Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors belonging to the nuclear hormone receptor superfamily that functions as critical regulators of lipid and energy homeostasis. Although intensively studied in mammals, their basic biological functions are still poorly understood. The objective of this work was to characterize PPARb subtypes in a fish, the Atlantic salmon (Salmo salar), in order to address PPAR function and the regulation of lipid

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... asis in lower vertebrates. The screening of an Atlantic salmon genomic library revealed the presence of four genes for PPARb subtypes. Based on comparisons of exons and exon-flanking regions, these genes were assigned into two families, ssPPARb1 and ssPPARb2, each family containing two isotypes: ssPPARb1A and b1B and ssPPARb2A and b2B. Two full-length cDNAs for ssPPARb1A and ssPPPARb2A were isolated. Transcripts for ssPPARb1A and ssPPARb2A have distinct tissue expression profiles, with ssPPARb1A predominating in liver and ssPPARb2A predominating in gill. Expression levels of mRNA of either isotypes were up to tenfold lower in kidney, heart, spleen, muscle, and brain. In cellular transfection assays, ssPPARb1A is activated by monounsaturated fatty acids, 2-bromopalmitate, and mammalian PPARb-specific ligand GW501516. In contrast, PPARb2A was not activated by any of the compounds tested. Furthermore, ssPPARb2A repressed both the basal reporter gene activity and the GW501516induced activity of ssPPARb1A. The results indicate unexpected levels of variety and complexity in PPAR subtype and mechanism of action in lower vertebrates.