Albanese, J. and Dainiak, N. Ionizing Radiation Alters Fas Antigen Ligand at the Cell Surface and on Exfoliated Plasma Membrane-Derived Vesicles: Implications for Apoptosis and Intercellular Signaling.

Resident proteins that reside on the plasma membrane are continually exfoliated from the cell surface. Exfoliation is a selective, energy-dependent process that mediates intercellular communication. Ionizing radiation modulates the expression of many plasma membrane-bound growth regulators, including the “death” ligand, TNFSF6 (formerly known as FasL, CD95L). Here we report that ionizing radiation induces dose-dependent up-regulation of TNFSF6 on plasma membranes purified from SW620 cells, a TNFSF6-expressing colon cancer cell line. Serum-free medium conditioned by exposed and control cells was collected and exfoliated vesicles were obtained by ultracentrifugation. Western blot analysis of vesicles from unexposed cells and from cells treated with 10 Gy showed increased amounts of TNFSF6 compared to that on vesicles from unexposed cells. Cells treated with 4 Gy released vesicles having a low level of TNFSF6 on their surface relative to that on vesicles exfoliated from unexposed cells. When assayed for bioactivity, vesicles from unexposed cells induced the greatest level of apoptosis in TNFRSF6 (formerly known as FAS) receptor-bearing Jurkat cells (cell surviving fraction of 43.7 ± 6.1; P < 0.05), followed by vesicles collected from cells treated with 4 Gy (79.6 ± 2.6%; P < 0.05). Despite having a high level of TNFSF6 by Western analysis, vesicles collected from cells exposed to 10 Gy display minimal biological activity (77.9 ± 3.2%; P < 0.05), suggesting that modification of the vesicle-associated ligand has occurred. Our results indicate that ionizing radiation increases the level of TNFSF6 exfoliated on extracellular vesicles. The data may provide a mechanism for abscopal and bystander effects after irradiation.