We investigated the effects of two natural dietary retinoid X receptor (RXR) ligands, phytanic acid (PA) and docosahexaenoic acid (DHA), on proliferation and on the metabolism of retinol (vitamin A) in both cultured normal human prostate epithelial cells (PrECs) and PC-3 prostate carcinoma cells. PA and DHA inhibited the proliferation of the parental PC-3 cells and PC-3 cells engineered to overexpress human lecithin:retinol acyltransferase (LRAT) in both the absence and presence of retinol. A synthetic RXR-specific ligand also inhibited PC-3 cell proliferation, whereas all-trans retinoic acid (ATRA) did not. PA and DHA treatment increased the levels of retinyl esters (REs) in both PrECs and PC-3 cells and generated novel REs that eluted on reverse-phase HPLC at 54.0 and 50.5 min, respectively. Mass spectrometric analyses demonstrated that these novel REs were retinyl phytanate (54.0 min) and retinyl docosahexaenoate (50.5 min). Neither PA nor DHA increased LRAT mRNA levels in these cells. In addition, we demonstrate that retinyl phytanate was generated by LRAT in the presence of PA and retinol; however, retinyl docosahexaenoate was produced by another enzyme in the presence of DHA and retinol. Vitamin A (retinol) and its natural metabolites and synthetic derivatives, known as retinoids, play important roles in regulating cell proliferation, differentiation, and embryonic development (1-3). The physiological activities of retinoids are primarily carried out via nuclear receptors that transcriptionally activate certain retinoid-responsive genes. Two types of retinoid receptors have been identified, retinoic acid receptors (RARs) and retinoid X receptors (RXRs), each with three subtypes, a, b, and g (4-6). RAR/RXR heterodimers bind to specific DNA sequences known as retinoic acid response elements (7). Ligand binding of RARs and RXRs regulates the transcription of retinoic acid-responsive genes. The actions of RARs and RXRs on gene transcription also require a highly coordinated interaction with a large number of coactivators and corepressors (8).The appropriate intracellular retinoid concentrations are maintained by the activities of several metabolic enzymes (9-11). All-trans retinol (ROH; vitamin A) is the precursor of other natural retinoids. Retinol dehydrogenases oxidize ROH to all-trans retinaldehyde, and then all-trans retinaldehyde is further metabolized to all-trans retinoic acid (ATRA) by retinaldehyde dehydrogenases (11). Currently, four members of the retinaldehyde dehydrogenase family have been identified, retinaldehyde dehydrogenases 1, 2, 3, and 4 (12). ROH is converted to all-trans retinyl esters primarily by lecithin:retinol acyltransferase (LRAT), and retinyl esters (REs) are hydrolyzed to retinol by ester hydrolases (9,(13)(14)(15). REs are regarded as the storage forms of retinol (9). ATRA is oxidized to more polar metabolites, such as 4-oxo-retinoic acid, by a cytochrome P450 family member (CYP26) (16)(17)(18)(19).Retinoids are required for the appropriate differentiation of normal ro...