Organic cation/carnitine transporter, OCTN2, transcriptional activity is regulated by osmotic stress in epididymal cells

Clinical Significance of Carnitine in the Treatment of Cancer: From Traffic to the Regulation

Metabolic reprogramming is a common hallmark of cancer cells. Cancer cells exhibit metabolic flexibility to maintain high proliferation and survival rates. In other words, adaptation of cellular demand is essential for tumorigenesis, since a diverse supply of nutrients is required to accommodate tumor growth and progression. Diversity of carbon substrates fueling cancer cells indicate metabolic heterogeneity, even in tumors sharing the same clinical diagnosis. In addition to the alteration of glucose and amino acid metabolism in cancer cells, there is evidence that cancer cells can alter lipid metabolism. Some tumors rely on fatty acid oxidation (FAO) as the primary energy source; hence, cancer cells overexpress the enzymes involved in FAO. Carnitine is an essential cofactor in the lipid metabolic pathways. It is crucial in facilitating the transport of long-chain fatty acids into the mitochondria for β-oxidation. This role and others played by carnitine, especially its antioxidant function in cellular processes, emphasize the fine regulation of carnitine traffic within tissues and subcellular compartments. The biological activity of carnitine is orchestrated by specific membrane transporters that mediate the transfer of carnitine and its derivatives across the cell membrane. The concerted function of carnitine transporters creates a collaborative network that is relevant to metabolic reprogramming in cancer cells. Here, the molecular mechanisms relevant to the role and expression of carnitine transporters are discussed, providing insights into cancer treatment.

PGC-1α and MEF2 Regulate the Transcription of the Carnitine Transporter OCTN2 Gene in C2C12 Cells and in Mouse Skeletal Muscle

OCTN2 (SLC22A5) is a carnitine transporter whose main function is the active transport of carnitine into cells. In skeletal muscle and other organs, the regulation of the SLC22A5 gene transcription has been shown to depend on the nuclear transcription factor PPAR-α. Due to the observation that the muscle OCTN2 mRNA level is maintained in PPAR-α knock-out mice and that PGC-1α overexpression in C2C12 myoblasts increases OCTN2 mRNA expression, we suspected additional regulatory pathways for SLC22A5 gene transcription. Indeed, we detected several binding sites of the myocyte-enhancing factor MEF2 in the upstream region of the SLC22A5 gene, and MEF2C/MEF2D stimulated the activity of the OCTN2 promoter in gene reporter assays. This stimulation was increased by PGC-1α and was blunted for a SLC22A5 promoter fragment with a mutated MEF2 binding site. Further, we demonstrated the specific binding of MEF2 to the SLC22A5 gene promoter, and a supershift of the MEF2/DNA complex in electrophoretic mobility shift assays. In immunoprecipitation experiments, we could demonstrate the interaction between PGC-1α and MEF2. In addition, SB203580, a specific inhibitor of p38 MAPK, blocked and interferon-γ stimulated the transcriptional activity of the SLC22A5 gene promoter. Finally, mice with muscle-specific overexpression of OCTN2 showed an increase in OCTN2 mRNA and protein expression in skeletal muscle. In conclusion, we detected and characterized a second stimulatory pathway of SLC22A5 gene transcription in skeletal muscle, which involves the nuclear transcription factor MEF2 and co-stimulation by PGC-1α and which is controlled by the p38 MAPK signaling cascade.

Exogenous Melatonin Ameliorates the Negative Effect of Osmotic Stress in Human and Bovine Ovarian Stromal Cells

Ovarian tissue cryopreservation transplantation (OTCT) is the most flexible option to preserve fertility in women and children with cancer. However, OTCT is associated with follicle loss and an accompanying short lifespan of the grafts. Cryopreservation-induced damage could be due to cryoprotective agent (CPA) toxicity and osmotic shock. Therefore, one way to avoid this damage is to maintain the cell volume within osmotic tolerance limits (OTLs). Here, we aimed to determine, for the first time, the OTLs of ovarian stromal cells (OSCs) and their relationship with reactive oxygen species (ROS) and mitochondrial respiratory chain activity (MRCA) of OSCs. We evaluated the effect of an optimal dose of melatonin on OTLs, viability, MRCA, ROS and total antioxidant capacity (TAC) of both human and bovine OSCs in plated and suspended cells. The OTLs of OSCs were between 200 and 375 mOsm/kg in bovine and between 150 and 500 mOsm/kg in human. Melatonin expands OTLs of OSCs. Furthermore, melatonin significantly reduced ROS and improved TAC, MRCA and viability. Due to the narrow osmotic window of OSCs, it is important to optimize the current protocols of OTCT to maintain enough alive stromal cells, which are necessary for follicle development and graft longevity. The addition of melatonin is a promising strategy for improved cryopreservation media.

Sperm preparedness and adaptation to osmotic and pH stressors relate to functional competence of sperm in Bos taurus

The adaptive ability of sperm in the female reproductive tract micromilieu signifies the successful fertilization process. The study aimed to analyze the preparedness of sperm to the prevailing osmotic and pH stressors in the female reproductive tract. Fresh bovine sperm were incubated in 290 (isosmotic-control), 355 (hyperosmotic-uterus and oviduct), and 420 (hyperosmotic-control) mOsm/kg and each with pH of 6.8 (uterus) and 7.4 (oviduct). During incubation, the changes in sperm functional attributes were studied. Sperm kinematics and head area decreased significantly (p < 0.05) immediately upon exposure to hyperosmotic stress at both pH. Proportion of sperm capacitated (%) in 355 mOsm/kg at 1 and 2 h of incubation were significantly (p < 0.05) higher than those in 290 mOsm media. The magnitude and duration of recovery of sperm progressive motility in 355 mOsm with pH 7.4 was correlated with the ejaculate rejection rate (R² = 0.7). Using this information, the bulls were divided into good (n = 5) and poor (n = 5) osmo-adapters. The osmo-responsive genes such as NFAT5, HSP90AB1, SLC9C1, ADAM1B and GAPDH were upregulated (p < 0.05) in the sperm of good osmo-adapters. The study suggests that sperm are prepared for the osmotic and pH challenges in the female reproductive tract and the osmoadaptive ability is associated with ejaculate quality in bulls.

Microenvironment of the male and female reproductive tracts regulate sperm fertility: Impact of viscosity, pH, and osmolality

BACKGROUND

Terminally differentiated mammalian sperm are exposed to gradients of viscosity, pH, and osmolality both in the male and female reproductive tract during their perilous journey to quest the ovum. The complex physicochemical factors play an integral role in preparing sperm for the fertilization process.

OBJECTIVES

To elucidate the influence of the reproductive tract microenvironment especially viscosity, pH, and osmolality in regulating sperm functional and fertilization competence.

MATERIALS AND METHODS

The data used in this review were collected from the research papers and online databases focusing on the influence of viscosity, pH, and osmolality on sperm function.

DISCUSSION

The gradients of viscosity, pH, and osmolality exist across various segments of the male and female reproductive tract. The changes in the viscosity create a physical barrier, pH aid in capacitation and hyperactivation, and the osmotic stress selects a progressive sperm subpopulation for accomplishing fertilization. The sperm function tests are developed based on the concept that the male genotype is the major contributor to the reproductive outcome. However, recent studies demonstrate the significance of sperm genotype-environment interactions that are essentially contributing to reproductive success. Hence, it is imperative to assess the impact of physicochemical stresses and the adaptive ability of the terminally differentiated sperm, which in turn would improve the outcome of the assisted reproductive technologies and male fertility assessment.

CONCLUSION

Elucidating the influence of the reproductive tract microenvironment on sperm function provides newer insights into the procedures that need to be adopted for selecting fertile males for breeding, and ejaculates for the assisted reproductive technologies.

Carnitine Traffic in Cells. Link With Cancer

Metabolic flexibility is a peculiar hallmark of cancer cells. A growing number of observations reveal that tumors can utilize a wide range of substrates to sustain cell survival and proliferation. The diversity of carbon sources is indicative of metabolic heterogeneity not only across different types of cancer but also within those sharing a common origin. Apart from the well-assessed alteration in glucose and amino acid metabolisms, there are pieces of evidence that cancer cells display alterations of lipid metabolism as well; indeed, some tumors use fatty acid oxidation (FAO) as the main source of energy and express high levels of FAO enzymes. In this metabolic pathway, the cofactor carnitine is crucial since it serves as a “shuttle-molecule” to allow fatty acid acyl moieties entering the mitochondrial matrix where these molecules are oxidized via the β-oxidation pathway. This role, together with others played by carnitine in cell metabolism, underlies the fine regulation of carnitine traffic among different tissues and, within a cell, among different subcellular compartments. Specific membrane transporters mediate carnitine and carnitine derivatives flux across the cell membranes. Among the SLCs, the plasma membrane transporters OCTN2 (Organic cation transport novel 2 or SLC22A5), CT2 (Carnitine transporter 2 or SLC22A16), MCT9 (Monocarboxylate transporter 9 or SLC16A9) and ATB^{0, +} [Sodium- and chloride-dependent neutral and basic amino acid transporter B(0+) or SLC6A14] together with the mitochondrial membrane transporter CAC (Mitochondrial carnitine/acylcarnitine carrier or SLC25A20) are the most acknowledged to mediate the flux of carnitine. The concerted action of these proteins creates a carnitine network that becomes relevant in the context of cancer metabolic rewiring. Therefore, molecular mechanisms underlying modulation of function and expression of carnitine transporters are dealt with furnishing some perspective for cancer treatment.

Exogenous L-Carnitine Promotes Plant Growth and Cell Division by Mitigating Genotoxic Damage of Salt Stress

L-carnitine is a fundamental ammonium compound responsible for energy metabolism in all living organisms. It is an oxidative stress regulator, especially in bacteria and yeast and lipid metabolism in plants. Besides its metabolic functions, l-carnitine has detoxification and antioxidant roles in the cells. Due to the complex interrelationship of l-carnitine between lipid metabolism and salinity dependent oxidative stress, this study investigates the exogenous l-carnitine (1 mM) function on seed germination, cell division and chromosome behaviour in barley seeds (Hordeum vulgare L. cv. Bulbul-89) under different salt stress concentrations (0, 0.25, 0.30 and 0.35 M). The present work showed that l-carnitine pretreatment could not be successful to stimulate cell division on barley seeds under non-stressed conditions compared to stressed conditions. Depending on increasing salinity without pretreatment with l-carnitine, the mitotic index significantly decreased in barley seeds. Pretreatment of barley seeds with l-carnitine under salt stress conditions was found promising as a plant growth promoter and stimulator of mitosis. In addition, pretreatment of barley seeds with l-carnitine alleviated detrimental effects of salt stress on chromosome structure and it protected cells from the genotoxic effects of salt. This may be caused by the antioxidant and protective action of the l-carnitine. Consequently, this study demonstrated that the exogenous application of 1 mM l-carnitine mitigates the harmful effects of salt stress by increasing mitosis and decreasing DNA damage caused by oxidative stress on barley seedlings.

NHERF3 is necessary for Escherichia coli heat-stable enterotoxin-induced inhibition of NHE3: differences in signaling in mouse small intestine and Caco-2 cells

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of childhood death from diarrhea and the leading cause of Traveler's diarrhea. E. coli heat-stable enterotoxin (ST) is a major virulence factor of ETEC and inhibits the brush border Na/H exchanger NHE3 in producing diarrhea. NHE3 regulation involves multiprotein signaling complexes that form on its COOH terminus. In this study, the hypothesis was tested that ST signals via members of the Na/H exchanger regulatory factor (NHERF) family of scaffolding proteins, NHERF2, which had been previously shown to have a role, and now with concentration on a role for NHERF3. Two models were used: mouse small intestine and Caco-2/BBe cells. In both models, ST rapidly increased intracellular cGMP, inhibited NHE3 activity, and caused a quantitatively similar decrease in apical expression of NHE3. The transport effects were NHERF3 and NHERF2 dependent. Also, mutation of the COOH-terminal amino acids of NHERF3 supported that NHERF3-NHERF2 heterodimerization was likely to account for this dual dependence. The ST increase in cGMP in both models was partially dependent on NHERF3. The intracellular signaling pathways by which ST-cGMP inhibits NHE3 were different in mouse jejunum (activation of cGMP kinase II, cGKII) and Caco-2 cells, which do not express cGKII (elevation of intracellular Ca²⁺ concentration [Ca²⁺]_i). The ST elevation of [Ca²⁺]_i was from intracellular stores and was dependent on NHERF3-NHERF2. This study shows that intracellular signaling in the same diarrheal model in multiple cell types may be different; this has implications for therapeutic strategies, which often assume that models have similar signaling mechanisms.

Physiology of L-carnitine in plants in light of the knowledge in animals and microorganisms

L-carnitine is present in all living kingdoms where it acts in diverse physiological processes. It is involved in lipid metabolism in animals and yeasts, notably as an essential cofactor of fatty acid intracellular trafficking. Its physiological significance is poorly understood in plants, but L-carnitine may be linked to fatty acid metabolism among other roles. Indeed, carnitine transferases activities and acylcarnitines are measured in plant tissues. Current knowledge of fatty acid trafficking in plants rules out acylcarnitines as intermediates of the peroxisomal and mitochondrial fatty acid metabolism, unlike in animals and yeasts. Instead, acylcarnitines could be involved in plastidial exportation of de novo fatty acid, or importation of fatty acids into the ER, for synthesis of specific glycerolipids. L-carnitine also contributes to cellular maintenance though antioxidant and osmolyte properties in animals and microbes. Recent data indicate similar features in plants, together with modulation of signaling pathways. The biosynthesis of L-carnitine in the plant cell shares similar precursors as in the animal and yeast cells. The elucidation of the biosynthesis pathway of L-carnitine, and the identification of the enzymes involved, is today essential to progress further in the comprehension of its biological significance in plants.

NHERF2/NHERF3 protein heterodimerization and macrocomplex formation are required for the inhibition of NHE3 activity by carbachol

NHERF1, NHERF2, and NHERF3 belong to the NHERF (Na(+)/H(+) exchanger regulatory factor) family of PSD-95/Discs-large/ZO-1 (PDZ) scaffolding proteins. Individually, each NHERF protein has been shown to be involved in the regulation of multiple receptors or transporters including Na(+)/H(+) exchanger 3 (NHE3). Although NHERF dimerizations have been reported, results have been inconsistent, and the physiological function of NHERF dimerizations is still unknown. The current study semiquantitatively compared the interaction strength among all possible homodimerizations and heterodimerizations of these three NHERF proteins by pulldown and co-immunoprecipitation assays. Both methods showed that NHERF2 and NHERF3 heterodimerize as the strongest interaction among all NHERF dimerizations. In vivo NHERF2/NHERF3 heterodimerization was confirmed by FRET and FRAP (fluorescence recovery after photobleach). NHERF2/NHERF3 heterodimerization is mediated by PDZ domains of NHERF2 and the C-terminal PDZ domain recognition motif of NHERF3. The NHERF3-4A mutant is defective in heterodimerization with NHERF2 and does not support the inhibition of NHE3 by carbachol. This suggests a role for NHERF2/NHERF3 heterodimerization in the regulation of NHE3 activity. In addition, both PDZ domains of NHERF2 could be simultaneously occupied by NHERF3 and another ligand such as NHE3, α-actinin-4, and PKCα, promoting formation of NHE3 macrocomplexes. This study suggests that NHERF2/NHERF3 heterodimerization mediates the formation of NHE3 macrocomplexes, which are required for the inhibition of NHE3 activity by carbachol.

The SLC22 family with transporters of organic cations, anions and zwitterions

The SLC22 family contains 13 functionally characterized human plasma membrane proteins each with 12 predicted α-helical transmembrane domains. The family comprises organic cation transporters (OCTs), organic zwitterion/cation transporters (OCTNs), and organic anion transporters (OATs). The transporters operate as (1) uniporters which mediate facilitated diffusion (OCTs, OCTNs), (2) anion exchangers (OATs), and (3) Na(+)/zwitterion cotransporters (OCTNs). They participate in small intestinal absorption and hepatic and renal excretion of drugs, xenobiotics and endogenous compounds and perform homeostatic functions in brain and heart. Important endogeneous substrates include monoamine neurotransmitters, l-carnitine, α-ketoglutarate, cAMP, cGMP, prostaglandins, and urate. It has been shown that mutations of the SLC22 genes encoding these transporters cause specific diseases like primary systemic carnitine deficiency and idiopathic renal hypouricemia and are correlated with diseases such as Crohn's disease and gout. Drug-drug interactions at individual transporters may change pharmacokinetics and toxicities of drugs.

NHERF2 protein mobility rate is determined by a unique C-terminal domain that is also necessary for its regulation of NHE3 protein in OK cells

Na(+)/H(+) exchanger regulatory factor (NHERF) proteins are a family of PSD-95/Discs-large/ZO-1 (PDZ)-scaffolding proteins, three of which (NHERFs 1-3) are localized to the brush border in kidney and intestinal epithelial cells. All NHERF proteins are involved in anchoring membrane proteins that contain PDZ recognition motifs to form multiprotein signaling complexes. In contrast to their predicted immobility, NHERF1, NHERF2, and NHERF3 were all shown by fluorescence recovery after photobleaching/confocal microscopy to be surprisingly mobile in the microvilli of the renal proximal tubule OK cell line. Their diffusion coefficients, although different among the three, were all of the same magnitude as that of the transmembrane proteins, suggesting they are all anchored in the microvilli but to different extents. NHERF3 moves faster than NHERF1, and NHERF2 moves the slowest. Several chimeras and mutants of NHERF1 and NHERF2 were made to determine which part of NHERF2 confers the slower mobility rate. Surprisingly, the slower mobility rate of NHERF2 was determined by a unique C-terminal domain, which includes a nonconserved region along with the ezrin, radixin, moesin (ERM) binding domain. Also, this C-terminal domain of NHERF2 determined its greater detergent insolubility and was necessary for the formation of larger multiprotein NHERF2 complexes. In addition, this NHERF2 domain was functionally significant in NHE3 regulation, being necessary for stimulation by lysophosphatidic acid of activity and increased mobility of NHE3, as well as necessary for inhibition of NHE3 activity by calcium ionophore 4-Br-A23187. Thus, multiple functions of NHERF2 require involvement of an additional domain in this protein.

Pharmacological and pathophysiological roles of carnitine/organic cation transporters (OCTNs: SLC22A4, SLC22A5 and Slc22a21)

The carnitine/organic cation transporter (OCTN) family consists of three transporter isoforms, i.e. OCTN1 (SLC22A4) and OCTN2 (SLC22A5) in humans and animals and Octn3 (Slc22a21) in mice. These transporters are physiologically essential to maintain appropriate systemic and tissue concentrations of carnitine by regulating its membrane transport during intestinal absorption, tissue distribution and renal reabsorption. Among them, OCTN2 is a sodium-dependent, high-affinity transporter of carnitine, and a functional defect of OCTN2 due to genetic mutation causes primary systemic carnitine deficiency (SCD). Since carnitine is essential for beta-oxidation of long-chain fatty acids to produce ATP, OCTN2 gene mutation causes a range of symptoms, including cardiomyopathy, skeletal muscle weakness, fatty liver and male infertility. These functional consequences of Octn2 gene mutation can be seen clearly in an animal model, jvs mouse, which exhibits the SCD phenotype. In addition, although the mechanism is not clear, single nucleotide polymorphisms of OCTN1 and OCTN2 genes are associated with increased incidences of rheumatoid arthritis, Crohn's disease and asthma. OCTN1 and OCTN2 accept cationic drugs as substrates and contribute to intestinal and pulmonary absorption, tissue distribution (including to tumour cells), and renal excretion of these drugs. Modulation of the transport activity of OCTN2 by externally administered drugs may cause drug-induced secondary carnitine deficiency. Rodent Octn3 transports carnitine specifically, particularly in male reproductive tissues. Thus, the OCTNs are physiologically, pathologically and pharmacologically important. Detailed characterization of these transporters will greatly improve our understanding of the pathology associated with common diseases caused by functional deficiency of OCTNs.

The effect of carnitine on Arabidopsis development and recovery in salt stress conditions

Carnitine exists in all living organisms where it plays diverse roles. In animals and yeast, it is implicated in lipid metabolism and is also associated with oxidative stress tolerance. In bacteria, it is a major player in osmotic stress tolerance. We investigate the carnitine function in plants and our present work shows that carnitine enhances the development and recovery of Arabidopsis thaliana seedlings subjected to salt stress. Biological data show that exogenous carnitine supplies improve the germination and survival rates of seedlings grown on salt-enriched medium, in a manner comparable to proline. Both compounds are shown to improve seedling survival under oxidative constraint meaning that they may act on salt stress through antioxidant properties. A transcriptome analysis of seedlings treated with exogenous carnitine reveals that it modulates the expression of genes involved in water stress and abscisic acid responses. Analyses of the abscisic acid mutants, aba1-1 and abi1-1, indicate that carnitine and proline may act through a modulation of the ABA pathway.

The blood-testis and blood-epididymis barriers are more than just their tight junctions

The terms blood-testis barrier (BTB) or blood-epididymis barrier (BEB), are often described as Sertoli cell-Sertoli cell tight junctions (TJs) or TJs between the epithelial cells in the epididymis, respectively. However, in reality, the BTB and BEB are much more complex than just the TJ. The focus of this minireview is to remind readers that the complete BTB and BEB are comprised of three components: anatomical, physiological, and immunological. The TJs form the anatomical (physical) barrier that restricts passage of molecules and cells from entering or exiting the lumen. The physiological barrier is comprised of transporters that regulate movement of substances in or out of the lumen, thus creating a microenvironment, which is critical for the proper development and maturation of germ cells. The immunological barrier limits access by the immune system and sequesters the majority of the autoantigenic germ cells. Combined with the overall immune-privilege of the testis, this suppresses detrimental immune responses against the autoantigenic germ cells. These three components on their own do not create a complete functional barrier; instead, it is the interaction between all three components that create a barrier of maximal competence.