The nuclear receptors for the steroid hormones, including 1,25(OH)2D3, and for thyroid hormone are highly regulated gene transcription factors. As with membrane hormone receptors, the regulation of the activity of these proteins occurs through the binding of ligand and a consequent conformational change that alters the activity of the receptor. Stimulation (activation) or repression of the transcription of specific genes ensues, altering the biological activities of the cell.
One way in which members of the group of hormone-regulated nuclear receptors differ is their location in the cell in the absence of ligand. Originally it appeared that the receptors for the cholesterol-derived classic steroid hormones were localized in the cytoplasm of the cell in the absence of ligand where they are bound to molecular chaperone proteins, HSP90 (heat shock protein 90) and in some cases HSP70. Figure 1 illustrates this difference between intracellular localization of the two types of receptors. It is now understood that this is not a rigid classification scheme; both forms of the estrogen receptor (ERα and ERβ), the androgen receptor, and one form of the progesterone receptor are found in the nucleus and/or travel freely between cytoplasm and nucleus in the unliganded state. Receptors for thy roid hormone and 1,25(OH)2D3 localize in the nucleus upon completion of synthesis where they bind to other proteins and to DNA. What all these receptors have in common is that, regardless of intracellular localization, in the unoccupied state they are maintained in an inactive state and binding of the ligand activates them and, if necessary, causes or permits their translocation to the nucleus.
Fig1. Transcriptional activation by nuclear receptors. Nuclear receptors for some of the classical steroid hormones are typified in this figure by the glucocorticoid receptor, GR, and its interaction with cortisol (panel A). In the absence of ligand these receptors are in the cytosol complexed with heat shock proteins (green; HSP) that maintain them in an inactive state. The conformational change brought about by ligand binding causes the HSP to dissociate and the receptor translocates to the nucleus. GR, MR, ER, AR, and PR all form heterodimers prior to DNA binding. Nuclear receptors for 1,25(OH)2D3 (VDR) and thyroid hormone (TR) are in the nucleus (panel B) prior to ligand binding and form heterodimers with the retinoic X receptor (RXR). These are usually maintained in an inactive state by forming a complex with a corepressor (purple; CoR), which can bind to DNA and repress its transcription. In this case, ligand binding results in a conformational change that causes the corepressor to dissociate, activating the VDR/RXR heterodimer. The ligand-activated dimer of either type of nuclear receptor (hetero- or homodimer) binds to a specific sequence of DNA, a hormone response element (HRE). A variety of proteins, termed coactivators, are recruited to the complex to modify chromatin structure and recruit and stabilize the basal transcriptional machinery. This includes the general transcriptional factors (GTF) and DNA dependent-RNA polymerase.
The other feature that distinguishes receptors for thyroid hormone and 1,25(OH)2D3 from the other receptors is that these two form heterodimers with RXR, a general nuclear receptor heterodimerization partner, while the others form homodimers. In either case, dimerization is a necessary step of the activation process. This is followed by binding to a specific DNA sequence, often but not always, in the 5′ regulatory (promoter) region of the gene whose transcription is being regulated.
The activated DNA-bound receptor dimer recruits other proteins to the region of the transcriptional start site. These proteins are termed coactivators. Among the first of these to be recognized were the steroid receptor coactivators, SRC-1, SRC-2, and SRC-3. These proteins bind to the receptor dimer and recruit other coactivator proteins, some of which have the enzymatic activity to modify the structure of chromatin. For example, histone acetylases (HAC; p300/CBP) neutralize the positively charged histones forming the nucleosomes, rendering the gene to be regulated more available for the binding of transcriptional proteins. Other histone modifications catalyzed by coactivators, such as methylation by arginine methyl transferases (PRMT1/CARM) and phosphorylation, to play a role in chromatin modification. Still other coactivators recruited by SRC interact with and stabilize the basal transcriptional machinery including basal transcriptional factors, and RNA polymerase II. Not all coactivators are present in the regulatory complex at the same time, but join, carry out their function, and are released to make room for the next set of coactivators. The combined result of these sequential activities is an increased rate of transcription of the gene that is being regulated.
Figure 2 shows two typical DNA sequences of hormone response elements (HRE) for ER and VDR. HREs are composed of two hexameric half-site motifs. As can be seen in the figure, the directionality of both the relationship of the monomers to each other and the DNA half sites to each other differ according to homo- vs. heterodimerization. Recall from Figure 1 that it is the two zinc fingers of the receptor that are making contact with the DNA.
Fig2. Nuclear receptor binding to DNA. The binding of nuclear hormone receptors to hormone response elements (HRE) within DNA is depicted. Monomers in homodimeric receptors are arranged as mirror images (“head to head”) and bind to hormone response elements that are inverted repeats, as shown in the example of an estrogen response element (ERE) at the right. The heterodimers that the receptors for 1,25(OH)2D3 (VDR) and thyroid hormone (TR) form with RXR are arranged sequentially (“head to tail”) and bind to response elements that are direct repeats as shown for VDR and the VDRE in the lower part of the figure. In response elements for homodimeric receptors, the number of nucleotides between the two repeats (n) is three while those for the heterodimeric receptors may vary from 3–5 (represented by n) with the response element.
Nuclear receptors can also bring about repression of gene transcription. In some cases, notably that of the thyroid hormone receptor, the unliganded receptor is complexed wth the corepressor, SMRT (silencing mediator for retinoid/thyroid hormone receptors), which recruits proteins with histone deacetylase (HDAC) activity. The removal of acetyl groups from histones leads to their reassociation with DNA, re-formation of the nucleosome, and inaccessibility of the DNA to other binding and activating proteins.
