High-dose copper activates p53-independent apoptosis through the induction of nucleolar stress in human cell lines

Copper is an essential micronutrient involved in many redox reactions in human cells. However, a high concentration of copper, intake from the environment or abnormal accumulation within cells because of genetic mutation, leads to cell toxicity. This is attributable to oxidative damage, altered gene expression, and functional impairment of the mitochondria. Copper stress also alters the morphology of the nucleolus, but the process has not been fully elucidated. In this study, cells were treated with copper sulfate at 3–9 ppm and examined if a high dose of copper would block ribosome biogenesis. With the incorrect distribution of nucleolar proteins nucleophosmin and fibrillarin to the nucleoplasm, ribosomal RNA (rRNA) processing was impaired; 34S rRNA from an abnormal A2 cut increased, and downstream pre-rRNAs decreased. The under-accumulation of 60S subunits was detected using sucrose gradients. From transcriptome analysis, ribosome synthesis–related genes were misregulated. Blockage in ribosome synthesis under copper-treatment induced nucleolar stress and triggered p53-independent apoptosis pathways. Thus, nucleolar stress is one cause of cell death under copper exposure.