Characterization of teleost insulin receptor family members

Insulin from mammals and fish has been used to determine insulin-binding affinities and receptor numbers with remarkable similarities between these two vertebrates, suggesting functional conservation. Yet, the nature and structure of teleost insulin receptors are not known. Therefore, the cloning and mRNA characterization of rainbow trout insulin receptors were undertaken. Three insulin receptor cDNAs were isolated by screening a cDNA library, confirmed as separate genes by genomic Southern hybridization, and designated as rainbow trout insulin receptor a (rtIR a), rainbow trout insulin receptor b (rtIR b), and rainbow trout insulin receptor c (rtIR c). A high degree of amino acid identity was observed between rainbow trout insulin receptors (rtIRs) and their human homolog, confirming the structural similarities between mammalian and fish insulin receptors. Reverse transcription-polymerase chain reaction from total RNA using either oligo(dT) or random hexamer primers resulted in a diminished ability to detect rtIR a and rtIR b mRNA when oligo(dT) was used, suggesting developmental and tissue-specific polyadenylation. The highest steady-state levels of rtIR mRNAs were consistently detected in juvenile and adult pyloric caeca (which also contained adipose and pancreatic tissue), while the lowest levels were consistently found in muscle. A high level of rtIR b and rtIR c mRNA was also found in ovary, while a high level of rtIR a was found in adult brain. Significant differences were also found between steady-state rtIR mRNA levels in corresponding juvenile and adult tissues. These results suggest a complex expression pattern of insulin receptor mRNAs in partial tetraploid fish.