Circadian sleep–wake rhythm reversal in mice implanted with stomach cancer cell lines

The present study explored the phenotype and behavioral characteristics of mice implanted with the 85As2 human stomach cancer cell lines. Generally, mice are nocturnal; they are active during the dark phase and resting in the light phase. However, mice implanted with 85As2 cells demonstrated diurnal patterns, showing activity in the light phase. The similar light-dark behavioral reversal was noted in mice implanted with other cancer cell lines, such as the HCT116 human colon cancer cell lines. Furthermore, 85As2 implanted mice revealed significant shortening of the free-running period under constant dark conditions. To explore the underlying physiological mechanisms of this circadian rhythm reversal, diurnal variations in the suprachiasmatic nucleus (SCN) were analyzed with observation of c-Fos expression. Interestingly, no significant difference was found in the SCN activity between the control and 85As2-implanted mice, demonstrating rhythm reversal. It is suggested that the lesion causing this rhythm reversal exists downstream of the SCN.