Introduction

Apoptosis is a highly regulated process of programmed cell death in which cells will undergo a tightly regulated program that plays a critical role in the normal and pathological process [1]. Cellular and nuclear shrinkage, nuclear fragmentation, condensation, the formation of apoptotic bodies, and cellular budding are critical apoptotic features. Nucleus and cytoplasm are condensed to produce a membrane-bound apoptotic bodies that are phagocytized by macrophages [2]. Uncontrolled proliferation and loss of apoptosis are major factors for tumor formation. A compound that prevents the proliferation of tumor cells by the induction of apoptosis has been considered a potential anti-tumor compound [3].

Silver nanoparticles (AgNPs) have been used as nanomaterials due to its anti-bacterial properties. AgNPs have been commonly utilized in the medical field such as coatings for wound dressings, implants, bone prostheses, and surgical instruments [4–6]. Nanoparticles elicit a greater biological response than microparticles [7], and it could affect cellular activity significantly [8]. Lynch et al. [9] have reported that the cytotoxicity of nanoparticles due to its smaller size, high number per given mass, large specific surface area, and generation of reactive oxygen species ROS. AgNP coatings have been widely used in textile industries for the manufacture of clothes and socks [10–11]. AgNPs have been commercialized as deodorants, room sprays, water cleaners, wall paints, and laundry detergents. Therefore, exposure of AgNP could occur to these consumers and laborers at the manufacturing site. Ionic silver is highly toxic to bacteria. Due to adsorption to the bacterial cell wall, de-activation of enzymes and ROS production [12–15]. AgNPs inhibits bacterial growth through attachment to cell membranes, changes of membrane permeability, and accumulation of intracellular ROS [16–18].

Several studies have reported on the biological effects of AgNPs in mammalian cells. However, the exact mechanism of AgNPs is not yet completely understood. A selection of HeLa cells based on being monocytic lineage could mimic local immune responses [19], and cervical carcinoma is one of the most common neoplastic diseases affecting women [20]. Chemotherapy, radiotherapy, and surgery could damage the cancer cells and some healthy cells in the body. These treatments are highly expensive and produce severe side effects including bone marrow problems, hair loss, nausea, and fatigue. Therefore, development of novel and efficient anti-cancer drugs, which, also, should overcome resistance, has become a significant issue.

We evaluated the toxicity of AgNPs and expression levels of several apoptotic-related markers in the HeLa cells. SRB assay investigated the cytotoxicity of AgNPs on cells. Apoptotic changes were observed by an inverted microscope, fluorescence microscope, and confocal laser scanning microscope (CLSM). DNA fragmentation students and quantitative polymerase chain reaction (qPCR) were carried out on apoptosis-related gene messenger RNA (mRNA) expressions. Caspase 3 enzyme activity and ROS levels were also determined.

Materials and Methods

Materials

AgNPs (10 nm (TEM), cat no. 730,785, Sigma-Aldrich), dimethyl sulphoxide (DMSO), and sulforhodamine B (SRB) were purchased from Sigma. DMEM, fetal bovine serum (FBS), penicillin-streptomycin, and trypsin-EDTA were obtained from Welgene (Daegu, South Korea). Acridine orange (AO), ethidium bromide (EB), and 2′,7′-dichlorofluorescin diacetate (DCFH-DA) were purchased from Santa Cruz Biotechnology, Inc. (Delaware Avenue, California, USA). Primers have been obtained from Macrogen Inc. (South Korea).

Cell Culture

HeLa cells were purchased from the Korean Cell Line Bank (South Korea). Cells were kept in the growth medium supplemented with 10 % FBS and 1 % antibiotics (penicillin-streptomycin). The cells were grown in a CO2 incubator at 37 °C and 5 % CO2.

SRB Assay

A 96-well plate was used for the culturing cells and allowed to adhere for 24 h at 37 °C. Cells were treated with AgNPs at different concentrations (0.001, 0.01, 0.02, 0.04, 0.08, and 0.16 mg/ml) for 48 and 72 h. The cytotoxic effect of HeLa cells was measured by the SRB assay [21].

Inverted Microscope

A 6-well plate was used for the culturing HeLa cells at a density of 2 × 105 cells/well. The cells were treated with AgNPs of different concentrations (0.01, 0.02, and 0.04 mg/ml) for 48 and 72 h after 24 h of adherence. After treatments, the morphology of cells was examined by an inverted microscope (Nikon, Eclipse, 80i, Melville, NY 11747-3064, USA).

Fluorescence Microscope

A 6-well plate was used for the used for the culturing cells at a density of 2 × 104 cells/well. The cells were treated with AgNPs of different concentrations (0.01, 0.02, and 0.04 mg/ml) for 48 and 72 h. Cells were examined with fluorescence microscope (Axiovert 2000, Carl Zeiss, Germany) [22].

CLSM

HeLa cells were seeded in the confocal dish. After 24 h of adherence, the cells were treated with AgNP of different concentrations (0.01, 0.02, and 0.04 mg/ml). After 48 and 72 h, cells were washed thrice with PBS and stained with AO (20 μg/ml) for 5 min. Cells were viewed immediately under CLSM (1X81R Motorized Inverted Microscope, Olympus [22].

DNA Fragmentation Study

A 6-well plate was used for the culturing of HeLa cells. The cells were treated with AgNPs of different concentrations (0.01, 0.02, and 0.04 mg/ml) for 48 and 72 h. A DNA was isolated from the cells using an extraction method [23]).The evaluation of the size range of the fragmented DNA was performed by electrophoresis using a 1.5 % agarose gel in Tris/borate/EDTA (TBE) buffer at a constant of 100 mA for 120 min.

qPCR

Total RNA was isolated from the control and AgNP-treated samples [24]. Primers specific for p53 (forward:5′-TAACAGTTCCTGCATGGGCGGC-3′, reverse: 5′-AGGACAGGCACAAACACGCACC-3′), bax (forward:5′-TGG AGCTGCAGAGGATGATTG-3′, reverse: 5′-GAAGTTGCCGTCAGAAAACATG-3′), caspase 3 (forward: 5′-TTAATAAAGGTATCCATGGAGAACACT-3′, reverse: 5′-TTAGTGATAAAAATAGAGTTCTTTTGTGAG-3′) and a housekeeping gene GAPDH (forward: 5′-GGTCACCAGGGCTGCTTTT-3′, reverse: 5′-ATCTCGCTCCTGGAAGATGGT-3′) were used in this study. The relative ratios were determined based on the 2–△△ CT method (Pfaffl, 2001). PCR was monitored using the CFX96™ Real-Time System (Bio-Rad).

Caspase Activity Assay

HeLa cells were seeded in the culture dish. After 24 h adherence, the cells were treated with AgNPs of different concentrations (0.01, 0.02, and 0.04 mg/ml). After 48 and 72 h, caspase 3 enzyme activity was measured based on the method of Muthuraman [25].

Determination of ROS Production

ROS was determined using a fluorescent probe, 2,7,dichlorodihydrofluorescein diacetate (DCFH-DA) [22]. Cells were seeded in 96-well plates in growth medium at a cell density of 5000 cells/well. The cells were treated with AgNPs for 48 and 72 h. The medium was removed, and the cells were incubated with 5 μM of DCFH-DA in the growth medium for 30 min at 37 °C and 5 % CO2. The fluorescence was measured using a fluorescent plate reader, and images were taken using fluorescence microscope (Axiovert 2000, Carl Zeiss, Germany).

Statistical Analysis

Values are expressed as means ± SEM. The difference between control and AgNP-treated cells was evaluated using Student’s t test. A p value of less than 0.05 was considered statistically significant.

Results

Cytotoxicity of AgNP

AgNPs showed an apparent cytotoxic effect on HeLa cells, and clear concentration-response relationship was observed (Fig. 1). Stained cells were photographed at 48 h (Fig. 2) and 72 h (Fig. S1). However, 0.08 and 0.16 mg/ml AgNP could inhibit the growth of HeLa cells. This result suggested that 0.08 and 0.16 mg/ml AgNP could be toxic to normal cells. Therefore 0.01, 0.02, and 0.04 mg/ml of AgNP was used for further study.

Fig. 1
figure 1

Cytotoxic effect of AgNPs in HeLa cells by SRB assay at 48 and 72 h. Results have presented as a percentage of growth inhibition compared with the control. The percentage of growth inhibition was calculated as follows: Growth inhibition (%): (control − sample/control) × 100. Values are shown as means ± SEM

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Fig. 2
figure 2

Cytotoxic effect of AgNPs in HeLa cells by SRB assay at 48 h. The morphology of cells were observed by an inverted microscope (×40). The representative images were obtained from three independent experiments

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Observation by Inverted Microscope

Inverted microscope could be used to observe the HeLa cell shape and its morphological changes. Control cells were regular polygonal form and short cell antennas were seen. AgNPs treated HeLa cells showed significant morphological changes such as sporadic distribution, loss of adherence and rounding. There is clear concentration-dependent response relationship was observed (Fig. 3).

Fig. 3
figure 3

The morphological observation by an inverted microscope following AgNPs exposure at 48 and 72 h. The representative images were obtained from three independent experiments

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Observation by Fluorescence Microscope

Fluorescence microscopy was carried out to determine whether the cytotoxic effect of AgNP was related to the induction of apoptosis, morphological features of cell death. This method combines the dual uptake of fluorescent DNA-binding dyes AO and EB. Control cells have no apparent morphological changes. Viable cells possess a uniform bright green nucleus. Early apoptotic cells show bright green areas of fragmented chromatin in the nucleus, and necrotic cells show an identical bright orange nucleus. However, HeLa cells exposed to AgNP for 48 and 72 h exhibited fragmented and condensed chromatin, fragmented nuclei, and appearance of apoptotic bodies. The results were correlated with clear concentration-dependent response relationships (Fig. 4; Fig. S2).

Fig. 4
figure 4

The morphological observation with AO/EB double staining by a fluorescence microscope (×40). HeLa cells were seeded in 6-well plates, and over 24 of adherence, cells were treated with AgNPs (0.01, 0.02, and 0.04 mg/ml) for 48 h. The representative images were obtained from three independent experiments

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Observation by CLSM

CLSM has been widely applied to investigate morphological studies of apoptosis and apoptotic DNA fragmentation [26]. AO can interact with DNA (green) and RNA (red). Control HeLa cells have bigger and smoother nucleus than AgNP-treated cells. The morphological changes including rounding, compact granular masses in the nucleus, and reduced nuclear volume were seen following treatment. The appearance of bright green nucleus suggests the induction of apoptosis in AgNP-treated cells. Chromatin and nuclear membrane were aggregated, and condensed cytoplasm was also observed in the AgNP-treated cells. The level of morphological changes is in a concentration-dependent response relationship (Fig. 5; Fig. S3).

Fig. 5
figure 5

Morphological observation of HeLa cells by CLSM (×40). HeLa cells were grown on the cover glass confocal dish. After 24 h of adherence, cells were treated with AgNP (0.01, 0.02, and 0.04 mg/ml) for 48 h. The representative images were obtained from three independent experiments

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Quantification of Apoptotic Cells

AgNPs induced apoptosis in HeLa cells and are investigated by the fluorescence and confocal microscopes over 48 and 72 h. The hundred cells were counted for control and each concentration point. The percentage of apoptotic cells was found to be 42, 51, and 55 % in 0.01, 0.02, and 0.04 mg/ml AgNP-treated samples by fluorescence microscope, whereas it was 54, 60, and 62 %, respectively. The percentage of apoptotic cells was found to be 61, 62, and 65 % in 0.01, 0.02, and 0.04 mg/ml AgNP-treated samples by confocal microscope, whereas it was 67, 70, and 71 %, respectively. The percentage of apoptotic cells showed slight concentration-response relationships (Table 1).

Table 1 Induction of apoptosis in HeLa cells by AgNPs and observed by the fluorescence and confocal microscopes over 48 and 72 h
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AgNP-Induced DNA Fragmentation

DNA fragmentation is a hallmark of apoptosis. Cell death can be defined as morphological and biochemical feature changes that differentiate it from other forms of cell death. An endonuclease cleaves the DNA into small fragments. Cells were seeded at a density of 2.5 × 104 cells/well. Multiple oligomers and smear did not appear in the control cells, whereas smear and various bands were appearing in treated cells in a concentration-dependent manner (Fig. 6).

Fig. 6
figure 6

DNA fragmentation analysis by classical agarose gel electrophoresis. HeLa cells were grown in 6-well plates and allowed for the adherence for 24 h. Cells were treated with AgNPs (0.01, 0.02, and 0.04 mg/ml) for 48 and 72 h. At the end of treatments, DNA was isolated from the cells using extraction method. The evaluation of the size range of the fragmented DNA was performed by electrophoresis using a 1.5 % agarose gel in TBE buffer at a constant of 100 mA for 120 min. The representative images were obtained from three independent experiments

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AgNP-Induced Apoptotic Marker Gene Expression

To further confirm necrosis and apoptosis, we constituted apoptotic marker gene expression by qPCR. HeLa cells exposed to different concentrations of AgNPs (0.01, 0.02, and 0.04 mg/ml) for 48 and 72 h showed significant changes in the mRNA expression of p53, bax, and caspase 3 gene. The mRNA expression level of p53, bax, and caspase 3 were significantly up-regulated in AgNP-treated cells compared with controls. The mRNA expression of p53 was increased (0.2-, 0.39-, and 0.51-fold) at 48 h and (0.33-, 0.41-, and 0.6-fold) at 72 h, following 0.01, 0.02, and 0.04 mg/ml of AgNP treatments, respectively. The mRNA expression of bax was increased (0.11-, 0.19-, and 0.24-fold) at 48 h and (0.21-, 0.28-, and 0.34-fold) at 72 h, following 0.01, 0.02, and 0.04 mg/ml of AgNP treatments, respectively. The mRNA expression of caspase 3 was increased (0.19-, 0.23-, and 0.25-fold) at 48 h and (0.28-, 0.33-, and 0.42-fold) at 72 h, following 0.01, 0.02, and 0.04 mg/ml of AgNP treatments, respectively (Fig. 7).

Fig. 7
figure 7

mRNA expression of apoptotic p53, bax, and caspase 3 genes by qPCR. HeLa cells were grown in a T-25 flask and allowed for adherence for 24 h. Cells were treated with AgNPs (0.01, 0.02, and 0.04 mg/ml) for 48 and 72 h. Values are shown as means ± SEM.

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AgNP-Induced Effect on Caspase-3 Activity

Caspases are cysteine proteases and act as central executioners of the apoptotic pathway [27]. Caspase 3 activity was increased 10.1, 21.5, and 34 % in AgNP-treated cells at 48 h, following 0.01, 0.02, and 0.04 mg/ml of treatments, respectively, whereas 15.4, 26.2, and 39.8 % at 72 h of treatment (Fig. 8).

Fig. 8
figure 8

Changes in caspase 3 enzyme activity. HeLa cells were grown in a T-75 flask and allowed for adherence for 24 h. Cells were treated with AgNPs (0.01, 0.02, and 0.04 mg/ml) for 48 and 72 h. At the end of treatments, cells were removed and activity was determined as mentioned in the “Materials and Methods.” Values have shown as means ± SEM

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AgNP-Induced Intracellular ROS Production

The fluorescent probe DCFH-DA determined the intracellular ROS generation. The fluorescence study indicated that the green fluorescence intensity of DCF was enhanced in the AgNP-treated cells compared with the control cells. These results indicated that the AgNPs induced intracellular ROS generation in a dose-dependent manner (Figs. 9, 10, and 11; Fig. S4).

Fig. 9
figure 9

Changes in fluorescence intensity. Cells were treated with AgNPs (0.01, 0.02, and 0.04 mg/ml) for 48 and 72 h. Cells were treated with dye as mentioned in the “Materials and Methods.” Values are shown as means ± SEM

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Fig. 10
figure 10

Changes in fluorescence intensity. Cells were treated with AgNPs (0.01, 0.02, and 0.04 mg/ml) for 48 and 72 h. Cells were stained with dye, and spectrum was taken for 48-h treatment (a) and 72-h treatment (b)

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Fig. 11
figure 11

Changes in fluorescence intensity. HeLa cells were grown in 6-well plates and allowed for the adherence for 24 h. Cells were treated with AgNPs (0.01, 0.02, and 0.04 mg/ml) for 48 h. Cells were stained with dye, and images were taken using fluorescence microscope

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Discussion

Nanoparticles attracted several researchers due to their unique properties such as electronic, optical, mechanical, catalytic, and biological properties [28–29]. The metal nanoparticles, AgNPs, are one of the promising nanoparticles used in nanomedicine because of their unique properties. AgNPs are widely used in topical ointments to prevent infection against burn and open wounds [30] and in anti-microbial [31], anti-fungal [32], anti-inflammatory [33], anti-viral [34], anti-angiogenesis [35], and anti-platelet activity [36].

Cancer is one of the leading causes of deaths due to its profound complex in nature. Cervical cancer is the second most common cancer in females. Cervical cancer is due to the infection with the human papilloma virus [37]. Chemotherapy, radiotherapy, and surgery could damage the cancer cells and some healthy cells in the body. These treatments are highly expensive and produce severe side effects including bone marrow problems, hair loss, nausea, emesis, and fatigue [38]. Therefore, development of novel and efficient anti-cancer drugs, which, also, should overcome resistance, has become a significant issue.

The tumor cell apoptosis induction is an essential mechanism of anti-cancer compound [3]. Apoptosis process has been characterized by the morphological and biochemical changes, and apoptosis of different cells in the same tissue do not occur at the same time. The morphological observation is critical for apoptosis study in the early stage because observations can see DNA multiplies after initiation of apoptosis. The apoptotic effect of AgNPs was reported using mouse NIH3T 3 cells [39]. Formation of DNA fragments is regarded as a biochemical hallmark of apoptosis [40]. Classical methods to study the cell death are morphological and biochemical feature changes that differ from other forms of cell death. An endonuclease enzyme cleaves the DNA into small fragments. In agreement with the above finding, our results also showed DNA fragmentation in a dose-dependent manner in the HeLa cells.

The gene p53 triggers cell-cycle arrest to provide time for the repayment and self-medicated apoptosis during cellular stress [41]. The role of p53 is to up-regulate the expression of bax gene. AgNPs increased caspase 3 mRNA expression and activity in a concentration-dependent manner. The up-regulated caspase-3 activates auto-catalysis, cleaving other members of the caspase family leading to irreversible apoptosis [42]. Apoptosis-related genes were altered in the liver of AgNP-fed mice [43]. Apoptosis process could occur in response to oxidative stress [44]. AgNP-mediated generation of ROS was reported in an in vitro study [39]. In agreement with this finding, ROS level was up-regulated in our study due to oxidative stress. These results indicate that apoptosis induction by AgNPs may be carried out by ROS generation.

Conclusion

The AgNPs showed the right cytotoxic effect in cervical carcinoma cells. Our results suggest that metal-based nanoparticles might be a potential candidate for the treatment of cervical cancer. Taking all these data together, it can be concluded that the AgNP could exert cytotoxicity on HeLa cell through the apoptotic pathway.