YEAR IN REVIEW STEROID HORMONES IN 2013
Glucocorticoids—timing, binding and environment Stafford L. Lightman and Charlotte L. George
2013 has revealed interesting mechanisms that explain how glucocorticoid signalling responses can be influenced by childhood trauma, activity of other signalling molecules, glucocorticoid circadian rhythms and the sequence of DNA regulatory regions. In particular, studies this year highlight how different signalling environments can determine the molecular and physiological responses of glucocorticoids themselves, and how glucocorticoids can affect other signalling systems. Lightman, S. L. & George, C. L. Nat. Rev. Endocrinol. 10, 71–72 (2014); published online 24 December 2013; doi:10.1038/nrendo.2013.257
Glucocorticoids bind to the mineralocor ticoid receptor (MR) and glucocorticoid receptor (GR) to either induce rapid ‘non genomic’ effects that affect the interactions of existing proteins or molecules in signal ling cascades, or regulate the expression of target genes to alter protein expression. The majority of research into glucocorticoid actions has focused on the regulation of gene induction, whereas relatively little is known about how GR binds directly to DNA to repress transcription. Research has predomi nantly focused on how GR inhibits tran scription by ‘tethering’ to other DNA-bound proteins, or how two GR molecules interact ing as a dimer bind directly to specific DNA sequences (called glucocorticoid response elements [GREs]) to induce transcription. In 2013, Hudson et al.1 investigated how GR interacts with DNA at genes whose expression is inhibited by glucocorticoids. The researchers assessed the crystal struc ture of GR bound to a DNA sequence termed a negative GRE (nGRE), which has been found near many genes.2 The particu lar nGRE they studied inhibits transcrip tion of the thymic stromal lymphopoietin gene. Hudson et al. show that the DNA sequence of this nGRE forces GR to bind to this sequence as two separate molecules, each interacting with opposite sides of the DNA helix, thereby preventing GR dimeri zation interactions.1 Indeed, the nGRE DNA structure promotes GR to act as a monomer, which inhibits transcription. These results highlight a new model of GR‑mediated gene repression that might be relevant for other nGRE sequences.2 Miranda et al.3 have also emphasized the importance of GR and DNA binding inter actions (Figure 1). Chromatin describes DNA that is wrapped around histones,
creating a structure that when open or ‘acces sible’ is associated with gene transcription, but when compacted or ‘closed’ is associ ated with gene repression. Overwhelmingly, transcription factor binding occurs in acces sible chromatin, but GR and related recep tors, such as the estrogen receptor (ER) can ‘open’ closed regions to enable binding of tr anscriptional machinery to induce gene expression. GR and ER are known to modulate each other’s transcriptional responses. To inves tigate this crosstalk, Miranda et al. assessed the in vitro genome-wide DNA binding pat terns of GR and ER in mouse mammary cells.3 Co-treatment of the cells with estrogen and glucocorticoids, causing co-activation of Already accessible GR DNA binding site
both ER and GR, led to the rearrangement of numerous DNA binding sites of both receptors, compared to when they are acti vated individually. Importantly, upon induc tion, GR actively increased the chromatin accessibility of a number of regions, which then enabled the binding of ER to sites that were prev iously inaccessible (Figure 1). Similarly, ER could aid the binding of GR to new regions.3 Co-activation of glucocor ticoids with other signalling systems can, thus, reprogram the genomic response of both systems, which suggests that perturba tion to either system can affect the other sys tem’s function. As glucocorticoid signalling varies over a daily (circadian) and shorter ultradian timeframes,4 it will be interesting to see if these fluctuations in GR activity induce oscillations in the binding profile of ER and other DNA-interacting molecules. If so, these interacting signalling systems may be important in understanding pathological processes related to the circadian or stressrelated changes in glucocorticoid exposure that have been implicated in disease.4 The importance of circadian fluctua tions in glucocorticoid levels have also been highlighted in 2013.5 Liston et al. studied transgenic mice with fluorescent neurons in the motor cortex of the brain to enable the effects of circadian glucocorticoid exposure on the structure of the neurons to be moni tored in vivo. The particular phase of the circadian rhythm in glucocorticoid secre tion was found to be important in memory
Closed GR DNA binding site
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GR increases accessibility aiding ER binding
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GR GR
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Figure 1 | ER and GR bind to specific DNA binding sites to regulate the expression of their target genes (arrows indicate transcription). a | ER binding sites (blue DNA regions) when estrogen is present and GR is inactive. b | Most potential GR binding sites are in accessible regions of chromatin (green regions), but when activated by glucocorticoids, GR can also remodel tightly compacted GR binding sites (red regions) into accessible chromatin that other transcription factors can then bind. Simultaneous activation of ER and GR enables GR to bind DNA and regulate glucocorticoid-responsive genes and also allows ER to access newly ‘opened’ regions. Abbreviations: ER, estrogen receptor; GR, glucocorticoid receptor.
NATURE REVIEWS | ENDOCRINOLOGY
VOLUME 10 | FEBRUARY 2014 | 71 © 2014 Macmillan Publishers Limited. All rights reserved
YEAR IN REVIEW Key advances ■■ The DNA structure of negative glucocorticoid response elements can actively prevent glucocorticoid receptor (GR) dimerization and, thus, repress transcription 1 ■■ Activated GR is capable of making the chromatin structure of certain binding regions more accessible, which enables these regions to be bound by other transcription factors and, thereby, alter the transcriptional response of these other proteins3 ■■ In mice, daily circadian fluctuations of glucocorticoids are vital for the morphological neuronal changes associated with memory stabilization and learning a motor skill task 5 ■■ In human cells, glucocorticoids impair hippocampal neurogenesis by mechanisms mediated by SGK1, suggesting that targeting this pathway could be exploited for therapeutic benefit 7 ■■ Molecular mechanisms have been identified that explain how environmental factors, such as early-life experiences, can interact with a specific genetic polymorphism to dysregulate gene expression and increase an individual’s risk of developing post-traumatic stress disorder9
formation and retention. Temporarily high levels of glucocorticoids at the time of learning (for example, during the evening circadian peak) were required to increase memory of the motor skills required by the mice. These high levels of glucocorticoids also increased the number of new spines formed by neurons, via a rapid GR‑mediated nongenomic mechanism involving LIM kinase signalling. Low levels of glucocorticoids during cir cadian troughs were required in the days after learning, however, to retain and stabi lize the memory.5 This stabilization process depended on the ‘pruning’ of older spines by MR‑mediated changes in gene expres sion. The daily glucocorticoid oscillations and fluctuations, thus, appear to improve memory function. This study suggests that fluctuations in GR and MR signalling are vital for plasticity in memory-encoding brain regions, which raises important thera peutic questions about the optimal ligands and timing for glucocorticoid therapy. These findings also provide insight into the adverse cognitive effects observed in Cushing disease and in individuals treated with glucoc orticoids who lose normal glucocorticoid oscillations. Chronically elevated glucocorticoid levels are associated with memory dysfunction and impaired hippocampal neurogenesis, both of which have been implicated in the pathology of depression.6 In 2013, Anacker and colleagues7 revealed a novel mecha nism whereby glucocorticoids decrease hippocampal neurogenesis. The research ers assessed the proliferation and differen tiation of a human hippocampal progenitor cell line into neurons during glucocorticoid exposure. Anacker et al. found that elevated glucoc orticoids decreased hippocampal neurogenesis by inducing the activity of SGK1, a protein previously implicated in regulating neuronal excitability. This 72 | FEBRUARY 2014 | VOLUME 10
mechanism is notably impaired by a small molecule inhibitor of SGK1. Glucocorticoids both increased the expression of SGK1 and upregulated SGK1-enhanced GR activa tion for several hours after glucocorticoid removal. Increased SGK1 expression was also observed in rodent models of chronic stress and peripheral blood cells of drug-free patients with depression. These findings imply that chronic gluco corticoid exposure could result in over expression of SGK1, which in turn could further exacerbate glucocorticoid signalling dysfunction. From a therapeutic perspec tive, SGK1 inhibition may reduce unwanted adverse effects resulting from increased GR signalling. Further investigation of the behavioural response to glucocorticoidmediated dysr egulation of SGK1 and decreased neurogenesis in rodents and its potential translation to human depression will be exciting. Given the recently implied role of an increased salt concentration in induci ng pathological SGK1 signalling in mice,8 studies investigating whether high levels of dietary salt also dysregulate GR sig nalling via alterations in SGK1 activity are also eagerly awaited. Finally, in 2013, Klengel et al.9 revealed the molecular mechanisms that explain how environmental exposure to childhood trauma increases the risk of stress-related psychological disorders, including posttraumatic stress disorder (PTSD). These researchers investigated a polymorphism (rs1360780) that is associated with an increased risk of PTSD in individuals who have experienced childhood abuse. The rs1360780 polymorphism is close to a GRE binding site in the FKBP5 gene. This ‘risk’ genotype showed enhanced GR‑mediated FKBP5 transcription by increasing the contact of the GRE with the transcriptional start site. DNA from peripheral blood cells of participants who experienced childhood
abuse had decreased DNA methylation at another FKBP5 GRE. Nota bly, this demethylation was replicated in vitro by glucocorticoid exposure and also resulted in amplified GR‑mediated FKBP5 transcrip tion. FKBP5 can also negatively regulate the activity of GR to reduce its sensitivity to glucocorticoids. Indeed, individuals with a risk genotype and childhood trauma had altered glucocorticoid-regulated transcrip tion in immune cells and altered hippo campal mo rphology, which could alter GR signalling.9 2013 has highlighted molecular mecha nisms by which factors ranging from the composition of DNA sequences to life experiences can alter glucocorticoid sig nalling. These findings highlight signalling pathways that may be disrupted in disease and point to novel therapeutic strategies to improve glucocorticoid therapies. This new knowledge has implications for a wide spectrum of diseases. Henry Wellcome Laboratories for Integrative Neuroscience and Endocrinology, Dorothy Hodgkin Building, University of Bristol, Whitson Street, Bristol BS1 3NY, UK (S. L. Lightman, C. L. George). Correspondence to: S. L. Lightman
[email protected] Competing interests The authors declare no competing interests. 1.
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Hudson, W. H., Youn, C. & Ortlund, E. A. The structural basis of direct glucocorticoidmediated transrepression. Nat. Struct. Mol. Biol. 20, 53–58 (2013). Surjit, M. et al. Widespread negative response elements mediate direct repression by agonistliganded glucocorticoid receptor. Cell 145, 224–241 (2011). Miranda, T. B. et al. Reprogramming the chromatin landscape: interplay of the estrogen and glucocorticoid receptors at the genomic level. Cancer Res. 73, 5130–5139 (2013). Lightman, S. L. & Conway-Campbell, B. L. The crucial role of pulsatile activity of the HPA axis for continuous dynamic equilibration. Nat. Rev. Neurosci. 11, 710–718 (2010). Liston, C. et al. Circadian glucocorticoid oscillations promote learning-dependent synapse formation and maintenance. Nat. Neurosci. 16, 698–705 (2013). Sahay, A. & Hen, R. Adult hippocampal neurogenesis in depression. Nat. Neurosci. 10, 1110–1115 (2007). Anacker, C. et al. Role for the kinase SGK1 in stress, depression, and glucocorticoid effects on hippocampal neurogenesis. Proc. Natl Acad. Sci. USA 110, 8708–8713 (2013). Wu, C. et al. Induction of pathogenic TH17 cells by inducible salt-sensing kinase SGK1. Nature 496, 513–517 (2013). Klengel, T. et al. Allele-specific FKBP5 DNA demethylation mediates gene–childhood trauma interactions. Nat. Neurosci. 16, 33–41 (2013).
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