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Hormones and Signal Transduction: Introduction – Biochemistry - Lecturio

Buy Softcover. FAQ Policy. About this book Plant hormones play a crucial role in controlling the way in which plants grow and develop. Show all. Jasmonates Pages Howe, Gregg A. Show next xx. Read this book on SpringerLink. Recommended for you. In addition to scaffold proteins, ER lipid modifications are involved in ER membrane targeting. Specifically, palmitoylation appears required, with cysteine the apparent critical palmitoylation site Furthermore, ERs are targeted to lipid rafts and, in ECs, the specialized membrane signaling organelles known as caveolae are the preferential sites of ER-centered complexes in EC 24 — This molecular interaction is critical for ER plasma membrane localization Estrogen is able to evoke specific physiological responses in many tissues within seconds to minutes after ligand binding.

Although this is not proof of membrane receptor localization, the rapidity of consequent events is certainly consistent with such a distribution. Many intracellular signaling cascades have been shown to be triggered by estrogen. Activation of these pathways occurs in a cell-type-specific manner to alter downstream effectors and produce rapid physiological responses in target tissues. Because ERs do not have intrinsic kinase activity, these molecular interactions are critical to direct estrogen-stimulated rapid action and may occur in a cell-type-dependent fashion.

In addition to its potential as a docking protein for membrane-associated ER, E2 stimulates Shc phosphorylation, leading to Shc-Grb2-Sos complex formation, which in turn is critical to transduction of MAPK pathways 5. MNAR also known as PELP1 , is an important scaffold molecule in many tissues but may be preferentially expressed in rapidly proliferating cell types, as demonstrated in cancer-derived cell lines Consequent to this series of interactions is activation of c-Src, leading to downstream signaling in the MAPK pathway through Ras and Raf Within the cardiovascular system, rapid signaling pathways initiated by ER engagement have now been well defined.

One method by which estrogen exerts rapid modulation of the vascular endothelium is via the enhanced production of nitric oxide NO , a vasoprotective molecule important in maintaining vascular health. Akt phosphorylation then occurs on serine and threonine The relevant outcome is enhanced NO production, which promotes maintenance of vascular homeostasis through vasodilation, inhibition of platelet aggregation, leukocyte adhesion, and smooth muscle cell proliferation as well as playing a significant role in angiogenic responses In vivo correlates have been described, with estrogen-induced femoral arterial vasodilatory responses to estrogen occurring within minutes.

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In some models, these responses are MAPK dependent ER-centered, eNOS-activating molecular complex in caveolae within vascular endothelium. Depicted at the bottom are molecular features of plasma membrane targeting, including palmitoylation, caveolin-1, and c-Src interactions. Kim and J. Hormonal regulation of epithelial cells, such as breast and ovarian tissue, contributes to the proliferative responses in these cell types. It has long been established that estrogen plays a role in initiation, development, and progression of most human breast cancers. In addition to important nuclear events induced by estrogen, it has now become clear that rapid membrane-initiated responses also affect cellular proliferation and apoptotic mechanisms.

Membrane-associated ERs have also been shown to contribute to prevention of chemotherapy- or radiation-induced apoptosis in breast cancer cells G protein-coupled receptor 30 GPR30 , an orphan receptor unrelated to classic ERs, has been uniquely localized to the endoplasmic reticulum. GPR30 can also bind estrogen and initiate rapid responses in breast cancer cells, resulting in mobilization of intracellular calcium and PI3K activation, both of which appear dependent on the epidermal growth factor receptor 44 , Phospho-STAT3 then may play a role in tumorigenesis through transcriptional enhancement of oncogenic, proproliferative genes, such as Cyclin D-1, c-Myc, and c-Fos Estrogen also exerts neuroprotective properties in the brain by means of rapid signaling, as well as by similar membrane-to-nuclear cross-talk events 52 , through a membrane-associated ER observed in many neuronal cell types.

Similarly, E2 can act via the Akt pathway to prevent injury-induced apoptosis in a model of ischemia using metabolically inhibited cortical explant cultures Estrogen-induced rapid modulation of intracellular calcium has been observed in multiple neuronal cell types, such as astrocytes 43 , granulosa cells 11 , and striatal neurons Estrogen-triggered rapid signaling events in osteocytes confers protection against bone loss.

The kinetics of ERK phosphorylation and length of time retained in the nucleus determine its pro- vs. Thus, there now exists a large amount of data supporting rapid responses to estrogen in a wide variety of cells and tissues. Furthermore, the diversity of responses is amplified by consequential effects on transcription, initiated by signals generated at the plasma membrane Table 1.

Tissue-specific splicing is a characteristic of many receptors, found in a variety of tissues, conferring the ability to direct physiological responses in a tissue-selective fashion. Characterization of all these splice variants and their engagement-related responses is beyond the scope of this review. It contains eight exons that encode a kDa full-length receptor. Like other steroid hormone receptors, ER can be divided into A-F motifs.

The C region is integral to DNA binding, whereas the D region functions in nuclear localization and dimerization. Multiple anti-ER antibody immunoreactive species have been described in numerous cell types. Subsequently, multiple transcripts of varying length have been correlated with the gene products of different sizes. In vivo gene targeting facilitated our understanding of ER variants.

Subsequently known as the ERKO Chapel Hill, both male and female mice unexpectedly displayed relatively normal sexual development, although fertility was affected This series of experiments, in multiple laboratories, demonstrated that ER variants are functional in vascular tissue.

Within the vascular endothelium, generation of a full-length, mature RNA transcript involves splicing of upstream promoter exons B—F to an acceptor site 70 nucleotides upstream of exon 1, resulting in a kDa receptor. However, a kDa splice variant is abundant in EC 58 , Creation of this variant likely involves splicing of upstream promoter exons E—F to a translation initiation ATG within exon 2. They can form heterodimers, potentially competitively inhibiting the maximally efficient DNA binding achieved by ligand-bound ER66 homodimers.

Although fully capable of mediating rapid signaling in these cells, ER46 is unable to support estrogen-dependent estrogen response element transactivation.

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Membrane-impermeant E2 stimulates rapid ER46 mobilization to caveolae microdomains, with consequent NO release. In fact, upon cotransfection, ER66 appears to reduce the noted ligand-triggered activation, suggesting a repressive or competitive effect of ER66 on rapid, ERmediated signaling. At the very least, these results confirm that there exists a signaling hierarchy and that selective receptor isoforms are likely to be more functional for specific responses in a given tissue Despite our growing understanding of the molecular controls on estrogen responses in numerous tissues, debates over the potentially beneficial and harmful effects of hormone replacement therapy HRT have intensified.

This controversy is perhaps most evident with regard to estrogen use in cardiovascular disease prevention. Earlier retrospective studies supported a cardioprotective effect of estrogen in postmenopausal women. However, more recent prospective, randomized clinical trials have challenged this concept.

In fact, there was an increase in cardiac events in the hormone-treated group within the first study year 70 , These two studies and others have directed the avoidance of HRT in postmenopausal women, at least in the context of cardiovascular disease prevention. Ligands that interact with cell-surface receptors do not have to enter the cell that they affect. Cell-surface receptors are also called cell-specific proteins or markers because they are specific to individual cell types. Because cell-surface receptor proteins are fundamental to normal cell functioning, it should come as no surprise that a malfunction in any one of these proteins could have severe consequences.

Errors in the protein structures of certain receptor molecules have been shown to play a role in hypertension high blood pressure , asthma, heart disease, and cancer. Each cell-surface receptor has three main components: an external ligand-binding domain, a hydrophobic membrane-spanning region, and an intracellular domain inside the cell. The ligand-binding domain is also called the extracellular domain.

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  • The size and extent of each of these domains vary widely, depending on the type of receptor. Cell-surface receptors are involved in most of the signaling in multicellular organisms. There are three general categories of cell-surface receptors: ion channel-linked receptors, G-protein-linked receptors, and enzyme-linked receptors. Ion channel-linked receptors bind a ligand and open a channel through the membrane that allows specific ions to pass through.

    To form a channel, this type of cell-surface receptor has an extensive membrane-spanning region. In order to interact with the phospholipid fatty acid tails that form the center of the plasma membrane, many of the amino acids in the membrane-spanning region are hydrophobic in nature. Conversely, the amino acids that line the inside of the channel are hydrophilic to allow for the passage of water or ions. When a ligand binds to the extracellular region of the channel, there is a conformational change in the proteins structure that allows ions such as sodium, calcium, magnesium, and hydrogen to pass through Figure 9.

    A closed gated ion channel. Gated ion channels form a pore through the plasma membrane that opens when the signaling molecule binds. The open pore then allows ions to flow into or out of the cell. G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein. The activated G-protein then interacts with either an ion channel or an enzyme in the membrane Figure 9. All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site.

    Cell signaling using G-protein-linked receptors occurs as a cyclic series of events. Before the ligand binds, the inactive G-protein can bind to a newly revealed site on the receptor specific for its binding. One or both of these G-protein fragments may be able to activate other proteins as a result. The subunits reassociate to form the inactive G-protein and the cycle begins anew. G-protein-linked receptors have been extensively studied and much has been learned about their roles in maintaining health.

    Bacteria that are pathogenic to humans can release poisons that interrupt specific G-protein-linked receptor function, leading to illnesses such as pertussis, botulism, and cholera. In cholera Figure 9. The toxin then enters these intestinal cells, where it modifies a G-protein that controls the opening of a chloride channel and causes it to remain continuously active, resulting in large losses of fluids from the body and potentially fatal dehydration as a result.

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    Transmitted primarily through contaminated drinking water, cholera is a major cause of death in the developing world and in areas where natural disasters interrupt the availability of clean water. The cholera bacterium, Vibrio cholerae, creates a toxin that modifies G-protein-mediated cell signaling pathways in the intestines. Modern sanitation eliminates the threat of cholera outbreaks, such as the one that swept through New York City in This poster from that era shows how, at that time, the way that the disease was transmitted was not understood. Enzyme-linked receptors are cell-surface receptors with intracellular domains that are associated with an enzyme.

    In some cases, the intracellular domain of the receptor itself is an enzyme. Other enzyme-linked receptors have a small intracellular domain that interacts directly with an enzyme. The enzyme-linked receptors normally have large extracellular and intracellular domains, but the membrane-spanning region consists of a single alpha-helical region of the peptide strand.

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    When a ligand binds to the extracellular domain, a signal is transferred through the membrane, activating the enzyme. Activation of the enzyme sets off a chain of events within the cell that eventually leads to a response. One example of this type of enzyme-linked receptor is the tyrosine kinase receptor Figure 9.

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    • A kinase is an enzyme that transfers phosphate groups from ATP to another protein. The tyrosine kinase receptor transfers phosphate groups to tyrosine molecules tyrosine residues. First, signaling molecules bind to the extracellular domain of two nearby tyrosine kinase receptors. The two neighboring receptors then bond together, or dimerize. Phosphates are then added to tyrosine residues on the intracellular domain of the receptors phosphorylation. The phosphorylated residues can then transmit the signal to the next messenger within the cytoplasm.

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      A receptor tyrosine kinase is an enzyme-linked receptor with a single transmembrane region, and extracellular and intracellular domains. Binding of a signaling molecule to the extracellular domain causes the receptor to dimerize. Tyrosine residues on the intracellular domain are then autophosphorylated, triggering a downstream cellular response. The signal is terminated by a phosphatase that removes the phosphates from the phosphotyrosine residues. Produced by signaling cells and the subsequent binding to receptors in target cells, ligands act as chemical signals that travel to the target cells to coordinate responses.

      Small hydrophobic ligands can directly diffuse through the plasma membrane and interact with internal receptors. Important members of this class of ligands are the steroid hormones. Steroids are lipids that have a hydrocarbon skeleton with four fused rings; different steroids have different functional groups attached to the carbon skeleton.

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      Steroid hormones include the female sex hormone, estradiol, which is a type of estrogen; the male sex hormone, testosterone; and cholesterol, which is an important structural component of biological membranes and a precursor of steriod hormones Figure 9. Other hydrophobic hormones include thyroid hormones and vitamin D. In order to be soluble in blood, hydrophobic ligands must bind to carrier proteins while they are being transported through the bloodstream.

      Steroid hormones have similar chemical structures to their precursor, cholesterol. Because these molecules are small and hydrophobic, they can diffuse directly across the plasma membrane into the cell, where they interact with internal receptors. Water-soluble ligands are polar and therefore cannot pass through the plasma membrane unaided; sometimes, they are too large to pass through the membrane at all. Instead, most water-soluble ligands bind to the extracellular domain of cell-surface receptors.

      This group of ligands is quite diverse and includes small molecules, peptides, and proteins. Nitric oxide NO is a gas that also acts as a ligand. It is able to diffuse directly across the plasma membrane, and one of its roles is to interact with receptors in smooth muscle and induce relaxation of the tissue. NO has a very short half-life and therefore only functions over short distances. Nitroglycerin, a treatment for heart disease, acts by triggering the release of NO, which causes blood vessels to dilate expand , thus restoring blood flow to the heart.

      NO has become better known recently because the pathway that it affects is targeted by prescription medications for erectile dysfunction, such as Viagra erection involves dilated blood vessels.

      Hormones mediate changes in target cells by binding to specific hormone receptors. In this way, even though hormones circulate throughout the body and come into contact with many different cell types, they only affect cells that possess the necessary receptors.