Countervailing effects of rapamycin (sirolimus) on nuclear factor-κB activities in neointimal and medial smooth muscle cells

Objective Local application of rapamycin (sirolimus) by drug-eluting stents prevents lumen obliteration after angioplasty by inhibition of neointimal hyperplasia. The effects of rapamycin on neointimal smooth muscle cells (niSMC) which are responsible for the occurrence of restenosis have not been investigated so far. Methods and results Rat niSMC and medial SMC (mSMC) were obtained from balloon catheter-injured arteries. The niSMC exhibited higher basal NF-κB activity and TNF-α mRNA levels. Nuclear protein binding to NF-κB-DNA was attenuated in niSMC by incubation with rapamycin (0.1 and 1 μg/ml) for 24 and 48 h. In contrast in mSMC, 0.1 μg/ml rapamycin had no effect and at 1 μg/ml even increased nuclear protein binding to NF-κB-DNA. After 12 h incubation, rapamycin (0.001–10 μg/ml) induced IκB-α protein in niSMC, whereas in mSMC it stimulated IκB-α at much lower levels. Prolonged rapamycin treatment (1 μg/ml for 72 h) had no effect on TNF-α mRNA level and NF-κB activity in niSMC, whereas it led to their increase in mSMC. Vascular endothelial growth factor (VEGF) secretion was higher in mSMC than in niSMC; rapamycin decreased VEGF levels in both cell types. Ultrastructual analysis suggested that rapamycin caused early signs of degeneration in niSMC, but enhanced protein synthesis in mSMC. Conclusions This study shows that rapamycin influences the inflammatory phenotypes of SMC in opposite directions: it reduces the high basal NF-κB activity in niSMC and enhances NF-κB activity and TNF-α expression in mSMC. In addition, rapamycin inhibits VEGF production regardless of the phenotype of SMC. These findings shed light on molecular mechanisms and structural changes underlying therapeutic applications of rapamycin in prevention of restenosis, inhibition of chronic transplant arteriosclerosis and reduction of secondary malignoma formation due to immunosuppression.