Accumulation of the insoluble PiZ variant of human alpha 1-antitrypsin within the hepatic endoplasmic reticulum does not elevate the steady-state level of grp78/BiP

Abstract

Greater than 85% of the transport-impaired PiZ variant of human alpha 1-antitrypsin is retained within cells and subsequently degraded within a pre-Golgi nonlysosomal compartment that is apparently separate from the endoplasmic reticulum (ER) (Le, A., Graham, K. S., and Sifers, R. N. (1990) J. Biol. Chem. 265, 14001-14007). Despite this phenomenon, human patients and PiZ-bearing transgenic mice exhibit an accumulation of the undegraded protein as insoluble aggregates within distended cisternae of the hepatic ER (Carlson, J. A., Rogers, B. B., Sifers, R. N., Finegold, M. J., Clift, S. M., DeMayo, F. J., Bullock, D. W., and Woo, S. L. C. (1989) J. Clin. Invest. 83, 1183-1190). Immunoprecipitation of the PiZ variant from pulse-radiolabeled hepatocytes from the transgenic animals has demonstrated that a minute quantity of the newly synthesized mutant protein is apparently resistant to degradation and accumulates gradually within the particulate fraction of the cell. Although the steady-state level of the resident ER protein grp78/BiP is elevated in response to the accumulation of malfolded proteins within that subcellular compartment, this phenomenon is not elicited by the accumulation of the insoluble PiZ variant. These results indicate that neither the accumulation of this malfolded protein within the ER nor even the distention of that subcellular compartment is sufficient to cause the up-regulation of grp78/BiP levels. The interpretation of these results with regard to the factors that regulate the levels of grp78/BiP in the ER is discussed.