| Apoptosis, a mechanism for modulating tissue growth, is triggered by signals (Fas ligand (FasL), tumor necrosis factor-alpha (TNF-alpha)) derived from other cells as well as external factors such as ultraviolet radiation (UV). We have investigated the possibility that FasL, Fas, M-CSF and flt3/flk2 ligand are released on shed vesicles from HuT 78 cells, tumor cell lines (MIP-101 and CX-1), and Chinese hamster ovary (CHO) cells transfected with cDNA encoding for M-CSF and flt3/flk2 ligand, respectively. Further, we examined the effects of UV-B irradiation on the capacity of CHO cells to shed vesicles per se, and on the capacity of CHO cells transfected with cDNA encoding flt3/flk2 ligand to express flt3/flk2 ligand on their cell surface. Vesicles and plasma membranes from CHO cells solubilized in N-octyl-beta-D-glycopyranoside, electrophoresed in polyacrylamide and reacted with anti-human M-CSF or anti-flt3/flk2 ligand antibody revealed bands of 90 and 45 kDal or 32 kDal bands, respectively. Following irradiation with UV-B (600 Joules/m2), CHO cells transfected with flt3/flk2 ligand cDNA failed to express the protein at their cell surface. When plasma membranes and shed vesicles collected from HuT 78 cells, solubilized, electrophoresed in polyacrylamide and reacted with anti-FasL IgG, immunoreactive FasL protein was detected in membranes and vesicles from HuT 78 cells. When exposed to FasL-bearing SVs 67.2 +/- 0.59% of Fas-expressing tumor cells survived as compared to cells treated with SVs derived from CX-1 cells (control), results which were comparable to the level of cell death observed following treatment with anti-Fas antibody (69.1 +/- 1.25%). Tumor cells treated with FasL- and Fas-bearing SVs simultaneously, exhibited 96.4 +/- 1.04% viability, indicating that FasL and Fas interact when expressed on SVs, in vitro. Our results suggest that diminished release of shed vesicles represents a potential early event in radiation-induced apoptosis occurring even prior to plasma membrane blebbing, and adds to the accumulating body of evidence that the signal in radiation-induced apoptosis is propagated from the cell surface to the nucleus. In addition, we provide evidence to suggest that both, ligand and Fas receptor are shed on extracellular vesicles that are derived from the plasma membrane. Since they retain a bioactive configuration, shed vesicles expressing FasL or Fas antigen may interact with the cell of origin or neighboring cells to trigger autocrine and paracrine apoptosis, respectively. (Abstract shortened by UMI.). |
