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Binxing Li, Evan W. George, Aruna Gorusupudi, Fu-Yen Chang, Gregory T. Rognon, Jeanne M. Frederick, Paul S Bernstein; HDL-Cholesterol Receptor SR-BI May Serve as a Macular Carotenoid Transporter. Invest. Ophthalmol. Vis. Sci. 2019;60(9):1739.
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Low content of macular carotenoid is associated with higher risk of age-related macular degeneration (AMD). Humans do not have the enzymes to synthesize carotenoids and must obtain them from the diet, making it important to understand the mechanism of macular carotenoid transport. Macular carotenoids lutein and zeaxanthin are thought to be transported into the retina from the bloodstream through the HDL–cholesterol transport pathway. In this work, we investigated whether HDL-cholesterol receptor SR-BI may serve as a macular carotenoid transporter.
We first detected SR-BI expression in the human macula, peripheral retina, macular RPE and peripheral RPE using western blot. We then investigated SR-BI’s distribution in the monkey retinal sections using immunohistochemistry (IHC). Next, cell culture assays were employed to determine if and how SR-BI participates in macular carotenoid transport. Carotenoid content of HEK293 cells expressing with and without SR-BI was measured by HPLC after 24-h incubation with lutein, zeaxanthin or β-carotene solution containing detergent Tween 40. To further examine whether carotenoids influx into cells though the cholesterol tunnel of SR-BI, a key amino acid cysteine 384 (C384) was substituted with tyrosine (Y), generating a SR-BI mutant, C384Y. Carotenoid content of HEK293 cells expressing with wild type SR-BI or C384 mutant was measured by HPLC.
Western blot detected a strong protein band at ~80 kD in each of the protein sample from the human macula, peripheral retina, macular RPE and peripheral RPE. IHC demonstrated that SR-BI is localized in the ganglion cell layer (GCL), photoreceptor layer and retinal pigment epithelium (RPE) on monkey retinal section. The contents of lutein, zeaxanthin and β-carotene in HEK293 cells expressing with SR-BI were about 1.2, 1.3 and 1.8 times higher than the controls. On the other hand, these three carotenoid content in HEK293 cells expressing with SR-BI mutant (C384Y) were decreased to around 68%, 70%, and 56% of the wild type SR-BI.
Our data showed that SR-BI may serve as a macular carotenoid transporter, and carotenoids influx into cells through the cholesterol tunnel in SR-BI. These results will facilitate our understanding on the mechanism of macular carotenoid transport.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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