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Metabolic defects in phytanic acid catabolism have been shown to be connected with a number of human diseases which can lead to lethal defects of the nervous system and other organs. These effects are probably a result of the very high accumulation of phytanic acid in tissues throughout the body, due to defects in phytanic acid oxidation, the peroxisome being a major site for this process. The nuclear hormone receptors peroxisome proliferator-activated receptor and retinoid X receptor (RXR) have been shown to function as transcription factors in the control of the peroxisomal enzyme expression. Known activators of peroxisome proliferator-activated receptor include polyunsaturated fatty acids and, for RXR, the 9-cis isomer of retinoic acid. Here we report that phytanic acid is also a natural ligahd for RXRa, being able to activate a RXR-responsive promoter. We present evidence that phytanic acid binds to RXRa, promotes formation of an RXRa/RXR response element complex (as detected by gel retardation), and induces a RXRa conformational change similar to that induced by 9-cis-retinoic acid (as detected by protease sensitivity). These results suggest an involvment of RXRa in the control of fatty acid metabolism and could simply that RXRs have a role in the disease effects resulting from defective phytanic acid catabolism. [ABSTRACT FROM AUTHOR]