Kathy R. Bailey

Expression of reverse cholesterol transport proteins ATP-binding cassette A1 (ABCA1) and scavenger receptor BI (SR-BI) in the retina and retinal pigment epithelium

Aims: Excessive lipid accumulation in Bruch’s membrane (BrM) is a hallmark of ageing, the major risk factor for age-related macular degeneration (AMD). Retinal pigment epithelial (RPE) cells may utilise reverse cholesterol transport (RCT) activity to move lipid into BrM, mediated through ATP-binding cassette A1 (ABCA1) and scavenger receptor BI (SR-BI). Methods: ABCA1 expression was assessed by reverse transcription polymerase chain reaction (RT-PCR) and western blotting of human RPE cell extracts. Lipid transport assays were performed using radiolabelled photoreceptor outer segments (POS). ABCA1 and SR-BI expression was examined in normal mouse eyes by immunofluorescence staining. BrMs of ABCA1 and SR-BI heterozygous mice were examined microscopically. Results: Human RPE cells expressed ABCA1 mRNA and protein. The ABCA1 and SR-BI inhibitor glyburide (also known as glibenclamide) abolished basal transport of POS-derived lipids in RPE cells in the presence of high-density lipoprotein. Mouse retina and RPE expressed ABCA1 and SR-BI. SR-BI was highly expressed in RPE. BrMs were significantly thickened in SR-BI heterozygous mice, but not in ABCA1 heterozygous mice. Conclusion: RPE cells express ABCA1 and SR-BI. This implies a significant role for SR-BI and ABCA1 in lipid transport and RCT in the retina and RPE.

High density lipoprotein mediated lipid efflux from retinal pigment epithelial cells in culture

Backgound/aim: The transport of radiolabelled photoreceptor outer segments (POS) lipids was investigated by cultured retinal pigment epithelial cells (RPE). Phagocytosis of POS by the RPE is essential to maintain the health and function of the photoreceptors in vivo. POS are phagocytised at the apical cell surface of RPE cells. Phagocytised POS lipids may be either recycled to the photoreceptors for reincorporation into new POS or they may be transported to the basolateral surface for efflux into the circulation. Results: The authors have demonstrated that high density lipoprotein (HDL) stimulates efflux of radiolabelled lipids, of POS origin, from the basal surface of RPE cells in culture. Effluxed lipids bind preferentially to HDL species of low and high molecular weight. Effluxed radiolabelled phosphotidyl choline was the major phospholipid bound to HDL, with lesser amounts of phosphatidyl ethanolamine, phosphatidyl inosotol. Effluxed radiolabelled triglycerides, cholesterol, and cholesterol esters also bound to HDL. Lipid free apolipoprotein A-I (apoA-I) and apoA-I containing vesicles also stimulate lipid efflux. Conclusion: The findings suggest a role for HDL and apoA-I in regulating lipid and cholesterol transport from RPE cells that may influence the pathological lipid accumulation associated with age related macular degeneration.

Hypothermia reduces secretion of vascular endothelial growth factor by cultured retinal pigment epithelial cells.

Aim Visual loss in age-related macular degeneration usually develops secondary to choroidal neovascularisation. Vascular endothelial growth factor (VEGF) is a critical regulator of retinal angiogenesis and vascular permeability, especially in hypoxic conditions. We hypothesise that hypothermia may reduce the retinal pigment epithelium (RPE) metabolism and, consequently, the levels of VEGF secretion by cultured RPE cells under hypoxic conditions. The effects of hypothermia were compared with the metabolic inhibiting effects of thiopental and nicotinamide. Methods ARPE-19 cells were grown in culture for up to 5 days under normoxic (20% O2) and hypoxic (1% O2) conditions at temperatures ranging from 27°C to 40°C. For experiments with pharmacological agents, thiopental and nicotinamide were added to the media. VEGF levels in the media were measured by ELISA and cell metabolic activity was measured by a fluorescent cell metabolic assay. Results We found that hypothermia reduced ARPE-19 cell metabolism in a temperature-dependent fashion. Hypothermia also reduced ARPE-19 cell VEGF secretion in a temperature-dependent fashion. ARPE-19 cell VEGF secretion was reduced by 38% at 34°C compared with cells grown at 37°C. Conversely, ARPE-19 cell VEGF secretion was increased by 32% at 40°C compared with cells grown at 37°C. Hypoxia increased ARPE-19 cell VEGF secretion by 84% at 37°C. However, hypothermia decreased the hypoxia-induced increase of ARPE-19 cell VEGF secretion by 30%. The effect of hypothermia on ARPE-19 cell VEGF secretion was reversible after 4 days. In contrast to hypothermia, thiopental and nicotinamide were able to reduce RPE cell metabolism but not VEGF secretion. Conclusion Hypothermia decreases both VEGF secretion and cellular metabolism in ARPE-19 cells. Hypothermia also mitigates the hypoxia-induced increase in ARPE-19 cell VEGF secretion. These effects of hypothermia are potentially unique and cannot be obtained by other pharmacological agents that slow cellular metabolism.

Human trabecular meshwork cells as a thyroid hormone target tissue: presence of functional thyroid hormone receptors

Abstract. · Purpose: To deter- mine whether human trabecular meshwork cells (HTM) are a po- tential target tissue for thyroid hormone (3,3′,5-triiodothyronine, T3). · Methods: Cultured HTM were assayed for the presence of thyroid hormone receptors (TRs) and retinoid X receptors (RXRs) by reverse-transcriptase polymerase chain reaction (RT-PCR) to detected TR and RXR mRNA, and by immunohistochemistry to detect nuclear TR and RXR proteins. Functionality of the TR was determined by analysis of 125I-T3 binding affinity and capacity in HTM nuclear extracts. Effects of T3 on modulation of hyaluronic acid (HA) levels in HTM were measured as a function of dose and duration of T3 administration. · Results: Analysis of RT-PCR and immunohistochemistry demonstrated that cultured HTM expressed TRα1, TRα2, and TRβ1 but not TRβ2; and RXRα but not RXRβ and RXRγ isoforms. Saturation binding and analysis of 125I-T3 to HTM nuclear extracts revealed Kd of 57 pM. The number of T3 binding sites extrapolated from a Scatchard plot was 7.3×1010/μg of HTM nuclear protein extract. T3 supplementation reduced the concentration of HA in the cell medium by 32–43% compared to cells grown in the absence of T3. · Conclusions: Cultured HTM express three TR isoforms and one RXR isoform, bind T3 with an affinity similar to that of TR in responsive cells, and modulate their HA production in response to T3. These findings indicate that the human trabecular meshwork tissue has the capacity to respond to thyroid hormones.

Advantages and Limitations of Androgen Receptor-Based Methods for Detecting Anabolic Androgenic Steroid Abuse as Performance Enhancing Drugs

Testosterone (T) and related androgens are performance enhancing drugs (PEDs) abused by some athletes to gain competitive advantage. To monitor unauthorized androgen abuse, doping control programs use mass spectrometry (MS) to detect androgens, synthetic anabolic-androgenic steroids (AASs) and their metabolites in an athlete’s urine. AASs of unknown composition will not be detected by these procedures. Since AASs achieve their anabolic effects by activating the Androgen Receptor (AR), cell-based bioassays that measure the effect of a urine sample on AR activity are under investigation as complementary, pan-androgen detection methods. We evaluated an AR BioAssay as a monitor for androgen activity in urine pre-treated with glucuronidase, which releases T from the inactive T-glucuronide that predominates in urine. AR BioAssay activity levels were expressed as ‘T-equivalent’ concentrations by comparison to a T dose response curve. The T-equivalent concentrations of androgens in the urine of hypogonadal participants supplemented with T (in whom all androgenic activity should arise from T) were quantitatively identical to the T measurements conducted by MS at the UCLA Olympic Analytical Laboratory (0.96 ± 0.22). All 17 AASs studied were active in the AR BioAssay; other steroids were inactive. 12 metabolites of 10 commonly abused AASs, which are used for MS monitoring of AAS doping because of their prolonged presence in urine, had reduced or no AR BioAssay activity. Thus, the AR BioAssay can accurately and inexpensively monitor T, but its ability to monitor urinary AASs will be limited to a period immediately following doping in which the active AASs remain intact.