Identification, characterization and nutritional regulation of two isoforms of acyl-coenzyme A oxidase 1 gene in Nile tilapia (Oreochromis niloticus)

• We identified two isoforms of ACOX1 gene in Nile tilapia (Cichlidae, perciformes).
• The protein sequence of isoform 1 was more conserved than that of isoform 2.
• Two isoforms expressed differently in kidney, white muscle, gill and brain.
• The expression of two isoforms was affected differently by nutritional stage.

In peroxisome, acyl-coenzyme A oxidase 1 (ACOX1) is the first rate-limiting enzyme of the fatty acid beta-oxidation pathway, which catalyzes the desaturation of acyl-CoAs to 2-trans-enoyl-CoAs. Two isoforms of acyl-coenzyme A oxidase 1 were firstly identified in Nile tilapia (Oreochromis niloticus) in this study. ACOX1 isoform1 (ACOX1i1) and ACOX1 isoform2 (ACOX1i2) were encoded by the single gene with 661 amino acids in length. The coding region of both isoforms consisted of 14 exons. The residues from 89 to 193 in ACOX1i1 were encoded by exon 3b, while in ACOX1i2 they were encoded by exon 3a. Homologous alignment analysis indicated that the varied region (the residues from 89 to 193) of ACOX1i1 was more conserved than ACOX1i2 in vertebrates (Mammalia, Aves, Amphibia and Pisces). The mRNA expression level of ACOX1i1 and ACOX1i2 was detected separately in eleven tissues and the results indicated that ACOXi1 expression was the highest in liver followed by kidney and brain, while the expression of ACOXi2 was the highest in kidney followed by liver. The normalized levels of both transcript variants were comparable in most tissues, however the level of ACOX1i2 was significantly higher than that of ACOX1i1 in white muscle and kidney (5.1 fold and 3.1 fold), and ACOX1i1 was significantly higher than ACOX1i2 in gill and brain (4.8 fold and 1.9 fold). In different nutritional states, the expression levels of both isoforms in liver were comparable between fasting and most of post-feeding time points, except that the expression at 3 h post-feeding was significantly lower than others. The expression of ACOX1i1 in the kidney also showed the similar pattern, indicating the lowest expression at 8 h post-feeding, however, no significant change was seen in ACOX2i2 among all nutritional states. These results suggested that ACOX1i1 and i2 may play different roles in tissues, and their expression levels were differently modulated by nutritional stage.