Bortezomib Causes ER Stress-related Death of Acute Promyelocytic Leukemia Cells Through Excessive Accumulation of PML-RARA.

The success of proteasome inhibitors in therapy of multiple myeloma has led to their use for other malignancies. For the proteasome inhibitor bortezomib, combination therapies with histone deacetylase inhibitors, which up-regulate ubiquitin-proteasome system (UPS)-related enzymes, produce a beneficial effect. 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. The NB4 cell line established from a patient with acute promyelocytic leukemia (APL) expressing the promyelocytic leukemia/retinoic acid receptor alpha (PML-RARA) fusion protein was used to assess changes in cell viability and apoptosis caused by bortezomib, as well as alterations in PML-RARA and UPS-related enzymes via western blotting and immunoprecipitation assays. Bortezomib time- and dose-dependently reduced cell viability and induced apoptosis. Bortezomib significantly increased the abundance of ubiquitinated-PML-RARA (Ub-PML-RARA), ubiquitin-conjugating human enzyme 8 (UbcH8), and Ub-UbcH8, indicating that UbcH8 is the E2 ubiquitin-conjugating enzyme for PML-RARA. Moreover, UbcH8 abundance was dose-dependently increased in the culture supernatant of bortezomib-treated cells. UbcH8 may have a utility as a biomarker of treatment response to bortezomib in patients with APL. Furthermore, bortezomib impairs the UPS that controls normal protein homeostasis by causing excessive accumulation of PML-RARA augmenting ER stress and leading to APL cell death. The study provides a rationale for incorporating proteasome inhibitors in the treatment of diseases expressing aberrant proteins. Furthermore, monitoring of UPS-related enzymes might have use in predicting the treatment response to proteasome inhibitors and in assessing their therapeutic effects.