Post-translational processing of the inter-α-trypsin inhibitor in the human hepatoma HepG2 cell line

In human hepatoma HepG2 cells, the serum inter-alpha-trypsin inhibitor (ITI)-like protein is synthesized from two protein precursors, the heavy chain (H) H2 and the light chain (L). Both of them carry sulphate groups involved in the chondroitin sulphate glycosaminoglycan (GAG) linkage, as demonstrated by [35S]sulphate labelling, chondroitinase digestion and inhibition with beta-D-xyloside, an artificial GAG acceptor. While inhibition of N-glycosylation prevented neither the maturation nor the secretion of the ITI-related entities, brefeldin A induced the accumulation of H and L precursors in the cells, therefore blocking subsequent association and maturation of the precursors before their secretion. The enzyme system involved in the ester linkage between H and L chains is localized in the trans-Golgi network since no ITI-like protein could be obtained in the presence of monensin; instead free heavy-chain protein forms and bikunin were secreted in culture supernatants. The ITI-like protein synthesized by HepG2 cells is therefore composed of two heavy chains HC2 linked to two bikunin chains by chondroitin sulphate bridges, although the GAG linkage between HC2 chains is presumably different. Further, a different maturation route leading to restricted heavy-chain forms, Hm and Hd, could be shown.