Cytotoxicity investigations of biogenic tellurium nanorods towards PC12 cell line

Abstract

The authors evaluated the cytotoxicity underlying mechanisms of biogenic tellurium (Te) nanorods (NRs) produced by the Pseudomonas pseudoalcaligenes strain Te on the PC12 cell line. The half‐maximal inhibitory concentration (IC₅₀) value was estimated at 5.05 ± 0.07 ng/ml for biogenic Te NRs and 2.44 ± 0.38 ng/ml for K₂ TeO₃, respectively. The viability of PC12 was inhibited concentration dependent at doses of 1, 2.5, 5, 10, and 20 ng/ml. Te NRs principally induced late apoptosis or necrosis at IC₅₀ concentration, without effect on caspase‐3 activities. Furthermore, Te NRs reduced glutathione and enhanced malondialdehyde levels, and also reduced superoxide dismutase and catalase activities. These findings revealed that biogenic Te NRs were less toxic than K₂ TeO₃. Additionally, they induced cytotoxity towards the PC12 cell line through the activation of late apoptosis independent of the caspase pathway, and may also enhance oxidative stress in the nervous system.

1 Introduction

Tellurium (Te) is a metalloid belonging to the chalcogens family. It is chemically related to important biological elements such as oxygen, selenium (Se), sulphur, and polonium that occasionally discovered in the Earth's crust as an elemental crystal [1, 2]. Te has been found in biomolecules like proteins as tellurocyteine and telluromethionine in some bacterial species [3, 4]. Telluromethionine has not been detected in animal cells and no physiological role has been indicated for Te. However, trace amounts of this element have been found in human body fluids in reported concentrations of 5 and 50 ng/ml in blood and urine, respectively [5, 6, 7, 8]. Behind Te, the similar counterpart Se has been also indicated as a biologically important chalcogen. For example, selenoenzyme glutathione peroxidase (GPx) could counteract reactive oxygen species (ROS) or reactive nitrogen species to catalyse the reduction of peroxynitrite [9]. Organoselenium and organotellurium compounds enhanced the reduction of hydroperoxides or peroxynitrite to protect DNA from breakage [9]. Enhancement of anti‐apoptotic phosphoinositide‐3‐kinase/Akt cascade is another anti‐oxidative function of these trace elements to stabilise proteins [9].

Recently, the use of Te has been increased in industry and agriculture. It is being applied in the manufacturing of optical blue‐ray discs, cadmium telluride solar panels and semiconductors, metal‐oxidising solutions, rubber industries, magnetic and photographic materials, insecticides, fluorescent materials, and as an intermediate of organic synthesis reactions [10, 11]. It may be used in medicines, production of nanotubes, cancer treatment, syphilis, leprosy, as well as in organ imaging and diagnosis [10, 11]. Therefore, investigations should look at long‐term exposure, possibility of toxicity, and molecular mechanisms underlying the negative effects of syntheses of new Te compounds. According to the toxicity mechanisms of Te compounds, abundance in intracellular calcium ion concentrations, induction of oxidative stress (OS), cell membrane damage, cell apoptosis through enhancement of caspase‐3/7 and ‐9 activities, and DNA damage have been reported [11, 12, 13, 14].

Recently, biotechnologically derived nanoparticles (NPs) by bacterial species have become an attractive subject as a biological source for the synthesis of trace element nanostructures. Nanobiosynthesis has provided many advantageous properties including uniformity in particle shape, size, and less toxicity [15]. Te has reported a nearly forgotten element in biology due to its high toxicity. With regard to this, there is not much information yet on the toxicity or risk assessment of these compounds. Therefore, in this study, the cytotoxicity mechanisms of biogenic Te nanorods (NRs) produced by the Pseudomonas pseudoalcaligenes Te strain as compared to K₂ TeO₃ were assayed by MTT, flow cytometry, and activated caspase‐3 to confirm the mode of apoptosis investigated in PC12 neuronal cells.

2 Materials and methods

2.1 Reagents and kits

Potassium tellurite (K₂ TeO₃ 3H₂ O), nutrient broth, nutrient agar, ethylenediamine tetra acetic acid and n ‐octanol were purchased from Cayman Chemicals (Ann Arbor, MI, USA). RPMI 1640 medium, Dulbecco's modified Eagle's medium, fetal bovine serum (FBS), trypsin, penicillin and streptomycin were provided from Biosera (Vienna, Austria). Western blot detection kit and polyvinylidenedifluoride (PVDF) membrane were prepared from Roche (Mannheim, Germany). An antibody against activated caspase‐3 was purchased from Cell Signaling Technology (Danvers, MA, USA). β‐Actin antibody was prepared from Santa Cruz Biotechnology (Santa Cruz, CA, USA). Malondialdehyde (MDA), hydrogen peroxide (H₂ O₂), super oxide dismutase (SOD) and catalase (CAT) assay kit from ZellBio (GmbH, Germany). Other chemicals and solvents were of analytical grade.

2.2 Biosynthesis, purification and characterisation of the Te NRs

Te NRs were biosynthesised using a recently described method [16]. Briefly, 500 ml Erlenmeyer flask containing 100 ml of sterile NB medium was supplemented with K₂ TeO₃ (final concentration 1 mM) and there was inoculated with 1 ml of the fresh inoculums of P. pseudoalcaligenes strain Te (OD₆₀₀ nm, 0.1). After 80 h incubation in a shaker incubator (30°C, 150 rpm), the culture media were centrifuged (4000 × g, 10 min) and the obtained biomass was washed using sterile NaCl solution (0.9%). The cells were frozen by liquid nitrogen in a mortar and were disrupted by a pestle. After ultrasonication (100 W, 5 min), the resulting slurry was washed three times by sequential centrifugation (10,000 × g, 5 min) with 1.5 M Tris–HCl buffer (pH 8.3) containing SDS (1%) and deionised water, respectively. Then, Te NRs were extracted and purified using an organic‐aqueous two partitioning system (n ‐octyl alcohol–water) as described earlier [17]. Transmission electron micrographs of NRs were obtained using a transmission electron microscope (TEM) apparatus (Zeiss 902A) operated at an accelerating voltage of 80 kV and elemental composition of Te NRs samples were analysed using an energy dispersive X‐ray (EDX) microanalyser. The charge and related size distribution pattern of Te NRs was plotted by using the Zetasizer MS2000 (Malvern Instruments). The XRD pattern of the prepared Te NRs was examined by the X‐ray diffractometer (Philips, PW1710) with CuKα radiation (λ  = 1.5405 A°) in the 2θ range of 0–80°.

2.3 Cell culture

Human PC12 cells were obtained from Iranian Biological Resource Center (IBRC, Tehran, Iran), and cultured in 75 cm² plastic bottles in RPMI 1640 supplemented with 10% bovine serum albumin (FBS), 100 IU/ml penicillin and 100 µg/ml streptomycin at 37°C in a 5% CO₂ humidified incubator.

2.4 Cell viability: MTT assay

Cell viability was assessed by MTT assay that reduced it to purple‐coloured insoluble formazan crystals by mitochondrial dehydrogenases. PC12 cells were seeded (1 × 10⁴ cells/well) in 96‐well microplates in order to evaluate the effects of various concentrations of biogenic Te NRs and K₂ TeO₃ exposure including 1, 2.5, 5, 10, and 20 ng/ml on cell viability after 24 h.

2.5 Flow cytometry: Annexin V‐fluorescein isothiocyanate (FITC) and 7‐aminoactinomycin D (7‐AAD) assay

Apoptosis and necrosis were observed using flow cytometry. A total number of 1 × 10⁶ PC12 cells were treated with an inhibitory concentration (IC₅₀) dose of biogenic Te NRs for 24 h. Thereafter, the cells were washed with phosphate‐buffered saline (PBS), fixed with ice‐cold 70% ethanol at room temperature, and stained with annexin V‐FITC and 7‐AAD, a DNA dye that distinguishes viable, apoptotic, and late apoptotic or dead cells in flow cytometry, according to the kit's instructions (Apogee Flow Systems, Hertfordshire, UK).

2.6 Western blot analyses

Western blotting was performed as previously described [18]. Briefly, after 24 h incubation periods, PC12 cells were trypsinised and collected by centrifugation. Then the cells were washed by PBS and lysed with lysis buffer containing 62.5 mM Tris–HCl (pH 6.8), dithiothreitol (DTT) 50 mM, glycerol 10% and bromophenol blue 0.25% (w/v). The lysate was then collected in a microfuge tube and kept in −80°C until use. After that, the lysates were boiled in water for 8 min and then the equal amount (40 µl) of protein samples were loaded into 12% SDS–PAGE. Separated proteins were transferred onto PVDF membranes at 120 V for 60 min and blocked with 1% blocking solution, and then incubated with anti‐activated caspase‐3 at 1:1000 dilution at 4°C, overnight. Blots were consequently probed with anti β‐actin antibody (1:2000) as internal control. The blots were then washed three times, and probed with horseradish peroxidase‐conjugated goat anti‐rabbit secondary antibody (1:10,000) for 60 min at room temperature. Proteins bands were detected on X‐ray film by using a Chemiluminescence kit, and then the densitometry was observed.

2.7 Statistical analyses

The results were represented as mean ± SD for the experiments mentioned. The statistical significant between the groups was estimated by one‐way Analysis of Variance (ANOVA) followed by Tukey post hoc test by Graphpad Prism software (version 6, San Diego, CA, USA), and p ‐value <0.05 was considered statistically significant.

3 Results

3.1 Biosynthesis and characterisation of Te NRs

The culture medium of P. pseudoalcaligenes strain Te exhibited a gradual change in colour towards black during 80 h incubation in the presence of K₂ TeO₃ (1 mM), which indicated the reduction of Te⁺⁴ ions to Te⁰ NPs. This was clear in Fig. 1 a, which shows the TEM image of the biogenic Te NRs, representing rod‐shape nanostructures with 185 nm length by <22 nm in width. Microanalysis of the purified Te NRs using EDX method represented Te absorption peak at 3.72 keV with the weight per cent equal to 100 without other atom signals (Fig. 1 b). The SEM image of purified Te NRs exhibited some aggregation between the biogenic NRs (Fig. 1 c). The ζ potential value for Te NRs measured by a Zetasizer was +16.8 Mv. The corresponding size distribution patterns of Te NRs revealed that the NPs were in the size range of 40–200 nm and the most frequent particles were in the size range of 80–100 nm (Fig. 1 d).

Fig. 1.

Characteristic properties of biogenic Te NRs

(a) Transmission electron micrograph, (b) Energy dispersive X‐ray spectrum, (c) SEM image, (d) Size distribution patterns

3.2 Effect of biogenic Te NRs on PC12 viability

As shown in Fig. 2, biogenic Te NRs at doses of 1, 2.5, 5, 10, and 20 ng/ml concentration‐dependent reduced the viability (% of control) after 24 h of exposure to cancer cell line PC 12. By increasing the concentration of Te NRs, reduction of cell viability was observed. The IC₅₀ value of Te NRs was estimated at 5.05 ± 0.07 ng/ml as compared to that of K₂ TeO₃ at 2.44 ± 0.38 ng/ml. Cell death, rate of apoptosis, and necrosis were characterised by annexin V and 7‐AAD stationing using flow cytometry.

Fig. 2.

Effects of biogenic Te NRs on PC12 cell viability as measured using MTT assay in PC12 neuronal cells. Cell viability was expressed as the percentage of control after treatment with increasing concentration of biogenic Te NRs after 24 h

3.3 Effect of biogenic Te NRs on cell apoptosis

PC12 cells were stained with annexin V‐FITC and 7‐AAD as phosphatidylserine and DNA staining residues, respectively, and subjected to flow cytometry to determine the proportion of live, apoptotic, and necrotic cells. As shown in Fig. 3, a significant percentage of cells was shifted to late apodoses or necrosis following treatment with IC₅₀ concentration after 24 h.

Fig. 3.

Characteristic flow cytometry of PC12 cells exposure to biogenic Te NRs at IC50 concentration. The numbers at the bottom right quadrant of each dot plot represent the percentage of cells in early apoptosis (annexin V‐positive, 7‐AAD‐negative). Numbers at the top right quadrant represent the percentage of cells in late apoptosis and/or secondary necrosis (annexin V‐positive, 7‐AAD‐positive) patterns

(a) Control, (b) Treated with Te NRs

To confirm the flow cytometry analyses, activated caspase‐3 expressions as the key regulator of the apoptosis pathway were determined using western blot assay.

3.4 Biogenic Te NRs caused cell death independent of caspase activation

The effects of biogenic Te NRs at two doses of 2.5 and 5 ng/ml, and K₂ TeO₃ at a dose of 2.5 ng/ml on PC12 cells were determined after 24 h of exposure. As shown in Fig. 4, biogenic Te NRs and K₂ TeO₃ could not induce the expression of activated caspase‐3 in two experimental doses as compared to control. However, doxorubicin (DOX) at a concentration of 8 µg/ml as positive control enhanced the expression of activated caspase‐3. The expression of β‐actin was reserved as an internal standard.

Fig. 4.

Characteristic the PC12 neuronal cells were treated with biogenic Te NRs (2.5 and 5 ng/ml), K₂ TeO₃ (2.5 ng/ml) and doxorubicin (DOX, 8 µM/ml) as positive control, and the expression of activated caspase‐3 and β‐actin was analysed by western blot

3.5 Oxidative stress damage

3.5.1 Effect of biogenic Te NRs on GSH content

GSH content of cells was evaluated as one of the indices of OS. As shown in Fig. 5 a, after exposure of PC12 cells to biogenic Te NRs, GSH levels were significantly reduced at a dose of 5 ng/ml as compared to control (p  = 0.023). However, no significant change was indicated following treatment with biogenic Te NRs at a dose of 2.5 ng/ml. Furthermore, biogenic Te NRs significantly enhanced GSH content at a lower dose of 1 ng/ml. A higher depletion was observed after exposure to a concentration of 8 µg/ml of DOX as compared to control (p  = 0.006).

Fig. 5.

Effects of biogenic Te NRs on oxidative stress (OS) biomarkers after 24 h treatment with biogenic Te NRs at three doses of 1, 2.5 and 5 ng/ml were determined in PC 12 neuronal cells

(a) GSH content, (b) MDA level, (c) SOD, (d) CAT activity. *p  < 0.05, **p  < 0.01, and ***p  < 0.001 compared to the control group

3.5.2 Effect of biogenic Te NRs on MDA level

The MDA level of cells was estimated as a lipid peroxidation (LPO) biomarker. As shown in Fig. 5 b, biogenic Te NRs significantly (p  = <0.001) elevated MDA levels at two doses of 2.5 and 5 ng/ml as compared to control. However, no significant change was indicated after exposure to a dose of 1 ng/ml of biogenic Te NRs as compared to control. A higher enhancement was observed after treatment with DOX at a concentration of 8 µg/ml as positive control.

3.5.3 Effect of biogenic Te NRs on SOD activity

Expression of SOD deregulated by cells (as antioxidant enzyme) is associated with the OS status. SOD activity was evaluated after exposure to three doses of biogenic Te NRs including 1, 2.5, and 5 ng/ml. As shown in Fig. 5 c, biogenic Te NRs significantly (p  = <0.001) depleted SOD activity at a dose of 5 ng/ml as compared to control. No significant change was observed following treatment with biogenic Te NRs at a dose of estimated after treatment with biogenic Te NRs at a dose of 1 ng/ml. A higher reduction was indicated after exposure to DOX at a concentration of 8 µg/ml as positive control.

3.5.4 Effect of biogenic Te NRs on CAT activity

The CAT activity of cells was also evaluated as another OS biomarker. CAT activity was estimated after exposure to three doses of biogenic Te NRs including 1, 2.5, and 5 ng/ml. As shown in Fig. 5 d, biogenic Te NRs significantly reduced CAT activity at two doses of 2.5 and 5 ng/ml as compared to control (p  = 0.016 and p  = 0.008, respectively). No significant differences were indicated following treatment with a dose of 1 ng/ml as compared to control. A higher depletion was observed after treatment with DOX at a concentration of 8 µg/ml as positive control.

4 Discussion

In this study, the characterisation of purified Te NRs produced by the P. pseudoalcaligenes strain Te, as a biogenic source, provided a relatively wide size range of 40–200 nm (Fig. 1 d). It has been previously reported that the XRD pattern of biologically synthesised Te NRs showed diffraction peaks which can be indexed as hexagonal phases of Te (space group: P 3121 (no. 152)), (JCPDS 36‐1452) [16]. From the toxicological perspective, the shape and size of NPs were important factors that influence their cytotoxicity. Since, as the size of NPs decreases and the more spherical shape allows a greater penetration of NPs into the cell that could lead to further toxicological effects [19]. The optimal size for NPs taken up into the cells has found to 50 nm and the spherical NPs with similar size has investigated to 500% more uptake than rod‐shaped NPs [19]. In addition to shape and size, the surface properties and charge of NPs could influence the cell interactions and affected the cytotoxicity [19]. Zeta potential results indicated that Te NPs have positive charge that more intended to cell‐membrane with a negative charge and cellular internalisation greater than negative particle, but on the other hand, the hexagonal rod shape of Te NRs may be counteracted to this property. Therefore, we expected to Te NPs more influence the changes in PC12 cell‐membrane than cytosol or nucleus to effect on proteins or gene expression. The flow cytometry and western blot analyses confirm these hypotheses. So that, Te NRs have shown to induce necrosis more than apoptosis behaviour through caspase‐3 activation. However, little known about the Te NRs behaviour on cell interaction and more study required in this regard.

The cytotoxicity of biogenic Te NRs was compared to K₂ TeO₃ by estimation of the IC₅₀ value of the PC12 cell line. The cytotoxicity of biogenic Te NRs was assessed at three doses of 1, 2.5, and 5 ng/ml with the purpose of evaluating the occurrence of cell apoptosis and possible underlying mechanisms that take place at the IC₅₀ dose through flow cytometry and investigation of the expression of caspase‐3 protein by the western blot method to confirm flow cytometry analyses. OS events, as other cytotoxicity mechanisms after exposure to experimental doses of biogenic Te NRs, were determined by ascertaining the levels of GSH, MDA, SOD, and CAT. According to a literature survey, only a few studies have been published on the cytotoxicity and IC₅₀ measurement of biologic Te NRs produced by different bacterial strains. The IC₅₀ values were estimated at 5.05 ± 0.07 ng/ml for biogenic Te NRs and 2.44 ± 0.38 ng/ml for K₂ TeO₃ on the PC12 cell line. It was indicated that biogenic Te NRs were less toxic than K₂ TeO₃. Similarly, previous work also indicated lower toxicity of NPs produced in bacterium species that named biotechnology‐derived NPs [15, 20]. These studies indicated that biogenic Se and Te NPs exhibited lower toxicity, as compared to their metalloidions [15, 20]. Biogenic Te NRs are approximately five‐fold and biogenic Se NPs (produced by Bacillus sp. MSh‐1) are almost 26‐fold less toxic in mice [15, 20]. Bacterial species have the potential to change oxidation state and convert the soluble ion source to insoluble and less toxic NPs [15, 20]. Therefore, these NPs have specific properties as compared to their ion sources. On the other hand, studies have reported the potential toxicity of NPs in interacting with the biological system [21, 22]. Therefore, methods used to manufacture NPs are important criteria. Due to specific physiochemical properties, NPs may be used as fillers, semiconductors, opacifiers, catalysts, and microelectronics, and also as cosmetics and drug carriers in pharmacology and pharmaceutics [22]. Thereafter, we need to consider risk assessment, establishment of safety manufacture, and the use of NPs in the marketplace and their potential toxicity [21, 22]. Next, we investigated the underlying mechanism of biogenic Te NRs‐induced cytotoxicity on the PC12 cell line. As shown in Fig. 2, cell viability was reduced in a concentration‐dependent manner. By increasing the concentration of biogenic Te NRs, cytotoxicity can be enhanced in PC12 cells. Significant reduction in rat hippocampal astrocytes viability was observed after exposure to 1.95–250 µM of diphenylditelluride (DPDT) and 0.97–250 µM of TeCl₄ through MTT assay; lethal concentration (LC₅₀) for both compounds was estimated at 62.5 µM [23]. Studies have indicated that NPs may induce cytotoxicity by disrupting the cell structure and cause dysfunction in the cells by binding to survival macromolecules [24]. It has been reported that these toxicities depend on particle size, surface properties, and then, on cellular uptake of NPs through the clathrin‐mediated endocytic pathway [25, 26]. According to the flow cytometric results, biogenic Te NRs induced cell death at the IC₅₀ value exposure in the mode of late apoptosis or necrosis towards PC 12 cells. It has been demonstrated that organotellurium compounds such as DPDT, 3,3′‐diaminodiphenyl ditelluride, and 4,4′‐diisopropyldiphenyl ditelluride induced apoptosis in a time‐dependent and dose‐dependent manner in a promyelocytic (HL‐60) cell line [27]. Treatment with 1 × 10(−6) M concentrations of these compounds apoptotic cells was obvious as early as 2 h after exposure [27]. Both apoptosis and necrosis have been reported following treatment with different Te compounds. It has been demonstrated that DPDT exposure induced apoptosis towards tellurium tetrachloride (TeCl₄), promoting necrosis [28]. Necrosis of Schwann cells, producing a myelin sheath around neuronal axons in the peripheral nervous system, occurred during exposure to Te [29]. DPDT and TeCl₄ have indicated the inducing of cytotoxicity in rat hippocampal astrocytes with different mechanisms [23]. This may lead to neurotoxicity following exposure to Te.

To confirm flow cytometry assays, the western blot of caspase‐3 expression (as a key regulator of apoptosis) was investigated. It demonstrated that treatment with DPDT, significantly enhanced in caspase‐3/7 and ‐9 activities at the concentrations of range 500–1000 µM in the HT‐29 cell line and CCD‐18Co cells at the concentration of 1000 µM indicating apoptosis. However, no significant enhancement in caspase activity was observed after exposure to TeCl₄, indicating necrosis [28]. Additionally, apoptotic cells reported after exposure to TeCl₄ at a concentration of 31.25–125 µM by TUNEL and cytochrome c assays, while non‐apoptotic cells were demonstrated following treatment with DPDT in rat hippocampal astrocytes [23]. Also, activity of caspase‐3/7 and ‐9 was enhanced in TeCl₄, while no changes were indicated in caspase activity after treatment of the cell with DPDT [23]. It has been further demonstrated that TeCl₄ induced cytotoxicity through an intrinsic apoptotic pathway [23]. Therefore, controversies were observed in the activated caspase pathways. Thus, Te compounds have different cytotoxicity mechanisms in various cells.

Oxidative mechanism and induction of ROS generation, OS events, and mitochondrial toxicity were also indicted as being involved in inducing cytotoxicity after exposure to Te‐containing components leading to DNA fragmentation and persistent apoptosis or necrosis [28, 30]. The Te‐containing compound induced ROS formation resulting in the enhanced gene expression of Bcl‐2 (pro‐apoptotic gene) and p53 (tumour suppressor gene), and reduced gene expression in Bax (anti‐apoptosis gene), indicating that the mitochondria‐regulated pathway was involved in inducing apoptosis [12, 13]. Cytosolic cytochrome c release, mitogen‐activated protein kinases such as c‐Jun N‐terminal kinases, p38, and extracellular signal‐regulated kinases, and NF‐E2‐related factor 2 are the other protein kinases and signalling pathways that indicated activation following treatment with Te‐containing compounds [12, 31]. A new Te‐containing amphiphilic molecule (DP41) has elevated levels of O²⁻ radical formation and OS damage in HCT116 colon cancer cells [32]. We demonstrated that GSH content was enhanced after PC 12 cells were treated with biogenic Te NRs at a lower concentration of 1 ng/ml, while significant reduction in the GSH level was observed at a higher dose of 5 ng/ml. It indicated that the GSH/GSSG ratio shifted following exposure to Te‐containing components [28]. We investigated that the MDA level was dose‐dependently elevated after exposure to biogenic Te NRs. Significant enhancement was found at two doses of 2.5 and 5 ng/ml. Additionally, activities of intracellular antioxidant enzymes such as SOD and CAT were significantly reduced after treatment with higher doses (5 ng/ml) of biogenic Te NRs. However, the GSH content and SOD activity were promoted after treatment with lower doses (1 ng/ml) of biogenic Te NRs. It has referred to similarly effect of Te to Se belonging to chalcogens family as cofactor of GPx, glutathione‐S ‐transferase (GST) and SOD as antioxidative enzymes [1, 2, 9]. Both Se‐xylofuranosides and Te‐xylofuranosides (as organochalcogens) have enhanced the expression of SOD‐3, which is responsible for regulating OS, ageing, and metabolism [33]. Exposure to Te‐containing compounds has promoted ROS production, related to enhancement of LPO and MDA (as LPO biomarkers) in a dose‐dependent and time‐dependent manner, and has reduced the activity of antioxidant enzymes such as GSH, SOD, CAT, and GST [12, 13, 31]. The potential of Te‐containing compounds to induce OS, and interference with antioxidant defenses by reducing the GSH content and SOD and CAT, and LPO through enhancement of MDA levels and activate protein kinases, revealed that apoptosis through both extrinsic and intrinsic pathways may occur [30]. These results further confirmed the involvement of oxidative mechanisms in the cytotoxicity of biogenic Te NRs and other Te‐containing compounds.

5 Conclusion

The use of Te is being developed in industrial and medical applications. Hence, environmental exposure to its components will increase in future. Te is a highly toxic trace element, elucidated by only a few publications according to its biological effects. Bacterium species (P. pseudoalcaligenes strain Te) are an attractive source for the syntheses of Te, known as biogenic Te NRs (due to their rod shape). We investigated that biogenic Te NRs were less toxic than K₂ TeO₃. The results revealed that biogenic Te NRs cause OS and interfere with antioxidant defenses, independent of activation of the caspase pathway leading to late apoptosis or necrosis in PC12 neuronal cells. So the risk of neurotoxicity during exposure to Te‐containing components should be considered. However, further study is required to look into the toxicity mechanisms induced by different Te‐containing components in various cells.