Circadian rhythm and cell cycle: possible entraining mechanisms.

Circadian rhythms and cell cycles are endogenous self-sustained oscillations. They differ in several characteristics of the rhythmicity such as temperature dependency, reaction to external stimuli and the impact of protein synthesis. In multicellular organisms, the cell cycle very often is entrained by a circadian rhythm as has been analyzed in rats and mice, in particular. The same seems to hold true in cell cultures: Yoshida-ascites hepatoma cells, human embryonic fibroblasts, rat liver and rat hepatoma cells show circadian changes in the percentage of cells in G1-, S- and G2 + M-phases. The different phases within the cell cycle were determined by applying impulse cytophotometric methods. Some hypothetical mechanisms of entrainment of the cell cycle by circadian rhythms are discussed. Possible entrancing signals (Zeitgebers) are membrane and transport functions, cyclic nucleotides, nuclear non-histone proteins and their phosphorylation, and RNA synthesis. The maxima of the circadian rhythms of most of these functions in rat liver can be arranged in a certain temporal sequence.