Retinoid-binding proteins and retinol esterification in cultured retinal pigment epithelium cells

Uptake and isomerization of all-trans retinol by isolated bovine retinal pigment epithelial cells: further clues to the visual cycle

The site and substrate for all-trans to 11-cis isomerization in the visual cycle have remained obscure for several decades. Only recently studies on a subcellular level have begun to shed some light on these phenomena. We have addressed this system on a cellular level by utilizing intact isolated bovine retinal pigment epithelial cells, maintained during short-term incubation in vitro. Supplementation with labeled all-trans retinol incorporated in a lipid vesicle carrier, in a range of 1-6 nmol per 10(6) cells, resulted in a rapid uptake of retinol. The majority of the internalized retinol was processed prior to mixing with endogenous retinoid pools and most of it was converted into all-trans retinylester. Up to 10% of the incorporated label was isomerized yielding 11-cis retinol, 11-cis retinaldehyde and 11-cis retinylester. The kinetics of the 11-cis retinoid formation indicated that 11-cis retinol is the first isomerization product. The level of 11-cis retinol apparently 'triggered' further processing into other 11-cis retinoids. An updated model with discussion topics is presented for the retinoid pathway relevant to the visual cycle.

Requirement of insulin or IGF-1 for the maintenance of retinyl ester synthetase activity by cultured retinal pigment epithelial cells

Previous work from these laboratories showed that the retention of retinyl ester synthetase activity by cultured human retinal pigment epithelium is up to tenfold greater with PM medium (Medium 199 plus insulin, other added defined components, 1% serum and 1% retina extract) than with conventional culture media. The present work shows that insulin is the component of PM medium required for maintenance of ester synthetase activity and that insulin-like growth factor type 1 (IGF-1) also is effective at maintaining ester synthesis. In addition, insulin can maintain ester synthetase activity in cultured rat RPE. Preliminary dose-response measurements provide additional support for these findings and strongly suggest that both insulin and IGF-1 are maximally effective at physiological concentrations (1-10 ng ml-1).

Retinol esterification by mouse epidermal microsomes: evidence for acyl-CoA:retinol acyltransferase activity

In an attempt to characterize the enzyme(s) responsible for retinol esterification in hairless mouse epidermis, various subcellular fractions were incubated with [3H]retinol and the reaction products (retinyl esters) isolated by high-performance liquid chromatography. The microsomal fraction exhibited the highest esterifying activity and was stimulated by the addition of palmitoyl-CoA and dithiothreitol, but not by palmitic acid. Saturation kinetics with an apparent Km of about 6 microM for retinol were noted. Experiments with competitive and noncompetitive inhibitors of [3H]retinol esterification established that the epidermal enzyme was an acyl-CoA:retinol acyltransferase (ARAT; EC 2.3.1.76). The specificity for retinol was not absolute; a few closely related vitamin A alcohols were equally good substrates. The ARAT activity was not significantly altered by physiologic variations in the epidermal vitamin A content. In conclusion, mouse epidermis expresses ARAT activity which may be of importance for the regulation of vitamin A metabolism at the cellular level.