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

Mutant α 1 -antitrypsin Z (α 1 -ATZ) protein, which has a tendency to form aggregated polymers as it accumulates within the endoplasmic reticulum of the liver cells, is associated with the development of chronic liver injury and hepatocellular carcinoma in hereditary α 1 -antitrypsin (α 1 -AT) deficiency. Previous studies have suggested that efficient intracellular degradation of α 1 -ATZ is correlated with protection from liver disease in α 1 -AT deficiency and that the ubiquitin-proteasome system accounts for a major route, but not the sole route, of α 1 -ATZ disposal. Yet another intracellular degradation system, autophagy, has also been implicated in the pathophysiology of α 1 -AT deficiency. To provide genetic evidence for autophagy-mediated disposal of α 1 -ATZ, here we used cell lines deleted for the Atg5 gene that is necessary for initiation of autophagy. In the absence of autophagy, the degradation of α 1 -ATZ was retarded, and the characteristic cellular inclusions of α 1 -ATZ accumulated. In wild-type cells, colocalization of the autophagosomal membrane marker GFP-LC3 and α 1 -ATZ was observed, and this colocalization was enhanced when clearance of autophagosomes was prevented by inhibiting fusion between autophagosome and lysosome. By using a transgenic mouse with liver-specific inducible expression of α 1 -ATZ mated to the GFP-LC3 mouse, we also found that expression of α 1 -ATZ in the liver in vivo is sufficient to induce autophagy. These data provide definitive evidence that autophagy can participate in the quality control/degradative pathway for α 1 -ATZ and suggest that autophagic degradation plays a fundamental role in preventing toxic accumulation of α 1 -ATZ.