Homocysteine inhibits store-mediated calcium entry in human endothelial cells: evidence for involvement of membrane potential and actin cytoskeleton

Ca2+ Influx through TRPC Channels Is Regulated by Homocysteine-Copper Complexes

An elevated level of circulating homocysteine (Hcy) has been regarded as an independent risk factor for cardiovascular disease; however, the clinical benefit of Hcy lowering-therapy is not satisfying. To explore potential unrevealed mechanisms, we investigated the roles of Ca²⁺ influx through TRPC channels and regulation by Hcy-copper complexes. Using primary cultured human aortic endothelial cells and HEK-293 T-REx cells with inducible TRPC gene expression, we found that Hcy increased the Ca²⁺ influx in vascular endothelial cells through the activation of TRPC4 and TRPC5. The activity of TRPC4 and TRPC5 was regulated by extracellular divalent copper (Cu²⁺) and Hcy. Hcy prevented channel activation by divalent copper, but monovalent copper (Cu⁺) had no effect on the TRPC channels. The glutamic acids (E542/E543) and the cysteine residue (C554) in the extracellular pore region of the TRPC4 channel mediated the effect of Hcy-copper complexes. The interaction of Hcy-copper significantly regulated endothelial proliferation, migration, and angiogenesis. Our results suggest that Hcy-copper complexes function as a new pair of endogenous regulators for TRPC channel activity. This finding gives a new understanding of the pathogenesis of hyperhomocysteinemia and may explain the unsatisfying clinical outcome of Hcy-lowering therapy and the potential benefit of copper-chelating therapy.

Gut dysbiosis and homocysteine: a couple for boosting neurotoxicity in Huntington disease

Huntington's disease (HD), a neurodegenerative disorder caused by an expansion of the huntingtin triplet (Htt), is clinically characterized by cognitive and neuropsychiatric alterations. Although these alterations appear to be related to mutant Htt (mHtt)-induced neurotoxicity, several other factors are involved. The gut microbiota is a known modulator of brain-gut communication and when altered (dysbiosis), several complaints can be developed including gastrointestinal dysfunction which may have a negative impact on cognition, behavior, and other mental functions in HD through several mechanisms, including increased levels of lipopolysaccharide, proinflammatory cytokines and immune cell response, as well as alterations in Ca²⁺ signaling, resulting in both increased intestinal and blood-brain barrier (BBB) permeability. Recently, the presence of dysbiosis has been described in both transgenic mouse models and HD patients. A bidirectional influence between host brain tissues and the gut microbiota has been observed. On the one hand, the host diet influences the composition and function of microbiota; and on the other hand, microbiota products can affect BBB permeability, synaptogenesis, and the regulation of neurotransmitters and neurotrophic factors, which has a direct effect on host metabolism and brain function. This review summarizes the available evidence on the pathogenic synergism of dysbiosis and homocysteine, and their role in the transgression of BBB integrity and their potential neurotoxicity of HD.

Homocysteine Causes Endothelial Dysfunction via Inflammatory Factor-Mediated Activation of Epithelial Sodium Channel (ENaC)

Background

Hyperhomocysteinemia (HHcy) causes cardiovascular diseases via regulating inflammatory responses. We investigated whether and how the epithelial sodium channel (ENaC), a recently identified ion channel in endothelial cells, plays a role in HHcy-induced endothelial dysfunction.

Methods

Cell-attached patch-clamp recording in acute split-open aortic endothelial cells, western blot, confocal imaging, and wire myograph combined with pharmacological approaches were used to determine whether HHcy-mediated inflammatory signaling leads to endothelial dysfunction via stimulating ENaC.

Results

The data showed that 4 weeks after L-methionine diet the levels of plasma Hcy were significantly increased and the ENaC was dramatically activated in mouse aortic endothelial cells. Administration of benzamil, a specific ENaC blocker, ameliorated L-methionine diet-induced impairment of endothelium-dependent relaxation (EDR) and reversed Hcy-induced increase in ENaC activity. Pharmacological inhibition of NADPH oxidase, reactive oxygen species (ROS), cyclooxygenase-2 (COX-2)/thromboxane B2 (TXB2), or serum/glucocorticoid regulated kinase 1 (SGK1) effectively attenuated both the Hcy-induced activation of endothelial ENaC and impairment of EDR. Our in vitro data showed that both NADPH oxidase inhibitor and an ROS scavenger reversed Hcy-induced increase in COX-2 expression in human umbilical vein endothelial cells (HUVECs). Moreover, Hcy-induced increase in expression levels of SGK-1, phosphorylated-SGK-1, and phosphorylated neural precursor cell-expressed developmentally downregulated protein 4-2 (p-Nedd4-2) in HUVECs were significantly blunted by a COX-2 inhibitor.

Conclusion

We show that Hcy activates endothelial ENaC and subsequently impairs EDR of mouse aorta, via ROS/COX-2-dependent activation of SGK-1/Nedd4-2 signaling. Our study provides a rational that blockade of the endothelial ENaC could be potential method to prevent and/or to treat Hcy-induced cardiovascular disease.

Homocysteine augments BK channel activity and decreases exocytosis of secretory granules in rat GH3 cells

In this study, we investigated the effects of L-homocysteine (Hcy) on maxi calcium-activated potassium (BK) channels and on exocytosis of secretory granules in GH3 rat pituitary-derived cells. A major finding of our study indicates that short-term application of Hcy increased the open probability of oxidized BK channels in inside-out recordings. Whole-cell recordings show that extracellular Hcy also augmented BK currents during long-term application. Furthermore, Hcy decreased the exocytosis of secretory granules. This decrease was partially prevented by the BK channel inhibitor paxilline and fully prevented by N-acetylcysteine, a reactive oxygen species scavenger. Taken together, our data show that elevation of cellular Hcy level induces oxidative stress, increases BK channel activity, and decreases exocytosis of secretory granules. These findings may provide insight into some of the developmental impairments and neurotoxicity associated with Hyperhomocysteinemia (HHcy), a disease arising due to abnormally elevated levels of Hcy in the plasma.

Redox regulation of store-operated Ca2+ entry

SIGNIFICANCE

Store-operated Ca2+ entry (SOCE) is a ubiquitous Ca2+ signaling mechanism triggered by Ca2+ depletion of the endoplasmic reticulum (ER) and by a variety of cellular stresses. Reactive oxygen species (ROS) are often concomitantly produced in response to these stresses, however, the relationship between redox signaling and SOCE is not completely understood. Various cardiovascular, neurological, and immune diseases are associated with alterations in both Ca2+ signaling and ROS production, and thus understanding this relationship has therapeutic implications.

RECENT ADVANCES

Several reactive cysteine modifications in stromal interaction molecule (STIM) and Orai proteins comprising the core SOCE machinery were recently shown to modulate SOCE in a redox-dependent manner. Moreover, STIM1 and Orai1 expression levels may reciprocally regulate and be affected by responses to oxidative stress. ER proteins involved in oxidative protein folding have gained increased recognition as important sources of ROS, and the recent discovery of their accumulation in contact sites between the ER and mitochondria provides a further link between ROS production and intracellular Ca2+ handling.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Future research should aim to establish the complete set of SOCE controlling molecules, to determine their redox-sensitive residues, and to understand how intracellular Ca2+ stores dynamically respond to different types of stress. Mapping the precise nature and functional consequence of key redox-sensitive components of the pre- and post-translational control of SOCE machinery and of proteins regulating ER calcium content will be pivotal in advancing our understanding of the complex cross-talk between redox and Ca2+ signaling.

Inhibition of ATP-induced calcium influx by homocysteine in human umbilical vein endothelial cells

Mechanisms involved in the association between hyperhomocysteinemia and vascular occlusive diseases remain unclear. Homocysteine (Hcy) may disturb calcium (Ca(2+) ) cytosolic regulation in endothelial cells, a process that can directly affect the synthesis of vasoactive substances, such as nitric oxide (NO). We have investigated the effect of acute and chronic incubation with high concentrations of Hcy (100 and 500 μmol/L) on the changes in the intracellular Ca(2+) concentration ([Ca(2+) ]i ) induced by ATP, using primary cultures of human umbilical vein endothelial cells (HUVEC). The changes in [Ca(2+) ]i , expressed as ΔFt /Fb , were measured using the microspectrofluorimetric technique with Fluo-3 as Ca(2+) indicator. HUVEC acutely exposed to Hcy did not produce significant effects on any of the parameters studied. However, chronic exposition (24 h) caused a significant decrease in the speed of store-mediated Ca(2+) entry, expressed as (ΔFt /Fb )/t (s(-1) ). Exposure of HUVEC to 100 and 500 µmol/L Hcy gave significantly lower values (0.019 ± 0.002 s(-1) , n = 5 and 0.021 ± 0.004 s(-1) , n = 6, respectively) compared to the controls (0.046 ± 0.004 s(-1) , n = 8, P < 0.003). This was detected only when the sustained phase of the ATP-induced [Ca(+2) ]i increase was isolated. These results demonstrate that high concentrations of Hcy can affect the mechanisms involved in [Ca(2+) ]i regulation of HUVEC, and that alteration occurs specifically in the sustained phase, which has been directly associated with NO synthesis.

The methylenetetrahydrofolate reductase C677T mutation induces cell-specific changes in genomic DNA methylation and uracil misincorporation: a possible molecular basis for the site-specific cancer risk modification

The C677T polymorphism in the methylenetetrahydrofolate reductase (MTHFR) gene is associated with a decreased risk of colon cancer although it may increase the risk of breast cancer. This polymorphism is associated with changes in intracellular folate cofactors, which may affect DNA methylation and synthesis via altered one-carbon transfer reactions. We investigated the effect of this mutation on DNA methylation and uracil misincorporation and its interaction with exogenous folate in further modulating these biomarkers of one-carbon transfer reactions in an in vitro model of the MTHFR 677T mutation in HCT116 colon and MDA-MB-435 breast adenocarcinoma cells. In HCT116 cells, the MTHFR 677T mutation was associated with significantly increased genomic DNA methylation when folate supply was adequate or high; however, in the setting of folate insufficiency, this mutation was associated with significantly decreased genomic DNA methylation. In contrast, in MDA-MB-435 cells, the MTHFR 677T mutation was associated with significantly decreased genomic DNA methylation when folate supply was adequate or high and with no effect when folate supply was low. The MTHFR 677T mutation was associated with a nonsignificant trend toward decreased and increased uracil misincorporation in HCT116 and MDA-MB-435 cells, respectively. Our data demonstrate for the first time a functional consequence of changes in intracellular folate cofactors resulting from the MTHFR 677T mutation in cells derived from the target organs of interest, thus providing a plausible cellular mechanism that may partly explain the site-specific modification of colon and breast cancer risks associated with the MTHFR C677T mutation.

Effect of homocysteine on calcium mobilization and platelet function in type 2 diabetes mellitus

Type 2 diabetes mellitus induces a characteristic platelet hyperactivity that might be due to several factors including oxidative stress and abnormal intracellular Ca2+ homeostasis. Hyperhomocysteinaemia is considered a risk factor in the development of thrombosis although its effect on platelet function and the mechanisms involved are still poorly understood. Here we show that homocysteine (Hcy) induce a concentration-dependent increase in endogenous production of reactive oxygen species (ROS), which was significantly greater in platelets from diabetic patients than in controls. Platelet treatment with Hcy resulted in Ca2+ release from the dense tubular system and the acidic stores. Ca2+ mobilisation-induced by Hcy consisted in two components, an initial slow increase in intracellular free Ca2+ concentration ([Ca2+]i) and a rapid and marked increase in [Ca2+]i, the second leading to the activation of platelet aggregation. As well as ROS generation, Ca2+ mobilization and platelet aggregation were significantly greater in platelets from diabetic donors than in controls, which indicate that platelets from diabetic donors are more sensitive to Hcy. These findings, together with the hyperhomocysteinaemia reported in diabetic patients, strongly suggest that Hcy might be considered a risk factor in the development of cardiovascular complications associated to type 2 diabetes mellitus.

Effect of homocysteine on calcium mobilization and platelet function in type 2 diabetes mellitus

Type 2 diabetes mellitus induces a characteristic platelet hyperactivity that might be due to several factors including oxidativ stress and abnormal intracellular Ca²⁺ homeostasis. Hyperhomocysteinaemia is considered a risk factor in the development of thrombosis although its effect on platelet function and the mechanisms involved are still poorly understood. Here we show tha homocysteine induce a concentration-dependent increase in endogenous production of reactive oxygen species (ROS), which was significantly greater in platelets from diabetic patients than in controls. Platelet treatment with homocysteine resulted in Ca²⁺ release from the dense tubular system and the acidic stores. Ca²⁺ mobilization-induced by homocysteine consisted in two components, an initial slow increase in intracellular free Ca ⁺ concentration ([Ca ⁺]i) and a rapid and marked increase in [Ca²⁺]i, th second leading to the activation of platelet aggregation. As well as ROS generation, Ca²⁺ mobilization and platelet aggregation were significantly greater in platelets from diabetic donors than in controls, which indicate that platelets from diabetic donors are more sensitive to homocysteine. These findings, together with the hyperhomocysteinaemia reported in diabetic patients, strongly suggest that homocysteine might be considered a risk factor in the development of cardiovascular complications associated to type 2 diabetes mellitus.

Role of endoplasmic reticulum calcium disequilibria in the mechanism of homocysteine-induced ER stress

Our laboratory demonstrated that hyperhomocysteinemia accelerates atherosclerosis in mouse models through ER stress and activation of the unfolded protein response (UPR). In this study, we tested the hypothesis that homocysteine-induced ER stress may arise from ER-Ca(2+) disequilibria. We found that homocysteine-induced cytosolic Ca(2+) transients in T24/83 cells and human aortic smooth muscle cells (HASMCs). These calcium effects occurred at concentrations of homocysteine in the external medium (1-5 mM) that increase intracellular homocysteine in these cell types. Prolonged homocysteine treatment (5 h) at these exogenous concentrations reduced ER-Ca(2+) emptying evoked by thapsigargin. However, these homocysteine-induced effects on ER-Ca(2+) emptying were of a much smaller magnitude than those evoked by A23187 or thapsigargin (ER stressors known to induce ER stress through ER-Ca(2+) depletion). T24/83 cells stably overexpressing the Ca(2+)-binding ER chaperone GRP78 showed diminished cytosolic Ca(2+) transients induced by homocysteine and reduced ER-Ca(2+) emptying evoked by thapsigargin. Prevention of the homocysteine-induced UPR by cycloheximide pretreatment normalized GRP78 expression and ER-Ca(2+) emptying evoked by thapsigargin. These results are inconsistent with a mechanism of ER stress induction by homocysteine through ER-Ca(2+) depletion.

Long-term Administration of 3-deazaadenosine Does Not Alter Progression of Advanced Atherosclerotic Lesions in Apolipoprotein E-deficient Mice

Inflammatory mechanisms are involved in initiation and progression of atherosclerotic lesions. Previous studies demonstrated antiinflammatory and consecutive antiatherosclerotic effects of the adenosine analogue 3-Deazaadenosine (c(3) Ado) on early lesion development. The present study evaluated the effect of long-term administration of c(3) Ado in a mouse model of advanced atherosclerosis. Apolipoprotein E-deficient mice (age, 35 weeks; n = 31) with already established advanced atherosclerotic lesions were fed either a diet supplemented with c Ado or a regular chow diet for 21 weeks. Treatment resulted in a significant reduction of serum homocysteine levels. Lesion size and lesion morphology, such as frequency of intraplaque hemorrhage, size of necrotic cores, thickness of fibrous caps, and macrophage content within the plaque, were not different between the groups. Lesion calcification, expression of alpha-actin, and intercellular adhesion molecule-1, but not vascular cell adhesion molecule-1, were inhibited by treatment with c(3) Ado. We could not detect any effect on serum concentrations of interleukin-10 (IL-10) and interleukin-1beta (IL-1beta) or on soluble adhesion molecules intercellular adhesion molecule (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1). Electromobility shift assays of protein extracts isolated from aortas did not demonstrate different binding activities of nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1) after treatment with c Ado. Long-term treatment with the adenosine analogue 3-Deazaadenosine did not show significant effects on progression and stability of advanced atherosclerotic lesions in older apolipoprotein E-deficient mice. A potential antiatherosclerotic effect of c(3)Ado (eg, mediated through inhibition of adhesion molecules) might therefore be limited to prevention of early lesion formation and does not seem to play a relevant role in modifying advanced atherosclerotic disease.

Modulation by homocysteine of the iberiotoxin-sensitive, Ca2+ -activated K+ channels of porcine coronary artery smooth muscle cells

We evaluated the acute effect of homocysteine on the iberiotoxin-sensitive, Ca(2+)-activated K(+) (BK(Ca)) channels of the porcine coronary artery smooth muscle cells. NS 1619 (1 to 30 microM) caused a concentration-dependent enhancement of the BK(Ca) amplitude (recorded using the whole-cell, membrane-rupture configuration) only with an elevated [Ca(2+)](i) of approximately 444 nM, but not with [Ca(2+)](i) of approximately 100 nM. Homocysteine (30 microM) caused a small inhibition ( approximately 16%) of the BK(Ca) amplitude ([Ca(2+)](i)= approximately 444 nM), and a greater inhibition ( approximately 77%) was observed with 100 microM NADH present in the pipette solution. The inhibition persisted after washing. With NADPH (100 microM), a smaller magnitude of inhibition ( approximately 34%) of the BK(Ca) amplitude was recorded. The NS 1619-mediated enhancement of the BK(Ca) amplitude (with elevated [Ca(2+)](i) plus NADH in the pipette) was attenuated by homocysteine. The homocysteine-mediated inhibition of the BK(Ca) amplitude was suppressed by Tiron (10 mM) or diphenylene iodonium (30 nM), applied alone, but not by superoxide dismutase (500 U/ml) and catalase (500 U/ml). Generation of superoxide (O(2)(-)) of the smooth muscle cells (with NADH presence), measured using the lucigenin-enhanced chemiluminescence, was markedly increased by angiotensin II (100 nM) and homocysteine (30 microM). The chemiluminescence signal was sensitive to apocynin (300 microM) or Tiron, applied alone, but not to superoxide dismutase and catalase. In conclusion, our results demonstrate that acute homocysteine application inhibits the iberiotoxin-sensitive BK(Ca) channels (with elevated [Ca(2+)](i) and NADH present) which is probably caused by the NADH oxidase activation and the concomitant generation of intracellular superoxide.