We have performed studies using cultured human retinal pigment epithelial (RPE) cells as a model system to study gene expression and regulation of gene expression in the RPE. We have found that stearoyl-CoA desaturase (SCD), the rate limiting enzyme in the synthesis of unsaturated fatty acids, is expressed in RPE cells and that its expression is regulated by retinoic acid and TGF-b. All-trans retinoic acid increased SCD mRNA expression in a dose- and time-dependent manner, and analyses using retinoid receptor agonists and antagonists demonstrated that this regulation is mediated through the retinoid receptors RAR and RXR. We have also found that all three TGF-b isoforms, b1, b2 and b3 increase SCD mRNA expression in cultured human RPE cells and that a TGF-b signal transduction pathway involving Smad proteins regulates the cellular expression of the SCD gene. Thus, SCD may play an important role in the process of lipid metabolism and trafficking in the RPE, and because of their ability to regulate the expression of SCD, retinoic acid and TGF-b may play important regulatory roles in lipid metabolism and RPE pathophysiology. We have also performed studies on the effect of acute, intense visible light on the retinoid content and distribution in the eyes of normal and transgenic rats carrying the P23H mutation in the opsin gene that has been demonstrated to render photoreceptor cells more susceptible to light damage. We have found that upon rhodopsin bleaching the relative distribution of visual cycle retinoids was similar in cyclic-light and dark-reared rats, but that there were significant differences in the retinoid content of the neural retina and the RPE depending on the duration of the light exposure and whether DMTU, an antioxidant, was given to the rats. Most notably, no 11-cis retinal was detected in the RPE of DMTU treated dark -reared rats. Other differences in visual cycle retinoids were found based on the duration of light exposure and the P23H mutation in the opsin gene. We have isolated a retinoid-like compound that is present in the light exposed P23H retina and RPE/cup but not in the normal rat retina or RPE/eyecup.
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