9-cis-retinoids: Biosynthesis of 9-cis-retinoic acid

Retinoids function through conformational alterations of ligand-dependent n uclear transcription factors, the retinoic acid receptors, and retinoid X r eceptors, 9-cis-Retinoic acid is a known biological ligand for retinoid X r eceptors, but its synthesis pathway in vivo is largely unknown. Recently, w e identified a cis-retinol dehydrogenase (cRDH) that oxidizes 9-cis-retinol to 9-cis-retinal. Since both the expression of cRDH mRNA and its substrate are found ill liver, we studied 9-cis-retinol metabolism and 9-cis-retinoi c acid biosynthesis in two hepatic-derived cell types, Hep G2 hepatoma cell s and HSC-T6 stellate cells. Both cell lines accumulate similar amounts of 9-cis-retinol provided in the medium. However, Hep G2 cells preferentially incorporate all-trans-retinol when equimolar concentrations of all-trans- a nd 9-cis-retinol were provided. In contrast, HSC-T6 cells did not exhibit a preference between all-trans- and 9-cis-retinol under the same conditions. Esterification of 9-cis-retinol occurred in both cell types, likely by acy l-CoA:retinol acyltransferase and lecithin:retinol acyltransferase, In vitr o enzyme assays demonstrated that both cell types can hydrolyze 9-cis-retin yl esters via retinyl ester hydrolase(s), In Hep G2 cells, 9-cis-retinoic a cid synthesis was strongly inhibited by high concentrations of 9-cis-retino l, which may explain the low levels of 9-cis-retinol in liver of mice. Cell homogenates of Hep G2 can convert all-trans-retinol to 9-cis-retinal, sugg esting that the free form of all-trans-retinol may be used as a source for 9-cis-retinol and, thus, 9-cis-retinoic acid synthesis, Our studies provide the basis for identification of additional pathways fur the generation of 9-cis-retinoic acid in specialized tissues.