Breast cancer is the most common cancer in women. The cumulative risk that a womandeveloped breast cancer is around 5% worldwide, with a 1.4% risk of death. In 2018, more than 2 million new cases were diagnosed, which represents almost 25% of all cancer cases worldwide. In France, the Ligue contre le cancer claims that with approximately 54,062 new people affected each year, this is the most prevalent of female cancers. Various epidemiological studies carried out in recent years in France and abroad have made it possible to better understand the factors likely to increase the risks of developing breast cancer.
We know that less than 10% of cases are hereditary and have a genetic origin. In most cases, the different risk factors identifiedare behavioral factors lfor example, tobacco, poor diet, physical inactivity or alcohol consumption. There are also hormonal factors in connection with taking a very early or very prolonged contraceptive pill or taking hormonal treatments at menopause. Finally, scientists are becoming more and more interested environmental factors such as air pollution or altered light / dark cycles, as is the case with so-called “night workers”. “
Circadian rhythm disturbances disrupt the body
It’s precisely at this effect of chronic jet lag on the development of breast tumors of interest to Inserm researchers, in a study published in the journal Nature Communications. Indeed we all have a circadian rhythm, either an internal clock, a biological rhythm integrated taking the form of a continuous cycle and governing certain physiological processes such as the sleep / wake process and food. More specifically, Inserm explains that the circadian rhythm is defined by the alternation between waking, period of the day during which one is awake and sleep, that during which one sleeps. It’s around 24 hours.
In this field, several synchronizers act simultaneously, in particular light, the effect of which depends on the duration of the light exposure. Now, a work can disrupt this circadian rhythm due to changes in light exposure and sleep debt. For example, by obliging the person to stay awake at night and sleep during the day, or to work in different shifts which constantly modify the periods of sleep and wakefulness, which can have negative physical repercussions. Studies have thus evoked disturbances at the metabolic, cardiovascular, immune and cognitive level (sleep disorders …) but what about cancer?
Premenopausal women particularly at risk
To answer this question, the scientists used a mouse model that spontaneously developed breast tumors. Animals have been submittedat a continuous jet lag in order to experimentally reproduce a shifted work pace (alternating day and night work or straddling day and night periods). The researchers were then able to observe that the circadian disorder did have a significant impact on the development of breast tumors. “This disruption of the sleep / wake rhythm increased the spread of cancer cells and the formation of metastases in these animal models,” says Inserm.
The researchers also reveal that these chronic disturbances of the circadian rhythm make the immune system more “permissive” for the spread of cancer cells by modifying the tumor microenvironment. They have even found the mechanism behind these negative effects of this “circadian stress” on tumor progression. Namely, that the tumors express more strongly a molecule called “chemokine Cxcl5” having the characteristic “to attract” cells of the immune system called “myeloid CXCR2” whose role is to lower the immune response of the organism, which allows them thereafter to spread more easily.
This study thus reinforces the conclusions of previous epidemiological studies carried out on the subject. Taking into account these results, the scientific team specifies that a category of population is particularly at risk: premenopausal women exposed by their work to rhythms shifted over long periods would be particularly exposed to more aggressive breast cancers. Note that Inserm specifies that the International Agency for Research on Cancer (IARC), under the aegis of the WHO, has register shift work (organization of work in rotation in 3 x 8 hours a day) on the list of agents “probably carcinogenic” in 2007.