The pro-inflammatory molecule interleukin-6 (IL-6) drives the activation of a cellular pathway that promotes the development of endometriotic lesions, a study in animal models and human samples has found.
These findings shed new light on the cellular mechanisms that cause these lesions, which can lead to intense pain and infertility in women with endometriosis.
The study, “Interleukin-6 (IL-6) activates the NOTCH1 signaling pathway through E-proteins in endometriotic lesions,” was published in The Journal of Clinical Endocrinology & Metabolism.
An inflammatory and estrogen-dependent disease, endometriosis can lead to serious symptoms, including chronic pelvic pain and infertility. Its causes are, however, still poorly understood.
The condition is characterized by the presence of endometriotic lesions and adhesions found outside the uterus, or womb. These lesions have inflammatory activity and their development has been associated with cell proliferation, invasion, and inflammation.
Research has suggested the NOTCH signaling pathway is activated in these lesions, promoting fibrosis, or tissue scarring, lesion survival, and angiogenesis, or the formation of new blood vessels in these lesions.
Although the findings of these previous studies suggest this signaling pathway plays an important role in the development of endometriosis, the exact mechanisms leading up to the abnormal activation — called up-regulation — of the NOTCH1 gene in these lesions remain unclear.
It has been shown that IL-6, one of the cytokines or proteins secreted by immune cells during menstruation, is increased in the peritoneal (abdominal cavity) fluid, endometriotic lesions, and serum from women with endometriosis. Cytokines are molecules that mediate and regulate immune and inflammatory responses.
IL-6 stabilizes E-proteins, such as E2A and HEB, that code for transcription factors — proteins that control the activity of certain genes — that regulate the activity of NOTCH1 in the thymus, as well as during cancer development.
Based on these data, the researchers theorized that, using E-proteins as mediators, IL-6 might induce the overactivation of NOTCH1, contributing to the development of endometriotic lesions.
To test their idea, they first collected endometrium samples from nine women with endometriosis and five healthy women, as well as from five female baboons with endometriosis and control animals.
The researchers found the protein levels of E2A, HEB, and NOTCH1 were elevated in cells lining the ectopic endometrium — outside the uterus — of women and female baboons with endometriosis.
“These results suggest a correlation between the up-regulation of E-proteins and NOTCH1 in endometriotic lesions,” they said.
To further characterize the mechanism, the investigators then used cell assays to test the effect IL-6 could have on these molecules. They found that treatment with IL-6 increased the protein and RNA levels of NOTCH1, along with E2A and HEB. Of note, RNA is the molecule used as a template for the production of proteins.
When the activity of E2A and HEB both were blocked, or HEB alone was stopped, the protein and RNA levels of NOTCH1 decreased significantly, the study showed.
Moreover, the researchers found the binding efficiency of E2A and HEB to the human NOTCH1 promoter was increased following treatment with IL-6. A promoter is a region of a gene that controls its activity.
According to the researchers, this may be one of mechanisms by which IL-6 induces the activity of NOTCH1 in endometriotic lesions.
These findings strongly suggested that IL-6 increases the levels and activity of E2A, HEB, and NOTCH1 both in vivo, or in the body, and in vitro, or in the lab. To confirm these findings, the researchers treated mice with endometriosis with IL-6 for a period of two weeks.
That treatment led to a significant increase in the number of endometriotic lesions. This was associated with an increase in the protein levels of E2A, HEB and NOTCH1 in cells lining the lesions, similar to what had been previously observed in women and female baboons with endometriosis.
“To the best of our knowledge, this is the first report that associates IL-6 with the up-regulation of NOTCH1 in the endometriotic lesions,” the investigators said.
“IL-6 induced NOTCH1 expression [activity] is mediated by E-proteins in the ectopic GE [endometrium outside the uterus] cells, which may promote endometriotic lesion development,” they added.