Ambient Light Promotes Selective Subcellular Proteotoxicity after Endogenous and Exogenous Porphyrinogenic Stress

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.