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 2alpha. 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.