3.5.1. Overview of the Regulation of ATP-Mg/Pi Carrier
Phosphate is an abundant molecule that is incorporated into ATP and transferred from ATP to a large number of small biomolecules including nucleotides, sugars and lipids, and also macromolecules such as proteins.
Mitochondrial PiC transports Pi to the mitochondrial matrix to supply the orthophosphate (Pi) required for ADP phosphorylation. PiC exists as two isoforms (PiC A and PiC B) as a result of alternative splicing events that differ in the presence of two separate exons (3A and 3B) [47
The uptake of orthophosphate (Pi) into the mitochondrial matrix is essential for the oxidative phosphorylation of ADP to ATP. The mitochondrial phosphate transporter/carrier (MPT/PiC), which is located in the IMM, catalyzes the phosphate (H2PO4)/proton symport, the phosphate/hydroxyl ion antiport, and the exchange of the mitochondrial matrix with cytosolic phosphate. Isoform A is found abundantly in the heart, skeletal muscle, and diaphragm tissues to match the higher energy demands, whereas isoform B is present in all tissues to provide basic energy requirements.
Pi uptake can also be mediated by other carriers such as ATP-Mg/Pi. The transport properties and mitochondrial targeting of these carriers indicate that they are isoforms of the ATP-Mg/Pi carrier described in whole mitochondria [48
]. ATP-Mg/Pi carrier activity is known to be regulated by Ca2+
in isolated mitochondria and by Ca2+
In addition, ATP is rapidly synthetized from ADP and phosphate (Pi) by ATP synthase. Newly synthetized ATP in the mitochondrial matrix is exchanged for cytosolic ADP by the mitochondrial AAC. This carrier, which is named adenine nucleotide carrier (Ancp or ANC), is a nuclear encoded protein that catalyzes the exchange of ATP4-
(generated in the mitochondria by ATP synthase) with ADP3-
(produced in the cytosol by most energy-consuming reactions). Human AACs are involved in different genetic diseases and play a role in cancerogenesis. The following two specific inhibitors have greatly facilitated biochemical and biophysical studies of the carrier: carboxyatractyloside (CATR), which is known to inhibit respiration, and BKA [49
Mitochondria have ATP-Mg/Pi carriers (APC), which transport ATP, with or without magnesium and ADP in exchange for matrix Pi. This unbalanced exchange can rapidly alter the concentration of adenosine nucleotide in the matrix, which is fundamental for cellular growth and energy metabolism. The ATP-Mg/Pi carrier is saturable and activated by calcium [48
]. APC has the important role of altering the mitochondrial adenine nucleotide pool in order meet the fluctuating energy demands within the cell [8
Defects in human ANC isoforms can arise from transcriptional or translational deregulation or protein inactivation. A shortage of mitochondrial phosphate carriers can cause severe neonatal lactic acidosis, hypertrophic cardiomyopathy, and generalized muscular hypotonia [9
]. Autoantibodies against ANC1 have been described in patients suffering from dilated cardiomyopathy (DCM), a cardiac defect affecting both ventricles and septum [50
]. ANC dysfunction can affect different tissues at various levels, and for this reason the symptoms can vary from one patient to another.
3.5.2. SLC25 Family and Cancer
A direct link between abnormal SLC25 activity and disease has been demonstrated in cases of cancer. Concerning CIC, it has been found to be involved in inflammation and cancer [5
]. SLC25 members were identified as potential biomarkers for various cancers. In this context, SLC25 carriers are potentially exploitable targets for anticancer strategies.
SLC25A1 is overexpressed in most lung cancers relative to normal tissues and in metastatic sites. It has been demonstrated that SLC25A1 plays a key role in the adaptive mechanisms that allow some tumor cells to acquire drug resistance. Studies have shown that SLC25A1 is an essential component of the tumor cell metabolism and have enlightened novel mechanisms and therapeutic perspectives for the treatment of resistant non-small cell lung cancer tumors [51
]. CIC appears as an important determinant of the homeostatic control of tumor mitochondria, through which activity CIC becomes essential to the cancer promoting metabolic program. It has been reported in tumors that high CIC levels preserved a critical threshold of mitochondrial activity and amount that allows adaptation during metabolic and respiration stress. In this context, it appears rational to test the effects CIC inhibitors in human pathogenic conditions such as cancer [52
]. It has been reported that the frequency of p53 mutations was particularly high in lung, ovarian, and breast cancers, paralleling high expression levels of CIC in these tumors. In a therapeutic approach, CIC is an important target for cancer therapy; CIC promotes tumorigenesis while its inhibition reduces tumor growth [53
Concerning SLC25 A10, its major role is to transport the dicarboxylate substrates (malate and succinate) out of the mitochondria in exchange for phosphate, sulfate, and thiosulfate. The substrates of the SLC25A10 carrier are linked to NADPH synthesis and the regulation of cell metabolism, and they are involved in the regulation of redox homeostasis [54
]. Specifically, the SLC25A10 carrier helps to regulate redox homeostasis in order to protect confluent cells against oxidative stress. Interestingly, increased SLC25A10 expression has been seen in a variety of tumor types. A549 cell lines with downregulated SLC25A10 have an altered growth process and a decreased ability to respond to oxidative stress [54
]. Changes in energy metabolism and redox homeostasis are frequently identified in tumor cells, which are able to adjust their metabolism phenotypes to adapt to the microenvironment [55
]. The change in redox homeostasis has been linked to a shift in metabolic energy, going from glycolysis to mitochondrial oxidative phosphorylation [56
Many recent studies have confirmed that the circadian clock plays a key role in the daily metabolism of vital organs. In addition, the internal medium of a cell changes according to a ~24 h cycle that is regulated by a molecular clock [57
]. In mammals, the circadian rhythm is mainly maintained by three linked transcriptional feedback loops that contain transcriptional regulators. One of these regulators is the activator known as circadian locomotor output cycles kaput (CLOCK) [58
]. It has been recently reported that SLC25A10 is a site that responds to CLOCK binding. Rhythmic interactions between CLOCK and SLC25A10 lead to the circadian regulation of SLC25A10 and mitochondrial metabolism [59
]. The nuclear receptors REV-ERBα and REV-ERBβ are involved in the cell-autonomous circadian transcriptional/translational feedback loops and act as transcriptional repressors. Molecules acting on REV-ERBs have been used to elucidate the connections between circadian rhythm and breast cancer such as human epidermal growth factor receptor 2 HER2+ subtype [60
]. The circadian nature of mitochondrial morphology through fusion/fission mechanisms, and its relation to metabolic rhythm and clock regulation have been thoroughly studied in the literature [62
Two MCs for aspartate and glutamate, referred to as AGC1 (SLC25A12 or Aralar1) and AGC2 (SLC25A13 or citrine), have been identified in humans [63
]. The mitochondrial AGCs are important to supply aspartate to the cytosol with potential therapeutic value in carcinoma. In humans there are two AGC isoforms, i.e., AGC1 and AGC2 encoded by SLC25A12 and SLC25A13, respectively. SLC25A12 upregulation in hepatocellular carcinoma (HCC) cell lines is essential to promote HCC cell growth. The SLC25A12 gene, expressed in adult liver, is upregulated in HCC cell lines by epigenetic mechanisms and AGC1 is involved in HCC cell growth and migration by supplying cytosolic aspartate levels for nucleotide biosynthesis. In this field, aspartate has been described as a limiting metabolite for cancer growth [64
Among the other transporters known as MCF were investigated the properties of the following two human UCPs: UCP5 (BMCP1, brain mitochondrial carrier protein 1 encoded by SLC25A14) and UCP6 (KMCP1, kidney mitochondrial carrier protein 1 encoded by SLC25A30). UCP5 and UCP6 transport inorganic anions such as sulfate and thiosulfate. It has been demonstrated that UP5 overexpression lowers the accumulation of ROS in neuronal and neuroblastoma cells [66
Research has found that the gene for the SLC25A43 mitochondrial transporter is commonly deleted in HER2+ breast cancer, as well as in other cancers, and altered SLC25A33 expression influences the proliferation of breast cancer cells [67
]. Finally, it appears that cancer cells that alter mitochondrial function are able to modify energy metabolism to sustain uncontrolled proliferation.
When compared with normal cells, the mitochondria of cancer cells exhibit significantly increased transmembrane potentials and a number of their transporters are altered. These differences have been used as the basis for developing mitochondria-targeting compounds, such as triphenylphosphonium (TPP), which may be preferentially accumulated within the mitochondria of tumor cells [68
]. Doxorubicin, an anticancer drug that intercalates into DNA and inhibits tropoisomerase II [69
], has been conjugated with a selective mitochondria-localizing compound; the new agent enhances its selectivity towards cancer cells, acting on the properties of the mitochondrial transporters.
Inhibiting the expression of a carrier such as SLC25A10 can make it possible to reprogram cell metabolism, compromise cell growth, and increase sensitivity to traditional anticancer drugs. For instance, pharmacologic inhibition using butylmalonate (BMA) in combination with ionizing radiation was able to overcome the increased radioresistance induced by adaptation to chronic-cycling hypoxia [70
]. Pharmacologic inhibition of SLC25A10 has been proven to be a novel therapeutic strategy to counteract increased radioresistance induced by chronic-cycling hypoxia. It is an important topic because there is a high demand in the development of novel therapeutic advances.
On the basis of the evidence of altered expression of SLC25A10, it has been suggested as a novel target for anticancer strategies. Confluent non-small cell lung cancer (NSCLC) cell line A549 with downregulated SLC25A10 has altered growth behavior and decreased ability to respond to oxidative stress, especially in resting cells. This study confirms that SLC25A10 has an important role in regulating redox homeostasis. The inhibition of SLC25A10 expression is a potential strategy to reprogram cell metabolism, compromise cell growth, and increase sensitivity to anticancer drugs such as cisplatin [54
Members of the SLC25 family can be involved either directly or indirectly in physiological and pathological processes. Studies have demonstrated that PiC has a potential role in mitochondria-dependent cell death; the overexpression of this carrier triggers the intrinsic apoptosis pathway [33
Recent results in type A549 cells suggest a link between metabolic alterations and p21 expression with cumulative effects of decreased SLC25A10 expression and metformin treatment. SLC25A10 knockdown and metformin treatment in A549 cells worked together to influence metabolism and mitochondrial ROS production. Interestingly, metformin treatment decreased the expression of the SLC25A10 carrier, suggesting that this drug could inhibit tumor growth when used in cancer treatment [72