Molecular cloning and tissue expression of the insulin-like growth factor II prohormone in the bony fish Cottus...

The cDNA encoding pro-IGF-II of an advanced teleost fish, Cottus scorpius (Scorpaeniformes), the daddy sculpin, was isolated from liver by RT-PCR and molecular cloning. Like other IGFs, the deduced 168 amino acid peptide contains B-, C-, A-, D-, and E-domains and six cysteine residues (CysB9, CysB21, CysA6, CysA7, CysA11, and CysA20) necessary for the maintenance of tertiary structure. At the amino acid level, the sculpin IGF-II prohormone exhibits 85-92% homology to pro-IGF-II of other bony fish but only 51% homology to human. The mature sculpin IGF-II peptide comprises 70 amino acids. Its A-, B-, and D-domains exhibit homologies as high as 91, 91, and 100%, respectively, when compared with the other bony fish species studied. The high sequence homologies may indicate a particular physiological impact of IGF-II in bony fish. RT-PCR followed by Southern blotting revealed an IGF-II mRNA transcript of the expected size in liver, pyloric and splenic islets, stomach, small and large intestine, kidney, gill, testis, ovary, brain, and heart. The local production of IGF-II in many organs indicates that IGF-II is involved in organ-specific functions in a paracrine/autocrine manner. Furthermore, the results show that all bony fish organs which have been demonstrated to express IGF-I mRNA also express IGF-II mRNA.