Hyperhomocysteinemia and Age-related Macular Degeneration: Role of Inflammatory Mediators and Pyroptosis; A Proposal.

Singh M, Tyagi SC. Medical Hypotheses 105 (2017) 17 - 21 

Age-related macular degeneration (AMD) and pyroptosis cause irreversible vascular changes in the eyes leading to central vision loss in patients. It is the most common eye disease affecting millions of people aged 50years or older, and is slowly becoming a major health problem worldwide. The disease mainly affects macula lutea, an oval-shaped pigmented area surrounding fovea near the center of retina, a region responsible for visual acuity. It is fairly a complex disease as genetics of patients, environmental triggers as well as risk factors such as age, family history of CVDs, diabetes, gender, obesity, race, hyperopia, iris color, smoking, diabetes, exposure to sun light and pyroptosis have all been clubbed together as probable causes of macular degeneration. Among genes that are known to play a role include variant polymorphisms in the complement cascade components such as CFH, C2, C3, and CFB as potential genetic risk factors. So far, AMD disease hypothesized theories have not resulted into the anticipated impact towards the development of effective or preventive therapies in order to help alleviate patients' suffering because, as of today, it is still unclear what actually initiates or leads to this dreaded eye condition. Based upon our extensive work on the metabolism of homocysteine (Hcy) in various disease conditions we, therefore, are proposing a novel hypothesis for AMD pathogenesis as we strongly believe that Hcy and events such as pyroptosis make a greater contribution to the overall etiology of AMD disease in a target population of susceptible hosts by inciting and accelerating the inherent inflammatory changes in the retina of these patients (Fig. 2). In this context, we further state that Hcy and pyroptosis should be considered as legitimate and valuable markers of retinal dysfunction as they not only aid and abet in the development but also in the progression of AMD in older people as discussed in this paper. This discussion should open up new avenues in tackling inflammatory and pyroptosis centered pathways that are up-regulated or solely promoted by Hcy interaction within the ocular compartment of AMD susceptible hosts.

https://www.ncbi.nlm.nih.gov/pubmed/28735646

Homocysteine, folate, vitamin B-12, and 10-y incidence of age-related macular degeneration.

Gopinath B, Flood VM, Rochtchina E, Wang JJ, Mitchell P.  AM J Clin Nutr. 2013 Jul; 98(1): 129-35. 

BACKGROUND: Epidemiologic evidence of a relation between serum total homocysteine (tHcy), vitamin B-12, and folate and age-related macular degeneration (AMD) is inconsistent and unresolved.

OBJECTIVE: In this cohort study, we aimed to investigate associations between intakes and serum concentrations of folate and vitamin B-12 or serum tHcy and 10-y AMD incidence.

DESIGN: Serum folate, vitamin B-12, and tHcy were determined from blood samples drawn in 1997-1999 from cohort members aged ≥55 y. AMD was assessed in 1760 survivors from retinal photographs taken in 2002-2004 and 2007-2009. Total intakes of folate and vitamin B-12 were assessed by using a food-frequency questionnaire.

RESULTS: After adjustment for age, sex, current smoking, white blood cell count, and fish consumption, each 1-SD increase in serum tHcy was associated with increased risk of incident early and any AMD [ORs (95% CIs): 1.33 (1.09, 1.63) and 1.33 (1.11, 1.60), respectively]. Participants with a serum vitamin B-12 deficiency (<185 pmol/L) had higher risk of incident early and late AMD [ORs (95% CIs): 1.58 (1.06, 2.36) and 2.56 (1.38, 4.73), respectively]. Folate deficiency (<11 nmol/L) was associated with 75% and 89% increased risk of incident early and any AMD, respectively, 10 y later. Participants who reported supplementary vitamin B-12 intake had 47% reduced risk of incident any AMD (OR: 0.53; 95% CI: 0.33, 0.85).

CONCLUSION: Elevated serum tHcy and folate and vitamin B-12 deficiencies predicted increased risk of incident AMD, which suggests a potential role for vitamin B-12 and folate in reducing AMD risk.

https://www.ncbi.nlm.nih.gov/pubmed/23636242

Dietary folate, B vitamins, genetic susceptibility and progression to advanced nonexudative age-related macular degeneration with geographic atrophy: a prospective cohort study.

Merle BM, Silver RE, Rosner B, Seddon JM., AM J Clin Nutr. 2016 Apr;103(4):1135-44 

BACKGROUND: Epidemiologic evidence of a relation between serum total homocysteine (tHcy), vitamin B-12, and folate and age-related macular degeneration (AMD) is inconsistent and unresolved.

OBJECTIVE: In this cohort study, we aimed to investigate associations between intakes and serum concentrations of folate and vitamin B-12 or serum tHcy and 10-y AMD incidence.

DESIGN: Serum folate, vitamin B-12, and tHcy were determined from blood samples drawn in 1997-1999 from cohort members aged ≥55 y. AMD was assessed in 1760 survivors from retinal photographs taken in 2002-2004 and 2007-2009. Total intakes of folate and vitamin B-12 were assessed by using a food-frequency questionnaire.

RESULTS: After adjustment for age, sex, current smoking, white blood cell count, and fish consumption, each 1-SD increase in serum tHcy was associated with increased risk of incident early and any AMD [ORs (95% CIs): 1.33 (1.09, 1.63) and 1.33 (1.11, 1.60), respectively]. Participants with a serum vitamin B-12 deficiency (<185 pmol/L) had higher risk of incident early and late AMD [ORs (95% CIs): 1.58 (1.06, 2.36) and 2.56 (1.38, 4.73), respectively]. Folate deficiency (<11 nmol/L) was associated with 75% and 89% increased risk of incident early and any AMD, respectively, 10 y later. Participants who reported supplementary vitamin B-12 intake had 47% reduced risk of incident any AMD (OR: 0.53; 95% CI: 0.33, 0.85).

CONCLUSION: Elevated serum tHcy and folate and vitamin B-12 deficiencies predicted increased risk of incident AMD, which suggests a potential role for vitamin B-12 and folate in reducing AMD risk.

https://www.ncbi.nlm.nih.gov/pubmed/26961928

Folic acid, pyridoxine, and cyanocobalamin combination treatment and age-related macular degeneration in women: the Women's Antioxidant and Folic Acid Cardiovascular Study (WAFACS).

Christen WG, Glynn RJ, Chew EY, Albert CM, Manson JE. Arch Intern Med. 2009 Feb 23;169(4):335-41 

BACKGROUND: Observational epidemiologic studies indicate a direct association between homocysteine concentration in the blood and the risk of age-related macular degeneration (AMD), but randomized trial data to examine the effect of therapy to lower homocysteine levels in AMD are lacking. Our objective was to examine the incidence of AMD in a trial of combined folic acid, pyridoxine hydrochloride (vitamin B(6)), and cyanocobalamin (vitamin B(12)) therapy.

METHODS: We conducted a randomized, double-blind, placebo-controlled trial including 5442 female health care professionals 40 years or older with preexisting cardiovascular disease or 3 or more cardiovascular disease risk factors. A total of 5205 of these women did not have a diagnosis of AMD at baseline and were included in this analysis. Participants were randomly assigned to receive a combination of folic acid (2.5 mg/d), pyridoxine hydrochloride (50 mg/d), and cyanocobalamin (1 mg/d) or placebo. Our main outcome measures included total AMD, defined as a self-report documented by medical record evidence of an initial diagnosis after randomization, and visually significant AMD, defined as confirmed incident AMD with visual acuity of 20/30 or worse attributable to this condition.

RESULTS: After an average of 7.3 years of treatment and follow-up, there were 55 cases of AMD in the combination treatment group and 82 in the placebo group (relative risk, 0.66; 95% confidence interval, 0.47-0.93 [P = .02]). For visually significant AMD, there were 26 cases in the combination treatment group and 44 in the placebo group (relative risk, 0.59; 95% confidence interval, 0.36-0.95 [P = .03]).

CONCLUSIONS: These randomized trial data from a large cohort of women at high risk of cardiovascular disease indicate that daily supplementation with folic acid, pyridoxine, and cyanocobalamin may reduce the risk of AMD.

https://www.ncbi.nlm.nih.gov/pubmed/19237716

Homocysteine and the risk of age-related macular degeneration: a systematic review and meta-analysis.

Huang P, Wang F, Sah BK, Jiang J, NiZ, Wang J, Sun X. Sci Rep. 2015 Jul 21;5:10585   

Contrasting results have been reported regarding the associations between plasma total homocysteine (tHcy) and B vitamin levels and age-related macular degeneration (AMD) risk. Thus, we aimed to systematically evaluate these associations. Relevant case control studies in English were identified via a thorough search of the PubMed, Medline, and Embase databases from inception to June 2014. The results were pooled using Review Manager 5.2.1. Eleven studies (including 1072 cases and 1202 controls) were eligible for analysis of tHcy levels; additionally, 3 studies (including 152 cases and 98 controls) were eligible for analysis of folic acid and vitamin B12 levels. The cumulative results demonstrated that the plasma tHcy level among the AMD cases was 2.67 μmol/L (95% confidence interval [CI], 1.60-3.74) higher than that among the controls. In contrast, the vitamin B12 level among the AMD cases was 64.16 pg/mL (95% CI, 19.32-109.00) lower than that among the controls. Subgroup analyses showed that the folic acid level was 1.66 ng/mL (95% CI, 0.10-3.21) lower for the wet type. Together, the results demonstrated that AMD is associated with elevated tHcy levels and decreased vitamin B12 levels. Plasma tHcy may act as a modulator of the risk for AMD based on the current evidence.

https://www.ncbi.nlm.nih.gov/pubmed/26194346

Dietary Nutrient Intake and Progression to Late Age-Related Macular Degeneration in the Age-Related Eye Disease Studies 1 and 2

Elvira Agrón, MA, Julie Mares, PhD, Traci E. Clemons, PhD, Anand Swaroop, PhD, Emily Y. Chew, MD,
Tiarnan D.L. Keenan, BM BCh, PhD, for the AREDS and AREDS2 Research Groups

Purpose: To analyze associations between the dietary intake of multiple nutrients and risk of progression to late age-related macular degeneration (AMD), its subtypes, and large drusen.

Design: Post hoc analysis of 2 controlled clinical trial cohorts: Age-Related Eye Disease Study (AREDS) and AREDS2.

Participants: Eyes with no late AMD at baseline among AREDS participants (n = 4504) and AREDS2 participants (n = 3738) totaled 14 135 eyes. Mean age was 71.0 years (standard deviation, 6.7 years), and 56.5% of patients were women.

Methods: Fundus photographs were collected at annual study visits and graded centrally for late AMD. Dietary intake of multiple nutrients was calculated from food frequency questionnaires.

Main Outcome Measures: Progression to late AMD, geographic atrophy (GA), neovascular AMD, and (separate analyses) large drusen.

Results: Over median follow-up of 10.2 years, of the 14 135 eyes, 32.7% progressed to late AMD. For 9 nutrients, intake quintiles 4 or 5 (vs. 1) were associated significantly (P ≤ 0.0005) with decreased risk of late AMD: vitamin A, vitamin B6, vitamin C, folate, b-carotene, lutein and zeaxanthin, magnesium, copper, and alcohol. For 3 nutrients, quintiles 4 or 5 were associated significantly with increased risk: saturated fatty acid, monounsaturated fatty acid, and oleic acid. Similar results were observed for GA. Regarding neovascular AMD, 9 nutrients were associated nominally with decreased risk - vitamin A, vitamin B6, b-carotene, lutein and zeaxanthin, magnesium, copper, docosahexaenoic acid, omega-3 fatty acid, and alcohol - and 3 nutrients were associated with increased riskdsaturated fatty acid, monounsaturated fatty acid, and oleic acid. In separate analyses (n = 5399 eyes of 3164 AREDS participants), 12 nutrients were associated nominally with decreased risk of large drusen.

Conclusions: Higher dietary intake of multiple nutrients, including minerals, vitamins, and carotenoids, is associated with decreased risk of progression to late AMD. These associations are stronger for GA than for neovascular AMD. The same nutrients also tend to show protective associations against large drusen development. Strong genetic interactions exist for some nutrient-genotype combinations, particularly omega-3 fatty acids and CFH. These data may justify further research into underlying mechanisms and randomized trials of supplementation.

Ophthalmology 2021;128:425-442 Published by Elsevier on behalf of the American Academy of Ophthalmology

https://doi.org/10.1016/j.ophtha.2020.08.018

Association between vitamin D status and age-related macular degeneration by genetic risk

Amy E. Millen, Kristin J Meyers, Zhe Liu, Corinne D Engelman, Robert B Wallace, Erin S LeBlanc, Lesley F. Tinker, Sudha K Iyengar, Jennifer Robinson. Gloria E. Sarto and Jule A Mares, JAMA Ophthalmol. 2015 Oct; 133(10): 1171-117

IMPORTANCE: Deficient 25-hydroxyvitamin D (25[OH]D) concentrations have been associated with increased odds of age-related macular degeneration (AMD).

OBJECTIVE: To examine whether this association is modified by genetic risk for AMD and whether there is an association between AMD and single-nucleotide polymorphisms of genes involved in vitamin D transport, metabolism, and genomic function.

DESIGN, SETTING, AND PARTICIPANTS: Postmenopausal women (N = 913) who were participants of the Carotenoids in Age-Related Eye Disease Study (CAREDS) (aged 54 to <75 years) with available serum 25(OH)D concentrations (assessed October 1, 1993, to December 31, 1998), genetic data, and measures of AMD (n = 142) assessed at CAREDS baseline from May 14, 2001, through January 31, 2004, were studied.

MAIN OUTCOMES AND MEASURES: Prevalent early or late AMD was determined from graded, stereoscopic fundus photographs. Logistic regression was used to estimate odds ratios (ORs) and 95% CIs for AMD by the joint effects of 25(OH)D (<12, ≥12 to <20, ≥20 to <30, and ≥30 ng/mL) and risk genotype (noncarrier, 1 risk allele, or 2 risk alleles). The referent group was noncarriers with adequate vitamin D status (≥30 ng/mL). Joint effect ORs were adjusted for age, smoking, iris pigmentation, self-reported cardiovascular disease, self-reported diabetes status, and hormone use. Additive and multiplicative interactions were assessed using the synergy index (SI) and an interaction term, respectively. To examine the association between AMD and variants in vitamin D-related genes, age-adjusted ORs and 95% CIs were estimated using logistic regression.

RESULTS: Among the 913 women, 550 had adequate levels of vitamin D (≥20 ng/mL), 275 had inadequate levels (≥12 to <20 mg/mL), and 88 had deficient levels (<12 ng/mL). A 6.7-fold increased odds of AMD (95% CI, 1.6-28.2) was observed among women with deficient vitamin D status (25[OH]D <12 ng/mL) and 2 risk alleles for CFH Y402H (SI for additive interaction, 1.4; 95% CI, 1.1-1.7; P for multiplicative interaction = .25). Significant additive (SI, 1.4; 95% CI, 1.1-1.7) and multiplicative interactions (P = .02) were observed for deficient women with 2 high-risk CFI (rs10033900) alleles (OR, 6.3; 95% CI, 1.6-24.2). The odds of AMD did not differ by genotype of candidate vitamin D genes.

CONCLUSIONS AND RELEVANCE: In this study, the odds of AMD were highest in those with deficient vitamin D status and 2 risk alleles for the CFH and CFI genotypes, suggesting a synergistic effect between vitamin D status and complement cascade protein function. Limited sample size led to wide CIs. Findings may be due to chance or explained by residual confounding.

https://www.ncbi.nlm.nih.gov/pubmed/26312598

L-5-Methyltetrahydrofolate Supplementation Increases Blood Folate Concentrations to a Greater Extent than Folic Acid Supplementation in Malaysian Women

Amanda M Henderson, Rika E Aleliunas, Su Peng Lo, Geok Lin Khor, Sarah Harvey-Leeson, Melissa B Glier, David D Kitts, Tim J Green and Angela M Devlin, J Nutr. 2018 Jun 1M; 148(6): 885-890 

BACKGROUND: Folic acid fortification of grains is mandated in many countries to prevent neural tube defects. Concerns regarding excessive intakes of folic acid have been raised. A synthetic analog of the circulating form of folate, l-5-methyltetrahydrofolate (l-5-MTHF), may be a potential alternative.

OBJECTIVE: The objective of this study was to determine the effects of folic acid or l-5-MTHF supplementation on blood folate concentrations, methyl nutrient metabolites, and DNA methylation in women living in Malaysia, where there is no mandatory fortification policy.

METHODS: In a 12-wk, randomized, placebo-controlled intervention trial, healthy Malaysian women (n = 142, aged 20-45 y) were randomly assigned to receive 1 of the following supplements daily: 1 mg (2.27 μmol) folic acid, 1.13 mg (2.27 μmol) l-5-MTHF, or a placebo. The primary outcomes were plasma and RBC folate and vitamin B-12 concentrations. Secondary outcomes included plasma total homocysteine, total cysteine, methionine, betaine, and choline concentrations and monocyte long interspersed nuclear element-1 (LINE-1) methylation.

RESULTS: The folic acid and l-5-MTHF groups had higher (P < 0.001) RBC folate (mean ± SD: 1498 ± 580 and 1951 ± 496 nmol/L, respectively) and plasma folate [median (25th, 75th percentiles): 40.1 nmol/L (24.9, 52.7 nmol/L) and 52.0 nmol/L (42.7, 73.1 nmol/L), respectively] concentrations compared with RBC folate (958 ± 345 nmol/L) and plasma folate [12.6 nmol/L (8.80, 17.0 nmol/L)] concentrations in the placebo group at 12 wk. The l-5-MTHF group had higher RBC folate (1951 ± 496 nmol/L; P = 0.003) and plasma folate [52.0 nmol/L (42.7, 73.1 nmol/L); P = 0.023] at 12 wk than did the folic acid group [RBC folate, 1498 ± 580 nmol/L; plasma folate, 40.1 nmol/L (24.9, 52.7 nmol/L)]. The folic acid and l-5-MTHF groups had 17% and 15%, respectively, lower (P < 0.001) plasma total homocysteine concentrations than did the placebo group at 12 wk; there were no differences between the folic acid and l-5-MTHF groups. No differences in plasma vitamin B-12, total cysteine, methionine, betaine, and choline and monocyte LINE-1 methylation were observed.

CONCLUSION: These findings suggest differential effects of l-5-MTHF compared with folic acid supplementation on blood folate concentrations but no differences on plasma total homocysteine lowering in Malaysian women. This trial was registered at clinicaltrials.gov as NCT01584050.

https://www.ncbi.nlm.nih.gov/pubmed/29878267

Folate, folic acid and 5-methyltetrahydrofolate are not the same thing

Francesco Scaglione, Giscardo Panzavolta, Xenobiotica, 2014; 44(5): 480-488 

1. Folate, an essential micronutrient, is a critical cofactor in one-carbon metabolism. Mammals cannot synthesize folate and depend on supplementation to maintain normal levels. Low folate status may be caused by low dietary intake, poor absorption of ingested folate and alteration of folate metabolism due to genetic defects or drug interactions. 2. Folate deficiency has been linked with an increased risk of neural tube defects, cardiovascular disease, cancer and cognitive dysfunction. Most countries have established recommended intakes of folate through folic acid supplements or fortified foods. External supplementation of folate may occur as folic acid, folinic acid or 5-methyltetrahydrofolate (5-MTHF). 3. Naturally occurring 5-MTHF has important advantages over synthetic folic acid - it is well absorbed even when gastrointestinal pH is altered and its bioavailability is not affected by metabolic defects. Using 5-MTHF instead of folic acid reduces the potential for masking haematological symptoms of vitamin B12 deficiency, reduces interactions with drugs that inhibit dihydrofolate reductase and overcomes metabolic defects caused by methylenetetrahydrofolate reductase polymorphism. Use of 5-MTHF also prevents the potential negative effects of unconverted folic acid in the peripheral circulation. 4. We review the evidence for the use of 5-MTHF in preventing folate deficiency.

https://www.ncbi.nlm.nih.gov/pubmed/24494987

Relationship between homocysteine level and diabtetic retinopathy: a systematic review and meta-analysis

Xu et al. Diagnostic Pathology 2014, 13:167 

Background: The relationship between homocysteine (Hcy) and diabetic retinopathy (DR) remains unclear to date. Therefore, a systematic review and meta-analysis was performed on the relationship between Hcy level and DR.

Methods: Studies were identified by searching PubMed, Embase, and Web of Science databases until 5 May, 2014.

Results: A total of 31 studies involving 6,394 participants were included in the meta-analysis. After pooling the data from each included study, the blood Hcy concentration in the DR group was observed to be higher than that in the control group [WMD = 2.55; 95% confidence interval (CI), 1.70-3.40], and diabetes mellitus (DM) patients with hyperhomocysteinemia were at a risk for DR [odds ratio (OR) = 1.93; 95% CI, 1.46-2.53]. Considering the different DM types, hyperhomocysteinemia in T1DM (OR = 1.83, 95% CI, 1.28-2.62) was associated with DR rather than in T2DM (OR = 1.59, 95% CI, 0.72-3.51). Considerable statistical heterogeneity in the overall summary estimates was partly explained by the geographical differences.

Conclusions: Results from this current meta-analysis indicate that hyperhomocysteinemia is a risk factor for DR, especially proliferative DR. Differences between geographical regions were observed in the relationship between hyperhomocysteinemia with T1DM risk. Given the heterogeneous results, the relationship between high Hcy and DR needs further investigation.

https://diagnosticpathology.biomedcentral.com/articles/10.1186/s13000-014-0167-y

Vitamin Status as a Determinant of Serum Homocysteine Concetration in Type 2 Diabetic Retinopathy

Fotiou et al., Journal of Diabetes Research, Volume 2014, Article ID 807209, 7 

We investigated the association of serum homocysteine levels and vitamin status with type 2 diabetic retinopathy. This study included 65 patients with and 75 patients without diabetic retinopathy. Patients with diabetic retinopathy had significantly higher serum homocysteine levels, higher prevalence of hyperhomocysteinemia, lower serum folic acid, and vitamin B12 () levels than those without diabetic retinopathy. Regression analysis revealed that homocysteine was an independent risk factor for diabetic retinopathy and there was a threshold in its serum level (13.7 μmol/L), above which the risk of diabetic retinopathy greatly increases (, ). Folic acid was associated with decreased odds for diabetic retinopathy (, ). There was a threshold in serum vitamin B12 level (248.4 pg/mL), below which serum homocysteine concentration significantly increases with decreasing serum vitamin B12 (). Our findings suggest that hyperhomocysteinemia is an independent risk factor for the development and progression of diabetic retinopathy. Decreased serum levels of folic acid and vitamin B12, through raising serum homocysteine concentrations, may also affect the diabetic retinopathy risk.

https://www.hindawi.com/journals/jdr/2014/807209/

Status of B-Vitamins and Homocysteine in Diabetic Retinopathy: Assocaiation with Vitamin-B12 Deficiency and Hyperhomocysteinemia.

Satyanarayana A, Balakrishna N, Pitla S, Reddy PY, Mudili S, et al. (2011) Status of B-Vitamins and Homocysteine in Diabetic Retinopathy: Association with Vitamin-B12 Deficiency and Hyperhomocysteinemia. PLos one, 6(2011)1  

Diabetic retinopathy (DR) is a common cause of blindness. Although many studies have indicated an association between homocysteine and DR, the results so far have been equivocal. Amongst the many determinants of homocysteine, B-vitamin status was shown to be a major confounding factor, yet very little is known about its relationship to DR. In the present study, we, therefore, investigated the status of B-vitamins and homocysteine in DR. A cross-sectional case-control study was conducted with 100 normal control (CN) subjects and 300 subjects with type-2 diabetes (T2D). Of the 300 subjects with T2D, 200 had retinopathy (DR) and 100 did not (DNR). After a complete ophthalmic examination including fundus fluorescein angiography, the clinical profile and the blood levels of all B-vitamins and homocysteine were analyzed. While mean plasma homocysteine levels were found to be higher in T2D patients compared with CN subjects, homocysteine levels were particularly high in the DR group. There were no group differences in the blood levels of vitamins B1 and B2. Although the plasma vitamin-B6 and folic acid levels were significantly lower in the DNR and DR groups compared with the CN group, there were no significant differences between the diabetes groups. Interestingly, plasma vitamin-B12 levels were found to be significantly lower in the diabetes groups compared with the CN group; further, the levels were significantly lower in the DR group compared with the DNR group. Higher homocysteine levels were significantly associated with lower vitamin-B12 and folic acid but not with other B-vitamins. Additionally, hyperhomocysteinemia and vitamin-B12 deficiency did not seem to be related to subjects' age, body mass index, or duration of diabetes. These results thus suggest a possible association between vitamin-B12 deficiency and hyperhomocysteinemia in DR. Further, the data indicate that vitamin-B12 deficiency could be an independent risk factor for DR.

https://www.ncbi.nlm.nih.gov/pubmed/22069468

Hyperhomocysteinemia Alters Retinal Endothelial Cells Barrier Function and Angiogenic Potential via Activation of Oxidative Stress

Riyaz Mohamed, Isha Sharma, Ahmed S. Ibrahim, Heba Saleh, Nehal M. Elsherbiny, Sadanand Fuzele, Khaled Elmasry, Sylvia B. Smith, Mohamed Al-Shabrawey, Amany Tawfik, Nature Scientific Reports (2017), 7: 11952

Hyperhomocysteinemia (HHcy) is associated with several human visual disorders, such as diabetic retinopathy (DR) and age-related macular degeneration (AMD). Breakdown of the blood-retinal barrier (BRB) is linked to vision loss in DR and AMD. Our previous work revealed that HHcy altered BRB in retinal endothelial cells in vivo. Here we hypothesize that homocysteine (Hcy) alters retinal endothelial cell barrier function and angiogenic potential via activation of oxidative stress. Human retinal endothelial cells (HRECs) treated with and without different concentrations of Hcy showed a reduction of tight junction protein expression, increased FITC dextran leakage, decreased transcellular electrical resistance and increased angiogenic potential. In addition, HRECs treated with Hcy showed increased production of reactive oxygen species (ROS). The anti-oxidant N-acetyl-cysteine (NAC) reduced ROS formation and decreased FITC-dextran leakage in Hcy treated HRECs. A mouse model of HHcy, in which cystathionine-β-synthase is deficient (cbs −/−), was evaluated for oxidative stress by dichlolorofluorescein (DCF), dihydroethidium (DHE) staining. There was a marked increase in ROS production and augmented GSH reductase and antioxidant regulator NRF2 activity, but decreased antioxidant gene expression in retinas of hyperhomocysteinemic mice. Our results suggest activation of oxidative stress as a possible mechanism of HHcy induced retinal endothelial cell dysfunction.

https://www.nature.com/articles/s41598-017-09731-y

The Association between Vitamin D Deficiency and Diabetic Retinopathy in Type 2 Diabetes: A Meta-Analysis of Observational Studies

Qin et al., Nutrients 2017, 9, 307 

Emerging evidence from in vivo and in vitro studies have shown that vitamin D may play an important role in the development of diabetic retinopathy (DR), but individually published studies showed inconclusive results. The aim of this study was to quantitatively summarize the association between vitamin D and the risk of diabetic retinopathy. We conducted a systematic literature search of Pubmed, Medline, and EMBASE updated in September 2016 with the following keywords: “vitamin D” or “cholecalciferol” or “25-hydroxyvitamin D” or “25(OH)D” in combination with “diabetic retinopathy” or “DR”. Fifteen observational studies involving 17,664 subjects were included. In this meta-analysis, type 2 diabetes patients with vitamin D deficiency (serum 25(OH)D levels <20 ng/mL) experienced a significantly increased risk of DR (odds ratio (OR) = 2.03, 95% confidence intervals (CI): 1.07, 3.86), and an obvious decrease of 1.7 ng/mL (95% CI: −2.72, −0.66) in serum vitamin D was demonstrated in the patients with diabetic retinopathy. Sensitivity analysis showed that exclusion of any single study did not materially alter the overall combined effect. In conclusion, the evidence from this meta-analysis indicates an association between vitamin D deficiency and an increased risk of diabetic retinopathy in type 2 diabetes patients.

https://www.mdpi.com/2072-6643/9/3/307

Expression and Differential Polarization of the reduced-folate transporter-1 and the folate receptor alpha in mammalian retinal pigment epithelium

Smith, Chancy et al., J Biol Chem., Vol. 275, No. 27, Issue of July 7, pp. 20676-20684, 2000

The differential polarized distribution of the reduced- folate transporter (RFT-1) and folate receptor alpha (FRalpha), the two proteins involved in the transport of folate, has been characterized in normal mouse retinal pigment epithelium (RPE) and in cultured human RPE cells. RPE cells mediate the vectorial transfer of nutrients from choroidal blood to neural retina. Whereas FRalpha is known to be present in many cell types of the neural retina, in situ hybridization analysis in the present study demonstrated that RFT-1 is present only in RPE. Laser-scanning confocal microscopy using antibodies specific for RFT-1 demonstrated an apical distribution of this protein in cultured human and intact mouse RPE, which contrasts with the basolateral distribution of FRalpha in these cells. The expression of RFT-1 in the RPE cell apical membrane was confirmed by functional studies with purified apical membrane vesicles from bovine RPE. These studies, done with N(5)-methyltetrahydrofolate (the predominant folate derivative in blood) and folate as substrates, have shown that RFT-1 functions in a Na(+)- and C1(-)-independent manner. The transporter is specific for folate and its analogs. A transmembrane H(+) gradient influences the transport function of this protein markedly; the transport mechanism is likely to be either folate/H(+) co-transport or folate/OH(-) exchange. Based on the differential polarization of FRalpha and RFT-1 in RPE, we suggest that these two proteins work in a concerted manner to bring about the vectorial transfer of folate across the RPE cell layer from the choroidal blood to the neural retina. This constitutes the first report of the differential polarization of the two folate transport proteins in any polarized epithelium. 

https://www.ncbi.nlm.nih.gov/pubmed/10787414

Folate nutrition and blood-brain barrier dysfunction

Patrick J Stover, Jane Durga and Martha S Field, Current Opinion in Biotechnology 2017, 44:146-152

Mammals require essential nutrients from dietary sources to support normal metabolic, physiological and neuronal functions, to prevent diseases of nutritional deficiency as well as to prevent chronic disease. Disease and/or its treatment can modify fundamental biological processes including cellular nutrient accretion, stability and function in cells. These effects can be isolated to a specific diseased organ in the absence of whole-body alterations in nutrient status or biochemistry. Loss of blood-brain barrier function, which occurs in in-born errors of metabolism and in chronic disease, can cause brain-specific folate deficiency and contribute to disease co-morbidity. The role of brain folate deficiency in neuropsychiatric disorders is reviewed, as well as emerging diagnostic and nutritional strategies to identify and address brain folate deficiency in blood-brain barrier dysfunction.

https://www.ncbi.nlm.nih.gov/pubmed/28189938