Received for publication October 12, 2007.
Revised February 15, 2008.
Accepted for publication March 3, 2008.

Differential effect of the protein synthesis inhibitors puromycin and cycloheximide on vascular smooth muscle cell viability

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. Here, 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 C/EBP homologous protein (CHOP) protein expression, splicing of X-box binding protein 1 mRNA and phosphorylation of eukaryotic translation initiation factor 2. The ER stress inducer thapsigargin upregulated 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 while 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.

Key words: apoptosis, atherosclerosis, cycloheximide, macrophages, puromycin, vascular smooth muscle cells