The human hepatoma cell line PLC/PRF/5 synthesized and secreted a functioal al-antitrypsin (al-AT) glycoprotein with normal molecular size but retarded electrophoretic mobility. The total process of translation, glycosylation and export required about 40 min and followed the same synthetic pattern as seen in rat hepatocytes, i.e., a signal peptide is cleaved cotranslationally; a core-glycosylated protein in the high-mannose form is formed in the rough endoplasmic reticulum, trimmed, and a stable complex-glycosylated W-AT is found intracellularly prior to export. aI-AT export to medium was delayed by tunicamycin, inhibited by cycloheximide but unaffected by colchicine. After addition of exogenous al-AT to culture medium, neither negative nor positive feedback induction of synthesis could be demonstrated.
Electrophoretic techniques indicated the presence of atypical, highly branched but incompletely sialylated carbohydrate chains in the hepatoma cell-derived al-AT. The accumulation of intracellular al-AT inclusions seen in the endoplasmic reticulum may reflect an imbalance between a high rate of polypeptide synthesis and terminal glycosylation.
The association between al-antitrypsin (al-AT) and liver disease was noted by Sharp in 1971 (1) with the observation of the al-AT deficiency of phenotype Pi (protease inhibitor) ZZ in patients with juvenile cirrhosis and demonstration of periodic acid-Schiff (PAS) stainable inclusions after diastase treatment in hepatocytes. Abnormal frequencies of cirrhosis and hepatoma were later reported among Pi ZZ adults (2). al-AT immunoreactive inclusion bodies from a Pi ZZ liver were chemically characterized and consisted of a,-AT with an amino acid substitution (3) in the core-glycosylated (high mannose) state. It has been postulated that the amino acid substitution renders the molecule unstable or insoluble in the endoplasmic reticulum thus preventing transport to the Golgi (1, 3) for complex glycosylation and subsequent export.
Similar inclusion bodies have also been reported in selected patients with phenotype Pi M, high plasma