A. Heron et al., POSTTRANSLATIONAL PROCESSING OF THE INTER-ALPHA-TRYPSIN INHIBITOR IN THE HUMAN HEPATOMA HEPG2 CELL-LINE, Biochemical journal, 302, 1994, pp. 573-580
In human hepatoma HepG2 cells, the serum inter-alpha-trypsin inhibitor
(ITI)-like protein is synthesized from two protein precursors, the he
avy chain (H) H2 and the light chain (L). Both of them carry sulphate
groups involved in the chondroitin sulphate glycosaminoglycan (GAG) li
nkage, as demonstrated by [S-35]sulphate labelling, chondroitinase dig
estion 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 th
e accumulation of H and L precursors in the cells, therefore blocking
subsequent association and maturation of the precursors before their s
ecretion. The enzyme system involved in the ester linkage between H an
d L chains is localized in the trans-Golgi network since no ITI-like p
rotein could be obtained in the presence of monensin; instead free hea
vy-chain protein forms and bikunin were secreted in culture supernatan
ts. The ITI-like protein synthesized by HepG2 cells is therefore compo
sed of two heavy chains HC2 linked to two bikunin chains by chondroiti
n sulphate bridges, although the GAG linkage between HC2 chains is pre
sumably different. Further, a different maturation route leading to re
stricted heavy-chain forms, H-m and H-d, could be shown.