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J. W. Lee, E. F. Moreira, I. R. Rodriguez; Oxysterols Induce the Expression of ABCA1 and ABCG1 in Cultured RPE Cells. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5892.
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The ATP-binding cassette transporters ABCA1 and ABCG1 are known HDL transporters that play a significant role in cellular export of cholesterol and phospholipids in macrophages and other cells types. However, little is known about their function in the retina. The purpose of this study is to evaluate the effects of different oxysterols on the expression of ABCA1 and ABCG1 in cultured RPE cells.
ARPE19 and D407 cells were incubated with different oxysterols (10 µM) and total RNA was isolated after 24 hr of treatment. ABCA1 and ABCG1 mRNA expression was determined in human tissues and cultured cells by real time qRT-PCR. Immunohistochemical localization of ABCA1 and ABCG1 was performed on monkey retina vibrotome sections (100 µm). For immunoblot and immunohistochemical analyses commercially available rabbit polyclonal antibodies specific to ABCA1 and ABCG1 were purchased from Novus Biological.
The expression of ABCA1 and ABCG1 mRNA is 1.4- and 2.5-fold greater in human retina than in liver, respectively. ABCA1 mRNA was detected at 35-fold greater levels in ARPE19 cells than in D407 cells. By contrast, ABCG1 mRNA was detected in D407 cells at 13-fold greater levels than in ARP19 cells. Immunohistochemistry on monkey retina showed that both transporters were localized mainly to the ganglion cell layer, outer plexiform layer and RPE. Notably, ABCA1 seems to localize more intensively to the apical side of the RPE while ABCG1 localizes to the basal side. Treatment with different oxysterols revealed that 22(R)-hydroxycholesterol induced a 10-fold increase in ABCA1 mRNA in ARPE19 cells and a 18-fold increase of ABCG1 in D407 cells. Immunoblots also demonstrated a significant increase in protein expression for both transporters.
22(R)-hydroxycholesterol is a known activator of liver X- receptors and was the most potent transcription inducer of ABCA1 and ABCG1. The location of the ABCA1 and ABCG1 transporters in the retina suggests HDL-based intraretinal lipid transport. Their apparent location on opposite sides of the RPE suggests they may perform different functions.
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