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

Abstract

Key Points• Bortezomib induces the degradation of FLT3-ITD through an autophagydependent mechanism that contributes to cell death.• This finding provides a mechanism-based rationale for the study of proteasome inhibitors in FLT3-ITD-mutant acute myeloid leukemia.Internal tandem duplication of the Fms-like tyrosine kinase-3 receptor (FLT3) internal tandem duplication (ITD) is found in 30% of acute myeloid leukemia (AML) and is associated with a poor outcome. In addition to tyrosine kinase inhibitors, therapeutic strategies that modulate the expression of FLT3-ITD are also promising. We show that AML samples bearing FLT3-ITD mutations are more sensitive to proteasome inhibitors than wild-type samples and this sensitivity is strongly correlated with a higher FLT3-ITD allelic burden. Using pharmacologic inhibitors of autophagy, specific downregulation of key autophagy proteins including Vps34, autophagy gene (Atg)5, Atg12, Atg13, biochemical, and microscopy studies, we demonstrated that proteasome inhibitors induced cytotoxic autophagy in AML cells. FLT3-ITD molecules were detectable within autophagosomes after bortezomib treatment indicating that autophagy induction was responsible for the early degradation of FLT3-ITD, which preceded the inhibition of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK), PI3K/AKT, and STAT5 pathways, and subsequent activation of cell death. Moreover, proteasome inhibitors overcome resistance to quizartinib induced by mutations in the kinase domain of FLT3, suggesting that these compounds may prevent the emergence of mutant clones arising from tyrosine kinase inhibitor treatments. In xenograft mice models, bortezomib stimulated the conversion of LC3-I to LC3-II, indicating induction of autophagy in vivo, downregulated FLT3-ITD protein expression and improved overall survival. Therefore, selecting patients according to FLT3-ITD mutations could be a new way to detect a significant clinical activity of proteasome inhibitors in AML patients. (Blood. 2016;127(7):882-892)

FLT3 Inhibitors in Acute Myeloid Leukemia: Current Status and Future Directions

“…Proteasome inhibitors , such as bortezomib (Velcade®, Takeda), have been shown to induce autophagosomal degradation of FLT3-ITD and to be cytotoxic to cells with FLT3-ITD in vitro and in vivo , including FLT3-ITD cells with resistance to quizartinib associated with FLT3 D835 mutations [64]. Bortezomib and sorafenib are being evaluated together (NCT01371981) and also in combination with decitabine (NCT01861314) in Phase I trials.…”

Heparanase: From basic research to therapeutic applications in cancer and inflammation

“…Equally important is the ability of heparanase overexpression to confer resistance to stress, chemotherapy and targeted drugs (Ramani et al, 2016), mediated, at least in part, by enhancing autophagy (Shteingauz et al, 2015). Indeed, diverse classes of anticancer drugs induce autophagy (Larrue et al, 2016) as well as lysosomal biogenesis (Zhitomirsky and Assaraf, 2015; 2016) thus attenuating tumor cell elimination, while autophagy inhibitors overcome chemo-resistance (Levy and Thorburn, 2011; Wu et al, 2012). Based on this concept, chloroquine is currently evaluated in several clinical trials in combination with different classes of chemotherapeutic agents (Levy and Thorburn, 2011).…”

Mechanisms of Resistance to FLT3 Inhibitors and the Role of the Bone Marrow Microenvironment

“…Since the survival of FLT3-ITD leukemia cells depends on absolute levels of FLT3-ITD, decreasing oncoprotein stability using Hsp90 inhibitors 49 or activating autophagy via proteasome inhibitors like Bortezomib 50 can also resensitize resistant FLT3-ITD AML cells to TKI. Lastly, in a small case series, concomitant treatment with CsA benefitted patients with FLT3-ITD AML who were being treated with FLT3 inhibitors, perhaps via inhibition of NFATc1 51 .…”