The human NR3C1 gene spans a length of 157,582 bases. The NR3C1 structural gene is composed of nine exons and is located in chromosome 5 (5q31.3)(Hollenberg et al., 1985)(Figure 2A).

The NR3C1 gene expresses mainly two mRNAs through alternative use of exons 9alpha and 9beta, producing two highly homologous receptor isoforms, termed alpha and beta(N. Z. Lu and Cidlowski, 2005). They are identical through amino acid 727, with hGRalpha having an additional 50 amino acids and hGRbeta having an extra no homologous 15 amino acids (Fig. 2A). Their molecular weights of hGR9alpha and hGR9beta are 97 and 94 kDa, respectively. Except these products, the NR3C1 gene expresses GR? (gamma), which has one amino acid insertion due to splicing variation at exon 3-4 boundary,(Meijsing et al., 2009) and GR-P isoform, which has only 676 amino acids and is encoded by an mRNA expressed from exons 1-7, but lacking exons 8 and 9 and unknown biologic significance(Hagendorf et al., 2005). The human GR gene has eleven different promoters with their alternative first exons (1A1, 1A2, 1A3, 1B, 1C, 1D, 1E, 1F, 1H, 1I and 1J) (Figure 1). Therefore, the human GR gene can produce eleven different transcripts alternating the different promoters that encode the same GR proteins having a common exon 2, which contains the translating ATG codon. 1A1, 1A2, 1A3 and 1I are located in the distal promoter region spanning ~32,000-36,000 bps upstream of the translation initiation site, while 1B, 1C, 1D, 1E, 1F, 1H and 1J position in the proximal promoter region located upstream up to ~5,000 bps. Alternate use of these promoters, differentiate the levels of GR protein isoforms in various tissues(Presul, Schmidt, Kofler, and Helmberg, 2007). Different splicing and translational GR isoforms originating from alternate promoters constitute up to 256 different combinations of homo- and hetero-dimers with different expression levels and transcriptional activities. This marked diversity in the transcription/translation of the GR gene allows cells/tissues to accommodate appropriately to the circulating concentrations of glucocorticoids depending on their needs(Chrousos and Kino, 2005a) and is responsible for the highly stochastic nature of the glucocorticoid-signaling pathway (Chrousos and Kino, 2005b).

The NR3C1 gene has a pseudogene (NC3C1P1) in chromosome 16q (provided by RefSep 2012)

The human NR3C1 protein sequence (NP_000167.1)consists of 777 amino acids and has a MW of 85659 Da. Without the presence of the ligand, hGRalpha takes part in a heteromultimeric cytoplasmic complex with chaperone Heat Shock Proteins (HSP)90,70,50 immunophilins and other proteins. As soon as it binds to the ligand, dissociates from the previous complex and FK506-binding immunophilin heat shock protein 56 takes its place, thereby connecting to dynein and mediating the transportation to the nucleus, where it dissociates(Czar, Lyons, Welsh, Renoir, and Pratt, 1995). When it reaches the nucleus, the receptor binds as a homodimer to various Glucocorticoid Response Elements(GREs) sequences in the promoter region of many genes, and as a heterodimer with NR3C2 or the retinoid X receptor, thus regulating their expression either positively or negatively depending on GRE sequence and promoter context. The ligand-activated hGRalpha modulates gene expression without binding to GREs, by interracting also with other transcription factors, such as activator protein-1(AP-1), nuclear factor-?B(NF-?B), p53 and signal transducers and activators of transcription(STATs). For the transcription when hGRalpha uses its transcriptional activation domains, AF-1 and AF-2, as surfaces to interact with specific nuclear receptor coactivators and chromatin-remodeling complexes, then these coactivators form a bridge between DNA-bound hGRalpha and the transcription initiation complex, conveying the transmission of the glucocorticoid signal to the RNA polymerase II and its ancillary components leading to initiation and promotion of the transcription. As a member of the nuclear receptor superfamily, hGR has 3 major domains: the N-terminal domain (NTD), middle DNA-binding domain (DBD) and the C-terminal ligand-binding domain (LBD) as illustrated in(Fig. 2B). Using the typical nomenclature for NR subdomains, hGR consists of the amino-terminal A/B region (corresponding to NTD), C (DBD), D (hinge region) and E (LBD) regions, without having the F region. The N-terminal domain (NTD) consist of transactivation domain, termed activation function (AF)-1, located between amino acids 77 and 262, activated when is ligand-independent. It has an important role in the interaction of the receptor with molecules necessary for stimulation of transcription, such as coactivators, chromatin modulators and basal transcription factors. The DNA-binding domain (DBD) consists of amino acids 420-480, contains two zinc finger motifs through which binds to specific DNA sequences, such as the glucocorticoid response elements (GREs) in the promoter region(s) of specific genes. The DBD also contains sequences important for receptor dimerization and nuclear translocation. The hinge region gives flexibility connecting DBD with LBD by conferring structural flexibility in the receptor dimers, allowing single receptor dimmer to interact with multiple GREs and different sequences. The ligand-binding domain (LBD) consist of amino acids 481-777, binds ligand glucocorticoid with its ligand-binding pocket and contains a second transactivation domain, the ligand-depended AF-2 which plays an important role in the glucocorticoid-induced stimulation of hGR transcriptional activity, by interacting with coactivators containing LxxLL motifs. LBD takes part in the complex formation with heat shock proteins, in the process of nuclear translocation and receptor dimerization(Nicolaides, et al., 2010).

Ubiquitous. Almost all human tissues and cells are expressing the hGRalpha. The GR expression has been reported in the Bone Marrow, Monocytes, Dendritic Cells, NK Cells, T Cells (CD4+), T Cells (CD8+), B Lymphoblasts, B Cells, Lymph Node, Spleen, Thymus, Retina, Heart, Cardiac Myocytes, Atrioventricular Node, Smooth Muscle, Skeletal Muscle, Appendix, Pancreatic Islet, Small Intestine, Colon, Adipocyte, Kidney, Liver, Lung, Trachea, Bronchial Epithelium, Tongue, Thyroid, Salivary Gland, Adrenal Gland, Breast, Skin, Ovary, Uterus Corpus, Uterin Cervix, Placenta, Fetal Brain, Liver, Lung and thyroid, Tonsil, Prefrontal Cortex, Cingulate Cortex, Parietal Lobe, Temporal Lobe, Occipital Lobe, Ciliary Ganglion, Globus Pallidus, Olfactory Bulb, Thalamus, Hypothalamus, Subthalamic Nucleus, Caudate Nucleus, Amygdala, Pons, Medulla Oblongata, SupCervical Ganglion, Dorsal Root Ganglion, Trigeminal Ganglion, Spinal Cord, Pineal (Day)and Pineal (Night), Pituitary, Prostate, Testis Germ, Testis Intersitial, Testis Leydig cells.

In the absence of ligand, hGRalpha resides mostly in the cell cytoplasm, but upon ligand-induced activation, the receptor dissociates from the multiprotein complex and translocate into the nucleus. After binding to specific DNA responsive elements remains within the nucleus for a considerable length of time and is then exported to the cytoplasm. It is also present in the Mitochondrion and Plasma membrane. The isoform Beta is mainly expressed in the nucleus.

hGR has a dual mode of transcriptional activity: acting either as a glucocorticoid-dependent transcription factor, binding mainly to glucocorticoid response elements (GRE) or functioning as a modulator of other transcription factors through protein-protein interaction. Post-translational modifications (PTMs), such as phosphorylation, acetylation, ubiqutilation and nitrosylation, play also an important role in the regulation of GR activity, affecting the receptor stability, subcellular localization, and also interaction between GR and other proteins, influencing finally the transcriptional activity. Recent studies have demonstrated that the circadian rhythm transcription factors CLOCK and BMAL1 repress GR-induced transcriptional activity by acetylating several lysine residues(Kino and Chrousos, 2011).

Is a member of the nuclear hormone receptor family, NR3 subfamily.

Mutations or polymorphisms in the hGR gene impair one or more of the molecular mechanisms of hGRalpha action and as a final consequence alter the tissue sensitivity to glucocorticoids. Reduced affinity of the mutant receptors for the ligand, altered subcellular localization, delayed nuclear translocation after binding to the ligand, reduced ability to bind with GREs and decreased transcriptional activity, altering the exertion of a dominant negative effect upon the wild-type receptor, reduced interaction with other coactivators and reduced motility within the nucleus are some of the mechanisms of mutant hGR(Charmandari, Chrousos, and Kino, 2009). Although, most mutations of the NR3C1 gene exert generalized glucocorticoid resistance there is one mutation reported to date in the hGRalpha NTD region that replaces aspartic acid at amino acid 401 by histidine (D401H) facilitating the mediated gene expression(Charmandari et al., 2008). Interindividual variations in tissue sensitivity to glucocorticoids have been described within the normal population and are mainly attributed to polymorphisms in the hGR gene. A heterozygous polymorphism replacing aspartic acid to serine at amino acid 363 mildly increases transcriptional activity, and arginine to lysine replacement at amino acid 23 is associated with relative glucocorticoid resistance. A single nucleotide polymorphism that replaces A with G at the nucleotide 3669(A3669G) located in the 3 end of exon 9beta increases the stability of GRbeta mRNA, leading to greater inhibition of GRalpha-induced transcriptional activity and thus glucocorticoid resistance(Syed et al., 2006).
It is worth mentioning also the fact that there is a plethora of laboratory generated mutated GR proteins, which provides an interesting tool for exploring hGR structure-function relationships. (Beck, De Bosscher, and Haegeman, 2011)