Isolation of a cDNA clone encoding rat insulin-like growth factor-II precursor

Abstract

Insulin-like growth factor-I (IGF-I) and IGF-II are mitogenic polypeptides of relative molecular mass (Mr) ∼7,500 isolated from human plasma1,2 each containing four peptide domains in a single chain and identical at more than 60% of their amino acid loci. The B- and A-domains of the IGFs are ∼40% identical to the B-and A-chains of human insulin1,2. IGF-I and IGF-II have similar in vitro biological activities2 and receptor reactivity3, but are immunologically distinct4,5. IGF-I appears to mediate the effects of growth hormone on cartilage to promote skeletal growth5,6, whereas IGF-II may have a special role in fetal development7,8 and in the central nervous system9. To investigate the in vivo role of IGF-II, we have studied IGF-II biosynthesis in the BRL-3A rat liver cell line10. BRL-3A cells synthesize and secrete a 7,484 Mr protein 93% identical to human IGF-II and representing rat IGF-II (rIGF-II)11. Rat IGF-II is synthesized as a ∼22,000 Mr prepro-rIGF-II (ref. 12) from 12 S poly(A)+mRNA13. In addition, ∼20,000 Mr pro-rIGF-II has been identified in lysates of biosynthetically labelled intact BRL-3A cells14. We report here the isolation of an almost complete cDNA clone for rIGF-II. Our results indicate that pro-rIGF-II is synthesized as a 156 amino acid peptide precursor (17,619 Mr) containing mature rIGF-II 1–67 at its amino-terminus and an 89-residue carboxy-terminal peptide extension.