Estrogen Signaling Issues - Essay Example

Estrogen Signaling 1. Steroid receptors have been traditionally classified as type 1 nuclear receptors. With reference to models of estrogen signaling, explain the basis of this classification. Nuclear receptors are very important for the functioning of various molecules in our body. They are basically proteins found in the interior aspects of the cell. They sense the presence of small lipophilic molecules like steroids and various other hormones and regulate the expression of the genes concerned to these molecules, thus assisting in various functions the molecules are destined to perform (Novac and Heinzel, 2004). Thus nuclear receptors act as ligand-inducible transcription factors (Aranda and Pascual, 2001). The past two decades has seen enormous research pertaining to nuclear receptor super family that has led to an understanding of not only the interaction of about 40 different nuclear receptors with DNA, but also the mode of action of various coregulators that help in the signaling of information to the transcriptional machinery (Aranda and Pascual, 2001). It is now clear that nuclear receptors play a major role in adult homeostasis and embryonic development after binding to certain important molecules. Molecules which affect the behavior of receptors are known as ligands. Binding of a ligand to the specific receptor induces certain conformational changes in the receptor resulting in activation of the receptor to upregulate gene expression. The speciality of nuclear receptors is that they control the expression of the DNA based on the ligands which bind to them. Most identified nuclear receptors have specific ligands. There are however certain nuclear receptors which do not have any identified specific endogenous ligands. These are known as orphan receptors (Aranda and Pascual, 2001). Examples of orphan receptors are PPAR and LXR. Nuclear receptors with identified ligands and also orphan receptors are basically classified into 2 classes: type-1 NR and type-2 NR. Type-1 NR receptors are located in the cytosol and those belonging to type-2 NR are located in the nucleus (Aranda and Pascual, 2001). Traditionally, receptors of steroid hormones are classified under type-1 NR receptors. Sex hormone receptors like estrogen receptors also fall into the same category (Aranda and Pascual, 2001). However, recent reports have suggested various other models of estrogen signaling, questioning the very basis of classification of estrogen receptors under type-1 receptors. Estrogen is one of the important sex hormones. It has definite physiological roles, the most important of which are sexual and reproductive functions. Other biological roles include involvement in various functions attributing to the cardiovascular, immune, central nervous system and musculoskeletal systems (Gustafsson, 2003; cited in Heldring, 2007: 906). The body produces many estrogen types, the most potent of which is 12- beta estradiol or E2. E2, along with its 2 metabolites estriol and estrone exerts various biophysiological effects in the body (Heldring, 2007). These effects are mediated through binding of the molecules of estrogen to specific estrogen receptors. Currently, 2 specific estrogen receptors have been identified and they are ER-alpha and ER-beta. These receptors belong to class-1 nuclear receptors (Petterson and Gustafsson, 2001; cited in Heldring, 2007: 907). Binding of the ligands to these receptors induces certain conformation changes in the receptor which in turn leads to a series of changes in the receptors and ultimately ends in the preinitiation complex. The changes which occur in the receptors are migration of ER from cytosol to nucleus, dimerisation of the receptor molecules, binding and interactions between dimerised receptor protein and specific sequences of DNA, recruitment of various coregulator proteins concerned with the biological action and also recruitment of various transcription factors (Paech, Webb, Kuiper, et al,1997; cited in Heldring, 2007: 908). Recruitment of coregulator proteins is essential to transcript DNA to mRNA and then to protein. The end-result of these actions is regulation of gene expression. This mode of signaling is known as the genomic signaling which is characteristic of type-1 nuclear receptors. This is the basis for traditional incorporation of estrogen receptors under type-1 nuclear receptors. 2. Critically evaluate why this categorization is thought to be increasingly redundant. Like any other intracellular steroid hormone receptor, estrogen receptors have domains which can be divided into four units: variable domain, DNA binding domain, hinge region and hormone binding domain. These domains are structurally and functionally distinct domains which have been conserved evolutionarily (Heldring, 2007). The most conserved domain and which is central is the DNA-binding domain or the DBD. This domain is basically involved in the recognition and binding of DNA (Heldring, 2007). The hormone-binding domain or the ligand-binding domain or LBD is that domain which binds with the ligand. The ligand binds to the COOH-terminal of the LBD. The variable domain has a NH2 terminal. It is not conserved and the sequence and length of the domain is variable. Transcriptional activation occurs with the help of activation functions or AF. There are 2 AF. AF-1 is located at the NH2 terminus and AF-2 is located at the COOH terminal. AF-1 is constitutively active, but AF-2 is dependent on the ligand. ER-alpha and ER-beta have a good sequence homology. The main difference lies in the NH2-terminal domains (Heldring, 2007). The genes coding them are located on different chromosomes. While the gene for ER-alpha is located on the 6th chromosome; that for ER-beta is located on the fourteenth chromosome (Fan, Bigsby and newphew, 2003; cited in Heldring, 2007). Also, there lies some difference in their patterns of expression. While ER-alpha is found in hypothalamus, endometrium, ovarian stroma cells and breast cancer cells, ER-beta receptors are expressed in other systems like bone, heart, lung, kidneys, neural tissue, colon, prostate and endothelial cells (Aranda and Pascual, 2001). Also, each receptor has affinity towards different for different forms of estrogen. While E2 has affinity for both ER-alpha and ER-beta equally, estrone has affinity for ER alpha and estriol has affinity for ER-beta. Different ligand-receptor complexes in different regions trigger different reaction series culminating in either agonist or antagonist actions. This property has been used for pharmacological benefits. For example, tamoxifen-ER complex in breast is an antagonist combination and hence is used in the treatment of carcinoma breast (Zivadinovic, Gametchu and Watson, 2005). But the same combination is an agonist in bone, making it beneficial to treat osteoporosis. Thus binding of estrogen to the estrogen receptors triggers ligand-dependent signaling. However, transcriptional response, which is actually cell-specific is based on many factors, the most important and immediate one being composition of coregulatory proteins in the specific cell portion (Heldring, 2007). Another important factor that influences the transcriptional response is the estrogen responsive genes promoter complex (Heldring, 2007). Subsequent to the identification of ER-beta, several other isoforms have also been identified, hinting at the possible complex nature of estrogen signaling. Also, may splice variants of both the receptors have been discovered (Heldring, 2007). Another important observation worth mentioning at this juncture is, that though ER receptors mainly lie in the cytosol, some research has shown that part of the receptors hang into the nucleus (Htun, Holth, Walker et al, 1999). Since estrogen can pass through phospholipid membranes of any cell, they can easily bind to receptors in the cytosol. Current research has discovered another method of estrogen signaling, the non-genomic signaling. In non-genomic signaling, there is no alteration in gene expression. The effects are due to activity of nuclear receptors which are present outside the nucleus of the cell. This information came into light when researchers discovered that certain actions of estrogen were seen immediately, which cannot be caused by the traditional genomic signaling. Non-genomic signaling causes biological effects by either activating certain enzymes like phosphatases and kinases or by causing ionic fluxes across memebranes ((Heldring, 2007). It has been widely recognized that most of the rapid non-genomic signaling occurs through G-protein coupled and seven transmembrane receptor or GPR30 (Maggiolini and Picard, 2009). GPR30 mediates a variety of responses to estrogens in many cell types. Though the structure and function of this receptor is different from the classical ER, it is has been proposed that overlapping and interactions between the two receptors is possible to occur (Maggiolini and Picard, 2009). Filardo, Quinn, Frackelton et al (2002) reported that estrogen action through GPR30 occurs by means of stimulation of adenyl cyclase and attenuation of epidermal growth factor receptor mediated by cAMP. Heino, Chagin and Savendahl (2008) reported the presence of GPR30 in abundant quantities in a variety of cells in the bone tissue like osteocytes, osteoblasts and osteoclasts. Hogan, Kennely, Collins et al (2009) reported inhibition of motility of human colon by estrogen through non-genomic signaling mechanism. The researchers opined that the non-genomic mode of action is rapid occurs through cell membrane coupling. Similar effects were demonstrated previously in rat colon by Doolan and Harvey (2003). The researchers reported that E-2 induced stimulation occurred via Galphas protein-coupled membrane receptor which is distinct from the classical ER and that fatty acids and PKC delta were involved in the signaling pathway of E2. Other studies which reported similar signaling pathways of estrogen using non-classical ER are Harvey, Doolan, Condliffe et al (2002) and Doolan, Condliffe and Harvey (2000). It is interesting to note that the ligand cavity of ER alpha and ER beta is large and allows binding of ligands other than E2. Certain environmental pollutants like xenoestrogens and polycyclic aromatic hydrocarbons, and plant constituents like phytoestrogens are known to exert estrogenic actions by binding to the ERs (Heldring, 2007). This property of ER has been attributed to diverse implications like altered reproductive capacity of animals, carcinoma of the breast and endometrial tumors on one hand and decreased risk of hormone-induced cancers by eating unrefined grains on the other hand (Heldring, 2007). This feature of ER has also led to the development of various synthetic estrogen antagonists to treat deadly cancers like breast carcinoma (Heldring, 2007). Kim and Bender (2009) proposed a membrane-initiated mode of action of estrogen on human endothelial cells by interacting with an isoform of ER alpha which has truncated N-terminus. This receptor has been named ER46. Interaction between estrogens and this receptor is expected to elaborate nitric oxide which is athero-protective. Hirahara, Matsuda and Gao (2009), who erstwhile reported the presence of membrane-associated ER at the oligodendrocyte plasma membrane and also at the myelin, investigated the location and function of this receptor and opined that through of estrogens to this receptor, rapid phosphorylation of GSK-3beta, Akt and GSK-3beta occurred. Estrogen can also exert its effects by just being a trophic factor, triggering cytoplasmic signals. Varea, Garrido, Dopazo et al (2009) demonstrated the association between ER alpha and beta-catenin and glycogen synthase kinase 3 in the neural tissue. Other extracellular non-genomic signaling mechanisms have also been described. Singh, Setalo, Guan et al (1999) reported that rapid tyrosine phosphorylation and activation of mitogen-activated kinases is one of the signaling methods of E2 and it is through this that the hormone exerts selective enhancement of growth and differentiation of nerve cells in embryonic and growing central nervous system. Another enzyme has been found to be activated by E2 by extracellular mechanisms is B-Raf kinase activity. Cordey, Gundimeda, Gopalakrishna et al (2003) found that E2 activates an important enzyme protein kinase C which is present in a variety of cell types and plays an important role in many cellular events including cell death and apoptosis. Honda, Sawada, Kihara et al (2000) identified activation of Phosphatidylinositol 3-kinase as a means of neuroprotection by estrogen. From the evaluation of literature pertaining to signaling methods of estrogen, it is evident that estrogen exerts its effects by not only reacting with nuclear receptors and altering gene expression, but also through other mechanisms in which gene expression is not altered. Such signaling mechanisms are known as non-genomic signaling pathways. Mode of action through these pathways is very rapid. It is not yet clear whether there is interaction between genomic and non-genomic pathways. However, the genomic pathway is very slow, while the non-genomic pathway is very fast. It has been proposed that non-genomic signaling mechanism occurs through receptors which lie in the cell membranes. These receptors are either isoforms of ER, or are variants of ER. Whatever is their structure and function, they are receptors of E2 and its metabolites, but are not type-1 nuclear receptors. Current research has identified many critical biophysiological functions of estrogen which take place through non-genomic non-traditional signaling mechanisms, thus questioning the very basis of classification of estrogen receptors under type-1 nuclear receptors. References Aranda, A., and Pascual, A. (2001). Nuclear Hormone Receptors and Gene Expression. Physiological Reviews, 81, 1269- 1304. Cordey, M., Gundimeda, U., Gopalakrishna, R., Pike, C.J. (2003). Estrogen activates protein kinase C in neurons: role in neuroprotection. J Neurochem., 84(6), 1340–1348. Doolan, C.M., and Harvey, B.J. (2003). A Galphas protein-coupled membrane receptor, distinct from the classical oestrogen receptor, transduces rapid effects of oestradiol on [Ca2+]ion in female rat distal colon. Mol Cell Endocrinol., 199(1-2), 87-103. Doolan, C.M., Condliffe, S.B., Harvey, B.J. (2000). 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