EXPRESSION OF MULTIPLE INSULIN AND INSULIN-LIKE-GROWTH-FACTOR RECEPTOR GENES IN SALMON GILL CARTILAGE

In mammals, one of the major actions of insulin-like growth factor I ( IGF-I) is to increase skeletal growth by stimulating new cartilage for mation. IGF-I stimulates chondrocytes in vitro to synthesize new carti lage matrix, measured by enhanced uptake of S-35-sulfate, but the addi tion of insulin does not produce a similar effect except when added at high concentrations, However, recent studies have shown that, in tele osts, both insulin and IGF-I are potent activators of S-35-sulfate upt ake in gill cartilage, To further characterize the growth-promoting ac tivities of these hormones in fish, we have used reverse transcriptase -linked PCR to analyze the expression of insulin receptor family genes in salmon gill cartilage, Partial cDNA sequences encoding the tyrosin e kinase domains from six distinct members of the IR gene family were obtained, and sequence comparisons revealed that four of the cDNAs enc oded amino acid sequences that were highly homologous to human IR wher eas the encoded sequences from two of the cDNAs were more similar to t he human type I IGF receptor (IGF-R), Furthermore, a comparative rever se transcriptase-linked PCR assay revealed that the four putative IR m RNAs expressed in toto in gill cartilage were 56% of that found in liv er whereas the expressed amount of the two IGF-R mRNAs was 9-fold high er compared with liver, These results suggest that the chondrogenic ac tions of insulin and IGF-I in fish are mediated by the ligands binding to their cognate receptors, However, further studies will be required to characterize the binding properties and relative contribution of t he individual IR and IGF-R genes.