FBG1 Is the Final Arbitrator of A1AT-Z Degradation

PLoS One. 2015 Aug 21;10(8):e0135591. doi: 10.1371/journal.pone.0135591. eCollection 2015.

Abstract

Alpha-1 antitrypsin deficiency is the leading cause of childhood liver failure and one of the most common lethal genetic diseases. The disease-causing mutant A1AT-Z fails to fold correctly and accumulates in the endoplasmic reticulum (ER) of the liver, resulting in hepatic fibrosis and hepatocellular carcinoma in a subset of patients. Furthermore, A1AT-Z sequestration in hepatocytes leads to a reduction in A1AT secretion into the serum, causing panacinar emphysema in adults. The purpose of this work was to elucidate the details by which A1AT-Z is degraded in hepatic cell lines. We identified the ubiquitin ligase FBG1, which has been previously shown to degrade proteins by both the ubiquitin proteasome pathway and autophagy, as being key to A1AT-Z degradation. Using chemical and genetic approaches we show that FBG1 degrades A1AT-Z through both the ubiquitin proteasome system and autophagy. Overexpression of FBG1 decreases the half-life of A1AT-Z and knocking down FBG1 in a hepatic cell line, and in mice results in an increase in ATAT. Finally, we show that FBG1 degrades A1AT-Z through a Beclin1-dependent arm of autophagy. In our model, FBG1 acts as a safety ubiquitin ligase, whose function is to re-ubiquitinate ER proteins that have previously undergone de-ubiquitination to ensure they are degraded.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • Apoptosis Regulatory Proteins / genetics*
  • Apoptosis Regulatory Proteins / metabolism
  • Autophagy / genetics*
  • Beclin-1
  • Cell Line, Tumor
  • Disease Models, Animal
  • Endoplasmic Reticulum / metabolism
  • Female
  • Gene Expression Regulation
  • Half-Life
  • Hepatocytes / cytology
  • Hepatocytes / metabolism
  • Humans
  • Membrane Proteins / genetics*
  • Membrane Proteins / metabolism
  • Mice
  • Mice, Transgenic
  • Mutation
  • Proteasome Endopeptidase Complex / metabolism*
  • Protein Binding
  • Proteolysis
  • RNA, Small Interfering / genetics
  • RNA, Small Interfering / metabolism
  • SKP Cullin F-Box Protein Ligases / antagonists & inhibitors
  • SKP Cullin F-Box Protein Ligases / genetics*
  • SKP Cullin F-Box Protein Ligases / metabolism
  • Signal Transduction
  • Ubiquitination
  • alpha 1-Antitrypsin / genetics*
  • alpha 1-Antitrypsin / metabolism
  • alpha 1-Antitrypsin Deficiency / genetics*
  • alpha 1-Antitrypsin Deficiency / metabolism
  • alpha 1-Antitrypsin Deficiency / pathology

Substances

  • Apoptosis Regulatory Proteins
  • BECN1 protein, human
  • Beclin-1
  • Membrane Proteins
  • RNA, Small Interfering
  • alpha 1-Antitrypsin
  • SKP Cullin F-Box Protein Ligases
  • Proteasome Endopeptidase Complex

Grant support

Veteran Affairs Research Career Development Award to KAG. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.