Mariko Takenokuchi, Kazuhide Miyamoto, Katsuyasu Saigo and Taizo Taniguchi
Faculty of Pharmacological Sciences, Himeji Dokkyo University, Uskudar University, Istanbul, Turkey
Background: The ubiquitin-proteasome system (UPS) controls normal protein homeostasis in cells. Ubiquitination is catalyzed by E1 ubiquitin-activating enzymes, E2 ubiquitin-conjugating enzymes, and E3 ubiquitin ligases. The success of proteasome inhibitors in multiple myeloma (MM) has led to their use for other malignancies. However, the mechanisms underlying the effect of bortezomib are not completely understood. We hypothesized that bortezomib causes excessive accumulation of aberrant proteins, which augments endoplasmic reticulum (ER) stress, leading to death of malignant cells.
Methods: The NB4 cells expressing the PML-RARA fusion protein were cultured with or without 10 or 100 nM bortezomib and used for experiments.
Results: Bortezomib time- and dose-dependently decreased cell viability and induced apoptosis. Bortezomib significantly increased abundance of ubiquitinated-PML-RARA (Ub-PML-RARA), ubiquitin conjugating human enzyme 8 (UbcH8), and ubiquitinated-UbcH8 (Ub-UbcH8) in the cells, indicating that UbcH8 is the E2-conjugating enzyme for PML-RARA. Moreover, UbcH8 abundance was dose-dependently increased in the culture supernatant of bortezomib-treated cells. Meanwhile, XBP-1 and BiP/GRP78, ER stress markers, were increased by Bortezomib.
Conclusions: UbcH8 can be used as a biomarker of the treatment response to bortezomib in APL patients. Bortezomib impairs the UPS that controls normal protein homeostasis by causing excessive accumulation of PML-RARA fusion protein, augmenting ER stress and leading to APL cell death. Furthermore, monitoring of UPS-related enzymes can be used to predict the treatment response to proteasome inhibitors and assess their therapeutic effects.
Keywords: Ubiquitin-proteasome system, proteasome inhibitor, PML-RARA.