Autophagy activation clears ELAVL1/HuR-mediated accumulation of SQSTM1/p62 during proteasomal inhibition in human retinal pigment epithelial cells

PLoS One. 2013 Jul 29;8(7):e69563. doi: 10.1371/journal.pone.0069563. Print 2013.

Abstract

Age-related macular degeneration (AMD) is the most common reason of visual impairment in the elderly in the Western countries. The degeneration of retinal pigment epithelial cells (RPE) causes secondarily adverse effects on neural retina leading to visual loss. The aging characteristics of the RPE involve lysosomal accumulation of lipofuscin and extracellular protein aggregates called "drusen". Molecular mechanisms behind protein aggregations are weakly understood. There is intriguing evidence suggesting that protein SQSTM1/p62, together with autophagy, has a role in the pathology of different degenerative diseases. It appears that SQSTM1/p62 is a connecting link between autophagy and proteasome mediated proteolysis, and expressed strongly under the exposure to various oxidative stimuli and proteasomal inhibition. ELAVL1/HuR protein is a post-transcriptional factor, which acts mainly as a positive regulator of gene expression by binding to specific mRNAs whose corresponding proteins are fundamental for key cellular functions. We here show that, under proteasomal inhibitor MG-132, ELAVL1/HuR is up-regulated at both mRNA and protein levels, and that this protein binds and post-transcriptionally regulates SQSTM1/p62 mRNA in ARPE-19 cell line. Furthermore, we observed that proteasomal inhibition caused accumulation of SQSTM1/p62 bound irreversibly to perinuclear protein aggregates. The addition of the AMPK activator AICAR was pro-survival and promoted cleansing by autophagy of the former complex, but not of the ELAVL1/HuR accumulation, indeed suggesting that SQSTM1/p62 is decreased through autophagy-mediated degradation, while ELAVL1/HuR through the proteasomal pathway. Interestingly, when compared to human controls, AMD donor samples show strong SQSTM1/p62 rather than ELAVL1/HuR accumulation in the drusen rich macular area suggesting impaired autophagy in the pathology of AMD.

MeSH terms

  • Adaptor Proteins, Signal Transducing / genetics
  • Adaptor Proteins, Signal Transducing / metabolism*
  • Aminoimidazole Carboxamide / analogs & derivatives
  • Aminoimidazole Carboxamide / pharmacology
  • Autophagy
  • Blotting, Western
  • Cell Line
  • ELAV Proteins / genetics
  • ELAV Proteins / metabolism*
  • ELAV-Like Protein 1
  • Epithelial Cells / cytology*
  • Epithelial Cells / drug effects
  • Humans
  • Immunohistochemistry
  • Immunoprecipitation
  • Leupeptins / pharmacology
  • Microscopy, Electron, Transmission
  • Proteasome Endopeptidase Complex / drug effects
  • Proteasome Endopeptidase Complex / metabolism*
  • RNA Interference
  • Retinal Pigment Epithelium / cytology*
  • Retinal Pigment Epithelium / drug effects
  • Retinal Pigment Epithelium / metabolism*
  • Reverse Transcriptase Polymerase Chain Reaction
  • Ribonucleotides / pharmacology
  • Sequestosome-1 Protein

Substances

  • Adaptor Proteins, Signal Transducing
  • ELAV Proteins
  • ELAV-Like Protein 1
  • ELAVL1 protein, human
  • Leupeptins
  • Ribonucleotides
  • SQSTM1 protein, human
  • Sequestosome-1 Protein
  • Aminoimidazole Carboxamide
  • Proteasome Endopeptidase Complex
  • AICA ribonucleotide
  • benzyloxycarbonylleucyl-leucyl-leucine aldehyde

Grants and funding

This work was supported by the Kuopio University Hospital EVO grant 5503726, the Academy of Finland, the Finnish Cultural Foundation and its North Savo Fund, the Finnish Eye Foundation, Evald and Hilda Nissi Foundation and Blind Friends Society. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.