Role of photosensitization and oxygen in chromosome stability and "spontaneous" malignant transformation in culture.

Visible light and oxygen enhanced both chromosome instability and malignant transformation of mouse cells in culture. Nine cell lines were initiated from 8 pools of 10- to 13-day C3H embryos. Each cell line was divided into sublines, which were either maintained shielded from light or were exposed for 3 or 24 hours to fluorescent light (approximately 150 foot-candles) two or three times weekly. Cultures of the sublines were also maintained with either a gaseous phase of 0-1% oxygen or atmospheric (18%) oxygen. Each line was monitored for cytologic manifestations of malignant neoplastic transformation, and 8 lines were monitored for chromosome alterations. Seven lines were assayed for tumorigenicity by intraocular implantation into syngeneic hosts. Repeated light exposure and/or high oxygen increased the frequency of minute chromosomes, which result from chromatid breaks, and also increased the rate of shift from diploid to heteroploid state. Four cell lines showed no cytologic changes indicative of neoplastic change during the test period. Two of these were assayed in vivo and failed to grow as tumors. In the remaining 6 lines, cytologically neoplastic colonies appeared earlier or more abundantly in the light-exposed cultures and/or those gassed with high oxygen. In 3 of these lines, tumors developed only from the light-exposed cultures; in the other 2, tumor latency periods were significantly shorter in the cultures exposed to light or gassed with atmospheric oxygen.