Constanza Angulo

Hexose transporter expression and function in mammalian spermatozoa: Cellular localization and transport of hexoses and vitamin C

We analyzed the expression of hexose transporters in human testis and in human, rat, and bull spermatozoa and studied the uptake of hexoses and vitamin C in bull spermatozoa. Immunocytochemical and reverse transcription‐polymerase chain reaction analyses demonstrated that adult human testis expressed the hexose transporters GLUT1, GLUT2, GLUT3, GLUT4, and GLUT5. Immunoblotting experiments demonstrated the presence of proteins of about 50–70 kD reactive with anti‐GLUT1, GLUT2, GLUT3, and GLUT5 in membranes prepared from human spermatozoa, but no proteins reactive with GLUT4 antibodies were detected. Immunolocalization experiments confirmed the presence of GLUT1, GLUT2, GLUT3, GLUT5, and low levels of GLUT4 in human, rat, and bull spermatozoa. Each transporter isoform showed a typical subcellular localization in the head and the sperm tail. In the tail, GLUT3 and GLUT5 were present at the level of the middle piece in the three species examined, GLUT1 was present in the principal piece, and the localization of GLUT2 differed according of the species examined. Bull spermatozoa transported deoxyglucose, fructose, and the oxidized form of vitamin C, dehydroascorbic acid. Transport of deoxyglucose and dehydroascorbic acid was inhibited by cytochalasin B, indicating the direct participation of facilitative hexose transporters in the transport of both substrates by bull spermatozoa. Transport of fructose was not affected by cytochalasin B, which is consistent for an important role for GLUT5 in the transport of fructose in these cells. The data show that human, rat, and bull spermatozoa express several hexose transporter isoforms that allow for the efficient uptake of glucose, fructose, and dehydroascorbic acid by these cells. J. Cell. Biochem. 71:189–203, 1998.

Ascorbic acid participates in a general mechanism for concerted glucose transport inhibition and lactate transport stimulation

In this paper, we present a novel function for ascorbic acid. Ascorbic acid is an important water-soluble antioxidant and cofactor in various enzyme systems. We have previously demonstrated that an increase in neuronal intracellular ascorbic acid is able to inhibit glucose transport in cortical and hippocampal neurons. Because of the presence of sodium-dependent vitamin C transporters, ascorbic acid is highly concentrated in brain, testis, lung, and adrenal glands. In this work, we explored how ascorbic acid affects glucose and lactate uptake in neuronal and non-neuronal cells. Using immunofluorescence and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis, the expression of glucose and ascorbic acid transporters in non-neuronal cells was studied. Like neurons, HEK293 cells expressed GLUT1, GLUT3, and SVCT2. With radioisotope-based methods, only intracellular ascorbic acid, but not extracellular, inhibits 2-deoxyglucose transport in HEK293 cells. As monocarboxylates such as pyruvate and lactate, are important metabolic sources, we analyzed the ascorbic acid effect on lactate transport in cultured neurons and HEK293 cells. Intracellular ascorbic acid was able to stimulate lactate transport in both cell types. Extracellular ascorbic acid did not affect this transport. Our data show that ascorbic acid inhibits glucose transport and stimulates lactate transport in neuronal and non-neuronal cells. Mammalian cells frequently present functional glucose and monocarboxylate transporters, and we describe here a general effect in which ascorbic acid functions like a glucose/monocarboxylate uptake switch in tissues expressing ascorbic acid transporters.

The Presence and Function of Dopamine Type 2 Receptors in Boar Sperm: A Possible Role for Dopamine in Viability, Capacitation, and Modulation of Sperm Motility

Abstract Several studies have shown that dopamine and other catecholamines are present in oviduct luminal fluid. We recently reported that dopamine type 2 receptors (DRD2) are present in a wide range of mammalian sperm, suggesting a role for dopaminergic signaling in events such as fertilization, capacitation, and sperm motility. In the present study, we used Western blot analysis to show that boar sperm express DRD2 and that their activation with dopamine (100 nM) has a positive effect on cell viability that can be correlated with AKT/PKB phosphorylation. Bromocriptine (100 nM) and dopamine (100 nM and 10 μM) increased tyrosine phosphorylation during the capacitation period. Immunofluorescence analysis indicated that DRD2 localization is dynamic and depends on the capacitation stage, colocalizing with tyrosine phosphorylated proteins in the acrosome and midpiece region of capacitated boar sperm. This association was confirmed by coimmunoprecipitation analysis. We also showed that bromocriptine (100 nM) and low-concentration dopamine (100 nM and 10 μM) increased total and progressive motility of sperm. However, high concentrations of dopamine (1 mM) decreased tyrosine phosphorylation and motility in in vitro sperm capacitation assays. This can be explained by the presence of the dopamine transporters (DAT, official symbol SLC6A3) in sperm, as demonstrated by Western blot analysis and immunocytochemistry. Taken together, our results support the idea that dopamine may have a fundamental role during sperm capacitation and motility in situ in the female upper reproductive tract.

Molecular identification and functional characterization of the vitamin C transporters expressed by Sertoli cells

Vitamin C is an essential micronutrient for the development of male germ cells. In the gonad, the germ cells are isolated from the systemic circulation by the blood–testis barrier, which consists of a basal layer of Sertoli cells that communicate through an extensive array of tight junction complexes. To study the behavior of Sertoli cells as a first approach to the molecular and functional characterization of the vitamin C transporters in this barrier, we used the 42GPA9 cell line immortalized from mouse Sertoli cells. To date, there is no available information on the mechanism of vitamin C transport across the blood–testis barrier. This work describe the molecular identity of the transporters involved in vitamin C transport in these cells, which we hope will improve our understanding of how germ cells obtain vitamin C, transported from the plasma into the adluminal compartment of the seminiferous tubules. RT‐PCR analyses revealed that 42GPA9 cells express both vitamin C transport systems, a finding that was confirmed by immunocytochemical and immunoblotting analysis. The kinetic assays using radioactive vitamin C revealed that both ascorbic acid (AA) transporters, SVCT1 and SVCT2, are functionally active. Moreover, the kinetic characteristics of dehydroascorbic acid (DHA) and 3‐methylglucose (OMG) transport by 42GPA9 Sertoli cells correspond to facilitative hexose transporters GLUT1, GLUT2 and GLUT3 expressed in these cells. This data is consistent with the concept that Sertoli cells have the ability to take up vitamin C. It is an important finding and contributes to our knowledge of the physiology of male germ cells. J. Cell. Physiol. 217: 708–716, 2008.

Ascorbic acid-dependent GLUT3 inhibition is a critical step for switching neuronal metabolism.

Intracellular ascorbic acid is able to modulate neuronal glucose utilization between resting and activity periods. We have previously demonstrated that intracellular ascorbic acid inhibits deoxyglucose transport in primary cultures of cortical and hippocampal neurons and in HEK293 cells. The same effect was not seen in astrocytes. Since this observation was valid only for cells expressing glucose transporter 3 (GLUT3), we evaluated the importance of this transporter on the inhibitory effect of ascorbic acid on glucose transport. Intracellular ascorbic acid was able to inhibit 3H‐deoxyglucose transport only in astrocytes expressing GLUT3–EGFP. In C6 glioma cells and primary cultures of cortical neurons, which natively express GLUT3, the same inhibitory effect on 3H‐deoxyglucose transport and fluorescent hexose 2‐[N‐(7‐nitrobenz‐2‐oxa‐1,3‐diazol‐4‐yl)amino]‐2‐deoxyglucose (2‐NBDG) was observed. Finally, knocking down the native expression of GLUT3 in primary cultured neurons and C6 cells using shRNA was sufficient to abolish the ascorbic acid‐dependent inhibitory effect on uptake of glucose analogs. Uptake assays using real‐time confocal microscopy demonstrated that ascorbic acid effect abrogation on 2‐NBDG uptake in cultured neurons. Therefore, ascorbic acid would seem to function as a metabolic switch inhibiting glucose transport in neurons under glutamatergic synaptic activity through direct or indirect inhibition of GLUT3. J. Cell. Physiol. 226: 3286–3294, 2011.

Essential role of intracellular glutathione in controlling ascorbic acid transporter expression and function in rat hepatocytes and hepatoma cells

Although there is in vivo evidence suggesting a role for glutathione in the metabolism and tissue distribution of vitamin C, no connection with the vitamin C transport systems has been reported. We show here that disruption of glutathione metabolism with buthionine-(S,R)-sulfoximine (BSO) produced a sustained blockade of ascorbic acid transport in rat hepatocytes and rat hepatoma cells. Rat hepatocytes expressed the Na(+)-coupled ascorbic acid transporter-1 (SVCT1), while hepatoma cells expressed the transporters SVCT1 and SVCT2. BSO-treated rat hepatoma cells showed a two order of magnitude decrease in SVCT1 and SVCT2 mRNA levels, undetectable SVCT1 and SVCT2 protein expression, and lacked the capacity to transport ascorbic acid, effects that were fully reversible on glutathione repletion. Interestingly, although SVCT1 mRNA levels remained unchanged in rat hepatocytes made glutathione deficient by in vivo BSO treatment, SVCT1 protein was absent from the plasma membrane and the cells lacked the capacity to transport ascorbic acid. The specificity of the BSO treatment was indicated by the finding that transport of oxidized vitamin C (dehydroascorbic acid) and glucose transporter expression were unaffected by BSO treatment. Moreover, glutathione depletion failed to affect ascorbic acid transport, and SVCT1 and SVCT2 expression in human hepatoma cells. Therefore, our data indicate an essential role for glutathione in controlling vitamin C metabolism in rat hepatocytes and rat hepatoma cells, two cell types capable of synthesizing ascorbic acid, by regulating the expression and subcellular localization of the transporters involved in the acquisition of ascorbic acid from extracellular sources, an effect not observed in human cells incapable of synthesizing ascorbic acid.

Vitamin C and oxidative stress in the seminiferous epithelium

In this article, we focus on the fundamental role of vitamin C transporters for the normal delivery of vitamin C to germ cells in the adluminal compartment of seminiferous tubules. We argue that the redox status within spermatozoa or in semen is partly responsible for the etiology of infertility. In this context, antioxidant defence plays a critical role in male fertility. Vitamin C, a micronutrient required for a wide variety of metabolic functions, has long been associated with male reproduction. Two systems for vitamin C transport have been described in mammals. Facilitative hexose transporters (GLUTs), with 14 known isoforms to date, GLUT1-GLUT14, transport the oxidized form of vitamin C (dehydroascorbic acid) into the cells. Sodium ascorbic acid co-transporters (SVCTs), SVCT1 and SVCT2 transport the reduced form of vitamin C (ascorbic acid). Sertoli cells control germ cell proliferation and differentiation through cell-cell communication and form the blood-testis barrier. Because the blood-testis barrier limits direct access of molecules from the plasma into the adluminal compartment of the seminiferous tubule, one important question is the method by which germ cells obtain vitamin C. Some interesting results have thrown light on this matter. Expression of SVCT2 and some isoforms of GLUT transporters in the testis have previously been described. Our group has demonstrated that Sertoli cells express functionally active vitamin C transporters. Kinetic characteristics were described for both transport systems (SVCT and GLUT systems). Sertoli cells are able to transport both forms of vitamin C. These findings are extremely relevant, because Sertoli cells may control the amount of vitamin C in the adluminal compartment, as well as regulating the availability of this metabolite throughout spermatogenesis.

Muscle glycogen synthase isoform is responsible for testicular glycogen synthesis: Glycogen overproduction induces apoptosis in male germ cells

Glycogen is the main source of glucose for many biological events. However, this molecule may have other functions, including those that have deleterious effects on cells. The rate‐limiting enzyme in glycogen synthesis is glycogen synthase (GS). It is encoded by two genes, GYS1, expressed in muscle (muscle glycogen synthase, MGS) and other tissues, and GYS2, primarily expressed in liver (liver glycogen synthase, LGS). Expression of GS and its activity have been widely studied in many tissues. To date, it is not clear which GS isoform is responsible for glycogen synthesis and the role of glycogen in testis. Using RT‐PCR, Western blot and immunofluorescence, we have detected expression of MGS but not LGS in mice testis during development. We have also evaluated GS activity and glycogen storage at different days after birth and we show that both GS activity and levels of glycogen are higher during the first days of development. Using RT‐PCR, we have also shown that malin and laforin are expressed in testis, key enzymes for regulation of GS activity. These proteins form an active complex that regulates MGS by poly‐ubiquitination in both Sertoli cell and male germ cell lines. In addition, PTG overexpression in male germ cell line triggered apoptosis by caspase3 activation, proposing a proapoptotic role of glycogen in testis. These findings suggest that GS activity and glycogen synthesis in testis could be regulated and a disruption of this process may be responsible for the apoptosis and degeneration of seminiferous tubules and possible cause of infertility. J. Cell. Biochem. 114: 1653–1664, 2013.

Glutathione Depletion Induces Spermatogonial Cell Autophagy

The development and survival of male germ cells depend on the antioxidant capacity of the seminiferous tubule. Glutathione (GSH) plays an important role in the antioxidant defenses of the spermatogenic epithelium. Autophagy can act as a pro‐survival response during oxidative stress or nutrient deficiency. In this work, we evaluated whether autophagy is involved in spermatogonia‐type germ cell survival during severe GSH deficiency. We showed that the disruption of GSH metabolism with l‐buthionine‐(S,R)‐sulfoximine (BSO) decreased reduced (GSH), oxidized (GSSG) glutathione content, and GSH/GSSG ratio in germ cells, without altering reactive oxygen species production and cell viability, evaluated by 2′,7′‐dichlorodihydrofluorescein (DCF) fluorescence and exclusion of propidium iodide assays, respectively. Autophagy was assessed by processing the endogenous protein LC3I and observing its sub‐cellular distribution. Immunoblot and immunofluorescence analysis showed a consistent increase in LC3II and accumulation of autophagic vesicles under GSH‐depletion conditions. This condition did not show changes in the level of phosphorylation of AMP‐activated protein kinase (AMPK) or the ATP content. A loss in S‐glutathionylated protein pattern was also observed. However, inhibition of autophagy resulted in decreased ATP content and increased caspase‐3/7 activity in GSH‐depleted germ cells. These findings suggest that GSH deficiency triggers an AMPK‐independent induction of autophagy in germ cells as an adaptive stress response. J. Cell. Biochem. 116: 2283–2292, 2015.

Cytokine stimulation promotes increased glucose uptake via translocation at the plasma membrane of GLUT1 in HEK293 cells.

Interleukin‐3 (IL‐3) and granulocyte/macrophage colony‐stimulating factor (GM‐CSF) are two of the best‐characterized cell survival factors in hematopoietic cells; these factors induce an increase in Akt activity in multiple cell lines, a process thought to be involved in cellular survival. It is known that growth factors require sustained glucose metabolism to promote cell survival. It has been determined that IL‐3 and GM‐CSF signal for increased glucose uptake in hematopoietic cells. Interestingly, receptors for IL‐3 and GM‐CSF are present in several non‐hematopoietic cell types but their roles in these cells have been poorly described. In this study, we demonstrated the expression of IL‐3 and GM‐CSF receptors in HEK293 cells and analyzed their effect on glucose uptake. In these cells, both IL‐3 and GM‐CSF, increased glucose uptake. The results indicated that this increase involves the subcellular redistribution of GLUT1, affecting glucose transporter levels at the cell surface in HEK293 cells. Also the data directly demonstrates that the PI 3‐kinase/Akt pathway is an important mediator of this process. Altogether these results show a role for non‐insulin growth factors in the regulation of GLUT1 trafficking that has not yet been directly determined in non‐hematopoietic cells. J. Cell. Biochem. 110: 1471–1480, 2010.