Different temporal expressions of tilapia (Oreochromis mossambicus) insulin-like growth factor-I and IGF binding protein-3 after growth hormone induction

In vertebrates the insulin-like growth factor-I (IGF-I) is well recognized for its important roles in prenatal and postnatal processes. Liver is the major endocrine organ in mammals to produce IGF-I and its modulators, the insulin-like growth factor binding proteins (IGFBPs), which form a binary complex with IGF-I in circulation or in extracellular fluid. In the complex, IGFBP-3 accounts for the majority of IGFBPs that interact with IGF-I in circulation. To date, we know little of fish IGF-I regulation at the gene or protein level. In this preliminary experiment, we isolated tilapia IGFBP-3 complementary DNA sequence by degenerate polymerase chain reaction cloning. This identified clone contained a partial open reading frame of 635 bp and lacked both 5' and 3' end cDNA sequence information. The deduced 211 amino acid residues shared approximately 50% identity with mammalian counterparts. This tilapia IGFBP-3 transcript was expressed in every tissue examined with the highest level found in liver. In a growth hormone time course experiment, both IGF-I and IGFBP-3 message levels increased sharply, and after 24 hours of injection, IGF-I and IGFBP-3 was increased 1.97 and 2.15 fold, respectively. The relative message ratio of IGM to IGFBP-3 was 1.58, 1.09, 0.94, 0.91, and 0.92 for 3, 6, 9, 12, and 24 hours, respectively. After 9 hours of treatment, the IGF-I/IGFBBP-3 message ratio maintained a constant value of approximately 0.9 throughout the experiment. This is the first report to demonstrate a fish counterpart of IGFBP-3 at the cDNA level and its message expression ratio with IGF-I under growth hormone induction.