Intracellular Inclusions Containing Mutant alpha sub(1)-Antitrypsin Z Are Propagated in the Absence of Autophagic Activity

Mutant alpha sub(1)-antitrypsin Z ( alpha sub(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 alpha sub(1)-antitrypsin ( alpha sub(1)-AT) deficiency. Previous studies have suggested that efficient intracellular degradation of alpha sub(1)-ATZ is correlated with protection from liver disease in alpha sub(1)-AT deficiency and that the ubiquitin-proteasome system accounts for a major route, but not the sole route, of alpha sub(1)-ATZ disposal. Yet another intracellular degradation system, autophagy, has also been implicated in the pathophysiology of alpha sub(1)-AT deficiency. To provide genetic evidence for autophagy-mediated disposal of alpha sub(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 alpha sub(1)-ATZ was retarded, and the characteristic cellular inclusions of alpha sub(1)-ATZ accumulated. In wild-type cells, colocalization of the autophagosomal membrane marker GFP-LC3 and alpha sub(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 alpha sub(1)-ATZ mated to the GFP-LC3 mouse, we also found that expression of alpha sub(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 alpha sub(1)-ATZ and suggest that autophagic degradation plays a fundamental role in preventing toxic accumulation of alpha sub(1)-ATZ.