Breast cancer metastases form mainly during sleep

Breast cancer mainly spreads when we sleep. This was reported by a research group led by cell biologist Zoi Diamantopoulou from ETH Zurich in the journal »Nature«. In the process, the cells break away from the original tumor, enter the bloodstream, and form metastases elsewhere. Until now, these circulating cancer cells (CTCs) were assumed to constantly form or arise as a reaction to mechanical stimuli such as surgery.

The team drew a blood sample from 30 breast cancer patients twice: at 4:00 (sleep phase) and 10:00 (wake phase). He found that nearly 80 percent of the total CTCs came from the nocturnal blood sample.

To explore this surprising discovery in more detail, the scientists looked at mice that had been genetically engineered to develop breast cancer or that had been injected with human breast cancer cells. In rodents, CTCs are also formed mainly during sleep, but during the day, as mice are nocturnal. If the researchers kept the animals awake longer, significantly fewer circulating cancer cells formed during the day.

The administration of melatonin, on the other hand, led to more cancer cells in the bloodstream. The hormone regulates the sleep-wake cycle and has a sleep-inducing effect. In genetically engineered mice without a functioning circadian rhythm, CTCs formed regardless of the rest period.

The authors also injected resting and active CTCs into healthy, tumor-free mice at different stages of the circadian cycle. They found that cells in the resting phase form more aggressive tumors than those in the active phase. Furthermore, resting mice were more likely to develop tumors than active animals.

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