Cytotoxic effects of bortezomib in myelodysplastic syndrome/acute myeloid leukemia depend on autophagy-mediated lysosomal degradation of TRAF6 and repression of PSMA1

Abstract

IntroductionMyelodysplastic syndrome (MDS) is a hematologic malignancy defined by blood cytopenias due to ineffective hematopoiesis, a predisposition to acute myeloid leukemia (AML), and genomic instability. 1,2 Molecular-targeted therapies do not exist for MDS and the mechanisms of current therapies are largely unknown. More recently, bortezomib (Velcade), which is widely used for the treatment of multiple myeloma (MM) and lymphomas, is being evaluated as a single agent or in combination with chemotherapy in certain MDS and AML patients. [3][4][5] Bortezomib is a selective and reversible inhibitor of the 26S proteasome, and mechanistic studies have revealed that inhibition of the proteasome complex leads to accumulation of lysine (K)-48 ubiquitin-linked proteins and consequently to cytotoxic effects in malignant cells. 6 Proapoptotic and cell-cycle inhibitor proteins are stabilized after proteasome inhibition and thought to contribute to the anticancer effect by inducing apoptosis and inhibiting the cell cycle, respectively. 6 Nevertheless, the molecular and cellular mechanisms of bortezomib-induced cytotoxicity remain unknown, particularly in MDS/AML. Whereas the role of bortezomib in regulating cell-cycle entry and survival have been characterized partially in MDS/AML, 7-9 recent evidence has pointed to a more general cellular effect: bortezomib treatment results in the accumulation of nondegraded proteins, leading to endoplasmic reticulum stress and autophagy in cancer. [10][11][12] Under normal cellular stresses, autophagy, a catabolic pathway, degrades long-lived proteins and defective and superfluous organelles. 13 However, under conditions of extreme cellular stress, autophagy is used by the cell to undergo death. 14 Human miR-146a, a candidate gene in del(5q) MDS/AML, is reduced significantly in del(5q) and normal karyotype MDS/AML patients. [15][16][17] TRAF6 is a key target of miR-146a 15,18,19 and, as expected, miR-146a-knockout mice have a dramatic increase in TRAF6 protein within the hematopoietic compartment. 20,21 Retroviral overexpression of TRAF6 in mouse hematopoietic stem/ progenitor cells results in MDS-like hematopoietic defects and progression to AML. 15 Bortezomib has been shown previously to be effective for an MDS patient with del(5q) and was also reported to reduce directly TRAF6 mRNA and protein in osteoclast precursors from MM patients. 22,23 Because TRAF6 is implicated in MDS/AML and bortezomib has been shown to be effective in del(5q) MDS and to inhibit TRAF6 in MM, we hypothesized that one mechanism of bortezomib action is through inhibition of TRAF6. The online version of this article contains a data supplement.The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked ''advertisement'' in accordance with 18 USC section 1734. For personal use only. on May 12, 2018. by guest www.bloodjournal.org FromIn the present study, we identified TRAF6 as a relevant target of bortezomib-induced...

Synergistic antitumor effects of radiation and proteasome inhibitor treatment in pancreatic cancer through the induction of autophagy and the downregulation of TRAF6

“…Fang et al indicated that RNAi-mediated knockdown of TRAF6 sensitized both bortezomib-sensitive and -resistant cell lines, implying a key role for TRAF6 in bortezomib-induced cytotoxicity. Furthermore, reduced TRAF6 protein levels coincide with bortezomib-induced autophagy and subsequent apoptosis in myelodysplastic syndrome and acute myeloid leukemia [24]. In the current study, combined MG132 and IR treatment resulted in reduced TRAF6 protein and TAK1 phosphorylation levels (Fig.…”

“…More recently, it was demonstrated that bortezomib (a proteasome inhibitor) induces proteasomeindependent degradation of the TRAF6 protein in myelodysplastic syndrome; such as acute myeloid leukemia cell lines and primary cells. Notably, TRAF6 is necessary for maintaining the survival of cells, and its degradation by bortezomib-induced autophagy contributes to cell death [24]. However, whether TRAF6 plays a critical role in the control of autophagy in pancreatic cancer cells remains undetermined.…”

Proteasome inhibitors induce FLT3-ITD degradation through autophagy in AML cells

“…Indeed, BLOOD 23,24 During autophagy, the microtubule-associated protein 1 light chain 3 (LC3-I) is converted to membrane-bound LC3-II and specifically associates with autophagosomes. 25 Treatment of MV4-11 cell line and FLT3-ITD primary samples with bortezomib increased the number of autophagosomes, as indicated by the conversion of LC3-I to LC3-II ( Figure 3A-B).…”

Induction of myelodysplasia by myeloid-derived suppressor cells

“…Moreover, knockdown of these specific miRNAs or overexpression of Traf6 in murine HSPC recapitulates the hematologic features of del(5q) MDS in a transplant model (10), providing convincing evidence that sustained TLR activation is a critical factor driving the malignant phenotype. More recent findings indicate that TRAF6 is essential for survival and proliferation of MDS HSPC (11) and sustained TLR activation skews their commitment toward the myeloid lineage while suppressing osteoblast differentiation (12,13), analogous to the senescence-dependent changes observed with normal aging (14).…”