Background
Multiple myeloma, also referred as plasma cell myeloma, plasmacytic myeloma, myelomatosis, or Kahler disease, is a neoplastic malignancy characterized by the proliferation of abnormal plasma cells derived from B cells [
1,
2]. These plasma cells proliferate in the bone marrow and frequently invade into the adjacent bone, causing skeletal destruction that finally results in fractures [
3,
4]. In addition, multiple myeloma can cause anemia, renal insufficiency, frequent infection, and hypercalcemia and is associated with significant mortality and morbidity. Multiple myeloma accounts for about 1% of all types of malignancy and 10% of hematologic malignancies. In 2017, there were 30,280 new diagnoses and an estimated 12,590 deaths from myeloma in the USA alone. Existing therapies for multiple myeloma, including the proteasome inhibitors bortezomib (BTZ) and carfilzomib, have the potential of extending the overall survival but are not curative [
5]. Bortezomib is one of the major drugs in the treatment of multiple myeloma [
6]. Almost all myeloma patients will receive bortezomib during their course of treatment, yet nearly all patients will develop drug resistance to bortezomib [
7]. Thus, to unmask the failure to cure, drug resistance mechanisms associated with bortezomib must be characterized.
Thioredoxin (TXN) is a 12 kDa ubiquitous oxidoreductase. Along with NADPH and thioredoxin reductase, thioredoxin constitutes the thioredoxin system, one of the major disulfide reductase antioxidant systems in mammalian cells. Thioredoxin is crucial in defense against oxidative stress and in maintaining the redox environment in the cell. In addition, TXN has multi-faceted roles [
8]. TXN regulates a variety of redox-sensitive signaling pathways as well as ROS-independent genes and exerts cytoprotective effects. Overexpression of thioredoxin has been implicated in the pathogenesis of advanced malignancies, including solid cancer and hematological malignancy [
9]. Besides, there are increasing evidences of its role in the development of resistance to several chemotherapeutic agents including cisplatin and docetaxel [
10,
11].
Mitophagy, mitochondrial degradation by autophagic delivery to lysosomes, is the major degradative pathway in mitochondrial turnover. Mitochondria are the essential site of aerobic energy production in eukaryotic cells. Maintaining a healthy population of mitochondria is essential to the cellular redox homeostasis and the well-being of cell self-renewal [
12]. The role of mitophagy dysfunction in cancer pathogenesis is currently an area of active investigation but the findings varied [
13]. For instance, some studies suggested that a decrease in mitophagy led to the increase in the production of free radicals and subsequent genetic instability [
14,
15], thus, favoring the development of cancer. On the other hand, studies have found that mitophagy protected cancer cells from apoptosis [
16,
17]; thus, an increase in mitophagy will promote cancer cell survival and progression. It is highly likely that the double-edged roles of mitophagy dysfunction in cancer pathogenesis may change significantly depending on cancer cell types [
18]. However, the role of mitophagy in chemotherapeutics-induced drug resistance is still unknown and remains to be investigated.
We hypothesized that thioredoxin plays an important role in bortezomib drug resistance in multiple myeloma, providing a novel therapeutic target against cancer drug resistance in the treatment of multiple myeloma. Additionally, we aimed at investigating the effects of thioredoxin on mitophagy in bortezomib-resistant myeloma cells.
Methods
Cell culture and generation of bortezomib-resistant myeloma cells
The human multiple myeloma cell lines MM.1S, MM.1R, OPM1, RPMI8226/Dox, and NCIH929 were purchased from ATCC Company. Cells were grown in suspension in RMPI1640 medium supplemented with 10% fetal bovine serum, 1% (v/v) penicillin, and 100 μg/mL streptomycin. Cells were maintained at 37 °C in a 5% CO2 atmosphere with a proper humidity.
To generate bortezomib-resistant myeloma cell lines, bortezomib (BTZ) was added to the multiple myeloma cell culture medium starting at 0.03 nM. The culture medium was replaced with bortezomib containing medium twice weekly and the myeloma cells were cultured for 2–4 weeks until the cells survived and became resistant to that concentration of bortezomib. The bortezomib concentration was then increased by doubling the previous concentration until BTZ reached at a final concentration of up to 8.4 nM by the end of the second year. Myeloma cell lines were maintained on BTZ containing medium until 1 week before experiments.
Drugs and reagents
The proteasome inhibitor bortezomib (PS-341, Velcade®) was obtained from Selleckchem Chemicals LLC (Houston, TX). The thioredoxin inhibitor 2-[(1-methylpropyl) dithio]-1H-imidazole (PX12) was purchased from Tocris Bioscience (Bristol, UK). The mitophagy inhibitor bafilomycin was obtained from Sigma-Aldrich (St. Louis, Missouri, USA). 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT) were purchased from GIBCO BRL (Grand Island, NY). The mTOR activator (MHY1485) and ERK activator (tert-butylhydroquinone; tBHQ) were purchased from Sigma-Aldrich (St. Louis, Missouri, USA). Drugs were dissolved in dimethyl sulfoxide (DMSO; Sigma-Aldrich, St. Louis, Missouri, USA) at a concentration of 100 mM as a stock solution. Anti-PINK1 antibody (ab75487) and anti-LC3B antibody (ab51520) used for western blot analysis were purchased from Abcam (Cambridge, MA). Anti-beta-actin antibody (A2228) was obtained from Sigma-Aldrich. Anti-thioredoxin antibody (C63C6), anti-AKT antibody (9272), anti-mTOR antibody (2972), anti-phospho-Akt (Ser473, 4060) antibody, anti-phospho-mTOR (Ser2448, 5536) antibody, and anti-phospho-p44/42 MAPK (ERK1/2, Thr202/Tyr204, 9101) antibody were purchased from Cell Signaling Technology Inc. (Beverly, MA, USA). Human CD138 enrichment kit (EasySep™) was purchased from StemCell Technologies (Vancouver, BC, Canada).
MTT assay
Equal number of parental and BTZ-resistant multiple myeloma cells were seeded in 96-well plates. Different concentrations of BTZ with or without PX12 were added to the cells in each group (at least four replicates for each group). Cells were incubated in RPMI1640 medium at 37 °C in a 5% CO2 humidified atmosphere. After incubating for 48 or 72 h, 10 μL MTT (5 mg/mL) was added to each well and plates were incubated at 37 °C for another 4 h. Finally, 100 μL of 10% sodium dodecyl sulfate (with 0.01 N HCl) was added to dissolve the crystals and absorbance was determined at 570 nm in an EL340 microplate reader (BioTek Instruments, Winooski, VT). Ratios of the 50% inhibitory concentration (IC50) value in BTZ-resistant groups to the IC50 value of parental cells were calculated and considered to be the relative indicators of drug resistance in the experimental groups.
Gene expression analyses
For TXN mRNA expression (forward: 5′-GTAGTTGACTTCTCAGCCACGTG-3′, reverse: 5′-CTGACAGTCATCCACATCTACTTC-3′), total RNAs were extracted using TRIzol reagent (Invitrogen) according to standard procedures and reverse transcribed into complementary DNA (cDNA) using a BIO-RAD iScript ™ cDNA synthesis kit (Bio-Rad, Hercules, CA, USA). Samples were then analyzed using an Applied Biosystems Real-Time PCR (SYBR Green, Bio-Rad Laboratories, Hercules, CA, USA) in triplicate. Gene expression was normalized using 18S rRNA (forward: 5′-GTAACCCGTTGAACCCCATT-3′, reverse: 5′-CCATCCAATCGGTAGTAGCG-3′).
Western blot analysis
Western blot analysis was performed as previously described [
19]. Briefly, total protein was extracted using a RIPA buffer (50 mM Tris HCl, pH 7.4/150 mM NaCl/5 mM EDTA/1% NP-40/1% sodium deoxycholate/0.1% SDS/1% aprotinin, 50 mM NaF/0.1 mM Na
3VO
4), and equal amounts of proteins were separated using a SDS-PAGE electrophoresis. Separated proteins were then transferred to polyvinylidene difluoride membranes (PVDF; Millipore Corp., Bedford, MA, USA) and incubated with primary antibodies for thioredoxin (1:1000), PINK1 (1:200), LC3B (1:1000), AKT/pAKT (1:1000), mTOR/p-mTOR (1:1000), pERK1/2 (1:1000), or β-actin (1:10,000) overnight at 4 °C with gentle agitation. Membranes were washed and then incubated with HRP-conjugated secondary antibodies (1:10,000) for 2 h at room temperature before signal detection by chemiluminescence (Pierce Biotechnology, Rockford, IL, USA). Densitometric quantification was performed by Image-Pro Plus 6.0 software (Media Cybernetics, Silver Springs, MD 20910, USA).
Thioredoxin-specific shRNA knockdown
Plasmids targeting human thioredoxin (shTXN1-4, catalog number RHS4430-200171579, RHS4430-200174379, RHS4430-200273352, and RHS4430-200274993) were purchased from GE Healthcare (Piscataway, NJ, USA). Plasmid for non-targeting control (shNT) and the packing and envelope vectors psPAX2 and VSVG were obtained from Addgene (Cambridge, Massachusetts). HEK293T cells were transfected with shNT or shTXN, psPAX2, and VSV-G using Lipofectamine 2000 (Invitrogen, Carlsbad, CA, USA) for 24 h. The DMEM medium was changed and collected at 24 and 48 h after transfection, respectively. To obtain thioredoxin stably knockdown cells, the transduced cells were cultured with 1 μg/ml puromycin and the GFP+ cells were sorted and expanded.
Mitochondrial network by transmission electron microscopy
Conventional transmission electron microscopy analysis was performed as described previously [
20]. Briefly, human multiple myeloma cells with or without treatment were fixed by a solution containing 4% formaldehyde and 2% glutaraldehyde. Specimens were washed following by OSO
4 postfixed, alcohol dehydrated, and araldite embedded. Thin sections of samples were analyzed using a FEI Tecnai G
2 Twin electron microscope (FEI, Hillsboro, OR, USA).
Determination of mitochondrial membrane potential (Δψm)
JC-1 fluorescent probe kit (Molecular Probes, Eugene, OR, USA) was used to determine Δψm with two different staining spectra, the orange aggregates dye form for normally respiring cells and green monomers for cells with respiratory dysfunction (apoptotic cells). Briefly, cells with or without treatment were incubated in RPMI1640 media containing JC-1 (2 μM final concentration) at 37 °C for 15 min. Cell pellets were resuspended in cold PBS and analyzed on a flow cytometer with 488-nm excitation emission.
In vivo tumor xenograft model
All animal experiments were approved by the Animal Care Committee of Duke University Medical Center. NOD/LtSz-scid/scid (NOD/SCID) mice were purchased from Jackson Laboratories (Bar Harbor, ME, USA) and maintained in microisolator cages on laminar flow racks under pathogen-free conditions in the Division of Laboratory Animal Resources, Duke University. BTZ-resistant MM.1R multiple myeloma cells, 3 × 106 cells in 100 μL PBS, were injected subcutaneously into the dorsal lateral flank of NOD/SCID mice that had received a total body irradiation with 1.5 Gy from a 137Cs source. Engraftment of human myeloma was monitored twice per week. When the tumor was established and palpable after 10 days of xenograft, the mice were given control buffer, PX12 (12 mg/kg, i.p., twice weekly), bortezomib (0.5 mg/kg, i.p., twice weekly) or the combination of PX12, and bortezomib. Tumor length and width were measured with a caliper and tumor volume was calculated using the formula V = (L × W × W)/2, where V is the tumor volume, L is the tumor length, and W is the tumor width. At the end of the experiments, tumors were harvested and weighed. A portion of the tumors were homogenized and used for western blot analysis.
Microarray data mining
For oncomining gene expression analysis, mRNA level of
TXN in normal plasma cell and myeloma cells was queried using Oncomine database (
https://www.oncomine.org). The prognostic value of
TXN in multiple myeloma was assessed using Mulligan and Arkansas myeloma microarrays. Overall survival was compared between high and low expression of
TXN using median gene expression value as a bifurcating point.
Patient samples
Bone marrow aspirates were obtained from patients with myeloma with patients’ informed consent and IRB approval. The study was performed in compliance with the guidelines of the Ethical Committee of Duke University Medical Center. A total of 13 patients with myeloma, including three newly diagnosed myeloma, five relapsed myeloma, and five treated myeloma prior to stem cell transplant were enrolled in the study. Human CD138+ and CD138− cells were isolated from the bone marrow aspirates of these patients using Histopaque-1077 (Sigma) gradient separation followed by human CD138 enrichment kit (EasySep™, StemCell Technologies). The purity of the human CD138+ cells was > 95%.
Statistical analyses
All statistical analyses were performed using the Student’s t test and represented as mean ± standard error of the mean (SEM) unless noted otherwise. For in vivo experiments with ≥ 5 per group, statistical analyses were performed with two-way ANOVA followed by multiple comparisons. The p values were designated as *p < 0.05, **p < 0.01, ***p < 0.005, ****p < 0.001; n.s. non-significant (p > 0.05).
Discussion
Multiple myeloma remains an incurable disease and nearly all myeloma patients will eventually develop resistance to currently available therapeutic agents including proteasome inhibitors. Bortezomib is the first proteasome inhibitor used in the treatment of myeloma, and it has dramatically changed the landscape of the care of patients with multiple myeloma. Bortezomib can induce deeper response, lead to higher response rate, and extend the survival of patients with newly diagnosed multiple myeloma and of patients with relapsed myeloma. Unfortunately, development of drug resistance to bortezomib is inevitable and poses a major challenge in our continuous improvement of clinical outcomes for patients with multiple myeloma. Therefore, it is critical and imperative to elucidate the molecular mechanisms associated with or driving the development of bortezomib resistance. This information is essential in our efforts to overcome or re-sensitize bortezomib resistance.
In the current study, we generated over a period of 1.5 years several adaptive bortezomib-resistant myeloma cell lines (Fig.
1). We have also analyzed and compared the expression of thioredoxin in CD138
+ myeloma cells from newly diagnosed myeloma patients, myeloma patients that were treated with bortezomib containing regimen, and relapsed/refractory myeloma patients who had prior exposure to bortezomib (Fig.
7). We have demonstrated that over-expression of thioredoxin was associated with the development of drug resistance to bortezomib. Thioredoxin was previously found to promote tumor growth through inhibition of apoptosis, reduce sensitivity of the tumor to drugs [
28‐
30], and be associated with poor prognosis [
29]. Raninga et al. has recently shown that compared to normal plasma cells, multiple myeloma cells had higher intrinsic oxidative stress and higher levels of thioredoxin and thioredoxin reductase expression and that thioredoxin over-expression was associated with resistance to NF-κβ inhibitors [
28]. Using comparative proteomic profiling, Dytfeld et al. showed upregulation of thioredoxin expression in bortezomib-resistant myeloma cells [
29]. Our current studies were consistent with and validated these observations. Importantly, our current studies advance significantly from those studies. We indeed showed that inhibition of thioredoxin by shRNA knockdown or pharmacological approach with PX12 could re-sensitize the bortezomib-resistant myeloma cells to bortezomib in vitro and in vivo in myeloma xenograft mouse models (Figs.
2 and
3). Mechanistically, we have demonstrated that inhibition of thioredoxin re-sensitize bortezomib-resistant myeloma cells through the activation of mitophagy. Our studies provide molecular rationale and justification for targeting thioredoxin in the treatment of bortezomib relapsed/refractory myeloma.
We found that PX12-sensitized BTZ-resistant myeloma cells to bortezomib through the activation of mitophagy. We confirmed the activation of mitophagy using several methods including LC3 western blotting, PINK1 expression analysis, measurement of mitochondrial membrane potential, and the visualization of mitochondrial-autophagosome fusion with transmission electron microscopy (Fig.
4). Using bafilomycin to inhibit mitophagy, we have demonstrated the important role of mitophagy in PX12-mediated effects. Thioredoxin-specific shRNA knockdown revealed similar findings on the induction of mitophagy (Fig.
5). Thioredoxin and thioredoxin system maintain the intracellular redox homeostasis by scavenging ROS and regulating other redox proteins [
31,
32]. Inhibition of thioredoxin increases intracellular oxidative stress [
33,
34]. Increased production of ROS stimulates the initiation of mitophagy [
28]. Mitophagy is a cellular self-cannibalization process that captures and digests mitochondrial in lysosomes [
35]. The role of mitophagy in tumorigenesis remains a topic of debate. On one hand, mitophagy is activated in transformed cells and is beneficial for tumor maintenance and progression. On the other hand, excessive autophagy can act as a tumor-suppressive mechanism possibly through initiation of cell death or senescence [
36‐
40]. The outcome of mitophagy activation in cancer depends on the stage of the disease, cell types, oncogenic drivers, and the intensity of the activation signal [
35,
41,
42]. Mitophagy is also found to be involved in chemoresistance. Our current study suggested that reduced level of mitophagy is associated with the development of drug resistance in multiple myeloma cells and that activation of mitophagy could re-sensitize myeloma cells to bortezomib killing. Our study provides evidence for future exploring mitophagy pathway for the treatment of relapsed and refractory multiple myeloma.
In the current study, we delineated the molecular events involved in the PX12 + BTZ- induced mitophagy in MM cells. We described 2 signal proteins (p-mTOR and p-ERK1/2) that may play a role in PX12 + BTZ -induced mitophagy and anti-myeloma effects in MM cells. Using chemical activator of mTOR and ERK, we demonstrated an important role of mTOR and ERK in the anti-myeloma effects induced by thioredoxin inhibition. Previous studies have demonstrated that various signal pathways are involved in autophagy, including PI3K/Akt/mTOR, ERK1/2 and NF-
κB. mTOR (mechanistic target of rapamycin kinase) has been shown to control multiple cellular functions such as gene transcription, protein formation, cell proliferation and senescence and cell metabolism [
43]. mTOR pathway is the master regulator of cellular metabolism and has been shown to be the major pathway regulating mitophagy. AKT was previously reported to control mitochondrial biogenesis and autophagy. In our system, we did not observe changes in AKT expression or phosphorylation after PX12 treatment. In addition, we found that treatment with PX12 resulted in desphosphorylation of ERK1/2. Our study suggested that inhibition of thioredoxin affects several molecular pathways that are important in the regulation of mitophagy.
ERK1 (p44) and ERK2 (p42) are two isoforms of ERK that are activated downstream of Ras in response to extracellular cues. ERK has been shown to induce autophagy in response to a number of anti-tumor/cytotoxic agents [
44,
45]. Inhibition of ERK was associated with a decrease in autophagy and increased cellular sensitivity to tumor necrosis factor-α (TNF) in breast cancer MCF-7 cells [
46]. Interestingly, our data showed that dephosphorylation of ERK1/2 appears to be associated with the induction of mitophagy in BTZ-resistant MM cells. Additional studies are needed to further understand the ERK1/2 pathway in BTZ resistance in multiple myeloma. The discrepancy between reduced activity of ERK1/2 in our current study and the findings in breast cancer could be due to different cancer type or to different drug sensitivity.
Acknowledgements
The authors thank Dr. Benny Chen and his lab members for the technical support. Besides, we also want to thank Gene Expression Omnibus (GEO) and Oncomine databases for making their data readily available to the scientific community.