Magnetic nanoparticles of Fe₃O₄ enhance docetaxel-induced prostate cancer cell death

Abstract

Docetaxel (DTX) is one of the most important anticancer drugs; however, the severity of its adverse effects detracts from its practical use in the clinic. Magnetic nanoparticles of Fe₃O₄ (MgNPs-Fe₃O₄) can enhance the delivery and efficacy of anticancer drugs. We investigated the effects of MgNPs-Fe₃O₄ or DTX alone, and in combination with prostate cancer cell growth in vitro, as well as with the mechanism underlying the cytotoxic effects. MgNPs-Fe₃O₄ caused dose-dependent increases in reactive oxygen species levels in DU145, PC-3, and LNCaP cells; 8-hydroxydeoxyguanosine levels were also elevated. MgNPs-Fe₃O₄ alone reduced the viability of LNCaP and PC-3 cells; however, MgNPs-Fe₃O₄ enhanced the cytotoxic effect of a low dose of DTX in all three cell lines. MgNPs-Fe₃O₄ also augmented the percentage of DU145 cells undergoing apoptosis following treatment with low dose DTX. Expression of nuclear transcription factor κB in DU145 was not affected by MgNPs-Fe₃O₄ or DTX alone; however, combined treatment suppressed nuclear transcription factor κB expression. These findings offer the possibility that MgNPs-Fe₃O₄–low dose DTX combination therapy may be effective in treating prostate cancer with limited adverse effects.

Keywords:

• prostate cancer
• magnetic nanoparticles
• docetaxel
• reactive oxidative species

Introduction

Prostate cancer is the most common cancer affecting men, and the second leading cause of cancer death in the United States.¹ The incidence and mortality rates of prostate cancer vary greatly among different geographic areas and ethnic groups. Although the incidence of prostate cancer in Japan remains low compared with that in the United States, it has been increasing in recent years. However, by 2020, prostate cancer is projected to surpass stomach cancer as the most frequently diagnosed cancer in Japanese men.²

Several management options are available when prostate cancer is diagnosed at an early stage, including watchful waiting, surgery, cryosurgery, radiation therapy, and hormonal therapy. For advanced prostate cancers, surgical or medical ablation of androgens is regarded the optimal first-line treatment. In most patients treated by androgen deprivation, disease progression will continue until reaching a stage referred to as castration-resistant prostate cancer (CRPC). Progression to a hormonal refractory state is a complex process, involving both selection and outgrowth of preexisting clones of androgen-independent cells as well as adaptive upregulation of genes that help cancer cells survive and grow after androgen ablation.³

Although the effects of several anticancer drugs for prostate cancer have been evaluated in vitro and in animal experiments in vivo, most have little or no impact on the survival of patients with CRPC or metastatic prostate cancer.^4,5 Docetaxel (DTX), a semisynthetic toxoid produced from the needles of the European yew tree, is the first chemotherapy agent to improve survival in CRPC, and the US Food and Drug Administration has recommended a 3-week DTX-prednisone regimen as a first-line treatment option for CRPC patients.^6–8 Although DTX-based chemotherapy may provide some benefits, most CRPC patients do not realize them, and the average survival remains relatively brief. Moreover, the current regimen requires the administration of high doses of DTX, which causes toxic reactions and thereby precludes the use of DTX as a monotherapy.⁹ To reduce toxicity and to improve the survival and quality of life of CRPC patients, novel therapeutic strategies targeting the molecular basis of androgen- and chemo-resistance of prostate cancer using a reduced but equieffective dose of DTX should be developed.

Cancer nanotechnology offers great potential for cancer diagnosis, targeted treatment, and monitoring.¹⁰ Researchers are exploring the use of nanoparticles (NPs) ranging in length from 1 nm to 100 nm in two or three dimensions to detect, image, monitor, and treat cancers. Among the rapidly evolving types of NPs, magnetic NPs (MgNPs) – biocompatible and superparamagnetic nanomaterials with chemical stability and low toxicity – are especially promising.¹¹ The combination of MgNPs with anticancer agents has been applied to leukemia, lung, and pancreatic cancer cells in vitro and to xenograft-injected nude mice.^12–15 MgNPs composed of Fe₃O₄ (MgNPs-Fe₃O₄) are being widely investigated for use as targeted drug carriers. The aim of this study was to evaluate the effect of treatment with MgNPs-Fe₃O₄ or MgNPs-Fe₃O₄ combined with DTX on prostate cancer cell growth in vitro. We also explored the mechanism underlying MgNPs-Fe₃O₄-induced cell death, focusing on the effect of MgNPs-Fe₃O₄ treatment on the production of reactive oxygen species (ROS).

Materials and methods

Physical characterization of MgNPs-Fe₃O₄

MgNPs-Fe₃O₄ were obtained from the Toda Kogyo Corporation (Otake, Hiroshima, Japan) and had the following characteristics: spherical shape; an average particle size of 10 nm in powder and 8–10 nm as measured by transmission electron microscopy (TEM); a size of 60–100 nm as measured by dynamic light scattering (DLS); a zeta potential of −30 to −40 mV at a pH of 10; and a surface area in powder of 100–120 m²/g.

Preparation of MgNPs-Fe₃O₄

After ultraviolet sterilization of the particles, MgNPs-Fe₃O4 stocks were prepared by suspending particles in Roswell Park Memorial Institute (RPMI)-1640 with supplements to yield final concentrations of 1 μg/mL, 10 μg/mL, or 100 μg/mL, followed by sonication at 30 W for 10 minutes with an Ultrasonic Homogenizer VP-050 (TAITEC, Koshigaya, Saitama, Japan).

Docetaxel

DTX was purchased from Sigma-Aldrich (St Louis, MO, USA) and dissolved in dimethyl sulfoxide (DMSO; stock solution). Stock solutions were aliquoted and stored at −20°C to avoid repetitive freeze-thaw cycles. Stock solutions were serially diluted using culture medium to prepare working solutions.

Cell lines

LNCaP, DU 145, and PC-3 human prostate cancer cell lines were purchased from American Tissue Type Culture Collection (Manassas, VA, USA). Cells were cultured in RPMI-1640 medium with 10% fetal bovine serum (FBS) and 100 U/mL penicillin-streptomycin in 5% CO2 at 37°C. The human normal prostate stromal cell (PrSC) line was obtained from BioWhittaker® (Lonza Walkersville, Inc, Walkersville, MD, USA) and maintained in Dulbecco’s modified Eagle’s medium supplemented with 10% FBS, 100 U/mL of penicillin G, 100 μg/mL of streptomycin, ITH (5 μg/mL insulin, 5 μg/mL transferrin, and 1.4 μmol/L hydrocortisone), and 5 ng/mL of bFGF in 5% CO2 at 37°C.

Characterization of MgNPs-Fe₃O₄ suspension

MgNP-Fe₃O₄ suspensions and their cellular localization were characterized using the following methods.

Dynamic light scattering (DLS)

The average hydrodynamic size and size distribution of MgNPs-Fe₃O₄ in media were determined by DLS using a Fiber-Optics Particle Analyzer FPAR-1000 (Otsuka Electronics Co, Ltd Hirakata, Osaka, Japan). DU145 cells were incubated with MgNPs-Fe₃O₄ (1 μg/mL, 10 μg/mL, or 100 μg/mL).

Transmission electron microscopy (TEM)

DU145 cells were incubated with MgNPs-Fe₃O₄ (10 μg/mL). After incubation for 24 hours, cells were collected washed three times with phosphate buffered saline (PBS), and fixed with 3% glutaraldehyde in 0.1 M cacodylate buffer (pH 7.3) at 4°C for 4 hours. The resulting samples were postfixed with 2% osmium tetraoxide at 4°C for 2 hours, dehydrated and embedded in epoxy resin. Ultrathin sections (80 nm) were then stained with uranyl acetate and lead citrate, and observed by TEM.

Measurement of intracellular reactive oxygen species

ROS were measured using the CM-H2DCFDA assay (Life Technologies, Carlsbad CA, USA), according to the manufacturer’s instructions. DU145 cells (1.0 × 10⁵ cells/well) were incubated with MgNPs-Fe₃O₄ (1 μg/mL, 10 μg/mL, or 100 μg/mL) for 24 hours in the absence or presence of N-acetylcysteine (NAC; 10 mM) (Sigma-Aldrich Co); NAC was added 3 hours before treatment with MgNPs-Fe₃O₄. A stock solution of CM-H2DCFDA (5 mM) was freshly prepared in DMSO and diluted to a final concentration of 1 μM in PBS. Cells were washed with PBS followed by incubation with 50 μL of working solution of fluorochrome marker CM-H2 DCFDA for 30 minutes. Fluorescent imaging was recorded using an IX2 N-FL-1 microscope (Olympus Corporation, Tokyo, Japan), and analyzed using imaging software (Adobe Photoshop Elements 8; Adobe Systems Incorporated San Jose, CA, USA). As a positive control, cells were treated with H₂O₂ (100 μM) for 24 hours.

Analysis of 8-hydroxydeoxyguanosine in DNA

The MgNPs-Fe₃O₄ (1 μg/mL, 10 μg/mL, or 100 μg/mL) were added to wells containing DU145, PC-3, or LNCap cells (5.0 × 10⁶ cells), and incubated for 72 hours at 37°C (5% CO2). Nuclear deoxyribonucleic acid (DNA) of the cells was isolated by the sodium iodide method. Analysis of 8-hydroxydeoxyguanosine (8-OH-dG) was performed as previously described.¹⁶ The 8-OH-dG levels were measured by high performance liquid chromatography electrochemical detection. The amount of 8-OH-dG in the DNA was determined through comparisons with the authentic standards, and expressed as the number of 8-OH-dG per 10⁶ deoxyguanosine (dG).

AlamarBlue® assay

Cell viability was determined using the AlamarBlue® assay (Alamar Biosciences, Inc, Sacramento, CA, USA), according to the manufacturer’s instructions. Briefly, cells were seeded in 24-well plates (1.0 × 10⁴ cells/well); cells were treated with DTX (0.1 μM, 1 μM, 10 μM, or 100 μM) or DTX (1 nM) plus MgNPs-Fe₃O₄ (1 μg/mL, 10 μg/mL, or 100 μg/mL) for 48 hours at 37°C (5% CO2). AlamarBlue® was added to each well at 10% volume and was incubated for 200 minutes. Metabolically active cells reduced the dye into a fluorescent form; fluorescence intensity was measured using a plate reader (excitation/emission: 570 nm/600 nm; Viento XS, DS Pharma Biomedical Co, Ltd Suita, Osaka, Japan). Fluorescence intensity was used to estimate cell viability by linear interpolation between the emission from cells treated with 0.1% saponin (0% viability) and that from untreated cells (100% viability).

Flow cytometry (FCM) analysis for cell apoptosis

The apoptotic peak (sub-G1) of cells was measured using FCM. DU145 cells (1.0 × 10⁶ cells) were seeded in 100 mm culture dishes; cells were either untreated (control), or treated with DTX (1 nM) or MgNPs-Fe₃O₄ (10 μg/mL or 100 μg/mL) in the absence or presence of DTX (1 nM). Aspirated medium was collected to determine the amount of floating cells and cell debris as indicators of cell death. Cells were collected and fixed in ice-cold 70% ethanol and stored at −20°C before use. In preparation for use, cells were washed with PBS and resuspended in PBS before incubation with ribonuclease (0.5 mg/mL) at room temperature for 30 minutes. After the addition of 1 mg/mL of propidium iodide (PI; Sigma-Aldrich), the cells were passed through a 40 mm nylon mesh for analysis using an LSRII flow cytometer (BD Bioscience Franklin Lakes, NJ, USA). The fluorescence intensities of PI were measured by FCM, and the number of cells in the sub-G1 peak was determined. Quantification of the fraction was performed with ModFit LT for Mac 3.0 (Verity Software House, Topsham, ME, USA).

Annexin-V assay was used to detect the early phases of apoptosis. Apoptosis was assessed by monitoring the expression of phosphatidylserine on the outer leaflet - an early marker of apoptotic cell death. Phosphatidylserine was stained with fluorescein isothiocyanate (FITC)-labeled Annexin V Loss of membrane integrity as a consequence of necrosis was detected using PI staining of DNA. Briefly, DU145 cells (1.0 × 10⁶) were either untreated (control) or treated with DTX (1 nM), or with MgNPs-Fe₃O₄ (10 μg/mL or 100 μg/mL) for 24 hours in the absence or presence of DTX (1 nM). After incubation, cells were harvested gently washed twice in ice-cold PBS, collected by centrifugation, and then stained using an Annexin V-FITC Kit (Beckman Coulter, Inc, Fullerton, CA, USA) according to the manufacturer’s instructions. Cells were then stained with Annexin V and PI for analysis by FCM within 1 hour of staining using the FL1 (FITC) and FL3 (PI) lines.

Western blot analysis

Cells were lysed in Radioimmunoprecipitation assay buffer (Sigma-Aldrich) containing protease inhibitors (Sigma-Aldrich). Total protein concentration was determined by Bio-Rad protein assay reagent (Bio-Rad Laboratories, Hercules, CA, USA). Equal amounts of lysates were resolved on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to a polyvinylidene fluoride membrane (Merck Millipore, Billerica, MA, USA). Membranes were blocked with a blocking reagent (NOF Corporation, Tokyo, Japan) for 1 hour at room temperature, and incubated overnight at 4°C with the respective primary antibodies in Tris-buffered saline and Tween 20 (TBST). The membranes were washed with TBST three times and incubated with diluted horseradish peroxidase-conjugated secondary antibodies (1:3000 for nuclear factor κB [NFκB]; 1:10,000 for β-actin) for 1 hour at room temperature. After three additional washes, membranes were detected using an enhanced chemiluminescence kit (GE Healthcare UK Ltd, Little Chalfont, UK). Antibodies against NFκB and β-actin were purchased from Santa Cruz Biotechnology, Inc (Santa Cruz, CA, USA) and Sigma-Aldrich, respectively; antirabbit and antimouse horseradish peroxidase-conjugated secondary antibodies were purchased from GE Healthcare (GE Healthcare UK Ltd).

Statistical analysis

All experiments were repeated at least three times. Data are represented as the mean ± standard deviation. Data were analyzed using an unpaired Student’s t-test with or without Welch’s correction and ANOVA. Differences were considered statistically significant at P < 0.05.

Results

MgNPs-Fe₃O₄ characterization in cell culture medium

Figure 1 shows the mean hydrodynamic diameter of MgNPs-Fe₃O₄ in medium with supplements as measured by DLS. The mean hydrodynamic diameter of MgNPs-Fe₃O₄ increased with increasing concentration, suggesting that aggregation is enhanced at higher concentrations.

Figure 1 Measurement of MgNPs-Fe₃O₄ size by dynamic light scattering. DU145 cells were incubated with MgNPs-Fe₃O₄: (A) 1 μg/mL, (B) 10 μg/mL, and (C) 100 μg/mL.

Abbreviation: MgNPs-Fe₃O₄, Fe₃O₄ magnetic nanoparticles.

Cellular uptake

Cellular uptake of MgNPs-Fe₃O₄ was evident from TEM microphotographs (Figure 2). MgNPs-Fe₃O₄ were localized within intracellular vesicles.

Figure 2 Transmission electron microscopy imaging of DU145 cells treated with 10 μg/mL of Fe₃O₄ magnetic nanoparticles.

Notes: Arrow: extracellular magnetic nanoparticles; arrow head: intracellular magnetic nanoparticles.

Abbreviation: N, nucleus.

ROS production

MgNPs-Fe₃O₄ caused dose-dependent increases of ROS production in DU145 and PC-3 cells; a significant increase in LNCaP cells was evident only at the highest dose. Treatment with 100 μg/mL of MgNPs-Fe₃O₄ elicited a response comparable to that evoked by H₂O₂ (Figure 3). Among the three cell lines, ROS levels in the DU145 and PC-3 lines were higher than that in the LNCaP line. Pretreatment with NAC attenuated the MgNPs-Fe₃O₄-induced rise in ROS in all three prostate cancer cell lines (Figure 3).

Figure 3 Production of intracellular ROS in DU145, PC-3, and LNCaP cells after treatment with MgNPs-Fe₃O₄ for 24 hours in the absence or presence of NAC.

Notes: Data are presented as the mean ± SD of three independent experiments. *Significantly different from the untreated control at P < 0.05; **significantly different from the control at P < 0.01; ***significantly different from the untreated control at P < 0.001.

Abbreviations: ROS, reactive oxygen species; MgNPs-Fe₃O₄, Fe₃O₄ magnetic nanoparticles; NAC, N-acetylcysteine; SD, standard deviation.

8-OH-dG levels in DNA

The 8-OH-dG levels in the DNA in all three prostate cancer cell lines increased in a dose-dependent manner (Figure 4). The 8-OH-dG levels of DU145 and PC-3 cells exposed to 10 μg/mL of MgNPs-Fe₃O₄ were 13-fold to 14-fold greater than that of the untreated control cells.

Figure 4 8-OH-dG levels in DU145, PC-3, and LNCaP cells after 72 hours of treatment with MgNPs-Fe₃O₄.

Notes: Data are presented as the mean ± SD of three independent experiments. *Significantly different from the untreated control at P < 0.05; **significantly different from the control at P < 0.01; ***significantly different from the untreated control at P < 0.001.

Abbreviations: 8-OH-dG, 8-hydroxydeoxyguanosine; MgNPs-Fe₃O₄, Fe₃O₄ magnetic nanoparticles; SD, standard deviation.

Effect of MgNPs-Fe₃O₄, DTX, and MgNPs-Fe₃O₄–DTX combinations on cell viability

MgNPs-Fe₃O₄ alone reduced the viability of LNCaP and PC-3 cells, but had little or no effect on the viability of DU145 and PrSC cells (Figure 5). These results suggest that the cytotoxicity of MgNPs-Fe₃O₄ may be dependent on the cell type of the prostate cancer cell line. DTX alone decreased cell viability in a dose-dependent manner in all three cancer cell lines (Figure 6). Combined treatment with MgNPs-Fe₃O₄ and DTX enhanced the inhibitory effect of DTX; in PC-3 cells, 100 μg/mL of MgNPs-Fe₃O₄ plus 1 nM of DTX reduced cell viability so it was similar to that caused by 10 nM DTX alone. These data suggest that MgNPs-Fe₃O₄ may be beneficial in reducing the DTX dose it may and thereby overcome the safety limitations of DTX.

Figure 5 Effect of MgNPs-Fe₃O₄ exposure on cell line viability. Effect of MgNPs-Fe₃O₄ exposure on the viability of (A) DU145, (B) PC-3, (C) LNCaP, and (D) PrSC cell lines.

Notes: Data are presented as the mean ± SD of three independent experiments. *Significantly different from the control at P < 0.05.

Abbreviations: MgNPs-Fe₃O₄, Fe₃O₄ magnetic nanoparticles; SD, standard deviation.

Figure 6 Effect of DTX alone or in combination with MgNPs-Fe₃O₄ on cell viability. Effect of DTX alone or in combination with MgNPs-Fe₃O₄ on the viability of (A) DU145, (B) PC-3, and (C) LNCaP cell lines.

Notes: Data are presented as the mean ± SD of three independent experiments. *Significantly different from the control at P < 0.05; **significantly different from the control at P < 0.01.

Abbreviations: DTX, docetaxel; MgNPs-Fe₃O₄, Fe₃O₄ magnetic nanoparticles; SD, standard deviation.

Effect of MgNPs-Fe₃O₄, DTX, and MgNPs-Fe₃O₄–DTX combinations on cell death

An apoptotic fraction of cells containing subdiploid amounts of DNA was detected as a sub-G1 peak (Figure 7). MgNPs-Fe₃O₄ caused a dose-dependent increase in the percentage of DU145 cells in the sub-G1 fraction; similarly, DTX alone elicited a rise in the percentage of cells in the sub-G1 fraction. Combined treatment with MgNPs-Fe₃O₄ plus DTX augmented the effect compared to either treatment alone; this enhancement was dose-dependent.

Figure 7 Flow cytometry analysis of apoptosis of DU145 cells. Panels represent the following treatments: (A) untreated (control); (B) MgNPs-Fe₃O₄ (10 μg/mL); (C) MgNPs-Fe₃O₄ (100 μg/mL); (D) DTX (1 nM); (E) DTX (1 nM) + MgNPs-Fe₃O₄ (10 μg/mL); and (F) DTX (1 nM) + MgNPs-Fe₃O₄ (100 μg/mL).

Notes: Cells were incubated with each condition for 24 hours. The percentage of cells in the sub-G1 phase was quantified for each plot.

Abbreviations: MgNPs-Fe₃O₄, Fe₃O₄ magnetic nanoparticles; DTX, docetaxel.

Neither MgNPs-Fe₃O₄ nor DTX alone increased Annexin V/PI staining (Figure 8). Conversely, a significant increase in the percentage of apoptotic cells was observed during the combined treatment with NPs-Fe₃O₄ and DTX compared to the untreatment, the treatment with MgNPs-Fe₃O₄ alone, or DTX alone (P < 0.05).

Figure 8 Effects of MgNPs-Fe₃O₄ and docetaxel (DTX) alone or in combination on apoptosis in DU145 cells. (A) representative FCM using Annexin V/PI staining of one set of triplicate experiments. N10: MgNPs-Fe₃O₄ (10 μg/mL); N100: MgNPs-Fe₃O₄ (100 μg/mL); DTX: DTX (1nM); CB10: DTX (1nM) + MgNPs-Fe₃O₄ (10 μg/mL); and CB100: DTX (1 nM) + MgNPs-Fe₃O₄ (100 μg/mL). (B) Percentages of apoptotic cells from FCM analysis.

Notes: Data are presented as the mean ± SD of three independent experiments. Results show that the combination of 10 μg/mL or 100 μg/mL of MgNPs-Fe₃O₄ with 1 nM of DTX induced significant apoptosis in DU145 cells compared to untreated cells, cells treated with 10 μg/mL or 100 μg/mL of MgNPs-Fe₃O₄ alone, or 1nM of DTX alone (*P < 0.05).

Abbreviations: MgNPs-Fe₃O₄, Fe₃O₄ magnetic nanoparticles; DTX, docetaxel; PI, propidium iodide; DMSO, dimethyl sulfoxide; FCM, flow cytometry analysis; SD, standard deviation.

Effect of MgNPs-Fe₃O₄ and MgNPs-Fe₃O₄–DTX combinations on NFκB expression in DU145 cells

Treatment with MgNPs-Fe₃O₄ alone did not lower NFκB expression in DU145 cells; conversely, treatment with MgNPs-Fe₃O₄–DTX combinations inhibited NFκB expression in a dose-dependent manner (Figure 9).

Figure 9 Effects of MgNPs-Fe₃O₄ in the absence or presence of DTX on NFκB expression in DU145 cells. (A) Western blot analysis. (B) Densitometric analysis of NFkB/actin expression ratio.

Notes: Cells were treated for 48 hours. The ratio of NFκB expression/actin expression represents the mean ± SD of three independent experiments. Results show that NFkB expression decreased in DU145 cells treated with 100 μg/mL of MgNPs-Fe₃O₄ with 1 nM of DTX compared to untreated cells (*P < 0.05).

Abbreviations: MgNPs-Fe₃O₄, Fe₃O₄ magnetic nanoparticles; DMSO, dimethyl sulfoxide; DTX, docetaxel; NFκB, nuclear factor κB; SD, standard deviation.

Discussion

DTX remains the cornerstone of chemotherapy for treating prostate cancer when castration resistance is documented and secondary hormone therapy is ineffective. However, to be effective, DTX must be administered at such high doses that can induce significant toxicity.^9,17 To overcome this drawback, combination therapies have been developed; DTX combined with tyrosine kinase or bcl-2 inhibitors are currently in Phase II studies for treating CRPC.¹⁷ Drug-delivery assemblies consisting of a nanocarrier, a targeting agent, and DTX have also been developed. For example, NC-6301 – a polymeric micelle with DTX – shows less toxicity than native DTX in vivo; NC-6301 is a nanoscale drug delivery system approximately 100 nm in a diameter.¹⁸ In the present study, we found that DTX alone has a strong anticancer effect, and the cytotoxic effect of a low concentration (1 nM) is augmented by MgNPs-Fe₃O₄.

Many studies have focused on the use of NPs, especially MgNPs, in theranostics.^10,11 Due to their biocompatibility and stability, iron oxide MgNPs, particularly magnetic Fe₃O₄ and its oxidized and more stable form, maghemite γ-Fe₂O₃, are superior for biomedical applications compared to other metal oxide NPs. Moreover, iron oxide NPs may have additional utility as a contrast agent in magnetic resonance imaging or as a carrier for drug delivery.^11–15 In the present study, we focused on MgNPs-Fe₃O₄ because of their potential to treat CRPC. This stems from the intrinsic properties of the magnetic core combined with the drug loading capability and the biomedical properties of MgNPs conferred by different surface coatings. Iron oxide MgNPs have also effectively been used in combination with chemotherapy and hyperthermia to overcome drug resistance in a leukemia xenograft model,¹⁹ and with doxorubicin under a static magnetic field to enhance the doxorubicin-mediated cytotoxicity of MCF-7 cells.²⁰

Results from several studies suggest that the cytotoxic effects of MgNPs are dependent on the metal and target cell type.^21,22 CuO, ZnO, and CuZnFe₂O₄, but not Fe₂O₃ or Fe₃O₄, were highly toxic to the human lung epithelial cell line A549; CuO NPs were especially effective in inducing a significant increase in ROS production.²¹ In BRL 3A liver cells, only silver NPs were highly toxic; Fe₃O₄, tungsten, aluminum, and MnO₂ exhibited little or no toxicity.²³ Conversely, iron oxide MgNPs caused hepatic and renal damage when administered to mice,²² and the reduced viability of J774 macrophages in vitro.²⁴

ROS act as a second messenger in cell signaling and are involved in various biological processes, such as growth and survival in normal cells. Oxidative stress reflects a redox imbalance within the cells and usually results from the net accumulation of intracellular ROS, which are not detoxified by cellular antioxidative agents.²⁵ In cancer cells, the production of ROS is typically increased; since they play important roles in initiation, progression, and metastasis, ROS are considered oncogenic. However, ROS are also implicated in triggering cell death, including that of cancer cells; thus, their production is desirable in chemotherapy, radiotherapy, and photodynamic therapy. This dual role of ROS has led to the development of two paradoxical ROS-manipulation strategies in cancer treatment.²⁵ One strategy is to treat tumor cells with antioxidants, such as through the dietary administration of red wine and green tea polyphenols to prevent cancer. The other strategy is to provide pro-oxidant therapy, which consists of generating ROS directly and inhibiting antioxidative enzyme systems in tumor cells. In the present study, MgNPs-Fe₃O₄ exhibited mild cytotoxicity toward PC-3 and LNCaP, but not toward DU145 and PrSC cells. The LNCaP and PC-3 cell lines have previously been reported to have unique redox state properties, including the production of different levels of oxidative damage products and antioxidant proteins; these differences may provide new insights into the possible uses and dangers of using pro-oxidants or antioxidants as cancer therapeutic agents.^26–28 We found that the MgNPs-Fe₃O₄-induced increase in ROS was most robust in the DU145 and PC-3 cell lines; however, the levels of 8-OH-dG, an index of oxidative DNA damage, were comparably elevated in all three cell lines.

The transcription of antiapoptotic genes is activated by the NFκB signaling pathway, resulting in cell survival. The NFκB signaling pathway also plays a critical role in cancer development and progression, and in the development of tumor resistance to chemotherapy and radiation therapy,²⁹ particularly in the transition toward CRPC.³⁰ Previous studies demonstrating a relationship between elevated NFκB and a worse prognosis support this notion.^31,32 Thus, the NFκB pathway has become an important target in the development of novel anticancer treatments. The combination of magnetic NPs with either adriamycin or daunorubicin has been reported to increase p53 levels and decrease NFκB protein levels, leading to increased apoptosis in Raji lymphoma cells.³³ In the present study, treatment with MgNPs-Fe₃O₄ or DTX alone had no effect on the expression of NFκB in DU145 cells; however, treatment with MgNPs-Fe₃O₄–DTX combinations decreased expression in a dose-dependent manner. This result is unique because many NPs have been reported to activate the NFκB pathway via activation of mitogen-activated protein kinase cascades by an oxidative stress response.³⁴ Thus, our results suggest that the decrease in NFκB expression resulting from treatment with MgNPs-Fe₃O₄–DTX combinations may be uncoupled from ROS generation. Although the chemical components involved in NFκB inhibition and ROS production have been identified, the contribution of MgNPs-Fe₃O₄ exposure to the mechanisms of induction and action remains unclear. Further studies such as those measuring NFκB DNA-binding activity are needed.

Conclusion

We found that MgNPs-Fe₃O₄ significantly increased ROS production in prostate cancer cell lines and induced oxidative DNA damage; the cytotoxic effects of MgNPs-Fe₃O₄ alone were mild. Treatment with a combination of MgNPs-Fe₃O₄ and a low dose of DTX enhanced the inhibitory effect of DTX alone on prostate cancer cell growth in vitro, and also suppressed NFκB expression. These findings offer the possibility that MgNPs-Fe₃O₄ may allow the dose of DTX to be reduced without decreasing its antitumor activity.

Acknowledgments

We thank Dr T Yabana for TEM analysis, and Dr M Yoneda for his pathological preparation. This research was supported in part by a Grant-in-Aid for the Global COE Program from the Ministry of Education, Culture, Sports, Science and Technology of Japan, a Grant-in-Aid for Research on Risk of Chemical Substances from the Ministry of Health, Labor and Welfare of Japan, and a Research Grant-in-Aid from Magnetic Health Science Foundation.

Disclosure

The authors report no conflicts of interest in this work. The authors have no financial interests in or financial conflict with the subject matter discussed in this manuscript.