Chemosensitization by knockdown of adenine nucleotide translocase-2

Exploring the multifaceted role of adenosine nucleotide translocase 2 in cellular and disease processes: A comprehensive review

Adenosine nucleotide translocases (ANTs) are a family of proteins abundant in the inner mitochondrial membrane, primarily responsible for shuttling ADP and ATP across the mitochondrial membrane. Additionally, ANTs are key players in balancing mitochondrial energy metabolism and regulating cell death. ANT2 isoform, highly expressed in undifferentiated and proliferating cells, is implicated in the development and drug resistance of various tumors. We conduct a detailed analysis of the potential mechanisms by which ANT2 may influence tumorigenesis and drug resistance. Notably, the significance of ANT2 extends beyond oncology, with roles in non-tumor cell processes including blood cell development, gastrointestinal motility, airway hydration, nonalcoholic fatty liver disease, obesity, chronic kidney disease, and myocardial development, making it a promising therapeutic target for multiple pathologies. To better understand the molecular mechanisms of ANT2, this review summarizes the structural properties, expression patterns, and basic functions of the ANT2 protein. In particular, we review and analyze the controversy surrounding ANT2, focusing on its role in transporting ADP/ATP across the inner mitochondrial membrane, its involvement in the composition of the mitochondrial permeability transition pore, and its participation in apoptosis.

Diving into cancer OXPHOS - The application of metabolic control analysis to cell and tissue research

Knowing how the oxidative phosphorylation (OXPHOS) system in cancer cells operates differently from that of normal cells would help find compounds that specifically paralyze the energy metabolism of cancer cells. The first experiments in the study of mitochondrial respiration using the metabolic control analysis (MCA) method were done with isolated liver mitochondria in the early 80s of the last century. Subsequent studies have shown that the regulation of mitochondrial respiration by ADP in isolated mitochondria differs significantly from a model of mitochondria in situ, where the contacts with components in the cytoplasm are largely preserved. The method of selective permeabilization of the outer membrane of the cells allows the application of MCA to evaluate the contribution of different components of the OXPHOS system to its functioning while mitochondria are in a natural state. In this review, we summarize the use of MCA to study OXPHOS in cancer using permeabilized cells and tissues. In addition, we give examples of how this data fits into cancer research with a completely different approach and methodology.

Lipid metabolism as a target for cancer drug resistance: progress and prospects

Cancer is the world's leading cause of human death today, and the treatment process of cancer is highly complex. Chemotherapy and targeted therapy are commonly used in cancer treatment, and the emergence of drug resistance is a significant problem in cancer treatment. Therefore, the mechanism of drug resistance during cancer treatment has become a hot issue in current research. A series of studies have found that lipid metabolism is closely related to cancer drug resistance. This paper details the changes of lipid metabolism in drug resistance and how lipid metabolism affects drug resistance. More importantly, most studies have reported that combination therapy may lead to changes in lipid-related metabolic pathways, which may reverse the development of cancer drug resistance and enhance or rescue the sensitivity to therapeutic drugs. This paper summarizes the progress of drug design targeting lipid metabolism in improving drug resistance, and providing new ideas and strategies for future tumor treatment. Therefore, this paper reviews the issues of combining medications with lipid metabolism and drug resistance.

Genetic analysis of familial predisposition in the pathogenesis of malignant pleural mesothelioma

PURPOSE

Mesothelioma is the primary tumor of the mesothelial cell membrane. The most important etiology is asbestos exposure. The development of malignant mesothelioma in very few of the population exposed to asbestos and its frequent occurrence in some families may be significant in terms of genetic predisposition. Again, the presence of relatives with mesothelioma who did not have asbestos contact strengthens this argument. This disease, which has limited treatment options and has a poor prognosis, revealing a genetic predisposition, if any, may prolong survival with early diagnosis and effective treatment.

METHODS

Based on the genetic predisposition idea, we diagnosed and followed a total of ten individuals of relatives with mesothelioma. DNA was isolated from peripheral blood and whole genome sequencing analysis was done. Common gene mutations in ten individuals were filtered using bioinformatics. After this filter, from the remaining variants, very rare in the population and damaging mutations are selected.

RESULTS

Eight thousand six hundred and twenty-two common variants have been identified in ten individuals with this analysis. In total, 120 variants were found on 37 genes in 15 chromosomes. These genes are PIK3R4, SLC25A5, ITGB6, PLK2, RAD17, HLA-B, HLA-DRB1, HLA-DQB1, GRM, IL20RA, MAP3K7, RIPK2, and MUC16.

CONCLUSION

Our finding, PIK3R4 gene, is directly associated with mesothelioma development. Twelve genes, which are associated with cancer, were detected in literature. Additional studies, which scan first-degree relatives of individual, are needed to find the specific gene region.

Comprehensive Analysis and Validation of Solute Carrier Family 25 (SLC25) and Its Correlation with Immune Infiltration in Pan-Cancer

As the largest gene family functioning in protein transport among human solute carriers, the SLC25 family (mitochondrial carrier family) can participate in development of cancer. However, a comprehensive exploration for the exactly roles of SLC family remains lacking. In the present study, a total of 15 functional SLC25 family genes were retrieved from all current publications. And multidimensional analyses were systematically performed based on the transcriptome and genome data of SLC25 family from a variety of online databases for their expression, immune cell infiltration, and cancer prognosis. Validation by qPCR and immunohistochemistry were further conducted for the expression of partial SLC25 family members in some tumor tissue. We found that the SLC25 family had strong correlation with immune cells, such as macrophages M2, CD8+ T cell, CD4+ T cell memory activated, and memory resting. Among them, SLC25A6 was most correlated with Macrophage M1 in uveal melanoma (r = -0.68, P = 1.9e - 0.5). Expression of mRNA level showed that SLC25A4 was downregulated in stomach adenocarcinoma and colon adenocarcinoma. SLC25A7 was highly expressed in stomach adenocarcinoma and colon adenocarcinoma. SLC25A23 was decreased in colon adenocarcinoma. qPCR and immunohistochemistry validation results were consistent with our bioinformatics prediction. SLC25A8 was associated with the prognosis of cancer. All these findings suggested that the SLC25 family might affects the immune microenvironment of the cancer and then had the potential to be predictive biomarkers for early diagnosis and prognosis as well as novel targets for individualized treatment of cancer.

The mitochondrial adenine nucleotide transporters in myogenesis

Adenine Nucleotide Translocator isoforms (ANTs) exchange ADP/ATP across the inner mitochondrial membrane, are also voltage-activated proton channels and regulate mitophagy and apoptosis. The ANT1 isoform predominates in heart and muscle while ANT2 is systemic. Here, we report the creation of Ant mutant mouse myoblast cell lines with normal Ant1 and Ant2 genes, deficient in either Ant1 or Ant2, and deficient in both the Ant1 and Ant2 genes. These cell lines are immortal under permissive conditions (IFN-γ + serum at 32 °C) permitting expansion but return to normal myoblasts that can be differentiated into myotubes at 37 °C. With this system we were able to complement our Ant1 mutant studies by demonstrating that ANT2 is important for myoblast to myotube differentiation and myotube mitochondrial respiration. ANT2 is also important in the regulation of mitochondrial biogenesis and antioxidant defenses. ANT2 is also associated with increased oxidative stress response and modulation for Ca⁺⁺ sequestration and activation of the mitochondrial permeability transition (mtPTP) pore during cell differentiation.

Suppression of ANT2 by miR-137 Inhibits Prostate Tumorigenesis

Prostate cancer (PCa) is a serious disease that affects men’s health. To date, no effective and long-lasting treatment option for this condition is available in clinical practice. ANT2 is highly expressed in a variety of hormone-related cancers, but its relationship and regulatory mechanism with PCa are unclear. In this study, we found that ANT2 expression was significantly upregulated in PCa tissues relative to control samples. Genetic knockdown of ANT2 effectively inhibited, while overexpression promoted, proliferation, migration, and invasion of PCa cells. In addition, miR-137 expression was reduced in prostate cancer tissues relative to control tissues. We identified a regulatory site for miR-137 in the 3′-UTR of ANT2 mRNA; luciferase reporter assays indicated that ANT2 is a direct target gene for miR-137. Transfecting cells with miR-137 mimics and/or an ANT2-encoding plasmid revealed that ANT2 promotes proliferation, migration, and invasion of PCa, whereas co-expression of miR-137 mimics inhibited these behaviors. These observations suggest that miR-137 mimics inhibit development of PCa by antagonizing expression of ANT2. Furthermore, tumorigenic assays in nude mice showed that miR-137 inhibitors abolished the inhibitory effect of ANT2 knockdown on PCa tumor growth. Collectively, our findings suggest that ANT2, a target gene of miR-137, is intimately involved in development of PCa, providing new evidence for the mechanism underlying pathogenesis of PCa as well as new options for targeted therapy.

MITOCHONDRIAL CHOLESTEROL AND CANCER

Cholesterol is a crucial component of membrane bilayers that determines their physical and functional properties. Cells largely satisfy their need for cholesterol through the novo synthesis from acetyl-CoA and this demand is particularly critical for cancer cells to sustain dysregulated cell proliferation. However, the association between serum or tissue cholesterol levels and cancer development is not well established as epidemiologic data do not consistently support this link. While most preclinical studies focused on the role of total celular cholesterol, the specific contribution of the mitochondrial cholesterol pool to alterations in cancer cell biology has been less explored. Although low compared to other bilayers, the mitochondrial cholesterol content plays an important physiological function in the synthesis of steroid hormones in steroidogenic tissues or bile acids in the liver and controls mitochondrial function. In addition, mitochondrial cholesterol metabolism generates oxysterols, which in turn, regulate multiple pathways, including cholesterol and lipid metabolism as well as cell proliferation. In the present review, we summarize the regulation of mitochondrial cholesterol, including its role in mitochondrial routine performance, cell death and chemotherapy resistance, highlighting its potential contribution to cancer. Of particular relevance is hepatocellular carcinoma, whose incidence in Western countries had tripled in the past decades due to the obesity and type II diabetes epidemic. A better understanding of the role of mitochondrial cholesterol in cancer development may open up novel opportunities for cancer therapy.

Differential Expression of ADP/ATP Carriers as a Biomarker of Metabolic Remodeling and Survival in Kidney Cancers

ADP/ATP carriers (AACs) are mitochondrial transport proteins playing a strategic role in maintaining the respiratory chain activity, fueling the cell with ATP, and also regulating mitochondrial apoptosis. To understand if AACs might represent a new molecular target for cancer treatment, we evaluated AAC expression levels in cancer/normal tissue pairs available on the Tissue Cancer Genome Atlas database (TCGA), observing that AACs are dysregulated in most of the available samples. It was observed that at least two AACs showed a significant differential expression in all the available kidney cancer/normal tissue pairs. Thus, we investigated AAC expression in the corresponding kidney non-cancer (HK2)/cancer (RCC-Shaw and CaKi-1) cell lines, grown in complete medium or serum starvation, for investigating how metabolic alteration induced by different growth conditions might influence AAC expression and resistance to mitochondrial apoptosis initiators, such as "staurosporine" or the AAC highly selective inhibitor "carboxyatractyloside". Our analyses showed that AAC2 and AAC3 transcripts are more expressed than AAC1 in all the investigated kidney cell lines grown in complete medium, whereas serum starvation causes an increase of at least two AAC transcripts in kidney cancer cell lines compared to non-cancer cells. However, the total AAC protein content is decreased in the investigated cancer cell lines, above all in the serum-free medium. The observed decrease in AAC protein content might be responsible for the decrease of OXPHOS activity and for the observed lowered sensitivity to mitochondrial apoptosis induced by staurosporine or carboxyatractyloside. Notably, the cumulative probability of the survival of kidney cancer patients seriously decreases with the decrease of AAC1 expression in KIRC and KIRP tissues making AAC1 a possible new biomarker of metabolic remodeling and survival in kidney cancers.

ANTs and cancer: Emerging pathogenesis, mechanisms, and perspectives

Adenine nucleotide translocases (ANTs) are a class of transporters located in the inner mitochondrial membrane that not only couple processes of cellular productivity and energy expenditure, but are also involved in the composition of the mitochondrial membrane permeability transition pore (mPTP). The function of ANTs has been found to be most closely related to their own conformational changes. Notably, as multifunctional proteins, ANTs play a key role in oncogenesis, which provides building blocks for tumor anabolism, control oxidative phosphorylation and glycolysis homeostasis, and govern cell death. Thus, ANTs constitute promising targets for the development of novel anticancer agents. Here, we review the recent findings regarding ANTs and their important mechanisms in cancer, with a focus on the therapeutic potential of targeting ANTs for cancer therapy.

Adenine Nucleotide Translocase 2 as an Enzyme Related to [18F] FDG Accumulation in Various Cancers

PURPOSE

Although glucose transporter 1 (GLUT1) and hexokinase 2 (HK2) are known as major proteins involved in the molecular mechanisms for accumulating 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) in cancer cells, sometimes, [¹⁸F] FDG accumulation cannot be explained by the expression of these two proteins. We investigated the involvement of adenine nucleotide translocase 2 (ANT2), which catalyzes ADP/ATP exchange at the mitochondrial inner membrane, in [¹⁸F] FDG accumulation.

PROCEDURES

ANT2 expression was evaluated in various cancer cell lines and human cancer tissues (microarrays) using western blot and immunohistochemical (IHC) staining, respectively. The expression levels of ANT2 were compared to [¹⁸F] FDG accumulation and pathologic findings, including differentiation grade. Additionally, we modulated ANT2 expression levels using ANT2 siRNA and an ANT2 expression vector in cancer cells and murine xenografted tumors.

RESULTS

[¹⁸F] FDG accumulation correlated with ANT2 expression in various cancer cell lines; this was not explained by GLUT1 and/or HK2 expression. At both the cell and tissue levels, ANT2 expression was high in less-differentiated or more malignant type of cancers. [¹⁸F] FDG accumulation changed according to the modulation of the ANT2 expression level.

CONCLUSION

In various cancer cells and tissues, the expression levels of ANT2 explained [¹⁸F] FDG accumulation better than those of GLUT1 and HK2. ANT2 can be used as a marker of dedifferentiated pathology and aggressiveness of cancer.

Mitochondrial PAK6 inhibits prostate cancer cell apoptosis via the PAK6-SIRT4-ANT2 complex

Rationale: P21-activated kinase 6 (PAK6) is a member of the class II PAKs family, which is a conserved family of serine/threonine kinases. Although the effects of PAK6 on many malignancies, especially in prostate cancer, have been studied for a long time, the role of PAK6 in mitochondria remains unknown. Methods: The expression of PAK6, SIRT4 and ANT2 in prostate cancer and adjacent non-tumor tissues was detected by immunohistochemistry. Immunofuorescence and immunoelectron microscopy were used to determine the subcellular localization of PAK6. Immunoprecipitation, immunofuorescence and ubiquitination assays were performed to determine how PAK6 regulates SIRT4, how SIRT4 regulates ANT2, and how PAK6 regulates ANT2. Flow cytometry detection and xenograft models were used to evaluate the impact of ANT2 mutant expression on the prostate cancer cell cycle and apoptosis regulation. Results: The present study revealed that the PAK6-SIRT4-ANT2 complex is involved in mitochondrial apoptosis in prostate cancer cells. It was found that PAK6 is mainly located in the mitochondrial inner membrane, in which PAK6 promotes SIRT4 ubiquitin-mediated proteolysis. Furthermore, SIRT4 deprives the ANT2 acetylation at K105 to promote its ubiquitination degradation. Hence, PAK6 adjusts the acetylation level of ANT2 through the PAK6-SIRT4-ANT2 pathway, in order to regulate the stability of ANT2. Meanwhile, PAK6 directly phosphorylates ANT2 atT107 to inhibit the apoptosis of prostate cancer cells. Therefore, the phosphorylation and deacetylation modifications of ANT2 are mutually regulated, leading to tumor growth in vivo. Consistently, these clinical prostate cancer tissue evaluations reveal that PAK6 is positively correlated with ANT2 expression, but negatively correlated with SIRT4. Conclusion: These present findings suggest the pivotal role of the PAK6-SIRT4-ANT2 complex in the apoptosis of prostate cancer. This complex could be a potential biomarker for the treatment and prognosis of prostate cancer.

ANKRD22, a novel tumor microenvironment-induced mitochondrial protein promotes metabolic reprogramming of colorectal cancer cells

Background: The leading cause of poor prognosis in colorectal cancer (CRC) is the presence of colorectal cancer-initiating cells (CCICs). The interplay between the tumor microenvironment (TME) and CRC cells induces reacquisition of initiating cell characteristics, but the underlying mechanisms remain elusive.

Methods: Candidate molecules were screened by global differential cDNA expression profiles of CCICs, which were enriched from patient-derived tumor xenograft models. Luciferase reporters and chromatin immunoprecipitation assays were used to explore the mechanism of TME factors regulating the transcription of ANKRD22. The effects of Ankyrin repeat domain-containing protein 22 (ANKRD22) on energy metabolism were monitored by extracellular flux and ¹³C-based metabolic flux analysis. Mass spectrometry was used to identify the interacting partners of ANKRD22. Morphological changes of CCICs overexpressing ANKRD22 were observed by electron microscopy. The effects of ANKRD22 on mitochondrial lipid metabolism were analyzed by lipidomics.

Results: We identified a novel nucleus-encoded mitochondrial membrane protein, ANKRD22, which was upregulated in CCICs. We found that ANKRD22 was induced by the p38/MAX pathway activated by different TME stimuli. As a key transcription factor, MAX promoted the transcription of ANKRD22. Expression of ANKRD22 promoted glycolysis associated with a decrease in ATP/ADP and an increase in AMP/ATP levels, which were related to its interaction with pyruvate dehydrogenase kinase isoform 1 (PDK1) and multiple subunits of ATP synthase. Further, in CCICs, ANKRD22 cooperated with the lipid transport protein, Extended Synaptotagmin-1 (E-Syt1), to transport excess lipids into mitochondria and reduced the number of mitochondria in an autophagy-independent manner, thus meeting the metabolic requirements of CCICs.

Conclusion: ANKRD22 induced by TME promotes the metabolic reprogramming of CRC cells. Our study has identified ANKRD22/E-Syt1 as a potential target for eradicating CCICs.

Oxidative phosphorylation as a potential therapeutic target for cancer therapy

In contrast to prior belief, cancer cells require oxidative phosphorylation (OXPHOS) to strive, and exacerbated OXPHOS dependency frequently characterizes cancer stem cells, as well as primary or acquired resistance against chemotherapy or tyrosine kinase inhibitors. A growing arsenal of therapeutic agents is being designed to suppress the transfer of mitochondria from stromal to malignant cells, to interfere with mitochondrial biogenesis, to directly inhibit respiratory chain complexes, or to disrupt mitochondrial function in other ways. For the experimental treatment of cancers, OXPHOS inhibitors can be advantageously combined with tyrosine kinase inhibitors, as well as with other strategies to inhibit glycolysis, thereby causing a lethal energy crisis. Unfortunately, most of the preclinical data arguing in favor of OXPHOS inhibition have been obtained in xenograft models, in which human cancer cells are implanted in immunodeficient mice. Future studies on OXPHOS inhibitors should elaborate optimal treatment schedules and combination regimens that stimulate-or at least are compatible with-anticancer immune responses for long-term tumor control.

Mitochondria-Centric Review of Polyphenol Bioactivity in Cancer Models

SIGNIFICANCE

Humans are exposed daily to polyphenols in milligram-to-gram amounts through dietary consumption of fruits and vegetables. Polyphenols are also available as components of dietary supplements for improving general health. Although polyphenols are often advertised as antioxidants to explain health benefits, experimental evidence shows that their beneficial cancer preventing and controlling properties are more likely due to stimulation of pro-oxidant and proapoptotic pathways. Recent Advances: The understanding of the biological differences between cancer and normal cell, and especially the role that mitochondria play in carcinogenesis, has greatly advanced in recent years. These advances have resulted in a wealth of new information on polyphenol bioactivity in cell culture and animal models of cancer. Polyphenols appear to target oxidative phosphorylation and regulation of the mitochondrial membrane potential (MMP), glycolysis, pro-oxidant pathways, and antioxidant (adaptive) stress responses with greater selectivity in tumorigenic cells.

CRITICAL ISSUES

The ability of polyphenols to dissipate the MMP (Δψm) by a protonophore mechanism has been known for more than 50 years. However, researchers focus primarily on the downstream molecular effects of Δψm dissipation and mitochondrial uncoupling. We argue that the physicochemical properties of polyphenols are responsible for their anticancer properties by virtue of their protonophoric and pro-oxidant properties rather than their specific effects on downstream molecular targets.

FUTURE DIRECTIONS

Polyphenol-induced dissipation of Δψm is a physicochemical process that cancer cells cannot develop resistance against by gene mutation. Therefore, polyphenols should receive more attention as agents for cotherapy with cancer drugs to gain synergistic activity. Antioxid. Redox Signal.

Evaluation of cell damage induced by irradiated Zinc-Phthalocyanine-gold dendrimeric nanoparticles in a breast cancer cell line

BACKGROUND

Cancer is a non-communicable disease that occurs following a mutation in the genes which control cell growth. Breast cancer is the most diagnosed cancer among South African women and a major cause of cancer-related deaths worldwide. Photodynamic therapy (PDT) is an alternative cancer therapy that uses photochemotherapeutic agents, known as photosensitizers. Drug-delivery nanoparticles are commonly used in nanomedicine to enhance drug-therapeutic efficiency. This study evaluated the photodynamic effects following treatment with 0.3 μM multiple particles delivery complex (MPDC) and irradiated with a laser fluence of 10 J/cm2 using a 680 nm diode laser in a breast cancer cell line (MCF-7).

METHODS

Cell damage was assessed by inverted light microscopy for cell morphology; the Apoptox-Glo triple assay was used for cell viability, caspase activity and identification of cytodamage markers; flow cytometric analysis for cell death pathways and mitochondrial membrane potential; the enzyme linked immunosorbent assay (ELISA) for cytochrome C release; and real-time reverse transcriptase polymerase chain reaction (RT-PCR) array for gene expression.

RESULTS

Laser activated-MPDC induced a significant change in morphology of PDT-treated cells, with the appearance of apoptotic like morphological features. An increase in cytotoxicity, caspase activity, cell depolarization and cytochrome C release were identified in PDT-treated cells. Finally, the upregulation of BAX, BCL-2, CASP-2 and ULK-1 genes was observed.

CONCLUSION

The MPDC yielded a successful and stable hybrid agent with potent photodynamic abilities.

Human adenine nucleotide translocases physically and functionally interact with respirasomes

A network of interactions for human adenine nucleotide translocases, required for oxidative phosphorylation, is reported. Of particular interest is an evolutionarily conserved and functionally important association with respiratory supercomplexes, which is surprising because the respirasomes of yeast and mammals are different.

Members of the adenine nucleotide translocase (ANT) family exchange ADP for ATP across the mitochondrial inner membrane, an activity that is essential for oxidative phosphorylation (OXPHOS). Mutations in or dysregulation of ANTs is associated with progressive external ophthalmoplegia, cardiomyopathy, nonsyndromic intellectual disability, apoptosis, and the Warburg effect. Binding partners of human ANTs have not been systematically identified. The absence of such information has prevented a detailed molecular understanding of the assorted ANT-associated diseases, including insight into their disparate phenotypic manifestations. To fill this void, in this study, we define the interactomes of two human ANT isoforms. Analogous to its yeast counterpart, human ANTs associate with heterologous partner proteins, including the respiratory supercomplex (RSC) and other solute carriers. The evolutionarily conserved ANT–RSC association is particularly noteworthy because the composition, and thereby organization, of RSCs in yeast and human is different. Surprisingly, absence of the major ANT isoform only modestly impairs OXPHOS in HEK293 cells, indicating that the low levels of other isoforms provide functional redundancy. In contrast, pharmacological inhibition of OXPHOS expression and function inhibits ANT-dependent ADP/ATP exchange. Thus ANTs and the OXPHOS machinery physically interact and functionally cooperate to enhance ANT transport capacity and mitochondrial respiration.

The pleiotropic regulation of cyclin D1 by newly identified sesaminol-binding protein ANT2

The expression of cyclin D1 is upregulated in various cancer cells by diverse mechanisms, such as increases in mRNA levels, the promotion of the translation by mammalian target of rapamycin complex 1 (mTORC1) signaling and the protein stabilization. We here show that sesaminol, a sesame lignan, reduces the expression of cyclin D1 with decreasing mRNA expression levels, inhibiting mTORC1 signaling and promoting proteasomal degradation. We subsequently generated sesaminol-immobilized FG beads to newly identify sesaminol-binding proteins. As a consequence, we found that adenine nucleotide translocase 2 (ANT2), the inner mitochondrial membrane protein, directly bound to sesaminol. Consistent with the effects of sesaminol, the depletion of ANT2 caused a reduction in cyclin D1 with decreases in its mRNA levels, mTORC1 inhibition and the proteasomal degradation of its protein, suggesting that sesaminol negatively regulates the function of ANT2. Furthermore, we screened other ANT2-binding compounds and found that the proliferator-activated receptor-γ agonist troglitazone also reduced cyclin D1 expression in a multifaceted manner, analogous to that of the sesaminol treatment and ANT2 depletion. Therefore, the chemical biology approach using magnetic FG beads employed in the present study revealed that sesaminol bound to ANT2, which may pleiotropically upregulate cyclin D1 expression at the mRNA level and protein level with mTORC1 activation and protein stabilization. These results suggest the potential of ANT2 as a target against cyclin D1-overexpressing cancers.

Expression and putative role of mitochondrial transport proteins in cancer

Cancer cells undergo major changes in energy and biosynthetic metabolism. One of them is the Warburg effect, in which pyruvate is used for fermentation rather for oxidative phosphorylation. Another major one is their increased reliance on glutamine, which helps to replenish the pool of Krebs cycle metabolites used for other purposes, such as amino acid or lipid biosynthesis. Mitochondria are central to these alterations, as the biochemical pathways linking these processes run through these organelles. Two membranes, an outer and inner membrane, surround mitochondria, the latter being impermeable to most organic compounds. Therefore, a large number of transport proteins are needed to link the biochemical pathways of the cytosol and mitochondrial matrix. Since the transport steps are relatively slow, it is expected that many of these transport steps are altered when cells become cancerous. In this review, changes in expression and regulation of these transport proteins are discussed as well as the role of the transported substrates. This article is part of a Special Issue entitled Mitochondria in Cancer, edited by Giuseppe Gasparre, Rodrigue Rossignol and Pierre Sonveaux.

Changes in the expression of the human adenine nucleotide translocase isoforms condition cellular metabolic/proliferative status

Human cells express four mitochondrial adenine nucleotide translocase (hANT) isoforms that are tissue-specific and developmentally regulated. hANT1 is mainly expressed in terminally differentiated muscle cells; hANT2 is growth-regulated and is upregulated in highly glycolytic and proliferative cells; and hANT3 is considered to be ubiquitous and non-specifically regulated. Here, we studied how the expression of hANT isoforms is regulated by proliferation and in response to metabolic stimuli, and examined the metabolic consequences of their silencing and overexpression. In HeLa and HepG2 cells, expression of hANT3 was upregulated by shifting metabolism towards oxidation or by slowed growth associated with contact inhibition or growth-factor deprivation, indicating that hANT3 expression is highly regulated. Under these conditions, changes in hANT2 mRNA expression were not observed in either HeLa or HepG2 cells, whereas in SGBS preadipocytes (which, unlike HeLa and HepG2 cells, are growth-arrest-sensitive cells), hANT2 mRNA levels decreased. Additionally, overexpression of hANT2 promoted cell growth and glycolysis, whereas silencing of hANT3 decreased cellular ATP levels, limited cell growth and induced a stress-like response. Thus, cancer cells require both hANT2 and hANT3, depending on their proliferation status: hANT2 when proliferation rates are high, and hANT3 when proliferation slows.

ATP/ADP Turnover and Import of Glycolytic ATP into Mitochondria in Cancer Cells Is Independent of the Adenine Nucleotide Translocator

Non-proliferating cells oxidize respiratory substrates in mitochondria to generate a protonmotive force (Δp) that drives ATP synthesis. The mitochondrial membrane potential (ΔΨ), a component of Δp, drives release of mitochondrial ATP(4-) in exchange for cytosolic ADP(3-) via the electrogenic adenine nucleotide translocator (ANT) located in the mitochondrial inner membrane, which leads to a high cytosolic ATP/ADP ratio up to >100-fold greater than matrix ATP/ADP. In rat hepatocytes, ANT inhibitors, bongkrekic acid (BA), and carboxyatractyloside (CAT), and the F1FO-ATP synthase inhibitor, oligomycin (OLIG), inhibited ureagenesis-induced respiration. However, in several cancer cell lines, OLIG but not BA and CAT inhibited respiration. In hepatocytes, respiratory inhibition did not collapse ΔΨ until OLIG, BA, or CAT was added. Similarly, in cancer cells OLIG and 2-deoxyglucose, a glycolytic inhibitor, depolarized mitochondria after respiratory inhibition, which showed that mitochondrial hydrolysis of glycolytic ATP maintained ΔΨ in the absence of respiration in all cell types studied. However in cancer cells, BA, CAT, and knockdown of the major ANT isoforms, ANT2 and ANT3, did not collapse ΔΨ after respiratory inhibition. These findings indicated that ANT was not mediating mitochondrial ATP/ADP exchange in cancer cells [corrected]. We propose that suppression of ANT contributes to low cytosolic ATP/ADP, activation of glycolysis, and a Warburg metabolic phenotype in proliferating cells.

Alterations in voltage-sensing of the mitochondrial permeability transition pore in ANT1-deficient cells

The probability of mitochondrial permeability transition (mPT) pore opening is inversely related to the magnitude of the proton electrochemical gradient. The module conferring sensitivity of the pore to this gradient has not been identified. We investigated mPT’s voltage-sensing properties elicited by calcimycin or H₂O₂ in human fibroblasts exhibiting partial or complete lack of ANT1 and in C2C12 myotubes with knocked-down ANT1 expression. mPT onset was assessed by measuring in situ mitochondrial volume using the ‘thinness ratio’ and the ‘cobalt-calcein’ technique. De-energization hastened calcimycin-induced swelling in control and partially-expressing ANT1 fibroblasts, but not in cells lacking ANT1, despite greater losses of mitochondrial membrane potential. Matrix Ca²⁺ levels measured by X-rhod-1 or mitochondrially-targeted ratiometric biosensor 4mtD3cpv, or ADP-ATP exchange rates did not differ among cell types. ANT1-null fibroblasts were also resistant to H₂O₂-induced mitochondrial swelling. Permeabilized C2C12 myotubes with knocked-down ANT1 exhibited higher calcium uptake capacity and voltage-thresholds of mPT opening inferred from cytochrome c release, but intact cells showed no differences in calcimycin-induced onset of mPT, irrespective of energization and ANT1 expression, albeit the number of cells undergoing mPT increased less significantly upon chemically-induced hypoxia than control cells. We conclude that ANT1 confers sensitivity of the pore to the electrochemical gradient.

Modulation of apoptosis and viral latency - an axis to be well understood for successful cure of human immunodeficiency virus

Human immunodeficiency virus (HIV) is the causative agent of the deadly disease AIDS, which is characterized by the progressive decline of CD4(+)T-cells. HIV-1-encoded proteins such as envelope gp120 (glycoprotein gp120), Tat (trans-activator of transcription), Nef (negative regulatory factor), Vpr (viral protein R), Vpu (viral protein unique) and protease are known to be effective in modulating host cell signalling pathways that lead to an alteration in apoptosis of both HIV-infected and uninfected bystander cells. Depending on the stage of the virus life cycle and host cell type, these viral proteins act as mediators of pro- or anti-apoptotic signals. HIV latency in viral reservoirs is a persistent phenomenon that has remained beyond the control of the human immune system. To cure HIV infections completely, it is crucial to reactivate latent HIV from cellular pools and to drive these apoptosis-resistant cells towards death. Several previous studies have reported the role of HIV-encoded proteins in apoptosis modulation, but the molecular basis for apoptosis evasion of some chronically HIV-infected cells and reactivated latently HIV-infected cells still needs to be elucidated. The current review summarizes our present understanding of apoptosis modulation in HIV-infected cells, uninfected bystander cells and latently infected cells, with a focus on highlighting strategies to activate the apoptotic pathway to kill latently infected cells.

Identification of new highly selective inhibitors of the human ADP/ATP carriers by molecular docking and in vitro transport assays

Mitochondrial carriers are proteins that shuttle a variety of metabolites, nucleotides and coenzymes across the inner mitochondrial membrane. The mitochondrial ADP/ATP carriers (AACs) specifically translocate the ATP synthesized within mitochondria to the cytosol in exchange for the cytosolic ADP, playing a key role in energy production, in promoting cell viability and regulating mitochondrial permeability transition pore opening. In Homo sapiens four genes code for AACs with different tissue distribution and expression patterns. Since AACs are dysregulated in several cancer types, the employment of known and new AAC inhibitors might be crucial for inducing mitochondrial-mediated apoptosis in cancer cells. Albeit carboxyatractyloside (CATR) and bongkrekic acid (BKA) are known to be powerful and highly selective AAC inhibitors, able to induce mitochondrial dysfunction at molecular level and poisoning at physiological level, we estimated here for the first time their affinity for the human recombinant AAC2 by in vitro transport assays. We found that the inhibition constants of CATR and BKA are 4 nM and 2.0 μM, respectively. For finding new AAC inhibitors we also performed a docking-based virtual screening of an in-house developed chemical library and we identified about 100 ligands showing high affinity for the AAC2 binding region. By testing 13 commercially available molecules, out of the 100 predicted candidates, we found that 2 of them, namely suramin and chebulinic acid, are competitive AAC2 inhibitors with inhibition constants 0.3 μM and 2.1 μM, respectively. We also demonstrated that chebulinic acid and suramin are "highly selective" AAC2 inhibitors, since they poorly inhibit other human mitochondrial carriers (namely ORC1, APC1 and AGC1).

PsANT, the adenine nucleotide translocase of Puccinia striiformis, promotes cell death and fungal growth

Adenine nucleotide translocase (ANT) is a constitutive mitochondrial component that is involved in ADP/ATP exchange and mitochondrion-mediated apoptosis in yeast and mammals. However, little is known about the function of ANT in pathogenic fungi. In this study, we identified an ANT gene of Puccinia striiformis f. sp. tritici (Pst), designated PsANT. The PsANT protein contains three typical conserved mitochondrion-carrier-protein (mito-carr) domains and shares more than 70% identity with its orthologs from other fungi, suggesting that ANT is conserved in fungi. Immuno-cytochemical localization confirmed the mitochondrial localization of PsANT in normal Pst hyphal cells or collapsed cells. Over-expression of PsANT indicated that PsANT promotes cell death in tobacco, wheat and fission yeast cells. Further study showed that the three mito-carr domains are all needed to induce cell death. qRT-PCR analyses revealed an in-planta induced expression of PsANT during infection. Knockdown of PsANT using a host-induced gene silencing system (HIGS) attenuated the growth and development of virulent Pst at the early infection stage but not enough to alter its pathogenicity. These results provide new insight into the function of PsANT in fungal cell death and growth and might be useful in the search for and design of novel disease control strategies.

ANT2-defective fibroblasts exhibit normal mitochondrial bioenergetics

Adenine nucleotide translocase 2 (ANT2) transports glycolytic ATP across the inner mitochondrial membrane. Patients with ANT2 deletion were recently reported. We aimed at characterizing mitochondrial functions in ANT2-defective fibroblasts. In spite of ANT2 expression in fibroblasts, we observed no difference between ANT2-defective and control fibroblasts for mitochondrial respiration, respiratory chain activities, mitochondrial membrane potential and intracellular ATP levels. This indicates that ANT2 insufficiency does not alter fibroblasts basal mitochondrial bioenergetics.

ATP/ADP ratio, the missed connection between mitochondria and the Warburg effect

Non-proliferating cells generate the bulk of cellular ATP by fully oxidizing respiratory substrates in mitochondria. Respiratory substrates cross the mitochondrial outer membrane through only one channel, the voltage dependent anion channel (VDAC). Once in the matrix, respiratory substrates are oxidized in the tricarboxylic acid cycle to generate mostly NADH that is further oxidized in the respiratory chain to generate a proton motive force comprised mainly of membrane potential (ΔΨ) to synthesize ATP. Mitochondrial ΔΨ then drives the release of ATP(4-) from the matrix in exchange for ADP(3-) in the cytosol via the adenine nucleotide translocator (ANT) located in the mitochondrial inner membrane. Thus, mitochondrial function in non-proliferating cells drives a high cytosolic ATP/ADP ratio, essential to inhibit glycolysis. By contrast, the bioenergetics of the Warburg phenotype of proliferating cells is characterized by enhanced aerobic glycolysis and the suppression of mitochondrial metabolism. Suppressed mitochondrial function leads to lower production of mitochondrial ATP and hence lower cytosolic ATP/ADP ratios that favor enhanced glycolysis. Thus, the cytosolic ATP/ADP ratio is a key feature that determines if cell metabolism is predominantly oxidative or glycolytic. Here, we describe two novel mechanisms to explain the suppression of mitochondrial metabolism in cancer cells: the relative closure of VDAC by free tubulin and the inactivation of ANT. Both mechanisms contribute to low ATP/ADP ratios that activate glycolysis.

The protein disulfide isomerases PDIA4 and PDIA6 mediate resistance to cisplatin-induced cell death in lung adenocarcinoma

Intrinsic and acquired chemoresistance are frequent causes of cancer eradication failure. Thus, long-term cis-diaminedichloroplatine(II) (CDDP) or cisplatin treatment is known to promote tumor cell resistance to apoptosis induction via multiple mechanisms involving gene expression modulation of oncogenes, tumor suppressors and blockade of pro-apoptotic mitochondrial membrane permeabilization. Here, we demonstrate that CDDP-resistant non-small lung cancer cells undergo profound remodeling of their endoplasmic reticulum (ER) proteome (>80 proteins identified by proteomics) and exhibit a dramatic overexpression of two protein disulfide isomerases, PDIA4 and PDIA6, without any alteration in ER-cytosol Ca(2+) fluxes. Using pharmacological and genetic inhibition, we show that inactivation of both proteins directly stimulates CDDP-induced cell death by different cellular signaling pathways. PDIA4 inactivation restores a classical mitochondrial apoptosis pathway, while knockdown of PDIA6 favors a non-canonical cell death pathway sharing some necroptosis features. Overexpression of both proteins has also been found in lung adenocarcinoma patients, suggesting a clinical importance of these proteins in chemoresistance.

Mitochondrial DNA and functional investigations into the radiosensitivity of four mouse strains

We investigated whether genetic radiosensitivity-related changes in mtDNA/nDNA ratios are significant to mitochondrial function and if a material effect on mtDNA content and function exists. BALB/c (radiosensitive), C57BL/6 (radioresistant), and F1 hybrid mouse strains were exposed to total body irradiation. Hepatic genomic DNA was extracted, and mitochondria were isolated. Mitochondrial oxygen consumption, ROS, and calcium-induced mitochondrial swelling were measured. Radiation influenced strain-specific survival in vivo. F1 hybrid survival was influenced by maternal input. Changes in mitochondrial content corresponded to survival in vivo among the 4 strains. Calcium-induced mitochondrial swelling was strain dependent. Isolated mitochondria from BALB/c mice were significantly more sensitive to calcium overload than mitochondria from C57BL/6 mice. Maternal input partially influenced the recovery effect of radiation on calcium-induced mitochondrial swelling in F1 hybrids; the hybrid with a radiosensitive maternal lineage exhibited a lower rate of recovery. Hybrids had a survival rate that was biased toward maternal input. mtDNA content and mitochondrial permeability transition pores (MPTP) measured in these strains before irradiation reflected a dominant input from the parent. After irradiation, the MPTP opened sooner in radiosensitive and hybrid strains, likely triggering intrinsic apoptotic pathways. These findings have important implications for translation into predictors of radiation sensitivity/resistance.

Comparative analysis of some aspects of mitochondrial metabolism in differentiated and undifferentiated neuroblastoma cells

The aim of the present study is to clarify some aspects of the mechanisms of regulation of mitochondrial metabolism in neuroblastoma (NB) cells. Experiments were performed on murine Neuro-2a (N2a) cell line, and the same cells differentiated by all-trans-retinoic acid (dN2a) served as in vitro model of normal neurons. Oxygraphy and Metabolic Control Analysis (MCA) were applied to characterize the function of mitochondrial oxidative phosphorylation (OXPHOS) in NB cells. Flux control coefficients (FCCs) for components of the OXPHOS system were determined using titration studies with specific non-competitive inhibitors in the presence of exogenously added ADP. Respiration rates of undifferentiated Neuro-2a cells (uN2a) and the FCC of Complex-II in these cells were found to be considerably lower than those in dN2a cells. Our results show that NB is not an exclusively glycolytic tumor and could produce a considerable part of ATP via OXPHOS. Two important enzymes - hexokinase-2 and adenylate kinase-2 can play a role in the generation of ATP in NB cells. MCA has shown that in uN2a cells the key sites in the regulation of OXPHOS are complexes I, II and IV, whereas in dN2a cells complexes II and IV. Results obtained for the phosphate and adenine nucleotide carriers showed that in dN2a cells these carriers exerted lower control over the OXPHOS than in undifferentiated cells. The sum of FCCs for both types of NB cells was found to exceed significantly that for normal cells suggesting that in these cells the respiratory chain was somehow reorganized or assembled into large supercomplexes.

Combined RNA interference of adenine nucleotide translocase-2 and ganciclovir therapy in hepatocellular carcinoma

PURPOSE

The purpose of this study was to investigate the anticancer effects of combined RNA interference (RNAi) of the adenine nucleotide translocase-2 (ANT2) gene and ganciclovir (GCV) therapy for treatment of hepatocellular carcinoma cells (Huh 7) in an animal model.

METHODS

The Huh 7/NTG stable cell line was established by transfection of a vector with the human sodium iodide symporter (hNIS), HSV1-sr39 thymidine kinase (tk), and enhanced green florescent protein (EGFP) fusion gene into Huh 7 cells. mRNA expressions of these genes were evaluated by RT-PCR analysis. The functions of hNIS and HSV1-sr39tk were verified with (125)I uptake and (3)H-penciclovir (PCV) uptake tests. EGFP and hNIS expression was confirmed with confocal microscopy after immunocytochemical staining. We treated the tumor cells with ANT2 shRNA or GCV or both ANT2 shRNA and GCV and treated the in vivo mouse model with a Huh 7/NTG tumor xenograft. The therapeutic effects of the in vivo study were assessed with caliper measurements and gamma camera imaging using (99m)Tc-pertechnetate.

RESULTS

Huh 7/NTG cells showed a cell number-dependent increase in (125)I uptake and a 24-fold higher (3)H-PCV uptake compared to parent Huh 7 cells. Huh 7/NTG cells transfected with ANT2 shRNA had lower ANT2 mRNA expression and more impaired proliferation activity than cells transfected with scramble shRNA. Proliferation of Huh 7/NTG cells was also inhibited by GCV treatment. Combined GCV and ANT2 shRNA therapy further inhibited cell proliferation in the in vitro study. The combined therapy with GCV and ANT2 shRNA showed a further decrease in tumor growth in the mouse model.

CONCLUSIONS

Our results suggest that the combined RNA interference with ANT2 and GCV therapy inhibited hepatocellular carcinoma cell proliferation more than single GCV therapy or ANT2 shRNA therapy in vitro and in vivo. Therefore it could be applied treating incurable hepatocellular carcinoma.

Solute carriers (SLCs) in cancer

During tumor progression cells acquire an altered metabolism, either as a cause or as a consequence of an increased need of energy and nutrients. All four major classes of macromolecules are affected: carbohydrates, proteins, lipids and nucleic acids. As a result of the changed needs, solute carriers (SLCs) which are the major transporters of these molecules are differently expressed. This renders them important targets in the treatment of cancer. Blocking or activating SLCs is one possible therapeutic strategy. For example, some SLCs are upregulated in tumor cells due to the increased demand for energy and nutritional needs. Thus, blocking them and turning off the delivery of fuel or nutrients could be one way to interfere with tumor progression. Specific drug delivery to cancer cells via transporters is another approach. Some SLCs are also interesting as chemosensitizing targets because blocking or activating them may result in an altered response to chemotherapy. In this review we summarize the roles of SLCs in cancer therapy and specifically their potential as direct or indirect targets, as drug carriers or as chemosensitizing targets.

The mitochondrial ADP/ATP carrier (SLC25 family): pathological implications of its dysfunction

In aerobic eukaryotic cells, the high energy metabolite ATP is generated mainly within the mitochondria following the process of oxidative phosphorylation. The mitochondrial ATP is exported to the cytoplasm using a specialized transport protein, the ADP/ATP carrier, to provide energy to the cell. Any deficiency or dysfunction of this membrane protein leads to serious consequences on cell metabolism and can cause various diseases such as muscular dystrophy. Described as a decisive player in the programmed cell death, it was recently shown to play a role in cancer. The objective of this review is to summarize the current knowledge of the involvement of the ADP/ATP carrier, encoded by the SLC25A4, SLC25A5, SLC25A6 and SLC25A31 genes, in human diseases and of the efforts made at designing different model systems to study this carrier and the associated pathologies through biochemical, genetic, and structural approaches.

Adenine nucleotide translocase, mitochondrial stress, and degenerative cell death

Mitochondria are intracellular organelles involved in ATP synthesis, apoptosis, calcium signaling, metabolism, and the synthesis of critical metabolic cofactors. Mitochondrial dysfunction is associated with age-related degenerative diseases. How mitochondrial dysfunction causes cell degeneration is not well understood. Recent studies have shown that mutations in the adenine nucleotide translocase (Ant) cause aging-dependent degenerative cell death (DCD) in yeast, which is sequentially manifested by inner membrane stress, mitochondrial DNA (mtDNA) loss, and progressive loss of cell viability. Ant is an abundant protein primarily involved in ADP/ATP exchange across the mitochondrial inner membrane. It also mediates basal proton leak and regulates the mitochondrial permeability transition pore. Missense mutations in the human Ant1 cause several degenerative diseases which are commonly manifested by fractional mtDNA deletions. Multiple models have been proposed to explain the Ant1-induced pathogenesis. Studies from yeast have suggested that in addition to altered nucleotide transport properties, the mutant proteins cause a global stress on the inner membrane. The mutant proteins likely interfere with general mitochondrial biogenesis in a dominant-negative manner, which secondarily destabilizes mtDNA. More recent work revealed that the Ant-induced DCD is suppressed by reduced cytosolic protein synthesis. This finding suggests a proteostatic crosstalk between mitochondria and the cytosol, which may play an important role for cell survival during aging.

Targeting cancer cell mitochondria as a therapeutic approach

Mitochondria are double membrane-enveloped organelles that play a central role in cellular metabolism, calcium homeostasis, redox signaling and cell fates. They function as main generators of ATP, metabolites for the construction of macromolecules and reactive oxygen species. In many cancer cells, mitochondria seem dysfunctional, manifested by a shift of energy metabolism from oxidative phosphorylation to active glycolysis and an increase in reactive oxygen species generation. These metabolic changes are often associated with upregulation of NAD(P)H oxidase. Importantly, the metabolic reprogramming in a cancer cell is mechanistically linked to oncogenic signals. Targeting mitochondria as a cancer therapeutic strategy has attracted much attention in the recent years and multiple review articles in this area have been published. This article attempts to provide an update on recent progress in identification of mitochondria-associated molecules as potential anticancer targets and the respective targeting compounds.

B-cell translocation gene 2: expression in the rat ovary and potential association with adenine nucleotide translocase 2 in mitochondria

The B-cell translocation gene 2 (Btg2) is an anti-proliferative tumor suppressor gene that behaves as a transcriptional regulator. The present study investigated gonadotropin induction of Btg2 in the rat ovary and the mechanism of Btg2 action as a partner of mitochondrial protein adenine nucleotide translocase 2 (Ant2). Transient induction of Btg2 as well as Btg1 mRNA levels by LH/hCG was observed in ovarian granulosa cells. Btg2 protein levels were also stimulated by LH/hCG. LH-induced gene expression of Btg2 required ERK signal pathway. Studies of deletion mutants in HeLa cells showed that deletion of Btg2 C-terminus (Btg2/ΔC) abolished the interaction with Ant2. In fact, the expression levels of Btg2/ΔC construct were decreased in mitochondrial fraction. Btg2 was also expressed in mitochondria and interacted with Ant2 in preovulatory granulosa cells. Interestingly, a Btg2/ΔC construct inhibited an action of Btg2 wild-type on ATP and H(2)O(2) production. These findings demonstrate the gonadotropin stimulation of Btg2 in the ovary and, the physical interaction of Btg2 with Ant2 in mitochondria.

Apigenin sensitizes prostate cancer cells to Apo2L/TRAIL by targeting adenine nucleotide translocase-2

Apo2 ligand (Apo2L)/tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising cancer therapeutic agent. Recombinant human Apo2L/TRAIL has been under clinical trials, whereas various kinds of malignant tumors have resistance to Apo2L/TRAIL. We and others have shown that several anticancer agents and flavonoids overcome resistance to Apo2L/TRAIL by upregulating death receptor 5 (DR5) in malignant tumor cells. However, the mechanisms by which these compounds induce DR5 expression remain unknown. Here we show that the dietary flavonoid apigenin binds and inhibits adenine nucleotide translocase-2 (ANT2), resulting in enhancement of Apo2L/TRAIL-induced apoptosis by upregulation of DR5. Apigenin and genistein, which are major flavonoids, enhanced Apo2L/TRAIL-induced apoptosis in cancer cells. Apigenin induced DR5 expression, but genistein did not. Using our method identifying the direct targets of flavonoids, we compared the binding proteins of apigenin with those of genistein. We discovered that ANT2 was a target of apigenin, but not genistein. Similarly to apigenin, knockdown of ANT2 enhanced Apo2L/TRAIL-induced apoptosis by upregulating DR5 expression at the post-transcriptional level. Moreover, silencing of ANT2 attenuated the enhancement of Apo2L/TRAIL-induced apoptosis by apigenin. These results suggest that apigenin upregulates DR5 and enhances Apo2L/TRAIL-induced apoptosis by binding and inhibiting ANT2. We propose that ANT2 inhibitors may contribute to Apo2L/TRAIL therapy.

Mitochondria and mitophagy: the yin and yang of cell death control

Mitochondria are primarily responsible for providing the contracting cardiac myocyte with a continuous supply of ATP. However, mitochondria can rapidly change into death-promoting organelles. In response to changes in the intracellular environment, mitochondria become producers of excessive reactive oxygen species and release prodeath proteins, resulting in disrupted ATP synthesis and activation of cell death pathways. Interestingly, cells have developed a defense mechanism against aberrant mitochondria that can cause harm to the cell. This mechanism involves selective sequestration and subsequent degradation of the dysfunctional mitochondrion before it causes activation of cell death. Induction of mitochondrial autophagy, or mitophagy, results in selective clearance of damaged mitochondria in cells. In response to stress such as ischemia/reperfusion, prosurvival and prodeath pathways are concomitantly activated in cardiac myocytes. Thus, there is a delicate balance between life and death in the myocytes during stress, and the final outcome depends on the complex cross-talk between these pathways. Mitophagy functions as an early cardioprotective response, favoring adaptation to stress by removing damaged mitochondria. In contrast, increased oxidative stress and apoptotic proteases can inactivate mitophagy, allowing for the execution of cell death. Herein, we discuss the importance of mitochondria and mitophagy in cardiovascular health and disease and provide a review of our current understanding of how these processes are regulated.

ANT2 suppression by shRNA restores miR-636 expression, thereby downregulating Ras and inhibiting tumorigenesis of hepatocellular carcinoma

MicroRNAs (miRNAs) participate in diverse biological functions and carcinogenesis by inhibiting specific gene expression. We previously reported that suppression of adenine nucleotide translocase 2 (ANT2) by using the short hairpin RNA (shRNA) approach has an antitumor effect in several cancer cells. We here examined the influence of ANT2 on expression of miRNAs in hepatocellular carcinoma (HCC) to further elucidate the tumor-suppressive mechanism of ANT2 shRNA. We first carried out screening for miRNAs, whose expression is regulated by ANT2 suppression in the Hep3B HCC cell line using miRNA microarrays. Validation of candidate miRNAs was done by incorporating clinical samples, and their effects on the tumorigenesis of HCC were studied in vitro and in vivo. miR-636 was one of the miRNAs whose expression was highly upregulated by ANT2 suppression in miRNA microarray analysis, as confirmed by real-time reverse transcription-polymerase chain reaction. Notably, miR-636 was markedly downregulated in HCC tissues compared with matched non-neoplastic liver in clinical samples. Restoration of miR-636 in Hep3B cells led to significant reduction of cell proliferation and colony formation. miR-636 restoration resulted in a decreased level of Ras, one of the putative targets of miR-636, and inactivation of its signaling pathway. Moreover, tumorigenesis was efficiently suppressed by miR-636 in an in vivo tumor xenograft model of HCC. The data suggest that miR-636 might function as a tumor suppressor miRNA affecting HCC tumorigenesis via downregulation of Ras, and that ANT2 suppression by shRNA could exert an anticancer effect by restoring miR-636 expression in HCC.

ANT2 suppression by shRNA may be able to exert anticancer effects in HCC further by restoring SOCS1 expression

Suppressor of cytokine signaling 1 (SOCS1) is a negative regulator of Janus kinase and the signal transducer and activation of transcription (Jak-STAT) pathway. SOCS-1 is known to be silenced by aberrant promoter methylation in human hepatocellular carcinoma (HCC) during early tumorigenesis, therefore, a strategy to restore SOCS1 expression can be utilized for cancer therapy. Here, we examined the influence of adenine nucleotide translocase 2 (ANT2) suppression by short-hairpin RNA (shRNA) on SOCS1 expression and its downstream effect in HCC. ANT2 shRNA treatment led to restoration of SOCS1 expression along with its promoter demethylation in Hep3B cells, which was accompanied by decreased DNA methyltransferase 1 (DNMT1) activity through the suppression of Ras/PI3K/Akt signaling. Restoration of SOCS1 by ANT2 knockdown, subsequently, inhibited STAT3 activity and downregulated the expression of miR-21, which has been reported to be an important onco-miR in HCC. Downregulation of miR-21 efficiently suppressed Hep3B cell proliferation in vitro with a comparable level to ANT2 shRNA treatment. ANT2 suppression by shRNA may be able to exert anticancer effects in HCC further by restoring SOCS1 expression.

Adenovirus adenine nucleotide translocator-2 shRNA effectively induces apoptosis and enhances chemosensitivity by the down-regulation of ABCG2 in breast cancer stem-like cells

Cancer stem cells (CSCs) are resistant to chemo- and radio-therapy, and can survive to regenerate new tumors. This is an important reason why various anti-cancer therapies often fail to completely control tumors, although they kill and eliminate the bulk of cancer cells. In this study, we determined whether or not adenine nucleotide translocator-2 (ANT2) suppression could also be effective in inducing cell death of breast cancer stem-like cells. A sub-population (SP; CD44⁺/CD24^-) of breast cancer cells has been reported to have stem/progenitor cell properties. We utilized the adeno-ANT2 shRNA virus to inhibit ANT2 expression and then observed the treatment effect in a SP of breast cancer cell line. In this study, MCF7, MDA-MB-231 cells, and breast epithelial cells (MCF10A) mesenchymally-transdifferentiated through E-cadherin knockdown were used. ANT2 expression was high in both stem-like cells and non-stem-like cells of MCF7 and MDA-MB-231 cells, and was induced and up-regulated by mesenchymal transdifferentiation in MCF10A cells (MCF10A^EMT). Knockdown of ANT2 by adeno-shRNA virus efficiently induced apoptotic cell death in the stem-like cells of MCF7 and MDA-MB-231 cells, and MCF10A^EMT. Stem-like cells of MCF7 and MDA-MB-231, and MCF10A^EMT cells exhibited increased drug (doxorubicin) resistance, and expressed a multi-drug resistant related molecule, ABCG2, at a high level. Adeno-ANT2 shRNA virus markedly sensitized the stem-like cells of MCF7 and MDA-MB-231, and the MCF10A^EMT cells to doxorubicin, which was accompanied by down-regulation of ABCG2. Our results suggest that ANT2 suppression by adeno-shRNA virus is an effective strategy to induce cell death and increase the chemosensitivity of stem-like cells in breast cancer.

Metabolic control analysis of cellular respiration in situ in intraoperational samples of human breast cancer

The aim of this study was to analyze quantitatively cellular respiration in intraoperational tissue samples taken from human breast cancer (BC) patients. We used oxygraphy and the permeabilized cell techniques in combination with Metabolic Control Analysis (MCA) to measure a corresponding flux control coefficient (FCC). The activity of all components of ATP synthasome, and respiratory chain complexes was found to be significantly increased in human BC cells in situ as compared to the adjacent control tissue. FCC(s) were determined upon direct activation of respiration with exogenously-added ADP and by titrating the complexes with their specific inhibitors to stepwise decrease their activity. MCA showed very high sensitivity of all complexes and carriers studied in human BC cells to inhibition as compared to mitochondria in normal oxidative tissues. The sum of FCC(s) for all ATP synthasome and respiratory chain components was found to be around 4, and the value exceeded significantly that for normal tissue (close to 1). In BC cells, the key sites of the regulation of respiration are Complex IV (FCC = 0.74), ATP synthase (FCC = 0.61), and phosphate carrier (FCC = 0.60); these FCC(s) exceed considerably (~10-fold) those for normal oxidative tissues. In human BC cells, the outer mitochondrial membrane is characterized by an increased permeability towards adenine nucleotides, the mean value of the apparent K(m) for ADP being equal to 114.8 ± 13.6 μM. Our data support the two-compartment hypothesis of tumor metabolism, the high sum of FCC(s) showing structural and functional organization of mitochondrial respiratory chain and ATP synthasome as supercomplexes in human BC.

Casein kinase I epsilon interacts with mitochondrial proteins for the growth and survival of human ovarian cancer cells

Epithelial ovarian cancer is the leading cause of death among gynaecologic cancers in Western countries. Our studies have shown that casein kinase I-epsilon (CKIε), a Wnt pathway protein, is significantly overexpressed in ovarian cancer tissues and is associated with poor survival. Ectopic expression of CKIε in normal human ovarian surface epithelial cells and inhibition of CKIε in ovarian cancer cells and in xenografts demonstrated the importance of CKIε in regulating cell proliferation and migration. Interestingly, CKIε function did not seem to involve β-catenin activity. Instead, CKIε was found to interact with several mitochondrial proteins including adenine nucleotide translocase 2 (ANT2). Inhibition of CKIε in ovarian cancer cells resulted in suppression of ANT2, downregulation of cellular ATP and the resulting cancer cells were more susceptible to chemotherapy. Our studies indicate that, in the context of ovarian cancer, the interaction between CKIε and ANT2 mediates pathogenic signalling that is distinct from the canonical Wnt/β-catenin pathway and is essential for cell proliferation and is clinically associated with poor survival.

Increased expression of VDAC1 sensitizes carcinoma cells to apoptosis induced by DNA cross-linking agents

A major clinical problem regarding antitumoral treatment with DNA cross-linking agents such as cisplatin (Cisp), mechlorethamine (HN2) or its derivative melphalan (MLP) is intrinsic or acquired resistance to therapy, which frequently results from a resistance to apoptosis induction. In this study, aimed to identify novel sensitizing targets to DNA cross-linker-induced cell death, we demonstrated that MLP, Cisp and HN2 induce mitochondrial permeability transition pore (PTP)-mediated apoptosis in cervical and colon carcinoma cells. This apoptotic pathway is characterized by dissipation of the mitochondrial membrane potential, production of ROS, mitochondrial translocation of Bax, release of apoptogenic factors, caspase activation and nuclear alterations. The opening of PTP and subsequent apoptosis was reduced in Bax deficient cells and in cells with elevated Bcl-2 level, but not in cells invalidated for Bak. We further showed that, among the pro-apoptotic PTP regulators tested (VDAC1, creatine kinase, ANT1 and ANT3), exogenous overexpression of VDAC1 was the most effective in enhancing Cisp- and MLP-induced apoptosis. In addition, pharmacologically induced up-regulation of VDAC1 by the chemotherapeutic agent arsenic trioxide (As(2)O(3)) greatly sensitized HeLa cells to Cisp and MLP treatment. These data indicate that increased expression of VDAC1 appears as a promising strategy to improve DNA cross-linker-induced chemotherapy.

Mitochondrial metabolism inhibitors for cancer therapy

Cancer cells catabolise nutrients in a different way than healthy cells. Healthy cells mainly rely on oxidative phosphorylation, while cancer cells employ aerobic glycolysis. Glucose is the main nutrient catabolised by healthy cells, while cancer cells often depend on catabolism of both glucose and glutamine. A key organelle involved in this altered metabolism is mitochondria. Mitochondria coordinate the catabolism of glucose and glutamine across the cancer cell. Targeting mitochondrial metabolism in cancer cells has potential for the treatment of this disease. Perhaps the most promising target is the hexokinase-voltage dependent anion channel-adenine nucleotide translocase complex that spans the outer- and inner-mitochondrial membranes. This complex links glycolysis, oxidative phosphorylation and mitochondrial-mediated apoptosis in cancer cells. This review discusses cancer cell mitochondrial metabolism and the small molecule inhibitors of this metabolism that are in pre-clinical or clinical development.

Targeting the mitochondrial permeability transition: cardiac ischemia-reperfusion versus carcinogenesis

Cardiovascular diseases and cancer continue to be major causes of death worldwide, and despite intensive research only modest progress has been reached in reducing the morbidity and mortality of these awful diseases. Mitochondria are broadly accepted as the key organelles that play a crucial role in cell life and death. They provide cells with ATP produced via oxidative phosphorylation under physiological conditions, and initiate cell death through both apoptosis and necrosis in response to severe stress. Oxidative stress accompanied by calcium overload and ATP depletion induces the mitochondrial permeability transition (mPT) with formation of pathological, non-specific mPT pores (mPTP) in the mitochondrial inner membrane. Opening of the mPTP with a high conductance results in matrix swelling ultimately inducing rupture of the mitochondrial outer membrane and releasing pro-apoptotic proteins into the cytoplasm. The ATP level is the determining factor in deciding whether cells die through apoptosis or necrosis. Cardiac cells undergoing ischemia followed by reperfusion (IR) possess exactly the same conditions mentioned above to induce mPTP opening. Due to its critical role in cell death, inhibition of mPTP opening has been accepted as a major therapeutic approach to protect the heart against IR. In contrast to cardiac IR, cancer cells exhibit less sensitivity to pore opening which can be in part explained by increased expression of mPTP compounds/modulators and metabolic remodeling. Since the main goal of chemotherapy is to provoke apoptosis, mPT induction may represent an attractive approach for the development of new cancer therapeutics to induce mitochondria-mediated cell death and prevent cell differentiation in carcinogenesis. This review focuses on the role of the mPTP in cardiac IR and cancer, and pharmacological agents to prevent or initiate mPT-mediated cell death, respectively in these diseases.

Effects of NYGGF4 knockdown on insulin sensitivity and mitochondrial function in 3T3-L1 adipocytes

NYGGF4 is a recently discovered gene that is involved in obesity-associated insulin resistance. It has been suggested that mitochondrial dysfunction might be responsible for the development of insulin resistance induced by NYGGF4 overexpression. In the present study, we aimed to define the impact of down-regulating NYGGF4 expression by RNA interference (RNAi) on the insulin sensitivity and mitochondrial function of 3T3-L1 adipocytes. The results revealed that NYGGF4 knockdown enhanced the glucose uptake of adipocytes, which reconfirmed the regulatory function of NYGGF4 in adipocyte insulin sensitivity. However, an unexpected observation was that knockdown of NYGGF4 reduced intracellular ATP concentration and promoted an increase in mitochondrial transmembrane potential (ΔΨm) and reactive oxygen species (ROS) level without affecting mitochondrial morphology or mtDNA. Therefore, the role of NYGGF4 in mitochondrial function remains unclear, and further animal studies are needed to explore the biological function of this gene.