High-Density Lipoprotein (HDL) Can Transport Carotenoids to the Mouse Retina

Abstract

Purpose : Carotenoid supplementation can increase the macular carotenoid level and reduce the risk of age-related macular degeneration (AMD); however the mechanism of macular carotenoid transport remains unclear. It is thought that carotenoids are transported into the retina by LDL (this lipoprotein carries cholesterol to the peripheral tissues) rather than HDL (this lipoprotein transport cholesterol from the peripheral tissues); however, genetic studies demonstrate that variants in many genes related to the HDL metabolism are associated with macular carotenoid levels. Here, we investigated if deleting Apo A1, the key apolipoprotein of HDL, may reduce the carotenoids in the retina of “macular pigment mice” (BCO2 knockout mice).

Methods : Apo A1 knockout (apo A1^-/-) mice were crossed into BCO2 knockout (bco2^-/-) mice to create Apo A1/BCO2 double knockout (apo A1^-/-/bco2^-/-) mice. Twenty apo A1^-/-/bco2^-/- mice and eighteen bco2^-/- mice then were used to perform a carotenoid feeding experiment when they were two months old. They first were fed vitamin A-deficient chow for one month to increase carotenoid absorption, and then changed to mixed-carotenoid chow (lutein: zeaxanthin: β-carotene=1:1:1) for another one month; carotenoids were subsequently extracted from the retina, RPE/choroid, serum, and liver of these mice and analyzed by HPLC. In addition, we determined the binding affinities between human Apo A1 protein and various carotenoids using surface plasmon resonance (SPR) spectroscopy and examined the expression of HDL-cholesterol receptor SR-BI in the mouse eye by immunoblotting and immunohistochemistry (IHC).

Results : Carotenoid feeding experiments demonstrated that the contents of lutein, zeaxanthin, and β-carotene in the retinas of the apo A1^-/-/bco2^-/-mice were reduced to ~73%, 60% and 11% of the bco2^-/- mice, respectively. Apo A1 deletion also decreases carotenoid levels in the RPE/choroids and serum. Interestingly, Apo A1 deletion increases all three carotenoids in the livers of bco2^-/- mice. SPR data showed that Apo A1 binds zeaxanthin and β-carotene more strongly than lutein. Western blotting and IHC detected that SR-BI is expressed strongly in the mouse RPE/choroid but not the retina.

Conclusions : Our results demonstrate that HDL is responsible for carotenoid transport from the liver to the eye. This suggests that Apo A1 could be a therapeutic target for AMD prevention by increasing retinal carotenoids.

This is a 2020 ARVO Annual Meeting abstract.