AN ALTERNATIVELY SPLICED HUMAN INSULIN-LIKE GROWTH-FACTOR-I TRANSCRIPT WITH HEPATIC TISSUE EXPRESSION THAT DIVERTS AWAY FROM THE MITOGENIC IBE1 PEPTIDE

An alternatively spliced transcript of the human insulin-like growth f actor-I (IGF-I) gene is described. The transcript was identified in hu man liver RNA by reverse transcriptase-polymerase chain reaction, clon ing, and sequencing. It contained IGF-I exons 3 and 4, 49 basepairs of exon 5, then exon 6 (exon 4-5-6). The 5'-donor site at the exon 5-6 j unction was a cryptic 5'-donor splice site (IGF(633)). The 3'-acceptor site of the splice was the usual intron-exon 6 junction. A second pai r of primers across the exon 5-exon 6 junction was used to confirm the presence of the transcript by reverse transcriptase-polymerase chain reaction. Cloning and sequencing this second fragment confirmed the pr esence of this splice in human liver. The exon 4-5-6 transcript was qu antified at about 10% relative to the exon 4-6 transcript in human liv ers (n = 7 subjects), but was not detected in other tissues. The exon 4-5-6 transcript was found in cultured human hepatoma HepG2 cells and increased, relative to exon 4-6 transcripts, in response to GH, but no t in cultured human lymphoblast IM-9 cells. The exon 4-5-6 splice pred icts a prepro-IGF-I of 158 amino acid residues, with an E-peptide sequ ence of 24 residues (Ec). The deduced Ec peptide sequence is 73% homol ogous to the rat Eb-peptide sequence. The predicted final residues of the Ec peptide are frame-shifted exon 6 codons ending in an in-frame s top codon. The predicted peptide sequences of Ec and Eb differ at the cleavage site of the Eb-peptide fragment (IBE1) which has been shown t o have mitogenic activity. These data suggest that 1) the exon 4-5-6 s plice has hepatic tissue expression and occurs by the use of a cryptic 5'-donor consensus splice site (IGF(633)) in exon 5; 2) exon 4-5-6 ca n be hormonally regulated in cultured human HepG2 cells; 3) exon 4-5-6 is the human counterpart of the rat IGF-I Eb, because the complementa ry DNA and predicted sequences are homologous; and 4) the production o f IBE1 is potentially regulated by alternative splicing.