Mutation Research 597 (2006) 133–137 The effect of melanin on the bystander effect in human keratinocytes I. Mosse a,∗ , P. Marozik a,b , C. Seymour c , C. Mothersill c a Institute of Genetics and Cytology of the National Academy of Sciences, Academicheskaya Str. 27, Minsk, Belarus b Radiation and Environmental Science Centre, DIT, Dublin 8, Ireland c Department of Medical Physics and Applied Radiation Sciences, McMaster University, Hamilton, Ont., Canada Received 8 March 2005; received in revised form 17 June 2005; accepted 14 September 2005 Available online 18 January 2006 Abstract The influence of melanin on radiation-induced bystander effects has been studied. Melanin is known to be a natural substance with proved radioprotective properties in different organisms and cell lines. It is non-toxic and is effective against acute and chronic irradiation. The lower the radiation dose, the higher the relative impact of melanin protection. In this study influence of melanin on human keratinocytes (HPV-G cells) has been studied using the colony-forming assay. We have shown that bystander donor medium from 0.5 Gy irradiated cells when transferred to unirradiated cells, caused almost the same effect as direct irradiation. Melanin increased the colony-forming ability of bystander recipient cells when it was added into culture medium before irradiation. The effect of melanin added after irradiation was to produce less protection in both the directly irradiated and bystander medium treated groups. The absorption spectrum of the filtered medium is identical to one of the intact culture medium showing that melanin was not present in filtered medium. Thus, it cannot protect recipient cells but reduces the amount of the bystander effect. It is concluded that melanin added before irradiation effectively decreased the radiation dose. The reduction of the impact of the bystander signal on recipient cells when melanin was added to the donor medium after harvest but before filtration, may mean that the bystander signal has a physical component as melanin can absorb all types of physical energy. © 2005 Elsevier B.V. All rights reserved. Keywords: Bystander effect; Melanin; Radioprotector; Low radiation dose; Human keratinocytes; Culture media 1. Introduction Recently there has been interest in the phenomenon of information transfer mediated by intercellular signal- ing from irradiated to unirradiated cells. The nature of these signals and associated mechanisms are not clear. The phenomenon is known as “bystander” effect—the cellular damage is expressed in unirradiated neighboring or bystander cells, connected or not to an irradiated cell ∗ Corresponding author. Tel.: +375 172 841942; fax: +375 172 841917. E-mail address: i.mosse@igc.bas-net.by (I. Mosse). or cells. There is evidence that the bystander effect may have at least two separate pathways for the transfer of damage from irradiated cells to unirradiated neighbors: through gap junctions [1] or by cell culture-mediated fac- tors [2]. Mothersill and Seymour [3] reported data, which suggest that the bystander effect does not depend on com- munication through gap junctions formed between cells in contact. The use of appropriate radioprotectors able to mod- ify intercellular transduction of the signals may allow a measure of control of this phenomenon in the radiation therapy of cancer and in radioprotection. We studied the influence of the pigment melanin on the bystander effect. There is some evidence for a 0027-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mrfmmm.2005.09.006