Journal of Investigative Medicine:
EB Symposium Manuscript
Conserved Steroid Hormone Homology Converges on Nuclear Factor κB to Modulate Inflammation in Asthma
Payne, Asha S. MD, MPH*†‡; Freishtat, Robert J. MD, MPH*†‡§
From the *Division of Emergency Medicine and †Centers for Genetic Medicine Research, Children’s National Medical Center; and Departments of‡Pediatrics and §Integrative Systems Biology, George Washington University School of Medicine and Health Sciences, Washington, DC.
Received April 26, 2011, and in revised form October 11, 2011.
Accepted for publication October 12, 2011.
Reprints: Robert J. Freishtat, MD, MPH, Center for Genetic Medicine Research, Children’s National Medical Center, 111 Michigan Ave, NW, Washington, DC. E-mail: email@example.com.
Support by Grant K12HD001399-11 (A.S.P.).
Abstract: Asthma is a complex, multifactorial disease comprising multiple different subtypes, rather than a single disease entity, yet it has a consistent clinical phenotype: recurring episodes of chest tightness, wheezing, and difficulty breathing (Pediatr Pulmonol Suppl. 1997;15:9–12). Despite the complex pathogenesis of asthma, steroid hormones (eg, glucocorticoids) are ubiquitous in the short-term and long-term management of all types of asthma. Overall, steroid hormones are a class of widely relevant, biologically active compounds originating from cholesterol and altered in a stepwise fashion, but maintain a basic 17-carbon, 4-ring structure. Steroids are lipophilic molecules that diffuse readily through cell membranes to directly and/or indirectly affect gene transcription. In addition, they use rapid, nongenomic actions to affect cellular products. Steroid hormones comprise several groups (including glucocorticoids, sex steroid hormones, and secosteroids) with critical divergent biological and physiological functions relevant to health and disease. However, the conserved homology of steroid hormone molecules, receptors, and signaling pathways suggests that each of these is part of a dynamic system of hormone interaction, likely involving an overlap of downstream signaling mechanisms. Therefore, we will review the similarities and differences of these 3 groups of steroid hormones (ie, glucocorticoids, sex steroid hormones, and secosteroids), identifying nuclear factor κB as a common inflammatory mediator. Despite our understanding of the impact of individual steroids (eg, glucocorticoids, sex steroids and secosteroids) on asthma, research has yet to explain the interplay of the dynamic system in which these hormones function. To do so, there needs to be a better understanding of the interplay of classic, nonclassic, and nongenomic steroid hormone functions. However, clues from the conserved homology steroid hormone structure and function and signaling pathways offer insight into a possible model of steroid hormone regulation of inflammation in asthma through common nuclear factor κB–mediated downstream events.
STEROID HORMONE RECEPTORS
Most steroid hormone effects begin with binding of the hormone to steroid receptors. Each receptor can have multiple isoforms, either encoded by different genes or created via alternative splicing of the same gene. For example, the glucocorticoid receptor α (GRα) is more widely expressed in tissues and functions as the “classic” steroid hormone receptor in which binding of the ligand induces expression of glucocorticoid-responsive genes.2 Meanwhile, GRβ does not bind glucocorticoids but represses the GRα receptor by specifically inhibiting its activation of gene transcription through glucocorticoid response elements.3 It is suspected that increased expression of the GRβ may contribute to steroid-resistant asthma in this manner.4
Similar to GRs, estrogen receptors (ERs) have 2 isoforms. Each subtype is differentially expressed, but in tissues coexpressing both receptors, ERβ can inhibit ERα5 while regulating different genes.6 In contrast, progesterone receptor B is the more active isoform but can be repressed by progesterone receptor A.7,8 Vitamin D (1,25-dihydroxycholecalciferol) receptors (VDRs) also have 2 isoforms,9 but their respective functions are not clear.
Regardless of isoform, all steroid hormone receptors consist of an N-terminal domain, a DNA-binding domain, and a variable-length hinge region connected to the C-terminal, ligand-binding domain. The GR, ER, progesterone receptor, and VDR all maintain this basic structure.2,10–12 The N-terminal domain has a variable length and homology, even among the same receptor types.12 It contains the ligand-independent transcriptional activation function 1, which interacts with molecules important for transcription. Glucocorticoid, ER, progesterone receptor, and VDR all have activation function 1 in the N-terminal domain.12–14
The DNA-binding domain is the site of genomic interaction and signaling, but it also contains components important for receptor homodimerization and nuclear translocation.14,15 Although ER isoforms have significant structural divergence in the N-terminal domain, they conserve homology between themselves and other steroid hormone receptors in the DNA-binding domain.12,14 The C-terminal end contains the ligand-binding domain, the site for steroid hormone binding. It also contains another transactivation domain, activation function 2, which is ligand dependent. The ligand-binding domain is important for interaction with transcription factors, but it also maintains a role in nuclear localization and dimerization.15–21 Overall, the conserved hormone receptor homology lays the framework of conserved steroid hormone function, including that of downstream signaling.
CLASSIC GENOMIC SIGNALING OF STEROID HORMONE RECEPTORS
All steroid hormone receptors conserve basic functions: binding of steroid hormones to receptors causes hormone receptor complexes to translocate to the nucleus, attach to DNA at hormone-specific response elements, and alter gene transcription. This process is often referred to as “classic” steroid hormone genomic signaling (Fig. 1). For example, the GRα exists in the cytosol bound to several associated proteins, including heat shock proteins and kinases of the mitogen-activated protein kinase (MAPK) signaling cascade.22 When glucocorticoids diffuse into the cell, they bind with high affinity to this receptor, leading to conformational changes and dissociation of the associated proteins. The glucocorticoid-GR complex then translocates to the nucleus and binds as a homodimer to glucocorticoid response elements inducing changes in transcription of the corresponding genes. The process is similar for estrogen and progesterone, with estrogen- or progesterone-bound receptor homodimers binding to their respective gene response elements. Similarly, 1,25-dihydroxycholecalciferol binds to the VDR to alter the transcription of genes with vitamin D response elements; however, it does so not as a homodimer but as a heterodimer with the retinoid X receptor.
Binding of steroid-receptor complexes to steroid-specific response elements may induce gene transcription or gene repression altering the manifestations and pathogenesis of asthma. For example, glucocorticoid-GR complexes upregulate the transcription of anti-inflammatory genes such IκB, which inhibits NF-κB.23 Similarly, 1,25-dihydroxycholecalciferol also upregulates IκB in airway epithelial cells.24 The glucocorticoid-GR complexes also suppress transcription of key inflammatory genes, such as interleukin 1β (IL-1β).25 Through these anti-inflammatory mechanisms, glucocorticoids decrease proinflammatory cytokine release and airway recruitment of inflammatory cells (eosinophils, T lymphocytes, and mast cells).26,27
NONCLASSIC GENOMIC SIGNALING OF STEROID HORMONE RECEPTORS
In nonclassic genomic signaling, steroid hormone receptor complexes influence transcription without direct binding to their steroid-specific DNA response element. Instead, steroid hormone receptor complexes interact with transcription factors such as NF-κB or activator protein 1 (AP-1) to effect transcription. The glucocorticoid-GR complex can physically bind to AP-1, a proinflammatory transcription factor important in asthma,28 leading to transrepression of AP-1–induced genes.29 Similarly, transrepression occurs when the glucocorticoid-GR complex interacts with the p65 subunit of NF-κB.30 Further, estrogen and progesterone-receptor complexes transrepress through AP-1 and NF-κB binding.5,31–35 Vitamin D receptors also inhibit NF-κB through direct interaction, preventing NF-κB translocation to the nucleus.36 Additional evidence exists for the transrepression of AP-1by the VDR.37 Cumulatively, glucocorticoids, estrogen, progesterone, and 1,25-dihydroxyvitamin D all mediate transrepression of inflammation via NF-κB and AP-1.
RAPID NONGENOMIC EFFECTS OF STEROID HORMONES
Current literature indicates that steroid hormones also have nongenomic effects. Glucocorticoids, sex steroids, and secosteroids all produce cellular changes too rapidly to be explained by de novo transcription and translation. Within minutes, glucocorticoids inhibit Mek1 and Erk1, members of MAPK pathways.38 Similarly, the rapid effects of estrogen include activation of signaling cascades leading to ion fluxes and activation of kinases and phosphatases or other second messengers via G protein–coupled receptors.39,40 The rapid effects seen with progesterone signaling also occur with activation of the MAPK or other kinase families.41 1,25-Dihydroxyvitamin D also induces rapid nongenomic responses similar to that of glucocorticoids42 by mobilizing intracellular calcium43 and increasing cyclic GMP.44 Several mechanisms have been proposed to explain the nongenomic actions of all the steroid hormones: (1) physiochemical interactions of hormones with cellular membranes leading to altered changes in ion fluxes across the membrane, (2) binding to a membrane-located steroid hormone or G protein–coupled receptor leading to rapid effects via second messenger systems (Ca, IP3, cyclic AMP, and protein kinase C), and (3) via proteins dissociated after steroid-receptor complex formation (heat shock proteins and proteins of the MAPK signaling system).22,45,46
Although the exact mechanism requires further study, nongenomic signaling may explain the conflicting evidence for estrogen and progesterone effects on asthmatic inflammation. Early menarche is a risk factor for developing asthma.47 In addition, increased female airway hyperresponsiveness to an inhaled methacholine challenge emerges coincident with secondary sex characteristics,48 suggesting that asthmatic inflammation may be propagated by estrogen. This proinflammatory concept is reinforced because adult asthmatic women show more airway hyperresponsiveness to allergic stimuli,49,50 are more likely to be categorized with severe asthma,51 and have more hospitalizations for asthma than men.52 However, some studies suggest an anti-inflammatory role of estrogen because exogenous administration of estradiol is associated with improvement in asthma symptoms,53,54 especially in women with severe premenstrual asthma.53,55 With respect to progesterone, administration of exogenous progesterone lessened the premenstrual dips in peak flow of asthmatic women56 suggesting an anti-inflammatory role. However, progesterone seems to be proinflammatory in some mouse models by increasing total immunoglobulin E,57 airway eosinophilia, IL-4, and interferon α.58 The difference in the proinflammatory and anti-inflammatory effects of estrogen and progesterone may be explained by the combined effect of rapid nongenomic signaling and nonclassic signaling on NF-κB. Estrogen and progesterone fluctuate frequently during the normal menstrual cycle. Rapid changes in sex hormone concentrations and/or ratios may activate or deactivate kinases leading to altered phosphorylation of NF-κB or its inhibitory cofactor, IκB. This may tip the balance of downstream signaling to either a proinflammatory or an anti-inflammatory state.
OVERLAP AND INTERPLAY OF SIGNALING MECHANISMS IN ASTHMA
Either via classic, nonclassic signaling, or rapid genomic affects, current data examining glucocorticoids, sex steroid hormones, and 1,25-dihydroxyvitamin D show commonality of downstream signaling pathways, namely through NF-κB. We propose that through NF-κB, glucocorticoids, estrogen, progesterone, and 1,25-dihydroxyvitamin D work to modulate inflammation (Fig. 1). In this model, NF-κB is a common target for inhibition by glucocorticoids, sex steroid, and secosteroids because studies show that NF-κB directly interacts with each of the steroid hormone receptors.59 Whereas this model highlights the known effects of glucocorticoids on asthma, it also integrates the anti-inflammatory evidence of estrogen and 1,25-dihydroxyvitamin D on asthma. This is a novel model because it encompasses a systems biology approach, integrating the simultaneous effects of steroid hormones as would be seen in natural biological systems.
In addition, our model incorporates the variable effects of sex steroid hormones. We propose that estrogen may alter the inflammatory “balance” such that glucocorticoids and estrogen work antagonistically in the asthmatic lung with estrogen increasing inflammation, but its effects are modulated by glucocorticoids, progesterone, and 1,25-dihydroxyvitamin D. The inflammatory balance may be mediated through rapid nongenomic actions of estrogen. Further, it is the steroid hormone balance between the “proinflammatory” estrogen and the “anti-inflammatory” glucocorticoids and progesterone that contributes to the inflammatory process in asthma. Further, we believe that 1,25-dihydroxyvitamin D serves a supporting but critical role by augmenting the anti-inflammatory actions of glucocorticoids. From a clinical perspective, this refined model incorporating sex steroid hormones and 1,25-dihydroxyvitamin D accurately represents the literature. As mentioned previously, sex differences in airway hyperresponsiveness to allergic stimuli occur when secondary sex characteristics are becoming apparent.48 It is possible that the introduction of sex hormones serves as a potent inflammatory signal, releasing NF-κB from IκB, allowing for the transcription and release of inflammatory signals. Finally, low 1,25-dihydroxyvitamin D levels are associated with increased airway hyperresponsiveness and reduced glucocorticoid response.60 Increased levels of 1,25-dihydroxyvitamin D, resulting in inhibition of VDR on NF-κB, may improve asthma symptoms and sensitivity to glucocorticoids.
Asthma is a complex condition whose phenotype is modulated by steroid hormones. Despite our understanding of the impact of individual steroids (eg, glucocorticoids, sex steroids, and secosteroids) on asthma, research has yet to explain the interplay of the dynamic system in which these hormones function. To do so, there needs to be a better understanding of the interplay of classic, nonclassic, and nongenomic steroid hormone functions. However, clues from the conserved homology steroid hormone structure and function and signaling pathways offer insight into a possible model of steroid hormone regulation of inflammation in asthma through common NF-κB–mediated downstream events.
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asthma; hormones; inflammation; nuclear factor κB
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