Citrate transport in the human prostate epithelial PNT2-C2 cell line: electrophysiological analyses

Evaluation of the Links between Lamb Feed Efficiency and Rumen and Plasma Metabolomic Data

Feed efficiency is one of the keystones that could help make animal production less costly and more environmentally friendly. Residual feed intake (RFI) is a widely used criterion to measure feed efficiency by regressing intake on the main energy sinks. We investigated rumen and plasma metabolomic data on Romane male lambs that had been genetically selected for either feed efficiency (line rfi−) or inefficiency (line rfi+). These investigations were conducted both during the growth phase under a 100% concentrate diet and later on under a mixed diet to identify differential metabolite expression and to link it to biological phenomena that could explain differences in feed efficiency. Nuclear magnetic resonance (NMR) data were analyzed using partial least squares discriminant analysis (PLS-DA), and correlations between metabolites’ relative concentrations were estimated to identify relationships between them. High levels of plasma citrate and malate were associated with genetically efficient animals, while high levels of amino acids such as L-threonine, L-serine, and L-leucine as well as beta-hydroxyisovalerate were associated with genetically inefficient animals under both diets. The two divergent lines could not be discriminated using rumen metabolites. Based on phenotypic residual feed intake (RFI), efficient and inefficient animals were discriminated using plasma metabolites determined under a 100% concentrate diet, but no discrimination was observed with plasma metabolites under a mixed diet or with rumen metabolites regardless of diet. Plasma amino acids, citrate, and malate were the most discriminant metabolites, suggesting that protein turnover and the mitochondrial production of energy could be the main phenomena that differ between efficient and inefficient Romane lambs.

Citric Acid as a Potential Prostate Cancer Biomarker Determined in Various Biological Samples

Despite numerous studies, the molecular mechanism of prostate cancer development is still unknown. Recent investigations indicated that citric acid and lipids—with a special emphasis on fatty acids, steroids and hormones (ex. prolactin)—play a significant role in prostate cancer development and progression. However, citric acid is assumed to be a potential biomarker of prostate cancer, due to which, the diagnosis at an early stage of the disease could be possible. For this reason, the main goal of this study is to determine the citric acid concentration in three different matrices. To the best of our knowledge, this is the first time for citric acid to be determined in three different matrices (tissue, urine and blood). Samples were collected from patients diagnosed with prostate cancer and from a selected control group (individuals with benign prostatic hyperplasia). The analyses were performed using the rapid fluorometric test. The obtained results were correlated with both the histopathological data (the Gleason scale as well as the Classification of Malignant Tumors (pTNM) staging scale) and the biochemical data (the values of the following factors: prostate specific antigen, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, triglyceride, total cholesterol, creatinine and prolactin) using chemometric methods. For tissue samples, the results indicated a decreased level of citric acid in the case of prostate cancer. The analyte average concentrations in serum and urine appeared to be corresponding and superior in the positive cohort. This trend was statistically significant in the case of urinary citric acid. Moreover, a significant negative correlation was demonstrated between the concentration of citric acid and the tumor stage. A negative correlation between the total cholesterol and high-density lipoprotein and prolactin was particularly prominent in cancer cases. Conversely, a negative association between low-density lipoprotein and prolactin levels was observed solely in the control group. On the basis of the results, one may assume the influence of hormones, particularly prolactin, on the development of prostate cancer. The present research allowed us to verify the possibility of using citric acid as a potential biomarker for prostate cancer.

Extracellular Citrate Affects Critical Elements of Cancer Cell Metabolism and Supports Cancer Development In Vivo

Glycolysis and fatty acid synthesis are highly active in cancer cells through cytosolic citrate metabolism, with intracellular citrate primarily derived from either glucose or glutamine via the tricarboxylic acid cycle. We show here that extracellular citrate is supplied to cancer cells through a plasma membrane-specific variant of the mitochondrial citrate transporter (pmCiC). Metabolomic analysis revealed that citrate uptake broadly affected cancer cell metabolism through citrate-dependent metabolic pathways. Treatment with gluconate specifically blocked pmCiC and decreased tumor growth in murine xenografts of human pancreatic cancer. This treatment altered metabolism within tumors, including fatty acid metabolism. High expression of pmCiC was associated with invasion and advanced tumor stage across many human cancers. These findings support the exploration of extracellular citrate transport as a novel potential target for cancer therapy.Significance: Uptake of extracellular citrate through pmCiC can be blocked with gluconate to reduce tumor growth and to alter metabolic characteristics of tumor tissue. Cancer Res; 78(10); 2513-23. ©2018 AACR.

Molecular features and physiological roles of K+-Cl- cotransporter 4 (KCC4)

A K⁺-Cl^- cotransport system was documented for the first time during the mid-seventies in sheep and goat red blood cells. It was then described as a Na⁺-independent and ouabain-insensitive ion carrier that could be stimulated by cell swelling and N-ethylmaleimide (NEM), a thiol-reacting agent. Twenty years later, this system was found to be dispensed by four different isoforms in animal cells. The first one was identified in the expressed sequence tag (EST) database by Gillen et al. based on the assumption that it would be homologous to the Na⁺-dependent K⁺-Cl^- cotransport system for which the molecular identity had already been uncovered. Not long after, the three other isoforms were once again identified in the EST databank. Among those, KCC4 has generated much interest a few years ago when it was shown to sustain distal renal acidification and hearing development in mouse. As will be seen in this review, many additional roles were ascribed to this isoform, in keeping with its wide distribution in animal species. However, some of them have still not been confirmed through animal models of gene inactivation or overexpression. Along the same line, considerable knowledge has been acquired on the mechanisms by which KCC4 is regulated and the environmental cues to which it is sensitive. Yet, it is inferred to some extent from historical views and extrapolations.

Development of a LC-MS/MS Method for the Simultaneous Detection of Tricarboxylic Acid Cycle Intermediates in a Range of Biological Matrices

It is now well-established that perturbations in the tricarboxylic acid (TCA) cycle play an important role in the metabolic transformation occurring in cancer including that of the prostate. A method for simultaneous qualitative and quantitative analysis of TCA cycle intermediates in body fluids, tissues, and cultured cell lines of human origin was developed using a common C18 reversed-phase column by LC-MS/MS technique. This LC-MS/MS method for profiling TCA cycle intermediates offers significant advantages including simple and fast preparation of a wide range of human biological samples. The analytical method was validated according to the guideline of the Royal Society of Chemistry Analytical Methods Committee. The limits of detection were below 60 nM for most of the TCA intermediates with the exception of lactic and fumaric acids. The calibration curves of all TCA analytes showed linearity with correlation coefficients r² > 0.9998. Recoveries were >95% for all TCA analytes. This method was established taking into consideration problems and limitations of existing techniques. We envisage that its application to different biological matrices will facilitate deeper understanding of the metabolic changes in the TCA cycle from in vitro, ex vivo, and in vivo studies.

Voltage-gated sodium channels and cancer: is excitability their primary role?

Voltage-gated sodium channels (Na_V) are molecular characteristics of excitable cells. Their activation, triggered by membrane depolarization, generates transient sodium currents that initiate action potentials in neurons and muscle cells. Sodium currents were discovered by Hodgkin and Huxley using the voltage clamp technique and reported in their landmark series of papers in 1952. It was only in the 1980's that sodium channel proteins from excitable membranes were molecularly characterized by Catterall and his collaborators. Non-excitable cells can also express Na_V channels in physiological conditions as well as in pathological conditions. These Na_V channels can sustain biological roles that are not related to the generation of action potentials. Interestingly, it is likely that the abnormal expression of Na_V in pathological tissues can reflect the re-expression of a fetal phenotype. This is especially true in epithelial cancer cells for which these channels have been identified and sodium currents recorded, while it was not the case for cells from the cognate normal tissues. In cancers, the functional activity of Na_V appeared to be involved in regulating the proliferative, migrative, and invasive properties of cells. This review is aimed at addressing the non-excitable roles of Na_V channels with a specific emphasis in the regulation of cancer cell biology.

Tumor microenvironment promotes dicarboxylic acid carrier-mediated transport of succinate to fuel prostate cancer mitochondria

Prostate cancer cells reprogram their metabolism, so that they support their elevated oxidative phosphorylation and promote a cancer friendly microenvironment. This work aimed to explore the mechanisms that cancer cells employ for fueling themselves with energy rich metabolites available in interstitial fluids. The mitochondria oxidative phosphorylation in metastatic prostate cancer DU145 cells and normal prostate epithelial PrEC cells were studied by high-resolution respirometry. An important finding was that prostate cancer cells at acidic pH 6.8 are capable of consuming exogenous succinate, while physiological pH 7.4 was not favorable for this process. Using specific inhibitors, it was demonstrated that succinate is transported in cancer cells by the mechanism of plasma membrane Na(+)-dependent dycarboxylic acid transporter NaDC3 (SLC13A3 gene). Although the level of expression of SLC13A3 was not significantly altered when maintaining cells in the medium with lower pH, the respirometric activity of cells under acidic condition was elevated in the presence of succinate. In contrast, normal prostate cells while expressing NaDC3 mRNA do not produce NaDC3 protein. The mechanism of succinate influx via NaDC3 in metastatic prostate cancer cells could yield a novel target for anti-cancer therapy and has the potential to be used for imaging-based diagnostics to detect non-glycolytic tumors.

Molecular origin of plasma membrane citrate transporter in human prostate epithelial cells

The main function of the prostate gland is to produce and release large amounts of citrate into the prostatic fluid in order to sustain sperm motility and vitality. Mycielska and colleagues have now cloned the citrate transporter responsible for citrate release from prostatic cells. Interestingly, they find that it is an isoform of the transporter that is expressed in the mitochondrial membrane.

The prostate is a highly specialized mammalian organ that produces and releases large amounts of citrate. However, the citrate release mechanism is not known. Here, we present the results of molecular cloning of a citrate transporter from human normal prostate epithelial PNT2-C2 cells shown previously to express such a mechanism. By using rapid amplification of cDNA ends PCR, we determined that the prostatic carrier is an isoform of the mitochondrial transporter SLC25A1 with a different first exon. We confirmed the functionality of the clone by expressing it in human embryonic kidney cells and performing radiotracer experiments and whole-cell patch-clamp recordings. By using short interfering RNAs targeting different parts of the sequence, we confirmed that the cloned protein is the main prostatic transporter responsible for citrate release. We also produced a specific antibody and localized the cloned transporter protein to the plasma membrane of the cells. By using the same antibody, we have shown that the cloned transporter is expressed in non-malignant human tissues.

Citrate enhances in vitro metastatic behaviours of PC-3M human prostate cancer cells: Status of endogenous citrate and dependence on aconitase and fatty acid synthase

Prostate is a unique organ that produces and releases large amounts of citrate. This is reduced significantly in cancer and it is possible that citrate is (re)taken up and used as a metabolite to enhance cellular activity. The main purpose of this study was to determine how cytosolic citrate might affect in vitro metastatic cell behaviours (lateral motility, endocytosis and adhesion). Normal (PNT2-C2) and metastatic (PC-3M) human prostate cancer cells were used in a comparative approach. As regards intermediary metabolic enzymes, aconitase and fatty acid synthase, already implicated in prostate cancer, were evaluated. The level of intracellular citrate was significantly higher in PNT2-C2 cells under both control conditions and following preincubation in extracellular citrate. Supply of exogenous citrate enhanced endocytosis, lateral motility, decreased cell adhesion of PC-3M cells but failed to produce any effect on normal cells. Real-time PCR measurements showed that the mRNA levels of mitochondrial and cytosolic aconitases and fatty acid synthase were significantly higher in PC-3M cells. Correspondingly, aconitase activity was also higher in PC-3M cells. Using cerulenin (an inhibitor of fatty acid synthase), oxalomalate and fluorocitrate (inhibiting aconitases), we investigated the dependence of citrate-induced down-regulation of cellular adhesion on aconitase and fatty acid synthase activities. It was concluded: (1) that strongly metastatic PC-3M cells stored less/utilised more cytosolic citrate than the normal PNT2-C2 cells and (2) that cancer cells could metabolise cytoplasmic citrate via aconitase and fatty acid synthase to enhance their metastatic behaviour.

Endogenous and exogenous citrate transport and release in prostatic preparations: semi-polarized two-dimensional cultures of human PNT2-C2 cells and isolated tubules and segments of rat prostate

BACKGROUND

Electrophysiological characterization of normal human prostate epithelial cells showed exogenous trivalent citrate transport (release) to be K(+)-dependent.

METHODS

(1) Ussing chamber recordings of short circuit current (SCC) were used to study citrate transport in the same (PNT2-C2) cell line grown on micro-pore filters as a monolayer. (2) Release of endogenous citrate from confluent cultures and tubules and segments of rat prostate was measured using a fluorescence technique. (3) Enzyme-spectrophotometry was employed to detect citrate release from segments of rat prostate.

RESULTS

Citrate transport across the PNT2-C2 monolayer was asymmetrical, consistent with release into the lumen-side. Fluorescence and/or enzyme-spectrophotometric measurements showed that time-dependent citrate release (endogeneous and preabsorbed) occurred from rat prostate (tubules and segments), but not kidney or lung. The release was dependent on extracellular K(+) but not Na(+).

CONCLUSIONS

Citrate release from prostatic cells and tissues (rat and human) was K(+)-dependent, consistent with the previous electrophysiological data.

Expression of Na+-dependent citrate transport in a strongly metastatic human prostate cancer PC-3M cell line: regulation by voltage-gated Na+ channel activity

Prostate is a unique organ which synthesizes and releases large amounts of citrate. It has been shown that in metastatic prostate cancer, the amount of citrate in prostatic fluid is significantly reduced, approaching the level normally found in blood. In our previous study, we characterized electrophysiologically the mechanism of citrate transport in a normal prostatic epithelial (PNT2-C2) cell line. It was concluded that the cells expressed a novel transporter carrying 1 citrate3- together with 4 K+, primarily out of cells. In the present study, we aimed similarly to characterize the mechanism(s) of citrate transport in a strongly metastatic human prostate cancer (PC-3M) cell line and to compare this with the previous data. Citrate transport in PC-3M cells was found to be both Na+ and K+ dependent. Intracellular application of citrate produced an outward current that was primarily K+ dependent whilst extracellular citrate elicited an inward current that was mainly Na+ dependent. The electrophysiological and pharmacological characteristics of the citrate outward current were similar to the K+-dependent citrate transporter found in the PNT2-C2 cells. On the other hand, the inward citrate current had a markedly different reversal potential, ionic characteristics, inhibitor profile and pH sensitivity. Preincubation of the PC-3M cells (24 or 48 h) with the voltage-gated Na+ channel (VGSC) blocker tetrodotoxin (TTX) significantly reduced the Na+ sensitivity of the citrate current, up-regulated VGSC mRNA expression but did not change the partial permeability of the membrane to Na+. It was concluded (a) that PC-3M cells express a K+-dependent transporter (carrying citrate outward), similar to that found in normal prostate epithelial cells, as well as (b) a Na+-dependent transporter (carrying citrate inward). The molecular nature of the latter was investigated by RT-PCR; the three known Na+-dependent citrate/dicarboxylate transporters could not be detected. VGSC activity, which itself has been associated with metastatic prostate cancer, had a differential effect on the two citrate transporters, down-regulating the expression of the Na+-dependent component whilst enhancing the K+-dependent citrate transporter.