The influence of ventral UVA exposure on subsequent tumorigenesis in mice by UVA or UVB irradiation.

Exposure to ultraviolet-B radiation (UVB: 280-315 nm) can result in a decreased immune response. This immune suppression can be restricted to the exposed skin site (local immune suppression) but may also be systemic. To investigate whether ultraviolet-A radiation (UVA: 315-400 nm) could also exert such a systemic effect, we performed the present investigation. The study consisted of two parts. Experiment I: 24 albino hairless mice (SKH:HRI) were ventrally exposed to UVA radiation for 300 days (glass-filtered Philips TLK09 fluorescent tubes, daily dose: 350 kJ/m2), while 24 control mice were left unexposed. After this period the control animals were still tumour free, but 60% of the exposed animals had developed abdominal tumours. Subsequently ventral exposures were stopped and both groups were dorsally exposed to identical UVB regimens (Westinghouse FS40, daily dose: 900 J/m2). Experiment II: this was virtually the same as experiment I, but here the mice were dorsally exposed to UVA radiation (glass-filtered Philips TLK09, daily dose: 290 kJ/m2) instead of UVB radiation. If we look at all tumours induced dorsally, we find no significant influence of pre-exposures to UVA radiation. This holds for dorsal UVB as well as for dorsal UVA exposures. In contrast to UVB, however, the UVA radiation induced many papillomas. Excluding the papillomas from the analysis we find that the induction of non-papillomas (mainly squamous cell carcinomas) under dorsal UVA exposure, is slightly enhanced in the ventrally pre-exposed group (difference significant at the P < 0.05 level). This suggests that UVA radiation induced only a weak systemic effect. Ventral UVA pre-exposure did not appear to affect dorsal skin irritation as expressed by scratch marks. The induction period for hyperkeratosis, however, was significantly shortened by the ventral UVA pre-exposure; this applied to dorsal UVB as well as dorsal UVA exposures.