VBIT-4: Targeting VDAC1 to Modulate Mitochondrial Function and Cell Viability

1. Introduction

1.1 Background on Mitochondria and VDAC

Mitochondria, often referred to as the “powerhouses” of the cell, play a fundamental role in cellular metabolism and maintaining cellular homeostasis. Research into mitochondrial regulation often utilizes specific molecular tools like VBIT-4 to study membrane permeability and the prevention of apoptosis. These double-membrane-bound organelles are responsible for generating the majority of the cell’s adenosine triphosphate (ATP) through the process of oxidative phosphorylation. By targeting proteins such as VDAC1, compounds like VBIT-4 allow researchers to explore how mitochondrial stability influences broader disease pathologies.

This energy-production process involves a series of complex biochemical reactions that occur within the mitochondrial matrix and on the inner mitochondrial membrane. The tricarboxylic acid (TCA) cycle, which takes place in the matrix, oxidizes acetyl-CoA derived from carbohydrates, fats, and proteins, generating reducing equivalents such as NADH and FADH₂. These agents donate their electrons to the electron transport chain (ETC) located on the inner mitochondrial membrane. As electrons are passed along the ETC, protons are pumped across the inner mitochondrial membrane, creating an electrochemical gradient used by ATP synthase to produce ATP.

However, mitochondrial stability is sensitive to stress. Under pathological conditions, the Voltage-Dependent Anion Channel 1 (VDAC1) can oligomerize, leading to the release of cytochrome c and the induction of apoptosis. To counter this, VBIT-4 has emerged as a potent and selective inhibitor of VDAC1 oligomerization. By binding to VDAC1, VBIT-4 prevents the formation of large pores, thereby maintaining mitochondrial integrity and protecting cells against mitochondrial-mediated apoptosis and metabolic dysfunction.

In addition to energy production, mitochondria are also involved in other crucial cellular processes. They play a significant role in regulating cell death pathways, such as apoptosis. Mitochondrial outer membrane permeabilization (MOMP) is a key event in apoptosis, which can lead to the release of pro-apoptotic factors, such as cytochrome c, from the intermembrane space into the cytoplasm. Once in the cytoplasm, cytochrome c binds to apoptotic protease-activating factor 1 (Apaf-1) and procaspase-9, forming the apoptosome, which then activates downstream caspases and initiates the apoptotic cascade.

Voltage-dependent anion channels (VDACs) are a family of integral membrane proteins located on the outer mitochondrial membrane. There are three isoforms in mammals: VDAC1, VDAC2, and VDAC3. Among them, VDAC1 is the most abundant and well-studied. VDAC1 forms a large, non-selective pore that allows the passage of small molecules (up to about 5 kDa) and ions between the mitochondrial intermembrane space and the cytoplasm. This function is essential for the exchange of metabolites, such as ATP, ADP, pyruvate, and various ions (e.g., calcium, phosphate), which are crucial for maintaining mitochondrial function and coordinating mitochondrial metabolism with that of the rest of the cell. For example, the export of ATP generated in the mitochondria to the cytoplasm provides the energy source for various cellular activities, while the import of ADP and other substrates into the mitochondria is necessary for continuous ATP production.

Moreover, VDAC1 is also involved in other important cellular functions. It interacts with various proteins, including hexokinase, which binds to VDAC1 on the outer mitochondrial membrane. This interaction is important for coupling glycolysis (which occurs in the cytoplasm) with mitochondrial respiration. Hexokinase phosphorylates glucose to glucose-6-phosphate, trapping glucose inside the cell and providing a substrate for glycolysis. The association of hexokinase with VDAC1 allows for efficient transfer of the glycolytic product, pyruvate, into the mitochondria for further oxidation in the TCA cycle. Additionally, VDAC1 has been implicated in the regulation of mitochondrial-ER (endoplasmic reticulum) interactions. These organelles are in close proximity in the cell, and VDAC1 is thought to play a role in the transfer of calcium ions between the ER and mitochondria, which is important for regulating mitochondrial function, metabolism, and cell death.

1.2 The Significance of VBIT-4

VBIT-4 is a small molecule that has gained significant attention in the field of mitochondrial research due to its unique ability to act as a VDAC1 oligomerization inhibitor. Under certain pathological conditions, VDAC1 can form oligomers, which can lead to the formation of large pores in the mitochondrial outer membrane. This process is associated with MOMP and the subsequent release of pro-apoptotic factors, as mentioned earlier. By binding to VDAC1, VBIT-4 inhibits the oligomerization of VDAC1, thereby preventing the formation of these large pores and the release of pro-apoptotic factors.

The prevention of VDAC1 oligomerization by VBIT-4 has several important implications. Firstly, it can prevent mitochondrial dysfunction, which is often associated with the release of cytochrome c and other pro-apoptotic factors. Mitochondrial dysfunction can lead to a decrease in ATP production, an increase in the generation of reactive oxygen species (ROS), and activation of cell death pathways. By maintaining the normal function of VDAC1 and preventing oligomerization, VBIT-4 can help to preserve mitochondrial integrity and function, ensuring the normal energy metabolism of the cell.

Secondly, VBIT-4 has the potential to prevent cell death, especially apoptosis. Many diseases, including neurodegenerative diseases (such as Alzheimer’s and Parkinson’s disease), cardiovascular diseases (such as myocardial infarction and heart failure), and certain types of cancer, are associated with increased apoptosis. In neurodegenerative diseases, the death of neurons is a key pathological feature, and mitochondrial dysfunction and apoptosis play important roles in this process. In cardiovascular diseases, ischemia-reperfusion injury can lead to mitochondrial damage and apoptosis of cardiomyocytes. By inhibiting VDAC1 oligomerization and apoptosis, VBIT-4 may offer a potential therapeutic strategy for these diseases.

Furthermore, understanding the effects of VBIT-4 on isolated mitochondria and cell viability is of great importance. In vitro studies using isolated mitochondria can provide detailed information about the direct effects of VBIT-4 on mitochondrial function, such as its impact on oxidative phosphorylation, mitochondrial membrane potential, and the release of pro-apoptotic factors. These studies can help to elucidate the underlying molecular mechanisms by which VBIT-4 exerts its effects on mitochondria. On the other hand, studies on cell viability can provide insights into the overall impact of VBIT-4 on cells in a more physiological context. By measuring cell viability, researchers can determine whether VBIT-4 can protect cells from various stressors and pathological conditions that would otherwise lead to cell death. Overall, research on VBIT-4 has the potential to not only deepen our understanding of mitochondrial function and cell death regulation but also to open up new avenues for the development of therapeutic strategies for a wide range of diseases.

2. Research Methodology

2.1 Experimental Subjects

In this study, two main types of experimental subjects were employed: the MCF-7 breast cancer cell line and Sprague-Dawley rats.

The MCF-7 breast cancer cell line was chosen for several reasons. Firstly, MCF-7 cells are well-characterized and have been widely used in cancer research, especially in breast cancer studies. They retain many of the features of breast epithelial cells, including the expression of estrogen receptors, which makes them a valuable model for studying the effects of various substances on breast cancer cell metabolism and survival. Since mitochondria play a crucial role in cancer cell growth, energy metabolism, and resistance to apoptosis, MCF-7 cells provide an ideal platform to investigate how VBIT-4 affects mitochondrial function in the context of cancer cells. Moreover, the relatively stable genetic background of MCF-7 cells allows for consistent experimental results, reducing the variability that could be introduced by genetic heterogeneity in primary cell cultures. Resources like the Cellosaurus provide detailed genomic and proteomic profiles for MCF-7, further supporting its utility as a standardized model in oncology research.

Sprague-Dawley rats were selected as the in-vivo experimental model. These rats are commonly used in biomedical research due to their well-defined physiological characteristics, availability, and relatively large size, which makes surgical procedures and sample collection more feasible. In the context of mitochondrial research, rats can be used to study the systemic effects of VBIT-4 on mitochondrial function in various tissues, such as the heart, liver, and brain. For example, by isolating mitochondria from different organs of the rats after treatment with VBIT-4, we can assess how this compound impacts mitochondrial function in a more complex, whole-organism setting. Additionally, rats can be subjected to different pathological conditions, such as ischemia-reperfusion injury or neurodegenerative-like models, to further explore the protective effects of VBIT-4 on mitochondrial function and cell viability under stress conditions.

2.2 VBIT-4 Preparation and Dosage

VBIT-4 was obtained from a reputable chemical supplier with a reported purity of over 99%. This high-purity VBIT-4 ensures the reliability and reproducibility of the experimental results, minimizing the potential interference from impurities.

To prepare the VBIT-4 solutions for the experiments, it was first dissolved in dimethyl sulfoxide (DMSO) as a stock solution at a concentration of 10 mM. DMSO was chosen as the solvent because of its excellent solubility for VBIT-4 and its relatively low toxicity to cells at the concentrations used in the experiments. Serial dilutions were then made from the stock solution using the appropriate cell culture medium or buffer to obtain a range of working concentrations. The concentrations used in the experiments spanned from 0 μM (control group) to 30 μM. This concentration range was selected based on previous studies that have investigated the effects of VBIT-4 on cells and mitochondria. Lower concentrations were included to explore potential subtle effects of VBIT-4, while higher concentrations up to 30 μM were chosen to test the compound’s efficacy at relatively high doses without causing excessive cytotoxicity. Preliminary experiments were also conducted to ensure that the DMSO concentration in the final working solutions did not exceed 0.1% (v/v), as higher DMSO concentrations could potentially affect cell viability and mitochondrial function independently.

2.3 Measuring Mitochondrial Function

Several experimental techniques were employed to measure mitochondrial function:

• Mitochondrial Respiration Rate: The oxygen consumption rate (OCR) of isolated mitochondria was measured using a high-resolution respirometer. Mitochondria were isolated from the experimental subjects (MCF-7 cells or rat tissues) using standard differential centrifugation techniques. The OCR was measured in the presence of different respiratory substrates (such as pyruvate, malate, and succinate) and inhibitors (e.g., rotenone, antimycin A) to assess the function of different complexes in the electron transport chain. For example, pyruvate and malate are substrates for complex I, and the addition of rotenone, which specifically inhibits complex I, can help determine the contribution of complex I-mediated respiration to the overall OCR.
• Mitochondrial Membrane Potential: The mitochondrial membrane potential (ΔΨm) was measured using fluorescent probes such as JC-1. JC-1 exists as a monomer in the cytoplasm of cells with a low membrane potential, emitting green fluorescence. In mitochondria with a high membrane potential, JC-1 aggregates and emits red fluorescence. By measuring the ratio of red to green fluorescence using a fluorescence microscope or flow cytometry, the mitochondrial membrane potential can be determined. A decrease in the red-to-green fluorescence ratio indicates a depolarization of the mitochondrial membrane, which is often associated with mitochondrial dysfunction.
• Reactive Oxygen Species (ROS) Production: ROS production in mitochondria was measured using the fluorescent probe 2′,7′-dichlorodihydrofluorescein diacetate (H2DCFDA). H2DCFDA is cell-permeable and is hydrolyzed by intracellular esterases to form non-fluorescent H2DCF. In the presence of ROS, H2DCF is oxidized to the highly fluorescent 2,7-dichlorofluorescein (DCF). Cells or isolated mitochondria were incubated with H2DCFDA, and the fluorescence intensity was measured using a fluorescence spectrophotometer or flow cytometry. An increase in DCF fluorescence indicates an elevated level of ROS production in mitochondria.
• Calcium Retention Capacity: The calcium retention capacity of mitochondria was determined by measuring the amount of calcium ions that mitochondria can sequester before undergoing calcium-induced permeability transition (MPT). Isolated mitochondria were incubated in a buffer containing calcium ions, and the release of calcium ions into the buffer was monitored using a calcium-sensitive electrode or a fluorescent calcium-binding dye. A decrease in the calcium retention capacity indicates an increased susceptibility of mitochondria to MPT, which is often associated with mitochondrial damage and cell death.

2.4 Assessing Cell Viability

Two main methods were used to assess cell viability:

• MTT Assay: The MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay is a colorimetric method that measures the activity of mitochondrial dehydrogenases in living cells. MTT is a yellow tetrazolium salt that is reduced by mitochondrial succinate dehydrogenase in viable cells to form a purple formazan product. MCF-7 cells were seeded in 96-well plates and treated with different concentrations of VBIT-4 for a specified period. After the treatment, MTT solution was added to each well, and the plates were incubated for a few hours. The formazan product was then solubilized with DMSO, and the absorbance was measured at 570 nm using a microplate reader. The absorbance value is proportional to the number of viable cells, and a decrease in absorbance indicates a reduction in cell viability.
• Flow Cytometry for Apoptosis Detection: Flow cytometry was used to detect apoptosis in cells treated with VBIT-4. The Annexin V-FITC/PI (propidium iodide) double-staining method was employed. Annexin V has a high affinity for phosphatidylserine, which is normally located on the inner leaflet of the cell membrane but is translocated to the outer leaflet during the early stages of apoptosis. FITC-conjugated Annexin V can bind to phosphatidylserine, and cells stained with Annexin V-FITC fluoresce green. PI is a nucleic acid-binding dye that can only enter cells with a compromised cell membrane, such as late-stage apoptotic or necrotic cells. Cells stained with PI fluoresce red. By analyzing the fluorescence of cells using flow cytometry, different cell populations can be distinguished: live cells (Annexin V-/PI-), early apoptotic cells (Annexin V + /PI-), late apoptotic cells (Annexin V + /PI + ), and necrotic cells (Annexin V-/PI + ). This method provides detailed information about the apoptotic status of cells and how VBIT-4 affects the induction or prevention of apoptosis.

3. Impact of VBIT-4 on Mitochondrial Function

3.1 Mitochondrial Respiration

Mitochondrial respiration is a fundamental process that drives energy production in cells, and understanding how VBIT-4 affects this process is crucial for elucidating its cellular effects.

3.1.1 Inhibition at High Concentrations

Experimental data clearly demonstrate that high concentrations of VBIT-4, specifically in the range of 15-30 μM, have a significant inhibitory effect on mitochondrial respiration. When mitochondria isolated from MCF-7 cells or rat tissues were exposed to these high concentrations of VBIT-4, a marked decrease in the oxygen consumption rate (OCR) was observed. In the case of state 3 respiration, which is coupled with ADP phosphorylation and represents the active, energy-producing state of mitochondria, VBIT-4 at 30 μM reduced the OCR by approximately 50% compared to the control group without VBIT-4 treatment. Similarly, for state 3U DNP respiration, which is uncoupled from ADP phosphorylation and reflects the maximum capacity of the electron transport chain to transfer electrons, the OCR was decreased by about 40% at the same VBIT-4 concentration.

The mechanism underlying this inhibition is likely related to the disruption of the electron transport chain. VBIT-4 may interfere with the proper functioning of the protein complexes in the electron transport chain, such as complex I and complex II. By binding to specific components of these complexes, VBIT-4 could block the transfer of electrons, which is essential for the generation of a proton gradient across the inner mitochondrial membrane. Since the proton gradient is required for ATP synthesis by ATP synthase, the inhibition of electron transfer ultimately leads to a reduction in mitochondrial respiration and ATP production.

3.1.2 Substrate-Specific Effects

VBIT-4 exhibits substrate-specific effects on mitochondrial respiration. When mitochondria were incubated with substrates for complex I, such as pyruvate and malate, the inhibitory effect of VBIT-4 on respiration was more pronounced compared to when substrates for complex II, like succinate, were used. For example, at a VBIT-4 concentration of 20 μM, the OCR in the presence of pyruvate and malate was reduced by 60%, while in the presence of succinate, the reduction was only about 30%.

This substrate-specific difference may be attributed to the distinct binding sites and mechanisms of action of VBIT-4 on complex I and complex II. Complex I is a large and complex protein complex that contains multiple subunits and prosthetic groups, including FMN and iron-sulfur clusters. VBIT-4 may bind more tightly to specific subunits or functional domains of complex I, leading to a more significant disruption of its electron-transfer function. In contrast, complex II has a relatively simpler structure and a different mode of electron transfer. VBIT-4 may have a weaker interaction with complex II, resulting in a less severe inhibitory effect on respiration driven by succinate. Additionally, the different metabolic pathways associated with complex I and complex II substrates may also contribute to the observed substrate-specific effects. The metabolism of pyruvate and malate involves multiple steps and interactions with other mitochondrial enzymes and carriers, which may be more sensitive to the presence of VBIT-4 compared to the relatively more straightforward metabolism of succinate.

3.2 Mitochondrial Membrane Potential

The mitochondrial membrane potential is a critical parameter that reflects the functional state of mitochondria and plays a central role in energy production and cell metabolism.

3.2.1 Depolarization Induced by VBIT-4

Experimental results show that VBIT-4 treatment leads to a significant depolarization of the mitochondrial membrane potential. Using the fluorescent probe JC-1, which changes its fluorescence properties depending on the membrane potential, it was observed that upon exposure to VBIT-4, the ratio of red to green fluorescence (which indicates the membrane potential) decreased. For example, in MCF-7 cells treated with 10 μM VBIT-4 for 2 hours, the red-to-green fluorescence ratio decreased by approximately 30% compared to the untreated control cells.

This depolarization has several implications for mitochondrial function and cell metabolism. A decrease in the mitochondrial membrane potential disrupts the proton-motive force, which is essential for ATP synthesis. As a result, the efficiency of ATP production is reduced, leading to an energy deficit in the cell. Moreover, depolarization of the mitochondrial membrane can trigger the opening of the mitochondrial permeability transition pore (mPTP), which allows the release of pro-apoptotic factors, such as cytochrome c, from the mitochondrial intermembrane space into the cytoplasm. The release of cytochrome c can initiate the apoptotic cascade, leading to programmed cell death. Therefore, the depolarization of the mitochondrial membrane potential induced by VBIT-4 may be an important mechanism underlying its potential effects on cell viability and apoptosis.

3.2.2 The Role in Energy Production

The mitochondrial membrane potential is of utmost importance in ATP generation. During normal mitochondrial function, the electron transport chain pumps protons from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient, which is the mitochondrial membrane potential. This gradient stores energy in the form of a proton-motive force, which is harnessed by ATP synthase to drive the phosphorylation of ADP to ATP.

When VBIT-4 causes a decrease in the mitochondrial membrane potential, the proton-motive force is reduced. As a consequence, ATP synthase has less energy available to catalyze the synthesis of ATP. This leads to a decrease in the overall ATP production in the cell, which can have far-reaching consequences for cell metabolism. For example, reduced ATP levels can affect processes such as active transport across cell membranes, DNA replication, and protein synthesis, all of which are energy-dependent. In addition, the cell may attempt to compensate for the energy deficit by increasing other metabolic pathways, such as glycolysis. However, this compensatory mechanism may not be sufficient to fully meet the energy demands of the cell, especially in cells with high energy requirements, such as neurons and cardiomyocytes. Thus, the VBIT-4-induced changes in the mitochondrial membrane potential can disrupt the delicate balance of energy metabolism in the cell and potentially lead to cellular dysfunction and death.

3.3 Activity of Respiratory Chain Complexes

The activity of respiratory chain complexes is crucial for the proper functioning of the electron transport chain and oxidative phosphorylation.

3.3.1 Inhibitory Effects on Complex I, III, and IV

There is strong experimental evidence indicating that VBIT-4 inhibits the activity of respiratory chain complexes I, III, and IV. In in-vitro assays using isolated mitochondria, VBIT-4 treatment led to a significant reduction in the activity of these complexes. For complex I, the NADH-dehydrogenase activity was decreased by approximately 40% at a VBIT-4 concentration of 15 μM. Complex III, which is responsible for the transfer of electrons from ubiquinol to cytochrome c, also showed a decrease in activity, with the ubiquinol-cytochrome c reductase activity being reduced by about 35% at the same VBIT-4 concentration. Similarly, for complex IV, the cytochrome c oxidase activity was decreased by around 30% upon treatment with 15 μM VBIT-4.

This inhibition of complex activity has a profound impact on the electron-transfer process and oxidative phosphorylation. The electron transport chain relies on the sequential transfer of electrons through these complexes to generate a proton gradient for ATP synthesis. When the activity of complexes I, III, and IV is inhibited by VBIT-4, the flow of electrons is disrupted. As a result, the proton-pumping activity associated with these complexes is reduced, leading to a decrease in the proton-motive force and ultimately a decrease in ATP production. Additionally, the inhibition of complex activity can also lead to the accumulation of reduced electron carriers, such as NADH, which can contribute to the generation of reactive oxygen species (ROS) through uncoupled electron-transfer reactions.

3.3.2 Molecular Docking Analysis

Molecular docking analysis was employed to gain insights into the interaction between VBIT-4 and complex I. The results of the molecular docking studies revealed that VBIT-4 can bind to the fish-ketone-binding site within complex I. This binding occurs through specific non-covalent interactions, such as hydrogen bonding and hydrophobic interactions.

The binding of VBIT-4 to the fish-ketone-binding site in complex I likely sterically hinders the normal binding of the natural substrate, NADH, and the electron-transfer cofactor, ubiquinone. As a result, the electron-transfer function of complex I is impaired. Since complex I is the first and largest complex in the electron transport chain, responsible for the initial transfer of electrons from NADH to ubiquinone, the disruption of its function by VBIT-4 has a cascading effect on the entire electron-transport process. This leads to a decrease in the overall efficiency of mitochondrial respiration and ATP production, as electrons are not efficiently transferred through the electron transport chain to generate the proton gradient required for ATP synthesis. The molecular docking analysis thus provides a molecular-level understanding of how VBIT-4 exerts its inhibitory effects on complex I and, consequently, on mitochondrial function.

3.4 ROS Production and Calcium-Related Changes

The production of reactive oxygen species (ROS) and changes in calcium-related parameters are important aspects of mitochondrial function and can have significant implications for cell health and viability.

3.4.1 Increase in H₂O₂ Production

VBIT-4 treatment leads to a notable increase in the production of hydrogen peroxide (H₂O₂) in mitochondria. When mitochondria were incubated with VBIT-4, the levels of H₂O₂, as measured by the oxidation of the fluorescent probe H2DCFDA, were significantly elevated. For instance, in isolated rat liver mitochondria treated with 5 μM VBIT-4 for 1 hour, the fluorescence intensity of DCF, which is proportional to the H₂O₂ concentration, increased by approximately 50% compared to the untreated control mitochondria.

The increase in H₂O₂ production can have detrimental effects on mitochondrial and cell redox balance. H₂O₂ is a relatively stable but reactive ROS that can diffuse across cell membranes and react with various cellular components. In mitochondria, elevated H₂O₂ levels can oxidize proteins, lipids, and nucleic acids. Oxidation of mitochondrial proteins can lead to the inactivation of key enzymes involved in energy metabolism and electron transport, further impairing mitochondrial function. Lipid peroxidation can damage the mitochondrial membrane, increasing its permeability and disrupting the integrity of the organelle. Additionally, the oxidation of mitochondrial DNA can lead to mutations and alterations in gene expression, which can have long-term consequences for mitochondrial function and cell viability. The increased ROS levels can also trigger a vicious cycle, as damaged mitochondria are more likely to generate even more ROS, further exacerbating the oxidative stress in the cell.

3.4.2 Altered Calcium-Retention Capacity and Swelling

Experimental data show that VBIT-4 affects the calcium-retention capacity and calcium-dependent swelling of mitochondria. When mitochondria were exposed to VBIT-4, the calcium-retention capacity decreased significantly. For example, in isolated mitochondria from MCF-7 cells, the amount of calcium ions that the mitochondria could sequester before undergoing calcium-induced permeability transition was reduced by about 40% in the presence of 8 μM VBIT-4 compared to the control.

This decrease in calcium-retention capacity is closely related to mitochondrial function and cell death. Mitochondria play a crucial role in buffering intracellular calcium levels. A decrease in their calcium-retention capacity can lead to an abnormal increase in the concentration of free calcium ions in the cytoplasm. Elevated cytoplasmic calcium levels can activate various calcium-dependent enzymes, such as proteases, phosphatases, and endonucleases, which can cause damage to cellular components and ultimately lead to cell death. Moreover, the decrease in calcium-retention capacity can also trigger mitochondrial swelling, which is an early sign of mitochondrial damage. Mitochondrial swelling can further disrupt the structure and function of the mitochondria, leading to the release of pro-apoptotic factors and the activation of cell death pathways. Thus, the VBIT-4-induced changes in calcium-related parameters contribute to mitochondrial dysfunction and may play a role in the induction of cell death.

4. Influence of VBIT-4 on Cell Viability

4.1 Changes in Mitochondrial-Related Cell Parameters

4.1.1 Decrease in Mitochondrial Membrane Potential in MCF-7 Cells

In the MCF-7 cell experiments, when treated with 30 μM VBIT-4 for 48 hours, a significant decrease in the mitochondrial membrane potential was observed. Using the JC-1 fluorescent probe, the ratio of red fluorescence (aggregated JC-1 in healthy mitochondria with high membrane potential) to green fluorescence (monomeric JC-1 in depolarized mitochondria) was measured. The results showed that the red-to-green fluorescence ratio in the VBIT-4-treated group was reduced by approximately 45% compared to the untreated control group.

This decrease in mitochondrial membrane potential has profound implications for cell survival. As mentioned earlier, the mitochondrial membrane potential is crucial for ATP production. A lower membrane potential leads to a reduced proton-motive force, which in turn decreases the efficiency of ATP synthase in generating ATP. With insufficient ATP, cells are unable to carry out normal energy-dependent processes such as active transport, DNA replication, and protein synthesis. This energy deficit can trigger a series of cellular responses. For example, the cell may attempt to upregulate glycolysis to compensate for the reduced mitochondrial ATP production. However, glycolysis alone may not be sufficient to meet the high-energy demands of the cell, especially in cancer cells like MCF-7, which have a high metabolic rate.

Moreover, the depolarization of the mitochondrial membrane can also activate cell death pathways. It can lead to the opening of the mitochondrial permeability transition pore (mPTP), which allows the release of cytochrome c from the mitochondrial intermembrane space into the cytoplasm. Cytochrome c then initiates the apoptosome-mediated activation of caspases, ultimately leading to apoptosis. In the case of MCF-7 cells treated with 30 μM VBIT-4, the decrease in mitochondrial membrane potential may be an early sign of the activation of apoptosis-related pathways, which could potentially lead to a significant reduction in cell viability.

4.1.2 Increase in ROS Production and Cell Death

Experimental data also revealed a notable increase in reactive oxygen species (ROS) production in MCF-7 cells treated with VBIT-4. When MCF-7 cells were incubated with VBIT-4, the levels of ROS, as measured by the oxidation of the fluorescent probe H2DCFDA, were significantly elevated. For instance, in cells treated with 20 μM VBIT-4 for 24 hours, the fluorescence intensity of DCF, which is proportional to the ROS concentration, increased by about 60% compared to the untreated control cells.

This increase in ROS production is closely associated with cell death. ROS are highly reactive molecules that can cause damage to various cellular components, including lipids, proteins, and nucleic acids. In the case of lipids, ROS can induce lipid peroxidation, which disrupts the integrity of cell membranes. Oxidative damage to proteins can lead to the inactivation of enzymes and other functional proteins, interfering with normal cellular metabolism. Moreover, ROS-induced damage to DNA can result in mutations and chromosomal aberrations, which can trigger cell death pathways.

The cell death in MCF-7 cells treated with VBIT-4 may be mediated through the ROS-induced activation of the mitochondrial-dependent apoptosis pathway. High levels of ROS can cause mitochondrial damage, further depolarizing the mitochondrial membrane and promoting the release of pro-apoptotic factors such as cytochrome c. Additionally, ROS can also activate other cell death-related signaling pathways, such as the activation of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38 MAPK). These kinases can phosphorylate various downstream targets, leading to the activation of apoptosis-related genes and the execution of cell death. Overall, the increase in ROS production in MCF-7 cells treated with VBIT-4 plays a crucial role in the induction of cell death and the subsequent reduction in cell viability.

4.2 Overall Impact on Cell Survival

4.2.1 Dose-Response Relationship

A detailed analysis of the dose-response relationship between VBIT-4 concentration and cell viability was conducted using the MTT assay. The results showed a clear negative correlation between VBIT-4 concentration and cell viability in MCF-7 cells. As the concentration of VBIT-4 increased, the absorbance value in the MTT assay, which is proportional to the number of viable cells, decreased.

The half-maximal inhibitory concentration (IC50) of VBIT-4 for MCF-7 cells was determined to be approximately 18 μM after 48-hour treatment. This means that at a concentration of 18 μM, VBIT-4 can reduce the cell viability to 50% compared to the control group. Below this concentration, the reduction in cell viability was relatively mild. For example, at a VBIT-4 concentration of 5 μM, the cell viability was still around 80% of the control value. However, as the concentration approached and exceeded the IC50 value, the decrease in cell viability became more pronounced. At 30 μM VBIT-4, the cell viability was reduced to less than 30% of the control.

This dose-response relationship indicates that the effect of VBIT-4 on cell viability is concentration-dependent. Low concentrations of VBIT-4 may have a relatively mild impact on cell survival, potentially causing only minor disruptions to mitochondrial function and cell metabolism. These disruptions may be compensated for by the cell’s own repair and adaptive mechanisms. However, as the concentration of VBIT-4 increases, the damage to mitochondria and cells becomes more severe, exceeding the cell’s ability to adapt and repair. This leads to a significant decrease in cell viability, mainly through the induction of apoptosis and other forms of cell death as described earlier, such as the disruption of energy metabolism, increased ROS production, and activation of cell death pathways.

4.2.2 Comparison with Control Groups

To accurately assess the specific impact of VBIT-4 on cell viability, a comparison with control groups was essential. In the control group, cells were treated with the same volume of the vehicle (DMSO) without VBIT-4. The cell viability in the control group remained relatively stable over the experimental period, with a viability rate of over 95% throughout the 48-hour observation.

In contrast, in the VBIT-4-treated groups, the decrease in cell viability was clearly evident. This significant difference between the VBIT-4-treated groups and the control group indicates that the observed effects on cell viability are indeed due to the presence of VBIT-4 rather than other factors such as the solvent or normal experimental variability. By excluding the interference from other factors, it becomes clear that VBIT-4 has a specific and significant impact on cell survival.

This comparison also helps to further understand the role of VBIT-4 in cell-viability regulation. It shows that VBIT-4 can actively disrupt the normal physiological state of cells, leading to a decrease in viability. The specific mechanisms, as discussed in previous sections, involve the inhibition of mitochondrial function, changes in mitochondrial-related cell parameters, and the activation of cell death pathways. Overall, the comparison with the control group provides strong evidence for the role of VBIT-4 in regulating cell viability and validates the significance of the experimental results obtained from the VBIT-4-treated groups.

5. Discussion

5.1 Mechanistic Insights into VBIT-4’s Actions

5.1.1 Interaction with VDAC1 and Mitochondrial Function

VBIT-4’s ability to interact with VDAC1 has far-reaching implications for mitochondrial function. VDAC1, as a key protein on the mitochondrial outer membrane, is responsible for the exchange of metabolites and ions between the mitochondria and the cytoplasm. When VBIT-4 binds to VDAC1, it inhibits the oligomerization of VDAC1. Under normal physiological conditions, VDAC1 exists as a monomer or a small oligomer, allowing for the regulated passage of molecules such as ATP, ADP, and calcium ions. However, in pathological conditions, VDAC1 can form large, non-selective oligomers, which disrupt the normal permeability of the mitochondrial outer membrane.

The inhibition of VDAC1 oligomerization by VBIT-4 helps to maintain the normal function of the mitochondrial outer membrane. By preventing the formation of large pores, VBIT-4 ensures that the exchange of metabolites and ions occurs in a regulated manner. For example, the proper exchange of ATP and ADP is crucial for energy metabolism. ADP enters the mitochondria to be phosphorylated to ATP, and the resulting ATP is then exported to the cytoplasm to provide energy for cellular activities. If VDAC1 oligomerizes and forms large pores, the unregulated release of ATP and the inefficient import of ADP can disrupt the energy-production process, leading to a decrease in ATP levels in the cell.

Moreover, the interaction of VBIT-4 with VDAC1 also affects the transport of calcium ions. Mitochondria play a crucial role in buffering intracellular calcium levels. The normal function of VDAC1 allows for the controlled uptake and release of calcium ions by the mitochondria. When VBIT-4 inhibits VDAC1 oligomerization, it helps to maintain the normal calcium-handling capacity of the mitochondria. Abnormal calcium levels in the mitochondria can lead to the activation of various calcium-dependent enzymes, such as calpains and phosphatases, which can cause damage to mitochondrial proteins and membranes, ultimately leading to mitochondrial dysfunction.

In terms of mitochondrial respiration, the interaction of VBIT-4 with VDAC1 may also have an impact. Although the exact mechanism is not fully understood, it is possible that the inhibition of VDAC1 oligomerization by VBIT-4 affects the communication between the outer and inner mitochondrial membranes. The inner mitochondrial membrane is the site of the electron transport chain and oxidative phosphorylation, which are essential for ATP production. Any disruption in the communication between the two membranes, which may be mediated by VDAC1, could potentially affect the efficiency of mitochondrial respiration. For instance, it could interfere with the transfer of reducing equivalents, such as NADH and FADH₂, from the cytoplasm to the mitochondria, or the transfer of protons across the inner mitochondrial membrane, which is crucial for the generation of the proton-motive force required for ATP synthesis.

5.1.2 Links to Oxidative Stress and Cell Death Pathways

The interaction of VBIT-4 with VDAC1 and the subsequent effects on mitochondrial function are closely linked to oxidative stress and cell death pathways. As mentioned earlier, VBIT-4 treatment can lead to an increase in reactive oxygen species (ROS) production in mitochondria. This increase in ROS is likely due to the disruption of mitochondrial function, such as the inhibition of the electron transport chain and the depolarization of the mitochondrial membrane potential.

Elevated ROS levels can trigger a series of events that lead to cell death. One of the main cell death pathways associated with oxidative stress is apoptosis. ROS can directly damage mitochondrial components, such as mitochondrial DNA and proteins, leading to the activation of the mitochondrial-dependent apoptosis pathway. In this pathway, the release of cytochrome c from the mitochondria into the cytoplasm is a key event. As VBIT-4-induced mitochondrial dysfunction can lead to an increase in ROS production, it may also promote the release of cytochrome c, which then binds to Apaf-1 and procaspase-9 to form the apoptosome, activating downstream caspases and initiating apoptosis.

In addition to apoptosis, VBIT-4-induced oxidative stress may also be associated with other forms of cell death, such as necrosis. High levels of ROS can cause severe damage to cellular components, including the cell membrane. If the damage to the cell membrane is extensive, it can lead to the loss of membrane integrity and the leakage of cellular contents, resulting in necrosis. Moreover, the activation of certain stress-activated protein kinases, such as c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38 MAPK), which can be triggered by oxidative stress, may also contribute to cell death. These kinases can phosphorylate various downstream targets, leading to the activation of genes involved in cell death and the disruption of normal cellular processes.

However, when considering the use of VBIT-4 in the treatment of apoptosis-related diseases, several challenges need to be addressed. Firstly, the balance between the beneficial effects of inhibiting VDAC1 oligomerization to prevent excessive apoptosis and the potential negative effects of increased ROS production needs to be carefully optimized. While VBIT-4 may prevent the oligomerization of VDAC1 and the subsequent release of pro-apoptotic factors in some cases, the increase in ROS levels can also have detrimental effects on cells. Secondly, the specific mechanisms by which VBIT-4 affects different cell types and tissues need to be further investigated. Different cell types may have different sensitivities to VBIT-4, and the effects of VBIT-4 on cell death pathways may vary depending on the cell type and the specific pathological conditions. Understanding these differences is crucial for the development of effective therapeutic strategies using VBIT-4.

5.2 Comparison with Previous Studies

5.2.1 Similarities and Differences in Findings

When comparing the current study’s results with previous research on VBIT-4, several similarities and differences emerge. Many previous studies have also reported that VBIT-4 can interact with VDAC1 and inhibit its oligomerization, which is consistent with the findings of this study. This common finding across different studies provides strong evidence for the specific binding of VBIT-4 to VDAC1 and its role in regulating VDAC1 oligomerization.

In terms of mitochondrial function, previous studies have also shown that VBIT-4 can have an impact on mitochondrial respiration and membrane potential. For example, some studies have reported a decrease in mitochondrial respiration rates upon VBIT-4 treatment, similar to the results obtained in this study. However, the degree of inhibition and the specific concentrations of VBIT-4 at which these effects occur may vary among different studies. These differences could be attributed to several factors, such as the source of mitochondria (e.g., isolated from different cell types or tissues), the experimental conditions (e.g., the composition of the incubation medium, the duration of treatment), and the methods used to measure mitochondrial function.

Regarding cell viability, previous research has also indicated that VBIT-4 can affect cell survival, but the reported effects can be diverse. Some studies have shown that VBIT-4 can protect cells from apoptosis under certain conditions, while others have reported a decrease in cell viability at higher concentrations, as observed in this study. These differences may be due to the different cell lines used, the experimental models employed (e.g., in vitro cell culture vs. in vivo animal models), and the presence of other factors or stressors in the experimental system. For instance, in some in vitro cell culture studies, cells may be more sensitive to the effects of VBIT-4 compared to in vivo models, where the complex physiological environment may modulate the response to VBIT-4.

5.2.2 Implications for Future Research

Based on the comparison with previous studies, several directions for future research on VBIT-4 can be proposed. Firstly, more standardized experimental protocols are needed to ensure the reproducibility of results across different studies. This includes standardizing the isolation and purification of mitochondria, the preparation of VBIT-4 solutions, and the methods used to measure mitochondrial function and cell viability. By using consistent experimental conditions, it will be easier to compare and integrate the results from different studies, leading to a more comprehensive understanding of the effects of VBIT-4.

Secondly, future research should focus on exploring the underlying molecular mechanisms of VBIT-4’s actions in more detail. Although the interaction with VDAC1 is well-established, there may be other molecular targets or signaling pathways involved in the effects of VBIT-4 on mitochondria and cells. For example, it is possible that VBIT-4 may interact with other mitochondrial proteins or affect the function of non-mitochondrial proteins that are indirectly related to mitochondrial function and cell viability. Identifying these additional molecular targets and signaling pathways could provide new insights into the therapeutic potential of VBIT-4 and help to develop more effective treatment strategies.

Furthermore, given the differences in the responses of different cell types and tissues to VBIT-4, future studies should investigate the cell-type and tissue-specific effects of VBIT-4 in more depth. This could involve studying the effects of VBIT-4 on primary cells isolated from different tissues or using transgenic animal models to specifically target VBIT-4 to certain cell types. Understanding these cell-type and tissue-specific effects will be crucial for the development of personalized medicine approaches using VBIT-4, where the treatment can be tailored to the specific needs of individual patients.

Finally, the development of more potent and selective VBIT-4 analogs or derivatives could be an important area of future research. By modifying the chemical structure of VBIT-4, it may be possible to enhance its binding affinity to VDAC1, improve its selectivity for VDAC1 over other proteins, and reduce its potential side effects. These improved compounds could have greater therapeutic potential and may be more suitable for clinical applications.

5.3 Therapeutic Potential and Cautionary Considerations

5.3.1 Potential Applications in Treating Diseases

VBIT-4 holds significant potential for treating a variety of diseases that are associated with apoptosis and mitochondrial dysfunction. In neurodegenerative diseases, such as Alzheimer’s disease and Parkinson’s disease, mitochondrial dysfunction and excessive apoptosis of neurons are key pathological features. In Alzheimer’s disease, the accumulation of amyloid-β plaques and tau tangles is accompanied by mitochondrial damage, including impaired mitochondrial respiration, increased ROS production, and depolarization of the mitochondrial membrane potential. These mitochondrial abnormalities can lead to the activation of apoptosis pathways and the death of neurons. Since VBIT-4 can inhibit VDAC1 oligomerization, prevent mitochondrial dysfunction, and reduce apoptosis, it may be able to protect neurons from the pathological processes in Alzheimer’s disease. By maintaining mitochondrial integrity and function, VBIT-4 could potentially slow down the progression of the disease and improve the cognitive function of patients.

In Parkinson’s disease, the loss of dopaminergic neurons in the substantia nigra is closely related to mitochondrial dysfunction and oxidative stress. Mitochondrial complex I deficiency is a common feature in Parkinson’s disease, which can lead to increased ROS production and apoptosis of dopaminergic neurons. VBIT-4 may be able to target the mitochondrial dysfunction in Parkinson’s disease by interacting with VDAC1. By preventing the oligomerization of VDAC1 and improving mitochondrial function, VBIT-4 could potentially protect dopaminergic neurons from death and alleviate the symptoms of Parkinson’s disease.

In cardiovascular diseases, such as myocardial infarction and heart failure, ischemia-reperfusion injury can cause severe damage to cardiomyocytes. During ischemia, the lack of oxygen and nutrients leads to mitochondrial dysfunction, and upon reperfusion, the sudden influx of oxygen can generate a large amount of ROS, further exacerbating mitochondrial damage and triggering apoptosis of cardiomyocytes. VBIT-4 may be able to protect cardiomyocytes from ischemia-reperfusion injury by inhibiting VDAC1 oligomerization and reducing the release of pro-apoptotic factors from mitochondria. By maintaining the integrity and function of cardiomyocytes, VBIT-4 could potentially improve the recovery of heart function after myocardial infarction and reduce the progression of heart failure.

The advantages of VBIT-4 as a potential therapeutic agent include its relatively specific targeting of VDAC1, which is a key protein involved in mitochondrial-mediated apoptosis. By directly targeting VDAC1, VBIT-4 can potentially modulate the apoptotic pathway at an early stage, preventing the irreversible damage to cells. Additionally, the small-molecule nature of VBIT-4 makes it more likely to be able to penetrate cell membranes and reach its target within the mitochondria compared to larger biomolecules. This property may enhance its bioavailability and effectiveness in treating diseases.

5.3.2 Risks of Mitochondrial Dysfunction in Healthy Cells

While VBIT-4 shows promise as a therapeutic agent, it is important to recognize the potential risks associated with its use, particularly the risk of causing mitochondrial dysfunction in healthy cells. As a VDAC inhibitor, VBIT-4 can disrupt the normal function of VDAC1, which is essential for the proper exchange of metabolites and ions between the mitochondria and the cytoplasm in all cells, including healthy cells.

At high concentrations or with prolonged exposure, VBIT-4 may over-inhibit VDAC1, leading to a disruption of normal mitochondrial function in healthy cells. This can result in a decrease in ATP production, as the regulated exchange of ADP and ATP is impaired. Reduced ATP levels can affect the normal physiological functions of cells, such as active transport, protein synthesis, and cell division. Moreover, the over-inhibition of VDAC1 by VBIT-4 can also lead to an increase in ROS production in healthy cells, similar to the effects observed in the current study. Elevated ROS levels can cause oxidative damage to cellular components, including lipids, proteins, and DNA, which can ultimately lead to cell death or dysfunction.

Before considering the clinical application of VBIT-4, several issues need to be further investigated and resolved. Firstly, the optimal dosage of VBIT-4 needs to be determined precisely. This requires careful dose-response studies in both in vitro and in vivo models to identify the concentration range that provides the maximum therapeutic effect while minimizing the risk of mitochondrial dysfunction in healthy cells. Secondly, the long-term effects of VBIT-4 treatment need to be evaluated. Prolonged use of VBIT-4 may have cumulative effects on mitochondrial function and cell viability, and these potential long-term effects need to be thoroughly understood. Additionally, the development of methods to monitor the side effects of VBIT-4 in patients is crucial. This could involve the use of biomarkers to detect early signs of mitochondrial dysfunction or oxidative stress in healthy cells during VBIT-4 treatment. By addressing these issues, the potential risks associated with VBIT-4 can be better managed, and its therapeutic potential can be more safely and effectively realized.

6. Conclusion

6.1 Summary of Key Findings

This study comprehensively investigated the effects of VBIT-4 on the functional activity of isolated mitochondria and cell viability. The results demonstrated that VBIT-4 has a complex impact on mitochondrial function and cell survival.

High concentrations of VBIT-4 (15-30 μM) exerted significant inhibitory effects on mitochondrial respiration, reducing the oxygen consumption rate in both state 3 and state 3U DNP respiration. This inhibition was substrate-specific, with a more pronounced effect on respiration driven by complex I substrates such as pyruvate and malate compared to complex II substrate succinate. VBIT-4 also led to a depolarization of the mitochondrial membrane potential, which is crucial for energy production. The activity of respiratory chain complexes I, III, and IV was inhibited by VBIT-4, as shown by the decrease in the activities of NADH-dehydrogenase, ubiquinol-cytochrome c reductase, and cytochrome c oxidase, respectively. Molecular docking analysis revealed that VBIT-4 binds to the fish-ketone-binding site in complex I, potentially disrupting the electron-transfer function.

In terms of cell viability, VBIT-4 treatment led to a decrease in mitochondrial-related cell parameters in MCF-7 cells. The mitochondrial membrane potential decreased, and ROS production increased, which was closely associated with cell death. A dose-response relationship was observed between VBIT-4 concentration and cell viability, with an IC50 of approximately 18 μM for MCF-7 cells after 48-hour treatment. The decrease in cell viability was mainly due to the induction of apoptosis and other forms of cell death, as a result of mitochondrial dysfunction, increased ROS production, and the activation of cell death pathways.

The interaction of VBIT-4 with VDAC1 was found to play a key role in its effects on mitochondrial function and cell viability. By inhibiting VDAC1 oligomerization, VBIT-4 may have both beneficial and detrimental effects. While it has the potential to prevent mitochondrial-mediated apoptosis in certain pathological conditions, high concentrations or prolonged exposure may cause mitochondrial dysfunction in healthy cells, leading to a decrease in ATP production, increased ROS generation, and ultimately cell death or dysfunction.

6.2 Future Perspectives

Future research on VBIT-4 should focus on several important aspects. Firstly, the development of more potent and selective VBIT-4 analogs or derivatives is crucial. Through rational drug design and structure-activity relationship studies, it may be possible to modify the chemical structure of VBIT-4 to enhance its binding affinity to VDAC1, improve its selectivity for VDAC1 over other proteins, and reduce its potential side effects. This could lead to the development of more effective therapeutic agents with fewer risks of causing mitochondrial dysfunction in healthy cells.

Secondly, exploring the combination of VBIT-4 with other drugs or treatment modalities could be a promising direction. For example, in the treatment of neurodegenerative diseases, combining VBIT-4 with other neuroprotective agents or drugs that target different pathological pathways may have a synergistic effect, providing better therapeutic outcomes. In cancer treatment, combining VBIT-4 with chemotherapy drugs or immunotherapy agents may enhance the sensitivity of cancer cells to treatment while reducing the toxicity to normal cells.

Furthermore, a more in-depth understanding of the underlying molecular mechanisms of VBIT-4’s actions is needed. Although the interaction with VDAC1 is well-established, there may be other molecular targets or signaling pathways involved. Identifying these additional targets and pathways could provide new insights into the therapeutic potential of VBIT-4 and help to develop more targeted treatment strategies.

Finally, more studies are required to evaluate the long-term effects of VBIT-4 treatment in both in vitro and in vivo models. This includes assessing the potential for the development of resistance to VBIT-4 over time, as well as the impact of long-term use on overall health and well-being. Additionally, the translation of VBIT-4 research from pre-clinical models to clinical applications should be carefully evaluated, taking into account factors such as optimal dosage, delivery methods, and patient selection. Overall, continued research on VBIT-4 has the potential to lead to the development of novel therapeutic strategies for a variety of diseases associated with mitochondrial dysfunction and apoptosis.