**The Tissue Specific Role of Estrogen and Progesterone in Human Endometrium and Mammary Gland**

Karin Tamm1,2,3, Marina Suhorutshenko1,

Miia Rõõm1, Jaak Simm1 and Madis Metsis1,2

*1Centre for Biology of Integrated Systems, Tallinn University of Technology, Tallinn, 2Competence Centre on Reproductive Medicine and Biology, Tartu, 3Nova Vita Clinic, Tallinn, Estonia* 

**1. Introduction** 

34 Steroids – Basic Science

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stimulation by transforming growth factor-beta of follicle-stimulating hormonedependent aromatase activity and messenger ribonucleic acid expression in rat ovarian granulosa cells. *Biology of Reproduction,* Vol. 61, No. 4, (October 1999), pp. The purpose of this chapter is to review the tissue-specific role of estrogen (E2) and progesterone (P4) in human endometrium and mammary gland. It is well known that both E2 and P4 are essential for the development and differentiation of human endometrium and mammary gland, but the exact basis for differential tissue-specific signalling of E2 and P4 are still not fully understood. This chapter explores observed functions of two major female steroid hormones and their cognate receptors in normal physiology of human reproductive system but also in assisted reproductive technology and breast cancer treatment.

The normal reproductive physiology requires tightly coordinated action of hypothalamus, pituitary gland, ovaries and endometrium. Also functioning of other endocrine units such as the thyroid and adrenal glands are essential for regular ovulation and cyclic changes. The production of ovarian steroid hormones is coordinated by the hypothalamic-pituitarygonadal axis which is activated in puberty (Figure 1). The hypothalamus produces and secretes luteinizing hormone-releasing hormone (LHRH), which binds to its receptors in pituitary gland. This causes cascade of biochemical events culminating in the production of two hormones in pituitary gland, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH are secreted into the general blood circulation and attach to receptors on the ovary, where they trigger ovulation and stimulate the production of E2 and P4. Ovarian steroid hormones themselves have direct role in the development of the inner lining of the uterus but they also act as a positive feedback system to hypothalamus and pituitary gland for continuous cyclic changes until the beginning of menopause (Kanis and Stevenson, 1994).

Cholesterol is the building block for all steroid hormones, which is carried into the bloodstream and through a sequence of enzymatic changes is synthesized into final products. In the bloodstream steroid hormones are distributed rapidly throughout the tissues and act on distant targets. This secretory process is called endocrine action and the function of many target tissues as mammary gland, brain, bones, liver and heart are affected by circulating hormones. Steroid hormones can also act very close to their site of secretion

The Tissue Specific Role

(Seifert-Kaluss and Prior, 2010).

of Estrogen and Progesterone in Human Endometrium and Mammary Gland 37

Beside the reproductive system, one of the most widely recognized effects of E2 is the prevention of the osteoporosis. Adequate E2 levels through E2 replacement therapy has shown to prevent or diminish calcium loss from bones in menopausal women (Venken et al., 2008). In the nervous system both estrogens and androgens have been reported to influence verbal fluency, performance of spatial tasks, verbal memory capacity and fine motor skills (Kelly and Ronnekleiv, 2008). The major role for P4 in humans is related to initiation and maintenance of the pregnancy. P4 is essential for milk preparation and secretion in mammary gland and for mediating signals required for sexually responsive behaviour. Recent evidence also supports a role for P4 in the modulation of bone mass

NRs function as transcription factors. The biological activities of E2 and P4 are mediated mainly by nuclear receptors (NRs). Binding of a steroid hormone to its cognate receptor results in a conformational change in the nuclear receptor that allows the ligand-receptor complex to bind with high affinity to response elements in DNA and regulate transcription of target genes. In the absence of ligand, NRs are held in a multi-subunit complex containing heatshock proteins such as Hsp90, SP70, HSP40, Hop, and p23 (Wolf et al., 2008). After binding to ligand, these receptors, undergo conformational changes, dissociate themselves from chaperone proteins, dimerize and in some cases translocate into the nucleus (if not already locked into the nucleus) (Bain et al., 2007). The differences in specificity of molecular mechanisms result from receptor subcellular location and binding to genomic DNA as homo- or heterodimers in either head-to-tail or head-to-head orientation to different consensus sequences known as hormone response elements (HREs) (Bain et al., 2007). Upon NR activation a hydrophobic pocket is created in their tertiary structure for interaction with co-activators such as members of the steroid receptor co-activator (SRC) protein family or co-repressors such as NR co-repressor (NCoR) and silencing mediator for retinoic acid and thyroid hormone receptor (SMRT) ( Hall et al., 2005). The recruitment of co-regulators leads to alterations in the rate of gene expression via modification of initiation complex formation process. Two types of estrogen receptors, ERα and ERβ, encoded by separate genes, are found in humans (Enmark etal., 1997; Kuiper and Gustafsson, 1997). P4 signalling is also mediated by two receptors, PRA and PRB, which are encoded by the same gene but transcribed from different promoters, resulting in a PRB that has an additional 164 amino acids at the N-terminus (Wen et al., 1994; Kastner et al., 1990). PRB is a stronger transcriptional activator in most cell types, while PRA acts often as a dominant negative

In addition to operating as TFs in the nucleus, NRs have been shown to possess nongenomic action which is usually characterized by a shorter lag time required to elicit a biological response following steroid hormone stimulation. For instance ERs can regulate gene expression independent of estrogen responsive element (ERE) through tethering different TFs and by membrane- initiated ER interference with other intracellular pathways. Examples of motifs recognized by ER other than ERE is the activator protein-1 site (AP-1

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

site) commonly occupied by the TFs c-Fos/c- Jun B (Björnström and Sjöberg., 2004).

repressor for PRB activity (Tung et al., 1993; Vegeto et al., 1993).

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

Fig. 1. The female hypothalamic–pituitary–gonadal axis. The hypothalamus produces and secretes luteinizing hormone–releasing hormone (LHRH) into a system of blood vessels that link the hypothalamus and the pituitary gland. LHRH stimulates the pituitary gland by attaching to specific molecules (i.e., receptors). After the coupling of LHRH with these receptors, a cascade of biochemical events causes the pituitary gland to produce and secrete two hormones, luteinizing hormone (LH) and follicle–stimulating hormone (FSH). LH and FSH are two of a class of hormones commonly known as gonadotropins. They are secreted into the general circulation and attach to receptors on the ovary, where they trigger ovulation and stimulate ovarian production of the hormones estrogen and progesterone. These female hormones cause monthly menstrual cycling and have multiple effects throughout the body. In particular, estrogen has profound effects on the skeletal system and is crucial to maintaining normal bone health (Figure adapted from Kanis and Stevenson, 1994).

on adjacent cells and tissues as it happens in gonads, testis and ovaries- paracrine action. Gonads produce only three classes of steroids: progestins, androgens and estrogens where progestins are obligatory precursors of both androgens and estrogens. Likewise, androgens are obligatory precursors of estrogens. Steroidogenesis in the ovary is compartmentalized in a cell-specific manner: the theca cells primarily producing androstenedione and the granulosa cells completing the synthesis of E2. After the ovulation the corpus luteum of the ovary starts to produce P4. Albeit the vast amount of sex steroids are synthesized locally in peripheral tissue, providing individual target tissues with the means to adjust synthesis and metabolism to their local requirements (Venken et al., 2008).

Fig. 1. The female hypothalamic–pituitary–gonadal axis. The hypothalamus produces and secretes luteinizing hormone–releasing hormone (LHRH) into a system of blood vessels that link the hypothalamus and the pituitary gland. LHRH stimulates the pituitary gland by attaching to specific molecules (i.e., receptors). After the coupling of LHRH with these receptors, a cascade of biochemical events causes the pituitary gland to produce and secrete two hormones, luteinizing hormone (LH) and follicle–stimulating hormone (FSH). LH and FSH are two of a class of hormones commonly known as gonadotropins. They are secreted into the general circulation and attach to receptors on the ovary, where they trigger ovulation and stimulate ovarian production of the hormones estrogen and progesterone. These female hormones cause monthly menstrual cycling and have multiple effects

throughout the body. In particular, estrogen has profound effects on the skeletal system and is crucial to maintaining normal bone health (Figure adapted from Kanis and Stevenson,

on adjacent cells and tissues as it happens in gonads, testis and ovaries- paracrine action. Gonads produce only three classes of steroids: progestins, androgens and estrogens where progestins are obligatory precursors of both androgens and estrogens. Likewise, androgens are obligatory precursors of estrogens. Steroidogenesis in the ovary is compartmentalized in a cell-specific manner: the theca cells primarily producing androstenedione and the granulosa cells completing the synthesis of E2. After the ovulation the corpus luteum of the ovary starts to produce P4. Albeit the vast amount of sex steroids are synthesized locally in peripheral tissue, providing individual target tissues with the means to adjust synthesis and

metabolism to their local requirements (Venken et al., 2008).

1994).

Beside the reproductive system, one of the most widely recognized effects of E2 is the prevention of the osteoporosis. Adequate E2 levels through E2 replacement therapy has shown to prevent or diminish calcium loss from bones in menopausal women (Venken et al., 2008). In the nervous system both estrogens and androgens have been reported to influence verbal fluency, performance of spatial tasks, verbal memory capacity and fine motor skills (Kelly and Ronnekleiv, 2008). The major role for P4 in humans is related to initiation and maintenance of the pregnancy. P4 is essential for milk preparation and secretion in mammary gland and for mediating signals required for sexually responsive behaviour. Recent evidence also supports a role for P4 in the modulation of bone mass (Seifert-Kaluss and Prior, 2010).

NRs function as transcription factors. The biological activities of E2 and P4 are mediated mainly by nuclear receptors (NRs). Binding of a steroid hormone to its cognate receptor results in a conformational change in the nuclear receptor that allows the ligand-receptor complex to bind with high affinity to response elements in DNA and regulate transcription of target genes. In the absence of ligand, NRs are held in a multi-subunit complex containing heatshock proteins such as Hsp90, SP70, HSP40, Hop, and p23 (Wolf et al., 2008). After binding to ligand, these receptors, undergo conformational changes, dissociate themselves from chaperone proteins, dimerize and in some cases translocate into the nucleus (if not already locked into the nucleus) (Bain et al., 2007). The differences in specificity of molecular mechanisms result from receptor subcellular location and binding to genomic DNA as homo- or heterodimers in either head-to-tail or head-to-head orientation to different consensus sequences known as hormone response elements (HREs) (Bain et al., 2007). Upon NR activation a hydrophobic pocket is created in their tertiary structure for interaction with co-activators such as members of the steroid receptor co-activator (SRC) protein family or co-repressors such as NR co-repressor (NCoR) and silencing mediator for retinoic acid and thyroid hormone receptor (SMRT) ( Hall et al., 2005). The recruitment of co-regulators leads to alterations in the rate of gene expression via modification of initiation complex formation process. Two types of estrogen receptors, ERα and ERβ, encoded by separate genes, are found in humans (Enmark etal., 1997; Kuiper and Gustafsson, 1997). P4 signalling is also mediated by two receptors, PRA and PRB, which are encoded by the same gene but transcribed from different promoters, resulting in a PRB that has an additional 164 amino acids at the N-terminus (Wen et al., 1994; Kastner et al., 1990). PRB is a stronger transcriptional activator in most cell types, while PRA acts often as a dominant negative repressor for PRB activity (Tung et al., 1993; Vegeto et al., 1993).

In addition to operating as TFs in the nucleus, NRs have been shown to possess nongenomic action which is usually characterized by a shorter lag time required to elicit a biological response following steroid hormone stimulation. For instance ERs can regulate gene expression independent of estrogen responsive element (ERE) through tethering different TFs and by membrane- initiated ER interference with other intracellular pathways. Examples of motifs recognized by ER other than ERE is the activator protein-1 site (AP-1 site) commonly occupied by the TFs c-Fos/c- Jun B (Björnström and Sjöberg., 2004).
