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Estrogen genomic and membrane actions at an intersection
Update
TRENDS in Endocrinology and Metabolism
Vol.19 No.1
Research Focus
Estrogen genomic and membrane actions at an intersection Lydia A. Arbogast Department of Physiology, Southern Illinois University School of Medicine, Carbondale, IL 62901, USA
Estradiol is a primary hormonal signal that synchronizes central neuronal activity changes during the female reproductive cycle. The cellular and molecular mechanisms for many of these changes are still not well understood. Exciting new findings of Roepke and colleagues indicate that estradiol regulates expression of key potassium channels as well as modulatory intracellular signaling molecules. This regulation might contribute to regionspecific changes in neuronal excitability in the hypothalamic arcuate nucleus and to the integration of neuronal responses that influence homeostatic functions.
Introduction Women are exposed to high levels of steroid hormones from endogenous and exogenous sources throughout their lives. The synchronized oscillation of endogenous estradiol exerts both negative and positive feedback effects on gonadotropin secretion during the female reproductive cycle. Exogenous estrogens are components of oral contraceptives and hormone replacement therapy. In spite of the importance of estrogens in women’s health, the molecular mechanisms and neuronal circuitry for the multiple and diverse actions of estrogens in the central nervous system are not completely understood. The importance of estradiol in modulating brain function extends beyond the control of reproductive functions to other hypothalamic and extrahypothalamic neuronal systems. Overall, findings indicate that estradiol does not act by just one mechanism but that there might be functionally specific mechanisms that influence synaptic transmission in the central nervous system. Exciting new findings by Roepke et al. [1] indicate that estrogen regulates expression of potassium channels and key signaling molecules associated with channel function. Although these authors focused on the hypothalamic arcuate nucleus and neurons specifically related to energy homeostasis and feeding behaviors, these findings point to a mechanism by which estrogen could affect neuronal excitability; this mechanism might be broadly applicable to the excitability of other estrogen-responsive neurons throughout the brain. Different avenues for estrogen receptor actions The pioneering work by Jensen’s laboratory in the 1950s started the scientific journey to understanding the mechanism of action of estrogen with the identification of the Corresponding author: Arbogast, L.A. ([email protected]). Available online 26 November 2007. www.sciencedirect.com
classical estrogen receptor (ER) denoted ERa [2]. The development of molecular techniques in the 1980s led to cloning of ERa and a greater understanding of its structure and action as a transcriptional regulator [3,4]. In the 1990s, the field became more complex with the expansion of the receptor family to include ERß and an increase in the understanding of ER interactions with co-regulatory molecules [3–5]. During this time, a wealth of information accumulated, which indicated that estradiol might have rapid effects in the central nervous system involving nongenomic mechanisms [6,7]. Evidence for a putative plasma membrane ER and interaction of estradiol with second messenger signaling pathways focused the journey in new directions. Proteins identical or similar to the classical nuclear receptors [8,9] as well as a distinct membrane receptor, ER-X [10], have been localized to the plasma membrane. In addition, the G-protein-coupled receptor GPR30 has been localized to the cell membrane and associated with specific estradiol binding and rapid estrogendependent G-protein-coupled signaling [11,12]. The wide-ranging influences of estradiol on neuronal function are apparent from the supplementary table included in the study by Roepke and colleagues [1]. These scientists used suppressive subtractive hybridization to identify hundreds of important estrogen-regulated brainspecific genes in the guinea-pig hypothalamus. Although not surprising, the broad scope of functions associated with these genes supports the notion that estrogen has diverse actions within the central nervous system. Although some of these genes have been identified previously by more focused approaches, there is still a significant scientific challenge to put these genes into a physiological perspective and to identify the complex estradiol-initiated interactions in particular neurons. Estradiol effects on neuronal excitability The genomic/gene-regulatory approach used by Roepke et al. [1] identified estradiol-induced changes in the expression of three prominent potassium channel family members from among the great number of genes encoding potassium channel proteins. The study focused on the identified potassium channels and those signaling molecules important in regulating these channels. The context being that the coordinated induction or suppression of these genes by estradiol may affect the functional interactions between potassium channel subunits and second messenger signals, thus altering neuronal excitability. It is notable that even within the arcuate nucleus there were regional variations in the effect of estrogen on the expression of specific channels. The
1043-2760/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tem.2007.10.002
2
Update
TRENDS in Endocrinology and Metabolism Vol.19 No.1
estrogen-regulated channels identified by this approach belong to two voltage-gated channel families involved with generation of M-current (KCNQ) or A-current (Kv4) and one inward rectifier channel family (Kir2). These data link the transcriptional actions of estrogen to proteins involved in membrane-associated events during the period of estrogen negative feedback and point to a complex expression pattern for different potassium channel subunits. This expression pattern might be resolved with a cell-based analysis. The functionality of potassium channels is more complex than just their expression. Post-translational modifications such as phosphorylation of the channels or the availability or propensity of effector molecules to associate directly with the channels play important modulatory roles. Roepke et al. [1] report regional differences in the arcuate nucleus in the expression of A-kinase anchor protein (AKAP) and calmodulin (CaM), protein kinase C (PKCe), phospholipase C (PLCß4) and phosphatidylinositol-3-kinase (PI3K p55g). The integration of these molecular expression findings with electrophysiological and functional studies of neurons in the arcuate nucleus will be necessary to support the contention that there are regional differences in neuronal excitability within the arcuate nucleus. The contribution of the membrane ER and non-transcriptional events must be included in the equation. The complement of potassium channel proteins and signaling molecules might allow for a prediction of neuronal excitability and of how a specific neuronal type would respond to neurotransmitter or neurohormone input. Functional implications for estrogen signaling in the arcuate nucleus The arcuate nucleus is an area intimately associated with controlling reproductive functions as well as energy homeostasis, and there are distinct regional distributions for specific neuronal phenotypes. The arcuate nucleus contains a diverse population of neurons that are differentially regulated and have complementary or opposing functions. Roepke and colleagues [1] detailed the expression of the potassium channels in two types of neurons, proopiomelanocortin (POMC) and neuropeptide Y (NPY), which release peptides exerting opposite anorectic and orexigenic effects, respectively. ERa expressed at a high level in POMC neurons would be a potential mediator of the estrogen signal, but the involvement of a membrane ER in modulating potassium receptors or signaling molecules by genomic or non-genomic mechanisms cannot be discounted. Indeed, some actions of estradiol to increase the neuronal excitability of POMC neurons are mediated by a novel G-protein-coupled ER-mediated mechanism that involves activation of PKC [13]. Only a moderate number of cells in the arcuate nucleus of rats express GPR30 immunoreactivity [14]. It is unknown whether a receptor similar or identical to ERa is localized to hypothalamic cell membranes, but such a receptor has been identified in transfected cells [8,15].
www.sciencedirect.com
The complexity of the system is evidenced by the diverse responses to estradiol within hypothalamic cells. It will be important to determine how nuclear and membrane ERmediated mechanisms interact to elicit a biological response through actions on specific neuronal types involved with energy homeostasis or reproductive functions. As new tools are developed to differentiate membrane versus nuclear ER actions, the ability to dissect the relative contributions of each of these mechanisms will be possible. Cell-based approaches will help in understanding the action of estradiol in neurons with opposing functions. As these molecular and cellular mechanisms for estrogen action are defined within specific neuronal types, it will also be important to relate them to the physiological responses in vivo. Although additional studies will be required to dissect the molecular mechanisms for these diverse actions of estrogen within the hypothalamus, the work of Roepke et al. [1] provides an important link between genomic actions of estradiol and membrane proteins to modulate neuronal excitability. References 1 Roepke, T.A. et al. (2007) Estrogen regulation of genes important for K+ channel signaling in the arcuate nucleus. Endocrinology 148, 4937– 4951 2 Jensen, E.V. and DeSombre, E.R. (1973) Estrogen-receptor interaction. Science 182, 126–134 3 O’Malley, B.W. (2005) A life-long search for the molecular pathways of steroid hormone action. Mol. Endocrinol. 19, 1402–1411 4 Chambon, P. (2005) The nuclear receptor superfamily: A personal retrospect on the first two decades. Mol. Endocrinol. 19, 1418–1428 5 Kuiper, G.G.J.M. et al. (1996) Cloning of a novel estrogen receptor expressed in rat prostate and ovary. Proc. Natl. Acad. Sci. U. S. A. 93, 5925–5930 6 Vasudevan, N. and Pfaff, D.W. (2007) Membrane-initiated actions of estrogens in neuroendocrinology: Emerging principles. Endocr. Rev. 28, 1–19 7 Zhang, D. and Trudeau, V.L. (2006) Integration of membrane and nuclear estrogen receptor signaling. Comp. Biochem. Physiol. 144, 306–315 8 Razandi, M. et al. (1999) Cell membrane and nuclear estrogen receptors (ERs) originate from a single transcript: Studies of ERa and ERß expressed in Chinese hamster ovary cells. Mol. Endocrinol. 13, 307– 319 9 Powell, C.E. et al. (2001) Identification and characterization of membrane estrogen receptor from MCF7 estrogen-target cell. J. Steroid Biochem. Mol. Biol. 77, 97–108 10 Toran-Allerand, C.D. et al. (2002) ER-X: A novel plasma membraneassociated, putative estrogen receptor that is regulated during development and after ischemic brain injury. J. Neurosci. 22, 8391– 8401 11 Thomas, P. et al. (2005) Identity of an estrogen membrane receptor coupled to a G protein in human breast cancer cells. Endocrinology 146, 624–632 12 Revankar, C.M. et al. (2005) A transmembrane intracellular estrogen receptor mediates rapid cell signaling. Science 307, 1625–1630 13 Qiu, J. et al. (2003) Rapid signaling of estrogen in hypothalamic neurons involves a novel G-protein-coupled estrogen receptor that activates protein kinase C. J. Neurosci. 23, 9529–9540 14 Brailoiu, E. et al. (2007) Distribution and characterization of estrogen receptor G protein coupled receptor 30 in the rat central nervous system. J. Endocrinol. 193, 311–321 15 Dewing, P. et al. (2007) Membrane estrogen receptor-a interactions with metabotropic glutamate receptor 1a modulate female sexual receptivity in rats. J. Neurosci. 27, 9294–9300
TRENDS in Endocrinology and Metabolism
Vol.19 No.1
Research Focus
Estrogen genomic and membrane actions at an intersection Lydia A. Arbogast Department of Physiology, Southern Illinois University School of Medicine, Carbondale, IL 62901, USA
Estradiol is a primary hormonal signal that synchronizes central neuronal activity changes during the female reproductive cycle. The cellular and molecular mechanisms for many of these changes are still not well understood. Exciting new findings of Roepke and colleagues indicate that estradiol regulates expression of key potassium channels as well as modulatory intracellular signaling molecules. This regulation might contribute to regionspecific changes in neuronal excitability in the hypothalamic arcuate nucleus and to the integration of neuronal responses that influence homeostatic functions.
Introduction Women are exposed to high levels of steroid hormones from endogenous and exogenous sources throughout their lives. The synchronized oscillation of endogenous estradiol exerts both negative and positive feedback effects on gonadotropin secretion during the female reproductive cycle. Exogenous estrogens are components of oral contraceptives and hormone replacement therapy. In spite of the importance of estrogens in women’s health, the molecular mechanisms and neuronal circuitry for the multiple and diverse actions of estrogens in the central nervous system are not completely understood. The importance of estradiol in modulating brain function extends beyond the control of reproductive functions to other hypothalamic and extrahypothalamic neuronal systems. Overall, findings indicate that estradiol does not act by just one mechanism but that there might be functionally specific mechanisms that influence synaptic transmission in the central nervous system. Exciting new findings by Roepke et al. [1] indicate that estrogen regulates expression of potassium channels and key signaling molecules associated with channel function. Although these authors focused on the hypothalamic arcuate nucleus and neurons specifically related to energy homeostasis and feeding behaviors, these findings point to a mechanism by which estrogen could affect neuronal excitability; this mechanism might be broadly applicable to the excitability of other estrogen-responsive neurons throughout the brain. Different avenues for estrogen receptor actions The pioneering work by Jensen’s laboratory in the 1950s started the scientific journey to understanding the mechanism of action of estrogen with the identification of the Corresponding author: Arbogast, L.A. ([email protected]). Available online 26 November 2007. www.sciencedirect.com
classical estrogen receptor (ER) denoted ERa [2]. The development of molecular techniques in the 1980s led to cloning of ERa and a greater understanding of its structure and action as a transcriptional regulator [3,4]. In the 1990s, the field became more complex with the expansion of the receptor family to include ERß and an increase in the understanding of ER interactions with co-regulatory molecules [3–5]. During this time, a wealth of information accumulated, which indicated that estradiol might have rapid effects in the central nervous system involving nongenomic mechanisms [6,7]. Evidence for a putative plasma membrane ER and interaction of estradiol with second messenger signaling pathways focused the journey in new directions. Proteins identical or similar to the classical nuclear receptors [8,9] as well as a distinct membrane receptor, ER-X [10], have been localized to the plasma membrane. In addition, the G-protein-coupled receptor GPR30 has been localized to the cell membrane and associated with specific estradiol binding and rapid estrogendependent G-protein-coupled signaling [11,12]. The wide-ranging influences of estradiol on neuronal function are apparent from the supplementary table included in the study by Roepke and colleagues [1]. These scientists used suppressive subtractive hybridization to identify hundreds of important estrogen-regulated brainspecific genes in the guinea-pig hypothalamus. Although not surprising, the broad scope of functions associated with these genes supports the notion that estrogen has diverse actions within the central nervous system. Although some of these genes have been identified previously by more focused approaches, there is still a significant scientific challenge to put these genes into a physiological perspective and to identify the complex estradiol-initiated interactions in particular neurons. Estradiol effects on neuronal excitability The genomic/gene-regulatory approach used by Roepke et al. [1] identified estradiol-induced changes in the expression of three prominent potassium channel family members from among the great number of genes encoding potassium channel proteins. The study focused on the identified potassium channels and those signaling molecules important in regulating these channels. The context being that the coordinated induction or suppression of these genes by estradiol may affect the functional interactions between potassium channel subunits and second messenger signals, thus altering neuronal excitability. It is notable that even within the arcuate nucleus there were regional variations in the effect of estrogen on the expression of specific channels. The
1043-2760/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tem.2007.10.002
2
Update
TRENDS in Endocrinology and Metabolism Vol.19 No.1
estrogen-regulated channels identified by this approach belong to two voltage-gated channel families involved with generation of M-current (KCNQ) or A-current (Kv4) and one inward rectifier channel family (Kir2). These data link the transcriptional actions of estrogen to proteins involved in membrane-associated events during the period of estrogen negative feedback and point to a complex expression pattern for different potassium channel subunits. This expression pattern might be resolved with a cell-based analysis. The functionality of potassium channels is more complex than just their expression. Post-translational modifications such as phosphorylation of the channels or the availability or propensity of effector molecules to associate directly with the channels play important modulatory roles. Roepke et al. [1] report regional differences in the arcuate nucleus in the expression of A-kinase anchor protein (AKAP) and calmodulin (CaM), protein kinase C (PKCe), phospholipase C (PLCß4) and phosphatidylinositol-3-kinase (PI3K p55g). The integration of these molecular expression findings with electrophysiological and functional studies of neurons in the arcuate nucleus will be necessary to support the contention that there are regional differences in neuronal excitability within the arcuate nucleus. The contribution of the membrane ER and non-transcriptional events must be included in the equation. The complement of potassium channel proteins and signaling molecules might allow for a prediction of neuronal excitability and of how a specific neuronal type would respond to neurotransmitter or neurohormone input. Functional implications for estrogen signaling in the arcuate nucleus The arcuate nucleus is an area intimately associated with controlling reproductive functions as well as energy homeostasis, and there are distinct regional distributions for specific neuronal phenotypes. The arcuate nucleus contains a diverse population of neurons that are differentially regulated and have complementary or opposing functions. Roepke and colleagues [1] detailed the expression of the potassium channels in two types of neurons, proopiomelanocortin (POMC) and neuropeptide Y (NPY), which release peptides exerting opposite anorectic and orexigenic effects, respectively. ERa expressed at a high level in POMC neurons would be a potential mediator of the estrogen signal, but the involvement of a membrane ER in modulating potassium receptors or signaling molecules by genomic or non-genomic mechanisms cannot be discounted. Indeed, some actions of estradiol to increase the neuronal excitability of POMC neurons are mediated by a novel G-protein-coupled ER-mediated mechanism that involves activation of PKC [13]. Only a moderate number of cells in the arcuate nucleus of rats express GPR30 immunoreactivity [14]. It is unknown whether a receptor similar or identical to ERa is localized to hypothalamic cell membranes, but such a receptor has been identified in transfected cells [8,15].
www.sciencedirect.com
The complexity of the system is evidenced by the diverse responses to estradiol within hypothalamic cells. It will be important to determine how nuclear and membrane ERmediated mechanisms interact to elicit a biological response through actions on specific neuronal types involved with energy homeostasis or reproductive functions. As new tools are developed to differentiate membrane versus nuclear ER actions, the ability to dissect the relative contributions of each of these mechanisms will be possible. Cell-based approaches will help in understanding the action of estradiol in neurons with opposing functions. As these molecular and cellular mechanisms for estrogen action are defined within specific neuronal types, it will also be important to relate them to the physiological responses in vivo. Although additional studies will be required to dissect the molecular mechanisms for these diverse actions of estrogen within the hypothalamus, the work of Roepke et al. [1] provides an important link between genomic actions of estradiol and membrane proteins to modulate neuronal excitability. References 1 Roepke, T.A. et al. (2007) Estrogen regulation of genes important for K+ channel signaling in the arcuate nucleus. Endocrinology 148, 4937– 4951 2 Jensen, E.V. and DeSombre, E.R. (1973) Estrogen-receptor interaction. Science 182, 126–134 3 O’Malley, B.W. (2005) A life-long search for the molecular pathways of steroid hormone action. Mol. Endocrinol. 19, 1402–1411 4 Chambon, P. (2005) The nuclear receptor superfamily: A personal retrospect on the first two decades. Mol. Endocrinol. 19, 1418–1428 5 Kuiper, G.G.J.M. et al. (1996) Cloning of a novel estrogen receptor expressed in rat prostate and ovary. Proc. Natl. Acad. Sci. U. S. A. 93, 5925–5930 6 Vasudevan, N. and Pfaff, D.W. (2007) Membrane-initiated actions of estrogens in neuroendocrinology: Emerging principles. Endocr. Rev. 28, 1–19 7 Zhang, D. and Trudeau, V.L. (2006) Integration of membrane and nuclear estrogen receptor signaling. Comp. Biochem. Physiol. 144, 306–315 8 Razandi, M. et al. (1999) Cell membrane and nuclear estrogen receptors (ERs) originate from a single transcript: Studies of ERa and ERß expressed in Chinese hamster ovary cells. Mol. Endocrinol. 13, 307– 319 9 Powell, C.E. et al. (2001) Identification and characterization of membrane estrogen receptor from MCF7 estrogen-target cell. J. Steroid Biochem. Mol. Biol. 77, 97–108 10 Toran-Allerand, C.D. et al. (2002) ER-X: A novel plasma membraneassociated, putative estrogen receptor that is regulated during development and after ischemic brain injury. J. Neurosci. 22, 8391– 8401 11 Thomas, P. et al. (2005) Identity of an estrogen membrane receptor coupled to a G protein in human breast cancer cells. Endocrinology 146, 624–632 12 Revankar, C.M. et al. (2005) A transmembrane intracellular estrogen receptor mediates rapid cell signaling. Science 307, 1625–1630 13 Qiu, J. et al. (2003) Rapid signaling of estrogen in hypothalamic neurons involves a novel G-protein-coupled estrogen receptor that activates protein kinase C. J. Neurosci. 23, 9529–9540 14 Brailoiu, E. et al. (2007) Distribution and characterization of estrogen receptor G protein coupled receptor 30 in the rat central nervous system. J. Endocrinol. 193, 311–321 15 Dewing, P. et al. (2007) Membrane estrogen receptor-a interactions with metabotropic glutamate receptor 1a modulate female sexual receptivity in rats. J. Neurosci. 27, 9294–9300