The proteasome participates in degradation of mutant α1-antitrypsin Z in the endoplasmic reticulum of hepatoma-derived hepatocytes

Because retention of mutant alpha (1)-antitrypsin (alpha (1)-AT) Z in the endoplasmic reticulum (ER) is associated with liver disease in alpha (1)-AT-deficient individuals, the mechanism by which this aggregated glycoprotein is degraded has received considerable attention. In previous studies using stable transfected human fibroblast cell lines and a cell-free microsomal translocation system, we found evidence for involvement of the proteasome in degradation of alpha (1)-ATZ (Qu, D., Teckman, J. H., Omura, S., and Perlmutter, D. H. (1996) J. Biol. Chem. 271,22791-22795). In more recent studies, Cabral et al. (Cabral, C. M., Choudhury, P., Liu, Y., and Sifers, R. N. (2000) J. Biol. Chem. 275, 25015-25022) found that degradation of alpha (1)-ATZ in a stable transfected marine hepatoma cell line was inhibited by tyrosine phosphatase inhibitors, but not by the proteasomal inhibitor lactacystin and concluded that the proteasome was only involved in ER degradation of alpha (1)-ATZ in nonhepatocytic cell types or in cell types with levels of alpha (1)-AT expression that are substantial lower than that which occurs in hepatocytes. To examine this important issue in further detail, in this study we established rat and murine hepatoma cell lines with constitutive and inducible expression of alpha (1)-ATZ. In each of these cell lines degradation of alpha (1)-ATZ was inhibited by lactacystin, MG132, epoxomicin, and clasto-lactacystin beta -lactone. Using the inducible expression system to regulate the relative level of alpha (1)-ATZ expression, we found that lactacystin had a similar inhibitory effect on degradation of alpha (1)-ATZ at high and low levels of al-AT expression. Although there is substantial evidence that other mechanisms contribute to ER degradation of alpha (1)-ATZ, the data reported here indicate that the proteasome plays an important role in many cell types including hepatocytes.