Polymers of Z α1-antitrypsin are secreted in cell models of disease

Alpha-1 antitrypsin (A1AT) is predominantly synthesised in the liver and secreted into the circulation, where it protects the lungs from the enzyme neutrophil elastase. Mutations in A1AT, particularly the Z mutation, cause A1AT deficiency, characterised by polymerisation and retention of A1AT within hepatocytes, low levels of circulating A1AT, uncontrolled elastase activity and emphysema. Patients also display variable amounts of circulating polymers in their plasma, but it is unknown whether polymers can be secreted from hepatocytes or form in plasma from secreted monomeric Z-A1AT. Here we investigated the origin of extracellular polymers in cell culture models of A1AT deficiency and showed that: all extracellular Z-A1AT contained only mature N-linked glycosylation, which excludes direct release of immature ER proteins upon cell death; polymers still appeared in the culture medium of transfected cells cultured in the presence of a polymerisation blocking antibody (4B12); most of the polymerised Z-A1AT co-localised with BiP but there was also partial co-localisation with proteins of the Golgi apparatus; and that cells co-transfected with tagged versions of Z-A1AT showed immunoprecipitation patterns compatible with intracellular polymerisation only. These results demonstrate the intracellular origin of secreted polymers in vitro, and support that plasma polymers derive at least in part from hepatocyte secretion.