Exosomes are released by bystander cells exposed to radiation-induced biophoton signals: Reconciling the mechanisms mediating the bystander effect

PLoS One. 2017 Mar 9;12(3):e0173685. doi: 10.1371/journal.pone.0173685. eCollection 2017.


Objective: The objective of our study was to explore a possible molecular mechanism by which ultraviolet (UV) biophotons could elicit bystander responses in reporter cells and resolve the problem of seemingly mutually exclusive mechanisms of a physical UV signal & a soluble factor-mediated bystander signal.

Methods: The human colon carcinoma cell line, HCT116 p53 +/+, was directly irradiated with 0.5 Gy tritium beta particles to induce ultraviolet biophoton emission. Bystander cells were not directly irradiated but were exposed to the emitted UV biophotons. Medium was subsequently harvested from UV-exposed bystander cells. The exosomes extracted from this medium were incubated with reporter cell populations. These reporter cells were then assayed for clonogenic survival and mitochondrial membrane potential with and without prior treatment of the exosomes with RNase.

Results: Clonogenic cell survival was significantly reduced in reporter cells incubated with exosomes extracted from cells exposed to secondarily-emitted UV. These exosomes also induced significant mitochondrial membrane depolarization in receiving reporter cells. Conversely, exosomes extracted from non-UV-exposed cells did not produce bystander effects in reporter cells. The treatment of exosomes with RNase prior to their incubation with reporter cells effectively abolished bystander effects in reporter cells and this suggests a role for RNA in mediating the bystander response elicited by UV biophotons and their produced exosomes.

Conclusion: This study supports a role for exosomes released from UV biophoton-exposed bystander cells in eliciting bystander responses and also indicates a reconciliation between the UV-mediated bystander effect and the bystander effect which has been suggested in the literature to be mediated by soluble factors.

MeSH terms

  • Bystander Effect / radiation effects*
  • Cell Survival / radiation effects*
  • Exosomes / physiology*
  • Exosomes / radiation effects
  • HCT116 Cells
  • Humans
  • Membrane Potential, Mitochondrial / radiation effects*
  • Photons*
  • Ultraviolet Rays*

Grants and funding

The authors would like to thank the following funding agencies for providing financial support for this research endeavour: The National Chronic Fatigue and Immune Dysfunction Syndrome (CFIDS) Foundation (url: http://www.ncf-net.org/; recipient: CEM), the Natural Sciences and Engineering Research Council of Canada (url: http://www.nserc-crsng.gc.ca/index_eng.asp; recipient: CEM; grant number RGPIN293153-12; recipient: FEM; grant number RGPIN203611-12), Canada Research Chairs Programme (url: http://www.chairs-chaires.gc.ca/home-accueil-eng.aspx; recipient: CEM; grant number 950-221284), CANDU Owners Group (url: http://www.candu.org/; recipient: CBS; grant number CRDPJ484381-15). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.