Retina lipoprotein receptors

The neural retina expresses several lipoprotein receptors including SR-BI, SR-BII (Tserentsoodol et al., 2006a,b), and VLDL receptor (VLDLR) (Hu et al., 2008). Thus, carotenoid flow through the RPE and further transport in the neural retina may also be mediated by lipoprotein receptors (Tserentsoodol et al., 2006a,b). SR-BI and SR-BII have been found to be localized mainly to the ganglion cell layer and POS in the monkey retina (Tserentsoodol et al., 2006a). 

Tserentsoodol, N, Sztein, J, Campos, M, Gordiyenko, NV, Fariss, RN, Lee, JW, Fhesler, SJ, and Rodriguez, IR, 2006b. Uptake of cholesterol by the retina occurs primarily via a low density lipoprotein receptor-... 

Recent data indicate that SR-BI is a nonspecific receptor for many lipophilic molecules (Lorenzi et al., 2008 Reboul et al., 2007b). Apart from HDLs, rodent SR-BI also binds to LDL, VLDL, acetylated LDL, oxidized LDL, and maleylated bovine serum albumin. SR-BII has a similar ligand specificity and function to that of SR-BI (Webb et al., 1998). However, it has been shown that vitamin E (which like carotenoids is carried in the bloodstream mainly by LDL and HDL) is transported more efficiently into the endothelial cells from HDLs than from LDLs (Balazs et al., 2004 Kaempf-Rotzoll et al., 2003 Mardones and Rigotti, 2004). This is in striking contrast to cholesterol, which is taken up much more efficiently from LDLs than HDLs by the RPE to the retina (Tserentsoodol et al., 2006b). It remains to be shown which lipoproteins are the main carriers for carotenoids transported from blood into the RPE. 

ApoC-I is expressed mainly in liver but also in lung, skin, testis, spleen, neural retina, and RPE. Its multiple functions include the activation of lecithin cholesterol acyltransferase (LCAT) and the inhibition, among others, of lipoprotein and hepatic lipases that hydrolyze triglycerides in particle cores. Notably, both LCAT and lipoprotein lipases are expressed in RPE and choroid (Li et al., 2006). Moreover ApoC-I has been shown to displace ApoE on the VLDL and LDL and thus hinder their binding and uptake via their corresponding receptors (Li et al., 2006). 

While it may be speculated that in the RPE both lipoprotein and/or scavenger receptors are likely to be involved in carotenoid uptake from the blood, it is not clear what mechanism(s) are responsible for carotenoid transport through the RPE into the neural retina. Also, it is not clear what mechanism(s) are responsible for selective accumulation in the retina of only two carotenoids. 

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