A biochemical mechanism for resistance of intervertebral discs to metastatic cancer: Fas ligand produced by disc cells induces apoptotic cell death of cancer cells

Metastatic spinal cancer is characterized by the maintenance of normal disc structure until the vertebral body is severely destroyed by cancer cells. Anatomic features of the discs have been thought to be the main factor which confer the discs their resistance to metastatic cancer. However, little is known about the biochemical mechanism to prevent or attenuate the local infiltration of cancer cells into the discs. The purpose of this study was to investigate whether Fas ligand (FasL) produced by disc cells can kill Fas-bearing breast cancer cells by Fas and FasL interaction. Two human breast cancer cells (MCF-7 and MDA-MB-231) were obtained and cultured (1 × 10⁶ cells/well), and the expression of Fas was investigated by western blot analysis. Annulus fibrosus cells were isolated and cultured, and the presence of FasL was quantified in the supernatants of three different numbers of annulus fibrosus cells (1×, 2×, and 4 × 10⁶ cells/well) by ELISA assay. The MCF-7 and MDA-MB-231 cancer cells were cultured with supernatants of annulus fibrosus cells for 48 h. As controls, MCF-7 and MDA-MB-231 cancer cells were also cultured by themselves for 48 h. Finally, we determined and quantified the apoptosis rates of MCF-7 and MDA-MB-231 cancer cells by Annexin V–FITC and PI and TUNEL at 48 h, respectively. The expression of Fas was identified in MCF-7 and MDA-MB-231 cancer cells. The mean concentrations of FasL in supernatants of annulus fibrosus cells (1×, 2×, and 4 × 10⁶ cells/well) were 10.8, 29.6, and 56.4 pg/mL, respectively. After treatment with the supernatant of three different numbers of annulus fibrosus cells, the mean apoptosis rate of MCF-7 cancer cells was increased (2.8%, P < 0.01; 6.7%, P < 0.001; 31.0%, P < 0.001) in a dose-dependent manner of FasL compared to that of control (1.1%). The mean apoptosis rate of MDA-MB-231 cancer cells was also increased (5.7%, P < 0.01; 11.1%, P < 0.001; 25.3%, P < 0.001) in a dose-dependent manner of FasL compared to that of control (2.1%). TUNEL also demonstrated direct evidence of apoptoses of MCF-7 and MDA-MB-231 cancer cells. Our results demonstrate that Fas-bearing cancer cells undergo apoptosis by FasL produced by disc cells, which may be considered as a potential biochemical explanation for the disc’s resistance to metastatic cancer.