Regulation of reduced-folate transporter-1 (RFT-1) by homocysteine and identity of transport systems for homocysteine uptake in retinal pigment epithelial (RPE) cells

Reduced folate carrier 1 is present in retinal microvessels and crucial for the inner blood retinal barrier integrity

BACKGROUND

Reduced folate carrier 1 (RFC1; SLC19a1) is the main responsible transporter for the B9 family of vitamins named folates, which are essential for normal tissue growth and development. While folate deficiency resulted in retinal vasculopathy, the expression and the role of RFC1 in blood-retinal barrier (BRB) are not well known.

METHODS

We used whole mount retinas and trypsin digested microvessel samples of adult mice. To knockdown RFC1, we delivered RFC1-targeted short interfering RNA (RFC1-siRNA) intravitreally; while, to upregulate RFC1 we delivered lentiviral vector overexpressing RFC1. Retinal ischemia was induced 1-h by applying FeCl₃ to central retinal artery. We used RT-qPCR and Western blotting to determine RFC1. Endothelium (CD31), pericytes (PDGFR-beta, CD13, NG2), tight-junctions (Occludin, Claudin-5 and ZO-1), main basal membrane protein (Collagen-4), endogenous IgG and RFC1 were determined immunohistochemically.

RESULTS

Our analyses on whole mount retinas and trypsin digested microvessel samples of adult mice revealed the presence of RFC1 in the inner BRB and colocalization with endothelial cells and pericytes. Knocking down RFC1 expression via siRNA delivery resulted in the disintegration of tight junction proteins and collagen-4 in twenty-four hours, which was accompanied by significant endogenous IgG extravasation. This indicated the impairment of BRB integrity after an abrupt RFC1 decrease. Furthermore, lentiviral vector-mediated RFC1 overexpression resulted in increased tight junction proteins and collagen-4, confirming the structural role of RFC1 in the inner BRB. Acute retinal ischemia decreased collagen-4 and occludin levels and led to an increase in RFC1. Besides, the pre-ischemic overexpression of RFC1 partially rescued collagen-4 and occludin levels which would be decreased after ischemia.

CONCLUSION

In conclusion, our study clarifies the presence of RFC1 protein in the inner BRB, which has recently been defined as hypoxia-immune-related gene in other tissues and offers a novel perspective of retinal RFC1. Hence, other than being a folate carrier, RFC1 is an acute regulator of the inner BRB in healthy and ischemic retinas.

Exogenous administration of hydrogen sulfide alleviates homocysteine induced inflammation in ARPE-19 cells

Plasma homocysteine (Hcy) is an independent risk factor for Age related macular degeneration (AMD) and an inducer of inflammation. Homocysteine catabolism releases hydrogen sulfide (H₂S). H₂S has controversial effects on inflammation. In this study we have analysed the endogenous and exogenous H₂S in modulating inflammation using adult retinal pigment epithelial (ARPE-19) cells as an in vitro model for AMD. ARPE-19 cells were treated with various concentrations of Hcy (15, 30 and 50 μM) for 3 h. Expression of Hcy transulfuration genes (CBS, CSE) by qPCR and western blot. H₂S levels were measured using Free Radical Analyzer System (WPI, USA). The inflammatory markers (IL-6 and IL-8) were evaluated using real-time PCR and ELISA. Hcy exposure increased CBS protein expression, hydrogen sulfide levels and pro-inflammatory cytokines, modulating CBS by silencing did not alter H₂S levels, but inhibition of CSE with PAG inhibited H₂S production and decreased cytokine (IL-6 and IL-8) levels. On the contrary exogenous supply of hydrogen sulfide with NaHS and by compound 1c showed anti-inflammatory effects even in the presence of Hcy. This study shows that exogenous delivery of H₂S decreases inflammation in retinal pigment epithelial cells on exposure to Hcy in ARPE-19 cells.

Homocysteine is transported by the microvillous plasma membrane of human placenta

Elevated maternal plasma concentrations of homocysteine (Hcy) are associated with pregnancy complications and adverse neonatal outcomes. The postulate that we wish to advance here is that placental transport of Hcy, by competing with endogenous amino acids for transporter activity, may account for some of the damaging impacts of Hcy on placental metabolism and function as well as fetal development. In this article, we provide an overview of some recent studies characterising the transport mechanisms for Hcy across the microvillous plasma membrane (MVM) of the syncytiotrophoblast, the transporting epithelium of human placenta. Three Hcy transport systems have been identified, systems L, A and y(+)L. This was accomplished using a strategy of competitive inhibition to investigate the effects of Hcy on the uptake of well-characterised radiolabelled substrates for each transport system into isolated MVM vesicles. The reverse experiments were also performed, examining the effects of model substrates on [³⁵S]L-Hcy uptake. This article describes the evidence for systems L, A and y(+)L involvement in placental Hcy transport and discusses the physiological implications of these findings with respect to placental function and fetal development.

Molecular and biochemical characterization of folate transport proteins in retinal Müller cells

PURPOSE

To analyze the mechanisms of folate uptake in retinal Müller cells.

METHODS

RT-PCR and Western blot analysis were performed in freshly isolated neural retina and RPE/eyecup, primary mouse Müller cells, and rMC-1 cells for the three known folate transport proteins folate receptor alpha (FRalpha), proton-coupled folate transporter (PCFT), and reduced folate carrier (RFC). Laser scanning confocal microscopy (LSCM) and immunoelectron microscopy were used to determine the subcellular location of FRalpha and PCFT in primary Müller cells. The pH dependence of the uptake of [(3)H]-methyltetrahydrofolate ([(3)H]-MTF) was assayed in Müller cells in the presence/absence of thiamine pyrophosphate, an inhibitor of RFC.

RESULTS

FRalpha and PCFT are expressed abundantly in the retina in several cell layers, including the inner nuclear layer; they are present in primary mouse Müller cells and rMC-1 cells. LSCM localized these proteins to the plasma membrane, nuclear membrane, and perinuclear region. Immunoelectron microscopic studies revealed the colocalization of FRalpha and PCFT on the plasma membrane and nuclear membrane and within endosomal structures. Müller cell uptake of [(3)H]-MTF was robust at pH 5.0 to 6.0, consistent with PCFT activity, but also at neutral pH, reflecting RFC function. RFC was expressed in mouse Müller cells that had been allowed to proliferate in culture, but not in freshly isolated primary cells.

CONCLUSIONS

FRalpha and PCFT are expressed in retinal Müller cells and colocalize in the endosomal compartment, suggesting that the two proteins may work coordinately to mediate folate uptake. The unexpected finding of RFC expression and activity in cultured Müller cells may reflect the upregulation of this protein under proliferative conditions.

Homocysteine transport by systems L, A and y+L across the microvillous plasma membrane of human placenta

Elevated maternal plasma levels of homocysteine (Hcy) are associated with pregnancy complications and adverse neonatal outcomes, suggesting placental transport of Hcy may impact on fetal development. However, such transport mechanisms have not been defined. In this study we characterise Hcy transport mechanisms across the microvillous plasma membrane (MVM) of the syncytiotrophoblast, the transporting epithelium of human placenta. Three candidate transport systems, systems L, A and y(+)L, were examined utilising competitive inhibition to investigate the effects of Hcy on the uptake of well-characterised radiolabelled substrates for each system into isolated MVM vesicles, and that of model substrates on 10 microm [(35)S]l-Hcy uptake. System L activity was inhibited by both l-Hcy and dl-Hcy, comparable to model substrates including 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH). System L constituted the major transport mechanism, with significant BCH inhibition (69%) of [(35)S]l-Hcy uptake. System A activity was also inhibited by l-Hcy and dl-Hcy with a smaller contribution (21%) to [(35)S]l-Hcy uptake. Inhibition by l-Hcy and dl-Hcy of system y(+)L activity was Na(+) sensitive with a significant inhibition constant (K(i)) shift observed following K(+) replacement; l-arginine reduced [(35)S]l-Hcy uptake by 19%. Kinetic modelling of [(35)S]l-Hcy uptake resolved two, Na(+)-independent, transport components (K(m) 72 microm and 9.7 mm). This study provides evidence for the involvement of systems L, A and y(+)L in placental Hcy transport. Such transport, by competing with endogenous amino acids for transporter activity, could have major implications for syncytiotrophoblast metabolism and function as well as fetal development.

Transport of thiol-conjugates of inorganic mercury in human retinal pigment epithelial cells

Inorganic mercury (Hg(2+)) is a prevalent environmental contaminant to which exposure to can damage rod photoreceptor cells and compromise scotopic vision. The retinal pigment epithelium (RPE) likely plays a role in the ocular toxicity associated with Hg(2+) exposure in that it mediates transport of substances to the photoreceptor cells. In order for Hg(2+) to access photoreceptor cells, it must first be taken up by the RPE, possibly by mechanisms involving transporters of essential nutrients. In other epithelia, Hg(2+), when conjugated to cysteine (Cys) or homocysteine (Hcy), gains access to the intracellular compartment of the target cells via amino acid and organic anion transporters. Accordingly, the purpose of the current study was to test the hypothesis that Cys and Hcy S-conjugates of Hg(2+) utilize amino acid transporters to gain access into RPE cells. Time- and temperature-dependence, saturation kinetics, and substrate-specificity of the transport of Hg(2+), was assessed in ARPE-19 cells exposed to the following S-conjugates of Hg(2+): Cys (Cys-S-Hg-S-Cys), Hcy (Hcy-S-Hg-S-Hcy), N-acetylcysteine (NAC-S-Hg-S-NAC) or glutathione (GSH-S-Hg-S-GSH). We discovered that only Cys-S-Hg-S-Cys and Hcy-S-Hg-S-Hcy were taken up by these cells. This transport was Na(+)-dependent and was inhibited by neutral and cationic amino acids. RT-PCR analyses identified systems B(0,+) and ASC in ARPE-19 cells. Overall, our data suggest that Cys-S-Hg-S-Cys and Hcy-S-Hg-S-Hcy are taken up into ARPE-19 cells by Na-dependent amino acid transporters, possibly systems B(0,+) and ASC. These amino acid transporters may play a role in the retinal toxicity observed following exposure to mercury.

Disturbed visual system function in methionine synthase deficiency

BACKGROUND

Isolated functional methionine synthase deficiency occurs in the cblE and cblG defects of methylcobalamin metabolism and is one of a number of causes of severely elevated plasma homocysteine. Clinical features are predominantly of a neurological nature but also include functional restriction of the visual system manifesting as loss of visual acuity and nystagmus. As yet, the origin and pathogenesis of impaired vision have not been explained.

MATERIALS AND METHODS

We investigated a patient who was proven by complementation analysis in cultured fibroblasts to belong to the cblG complementation group. Ganzfeld electroretinograms (ERG) and flash visual evoked potentials (VEP) were recorded over a period of 4 years.

RESULTS

Amplitudes of all International Society for Clinical Electrophysiology of Vision (ISCEV) standard responses were below normal. The greatest reductions were of rod response to 24 microV, of standard combined response (SC) b-wave to 120 microV, of oscillatory potentials (OP) to 5 microV, of cone response b-wave to 35 microV, and of 30 Hz flicker response to 8 microV. Except for SC and cone a-waves at age 2.5 and 3.5 years, as well as cone b-wave at 3.5 years, amplitudes remained at a subnormal level at follow-up examinations. Implicit times were slightly prolonged (SC b-wave 6 ms, OPs 2 ms, cone b-wave 2 ms, 30 Hz flicker 4 ms) or fell within the normal range. Responses of the flash VEP were severely deformed but reproducible.

CONCLUSIONS

This is the first report of detailed investigations of the visual system in a patient with isolated methionine synthase deficiency. Reduced oscillatory potentials suggest microvascular damage to the retina through homocysteine. Decreased photoreceptor function as well as ganglion cell loss as indicated by pathological flash VEPs may reflect a cytotoxic impact of homocysteine on neurons of the visual pathway.

Homocysteine, system b0,+ and the renal epithelial transport and toxicity of inorganic mercury

Proximal tubular epithelial cells are major sites of homocysteine (Hcy) metabolism and are the primary sites for the accumulation and intoxication of inorganic mercury (Hg(2+)). Previous in vivo data from our laboratory have demonstrated that mercuric conjugates of Hcy are transported into these cells by unknown mechanisms. Recently, we established that the mercuric conjugate of cysteine [2-amino-3-(2-amino-2-carboxy-ethylsulfanylmercuricsulfanyl)propionic acid; Cys-S-Hg-S-Cys], is transported by the luminal, amino acid transporter, system b(0,+). As Cys-S-Hg-S-Cys and the mercuric conjugate of Hcy (2-amino-4-(3-amino-3-carboxy-propylsulfanylmercuricsulfanyl)butyric acid; Hcy-S-Hg-S-Hcy) are similar structurally, we hypothesized that Hcy-S-Hg-S-Hcy is a substrate for system b(0,+). To test this hypothesis, we analyzed the saturation kinetics, time dependence, temperature dependence, and substrate specificity of Hcy-S-Hg-S-Hcy transport in Madin-Darby canine kidney (MDCK) cells stably transfected with system b(0,+). MDCK cells are good models in which to study this transport because they do not express system b(0,+). Uptake of Hg(2+) was twofold greater in the transfectants than in wild-type cells. Moreover, the transfectants were more susceptible to the toxic effects of Hcy-S-Hg-S-Hcy than wild-type cells. Accordingly, our data indicate that Hcy-S-Hg-S-Hcy is transported by system b(0,+) and that this transporter likely plays a role in the nephropathy induced after exposure to Hg(2+). These data are the first to implicate a specific, luminal membrane transporter in the uptake and toxicity of mercuric conjugates of Hcy in any epithelial cell.

Homocysteine increases the expression of vascular endothelial growth factor by a mechanism involving endoplasmic reticulum stress and transcription factor ATF4

Vascular endothelial growth factor (VEGF) plays a key role in the development and progression of diabetic retinopathy. We previously demonstrated that amino acid deprivation and other inducers of endoplasmic reticulum-stress (ER stress) up-regulate the expression of VEGF in the retinal-pigmented epithelial cell line ARPE-19. Because homocysteine causes ER stress, we hypothesized that VEGF expression is increased by ambient homocysteine. dl-Homocysteine-induced VEGF expression was investigated in confluent ARPE-19 cultures. Northern analysis showed that homocysteine increased steady state VEGF mRNA levels 4.4-fold. Other thiol-containing compounds, including l-homocysteine thiolactone and DTT, induced VEGF expression 7.9- and 8.8-fold. Transcriptional run-on assays and mRNA decay studies demonstrated that the increase in VEGF mRNA levels was caused by increased transcription rather than mRNA stabilization. VEGF mRNA induction paralleled that of the ER-stress gene GRP78. Homocysteine treatment caused transient phosphorylation of eIF2alpha and an increase in ATF4 protein level. Overexpression of a dominant-negative ATF4 abolished the VEGF response to homocysteine treatment and to amino acid deprivation. VEGF mRNA expression by ATF4-/- MEF did not respond to homocysteine treatment and the response was restored with expression of wild-type ATF4. These studies indicate that expression of the pro-angiogenic factor VEGF is increased by homocysteine and other thiol-containing reductive compounds via ATF4-dependent activation of VEGF transcription.