Exosomes are released by bystander cells exposed to radiation-induced biophoton signals

Researchers from McMaster University set out 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.

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.

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.

Transmission electron microscopy images illustrating the exosomes
that were extracted from HCT116 p53 +/+ cells via ultracentrifugation

Exosomes are indicated by arrowheads. Scale bars in each of the four panels represent 100 nm. (A) Exosomes extracted from UV-exposed cells, direct magnification: 120 000x. (B) Exosomes extracted from non-UV-exposed control cells, direct magnification: 100 000x. (C) Exosomes from UV-exposed cells, direct magnification: 140 000x. (D) Exosome from UV-exposed cells, direct magnification: 160 000x.

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.