IGF-I in the bony fish Cottus scorpius: cDNA, expression and differential localization in brain and islets.

The cDNA encoding prepro-insulin-like growth factor (IGF)-I of a teleost, Cottus scorpius, (Scorpaeniformes) was established from liver by RT-PCR and molecular cloning. Typically, the deduced 184 amino acid protein contains a signal peptide, B-, C-, A-, D- and E-domains and all residues necessary for maintenance of tertiary structure. C. scorpius IGF-I shares only approximately 57% identity with C. scorpius insulin in the A-domain and 7% in the B-domain. RT-PCR followed by Southern blotting revealed a transcript in liver, pancreatic islets, stomach, small and large intestine, kidney, gill, testis, ovary, heart and brain indicating paracrine/autocrine actions of locally produced IGF-I. IGF-I- and insulin-immunoreactivities coexisted in the islets, but did not in other sites such as brain. Thus, in contrast to other bony fish, sculpin insulin cells most probably produce IGF-I. The results also challenge the current model of the IGF/insulin evolution.