Abstract
Recent evidence indicates that the protein synthesis inhibitor cycloheximide triggers selective macrophage death in rabbit atheroma-like lesions without affecting smooth muscle cells (SMCs) or the endothelium, thereby favoring a stable plaque phenotype. In this study, we report that puromycin, a protein synthesis inhibitor with a different mode of action but with similar ability to inhibit de novo protein synthesis, did not reveal plaque-stabilizing effects. The macrophage and the SMC content readily decreased in puromycin-treated atheroma-like lesions in rabbit carotid arteries. Moreover, puromycin induced apoptosis in macrophages and SMCs in vitro. Puromycin-treated SMCs showed signs of endoplasmic reticulum (ER) stress, as demonstrated by CCAAT/enhancer-binding protein homologous protein (CHOP) protein expression, splicing of X-box-binding protein 1 mRNA, and phosphorylation of eukaryotic translation initiation factor 2α. The ER stress inducer thapsigargin up-regulated CHOP protein expression in SMCs without affecting their viability, indicating that ER stress not necessarily results in cell death. Puromycin, but not thapsigargin, activated the ER stress-related caspase-12. Treatment of SMCs with a combination of cycloheximide and puromycin inhibited ER stress and partially improved SMC viability. In addition, puromycin, but not cycloheximide or thapsigargin, induced intracellular accumulation of polyubiquitinated proteins in SMCs, whereas the proteasome function was not affected. Taken together, puromycin, in contrast to cycloheximide, induces SMC apoptosis, thereby favoring an unstable plaque phenotype. SMC death upon puromycin treatment could only be partially prevented by cycloheximide, which completely blocked ER stress. However, other or additional mechanisms, such as increased polyubiquitination of proteins, might be involved in puromycin-induced SMC death.
Footnotes
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This research was supported by the Fonds voor Wetenschappelijk Onderzoek (FWO)-Flanders (Belgium) (projects G.0308.04, G.0113.06, and G.0112.08), the University of Antwerp (Nieuwe Onderzoeksinitiatieven-Bijzonder Onderzoeksfonds), and the Bekales Foundation. W.M. is a postdoctoral fellow of the FWO-Flanders (Belgium).
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.107.132944.
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ABBREVIATIONS: SMC, smooth muscle cell; ER, endoplasmic reticulum; TUNEL, terminal deoxynucleotidyl transferase dUTP nick-end labeling; 4-PBA, sodium 4-phenylbutyrate; zATADfmk, z-Ala-Thr-Ala-Asp-fluoromethyl ketone; eGFP, enhanced green fluorescent protein; PCR, polymerase chain reaction; C/EBP, CCAAT/enhancer-binding protein; CHOP, C/EBP homologous protein; XBP, X-box-binding protein; eIF2α, eukaryotic translation initiation factor 2α; MODC, mouse ornithine decarboxylase; MG132, N-benzoyloxycarbonyl (Z)-Leu-Leu-leucinal; UPR, unfolded protein response; ANOVA, analysis of variance; PM, puromycin; CHX, cycloheximide.
- Received October 12, 2007.
- Accepted March 3, 2008.
- The American Society for Pharmacology and Experimental Therapeutics
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