Immunology, Endocrine & Metabolic Agents in Medicinal Chemistry (Discontinued) - Volume 6, Issue 3, 2006

This Theme Issue is dedicated to the emerging field of nongenomic or extranuclear effects of the nuclear receptor hormone family. It is nowadays recognized that most hormones working through nuclear receptors have a wider pleiotropic action than the one elicited by the interaction within the nucleus. For these hormones also extranuclear effects have been reported, with a rapid time course that cannot be explained by a genomic mechanism. Much of the pioneering work was done by of Paul J. Davis and his group, and it is appropriate that their contribution on the state of the art of nongenomic effects of thyroid hormones open this issue. They have recently identified the long-searched plasma membrane receptor for thyroxine as the αVβ3 integrin; the hormone binds at the Arg-Gly-Asp recognition site as shown by the capability of RGD peptides to inhibit thyroid hormone action at the cell surface. This finding explains several, if not all, of the extranuclear effects of this class of hormones, and opens a new scenario for the years to come. Membrane transport systems account for the largest group of nongenomic effects of thyroid hormones and the review by Farias et al. covers this topic with a wide survey of this important field. Thyroid hormones cause weight reduction by metabolic rate and cholesterol reduction through enhanced expression of LDL receptors and cholesterol metabolism, but the administration of thyroid hormones also gives rise to side effects that may induce thyreotoxicosis with particularly important effects on the cardiovascular system (tachycardia and atrial fibrillation). Pharmacological research has been aimed at the development of thyroid hormone analogs to be used in case of obesity that produce the positive effects on lipid metabolism without the deleterious effects on the heart. The contribution by Goglia's group focuses on the effects of diiodothyronines on mitochondria, and on the novel finding that 3,5-diiodothyronine appears to be devoid of the side effects causing thyreotoxicosis. The last contribution on thyroid hormones comes from Zhao et al. and deals with the complicated interaction between estrogens and thyroid hormones, that appears to be particularly relevant for neuroendocrine feedback and reproductive behavior. In analogy to estrogens, thyroid hormones exhibit a large range of actions, in particular they are critical for growth, development and differentiation. Neonatal hypothyroidism results in cretinism, a disorder characterized by mental retardation and skeletal defects. Hormone therapy during pregnancy or to the child immediately after birth, may reverse a situation that otherwise will become irreversible, resulting in a more or less serious cognitive deficiency. A plasma membrane receptor for estrogens has not been identified yet and contradictory results are reported in the literature, suggesting either that the membrane receptor is identical or very similar to a nuclear receptor (ERα or ERβ) or that it is a totally different protein. Nongenomic effects of estrogens have been found in many different tissues, and the paper by Zhao et al. reports on these controversial findings. It is possible that many actions of estrogens at the membrane of the neuroendocrine system are mediated through modulation of nongenomic responses such as the effects on voltage-operated calcium channels and intracellular calcium stores. Another contribution on estrogens, and a step forward in the understanding of both the mechanisms and roles of their extranuclear effects, is given by Marino and Ascenzi. They recently showed that ERα can undergo Spalmitoylation, a modification that represents a major determinant for ERα at the plasma membrane and is important for the modulation of nongenomic effects of estrogens. Palmitoylation enables the plasma membrane exposure of ERα and promotes downstream signaling for estradiol-mediated proliferation and survival of cancer cells, offering a new target for anti-tumor therapy. Glucocorticoid receptors have been studied for many years and the multi-billion-dollar market for this class of compounds is growing rapidly as side effect profiles are reduced and newly developed molecules travels from the laboratories to the pill boxes. The contribution from Daufeldt and Allera deals with the physiological and clinical significance of the interaction of natural and synthetic glucocorticoids with plasma-membrane-residing glucocorticoid receptors. Glucocorticoids have powerful anti-inflammatory and immunomodulatory effects as well as apoptotic competence in many lymphoid malignancies, and various glucocorticoids have been essential therapeutic tools for the past 50 years despite the significant risks associated with their long-term application, e.g. rapid bone loss, clinical osteoporosis, joint necrosis, metabolic effects or Cushing syndrome......

The higher affinity of the nuclear thyroid hormone receptor TR for 3,5,3'-triiodo-L-thyronine (T3) relative to Lthyroxine (T4), the well-characterized genomic actions of T3 and the existence of cellular deiodinases that convert T4 to T3 support the concept that T4 is a prohormone for T3. In the past decade, however, a number of actions of thyroid hormones, initiated at the plasma membrane and involving the cytoskeleton or specific events in the cell nucleus, have been described that are primary responses to T4. Two prototypes of such nongenomic actions are 1) actin polymerization and its consequences in terms of neuritogenesis and interactions of astrocytes with extracellular matrix proteins and 2) stimulation of the mitogen-activated protein kinase signal transduction pathway with subsequent serine phosphorylation of transactivator nucleoproteins, including TR, p53 and STAT1α. These actions of T4 appear to be independent of entry of thyroid hormone into the cell. From the cell surface, T4-dictated serine phosphorylation of transactivators such as TRβ1 modulate the transcriptional activity of these proteins. This is a postulated cooperative interface between nongenomic and genomic effects of thyroid hormone, prompted by action of T4 at the plasma membrane. Biological endpoints of such actions of T4 in model systems are angiogenesis and increased growth of certain tumor cells. The membrane receptor for activation by T 4 of mitogen-activated protein kinase is on an integrin, αVβ3. From its cell surface receptor, T4 can also influence intracellular protein trafficking and modulate activities of plasma membrane ion pumps or channels. Thus, T4 has plasma membrane-initiated actions as a hormone, as well as its function as a source of the more metabolically important iodothyronine analogue, T3.

Extranuclear or nongenomic effects of thyroid hormones are unaffected by inhibitors of protein synthesis, and their rapid time course cannot be explained by interaction of the hormone molecule with nuclear receptors. Their origin has been localized at the plasma membrane, but also at organelles such as the endoplasmatic reticulum and mitochondria. Thyroid hormone has been reported to activate, by both genomic and non genomic mechanisms, the Ca2+-ATPase that stores calcium from the cytosol in the sarcoplasmic reticulum; the decrease in intracellular Ca2+ leads to muscle relaxation. Considering the important effects on the cardiovascular system, T3 can actually be envisaged as a potent inotropic drug. T3 is also a major regulator of the plasma membrane Na+/K+-ATPase activity; T3 and its analog 3,5-diiodothyronine rapidly inhibits Na+/K+-ATPase in chick embryo hepatocytes, whereas the activity is up-regulated in alveolar epithelial cells. Also the ubiquitous plasma membrane Na+/H+ exchanger, that regulates cell volume and pH by exchanging extracellular Na+ with cytoplasmic H+ according to the concentration gradient, is activated by T3 via both genomic and nongenomic mechanisms. A growing number of natural and synthetic thyroid hormone analogs are available to study the physiological importance of extranuclear effects; this may lead to compounds that selectively target either genomic or nongenomic receptors. Such drugs may make it possible to activate separately only a part of the complex effects normally induced by thyroid hormones, this could be of clinical relevance for the cardiovascular system, bone tissue and the Central Nervous System.

The purpose of this review is to summarize the current state of knowledge concerning the biological activities of 3, 5-diiodothyronine (T 2) and its potential use as a pharmacological agent. Until recent years, T2 was considered an inactive metabolite of thyroid hormones thyroxine (T4) and triiodo-L-thyronine (T3). Several observations, however, led to a reconsideration of this idea. Early studies dealing with the biological activities of this iodothyronine revealed its ability to stimulate cellular/mitochondrial respiration, essentially by a nuclear-independent pathway. Mitochondria and the energytransduction apparatus seem to be major targets of T2, although outside the mitochondria T2 also has effects on carriers, ion-exchangers and enzymes. Recent studies suggest that T 2 may also affect the transcription of some genes, but again the underlying mechanisms seem to differ from those actuated by T3. The accumulated evidence permits the conclusion that the actions of T2 do not simply mimic those of T3 but instead are specific actions exerted through mechanisms that are independent of those actuated by T3 and do not involve thyroid hormone receptors. In addition, very recent evidence leads us to suggest that T2 may be a potentially useful agent for the treatment of diet-dependent overweight (and the consequent hypertriglyceridemia and high cholesterol level) without inducing thyrotoxicosis.

Estrogens and thyroid hormones are regulators of important diverse physiological processes such as reproduction, thermogenesis, neural development, neural differentiation and cardiovascular functions. Both are ligands for receptors in the nuclear receptor superfamily, which act as ligand-dependent transcription factors, regulating transcription. However, estrogens and thyroid hormones also rapidly (within minutes or seconds) activate kinase cascades and calcium increases, presumably initiated at the cell membrane. We discuss the relevance of both modes of hormone action, including the membrane estrogen receptor, to physiology, with particular reference to lordosis behavior. We first showed that estrogen restricted to the membrane can, in fact, lead to subsequent increases in transcription from a consensus estrogen response element-based reporter in the neuroblastoma cell line, SK-N-BE(2)C. Using a novel hormonal paradigm, we also showed that the activation of protein kinase A, protein kinase C, mitogen activated protein kinase and increases in calcium were important in the ability of the membrane-limited estrogen to potentiate transcription. We discuss the source of calcium important in transcriptional potentiation. Since estrogens and thyroid hormones have common effects on neuroprotection, cognition and mood, we also hypothesized that crosstalk could occur between the rapid actions of thyroid hormones and the genomic actions of estrogens. In neural cells, we showed that triiodothyronine acting rapidly via MAPK can increase transcription by the nuclear estrogen receptor ERα from a consensus estrogen response element, possibly by the phosphorylation of the ERα. Novel mechanisms that link signals initiated by hormones from the membrane to the nucleus are physiologically relevant and can achieve neuroendocrine integration.

The knowledge of the molecular mechanism by which estrogens exert pleiotropic functions in different tissues and organs has evolved rapidly during the past two decades. It is now well established that 17β-estradiol (E2) induces the transcriptional regulation of target gene expression upon binding to the intracellular estrogen receptor-α (ERα) or -β (ERb). In addition E2 modulates cell functions through rapid non-genomic actions. Stimulation of G-proteins, Ca2+ influx as well as phospholipase C, ERK/MAPK, and PI3K/AKT activation occur within seconds to minutes upon E2 binding to a small population of ERα located at the plasma membrane. Several laboratories have recently examined the structural requirements for the localization and function(s) of plasma membrane ERα. We have shown that human ERα location at the plasma membrane and its interaction with caveolin-1 is mediated by S-palmitoylation of the Cys447 residue. Moreover, E2 also modulates Cys447 S-(de)palmitoylation enabling ERα association with signal transduction proteins (e.g. Src and G-proteins) in a cell context-related fashion. This leads to downstream signaling important for cell growth and survival. Here, the structural bases and mechanisms for ERα localization and action(s) are discussed along with potential implications for development of new drugs.

Steroid hormones are indispensable in physical development, control of vital processes, reproduction and modulation of behavior. Lack or complete dysfunction of glucocorticoids (GC), in particular, have lethal consequences. Even a minor change in the level of circulating cortisol may have physiological and clinical significance in GChomeostasis and GC-related disorders. Knowledge of the action mechanisms of GC and analysis of their effects is therefore of essential importance, especially since natural and synthetic GC are widely used in the therapy of GC-responsive diseases. Most GC effects are assigned to the nuclear GC receptors (GR) and their co-modulators by activating or repressing gene expression. Whereas activation mainly requires DNA binding of the GR-ligand complex, repression is usually mediated by protein-protein interaction with other transcription factors, termed transcriptional cross-talk. In addition to the classical and the cross-talk mode of GR actions, GC also functionally interact with plasma membrane (PM) binding sites at the surface of target cells and initiate a number of so called rapid non-genomic effects similar to signaling by peptide hormones. It is generally believed that no functional link exists between the PM-related non-genomic steroid responses and the nuclear receptor-related genomic effects. However, some reports indicate that rapid responses can modulate the genomic pathway of steroids or enhance/impair gene activation. The corresponding PM-receptors may be (splice) variants of the GR or GR-unrelated proteins that cooperate with the GR or not. A PM-located protein in rat and human liver cells, SHREC (Steroid Hormone Recognition and Effector Complex), represents a pivotal link in nongenomic and genomic modes of GC-signal propagation. The different modes of action of GC and the various PMreceptors are reviewed with emphasis on the basic biological functions as well as the physiological and clinical significance of these "extranuclear" proteins. In addition, a three-dimensional computational model of SHREC is presented, which may provide the possibility for rational design of tailor made GC to improve therapy of GC related diseases.

Oxysterols are oxygenated derivatives of cholesterol with a very short life-time relative to cholesterol. Oxysterols are present in nanomolar concentrations in biological fluids underscoring their role as important intermediates in a number of biochemical pathways including bile acid synthesis, reverse cholesterol transport, control of cholesterol synthesis in the brain, and oxidative stress. Most oxysterols are produced by enzymes of the cytochrome P450 family while others are produced by free radical reactions. This last group of oxysterols (stress oxysterols, or SOX) , which are mainly oxygenated in the C6 or C7 position, have attracted interest for mechanistic studies in the context of oxidative stress, and for probing oxidative stress in vivo . Sensitive and specific mass spectrometric methods have been prepared to measure SOX in a number of clinical settings, and to follow the changes induced by pharmacological treatments. Additional interest in oxidative stress oxysterols is linked to the increasing number of biological effects, obtained at cellular level and in animal models, implicated in the pathophysiological mechanisms that play a role in several diseases, including atherosclerosis, neurodegeneration, and cancer. Oxysterols have in fact been shown to induce apoptosis, cell differentiation, cytotoxicity, and impairment of endothelial function. This review is an evaluation of the recent literature on oxysterols, in particular on the role of oxysterols as bioactive compounds.