ReviewTranscriptional control of the glucocorticoid receptor: CpG islands, epigenetics and more
Graphical abstract
Glucocorticoid receptor levels are transcriptionally controlled. The variable 5′ region produces multiple splice variants coding a single protein, and is regulated by epigenetic methylation and tissue-specific transcription factor usage.
Introduction
Stress causes an increasing disease burden at least in Western societies. Economic losses due to stress may be as high as 3–4% of the European gross national product. In addition, stress has detrimental effects on behaviour, mood, learning and memory that are difficult to quantify. Glucocorticoids (GCs), the key stress mediators, exert profound effects on a wide range of physiological and developmental processes that are crucial for the adaptation to stress. Psychosocial stress has also been implicated in the development of mental disorders, including schizophrenia, anxiety disorders, and depression.
GCs act via binding to two types of intracellular nuclear receptors, i.e. the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). Upon binding of GCs, these receptors translocate to the nucleus where they regulate the activity of specific target genes, in a cell-type specific manner, as transcription factors. This review focuses on the classical GR, NR3C1. Numerous factors have been demonstrated to affect the responsiveness to GC by regulating GR activity, such as GR co-activators and co-repressors [1], GR splice variants [2], [3], [4], and GR isoforms [5], [6], [7]. In addition, and perhaps most important for GC responsiveness is the expression level of GR protein [8], [9], [10], [11], determined by the mRNA level.
In eukaryotic cells, gene expression is controlled by a variety of mechanisms, both at transcriptional and translational levels, including chromatin condensation, transcription initiation, DNA methylation, alternative RNA splicing, mRNA stability and others. The GR is an ubiquitously expressed nuclear hormone receptor, however, levels of both mRNA and protein vary widely between cell and tissue types. Over the last few years we and others have contributed to the significant progress that has been made to unravel the transcriptional mechanisms determining the tissue specific control of GR levels, that will be reviewed here.
Section snippets
Structure of the NR3C1 gene
The human GR gene (OMIM + 138040; NR3C1) is located on chromosome 5q31–q32 [12] and contains 8 translated exons (2–9) and 9 untranslated alternative first exons. We and others have shown that GR levels are under the transcriptional control of a complex 5′ structure of the gene, containing the untranslated first exons important for differential expression of the GR. All of the alternative first exons identified are located in one of the two promoter regions: the proximal or the distal promoter
Alternative first exon usage and 3′ splice variants
The recent observation that transcription factors binding to pol II transcribed promoters modulate alternative splicing, supports a physical and functional link between transcription and splicing [22]. Several factors were identified that were critical for the recruitment of a specific set of co-regulators to pol II transcribed gene promoters and the production of a specific splice variants. The splice variant produced depends on the structural organisation of the gene and the nature of the
Transcription factors and transcriptional control within the CpG island
The hGR was initially described as a housekeeping or constitutively expressed gene with promoters that contain multiple GC boxes and no TATA or TATA-like box [27]. A wide variety of transcription factors have been identified that bind in the CpG island upstream of the gene. The description of the transcription factors active within this region is complicated by their tissue-specific usage. These transcription factors were not assigned to the different exon 1 promoters since most of this work
Transcriptional control upstream of the CpG island: exons 1A and 1I
Whilst the majority of the GR first exons and their promoters are located within the CpG island, exons 1A and 1I map 25 kb upstream of the CpG island and 32 kb upstream of the main GR ORF in the distal promoter region [16], [20]. Exon 1A has also been identified in the mouse, and three possible homologues 11, 12, and 13 have been found in the rat [39], [40]. The human promoter 1A generates 3 alternatively spliced transcripts, 1A1, 1A2 and 1A3 [16]. Expression of the 1A transcripts appears to be
Epigenetic programming of GR promoters
Epigenetic methylation of the 5′-cytosine of a CpG dinucleotide is associated with gene silencing either by inhibition of transcription factor binding (Fig. 2) or by chromatin inactivation [51], [52], [53]. For instance, prenatal epigenetic methylation governs genomic imprinting and inactivation of one X-chromosome [54]. The epigenetic chromatin status is sensitive to the host environment. Thus, epigenetic methylation represents a link between the environment and gene activity. In particular,
The future—GR post-transcriptional regulation by miRNAs?
MicroRNAs (miRNAs) were discovered in 2001 [64], [65]. In mammalian, cells miRNAs were predicted to regulate up to 30% of all genes [66]. So far they have been shown to be involved in almost every cellular process investigated [67], [68]. It is now recognised that miRNAs account for about 1% of the human genome and that they play a key role in many regulatory pathways such as development timing, cell differentiation and apoptosis [69]. MicroRNAs are single-stranded RNA molecules of about 21
Summary
The studies presented in this review demonstrate the multiple layers of complexity involved in the maintenance of the homeostasis of the ubiquitously expressed GR. Data from our laboratory and others have shown that the multiple first exons represent a first layer of complexity playing a particular role in tissue-specific transcriptional regulation. The abundance of the alternative exons is modulated by epigenetic methylation of their promoters. Finally, we suggest that the mature mRNA will be
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