Introduction
Terahertz (THz) radiation refers to electromagnetic waves with frequencies ranging from 0.1 to 10 THz (∼3 mm to 100 µm), lying between the infrared and microwave regions of the electromagnetic spectrum.1 Due to its expanding applications in pharmaceutical technologies,2 civil security systems,3 and biomedical research,4 interest in its biological effects has grown substantially. THz radiation can exert both thermal and non-thermal effects depending on power and exposure conditions.5–7 Under non-thermal conditions, pulsed THz radiation interacts with biomolecules without measurable heating, suggesting specific molecular and structural responses.8 Thermal effects are primarily associated with high-power millimeter-wave sources, whereas pulsed THz exposures produce negligible heating.9,10 Non-thermal mechanisms may involve linear or nonlinear resonances at the molecular level; for instance, local disruption of hydrogen bonds in DNA may lead to altered gene expression.11
On the one hand, most of the commonly used THz radiation intensities are not harmful to cells and do not reduce their viability. This has been confirmed in a study on human dermal fibroblasts, where no signs of apoptosis or oxidative stress were observed.12 On the other hand, there are reports of THz-induced adipogenic differentiation of mesenchymal stem cells, as well as indications that THz radiation may influence gene transcription.13 In addition, a research group led by Dr. Peltek has been actively studying the effects of THz radiation on a range of biological objects, including E. coli,14,15G. icigianus,16 and human embryonic stem cells,17 under exposure to a frequency of 2.3 THz.
Contemporary studies indicate that cell membranes are a primary target of THz radiation. In HeLa cells, exposure to 0.10–0.29 THz increased lateral lipid diffusion and induced an order–disorder transition in the bilayer under non-thermal conditions.18 In HT22 neuronal cells, continuous-wave 0.1 THz irradiation enhanced membrane permeability and facilitated transmembrane transport of small molecules.19 These findings are consistent with the underlying physics: phospholipid bilayers and their hydration shells support collective motions in the THz band, and THz spectroscopy detects a long-range hydration layer extending approximately four to five water layers beyond the membrane, providing a direct physical coupling between THz fields and the lipid–protein matrix.20 Similar effects have been demonstrated under exposure to radiation with a wavelength of 130 µm, which may induce reversible impairments in the barrier properties of neuronal membranes.21 Previously, we reported alterations in the metabolic profile after a 45-min exposure of SK-MEL-28 tumor cells to THz radiation, with no signs of cytotoxicity but with metabolomic and bioinformatic evidence of changes in mitochondrial membrane rafts.22 These observations support the concept that membranes and their lipid rafts might act as primary sensors of THz radiation, mediating subsequent metabolic and regulatory responses.
Metabolomics is a field of bioanalytical chemistry aimed at studying biological systems, which enables comprehensive monitoring of metabolic pathways and their responses to various stimuli.23 It enables the identification of pathway-level changes and the discovery of adaptive mechanisms underlying cellular responses. By integrating metabolomic data with gene network reconstruction, it is possible to connect metabolite-level changes to regulatory processes at the protein and signaling levels.24 The ANDSystem software platform is an automated bioinformatics tool that reconstructs molecular interaction networks from biomedical databases and scientific literature using ontology-based knowledge extraction.25–27 This approach has been successfully applied in various studies to identify molecular mechanisms of pathological processes and to interpret omics data.28–30 In our previous study on THz-irradiated melanoma cells, ANDSystem revealed strong connectivity between mitochondrial membrane proteins and enzymes involved in purine metabolism.22
The aim of the present study was to analyze high-performance liquid chromatography (HPLC)–tandem mass spectrometry (MS/MS) metabolomic data of SK-MEL-28 melanoma cells exposed to 2.3 THz radiation for 45 min, identify key metabolic pathways affected, and determine the role of membrane rafts in regulating these THz-induced metabolic and signaling changes.
Materials and methods
Cell culture
Cells (SK-MEL-28) were seeded two days before irradiation. Cells were cultured in T-25 flasks (TPP, Switzerland) (n = 5) in DMEM/F12 (1:1) medium supplemented with L-glutamine (Biolot, Russia), 10% fetal bovine serum (Hyclone, New Zealand), and gentamicin at a concentration of 50 µg/mL (Dalchimpharm, Russia). Incubation was carried out at 37°C in a humidified atmosphere containing 5% CO2. Cells were seeded at a density of 4.0 × 104 cells/cm2 (1 × 106 per flask) and reached approximately 1.2–1.6 × 105 cells/cm2 (3–4 × 106 per flask) by the time of irradiation, in 5 mL of culture medium to ensure uniform exposure across the monolayer surface (90–93%).
Cell viability and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays
Cell viability was assessed 72 h after THz exposure. Cells from both control and irradiated groups were detached from the substrate using a trypsin–versene solution (1:1) (Biolot, Russia), after which 0.4% trypan blue solution (Applichem, Germany) was added to the resulting cell suspension. The numbers of viable and necrotic cells were determined using a Countess automated cell counter (Invitrogen, USA) in accordance with the manufacturer’s instructions. Cell survival 72 h after irradiation was assessed using the MTT assay. Cells were seeded into 96-well plates at a density of 1 × 104 cells per well, with five replicates for each experimental point. After 72 h, 20 µL of the phenazine methosulfate-based reagent (Cell Titer 96® AQueous One Solution, Promega, USA) was added to 100 µL of culture medium in each well. Incubation was carried out under standard conditions for 4 h. Optical density was measured at 490 nm using a Multiskan SkyHigh Microplate Spectrophotometer (Thermo Fisher, USA).
THz-irradiation
THz irradiation was carried out at the Novosibirsk Free Electron Laser (NovoFEL) facility of the Budker Institute of Nuclear Physics, SB RAS. THz radiation was generated at a wavelength of 130 µm (2.3 THz). The power density of the radiation was 0.056 W/cm2. The total irradiation time for each sample was 45 min. The irradiation scheme and methodology are shown in Figure 1. All parameters of the THz laser are outlined in Table 1.31
Table 1The main parameters of the 1st Novosibirsk Free Electron Laser31
| Parameter | Value |
|---|
| Frequency, THz; wavelength, µm | 3.3–0.75 (90–400) |
| Pulse repetition frequency, MHz | 5.64 |
| Pulse duration, ps | 100 |
| Average power, kW | 0.5 |
| Peak power, MW | 1 |
| Spectrum width, % | <1% |
Sample preparation for HPLC–MS/MS analysis
Metabolomic analysis was carried out using five independent biological replicates (n = 5), each analyzed in two technical replicates to ensure reproducibility. Prior to sample preparation, cells were collected as a suspension, and their concentration was determined using a Countess automated cell counter (Invitrogen). Based on the obtained data, a suspension volume corresponding to 1 × 106 cells was transferred into 1.5 mL tubes. Centrifugation was performed at 1,000 g for 5 min. The supernatant was removed, and the cell pellet was resuspended in 1 mL phosphate-buffered saline. After a second centrifugation under the same conditions, phosphate-buffered saline was removed, and 100 µL of Milli-Q water was added to the pellet. The samples were thoroughly mixed until complete resuspension, frozen, and stored at −80°C until analysis. Before analysis, the samples were thawed at room temperature and normalized to contain 1 × 106 cells per 100 µL of Milli-Q water. To disrupt membrane structures, two cycles of freezing (−70°C) and thawing at room temperature were performed, followed by sonication using a 25 kHz ultrasonic system (model Elma Sonic S30H, Germany) at room temperature for 5 min. Then, 400 µL of pre-cooled methanol/acetonitrile mixture (1:1, v/v) was added to the resulting lysate. Extraction was carried out on a thermoshaker at 22°C, 900 rpm for 20 min, followed by centrifugation at 4°C and 16,000 g for 15 min. The obtained supernatant was transferred into vials and subjected to HPLC–MS/MS analysis. Each sample was analyzed in two analytical replicates. The detailed extraction and HPLC–MS/MS protocol was previously described in our publication.32
HPLC
Chromatographic separation was performed on an LC-20AD Prominence system (Shimadzu, Kyoto, Japan) equipped with an SIL-20AC autosampler (Shimadzu, Japan) thermostated at 10°C. Eluent A consisted of 5% acetonitrile in an aqueous solution of 20 mM (NH4)2CO3, adjusted to pH 9.8 with aqueous ammonia. Eluent B was 100% acetonitrile. Each sample was analyzed twice, namely in hydrophilic interaction liquid chromatography and reversed-phase chromatography modes. The hydrophilic interaction liquid chromatography gradient was as follows: 0 min – 98% B, 2 min – 98% B, 6 min – 0% B, and 10 min – 0% B. The column was then re-equilibrated for 4 min. The reversed-phase chromatography gradient was as follows: 0 min – 0% B, 1 min – 0% B, 6 min – 98% B, and 16 min – 98% B. The column was then re-equilibrated for 3 min. The flow rate for both methods was 300 µL/min. The injection volume was 10 µL. Separations in both chromatographic modes were conducted using a monolithic column based on 1-vinyl-1,2,4-triazole, 2 × 60 mm. The monolithic column material was synthesized according to the procedure described in reference33: copolymerization was carried out in a glass tube with an internal diameter of 2 mm using a monomer mixture of styrene/divinylbenzene/1-vinyl-1,2,4-triazole taken at a volume ratio of 10:50:40, respectively.
Mass spectrometry
Metabolite detection was performed in multiple reaction monitoring mode in both positive and negative ion modes using an API 6500 QTRAP mass spectrometer (AB SCIEX, USA) equipped with an electrospray ionization source. The main mass spectrometric parameters were as follows: ion source voltage 5,500 V for positive and −4,500 V for negative ionization; desolvation gas temperature, 475°C; collision gas, “high”; nebulizer gas, desolvation gas, and curtain gas pressures, 33, 33, and 30 psi, respectively. Declustering potential was ±91 V, entrance potential ±10 V, and collision cell exit potential ±9 V. The multiple reaction monitoring dwell time was 3 ms. Instrument control and data acquisition were carried out using Analyst 1.6.3 software (AB SCIEX). Precursor-to-fragment ion transitions, metabolite names, fragmentation times, and corresponding collision energies were adapted from reference.34
Gene network reconstruction
To reconstruct the gene networks, regulatory pathways were considered between significantly altered metabolites and two types of their regulators, human cellular receptors and lipid raft proteins. Human enzymes involved in the transformation of identified metabolites were retrieved from the KEGG Pathway database (https://www.genome.jp/kegg/pathway.html ) by querying their KEGG compound identifiers to obtain associated metabolic pathways.35 Only human pathways (prefix “hsa”) were considered. The analysis included enzymes directly catalyzing reactions involving the target metabolite, as well as those involved in one upstream and one downstream reaction step. KEGG compound identifiers of identified metabolites were extracted from the HMDB database (https://hmdb.ca/ ).36 The list of cellular receptors was obtained from the RaftProt database (https://raftprot.org/ ).37 The list of cellular receptors was extracted from the CellTalkDB database (http://tcm.zju.edu.cn/celltalkdb/browser.php?list=receptors ).38 The ANDSystem cognitive platform with its graphical interface ANDVisio was used to reconstruct and analyze gene networks. Using the “Pathway Master” module of the ANDVisio program,25–27 the regulatory molecular–genetic pathways were constructed. UniProt identifiers of cellular receptors, raft proteins, and enzymes were explicitly submitted to the “Pathway Master” query. Types of molecular interactions, including protein–protein interactions, regulation of protein activity, degradation, transport, post-translational modifications, cleavage, and catalytic reactions, were ticked as parameters in the interaction filter. The object “human proteins” for templates 3 and 4 was passed implicitly to the query. ANDVisio automatically extracted interactions between objects from the ANDSystem knowledgebase according to the templates in Table 2. The templates describe such relationships between proteins as protein–protein interactions, regulation of activity, degradation, transport, post-translational modifications, proteolysis, and catalytic reactions.
Table 2Schemes of template regulatory molecular–genetic pathways between enzymes and cellular receptors, as well as lipid raft proteins of human cells
| № | Template schematic description |
|---|
| 1 | Cellular receptors – protein–protein interactions → enzymes. Lipid rafts – protein–protein interactions → enzymes |
| 2 | Cellular receptors – regulation of activity/degradation/proteolysis/post-translational modifications (PTMs)/transport → enzymes. Lipid rafts – regulation of activity/degradation/proteolysis/PTMs/transport → enzymes |
| 3 | Cellular receptors – regulation of activity/degradation/proteolysis/PTMs/transport → human proteins – regulation of activity/degradation/proteolysis/PTMs/transport → enzymes. Lipid rafts – regulation of activity/degradation/proteolysis/PTMs/transport → human proteins – regulation of activity/degradation/proteolysis/PTMs/transport → enzymes |
| 4 | Cellular receptors – protein–protein interactions → human proteins – protein–protein interactions → enzymes. Lipid rafts – protein–protein interactions → human proteins – protein–protein interactions → enzymes |
Gene network analysis
For templates No. 1 and No. 2, manual verification of connections was performed to exclude errors arising from automated text analysis by the ANDSystem. For templates No. 3 and No. 4, no additional filtering steps were applied, as these templates yielded only the most highly connected proteins. Using ANDVisio, statistical analysis of the gene network was conducted, in which the topological properties of the network were examined based on the degree centrality of graph vertices. Degree centrality is defined as the ratio of the number of connections of a given vertex to the total number of other vertices in the network. This measure makes it possible to assess how strongly a particular vertex is connected to the rest of the network.
A Python 3.11 script incorporating Pandas 2.2.3 and Matplotlib 3.9 was used to calculate the number of regulatory connections for each vertex in each of the regulatory pathway templates and visualize the histograms.
The overrepresentation analysis of Gene Ontology biological processes within the gene network was conducted using the DAVID web tool (https://david.ncifcrf.gov/tools.jsp ).39 UniProt IDs of the gene network participants were uploaded to the DAVID Functional Annotation Tool. The analysis was conducted with Benjamini-Hochberg multiple comparison correction and a significance threshold of 0.05. The data on ages of the gene network participants and overrepresented Gene Ontology biological processes were obtained from the GenOrigin Database (http://genorigin.chenzxlab.cn/ ).40 The Age Coefficient for each overrepresented biological process (OBP) was calculated as the ratio of the average age of the gene network participants within the OBP to the average age of all genes annotated in the OBP.
Statistical analysis
Statistical tests were conducted with Python 3.11 using its SciPy 1.15.0 stats module. Metabolome coverage of the analytical approach was assessed using Student’s t-test. Significantly altered metabolomic profiles were revealed through analysis of the experimental data using the Mann–Whitney U test with multiple-testing correction (Benjamini–Hochberg procedure).
Overrepresentation analysis of metabolic processes was conducted using the MetaboAnalyst 6.0 web tool (https://www.metaboanalyst.ca/ ).41 For the participants of the reconstructed gene networks, statistically significant biological processes (adjusted P-value, or q-value < 0.05) were identified using the DAVID web tool (DAVID knowledgebase version: v2024q1).
Results
Cell viability and MTT assay
After 72 h, both groups were subjected to the MTT assay to assess cell survival; the results are presented in Table 3. As shown in Table 3, the survival rate of the experimental group was high (87–89%) and comparable to that of the control.
Table 3Cell viability and MTT assay results for a 45-min exposure evaluated 72 h post-irradiation
| Group | THz-45 | IR-45 | Control-45 |
|---|
| Cell viability (%) | 96.6 ± 1.2 | 93.4 ± 1.7 | 97.6 ± 0.8 |
| MTT assay (OD) | 87.1 ± 3.2 | 97.7 ± 5.7 | 100.0 ± 2.7 |
Metabolomic screening by HPLC–MS/MS
A total of 40 statistically significant metabolites were identified whose levels changed in cells exposed to THz radiation (Table 4). The P-values for these metabolites are provided in Supplementary Table 1.
Table 4Metabolites significantly altered by THz irradiation relative to the control group
Metabolites and changes in their levels
|
|---|
Increase (↑)
| Decrease (↓)
|
|---|
| Purine metabolism | Lipid metabolism | Energy metabolism | Purine and related metabolites |
|---|
| Uracil* | Cardiolipin (18:2/18:2/18:2/22:6) | Gluconic acid | Adenosine monophosphate (AMP)* |
| 7-methylguanine* | Ceramide (d18:1/16:1 OH) | Malic acid | 1-methyladenine* |
| Deoxyinosine* | Ceramide (d18:1/16:2) | Glyceric acid | Adenosine* |
| Inosine triphosphate (ITP)* | Ceramide (d18:1/16:0) | Adenylsuccinic acid | Adenine* |
| Hypoxanthine* | Ceramide (d18:1/18:1) | Salicyluric acid | Deoxyguanosine diphosphate (dGDP)* |
| Xanthine* | Ceramide (d18:1/24:1) | 1-methylnicotinamide | Methylcysteine |
| 7-methylguanosine* | Phosphatidylcholine (16:0/20:4) | N-acetylglucosamine | 5-hydroxyindoleacetic acid (5-HIAA) |
| Guanine* | Phosphatidylcholine (16:0/22:6) | N-oleoylethanolamine | Cytidine diphosphate (CDP)* |
| Cytosine* | Phosphatidylcholine (38:5) | Acadesine | |
| Anthranilate | Phosphatidylcholine (40:6) | Histamine | |
| Dephospho-CoA | Aminoimidazole carboxamide ribonucleotide (AICAR) | | |
A list of metabolic pathways with varying levels of significance was obtained, from which the pathways with P-value < 0.05 were selected (Fig. 2, Table 5).
Table 5Annotated metabolic pathways
| Metabolic pathway | The level of significance (p-value) |
|---|
| Purine metabolism | 9.47 × 10−11 |
| Pantothenate and CoA biosynthesis | 2.65 × 10−2 |
| Pentose phosphate pathway | 3.45 × 10−2 |
Reconstruction and analysis of gene networks
We hypothesized that THz radiation affected the supramolecular structures of lipid rafts as well as the signaling pathways originating from cellular receptors. Absorption of radiation by lipid rafts may have led to their enlargement or disintegration, as well as to changes in their localization on the membrane surface. An electromagnetic pulse acting on large receptor structures may have triggered signaling pathways that were not normally activated under physiological conditions.
Within this hypothesis, we reconstructed gene networks regulating enzymes through two types of regulators, cellular receptors and lipid raft proteins. Gene networks were constructed using the “Pathway Master” module of the ANDVisio program. Database queries were generated according to the templates presented in Table 1.
Gene networks of enzyme regulation by cellular receptors
Template 1, representing protein–protein interactions between enzymes and cellular receptors, was combined with Template 2, describing the regulation of enzyme biological functions (Fig. 3). The resulting gene network included 30 enzymes and 73 cellular receptors, with 118 protein–protein interaction links, 4 links of protein activity regulation, 2 catalytic links, and 2 proteolysis links.
In the gene network obtained by combining Templates 1 and 2, we highlighted enzymes catalyzing reactions involving guanine. According to the metabolomic analysis, guanine levels were increased in the group of cells after THz irradiation. The gene network reconstructed by combining Templates 1, 2, 3, and 4 was too extensive for visual representation; instead, a quantitative analysis of the regulatory influences from cell receptors to enzymes was performed for it (Fig. 4).
The histogram in Figure 4 illustrates the number of regulatory influences from cellular receptors to the 25 most highly regulated enzymes involved in the biosynthesis and degradation of significant metabolites.
Gene network of enzyme function regulation by lipid raft proteins
To construct the gene network of enzyme regulation by lipid raft proteins, Template 1 (representing protein–protein interactions) was combined with Template 2 (describing the regulation of enzyme biological functions by lipid raft proteins) (Fig. 5). The resulting gene network included eightenzymes and nine lipid raft proteins, with eight protein–protein interaction links and three links of protein degradation regulation.
In the gene network in Figure 5, we identified key lipid raft proteins: ALBU, PL4, and PGK1. In addition, by combining Templates 1, 2, 3, and 4, a large gene network was constructed (not shown in the paper), for which a quantitative analysis of the regulatory influences from lipid rafts to the enzymes was performed. The histogram in Figure 6 illustrates the number of regulatory connections from lipid raft proteins to the 25 most strongly regulated enzymes participating in the biosynthesis and degradation of key metabolites.
Overrepresentation analysis of biological processes
Overrepresentation analysis of biological processes was performed using the DAVID web tool. For the gene network of enzyme regulation by cellular receptors, 577 OBPs were identified, among which we highlighted processes of chromatin organization regulation, G protein–related signaling pathways, and regulation of post-translational modifications. For the gene network of enzyme regulation by lipid rafts, 91 OBPs were identified, including G protein–related signaling pathways, chromatin organization regulation, and protein post-translational modification processes.
Discussion
The lack of cytotoxicity under prolonged exposure is, in our view, an expected outcome, since Wilmink et al.42 demonstrated that differences between irradiated groups at a frequency of 2.52 THz in human dermal fibroblast cells and the sham-control group were not statistically significant. It was also shown that exposure to THz radiation at three different frequencies (1.4 THz, 2.52 THz, 3.11 THz) in human keratinocytes resulted in cell viability that did not differ significantly from the sham-control.43
Human neural progenitor cells were irradiated for 30 min with pulsed THz radiation at 100 Hz. The study demonstrated no changes in proliferative activity following THz exposure in cultures of both healthy and tumor cells (neuroblastoma).44
In another study, broadband THz radiation (0.05–1.2 THz) at powers of 30, 2, and 0.1 µW was shown to have no substantial effect on mitochondrial functional activity in either adherent cell cultures (human lung carcinoma A-549, human breast adenocarcinoma BT-20, human colon carcinoma COLO 320 HSR), suspension cultures (NMC-1, human histiocytic leukemia U937, human promyelocytic leukemia HL-60), or primary cultures (thymocytes and splenocytes of CBA mice).45 Importantly, no disruption of the integrity of their bilipid surface membranes was observed.
These findings confirm that THz exposure does not induce significant cytotoxicity, thereby allowing investigation of subtle metabolic and signaling rearrangements without confounding effects of cell death.
One of the most distinct and pronounced metabolomic changes observed after 45 min of irradiation was a decrease in adenine levels accompanied by an increase in hypoxanthine, inosine triphosphate, and deoxyinosine. Such shifts are characteristic of a transition from de novo synthesis to the purine salvage pathway,46 which is more energy-efficient and indicates an adaptive cellular response to THz radiation. In the reconstructed gene network, adenosine deaminase (ADA) forms five direct protein–protein interactions with membrane receptors; these interactions represent physical contacts, meaning that the enzyme is “recruited” to the plasma membrane and operates in close proximity to receptor complexes. Such localization is optimal for rapid processing of extracellular adenosine as well as the intracellular deoxyadenosine pool, consistent with the metabolomic profile.
Dipeptidyl peptidase-4 (DPP4) is known as an anchoring protein on the surface of immune and tumor cells: binding increases the catalytic constant of ADA and stabilizes it on the membrane.47 The most significant interaction was the DPP4–ADA pair. We hypothesize that DPP4 activation under THz exposure accelerates adenine/adenosine deamination, producing hypoxanthine/inosine, thereby restarting the salvage cascade through hypoxanthine-guanine phosphoribosyltransferase and inosine monophosphate dehydrogenase, shifting the cell to a more “cost-efficient” nucleotide resynthesis mode.48 Moreover, the DPP4–ADA complex is localized in lipid rafts, where the co-receptor CD45 forms a signaling metaplex, leading even to raft domain redistribution.49 Thus, secondary utilization of purines reduces the cellular demand for adenosine triphosphate (ATP) and amino acids, which is consistent with both the absence of pronounced cytotoxicity and the increase in the AMP-activated protein kinase marker 5-aminoimidazole-4-carboxamide ribonucleotide. In addition, decreased adenosine and adenine levels prevent accumulation of S-adenosylhomocysteine, a methyltransferase inhibitor, thereby providing a “window” for cellular energy landscape reorganization without the risk of hypermethylation. At the same time, studies on the demethylating potential of THz radiation open the hypothesis that, in melanoma, such demethylation may have both pro- and anti-tumor effects depending on the specific loci. This rearrangement should therefore be interpreted as an adaptive and non-thermal response to THz stress.
Gene network analysis also revealed that the hexameric receptor complement component 1 Q subcomponent-binding protein (C1QBP, also known as p32/HABP1) formed five direct protein–protein interactions with enzymes of the tricarboxylic (TCA) acid cycle and oxidative catabolism—monoamine oxidase B (MAOM (MAO-B)), malate dehydrogenase 2 (mitochondrial) (MDHM (MDH2)), aldehyde dehydrogenase 1 family member B1 (ALDH1B1), aldehyde dehydrogenase 2 (mitochondrial) (ALDH2), and the mitochondrial nucleoside diphosphate kinase (NME4/NDKM). C1QBP is known to form high-stoichiometry complexes with MDH2 and ALDH2, stabilizing them at the inner membrane and increasing electron transfer efficiency to Complex I.50 Metabolomic analysis showed elevated malic and glyceric acid levels, consistent with enhanced activity of the malate–aspartate shuttle catalyzed by MDH2. Increased ITP and deoxyinosine further point to activation of the ATP-independent salvage pathway (via NME4), which is energetically advantageous under conditions of enhanced respiration. In addition to purine metabolism, THz exposure also influenced pantothenate/CoA biosynthesis and the pentose phosphate pathway. Both pathways are closely linked to energy and redox regulation. Enhanced CoA biosynthesis may indicate an increased demand for acyl-group transfer and mitochondrial oxidation, while activation of the pentose phosphate pathway supplies NADPH for antioxidant defense and anabolic reactions. Together, these alterations suggest that THz irradiation induces a coordinated metabolic response aimed at maintaining redox balance and energetic stability under non-thermal electromagnetic stimulation.
The receptor–ubiquitin ligase 5′-nucleotidase domain containing 2 (5NTD (NT5DC2)) reportedly proteolytically cleaves ectonucleoside triphosphate diphosphohydrolase 1 (ENTP1) while simultaneously inhibiting guanine deaminase activity. This combination of effects explains the accumulation of ITP and the decrease in guanine.
The guanine nucleotide-binding protein, alpha stimulating activity polypeptide 2 (GNAS2 (Gas)), and guanine nucleotide-binding protein, alpha q polypeptide (GNAQ (Gaq)) proteins receive the largest number of inputs from plasma membrane receptors in the gene network. Notably, p32 and Gα proteins colocalize within membrane domains, and their convergence during THz-induced raft rearrangement may activate two possible signaling cascades: (1) the cyclic adenosine monophosphate/protein kinase A signaling pathway (cAMP/PKA via Gas), leading to phosphorylation of the MDH2/ALDH2 complex; and (2) the phospholipase C–inositol trisphosphate–calcium signaling pathway (PLC–IP3–Ca2+ via Gaq), causing activation of the mitochondrial calcium uniporter and an additional influx of NADH.51 Both cascades accelerate TCA cycle activity and explain the excess production of malic acid.
A study on the effect of THz radiation on the proteome of the thermophilic bacterium Geobacillus icigianus demonstrated changes in the expression of numerous proteins after 15 min of exposure to THz radiation generated by a free-electron laser.16 Alterations were observed in electron transport chain components, regulators of transcription, translation, DNA repair, cell growth, and chemotaxis, as well as in pathways of peptidoglycan synthesis, riboflavin, NAD, FAD, and pyridoxal phosphate biosynthesis, TCA cycle metabolites, and ATP biosynthesis, indicating a rapid cellular response to THz radiation.
The gene network of cellular responses to THz radiation revealed that epidermal growth factor receptor (EGFR) and a group of Gα subunits formed the densest cluster of interactions with enzymes processing guanine/adenine-containing metabolites. THz-induced rearrangement of lipid rafts may have promoted ligand-independent EGFR clustering, consistent with previous reports demonstrating that EGFR can undergo membrane-driven clustering and activation in response to non-chemical physical perturbations, independent of ligand binding.52 In the network, EGFR was connected to APT, ALDH7A1, NT5D2, and GNAS2. These enzymes govern the terminal steps of IMP/XMP reutilization and the glycerol shunt; thus, their spatial proximity to EGFR provides the cell with a “short” metabolic loop for rapid replenishment of the nucleotide pool.
GNAI1, GNAQ, and GNAS-short isoform (GNAS2) were identified as high-degree nodes (11 to 29 receptors). Through these, receptors from adrenergic, purinergic, lysophosphatidic acid, and other families can transactivate EGFR (the so-called GPCR–RTK transactivation). The GNAS family elevates intracellular cAMP levels, and increased cAMP has already been shown to trigger expression of several salvage pathway enzymes (e.g., APRT, PNP).53 In essence, Gα nodes act as a “gateway”: the higher the density of GPCR inputs, the stronger the shift from de novo synthesis toward purine salvage, which in our experiments was reflected as increased hypoxanthine, ITP, and deoxyinosine levels, accompanied by decreased adenine.
The most highly organized structures of the plasma membrane, as well as of mitochondria, are lipid rafts. Lipid rafts are dynamic nanoscale assemblies of sphingolipids, cholesterol, and proteins that can stabilize and coalesce into larger platforms.54 A key function of lipid rafts is signal transduction, particularly the regulation of metabolic processes. Albumin (regulator) inhibits the degradation of purine nucleoside phosphorylase (hereinafter referred to as PNPH), which catalyzes the phosphorolytic cleavage of the N-glycosidic bond in deoxyadenosine, a metabolite whose levels increased in the metabolomic profiles of cells exposed to THz laser irradiation.55 According to our calculations, the enzymes GNAS2 and MDHM are influenced by the largest number of corresponding lipid raft proteins through protein–protein interactions. THz-induced “compaction” of rafts increases the affinity of the ALBU→PNPH interaction and shifts the cell toward deoxyadenosine degradation.
The OBPs we identified are consistent with those reported by other authors studying the effects of THz radiation on cells. Exposure to THz radiation at the molecular level can lead to changes in the regulation of chromatin organization, G protein–related signaling pathways, and post-translational modifications. In a study,6 THz radiation was shown to affect chromatin organization and gene expression in microbial and eukaryotic cells, suggesting possible mechanisms of non-thermal action of THz radiation. Thus, THz radiation of appropriate intensity and frequency can induce epigenetic changes such as histone modifications,56 altered binding conditions of methylated DNA, or DNA demethylation, ultimately leading to changes in gene expression and chromatin structure. This is particularly relevant to cancer therapy, where demethylation can be used to suppress oncogenes.57
A large body of research has focused on aberrant DNA methylation in melanoma.58,59 Melanoma is one of the most promising cancer types for biomedical applications of THz radiation in therapy due to the easy accessibility of malignant lesions. Manipulating DNA methylation to correct alterations in tumors may become an effective strategy for molecular cancer therapy.
THz radiation may also induce conformational changes in chromatin, influencing its compactness and accessibility for transcription and DNA repair.6 In addition to van der Waals interactions, THz radiation can affect hydrogen bonds that stabilize chromatin structure, leading to either decondensation or condensation.60
The OBPs we identified are also consistent with the findings of a study, which reported that THz radiation can influence neuronal morphology and dynamic properties, inducing nonlinear resonance effects in proteins that may, in turn, impact chromatin organization and signaling pathways.21
The results obtained show that both the gene network of enzyme regulation by cellular receptors and the network associated with lipid rafts include numerous OBPs. This indicates a complex interplay between these elements and their significance in cellular physiology. The identified processes may serve as a basis for further research in molecular biology and medicine. The study highlights the need for continued analysis of interactions between different signaling pathways and their impact on cellular functions.