Intracellular inclusions containing mutant α₁-antitrypsin Z are propagated in the absence of autophagic activity

Mutant α₁-antitrypsin Z (α₁-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 α₁-antitrypsin (α₁-AT) deficiency. Previous studies have suggested that efficient intracellular degradation of α₁-ATZ is correlated with protection from liver disease in α₁-AT deficiency and that the ubiquitin-proteasome system accounts for a major route, but not the sole route, of α₁-ATZ disposal. Yet another intracellular degradation system, autophagy, has also been implicated in the pathophysiology of α₁-AT deficiency. To provide genetic evidence for autophagy-mediated disposal of α₁- 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 α₁-ATZ was retarded, and the characteristic cellular inclusions of α₁-ATZ accumulated. In wild-type cells, colocalization of the autophagosomal membrane marker GFP-LC3 and α₁-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 α₁-ATZ mated to the GFP-LC3 mouse, we also found that expression of α₁-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 α₁- ATZ and suggest that autophagic degradation plays a fundamental role in preventing toxic accumulation of α₁-ATZ.