Category Archives: Glucocorticoid Receptor

Glucocorticoids and Fetal Development

Figure 1. The word Gluco-corticoids is derived from the Greek word "γλυκός" for sweet and from the Latin word for cortex.

1. What are Glucocorticoids?

Glucocorticoids (from Greek “γλυκός” meaning ”sweet” and cortex) are a class of steroid hormones that are produced in the adrenal cortex. In human the main glucocorticoid (GC) is cortisol, whereas in rodents, the main glucocorticoid  is corticosterone. GC hormones are vital for the homeostatic regulation of the bodily functions, and their actions span a hierarchy of control levels. Accordingly, GCs act on the basic molecular level to control critical functions such the cell cycle, cellular metabolism, viability, synaptic plasticity and the immune response.  At the top of the hierarchical pyramid, GCs fine-tune the stress behavior and cognitive function. It is well-known and as every human has experienced, high levels of GCs caused by stressful situations impair memory. Also, GCs are very important for fetal and brain development.

2. The Glucocorticoid Receptor

GCs bind onto their receptor, the Glucocorticoid Receptor (GR), which is located in the cytoplasm of the cell. GR belongs to the superfamily of the ligand-activated nuclear transcription factors. Hence, in the absence of ligand, GR is located in the cellular cytoplasm in a complex with chaperone proteins such as HSP90, HSP60 and FKBP51 (1). Upon hormone binding, the receptor is shuttled in the nucleus (Fig.2).  There it recognizes and associates with partially pallidromic regulatory DNA areas in target genes, called Glucocorticoid Responsive Elements (i.e GREs) (2,3) . GR can either act as a transcriptional activator, by directly binding onto the promoter of the target gene. Alternatively, GR interacts with other co-factors and causes trans-repression of target genes (1, 4) .  Structure of the GR: GR shares the modular structure common among the steroid receptors. It consists of a variable N-terminal domain (NTD), a highly conserved DNA binding domain with two zinc finger motifs (DBD), a hinge region, and a C-terminal hormone binding domain (5).  The DBD is conserved throughout the members of the nuclear receptorsuperfamily and almostall vertebrate species (6). The hinge region participates in receptor–ligand binding.

Figure 2. The ligated GR can either activate or repress gene transcription. (by Holgate and Polosa, 2008) Figure 2. The ligated GR can either activate or repress gene transcription. (by Holgate and Polosa, 2008)

3. GCs and Adverse Effects on Fetal and Infant Brain Development

GC hormones are potent stimulators of organ maturation. Due to this property,  synthetic GC analogs are vastly used in prenatal medicine.   Specifically,  in pregnancies under the risk of pre-term delivery (10% of pregnancies in North America) .  GC administration is the preferred route followed, to accelerate fetal lung maturation (12,13). Moreover, other clinical conditions of the mother or the fetus make use of synthetic GC administration such as allergies, asthma, and when the fetus is suspected to suffer from  Congenital Adrenal Hyperplasia (CAH) (14). Synthetic GCs are marginally different from their endogenous equivalents and more potent agonists of the GR. Specifically, dexamethasone (DEX) one of the most widely used synthetic GC is a very potent GR agonist. DEX is less sensitive to degradation by 11β-hydroxysteroid dehydrogenase-2  (11β-HSD2) (8, 14); a key enzyme that transforms active GCs to inactive 11 keto-analogs in the fetoplacental area. Therefore, 11β-HSD2 reduces the exposure of the fetus to GCs.

Despite the benefits of DEX, accumulating data suggest a dark side of DEX.  Specifically, clinical follow-up studies have indicated that DEX can have detrimental effects on brain development. Specifically, in infants and school aged children having exposed to DEX around birth, cognitive and behavioral deficits such as reduced IQ, poor social interaction and difficulty in coping with stress have been documented. Moreover, imaging data have pointed to reduced cortical convolution and reduced head circumference of DEX exposed infants in respect to untreated controls. Even more significantly , animal studies on prenatal DEX treatment, have documented that brain development can be permanently impaired by DEX. For example, Uno et al.(1990 and 1994) demonstrated that the size of the  hippocampal structure is reduced by DEX treatment in utero in rhesus macaques (15, 16).

Importantly, the observed impairments in the brain architecture could be the result of changes in different processes. Hence, the formation of the brain can be disrupted by altered migration of neuronal cells,or by impairments in the proliferation of Neural Progenitor Cells from which neurons originate. Accordingly, a series of in vitro studies demonstrated that exposure to GCs inhibits neural progenitor and stem cell proliferation and changes the balance between proliferation and differentiation (17, 18). The anti- proliferative effects of GCs are not limited in embryonic NPCs as increased GCs due to stress, or DEX exposure cause the same effects in adult NPCs of the hippocampus. Moreover, GCs are potent inhibitors of the proliferation of tumor cell lines such as medulloblastoma, neuroblastoma and osteosarcoma cells (17, 18).

4. Conclusion

In the light of the clinical and experimental observations on GC’s  dynamics in the developing brain and NPCs, it  is very important to understand the mechanisms by which they can impair brain development. Results from this line of research could be applied for improving the current GC-based treatments and the mode of DEX use in perinatal medicine.

References

1. Lu, N.Z., et al. International Union of Pharmacology. LXV. The pharmacology and classification of the nuclear receptor superfamily: glucocorticoid, mineralocorticoid, progesterone, and androgen receptors. Pharmacol Rev 58, 782-797 (2006).

2. Strahle, U., Schmid, W. & Schutz, G. Synergistic action of the glucocorticoid receptor with transcription factors. EMBO J 7, 3389-3395 (1988).

3. Strahle, U., Klock, G. & Schutz, G. A DNA sequence of 15 base pairs is sufficient to mediate both glucocorticoid and progesterone induction of gene expression. Proc Natl Acad Sci U S A 84, 7871-7875 (1987).

4. Zanchi, N.E., et al. Glucocorticoids: Extensive physiological actions modulated through multiple mechanisms of gene regulation. Journal of Cellular Physiology 224, 311-315.

5. McMaster, A. & Ray, D.W. Modelling the glucocorticoid receptor and producing therapeutic agents with anti-inflammatory effects but reduced side-effects. Experimental Physiology 92, 299-309 (2007).

6. Stolte, E.H., van Kemenade, B.M., Savelkoul, H.F. & Flik, G. Evolution of glucocorticoid receptors with different glucocorticoid sensitivity. J Endocrinol 190, 17-28 (2006).

7. Cole, T.J., et al. Molecular genetic analysis of glucocorticoid signaling during mouse development. Steroids 60, 93-96 (1995).

8. Speirs, H.J., Seckl, J.R. & Brown, R.W. Ontogeny of glucocorticoid receptor and 11beta-hydroxysteroid dehydrogenase type-1 gene expression identifies potential critical periods of glucocorticoid susceptibility during development. J Endocrinol 181, 105-116 (2004).

9. Noorlander, C.W., De Graan, P.N., Middeldorp, J., Van Beers, J.J. & Visser, G.H. Ontogeny of hippocampal corticosteroid receptors: effects of antenatal glucocorticoids in human and mouse. J Comp Neurol 499, 924-932 (2006).

10. Pryce, C.R. Postnatal ontogeny of expression of the corticosteroid receptor genes in mammalian brains: inter-species and intra-species differences. Brain Res Rev 57, 596-605 (2008).

11. Patel, P.D., et al. Glucocorticoid and mineralocorticoid receptor mRNA expression in squirrel monkey brain. J Psychiatr Res 34, 383-392 (2000).

12. Effect of corticosteroids for fetal maturation on perinatal outcomes. NIH Consensus Development Panel on the Effect of Corticosteroids for Fetal Maturation on Perinatal Outcomes. JAMA 273, 413-418 (1995).

13. Effect of corticosteroids for fetal maturation on perinatal outcomes. NIH Consens Statement 12, 1-24 (1994).

14. Tegethoff, M., Pryce, C. & Meinlschmidt, G. Effects of intrauterine exposure to synthetic glucocorticoids on fetal, newborn, and infant hypothalamic-pituitary-adrenal axis function in humans: a systematic review. Endocr Rev 30, 753-789 (2009).

15.Uno H, Lohmiller L, Thieme C, Kemnitz JW, Engle MJ, Roecker EB et al. Brain damage induced by prenatal exposure to dexamethasone in fetal rhesus macaques. I. Hippocampus. Brain Res Dev Brain Res 1990 May 1; 53(2): 157-167.

16.Uno H, Eisele S, Sakai A, Shelton S, Baker E, DeJesus O et al. Neurotoxicity of glucocorticoids in the primate brain. Horm Behav 1994 Dec; 28(4): 336-348.

17.Glick RD, Medary I, Aronson DC, Scotto KW, Swendeman SL, La Quaglia MP. The effects of serum depletion and dexamethasone on growth and differentiation of human neuroblastoma cell lines. J Pediatr Surg 2000 Mar; 35(3): 465-472.

18. Sundberg M, Savola S, Hienola A, Korhonen L, Lindholm D. Glucocorticoid hormones decrease proliferation of embryonic neural stem cells through ubiquitin-mediated degradation of cyclin D1. J Neurosci 2006 May 17; 26(20): 5402-5410.

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Cell Cycle Regulators Related to the Glucocorticoid Receptor

Eukaryotic cell-cycle, with the relative duration of each phase

G1/S-specific cyclin-D1 (CD1) : The protein encoded by this gene belongs to the highly conserved cyclin family, whose members are characterized by a dramatic periodicity in protein abundance throughout the cell cycle. Cyclins function as regulators of CDK kinases. Different cyclins exhibit distinct expression and degradation patterns which contribute to the temporal coordination of each mitotic event. CD1 forms a complex with and functions as a regulatory subunit of CDK4 or CDK6, whose activity is required for cell cycle G1/S transition. This protein has been shown to interact with tumor suppressor protein Rb which also regulates the expression of CD1.

P21: p21 is a potent cyclin-dependent kinase inhibitor (CKI). The p21 (WAF1) protein binds to and inhibits the activity of cyclinCDK2 or –CDK4 complexes, and thus functions as a regulator of cell cycle progression at G1. The expression of this gene is tightly controlled by the tumor suppressor protein p53, through which this protein mediates the p53-dependent cell cycle G1 phase arrest in response to a variety of stress stimuli. This was a major discovery in the early 1990’s that revealed how cells stop dividing after being exposed to damaging agents such as radiation. In addition to growth arrest, p21 can mediate cellular senescence and one of the ways it was discovered was as a senescent cell-derived inhibitor. The p21(WAF1) protein can also interact with proliferating cell nuclear antigen (PCNA), a DNA polymerase accessory factor, and plays a regulatory role in S phase DNA replication and DNA damage repair. This protein was reported to be specifically cleaved by CASP3-like caspases, which thus leads to a dramatic activation of CDK2, and may be instrumental in the execution of apoptosis following caspase activation. However p21 may inhibit apoptosis and does not induce cell death on its own [4].

P27 or Cyclin-dependent kinase inhibitor 1B belongs to the Cip/Kip family of cyclin dependent kinase (Cdk) inhibitor proteins. The p27 protein binds to and prevents the activation of cyclin ECDK2 or cyclin DCDK4 complexes, and thus controls the cell cycle progression at G1. It is often referred to as a cell cycle inhibitor protein because its major function is to stop or slow down the cell division cycle.  p27Kip1 binds to cyclin D either alone, or when complexed to its catalytic subunit CDK4. In doing so p27Kip1 inhibits the catalytic activity of Cdk4, which means that it prevents Cdk4 from adding phosphate residues to its principal substrate, the retinoblastoma (pRb) protein. Increased levels of the p27Kip1 protein typically cause cells to arrest in the G1 phase of the cell cycle. Likewise, p27Kip1 is able to bind other Cdk proteins when complexed to cyclin subunits such as Cyclin E/Cdk2 and Cyclin A/Cdk2.

Retinoblastoma protein (pRb): pRb prevents the cell from replicating damaged DNA by preventing its progression along the cell cycle through G1 (first gap phase) into S (synthesis phase).[7] pRb binds and inhibits transcription factors of the E2F family, which are composed of dimers of an E2F protein and a DP protein.[8] The transcription activating complexes of E2 promoter-binding–protein-dimerization partners (E2F-DP) can push a cell into S phase.[9][10][11][12][13] As long as E2F-DP is inactivated, the cell remains stalled in the G1 phase. When pRb is bound to E2F, the complex acts as a growth suppressor and prevents progression through the cell cycle [3]. The pRb-E2F/DP complex also attracts a histone deacetylase (HDAC) protein to the chromatin, reducing transcription of S phase promoting factors, further suppressing DNA synthesis.

Source: Wikipedia (links are highlighted)

This list definitely does not cover the whole range of cell-cycle modulators, which have been shown or linked with GR . Albeit, it does include some critical molecules, linked to the antiproliferative effects of glucocorticoids and I intent to gradually expand the list as I go.

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