title: Ambient Light Promotes Selective Subcellular Proteotoxicity after Endogenous and Exogenous Porphyrinogenic Stress.
abstract: Hepatic accumulation of protoporphyrin-IX (PP-IX) in erythropoietic protoporphyria (EPP) or X-linked-dominant protoporphyria (XLP) cause liver damage. Hepatocyte nuclear lamin aggregation is a sensitive marker for PP-IX-mediated liver injury. We tested the hypothesis that extracellular or intracellular protoporphyria cause damage to different subcellular compartments, in a light-triggered manner. Three hepatoma cell lines (HepG2, Hepa-1, and Huh-7) were treated with exogenous PP-IX (mimicking XLP extrahepatic protoporphyria) or with the iron chelator deferoxamine and the porphyrin precursor 5-aminolevulinic acid (ALA) (mimicking intracellular protoporphyrin accumulation in EPP). Exogenous PP-IX accumulated predominantly in the nuclear fraction and caused nuclear shape deformation and cytoplasmic vacuoles containing electron-dense particles, whereas ALA+deferoxamine treatment resulted in higher PP-IX in the cytoplasmic fraction. Protein aggregation in the nuclear and cytoplasmic fractions paralleled PP-IX levels and, in cell culture, the effects were exclusively ambient light-mediated. PP-IX and ALA caused proteasomal inhibition, whereas endoplasmic reticulum protein aggregation was more prominent in ALA-treated cells. The enhanced ALA-related toxicity is likely due to generation of additional porphyrin intermediates including uroporphyrin and coproporphyrin, based on HPLC analysis of cell lysates and the culture medium, as well as cell-free experiments with uroporphyrin/coproporphyrin. Mouse livers from drug-induced porphyria phenocopied the in vitro findings, and mass spectrometry of liver proteins isolated in light/dark conditions showed diminished (as compared with light-harvested) but detectable aggregation under dark-harvested conditions. Therefore, PP-IX leads to endoplasmic reticulum stress and proteasome inhibition in a manner that depends on the source of porphyrin buildup and light exposure. Porphyrin-mediated selective protein aggregation provides a potential mechanism for porphyria-associated tissue injury.
authors.affiliation: From the Departments of Molecular and Integrative Physiology, dhimanm@med.umich.edu.
authors.forename: Dhiman
authors.initials: D
authors.lastname: Maitra
authors.affiliation: From the Departments of Molecular and Integrative Physiology.
authors.forename: Jared S
authors.initials: JS
authors.lastname: Elenbaas
authors.affiliation: From the Departments of Molecular and Integrative Physiology.
authors.forename: Steven E
authors.initials: SE
authors.lastname: Whitesall
authors.affiliation: Pathology, and.
authors.forename: Venkatesha
authors.initials: V
authors.lastname: Basrur
authors.affiliation: From the Departments of Molecular and Integrative Physiology.
authors.forename: Louis G
authors.initials: LG
authors.lastname: D'Alecy
authors.affiliation: From the Departments of Molecular and Integrative Physiology, Internal Medicine, University of Michigan Medical School, Ann Arbor, Michigan 48109 and the Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan 48105.
authors.forename: M Bishr
authors.initials: MB
authors.lastname: Omary
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D005260:Female; D006801:Humans; D008027:Light; D008297:Male; D051379:Mice; D011166:Porphyrins / Q000378:metabolism*; D013347:Subcellular Fractions
chemical_list: D011166:Porphyrins
keywords: Protoporphyrin; aggresome; endoplasmic reticulum stress (ER stress); lamin; lamins; porphyria; proteasome; protein aggregation