Nuclear receptors

The nuclear receptor family, also called nuclear or ligand-activated transcription factors, are special receptor proteins, many of which are located in the cytosol. Nuclear receptors act as transcription factors. As such, they regulate the expression of specific genes in the cell nucleus. They are responsible for the provision of proteins. At the same time, they themselves are subject to modulations through posttranslational modification. They can exert regulatory functions on other signal transduction pathways (cross talk). The totality of nuclear receptors subsumes the multitude of perceived signals and cause the reactions adequate for the organism by expression of defined genes.

Nuclear receptors bind directly to DNA and, through regulatory influences on the corresponding genes, play an important role in embryogenesis, cell differentiation, homeostasis, reproduction, growth, proliferation and apoptosis of cells. Furthermore, nuclear receptors play an important role in various diseases such as tumours, metabolic disorders (diabetes or obesity), malformation syndromes and other internal diseases; also in disorders of reproductive ability (Lazar MA 2017; Sun G et al. 2017).

Recognising the essential importance of nuclear receptors for the function of organisms in experimental animals which, due to genetic intervention, are unable to express one or more nuclear receptors (knock-out animals). They already die in utero or perinatal or show severe organ malformations, are infertile or have a shortened lifespan.

To date, 48 nuclear receptors (nuclear receptors) are known, but their ligands are only partially identified. Receptors whose ligands are still undiscovered are called orphan receptors. After phylogeny, nuclear receptors are divided into 6 subgroups:

The subfamily 1 (thyroid hormone receptor-like) contains the receptors which can form heterodimers with RXR. These include:

• Group A: Thyroid hormone receptor (thyroid hormones)
• Group B: retinoic acid receptor (vitamin A and related compounds)
• Group C: Peroxisome proliferator-activated receptors (fatty acids, prostaglandins)
• Group D: Rev-ErbA (heme)
• Group F: RAR-related orphan receptors (cholesterol, ATRA)
• Group H: Liver X receptor-like (oxysterol)
• Group I: Vitamin D receptor-like

Subfamily 2: Retinoid X receptor-like. These include:

• Group A: Hepatocyte nuclear factor-4 (HNF4) (fatty acids)
• Group B: Retinoid X receptors (RXRα) (Retinoids)
• Group C: Testicular receptors

Subfamily 3: Estrogen receptor-like. The classical steroid receptors like the androgen receptor (but not the estrogen receptor) are located in the cytoplasm in the inactive state bound to heat shock proteins . After binding their ligand, the heat shock protein is separated, the receptor translocates into the cell nucleus and binds to the DNA as a dimer. The subfamily 3 includes:

• Group A: Estrogen receptors (sex hormones: estrogens)
• Group B: Estrogen linked receptors
• Group C: 3-ketosteroid receptors
  • C1 glucocorticoid receptors (GR; NR3C1) (cortisol),
  • C2 Mineralocorticoid receptors (MR; NR3C2) (aldosterone)
  • C3 Progesterone receptors (PGR; NR3C3) (sex hormones: progesterone),
  • C4 Androgen receptor (AR; NR3C4) (sex hormones: testosterone)

Subfamily 4: Nerve growth factor IB-like. These include:

• the neuron-derived orphan receptor 1 (Nor-I)
• the Nuclear Receptor Related 1 (Nurr1)
• the Nerve Growth Factor IB (NGFI-B).

Subfamily 5: Steroidogenic factor-like. These include:

• the FTZ Transcription Factor 1 (FTZ-F1) from Drosophila
• the steroidogenic factor-1 (SF-1), which occurs in mammals.

Subfamily 6: Germ Cell nuclear factor-like. These include:

• The Germ Cell Nuclear Factor (GCNF)

Subfamily 0: Various

Group B: DAX/SHP

• Dosage-sensitive sex reverse, adrenal hypoplasia critical region, on chromosome X, gene 1 (DAX1, NR0B1)
• Small heterodimer partner (SHP; NR0B2)

Nuclear receptors are only activated by the binding of a specific (natural) ligand (DeLuca HF 2004).

The spectrum of these ligands includes substances such as:

• Vitamin D3 (associated receptor: vitamin D receptor, VDR, NR1I1)
• Retinoic acid (associated receptor: retinoic acid receptor, RARs, NR1B, the retinoic acid X-receptor, RXRs, NR2B).
• Bile acids (associated receptors: farnesoid X receptor, FXR, NR1H4; pregnanX receptor, PXR, NR1I2), fatty acids, eicosanoids (Peroxisome Proliferation Activating Receptor, PPAR, NR1C), oxysterols (Liver X Receptor, LXR, NR1H)
• Steroid hormones such as estrogens, progesterone, glucocorticoids, mineralocorticoids, androgens (associated receptors: estrogen receptor, ER, NR3A; progesterone receptor, PR, NR3C3; glucocorticoid receptor, GR, NR3C1; mineralocorticoid receptor, MR, NR3C2; androgen receptor, AR, NR3C4; constitutive androstane receptor, CAR, NR1I3).

In recent years it has been shown that non-cholesterol-based molecules such as phospholipids can also specifically bind and functionally regulate the activity of certain nuclear receptors, suggesting a crucial role for these molecules in transcriptional regulation (Crowder MK et al. 2017). These natural ligands must passively cross the cell membrane barrier. In most cases they do this by passive diffusion through the cell membrane. For this purpose, these ligands possess sufficient lipophilic or ambiphilic properties. Once at the site of action, they bind to the ligand binding domain of nuclear receptors.

As soon as the activated receptors are invaded into the cell nucleus, they interact with further proteins that contribute to the regulation of gene expression as co-activators, co-pressors and additional transcription factors. Nuclear receptors can cause both up- and down-regulation. They are also capable of interacting with other signal transduction pathways, which are controlled, for example, by the nuclear factor NF-κB or the activator protein AP-1.

Mechanism of action: The mechanism of action has to distinguish whether the nuclear receptor is located in the cytosol or already in the cell nucleus DNA at the time of ligand binding. Furthermore, it is important whether the DNA binding takes place as monomer or dimer. All nuclear receptors have a common modular structure consisting of 5-6 conserved domains that are responsible for specific functions. The hypervariable N-terminal A/B domain can act independently of ligands and contains the variable amino terminal transcriptional activation region (AF-1). The DNA binding domain (DBD) consists of 2 zinc fingers. It is located in the C region of the -receptor protein. The DNA binding domain is responsible for the recognition of DNA sequences. The D-domain of the receptor interacts with co-repressors (SaferJD et al. 1998) and connects the DNA binding region with the ligand binding region. This binding allows the DNA binding region to be rotated.

Nuclear receptors bind their corresponding response elements as monomers, homodimers or as heterodimers with other receptors. The dimerization and the resulting cooperativity can change the affinity and specificity of the DNA binding. While steroid hormone receptors generally bind as homodimers, RAR, RXR, TR and VDR can both homodimerize and heterodimerize. The 9-cis-retinoic acid receptors (RXRs) play a special role in signal transduction by nuclear receptors, since they can act as promiscuous heterodimerization partners for a number of receptors in addition to homodimerization (Gronemeyer H et al 2003). For example, steroid hormone receptors are present in the non-stimulated cell as relatively unstable monomers in the cytosol. Their binding to heat shock proteins (Hsp90) leads to a certain stabilisation of the molecule. Only after binding to its ligand does a stable hormone-receptor complex form. The heat-shock protein dissociates.

From the perspective of drug research, nuclear receptors are of particular interest because natural ligands have the typical size of various drugs. Thus, in 2003, 34 of the 200 most frequently prescribed drugs belonged to the group that unfolds its functions via a nuclear receptor.

Nomenclature: For simplification purposes, a nomenclature for the individual subfamilies and groups of receptors was drawn up (Committee for Nomenclature of Nuclear Receptors), which is based on the phylogenetic family tree of all known receptors, in particular on the evolutionary development of the DNA binding domain (DBD, C-domain) and the ligand binding domain (LBD, E-domain). This results in a systematic name for each receptor, e.g. NR2B1 for RXRalpha. For NR2B1, NR stands for "nuclear receptor", 2 for subfamily 2, B for group B and 1 for gene 1.

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