Theoretical and experimental considerations linking an environmental stress to expression of endogenous retroviruses in a mammalian tumour.

CBA mice, containing the transplantable CaNT adenocarcinoma were exposed to magnetic fields of 0.5T. Doubling times of the tumours were found to increase significantly from 71 to 123 hours. Electron microscopy of these tumours revealed that the numbers of intracisternal A-type viral-like particles increased from a total of 127 in control mice (counted in 97 fields from 4 different tumours), to 459 in mice exposed to magnetic fields (counted in 95 fields from 5 different tumours). Similarly, intra-cytoplasmic A-type particle numbers increased from 98 to 184. In contrast the number of budding and extracellular C-type virus particles did not differ significantly in mice exposed to the magnetic field. To our knowledge this is the first report linking an environmental stress (magnetic field) to the visible expression of endogenous retro-viral-like type-A particles. We therefore propose the hypothesis that environmental stress may be relevant to the expression of endogenous retroviruses in mammalian cells. Such a mechanism could be relevant in the mechanisms of carcinogenesis and in the rapid evolution of species.