Photodynamic therapy inactivates cell-associated basic fibroblast growth factor: a silent way of vascular smooth muscle cell eradication.

Abstract

Procedurally related vascular injury results in a smooth muscle cell (SMC) proliferative response which is in part initiated by SMC release of mitogens, including basic fibroblast growth factor (bFGF). This injury-induced proliferative response is believed to be a key event in intimal hyperplasia development. Photodynamic therapy (PDT), a novel approach found to be effective in inhibiting experimental intimal hyperplasia, produces cytotoxic free radicals resulting in localized SMC eradication. However, this form of SMC injury does not induce an inflammatory or proliferative response in the vessel wall. This study investigated whether PDT-generated free radicals could inactivate cell-associated bFGF normally released with cell injury. PDT of bovine SMC was performed in vitro with the photosensitizer CASPc (5 micrograms/ml) and 675 nm laser light using three different fluences: 10, 50, and 100 J/cm2. After PDT, SMC viability was determined with the tetrazolium salt (MTT) assay and cell-associated bFGF was quantitated by ELISA. A SMC mitogenesis assay was utilized to detect cell-associated bFGF activity released with SMC injury. In a dose-dependent manner, PDT-generated free radicals reduced cell-associated bFGF levels. After PDT with 100 J/cm2, cell-associated bFGF content was reduced by 88% (P < 0.0002). Of special interest was the finding that PDT with 10 J/cm2 significantly (P < 0.0002) reduced cell viability to around 50%, without affecting cellular bFGF levels. Consequently, a higher PDT dose (100 J/cm2) was needed to significantly (P < 0.001) inhibit the SMC mitogenic response associated with SMC injury. These results provide a mechanism to explain how, unlike mechanical or other forms of SMC injury, optimal doses of PDT can locally eradicate medial vascular SMC without resulting in a bFGF-induced initiation of cell proliferation.