50Hz Extremely Low Frequency Electromagnetic Fields Enhance Protein Carbonyl Groups Content in Cancer Cells: Effects on Proteasomal Systems
Electromagnetic fields are an assessed cause of prolonging free radicals lifespan. This study was carried out to investigate the influence of extremely low frequency electromagnetic fields on protein oxidation and on the 20S proteasome functionality, the complex responsible for the degradation of oxidized proteins. Caco 2 cells were exposed, for 24–72 hours, to 1 mT, 50 Hz electromagnetic fields. The treatment induced a time-dependent increase both in cell growth and in protein oxidation, more evident in the presence of TPA, while no changes in cell viability were detected. Exposing the cells to 50 Hz electromagnetic fields caused a global activation of the 20S proteasome catalytic components, particularly evident at 72 hours exposure and in the presence of TPA. The finding that EGCG, a natural antioxidant compound, counteracted the field-related pro-oxidant effects demonstrates that the increased proteasome activity was due to an enhancement in intracellular free radicals.
The effects of the exposure to extremely low frequency electromagnetic fields (ELF-EMFs), generated by power lines and household electric appliances, have been receiving increasing interest for the feasible harmful effects on human health. The abovementioned human-made fields have a different wave form, coherence, and frequency with respect to natural EMFs, and their effects on living biological systems are far from being completely understood. Substantially, human-made EMFs are above the naturally occurring electric and magnetic fields. The average B value of the static geomagnetic field is T approximately [1, 2].
A variety of biological hazards, partly attributable to an increased level of free radicals, were associated with the exposure to ELF-EMFs [3–5]. In cellular models, a significant postexposure increase in reactive oxygen species (ROS) has been reported, suggesting that ELF-EMFs are able to induce the activation of cellular processes in the investigated models [4, 5]. Additionally, ELF-EMFs were likely to cause an increase in free radicals lifetime in cells [6, 7]. Besides that, ELF-EMFs have been shown to alter second messenger pathways such as calcium signalling [1, 8], thus leading to downstream physiological effects. In fact, 50 Hz EMFs have been reported to produce an increase in the intracellular calcium level in the pituitary corticotrope-derived AtT20 D16 V cells, and a consequent rearrangement of the cytoskeletal network .
Several epidemiological studies reported evidences suggesting a link between ELF-EMFs exposure and cancer risk [1, 10, 11]. These observations were supported by cellular studies reporting the electromagnetic field-induced effects on biological systems, most notably on the activity of components implicated in the regulation of cell proliferation [1, 12, 13].
Nevertheless, it is still unclear whether and how the exposure to ELF-EMFs promotes carcinogenesis. Mutagenic effects have been extensively described, such as a global increase in the frequency of chromosomal aberrations , in micronuclei formation , or in DNA strand breaks .
Only limited data are currently available on the specific effects of ELF-EMFs on enzyme functionality, although it is believed that they can affect enzyme activity [1, 7, 13, 17–20]. In this perspective, our attention has been focused on the effects of ELF-EMFs exposure on enzymes that regulate cellular homeostasis, the proteasome in particular. Tumor suppressors, cell-cycle regulators, transcription factors, and antiapoptotic proteins are examples of proteasome substrates [21–23]. The cyclin dependent kinase p27, known as a pro-apoptotic protein, is an important regulator of the cell cycle progression, controlling the G1/S transition, where it reaches the highest concentration. Its cellular levels are modulated by the proteasome-dependent degradation, and its decrease has been related to an activation of the proteasome functionality, preventing the cell from undergoing apoptosis .
The 20S proteasome plays a central role in the cellular antioxidant defence, since partially folded, unfolded, and oxidized proteins [25–28], as well as proteins with the reduction of disulfide bonds, are susceptible to proteasomal degradation. The 20S proteasome is a member of the N-terminal nucleophile hydrolases family (Ntn-hydrolases), a class of enzymes that catalytically use an N-terminal residue as both a polarizing base and a nucleophile . In eukaryotic proteasomes only three subunits (1, 2, and 5) are catalytically active, each linked to a specific catalytic component: the “peptidylglutamyl-peptide hydrolizing” (PGPH, cleaving after acidic residues), the “trypsin-like” (T-L, cleaving after basic residues), and the “chymotrypsin-like” (ChT-L, cleaving after hydrophobic residues) activities, respectively [30–32]. In addition, two more catalytic activities, namely, the BrAAP (cleaving bonds after branched chain and aromatic amino acids) and the SNAAP (cleaving after small neutral amino acids), were reported .
Among the modulators of the 20S proteasome activity, the polyphenolic compound epigallocatechin-3-gallate (EGCG), the major component of green tea, has been demonstrated to possess a strong free radical scavenging effect in several cell lines [34–39].
Considering the key role of the proteasome in the removal of oxidized proteins and in the regulation of the cell cycle progression, in the present paper we have investigated the effect of ELF-EMFs on the 20S proteasome functionality. Furthermore, EGCG protective action towards ELF-EMFs mediated oxidative damage has been also evaluated.
2. Materials and Methods
The Oxidized Protein Detection Kit was purchased from Appligene-Oncor (Strasbourg, France). Substrates for assaying the ChT-L, T-L and PGPH activities (Suc-Leu-Leu-Val-Tyr-AMC, Z-Leu-Ser-Thr-Arg-AMC, and Z-Leu-Leu-Glu-AMC) were purchased from Sigma-Aldrich S.r.l. (Milano, Italy). The Z-Gly-Pro-Ala-Leu-Ala-pAB substrate was the kind gift of Professor M. Orlowski (Department of Pharmacology, Mount Sinai School of Medicine, New York, USA). 12-O-tetradecanoylphorbol-13-acetate (TPA) and (-)epigallocatechin-3-gallate (EGCG) were purchased from Sigma-Aldrich S.r.l. EGCG was used directly without further purification and was dissolved in assay buffer (Tris-HCl 50 mM, pH 8.0).
The mouse antihuman polyclonal antibody was obtained by Inalco S.p.A. (Milano, Italy). The rabbit anti-GAPDH (FL-335) polyclonal antibody was purchased from Santa Cruz Biotechnology Inc. Rabbit polyclonal antibodies to “core” subunits of the human 20S proteasome were purchased from Biomol International LP, UK. SigmaMarker, Wide Molecular Weight Range protein standards for SDS-PAGE were obtained from Sigma-Aldrich S.r.l. All chemicals and reagents were of the highest grade available.
2.2. Cell Culture
The human colon adenocarcinoma cell line Caco 2 (Istituto Zooprofilattico Sperimentale Lombardia ed Emilia Romagna, Italy) was cultured in D-MEM medium (Gibco Invitrogen Cell Culture, Milano, Italy) supplemented with 15% fetal calf serum, 0.4% L-glutamine, 1% non essential amino acids, penicillin, and streptomycin. Cells were maintained in a 5% CO2 atmosphere at 37°C.
Cells were grown in 100 mm tissue culture dishes at an initial concentration of cells/mL in a total volume of 15 mL. After a 24-hour starvation without serum, cells were continuously exposed to a sinusoidal 50 Hz magnetic field at a flux density of 1 mT (rms) in a solenoid for 24, 48, and 72 hours. Control samples were placed under the same conditions in a solenoid with no EM field applied. The stray field inside the nonpowered solenoid was 0.5 T. Both samples were placed into a water-jacketed, thermo-regulated and atmosphere-controlled incubator (37.0 ± 0.5°C and 5% CO2). All experiments were performed under blind conditions. Caco cells were cultured in the presence and absence of 150 M EGCG and in the presence and absence of 0.1 M TPA .
After removing the medium and washing with cold PBS, cells were harvested in 3 ml PBS. An aliquot was withdrawn for counting. After centrifuging at 1600 rpm for 5 minutes, the pellet was homogenized in a lysis buffer (20 mM Tris, pH 7.4, 250 mM sucrose, 1 mM EDTA, and 5 mM -mercaptoethanol) and passed through a 25-gauge needle for at least ten times. Lysates were centrifuged at for 15 minutes, and the supernatants were stocked at °C. Protein concentration in cell lysates was determined by Bradford method  using bovine serum albumine as a standard.
2.3. Electromagnetic Field Exposure
ELF-EMFs were generated by a 20 cm diameter 40 cm length solenoid. The solenoid was wound in three forward-backward-forward continuous layers with 600 turns of 2 mm diameter copper wire.
It was driven by the 50 Hz power line through with a variable autotransformer and generated a magnetic flux density of 1 mT (rms) for an applied voltage of 6 V (rms) [9, 41]. The solenoid was positioned into a water-jacketed temperature and atmosphere regulated incubator (37.0 ± 0.5°C and 5% CO2). The dishes were placed horizontally into the solenoid, so that the magnetic flux was parallel to the bottom. Moreover, dishes were placed in a volume determined by measurements of field variations inside the solenoid. In fact, field intensity, measured with a calibrated Chauvin Arnoux gaussmeter, was within % of centre value inside the cylindrical exposure volume of 11 cm by 17 cm along the solenoid axis. Cells were exposed in continuous to a 50 Hz magnetic field at a flux density of 1 mT (rms) for 24, 48 and 72 hours. Control samples were placed in a solenoid with no field applied positioned into a second identical water-jacketed temperature and atmosphere regulated incubator. The stray field inside the nonpowered solenoid was <0.5 T.
Direct monitoring of sample temperatures before, during, and after exposure for both the control and exposed conditions was performed in continuous using a digital multimeter (METEX Mod. M-3630D) controlled by ScopeView software (vers. 1.06) for data logging (the acquisition was carried out at 0.0028 Hz). The temperature probe was placed within the centre of the solenoid (either energized or not) in a culture dish containing medium with no cells: no significant temperature difference between culture media of exposed or unexposed dishes was revealed. Samples temperature was 37.0 ± 0.3°C; see Figure 1.
2.4. Cell Viability
Cell viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) conversion as previously described [42, 43]. Briefly, following experimental treatment, cells were washed in phosphate-buffered saline solution (PBS, pH 7.5), and then MTT (final concentration 0.5 mg/ml) was added to culture medium without fetal calf serum and incubated for 2 hours at 37°C. The medium was then removed and replaced with 100 L of DMSO. The optical density was measured at 550 nm with a microtiter plate reader. At least six cultures were utilized for each time point.
2.5. Measurements of 20S Proteasome Activities
The ChT-L, T-L, PGPH, and BrAAP activities in cell lysates were determined with a Shimadzu RF5300 spectrofluorometer as previously reported [33, 44, 45], using 5 M Suc-Leu-Leu-Val-Tyr-AMC, 5 M Z-Leu-Ser-Thr-Arg-AMC, 5 M Z-Leu-Leu-Glu-AMC, and 0.5 mM Z-Gly-Pro-Ala-Leu-Ala-pAB, respectively, as substrates. Cell lysates (5 g total proteins) were incubated with the synthetic substrates in the assay buffer (50 mM Tris-HCl, pH 8.0) up to a final volume of 500 l. After 1 hour of incubation at 37°C, the measurement of the hydrolyzed 7-amino-4-methyl-coumarin (AMC) or 4-aminobenzoic acid (PABA) was detected (AMC: = 365 nm, = 449 nm; PABA: = 304 nm, = 664 nm).
Since the BrAAP component cleaves exclusively the Leu-Ala bond in Z-Gly-Pro-Ala-Leu-Ala-pAB, a coupled two-step reaction in the presence of excess aminopeptidase N (APN), 20 g of protein, (EC 220.127.116.11), , was used for the determination of the activity. Aminopeptidase N was purified from pig kidney, as reported elsewhere [46, 47].
With the aim of evaluating the 20S proteasome contribution to the short peptide cleavage, control experiments were performed in the presence of specific inhibitors: Z-Gly-Pro-Phe-Leu-CHO (5 M) for the BrAAP activity and lactacystin (5 M) for the ChT-L component [48, 49].
2.6. Western Blotting
Immunoblot detection of carbonyl groups was performed with the OxyBlot oxidized protein detection kit (Appligene-Oncor, Strasbourg, France), according to the manufacturer. Briefly, 15 g of cell lysates were incubated for 15 minutes at room temperature with 2,4-dinitrophenylhydrazine to form the dinitrophenylhydrazone carbonyl derivate and separated on a 12% SDS/PAGE. The modified proteins were then transferred to a polyvinylidene fluoride membrane (Millipore Corporation, Bedford, MA, USA) and revealed by antidinitrophenylhydrazone antibodies as specified in the OxyBlot data sheet.
Western blotting analyses using antibodies to “core” subunits of the human 20S proteasome (1 : 1000) were carried out in order to reveal differences in the expression levels of the complex in cell lysates.
Immunoblot experiments using anti- human and mouse (Mouse) monoclonal antibody (1 : 100) were performed by electroblotting 30 g lysates samples per lane. Each gel was loaded with molecular weight markers including proteins with MW from 6.5 to 205 kDa (SigmaMarker Wide Molecular Weight Range, Sigma-Aldrich S.r.l.). After immunostaining, the two protein bands (50 and 70 kDa) were identified overlapping the film with the Comassie-Brilliant Blue-stained membrane. As a control for equal protein loading glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was utilized: membranes were stripped and reprobed for GAPDH using a polyclonal antibody diluted (Anti-GAPDH (FL-335) rabbit polyclonal antibody from Santa Cruz Biotechnology Inc.). The immunoblot detections were carried out with Enhanced ChemiLuminescence western blotting analysis system (GE-Healthcare, Milano, Italy) using peroxidase conjugated antimouse and antirabbit secondary antibodies. A densitometric algorithm was developed to quantitate the Western Blot results. Each Western Blot film was scanned (16 bits greyscale), and the obtained digital data were processed to calculate the background mean value and its standard deviation. The background-free image was then obtained subtracting the background intensity mean value from the original digital data. The integrated densitometric value associated to each band was then calculated as the sum of the density values over all the pixels belonging to the considered band having a density value higher than the background standard deviation. The band densitometric value was then normalized to the relative GAPDH signal intensity. The ratios of band intensities were calculated within the same Western Blot. All the calculations were carried out using the Matlab environment (The MathWorks Inc., MA, USA) .
2.7. Statistical Analysis
All data are expressed as mean values SE of results obtained from six independent experiments, in which all the activities measurements were carried out in triplicate, whereas for densitometric analysis six blots were considered. Analysis of variance (ANOVA) was used to assess differences among multiple sets of data obtained with untreated and treated cells at different times of exposure. When significant values were found (), post hoc comparisons of means were made using the Bonferroni-Dunn test .
3. Results and Discussion
It is widely accepted that ELF-EMFs can influence several biological functions of cells and tissues, modulating intracellular ROS levels and the cell cycle progression [1, 3–5, 12, 13]. The aim of this study was to investigate the effect of ELF-EMFs on cultured cells focalizing the attention on the 20S proteasome functionality, the main responsible for the degradation of oxidized proteins.
Exposing Caco cells to 1 mT, 50 Hz ELF-EMFs, resulted in a time-dependent increase in cell proliferation rate compared to unexposed control cells. In detail, at 24 hours a 10–12% increase in cell proliferation rate upon ELF-EMFs exposure was evident and after 48 hours there was an increase from 12 to 16%. Cell viability was essentially not affected by ELF-EMFs exposure, however several changes in the cytoplasmic proteolytic pathway have been highlighted. Results on cell proliferation are shown in Figure 2, where each data point is reported as percent of control unexposed cells within the same time exposure. Stimulating the cells with 0.1 M TPA, a tumor promoter able to activate protein kinase C in vivo and in vitro, caused a slighter increase in cell proliferation (from 8 to 10% at 24 hours and from 11 to 13% at 48 hours) and essentially no change in cell viability, as measured by the MTT reduction assay. Viability in exposed cells was not less than 85% of the respective controls (data not shown).
In order to test whether ELF-EMFs exposure causes an increase in cellular oxidative stress, thus favouring changes in protein structures, immunoblot analyses of protein carbonyl groups were performed. As shown in Figure 3, the exposure to ELF-EMFs led to a time dependent increase in protein oxidation. Particularly evident was the increase in carbonyl groups after ELF-EMFs exposure and in the presence of TPA, especially at 72 hours treatment (1.67-fold increase, ); see Figure 3.
Numerous publications reported on the ability of EGCG to act as a free radical scavenger [34, 36–39, 52]. The protective action of EGCG against the ELF-EMFs induced oxidative stress was assayed adding this polyphenol directly to the culture medium during exposure. The treatment with 150 M EGCG caused, essentially, no change in cell viability as measured by the MTT reduction assay (data not shown). As shown in Figure 4, the enhancement in protein carbonyl groups, evident in cells exposed to the fields, was lowered in EGCG treated cells, demonstrating that EGCG protected cells from protein oxidation induced by ELF-EMFs exposure.
As abovementioned, the 20S proteasome is the proteolytic system involved in the removal of oxidatively modified proteins [25–28]. The enzyme functionality was measured monitoring the cleavage of short synthetic peptides in whole cell extracts; in details, the ChT-L, T-L, BrAAP, and PGPH activities were assayed. The results obtained from the activity assays demonstrated that the most susceptible components to the fields were the ChT-L, T-L, and PGPH. In particular, the T-L and PGPH components presented a very similar behaviour. In fact, in the absence of TPA, the exposure to electromagnetic fields led to the activation of both the components with a 1.16-fold increase after 24 hours treatment (). The ChT-L activity presented the maximum activation at 72 hours (1.17-fold increase, ); see Figure 5. An evident activation of the components was also manifested after the simultaneous exposure to TPA. In detail, at 72 hours a 1.18-fold, a 1.2-fold and a 1.17-fold increase was evident for the T-L, PGPH, and ChT-L activity, respectively.
Interestingly, the BrAAP component presented a peculiar behaviour since it was not affected at 24 hours and 48 hours, while a 1.25-fold increase was evident at 72 hours exposure (); see Figure 5, (b).
In a recent study we have shown that EGCG has, besides its scavenging and antioxidant properties, a direct inhibitory effect on the 20S proteasome . When cells were exposed to the fields in the presence of EGCG the abovedescribed proteasome activation was blocked as shown in Figure 6 for the ChT-L activity. It is reasonable that the EGCG-dependent decrease in oxidized species results in a minor exposure of protein hydrophobic residues generating an environment not favorable for the proteasome to be activated.
In order to evaluate whether the differences observed in proteasome functionality were due to changes in 20S proteasome expression levels, we performed immunoblottings using an anti-20S proteasome antibody. As shown in Figure 7, no differences were detected in the proteasome content in lysates obtained from treated and control cells, suggesting that the fields do not affect proteasome expression.
The protein p27 is a cell cycle regulator with a pro-apoptotic effect and is also known as a 20S proteasome substrate. Its levels are therefore considered a hallmark of proteasome functionality. The immunoblot analysis performed with an antibody specific for this protein, showed a time dependent decrease of p27 expression following ELF-EMFs exposure. In the absence of TPA, a gradual decrease of p27 levels was detected reaching a 15% decrease after 48 hours and 72 hours exposure. The presence of TPA led to an overall decrease of protein levels, detectable both in control and ELF-EMFs exposed cells. Moreover, p27 levels in ELF-EMFs treated cells were a 10% lower than in unexposed control cells (Figure 8).
Previous findings suggest that conditions of mild oxidative stress, causing a rearrangement of cellular proteins structures with the consequent exposure of normally hidden hydrophobic residues, may induce an activation of the proteasome functionality [53, 54]. Considering this, the enhancement observed in the 20S proteasome activity, being not associated to an upregulation of its synthesis, as demonstrated by the 20S immunoblots, could be a consequence of the increase in the intracellular oxidative stress. It is possible that in order to counteract the toxicity associated to protein oxidation, cells promote the proteasome-mediated proteolytic pathway.
Furthermore, this increased activity could be related to the upregulation of the intracellular Ca2+ levels. An increase in [Ca2+] in cells upon ELF-EMFs exposure was reported by numerous groups [55, 56], and it is known that this divalent cation is able to modulate proteasome activity, as suggested by Reshetnyak et al. regarding the ChT-L component. The proteasome activation could be associated with a change in the 20S structure with the adoption of an ordered 7-fold symmetric pore conformation stabilized by interactions among a cluster of highly conserved residues (Tyr8, Asp9, Pro17 and Tyr26) . This change lets the proteasome achieve an open conformation thus allowing the substrate to easily enter the proteolytic cavity. The ELF-EMF mediated Vmax increase could be a Ca2+ dependent event. In fact, SDS and Ca2+ behave in a similar way toward proteasome activation  and it has been shown that SDS is able to increase proteasome Vmax .
Upon ELF-EMFs exposure there was a decrease in p27 expression levels most likely because of the activation of the 20S proteasome. Apoptosis is a normally occurring process which is used as a protective mechanism to remove damaged cells; therefore the observed downregulation of p27 expression, induced by both ELF-EMFs and TPA, may prevent the cells from utilizing that strategy in order to get rid of altered cells [60, 61].
ELF-EMFs effects are potentiated by stimulation with TPA, confirming the generally accepted notion that ELF-EMFs behave as a tumor promoter but not as a tumour initiator [1, 62]. In fact, it has been published that low TPA concentrations render the cells susceptible to other stimuli, and consequently the simultaneous exposure to ELF-EMFs lets the two factors act in a synergistic manner .
It is interesting to note that intracellular calcium concentration fluctuates upon ELF-EMFs exposure, affecting protein kinases activities and activating nuclear promoters. Recently, it has been demonstrated that [Ca2+] is able to trigger ubiquitination of endoplasmic reticulum key receptors, such as inositol 1,4,5-tris-phosphate receptor . This receptor binds to the antiapoptotic protein Bcl-X(L), resulting in reduced endoplasmic reticulum Ca2+ concentration, increased oscillations of cytoplasmic [Ca2+], and resistance to apoptosis . This pathway might potentiate the antiapoptotic effect, investigated in this study, derived from an increase of p27 degradation.
The goal of the present study was to elucidate the effects of ELF-EMFs on the functionality of the 20S proteasome, one of the major systems responsible for protein degradation, mainly involved in the removal of oxidized proteins. Our data show that the electromagnetic fields are able to affect the proteasome functionality inducing an increase in its proteolytic activity. The finding that EGCG, a natural compound with antioxidant properties, counteracts the ELF/EMFs related pro-oxidant effects demonstrate that this increase in proteasome activity is due to an increase in intracellular free radicals.
The modulation of the proteasome functionality may in turn affect indirectly various cellular pathways, that is, apoptosis in the human colon carcinoma cell line (Caco 2).
Further studies will be necessary to elucidate different susceptibilities to ELF-EMFs effects between normal and transformed cells in order to gain insight into the role of ELF-EMFs in the carcinogenetic process.
Starting from the potential effect on cell proliferation and differentiation, changes in proteasome functionality, which could be the cause of an altered cell cycle, and the degree of protein oxidation will also be considered among the detrimental effects of electromagnetic fields.
|ELF-EMFs:||Extremely low frequency electromagnetic fields|
|PGPH:||Peptidyl-glutamyl peptide hydrolyzing|
|BrAAP:||Branched chain amino acid preferring|
|SNAAP:||Small neutral amino acid preferring|
This work was supported by a Grant of (Istituto Superiore Prevenzione e Sicurezza Lavoro, Italy ISPESL).
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