Glucosidase and Mannosidase Inhibitors Mediate Increased Secretion of Mutant α1 Antitrypsin Z

Abstract

It is now well known that the addition and trimming of oligosaccharide side chains during post-translational modification play an important role in determining the fate of secretory, membrane, and lysosomal glycoproteins. Recent studies have suggested that trimming of oligosaccharide side chains also plays a role in the degradation of misfolded glycoproteins as a part of the quality control mechanism of the endoplasmic reticulum (ER). In this study, we examined the effect of several inhibitors of carbohydrate processing on the fate of the misfolded secretory protein ␣1 antitrypsin Z. Retention of this misfolded glycoprotein in the ER of liver cells in the classical form of ␣1 antitrypsin (␣1-AT) deficiency is associated with severe liver injury and hepatocellular carcinoma and lack of its secretion is associated with destructive lung disease/emphysema. The results show marked alterations in the fate of ␣1 antitrypsin Z (␣1-ATZ). Indeed, one glucosidase inhibitor, castanospermine (CST), and two mannosidase inhibitors, kifunensine (KIF) and deoxymannojirimycin (DMJ), mediate marked increases in secretion of ␣1-ATZ by distinct mechanisms. The effects of these inhibitors on secretion have interesting implications for our understanding of the quality control apparatus of the ER. These inhibitors may also constitute models for development of additional drugs for chemoprophylaxis of liver injury and emphysema in patients with ␣1-AT deficiency.Recent studies have provided further evidence that asparagine-linked oligosaccharide side chains play an important role in intracellular transport of glycoproteins. For one, mannose-6-phosphate modification is a key determinant of sorting to the lysosome (1). Second, a number of studies have now shown that transport of secretory and membrane glycoproteins from the ER 1 to their appropriate destination depends on the interaction of the innermost glucose residue of the oligosaccharide side chains with the resident ER molecular chaperones calnexin and calreticulin (2, 3). This means that trimming of the Nglycan by glucosidases I and II and interaction with calnexin and calreticulin facilitate the proper folding and translocation of wild type glycoproteins. There is also evidence that trimming of glucose residues by glucosidases and of mannose residues by ER mannosidases is involved in the degradation of misfolded, unassembled, or mutant glycoproteins (4 -10). Third, Nichols et al. (11) have shown that ERGIC-53, a lectin which is specifically localized in the ER-Golgi intermediate compartment, is mutated in the combined deficiency of coagulation factors V and VIII. These results suggest that a lectin-like mechanism involving the interaction of carbohydrate side chains with ER-GIC-53 is required for secretion of factors V and VIII.In the classic type of ␣1 antitrypsin (␣1-AT) deficiency, the most common genetic cause of emphysema in adults and of liver disease in children, the mutant glycoprotein ␣1-ATZ is retained in the ER of liver cells rather than secreted into the extracellular ...

Histone Deacetylase Inhibitor (HDACi) Suberoylanilide Hydroxamic Acid (SAHA)-mediated Correction of α1-Antitrypsin Deficiency

“…As previously demonstrated (91)(92)(93), kifunensine treatment resulted in a modest increase in Z-␣1AT secretion, whereas miglustat had no significant effect (Fig. 4B).…”

“…To monitor this activity, we exploited the ability of ␣1AT to form an SDS-resistant complex with HNE, leading to a slower migration of the ␣1AT-HNE complex on SDS-PAGE (91,96). Briefly, increasing quantities of HNE were incubated with a fixed amount of media recovered from WT-␣1AT-or Z-␣1AT-secreting cells.…”

Intracellular Inclusions Containing Mutant α1-Antitrypsin Z Are Propagated in the Absence of Autophagic Activity

“…The 52-kDa polypeptide was cleaved to 46 kDa, indicating that it represents the ER-glycosylated form of ␣ 1 -ATZ (Fig. 2D) (29). These data indicate that glycosylation and the degradative intermediate profile of ␣ 1 -ATZ are not altered in the absence of autophagy.…”

Glycoprotein Folding in the Endoplasmic Reticulum

“…PI Z was first trimmed by ER mannosidase II prior to elimination by a nonproteasomal, tyrosine-phosphatase-dependent pathway (Cabral et al, 2000). However, another group used the mannosidase inhibitors kifunensin and deoxymannojirimycin to conclude that secretion of the α 1 -antitrypsin Z mutant was increased when oligosaccharide trimming was prevented (Marcus and Perlmutter, 2000). Various other molecules have been shown to require mannose trimming in the ER prior to degradation, including CPY in yeast (Jakob et al, 1998) and the MHC class I molecule in man (Wilson et al, 2000).…”