After the first pregnancy in the epithelial cells of the mammary glands of mice there is a change in transcriptional programs and epigenetic restructuring. The result is molecular memory, and subsequent pregnancies (or when hormonal simulation of pregnancy) the pattern of gene expression was quickly rebuilt. In addition, due to epigenetic changes in the mammary glands of parous mice inhibit the carcinogenic effects of cMYC. At the same time the animal activated the molecular phenotype of cellular aging — the authors of the article in Nature Communications suggestthat it can protect the mammary glands against the development of tumors.
During pregnancy the mammary glands in women undergoing major restructuring: the cells expand, there is a lot of ducts that collect the milk. When lactation ends, the gland returns to its original state, but at the molecular level some of the changes remain forever. Transcription regulation of several genes in the mammary glands of women who gave birth at least once, differs; in cells occur epigenetic changes that underpin the molecular memory in future pregnancies cells of the mammary glands quickly rebuild their genes.
Lactation is not the only direction of the restructuring of epithelial cells of mammary glands during pregnancy. It is known that after birth changes in women the risk of breast cancer: in the early years it increased, but in the long term is reduced. Suppression of carcinogenesis in the mammary glands after pregnancy is observed in rodents, but the explanation for this effect is still not found.
Scientists from the United States under the leadership of Kamila DOS Santos (Camila dos Santos) from a Laboratory in cold spring Harbor, analyzed the expression of genes (in their RNA transcripts) in the epithelial cells of the mammary glands from nulliparous mice and after pregnancy. In addition, to simulate two consecutive pregnancies, animals were administered hormones. To test whether the expression of genes with epigenetic changes in the same groups of animals examined number of active histones (nuclear proteins responsible for epigenetic regulation of molecular processes) for specific labels.
After pregnancy the activity of some genes was increased, although the overall transcriptional program before and after pregnancy were similar. In the early stages of the second “artificial pregnancy” pattern of gene expression was similar to that in the later stages of the second hormonal cycle. So, after the first pregnancy really occurs molecular memory, and the second time the genome is reconstructed faster.
After pregnancy the breast cells occurred epigenetic restructuring: the number of active histones is increased 10 times, and they are located in areas of the genome that are responsible for other cellular processes.
To check the effect of pregnancy on tumor development in mice induced expression of the gene cMYC, which triggers the growth of tumors. As a result, nulliparous animals in the mammary glands a few days later she developed precancerous condition; after eight days of activation of cMYC mouse died. After pregnancy same cMYC did not cause abnormalities in the morphology of the tissue sections of the mammary glands of parous animals on the fifth day the expression of this gene looked just like the control mice.