1. Introduction
Considering the high-expression of telomerase activity in most cancer cells, cancer treatment specifically targeting telomerase has attracted people’s interest [
1,
2]. As one of the main subunits of telomerase, human telomerase reverse transcriptase catalytic subunit (hTERT) has been proved to be the rate limiting component of telomerase activity and thus become an important target for telomerase regulation [
3,
4]. Some studies have pointed out that the regulation of hTERT mRNA by anti-sense technology can effectively induce apoptosis of cancer cells [
5], and the drug–Imetelstat (GRN163L) based on this principle has long been in clinical trials [
6].
Many studies have shown that a variety of microRNAs in cancer cells, such as carcinogenic microRNAs including microRNA-19b, microRNA-346 and microRNA-21, could directly or indirectly up-regulate the expression of hTERT so as to promote cancer invasiveness, oxidative stress, genomic instability, and cell proliferation, as well as evasion of apoptosis [
7,
8,
9]. Among them, microRNA-21 was found to be indirectly associated with hTERT up-regulation in colorectal cancer and malignant melanoma cells by down-regulating PTEN (a tumor suppressor gene) and then activating PI3K/Akt pathway [
10]. Also, microRNA-21 was proved to enhance carcinogenesis through STAT3, and reduced expression of both hTERT and STAT3 as well as slowed tumor growth was observed when microRNA-21 was knocked down in murine glioblastoma xenografts [
11]. Moreover, if the inhibition of carcinogenic microRNAs is combined with telomerase therapy, the resistance effect in telomerase therapy can be reduced [
12]. Therefore, down-regulation of carcinogenic microRNAs and hTERT mRNA by anti-sense oligonucleotide technology will play a positive role in combating cancer. However, the nucleic acid fragment is not easy to enter the cell directly, appropriate carriers are needed to improve the intake rate of anti-sense oligonucleotides.
The structure of tumor vessels is abnormal with wide interendothelial junctions and a large number of fenestration. Therefore, after administered intravenously, nanoparticles easily extravasate through leaky vasculature and accumulate in the tumor [
13]. In addition, targeting ligands modified on the nanoparticles surface can specifically recognize tumor and bind to over-expressed receptors with a high affinity in the target region, which would induce nanomedicines extravasating into tumors through active trans-endothelial mechanisms [
14]. Among the nanoparticle-based drug delivery systems that have been developed, gold nanoparticles (AuNPs) have become the preferred carrier for researchers [
15,
16] due to their several advantages of good biosecurity, stability, surface functionization and fluorescence quenching property. Also, AuNPs have been proved to penetrate tumor vascular system and engineering tumoral vascular leakiness and increase tumoral accessibility of anti-tumor therapeutics and subsequently enhancing therapeutic efficiency [
17]. Altogether, targeting ligands modified AuNP-based drug delivery systems present great application potential in the field of targeted anti-tumor therapy.
To expand the application of AuNPs in the tumor theranostics, previously we designed two AuNP-based nucleic acid probes loaded with anti-sense oligonucleotide sequences to achieve in situ detection, fluorescence imaging and down-regulation of intracellular hTERT mRNA or microRNA-21 [
18,
19]. While the effectiveness of AuNP-based nucleic acid probes in inducing cancer cell apoptosis by anti-sense technology has been verified in the cell level, in vivo studies are needed to show their anti-tumor therapy potential. As a continuation of this series of research work, here we designed a gold nanoprobe (Au-nanoprobe) containing two types of anti-sense sequences, in which one type of anti-sense sequence is used for down-regulating hTERT mRNA and the other is designed for silencing microRNA-21. In addition, in order to achieve targeted delivery to tumors and improving cancer cell uptake of nanomedicines, the aptamer AS1411 that recognizes cancer cells through high affinity with the surface over-expressed nucleolin [
20] was modified on the surface of Au-nanoprobes with disulfide bonds as linkers. The in vitro and in vivo anti-cancer effect of Au-nanoprobes was investigated here.
2. Materials and Methods
2.1. Preparing Au NPs and Au-Nanoprobes
AuNPs were synthesized according to the literature previously reported [
21]. Firstly, 50 ml of HAuCl
4 (1 mM) solution was heated to 100 °C. Next, 10 ml of trisodium citrate solution (38.8 mM) is quickly added the above boiled HAuCl
4 solution and the mixed solution was stirred thoroughly at 100 °C for 15 min to obtain the dark red AuNPs solution. The size and morphology of AuNPs were characterized by the JEM-2100 transmission electron microscope (JEOL, Japan).
Au-nanoprobes are prepared according to the following procedure. All DNA sequences (1 OD) (
Table S1) are individually dissolved in 100 μL DEPC water. HS-anti-hTERT-DNA and Cy3-hTERT-DNA were mixed in the molar ratio of 1: 1.2 and the mixture was heated to 75 °C and hold for 10 min. Then the mixture was naturally cooled to room temperature and incubated under dark conditions for 12 h to obtain the hybridized hTERT-related DNA duplexes (HS-anti-hTERT-DNA/Cy3-hTERT-DNA). With the same procedure, the hybridized microRNA-21-related DNA duplexes (HS-miRNA-21-DNA/Cy5-AS1411-anti-miRNA-21-DNA) was also prepared. Next, the hybridized hTERT- and microRNA-21-related DNA duplexes systems were mixed together and further reacted with 4 mL AuNPs solution. The final mixture were incubated with rotation at room temperature for 24 h and further inactivated with PBS solution (400 μL) thrice with the interval time of 10 h. Then the reaction mixture was centrifuged and the precipitate was washed with PBS solution thrice to discard the unbound DNA sequences. Finally, the newly prepared Au-nanoprobes were dispersed in PBS solution (4 mL) and store at 4 °C for future studies. The ultraviolet visible (UV-Vis) absorption spectrum of Au-nanoprobes were determined and the concentration of Au-nanoprobes was calculated by measuring their extinction at 524 nm (
ε = 2.7 × 10
8 L·mol
-1·cm
-1).
2.2. Evaluating the Amount of DNA Duplexes Bound on Each Au-Nanoprobe
The amount of HS-anti-hTERT-DNA/Cy3-hTERT-DNA or HS-miRNA-21-DNA/Cy5-AS1411-anti-miRNA-21 duplexes bound on each Au-nanoprobe was evaluated according to the previously reported protocol [
22]. Briefly, different concentrations of mercaptoethanol (0, 1, 1.5, 2, 3, 5, and 10 mM) were individually added to the probe solutions (1.5 nM). After incubation overnight with shaking at room temperature, DNA duplexes were gradually released by the competitive binding of mercaptoethanol with AuNPs. Then the released DNA duplexes were separated from AuNPs through centrifugation and the fluorescence intensity of suspension was determined by using the F-7000 spectrofluorometer (Hitachi, Japan) with the excitation wavelengths of 530 and 630 nm. The standard linear calibration curve was prepared with known concentrations of two types of DNA duplexes (0, 10, 20, 40, 60, 80, 100, 120, 150, and 200 nM) with identical buffer pH, ionic strength and mercaptoethanol concentrations. The amount of HS-anti-hTERT-DNA/Cy3-hTERT-DNA or HS-miRNA-21-DNA/Cy5-AS1411-anti-miRNA-21 duplexes bound on each Au-nanoprobe was calculated by referring the fluorescence intensity of the supernatant containing DNA duplexes collected after the prepared probes were incubated with mercaptoethanol (10 mM) to the standard curve.
2.3. Determining the Fluorescence Response of Au-Nanoprobes to Target DNA
In order to verify the responsiveness of Au-nanoprobes to the hTERT mRNA or microRNA-21-related target DNA, a series of Target-hTERT-DNA or Target-miRNA-21-DNA (0, 100, 200, 600, and 1000 nM) were individually mixed with Au-nanoprobes (200 μL, 1.5 nM). All mixtures were incubated at 37 °C for 4 h. Then the fluorescence intensity of different system was determined by using the F-7000 spectrofluorometer (Hitachi, Japan) with the excitation wavelengths of 530 and 630 nm.
2.4. Determining the Fluorescence Response of Au-Nanoprobes to Intracellular MicroRNA-21 and hTERT mRNA
Two telomerase-positive cancer cell lines, including HeLa (human cervical cancer cells) and MCF-7 (human breast cancer cells), were cultured in the DMEM medium (GIBCO) containing fetal bovine serum and Penicillin-Streptomycin (100 µg·ml−1) with the ratio of 9 : 1 : 0.1 at 37 °C in a humidified atmosphere containing 5% CO2. The cell number was determined using the Petroff-Hausser cell counter (USA).
The fluorescence responsiveness of Au-nanoprobes to intracellular microRNA-21 and hTERT mRNA was studied with the cell lysates and living cells, respectively. The experimental process were shown as follows.
(i) Cell lysate analysis: Cell lysates were obtained by breaking down MCF-7 cells (1 × 106) by using ultrasonic disruptor. Au-nanoprobes (1.5 nM) were incubated with the freshly prepared cell extracts at 37 °C for 4 h. The fluorescence intensity of the experimental systems were determined by using the F-7000 spectrofluorometer (Hitachi, Japan) with the excitation wavelengths of 530 and 630 nm.
(ii) In situ fluorescence imaging: MCF-7 or HeLa cells (0.4 mL, 1 × 106 mL-1) were respectively seeded in a 20-mm glass-bottom confocal dish. After 24 h, Au-nanoprobes (1.5 nM) were incubated with cells for 4 h. Then, cells were washed with PBS thrice and observed by LSM880 confocal laser scanning microscopy (CLSM, Zeiss, Germany). The fluorescence signals of Cy3 and Cy5 of Au-nanoprobes responsive to hTERT mRNA and microRNA-21 were excited with the wavelengths of 543 and 633 nm, respectively.
2.5. Analyzing the Intracellular hTERT mRNA and microRNA-21 Level
HeLa and MCF-7 cells (5 × 106) in the logarithmic growth phase were treated with AuNP-probes (1.5 nM) or HS-control-DNA/Control-DNA duplex-functionalized probes (Control-Au-nanoprobes, 1.5 nM) for different times (12, 24, 48, or 72 h). Then, total RNA from the tested cells was extracted using Trizol total RNA isolation reagent (TIANGEN) according to the manufacturer’s instructions. The cDNA was reverse- transcribed using a QuantiNova Reverse Transcription Kit (Qiagen, Duesseldorf, Germany). The reactions were incubated in a thermal cycler for 60 min at 37 °C, 5 min at 95 °C, and then held at 4 °C. Real-time quantitative, reverse-transcription polymerase chain reaction (qRT-PCR) was performed using the QuantStudio™ 5 Real-Time PCR system (Applied Biosystems, USA) with specific microRNA-21 primers from the commercial kit (miScript Primer Assays, Qiagen) and miScript SYBR® Green PCR Kit (Qiagen). Relative level of microRNA-21 was calculated from the quantity of microRNA-21 PCR products and the quantity of RNU6B PCR products and normalized to the expression level in untreated cells using the 2-ΔΔCT method [ΔΔCT = (CTmiRNA-21−CTU6-RNA)experimental group − (CTmiRNA-21−CTU6-RNA)untreated group]. The reaction proceeded as follows: 1 cycle of 95 °C for 15 min, followed by 40 cycles of 94 °C for 15 s, 55 °C for 30 s, and 70 °C for 30 s.
With the similar procedure, the relative level of hTERT mRNA was also determined by using qRT-PCR. The sequences of forward and reverse primers of hTERT and GAPDH are given in
Table S1. The reaction proceeded as follows: 1 cycle of 50 °C for 2 min and 1 cycle of 95 °C for 2 min were followed by 40 cycles of 95 °C for 15 s, 55 °C for 15 s, and 72 °C for 1 min.
2.6. Analyzing the Intracellular hTERT Activity
MCF-7 cells (5 × 105 cells/well) were inoculated in 6-well plates and cultured for 24 h. Then all cell samples were divided into three groups and were incubated with PBS, Control-Au-nanoprobes (1.5 nM), or Au-nanoprobes (1.5 nM), respectively for 48 or 72 h. After the treatment, hTERT in different cell samples was extracted according to the following procedure. Firstly, 1 × 106 cells were dispensed in a 1.5 mL EP tube, washed thrice with ice-cold PBS (0.1 M, pH 7.4) through centrifugation, and resuspended in ice-cold CHAPS lysis buffer (200 µL) containing 10 mM Tris-HCl, pH 7.5, 1 mM MgCl2, 1 mM EGTA, 0.1 mM PMSF, 0.5% CHAPS and 10% glycerol. The mixture was incubated for 30 min on ice and centrifuged at 16000 rpm at 4 °C for 20 min. The supernatant was collected as cell extract for analysis. To quantify the hTERT activity in different samples, a standard curve was constructed using a commercial hTERT activity ELISA Kit (Shanghai Kepeirui Biotech. Co. Ltd.). The hTERT activity level in different cell extracts was determined according to the procedure given by the ELISA Kit and referring to the standard curve.
2.7. Determining the Pro-Apoptosis Effect and In Vitro Cytotoxicities of Au-Nanoprobes
Pro-apoptosis effect of Au-nanoprobes to MCF-7 cells was investigated by using the AnnexinV-FITC/PI method. MCF-7 (5 × 105 cells/well) were inoculated in 6-well plates. Cells were incubated with PBS (blank group) or Au-nanoprobes (1.5 nM), respectively, for 24, 48, or 72 h. Then, cells were stained according to the procedure given by the commercial Annexin V-FITC/PI apoptosis kit (Beyotime) and collected after the trypsinization treatment for the cell apoptosis analysis determined by Guava easyCyte 5HT flow cytometer (Millipore, USA). The apoptosis data were analyzed by FlowJo v10 software.
The in vitro cytotoxicities of Au-nanoprobes against HeLa and MCF-7 cells were determined by using the MTT method. Firstly, a certain number of cells were inoculated in 96-well plates (1 × 105 cells/well). 24 h later, cells were treated with Au-nanoprobes (1.5 or 2 nM) for 24, 36, 48, or 72 h. For the blank or control groups, cells were treated with PBS or Control-Au-nanoprobes (1.5 or 2 nM), respectively. Then, the cell medium was removed and replaced with 100 μl fresh medium containing 2.5 mg/ml of MTT. 4 h later, 100 μl DMSO was added to dissolve the formazan crystals after the removal of MTT solution. The absorbance at the wavelength of 490 nm was measured with the microplate reader. Cell survival was calculated from subtracting the optical density (OD) value of each well by that of blank group.
2.8. In Vivo and Ex Vivo Fluorescence Imaging
Xenograft tumor models of MCF-7 were built by subcutaneously injecting MCF-7 cells (1×106) in 200 µL Matrigel into the right flank of female balb/c nude mice (3 ~ 4 weeks old). MCF-7 tumor bearing balb/c nude mice with the tumor volume of ~300 mm3 were randomly divided into three groups (n = 3) and fasted 12 h before the experiment with free access to water. Then, biodistribution of Au-nanoprobes was investigated after a single intravenous injection at a dose of 50 μL (6.5 nM). As controls, the other two groups of mice were individually administered with PBS or Control-Au-nanoprobes (50 μL, 6.5 nM). At timed intervals, the mice were anesthetized and then imaged by using the PerkinElmer IVIS Spectrum In Vivo Imaging System for tracking the Cy5 modified anti-miRNA-21-DNA (excitation: 640 nm; emission: 680 nm; epi-illumination). In addition, at 60 min, two representative mice that were treated individually with PBS or Au-nanoprobes, were sacrificed by cervical dislocation and the tumors as well as main organs (hearts, livers, spleens, lungs, kidneys) were excised and imaged by the IVIS Spectrum system.
2.9. In Vivo Anti-Tumor Study
When the tumors reached approximately 100 mm3 (set as Day 0), MCF-7 tumor bearing balb/c nude mice were randomly divided into three groups (n = 5): PBS blank group, Au-nanoprobes group, and Control-Au-nanoprobes group. Next, at different time intervals (Day 0, 2, 4, 6, 8, 10, and 12), mice were injected via tail vein with 50 µL different systems: PBS, Au-nanoprobes (3 nM), and Control-Au-nanoprobes (3 nM). The tumor volumes and the mice body weights were determined synchronously every two days. On day 28, all mice was sacrificed by cervical dislocation. All experiments were carried out in accordance with the National Guide for Care and Use of Laboratory Animals.
2.10. Statistical Analysis
All experiments were performed in triplicate and all data presented as mean and standard deviation. Data were analyzed using IBM SPSS Statistics 25. Values of P < 0.05 and P < 0.01 were considered statistically significant.
Author Contributions
Conceptualization, M.H.; methodology, Q.J. and M.H.; software, Q.Y.; formal analysis, M.O.; investigation, Q.J. and Q.Y.; resources, Q.J.; data curation, M.O.; writing—original draft preparation, Q.J. and M.H.; writing—review and editing, M.H.; visualization, Q.Y.; supervision, M.H.; project administration, M.H.; funding acquisition, M.H. All authors have read and agreed to the published version of the manuscript.
Scheme 1.
Illustration of the construction of Au-nanoprobes, the pro-apoptosis and targeted anti-tumor mechanism that is involved with the simultaneous down-regulation of intracellular microRNA-21 and hTERT mRNA through anti-sense technology.
Scheme 1.
Illustration of the construction of Au-nanoprobes, the pro-apoptosis and targeted anti-tumor mechanism that is involved with the simultaneous down-regulation of intracellular microRNA-21 and hTERT mRNA through anti-sense technology.
Figure 1.
The fluorescence recovery phenomena of Cy3 (A) and Cy5 (B) after Au-nanoprobes (1.5 nM) being treated by different concentrations of mercaptoethanol (a to g: 0, 1, 1.5, 2, 3, 5, and 10 mM). Fluorescence spectra of Au-nanoprobes after incubating with different concentrations of (C) Target-hTERT-DNA (0, 100, 200, 600, and 1000 nM), (D) Target-miRNA-21-DNA (0, 100, 200, 600, and 1000 nM), or (E and F) cell lysate.
Figure 1.
The fluorescence recovery phenomena of Cy3 (A) and Cy5 (B) after Au-nanoprobes (1.5 nM) being treated by different concentrations of mercaptoethanol (a to g: 0, 1, 1.5, 2, 3, 5, and 10 mM). Fluorescence spectra of Au-nanoprobes after incubating with different concentrations of (C) Target-hTERT-DNA (0, 100, 200, 600, and 1000 nM), (D) Target-miRNA-21-DNA (0, 100, 200, 600, and 1000 nM), or (E and F) cell lysate.
Figure 2.
(A) CLSM imaging of HeLa and MCF-7 cells after incubating with Au-nanoprobes (1.5 nM) for 4 h. Relative expression level of microRNA-21 (B) and hTERT mRNA (C) in HeLa and MCF-7 cells after treated with Au-nanoprobes or Control-Au-nanoprobes for different times. (D) hTERT activity in MCF-7 cells after treated by PBS, Au-nanoprobes or Control-Au-nanoprobes for 48 or 72 h.
Figure 2.
(A) CLSM imaging of HeLa and MCF-7 cells after incubating with Au-nanoprobes (1.5 nM) for 4 h. Relative expression level of microRNA-21 (B) and hTERT mRNA (C) in HeLa and MCF-7 cells after treated with Au-nanoprobes or Control-Au-nanoprobes for different times. (D) hTERT activity in MCF-7 cells after treated by PBS, Au-nanoprobes or Control-Au-nanoprobes for 48 or 72 h.
Figure 3.
(A) Apoptosis analysis of MCF-7 cells treated with Au-nanoprobes (1.5 nM) for different times by flow cytometry. (B) Early and late apoptosis rate of MCF-7 shown in (A). Cell viabilities of (C) HeLa and (D) MCF-7 cells after treated with Au-nanoprobes (1.5 or 2.0 nM) or Control-Au-nanoprobes (1.5 or 2.0 nM) for different times (24, 36, 48, or 72 h).
Figure 3.
(A) Apoptosis analysis of MCF-7 cells treated with Au-nanoprobes (1.5 nM) for different times by flow cytometry. (B) Early and late apoptosis rate of MCF-7 shown in (A). Cell viabilities of (C) HeLa and (D) MCF-7 cells after treated with Au-nanoprobes (1.5 or 2.0 nM) or Control-Au-nanoprobes (1.5 or 2.0 nM) for different times (24, 36, 48, or 72 h).
Figure 4.
(A and B) In vivo real-time fluorescence imaging of balb/c nude mice after intravenous administration of PBS, Control-Au-nanoprobes (6.5 nM) or Au-nanoprobes (6.5 nM). (C) Ex vivo fluorescence imaging of the main organs and tumors dissected from the representative balb/c nude mice after oral administration of PBS or Au-nanoprobes for 60 min shown in panel (B).
Figure 4.
(A and B) In vivo real-time fluorescence imaging of balb/c nude mice after intravenous administration of PBS, Control-Au-nanoprobes (6.5 nM) or Au-nanoprobes (6.5 nM). (C) Ex vivo fluorescence imaging of the main organs and tumors dissected from the representative balb/c nude mice after oral administration of PBS or Au-nanoprobes for 60 min shown in panel (B).
Figure 5.
Anti-tumor study of Au-nanoprobes against subcutaneous MCF-7 xenograft mouse tumors: (A) representative tumor-bearing mouse images, (B) tumor growth curve, and (C) averaged tumor weight of each group during the experiments. During the first 12 days of the experiment, Au-nanoprobes at a dosage of 3 nM (50 μL) were intraveneously administered to mice every other day for seven times. (n.s. P > 0.05, *P < 0.05, **P < 0.01.).
Figure 5.
Anti-tumor study of Au-nanoprobes against subcutaneous MCF-7 xenograft mouse tumors: (A) representative tumor-bearing mouse images, (B) tumor growth curve, and (C) averaged tumor weight of each group during the experiments. During the first 12 days of the experiment, Au-nanoprobes at a dosage of 3 nM (50 μL) were intraveneously administered to mice every other day for seven times. (n.s. P > 0.05, *P < 0.05, **P < 0.01.).