Possible influences of circadian rhythms in experimental carcinogenesis.

The induction of tumours by chemical carcinogens depends not only on the chemical structure of the substance but also on biological properties of the target cell population, such as enzyme patterns, DNA repair mechanisms and the proliferative state of the cells. Interference with any of these processes can theoretically affect the tumour yield, as can variations due to biological rhythms. A series of investigations of circadian and circannual rhythms in the hairless mouse epidermis (hr/hr) is reviewed. DNA synthesis as well as the mitotic rate undergo great variations in the course of a 24-h period. The respiration and glycolysis in this epidermis undergo strong circannual variations. Evidence that the eventual tumour development after methylcholanthrene application to the skin is modified by circadian variations in DNA synthesis is presented and discussed in relation to pertinent data from other cell systems.