect of these agents has been reported on several solid tumors 19 , 20 and lymphoid malignancies. 21 , 22 , 23 , 24 , 25 These agents lead to a similar pattern to the expression of genes induced by 20S proteasome inhibitors 26 and show high susceptibility to bortezomib‐resistant cells. 27 , 28 …”

“…Novel proteasome DUBs inhibitors, b‐AP15 and VLX1570 (a derivative of b‐AP15), predominantly target ubiquitin‐specific protease 14 (USP14) and interfere with the proteasomal degradation by inhibiting the activities of the proteasome DUBs, such as USP14 and ubiquitin C‐terminal hydrolase L5 (UCHL5) localized on the 19S proteasome 17,18 . The antiproliferative effect of these agents has been reported on several solid tumors 19,20 and lymphoid malignancies 21‐25 . These agents lead to a similar pattern to the expression of genes induced by 20S proteasome inhibitors 26 and show high susceptibility to bortezomib‐resistant cells 27,28 …”