Role and mechanism of estrogen receptor beta in the physiology of the ovary and colon

Abstract

Estrogen regulates a variety of important physiological functions in both males and females, where the regulation of female reproduction and the development of sexual organs are typical examples. The effects of estrogen are predominantly exerted via signaling through the two nuclear receptors estrogen receptor α (ERα) and β (ERβ), or the membrane G protein-coupled estrogen receptor 1 (GPER1). While estrogen signaling is important for human health, dysregulation of signaling can have adverse effects and impact the development and progression of a wide range of diseases including reproductive disorders and cancer.

ERβ has been shown to be highly important for ovarian function by regulating folliculogenesis and ovulation but has also been implied to protect against the development of colorectal cancer (CRC) by mediating the effects of estrogen. Despite the known role of ERβ, there is a lack of mechanistic understanding regarding how ERβ acts under both normal conditions and during disease. The overall aim of this thesis was to characterize the function and molecular mechanism of endogenous ERβ and to decipher its role in the normal ovary as well as its impact on colitis and CRC development. To further understand the role of estrogen signaling in the colon, we also aimed to identify sex differences during CRC development.

In paper I we characterized the full cistrome of endogenous ovarian ERb in the mouse and explored its transcriptional impact. We confirmed a direct role for ERβ in the regulation of essential ovarian functions and identified a novel crosstalk with the nuclear receptor LRH-1.

In paper II we induced colitis-associated CRC (CAC) in intestinal epithelialspecific ERβ knockout mice and identified a protective effect by intestinal ERβ against tumor development in both male and female mice. We further characterized sex-dependent effects and proposed an underlying mechanism involving the regulation of TNFα/NFκB signaling.

In paper III we expanded the investigation of sex-dependent changes during chemically-induced colitis in wildtype mice and identified a sex-specific response related to inflammatory response. We further found that male mice have an enhanced response to induced colitis.

In paper IV the transcriptome of colitis-induced tumors and their immune cell infiltration was explored in wildtype and intestinal epithelial-specific ERβ knockout mice of both sexes. This showed that sex differences in the transcriptome appear to be dependent on the expression of ERβ. Also, the identified ERβ-dependent changes in the tumor transcriptome of female mice were specifically related to immune response. We corroborated an impact of ERβ on the infiltration of immune cells, especially a reduction of regulatory T cell and NK cell recruitment.

In summary, this thesis provides new mechanistic understanding of the transcriptional role of ERβ in the normal ovary and in the colon microenvironment. This includes the discovery of crosstalk with LRH-1 in the ovary and NFκB in the colon. Our characterization provides a foundation to develop targeted therapies for improved fertility and chemoprevention in CRC. This thesis also highlights the importance of including both sexes in colitis and CRC research to advance our knowledge and improve treatment development.