The retina is a multilayered organ composed of at least 12 different cell types, of which only Müller cells and RPE come in contact with the circulation. Müller cells surround the inner retinal vessels, and the RPE is fed by the choriocapillaris. Immunohistochemical localization of CYP27A1 performed on monkey retinas
31 showed strong protein expression in photoreceptor inner segments and faint expression throughout the retina, including Müller cells, ganglion cells, and nerve fibers (
Fig. 7). Faint CYP27A1 expression was also observed in the RPE and choriocapillaris.
31 If CYP27A1 is highly abundant in photoreceptor inner segments and this site is the major source of retinal 27-COOH, then the 27-oxygenated cholesterol metabolite has to leave the photoreceptor inner segments and be transported within the retina to the site(s) where it can enter systemic circulation. Studies with cultured human macrophages showed that the nature of the acceptor outside the cell affects whether intracellular cholesterol is metabolized to 27-OH or 27-COOH; the presence of HDL favors the production of 27-OH, whereas with albumin 27-COOH is the major secreted product.
39 Apolipoprotein A1, a constituent of HDL, was recently shown by immunohistochemistry to be expressed in the retina in multiple cell types, including photoreceptor inner segments.
19 Therefore, HDL is likely to be present in the retina, yet the predominant metabolite is 27-COOH. This suggests that factors other than acceptor presence in the interphotoreceptor matrix affect the metabolic fate of cholesterol in photoreceptor inner segments. One of these factors could be limited availability of cholesterol to CYP27A1, shown previously to facilitate the production of 27-COOH.
39 CYP27A1 resides in the inner mitochondrial membrane. Consequently, cholesterol has to be transported from the outer to the inner membrane to be metabolized by CYP27A1. Intramitochondrial cholesterol transport has been studied extensively in steroidogenic tissues,
52,53 but the nature of this process in photoreceptor inner segments is unknown. If it is similar to the process in steroidogenic tissues, the cholesterol amount should also be low in the inner mitochondrial membranes of photoreceptors. If it is, limited substrate availability coupled with high CYP27A1 expression creates a low substrate-to-enzyme ratio that also favors the production of 27-COOH.
51 Another contributing factor could be the unique composition of the retinal membranes rich in n-3 polyunsaturated fatty acids.
54 CYP27A1 activity depends on the phospholipid content of the membranes in which this enzyme is embedded.
55 We established that the catalytic efficiency of CYP27A1 in vitro is much higher in the presence of retinal mitochondrial phospholipids than in the presence of liver mitochondrial phospholipids (I. Pikuleva, unpublished, 2010). These results suggest that C27-oxygenation of cholesterol is more efficient in the retina than in the liver and are consistent with the data of the present study. Thus, the production of 27-COOH in the retina could be explained from the biochemical standpoint, whereas the physiological significance of the conversion to 27-COOH requires further investigation. It should also be noted that enzymes other than CYP27A1 may contribute to further oxidation of 27-OH. Cholesterol derivative 5β-cholestane-3α,7α,12α, 27-tetrol was shown to be efficiently oxidized into the corresponding C-27 acid by horse liver alcohol dehydrogenase combined with aldehyde dehydrogenase.
56 In addition, short-chain dehydrogenases/reductases are also known to metabolize a wide range of substrates in vitro, including steroids.
57 –61 Some of these enzymes are expressed in the retina
62,63 and, consequently, could be involved in the oxidation of 27-OH to 27-COOH.