Different effects of photodynamic therapy and γ-irradiation on vascular smooth muscle cells and matrix: Implications for inhibiting restenosis

Abstract

γ-Irradiation (γ-RT) and photodynamic therapy (PDT) are known to inhibit intimal hyperplasia. The common mechanism is that both modalities produce free radicals, but unlike γ-RT, PDT generates them through the absorption of light by photosensitizers. The purpose of this in vitro study was to assess the differences that PDT and γ-RT have on the fibroproliferative response after vascular injury by comparing their effects on vascular smooth muscle cells (SMCs) and on the extracellular matrix (ECM). Mitochondrial activity (tetrazolium salt), proliferation ([3H]thymidine incorporation), and the mechanisms of cell death (terminal deoxynucleotidyl transferase-mediated dUTP biotin nick end labeling [TUNEL] staining) were used to assess differences between PDT (100 J/cm2) and γ-RT (10 or 20 Gy) on SMC injury. The different effects on bioregulatory molecules were investigated by quantitating the proliferation of SMCs cultured with conditioned medium and on treated ECM. PDT of SMCs reduced proliferation and mitochondrial activity (0.5 ± 0.75% and 1.7 ± 4.25%, respectively, P < 0.0001), whereas γ-RT of SMCs decreased cell proliferation but did not affect metabolic activity. Stimulation with calf serum of γ-RT-treated SMCs did not affect proliferation but increased mitochondrial enzyme activity (160 ± 11%, P < 0.0005). The conditioned medium, derived from PDT- but not γ-RT- treated SMCs, did not stimulate effector SMC proliferation compared with γ- RT-treated SMCs (16 ± 4.1% versus 80 ± 16.8%, P < 0.0001). Apoptosis was the principle cytotoxic mechanism after PDT, whereas γ-RT cells were growth arrested but viable. PDT of the ECM reduced effector SMC proliferation compared with controls and γ-RT cells (18 ± 6.5% versus 100 ± 17.7% and 84 ± 8.9%, respectively, P < 0.0001). These data suggest that γ-RT and PDT may inhibit restenosis but by different mechanisms. The effects of PDT are more diverse and may result in improved outcome while avoiding the teratogenic exposure due to ionizing irradiation.