**2. The role of E2 and P4 in human endometrium**

Human endometrium is the inner tissue lining of uterine cavity that undergoes monthly cyclic changes dictated by ovarian steroid hormones E2 and P4 (Figure 2). As endometrium is a regenerative tissue it is subjected to proliferation, secretion and degeneration on

The Tissue Specific Role

of Estrogen and Progesterone in Human Endometrium and Mammary Gland 39

and ERβ are present in all endometrial cell types throughout the entire menstrual cycle, they are expressed at higher levels during the proliferative phase and show lower activity during the secretory phase because of the suppressive effect of P4. After the proliferative phase P4 takes the E2-primed endometrium towards a state of receptivity. P4, acting through its cognate receptors, is absolutely mandatory for successful implantation and postimplantation embryo survival. PRA and PRB levels are similar during the follicular phase of the menstrual cycle while the PRA is down-regulated at the time of implantation but higher stromal PRB levels during the mid-luteal phase have been reported (Arnett-Mansfield et al., 2004). The expression of the PR gene in endometrial glands is controlled by E2 and P4, where E2 induces PR synthesis and P4 down-regulates the expression of its own receptor (Graham et al., 1990). The actions of P4 counter the effects of P4 in the endometrium through paracrine regulators from the stromal part. Recent studies about small non-protein coding RNAs (microRNA, miRNA) have revealed their important role in gene regulation in endometrium (Kuokkanen et al., 2010; Li et al., 2011). The regulation of specific microRNAs is a mechanism that appears to fine tune gene expression by blocking cell proliferation at the

Fig. 3. **E2 and P4 genomic signalling in human endometrial cell.** E2 produced by ovary enters into the cell and binds to its specific receptors ERα or ERβ. Formed complex moves into the cell and has an impact on target gene expression. After the ovulation corpus luteum starts to produce P4 which also diffuses into the endometrial cells and through its receptors regulate gene expression. Steroid hormone analogues (Tamoxifen, Fluvestrant, Progestin,

In a restricted period, called implantation window (IW), endometrium is most receptive for the embryo attachment. In humans IW is temporally confined to days 20-24 of menstrual

Mifepristone) have a suppressive or repressive impact on ER , PR signalling.

**2.1 Endometrial gene expression during the time of embryo implantation** 

time of implantation P4 dependently (Lessey et al., 2010).

monthly basis. Nearly all morphologic and biochemical processes that the uterus undergoes during its acquisition of receptivity are directly or indirectly regulated by ovarian steroid hormones (Lim et al., 2002). The development of human endometrium is divided into follicular and luteal phase. During the follicular phase ovarian E2 is produced with increasing quantities until ovulation, stimulating the proliferation and growth of the epithelial and stromal components of the endometrium. During the luteal phase the increasing amounts of the P4 and secondary maintaining levels of E2 are both involved in the differentiation of the endometrium but P4 reverses the proliferative effects of E2 (Lim et al., 2002). Together, coordinated action of steroid hormones produced by the follicle and corpus luteum prepare the endometrium every month for potential embryo implantation. In the event of embryo implantation P4 predominantly facilitates and permits decidualization of the endometrium and supports maintenance of pregnancy. On the contrary, in the absence of implantation declining levels of E2 and P4 lead to degeneration of the endometrial tissue, which is followed by regeneration during the next cycle. In addition to cell differentiation, P4 plays the key role in the decision of cell survival or death prior to the menstruation. Three proteins related to apoptotic activation in endometrial cells are protooncogene p53, FOXO1 (forkhead box-O) and BIM which act as a switches between apoptosis and survival (Brosens and Gellersen, 2006).

Fig. 2. **E2 and P4 in human endometrium.** E2 causes the growth or proliferation of the endometrium during the first two weeks of the menstrual cycle. After ovulation, the corpus luteum produces P4. This hormone causes the endometrial glands to secrete nutritive substances required by the embryo and to allow it to implant into the endometrial lining (figure adapted from internet http://www.tubal-reversal.net/uterus-menstrual-cycle.htm).

Ovarian steroids mediate their signals through genomic or non-genomic pathways. The genomic signal is passed on by cognate receptors, ERs and PRs, in endometrial cells. As E2 is dominant hormone during the follicular phase of the cycle genes regulated by E2 are also more often related to tissue proliferation. Under the actions of E2 the epithelial cells respond by rapid induction of gene expression that promotes DNA synthesis and cell replication (Lessey et al., 2010). During the luteal phase P4 induces the genes related to differentiation*.*  Clinically used steroid hormone analogues (Tamoxifen, Fluvestrant, Progestin, Mifepristone) could have a suppressive or repressive impact on normal steroid hormone signalling in endometrial cells (Figure 2). The expression of ERs and PRs in spatiotemporal manner is crucial for the successful implantation process (Lessey et al., 2003). Although ERα

monthly basis. Nearly all morphologic and biochemical processes that the uterus undergoes during its acquisition of receptivity are directly or indirectly regulated by ovarian steroid hormones (Lim et al., 2002). The development of human endometrium is divided into follicular and luteal phase. During the follicular phase ovarian E2 is produced with increasing quantities until ovulation, stimulating the proliferation and growth of the epithelial and stromal components of the endometrium. During the luteal phase the increasing amounts of the P4 and secondary maintaining levels of E2 are both involved in the differentiation of the endometrium but P4 reverses the proliferative effects of E2 (Lim et al., 2002). Together, coordinated action of steroid hormones produced by the follicle and corpus luteum prepare the endometrium every month for potential embryo implantation. In the event of embryo implantation P4 predominantly facilitates and permits decidualization of the endometrium and supports maintenance of pregnancy. On the contrary, in the absence of implantation declining levels of E2 and P4 lead to degeneration of the endometrial tissue, which is followed by regeneration during the next cycle. In addition to cell differentiation, P4 plays the key role in the decision of cell survival or death prior to the menstruation. Three proteins related to apoptotic activation in endometrial cells are protooncogene p53, FOXO1 (forkhead box-O) and BIM which act as a switches between apoptosis

Fig. 2. **E2 and P4 in human endometrium.** E2 causes the growth or proliferation of the endometrium during the first two weeks of the menstrual cycle. After ovulation, the corpus luteum produces P4. This hormone causes the endometrial glands to secrete nutritive substances required by the embryo and to allow it to implant into the endometrial lining (figure adapted from internet http://www.tubal-reversal.net/uterus-menstrual-cycle.htm). Ovarian steroids mediate their signals through genomic or non-genomic pathways. The genomic signal is passed on by cognate receptors, ERs and PRs, in endometrial cells. As E2 is dominant hormone during the follicular phase of the cycle genes regulated by E2 are also more often related to tissue proliferation. Under the actions of E2 the epithelial cells respond by rapid induction of gene expression that promotes DNA synthesis and cell replication (Lessey et al., 2010). During the luteal phase P4 induces the genes related to differentiation*.*  Clinically used steroid hormone analogues (Tamoxifen, Fluvestrant, Progestin, Mifepristone) could have a suppressive or repressive impact on normal steroid hormone signalling in endometrial cells (Figure 2). The expression of ERs and PRs in spatiotemporal manner is crucial for the successful implantation process (Lessey et al., 2003). Although ERα

and survival (Brosens and Gellersen, 2006).

and ERβ are present in all endometrial cell types throughout the entire menstrual cycle, they are expressed at higher levels during the proliferative phase and show lower activity during the secretory phase because of the suppressive effect of P4. After the proliferative phase P4 takes the E2-primed endometrium towards a state of receptivity. P4, acting through its cognate receptors, is absolutely mandatory for successful implantation and postimplantation embryo survival. PRA and PRB levels are similar during the follicular phase of the menstrual cycle while the PRA is down-regulated at the time of implantation but higher stromal PRB levels during the mid-luteal phase have been reported (Arnett-Mansfield et al., 2004). The expression of the PR gene in endometrial glands is controlled by E2 and P4, where E2 induces PR synthesis and P4 down-regulates the expression of its own receptor (Graham et al., 1990). The actions of P4 counter the effects of P4 in the endometrium through paracrine regulators from the stromal part. Recent studies about small non-protein coding RNAs (microRNA, miRNA) have revealed their important role in gene regulation in endometrium (Kuokkanen et al., 2010; Li et al., 2011). The regulation of specific microRNAs is a mechanism that appears to fine tune gene expression by blocking cell proliferation at the time of implantation P4 dependently (Lessey et al., 2010).

Fig. 3. **E2 and P4 genomic signalling in human endometrial cell.** E2 produced by ovary enters into the cell and binds to its specific receptors ERα or ERβ. Formed complex moves into the cell and has an impact on target gene expression. After the ovulation corpus luteum starts to produce P4 which also diffuses into the endometrial cells and through its receptors regulate gene expression. Steroid hormone analogues (Tamoxifen, Fluvestrant, Progestin, Mifepristone) have a suppressive or repressive impact on ER , PR signalling.
