Non-genomic actions of sex steroid hormones.Sex steroid hormones are known to act through intracellular receptors and their cognate hormone response elements, located in the promoters non genomic mechanisms of action of steroid hormones hormone-regulated genes. However, this classical mechanism of action cannot account for a variety of rapid effects of steroids within seconds or minutes. In this review, non-genomic modes of oof cell responses to sex steroids are described. The prototypical example is the resumption of meiosis in amphibian oocytes, triggered by progesterone at the plasma membrane level. Membrane effects of progesterone may also account for sperm maturation.
Non-genomic mechanisms of action of steroid hormones. - PubMed - NCBI
According to the traditional model, steroid hormones bind to intracellular receptors and subsequently modulate transcription and protein synthesis, thus triggering genomic events finally responsible for delayed effects. Based upon similarities in molecular structure, specific receptors for steroids, vitamin D 3 derivatives, thyroid hormone, retinoids, and a variety of orphan receptors are considered to represent a superfamily of steroid receptors. In addition, very rapid effects of steroids mainly affecting intracellular signaling have been widely recognized that are clearly incompatible with the genomic model.
These rapid, nongenomic steroid actions are likely to be transmitted via specific membrane receptors. Evidence for nongenomic steroid effects and distinct receptors involved is presented for all steroid groups including related compounds like vitamin D 3 and thyroid hormones.
The physiological and clinical relevance of these rapid effects is still largely unclear, but their existence in vivo has been clearly shown in various settings including human studies. Drugs that specifically affect nongenomic steroid action may find applications in various clinical areas such as cardiovascular and central nervous disorders, electrolyte homeostasis, and infertility.
In addition to a short description of genomic steroid action, this review pays particular attention to the current knowledge and important results on the mechanisms of nongenomic steroid action. The modes of action are discussed in relation to their potential physiological or pathophysiological relevance and with regard to a cross-talk between genomic and nongenomic responses.
For more than 30 years steroids have been known to be involved in various physiological responses with a primary focus on the genomic aspects of action. According to the classic genomic theory of action, steroid hormones bind to specific receptors, which are intracellular transcription factors, and exert positive or negative effects on the expression of target genes Beato et al.
These effects are characterized by a specific delay and a sensitivity toward inhibitors of transcription and translation, e. Intracellular steroid receptors have been thoroughly characterized and, finally, cloned; they are composed of a ligand-binding domain, a DNA-binding domain, and several transactivation functions distributed along the molecule Evans, ; Beato, ; Fuller, In addition to the delayed genomic steroid actions, increasing evidence for rapid, nongenomic steroid effects has been demonstrated for virtually all groups of steroids, and transmission by so far hypothetical specific membrane receptors is very likely.
Nongenomic effects on cellular function involve conventional second messenger cascades, including phospholipase C PLC 2 Civitelli et al. They are clearly incompatible with the involvement of genomic mechanisms. Interestingly, the history of research on rapid responses of steroid hormones actually predates the knowledge on the existence of nuclear receptors. In , Hans Selye was the first to describe a rapid effect of progesterone, which following intraperitoneal application induces a prompt onset of anesthesia in rats Selye, In , acute cardiovascular effects of aldosterone in men were demonstrated Klein and Henk, Peripheral vascular resistance and blood pressure increased within 5 min, while cardiac output significantly decreased, suggesting a nongenomic mechanism of action because of the short time frame.
As a nucleus is absent, in vitro effects in these cells cannot be related to genomic mechanisms and, therefore, must be nongenomic in nature. Furthermore, rapid effects of glucocorticoids on isolated synaptosomes were demonstrated in the mids, being considered as cellular correlate for the long-known negative feedback mechanism between plasma cortisol and ACTH release occurring within a few minutes Edwardson and Bennett, In the following sections, important aspects of steroid action are summarized.
Two sections on mechanisms of steroid actions in principle and the proteins that mediate them are followed by a condensed summary on nongenomic actions of each particular steroid group.
Finally, clinical perspectives are discussed and an integrative model of steroid action is developed. According to the common theory of steroid action, steroids modulate gene transcription by interaction with intracellular, nuclear receptors, which act as ligand-dependent transcription factors. Steroids regulate expression of various genes in a network-like manner and initiate complex events involved in nearly every aspect of vertebrate development and physiological responses Evans, ; Beato et al.
The detailed characterization of steroid actions and mechanisms involved was the result of intensive long-term research on steroid hormones. In the beginning of the 20th century, abnormalities of embryonal development and various diseases had been associated with defects in steroid and thyroid hormone action Gudernatsch, The beginning of the modern era of steroid research is marked by a fundamental discovery by Clever and Karlson , who investigated the puff reactions of chromosomes in larvae of insects.
Injection of the steroid ecdysone induces changes in chromosomal structure within 2 h. These puff reactions disappear within 24 h, suggesting a link between steroid hormones and the activation of genes. Subsequently, research has focused on the analysis of cellular and molecular mechanisms involved in related steroid actions on specific target tissues for review see Beato, ; Fuller, ; Beato et al. Interestingly, orphan receptors may have no ligands or as yet undiscovered ligands Funder, ; they seem to open a particularly challenging field of future research.
These receptors act as transcription factors to regulate gene expression by recognizing palindromic hormone response elements HRE at the DNA after homo- or heterodimerization of the ligand-receptor complex. Subsequently, transcription is initiated in conjunction with the basal transcription complex, different coactivators, repressors, and transcription regulators Beato and Klug, The expression of steroid-induced genes is modulated at the protein level some hours after stimulation with the steroid, although immediate early genes are differentially expressed after aldosterone stimulation within 1 h Verrey, Unlike intracellular steroid receptors, membrane-bound receptors of other agonists such as peptide agonists, catecholamines, or platelet-derived growth factor affect cellular function by modulation of intracellular second messenger levels.
In addition to these direct effects of second messengers, agonist-induced changes of intracellular messengers modulate steroid-induced transcription by an intracellular cross-talk. Thus, activation of cells by peptide agonists may modulate steroid-induced nuclear transcription by second messengers induced with an intrinsic ability to modulate nuclear transcription [e. Furthermore, intracellular cross-talk may even occur in the absence of the steroid ligand.
In contrast to genomic steroid action, nongenomic steroid effects are principally characterized by their insensitivity to inhibitors of transcription and protein synthesis, and—representing the most obvious experimental evidence—by their rapid onset of action within seconds to minutes.
These rapid effects are likely to be mediated through receptors with pharmacological properties distinct from those of the intracellular steroid receptors see below. Discrepancies in pharmacological properties alone are not sufficient to support the hypothesis of separate receptor proteins for nongenomic action; however, this important issue is addressed in Section III.
In the past two decades, a growing body of reports dealing with nongenomic steroid action has emerged, which reflects the increasing interest in this field. In these studies a variety of potential mechanisms thought to be involved in rapid steroid action has been described, suggesting that the mechanisms of rapid steroid signaling are not uniform. This Mannheim classification scheme Fig. The scheme is divided into two major groups termed A direct steroid action and B indirect steroid action , which are subsequently split into a nonspecific I and a specific II category.
The latter is further divided into group a classic steroid receptor involved and b nonclassic steroid receptor involved. Mannheim classification of nongenomic steroid actions.
Solid arrows indicate examples for categories with examples given in the text. Dotted arrows indicate a hypothetical category with no example yet known. Reproduced with permission from Falkenstein et al. Each of the steroid and thyroid hormones displays its own facets of signaling and modulation of cellular functions. Specific nongenomic responses seem to depend on the type of steroid, cells, tissues, or species used.
The concept that steroids are involved in the regulation of cell function was originally triggered by the above-mentioned observation that the steroid hormone ecdysone induces puffs in giant chromosomes of insects Clever and Karlson, All steroid hormones, which are mainly formed in the gonads and adrenals of mammals, regulate a variety of functions in target cells equipped with the cognate steroid hormone receptors.
Although steroid hormones and retinoic acid, vitamin D 3 , and thyroid hormones are neither structurally nor biosynthetically related, receptors for steroids, retinoic acid, vitamin D 3 , and thyroid hormones have been characterized as nuclear receptors or a superfamily of steroid and thyroid hormone receptors due to their close structural homologies Evans, ; Beato and Klug, The binding characteristics of the GR are consistent with pharmacological properties of glucocorticoid-induced effects shown previously by in vivo and in vitro studies describing a high-affinity binding for the synthetic glucocorticoid dexamethasone and low-affinity binding for mineralocorticoids Lee et al.
Subsequently, the mineralocorticoid receptor MR Arriza et al. However, the distinctive role of each of those variants is currently not known in detail.
Primary amino acid sequences of these receptors have been aligned on the basis of regions of maximum amino acid similarity. The nuclear receptors are structurally organized in different domains: Furthermore, hormone-dependent transcriptional activation domains have been identified, which are embedded within the LBD and the N-terminal domain Beato, ; Beato and Klug, Fig.
General structure and functional organization of steroid and thyroid hormone receptors. Steroid receptors are structurally organized in different domains, which have been confirmed by results of cDNA cloning experiments: Most of the receptor antibodies described are directed against this part of the protein.
This part of the protein may be responsible for the DNA binding activity of the receptors Evans, The functional assignment of amino acid sequences derived from cDNA cloning experiments is indicated below. Modified according to Beato Historically, a new hormone has been discovered and characterized, and the partner receptor has been looked for, whereas here the sequence information of an assumed receptor may be obtained, and the ligands binding to these orphan receptors are unknown and remain to be identified.
Subsequently, a vitamin A metabolite 9-cis retinoic acid has been characterized as a high-affinity ligand for the three retinoic X receptor subtypes Heyman et al.
Furthermore, peroxisome proliferator-activated receptors PPARs have been detected as members of orphan nuclear receptors.
Their name reflects the fact that PPARs are activated by chemicals that increase the number and size of peroxisomes in rodents Issemann and Green, PPARs are regarded to be key regulators of glucose and lipid homeostasis Lemberger et al.
There is now compelling evidence that several of the orphans are a new generation of steroid receptors presumably revealing a broader biological role for steroid hormones than previously appreciated. It is assumed that the lipophilic steroid hormones enter the respective target cells by simple diffusion, although the matter of active transmembrane transport is still under debate but unsettled Allera and Wildt, Steroid hormone receptors are associated with a complex of chaperone proteins in the unliganded state Pratt and Toft, ; Defranco, Upon binding of steroids to the cognate steroid hormone receptor in the cytosol, the heat-shock protein Hsp 90 and the immunophilin Hsp 56, which maintain the receptors in an inactive form with high affinity for the steroid hormones, dissociate from the receptors Pratt and Toft, This transformation of the steroid hormone receptor is associated with an increased affinity of the receptor to DNA and a decrease in complex size.
The chaperones are probably necessary to keep the steroid hormone receptors functional Godowski and Picard, The cavity of the steroid hormone receptors, which is able to bind the steroid ligand, is created by the LBD and is covered by helix 12 of the LBD after hormone binding. Due to the relocation of helix 12 after hormone binding, coactivators are able to bind to respective parts of the LBDs Fig.
The knowledge of the atomic structure of nuclear receptors before and after hormone binding will allow researchers to specifically design compounds that exhibit specific and efficient features to differentially modulate nuclear receptor function. GRs and MRs translocate into the cell nucleus after hormone binding, probably due to the release of nuclear localization signals, whereas the majority of nuclear receptors, such as ER, AR, and PR, are located in the nucleus at equilibrium.
Constitutively expressed nuclear localization signals of those receptors at the DBD are thought to be required for nuclear pore recognition, whereas secondary nuclear localization signals at the LBD domains are ligand-dependent Guiochon-Mantel et al.
After translocation into the nucleus, the ligand-receptor complex binds to palindromic DNA sequences. The receptors for glucocorticoids, mineralocorticoids, androgens, and progestins bind to the same HRE, which have been originally described as glucocorticoid response elements. The sixth base pair of each half-palindrome is not well conserved, and its identity is not essential for specific binding Scheidereit et al. Initially it was believed that all members of the nuclear receptor family bind as homo- or heterodimers to the palindromic HREs in the promoter region of target genes, while each part of the HRE is recognized by one receptor monomer.
That the strength of a weak dimerization region within the DBD and the LBD is modified by cooperativity observed during DNA binding is obviously responsible for this dimerization Eriksson et al. Interaction of the steroid-receptor complex with the HRE is coordinated by the existence of two steroid hormone receptor-specific zinc fingers, a structure reminiscent but clearly different to the zinc finger motifs observed in the transcription factor IIIA of Xenopus laevis Beato, The steroid hormone receptor-specific zinc finger is formed by four cysteines in the cysteine-rich region of the DBD and few amino acids at the adhering region Fig.
While the proximal zinc finger P-box is responsible for specific interaction with the HRE, the distal box forms a weak dimerization area at the DNA-binding domain.
However, before interaction of the nuclear receptors with cognate HRE, the ligand-receptor complex must have the possibility to interact with the DNA, which is compacted into the chromatin. Genetic analysis has revealed that chromatin structure is essentially involved in gene regulation Beato and Eisfeld, The question is how nuclear transcription factors gain access to the target sequences in the chromatin.
Steroid hormone receptors can bind to regularly organized chromatin, probably due to the rotational orientation of HRE in nucleosomes, which has been observed in vitro and in vivo Pina et al. An interaction of the PR and the nucleosome-remodeling factor is involved in transactivation Di Croce et al.