Post-translational processing of the insulin-like growth factor-2 precursor -: Analysis of O-glycosylation and endoproteolysis

Insulin-like growth factor-2 (IGF-2) is expressed in most embryonic tissues and is required for normal development during gestation. After birth IGF-2 expression is extinguished in most tissues, but the gene is often reactivated during tumorigenesis. Tumors secrete high molecular weight forms of IGF-2 that result from aberrant post-translational processing of pro-IGF-2. As a first step toward understanding how high molecular weight IGF-P peptides might contribute to tumor progression, we have characterized the biosynthesis of IGF-P in a human embryonic cell line. We have found that pro-IGF-a can initially form two disulfide isomers that undergo rearrangement to a single conformation in vivo. The addition of N-acetylgalactosamine to Ser(71), Thr(72), Thr(75), and Thr(139) likely occurs in the cis-Golgi apparatus. Sialic acid addition begins in the trans Golgi apparatus, but IGF-2 peptides must reach the trans-Golgi network for oligosaccharide maturation to be completed. Endoproteolysis occurs concomitant to or slightly after oligosaccharide maturation. Cleavage was observed only at Arg(104), resulting in the secretion of IGF-2-(1-104) and free Et-peptide. Proteolysis required basic residues in the P1 (Arg(104)), and P4 (Arg(101)) positions, was completely blocked by a furin inhibitor, and was enhanced by coexpression with furin, PACE4, PC6A, PC6B, and LPC. These data suggest that members of the subtilisin-related proprotein convertase family mediate processing of pro-IGF-2 at Arg(104). We did not detect the IGF-S peptides that are most abundant in normal serum, mature IGF-S, and IGF-2-(1-87), in this expression system, which indicates that novel endoproteases are responsible for generating these products.