Regulation of human glucocorticoid receptor gene transcription by Sp1 and p53

Antenatal Hypoxia and Programming of Glucocorticoid Receptor Expression in the Adult Rat Heart

Glucocorticoid receptor (GR) signaling is critical for development and function of the heart. Our previous study demonstrated that gestational hypoxia induced epigenetic repression of the GR gene in the developing heart. The present study aims to determine that the alterations of promoter methylation level and epigenetic repression of the GR gene in the developing heart in response to maternal hypoxia is sustained in adult offspring and potential gender differences in the programming of GR gene. Pregnant rats were treated with 10.5% O₂ from gestational day 15 (E15) to 21 (E21). Hearts were isolated from 5-month-old male and female offspring with the developing stage being equivalent to 18-year-old human. GR mRNA and protein abundance was determined with real time qRT-PCR and Western blot. GR gene promoter methylation and binding of transcription factors were measured with methylated DNA immunoprecipitation (MeDIP) and Chromatin immunoprecipitation (ChIP). The results showed that antenatal hypoxia significantly decreased the expression of GR mRNA and protein in the hearts of adult male offspring, but not in females, which is ascribed to the differential changes of alternative exon1 mRNA variants of GR gene in male and female hearts in response to prenatal hypoxia. In addition, the downregulation of GR expression in the male heart was correlated with increased methylation levels of CpG dinucleotides in promoters of exon 1₄, 1₅, 1₆, 1₇, and 1₁₀, which resulted in a decrease in the binding of their transcription factors. Thus, the study reveals that antenatal hypoxia results in a reprogramming and long-term change in GR gene expression in the heart by hypermethylation of GR promoter in a sex-differential pattern, which provides a novel mechanism regarding the increased vulnerability of heart later in life with exposure of prenatal hypoxia.

C2H2-Type Zinc Finger Proteins: Evolutionarily Old and New Partners of the Nuclear Hormone Receptors

Nuclear hormone receptors (NRs) are evolutionarily conserved ligand-dependent transcription factors. They are essential for human life, mediating the actions of lipophilic molecules, such as steroid hormones and metabolites of fatty acid, cholesterol, and external toxic compounds. The C2H2-type zinc finger proteins (ZNFs) form the largest family of the transcription factors in humans and are characterized by multiple, tandemly arranged zinc fingers. Many of the C2H2-type ZNFs are conserved throughout evolution, suggesting their involvement in preserved biological activities, such as general transcriptional regulation and development/differentiation of organs/tissues observed in the early embryonic phase. However, some C2H2-type ZNFs, such as those with the Krüppel-associated box (KRAB) domain, appeared relatively late in evolution and have significantly increased family members in mammals including humans, possibly modulating their complicated transcriptional network and/or supporting the morphological development/functions specific to them. Such evolutional characteristics of the C2H2-type ZNFs indicate that these molecules influence the NR functions conserved through evolution, whereas some also adjust them to meet with specific needs of higher organisms. We review the interaction between NRs and C2H2-type ZNFs by focusing on some of the latter molecules.

Potential of Environmental Enrichment to Prevent Transgenerational Effects of Paternal Trauma

Adverse experiences in early life are risk factors for the development of behavioral and physiological symptoms that can lead to psychiatric and cognitive disorders later in life. Some of these symptoms can be transmitted to the offspring, in some cases by non-genomic mechanisms involving germ cells. Using a mouse model of unpredictable maternal separation and maternal stress, we show that postnatal trauma alters coping behaviors in adverse conditions in exposed males when adult and in their adult male progeny. The behavioral changes are accompanied by increased glucocorticoid receptor (GR) expression and decreased DNA methylation of the GR promoter in the hippocampus. DNA methylation is also decreased in sperm cells of exposed males when adult. Transgenerational transmission of behavioral symptoms is prevented by paternal environmental enrichment, an effect associated with the reversal of alterations in GR gene expression and DNA methylation in the hippocampus of the male offspring. These findings highlight the influence of both negative and positive environmental factors on behavior across generations and the plasticity of the epigenome across life.

Antenatal hypoxia induces epigenetic repression of glucocorticoid receptor and promotes ischemic-sensitive phenotype in the developing heart

Large studies in humans and animals have demonstrated a clear association of an adverse intrauterine environment with an increased risk of cardiovascular disease later in life. Yet mechanisms remain largely elusive. The present study tested the hypothesis that gestational hypoxia leads to promoter hypermethylation and epigenetic repression of the glucocorticoid receptor (GR) gene in the developing heart, resulting in increased heart susceptibility to ischemia and reperfusion injury in offspring. Hypoxic treatment of pregnant rats from day 15 to 21 of gestation resulted in a significant decrease of GR exon 14, 15, 16, and 17 transcripts, leading to down-regulation of GR mRNA and protein in the fetal heart. Functional cAMP-response elements (CREs) at -4408 and -3896 and Sp1 binding sites at -3425 and -3034 were identified at GR untranslated exon 1 promoters. Hypoxia significantly increased CpG methylation at the CREs and Sp1 binding sites and decreased transcription factor binding to GR exon 1 promoter, accounting for the repression of the GR gene in the developing heart. Of importance, treatment of newborn pups with 5-aza-2'-deoxycytidine reversed hypoxia-induced promoter methylation, restored GR expression and prevented hypoxia-mediated increase in ischemia and reperfusion injury of the heart in offspring. The findings demonstrate a novel mechanism of epigenetic repression of the GR gene in fetal stress-mediated programming of ischemic-sensitive phenotype in the heart.

IUGR increases chromatin-remodeling factor Brg1 expression and binding to GR exon 1.7 promoter in newborn male rat hippocampus

Intrauterine growth restriction (IUGR) increases the risk for neurodevelopment delay and neuroendocrine reprogramming in both humans and rats. Neuroendocrine reprogramming involves the glucocorticoid receptor (GR) gene that is epigenetically regulated in the hippocampus. Using a well-characterized rodent model, we have previously shown that IUGR increases GR exon 1.7 mRNA variant and total GR expressions in male rat pup hippocampus. Epigenetic regulation of GR transcription may involve chromatin remodeling of the GR gene. A key chromatin remodeler is Brahma-related gene-1(Brg1), a member of the ATP-dependent SWItch/Sucrose NonFermentable (SWI/SNF) chromatin remodeling complex. Brg1 regulates gene expression by affecting nucleosome repositioning and recruiting transcriptional components to target promoters. We hypothesized that IUGR would increase hippocampal Brg1 expression and binding to GR exon 1.7 promoter, as well as alter nucleosome positioning over GR promoters in newborn male pups. Further, we hypothesized that IUGR would lead to accumulation of specificity protein 1 (Sp1) and RNA pol II at GR exon 1.7 promoter. Indeed, we found that IUGR increased Brg1 expression and binding to GR exon 1.7 promoter. We also found that increased Brg1 binding to GR exon 1.7 promoter was associated with accumulation of Sp1 and RNA pol II carboxy terminal domain pSer-5 (a marker of active transcription). Furthermore, the transcription start site of GR exon 1.7 was located within a nucleosome-depleted region. We speculate that changes in hippocampal Brg1 expression mediate GR expression and subsequently trigger neuroendocrine reprogramming in male IUGR rats.

Fetal hypoxia increases vulnerability of hypoxic-ischemic brain injury in neonatal rats: role of glucocorticoid receptors

Gestational hypoxia is a common stress to the fetal development and increases the risk of neonatal morbidity. The present study tested the hypothesis that fetal hypoxia results in heightened brain vulnerability to hypoxic-ischemic (HI) injury in neonatal rats via down-regulation of glucocorticoid receptor (GR) in the developing brain. Time-dated pregnant rats were exposed to hypoxia (10.5% O2) from days 15 to 21 of gestation. Brain HI injury was determined in day 10 pups. Maternal hypoxia resulted in asymmetric intrauterine growth restriction in the fetus. The brain HI injury was significantly increased in maternal hypoxia-treated pups as compared with the normoxia control in both males and females. Activation of brain GR by dexamethasone injection into the right lateral ventricle produced a concentration-dependent reduction of HI-induced brain injury in control pups. Maternal hypoxia significantly decreased GR mRNA and protein abundance in the fetal brain and neonatal hippocampus and abolished the dexamethasone-mediated neuroprotective effect in pup brains. This decreased GR expression was resulted from increased DNA methylation, decreased binding of transcription factors Egr-1 and Sp1 to GR gene exon 17 and 111 promoters, and reduced expression of GR exon 17 and 111 mRNA variants. The results demonstrate that gestational hypoxia causes epigenetic repression of GR gene expression in the developing brain resulting in the heightened brain vulnerability to HI injury in neonatal rats.

p53 cooperates with Sp1 to regulate breed-dependent expression of glucocorticoid receptor in the liver of preweaning piglets

Previous studies indicate that Chinese indigenous pig breeds demonstrate distinct pattern of glucocorticoid receptor (GR) expression, which is associated with their unique growth and metabolic phenotypes. Here we sought to unravel the transcriptional mechanisms underlying the breed-specific hepatic GR expression in preweaning Chinese Erhualian (EHL) and Western Large White (LW) piglets. Total GR mRNA and the predominant GR mRNA variant 1-9/10 were expressed significantly higher in EHL compared with LW piglets (P<0.01), which was associated with more enriched histone H3 acetylation on 1-9/10 promoter (P<0.05). Nuclear content of transcription factor specificity protein 1 (Sp1) was significantly lower in EHL piglets, yet its binding to GR 1-9/10 promoter was significantly higher in EHL piglets, as revealed by chromatin immunoprecipitation assays. Although p53 binding to GR promoter 1-9/10 did not differ between breeds, expression of p53 mRNA and protein, as well as its binding to Sp1, were significantly higher in EHL piglets. Moreover, p53 activator doxorubicin significantly enhanced GR 1-9/10 promoter activity in HepG2 cells at 100 nM, which was associated with significantly higher protein content of p53 and GR. Sp1 inhibitor, mithramycin A, significantly inhibited (P<0.05) the basal activity of GR promoter 1-9/10 and completely blocked doxorubicin -induced activation of GR promoter 1-9/10. These data indicate that higher hepatic GR expression in EHL piglets attributes mainly to the enhanced transcription of GR promoter 1-9/10, which is achieved from breed-specific interaction of p53 and Sp1 on porcine GR 1-9/10 promoter.

Breed-dependent transcriptional regulation of 5'-untranslated GR (NR3C1) exon 1 mRNA variants in the liver of newborn piglets

Glucocorticoids are vital for life and regulate an array of physiological functions by binding to the ubiquitously expressed glucocorticoid receptor (GR, also known as NR3C1). Previous studies demonstrate striking breed differences in plasma cortisol levels in pigs. However, investigation into the breed-dependent GR transcriptional regulation is hampered by lacking porcine GR promoter information. In this study, we sequenced 5.3 kb upstream of the translation start codon of the porcine GR gene, and identified seven alternative 5′-untranslated exons 1–4, 1–5, 1–6, 1–7, 1–8, 1–9,10 and 1–11. Among all these mRNA variants, exons 1–4 and 1–5, as well as the total GR were expressed significantly (P<0.05) higher in the liver of newborn piglets of Large White (LW) compared with Erhualian, a Chinese indigenous breed. Overall level of CpG methylation in the region flanking exons 1–4 and 1–5 did not show breed difference. However, nuclear content of Sp1, p-CREB and GR in the liver was significantly (P<0.05) higher in LW piglets, associated with enhanced binding of p-CREB, and higher level of histone H3 acetylation in 1–4 and 1–5 promoters. In contrast, GR binding to promoters of exons 1–4 and 1–5 was significantly diminished in LW piglets, implicating the presence of negative GREs. These results indicate that the difference in the hepatic expression of GR transcript variants between two breeds of pigs is determined, at least partly, by the disparity in the binding of transcription factors and the enrichment of histone H3 acetylation to the promoters.

Tumor suppressor protein (p)53, is a regulator of NF-kappaB repression by the glucocorticoid receptor

Glucocorticoids can inhibit inflammation by abrogating the activity of NF-κB, a family of transcription factors that regulates the production of proinflammatory cytokines. To understand the molecular mechanism of repression of NF-κB activity by glucocorticoids, we performed a high-throughput siRNA oligo screen to identify novel genes involved in this process. Here, we report that loss of p53, a tumor suppressor protein, impaired repression of NF-κB target gene transcription by glucocorticoids. Additionally, loss of p53 also impaired transcription of glucocorticoid receptor (GR) target genes, whereas upstream NF-κB and glucocorticoid receptor signaling cascades remained intact. We further demonstrate that p53 loss severely impaired glucocorticoid rescue of death in a mouse model of LPS shock. Our findings unveil a new role for p53 in the repression of NF-κB by glucocorticoids and suggest important implications for treatment of the proinflammatory microenvironments found in tumors with aberrant p53 activity.

One function--multiple mechanisms: the manifold activities of p53 as a transcriptional repressor

Maintenance of genome integrity is a dynamic process involving complex regulation systems. Defects in one or more of these pathways could result in cancer. The most important tumor-suppressor is the transcription factor p53, and its functional inactivation is frequently observed in many tumor types. The tumor suppressive function of p53 is mainly attributed to its ability to regulate numerous target genes at the transcriptional level. While the mechanism of transcriptional induction by p53 is well characterized, p53-dependent repression is not understood in detail. Here, we review the manifold mechanisms of p53 as a transcriptional repressor. We classify two different categories of repressed genes based on the underlying mechanism, and novel mechanisms which involve regulation through noncoding RNAs are discussed. The complete elucidation of p53 functions is important for our understanding of its tumor-suppressor activity and, therefore, represents the key for the development of novel therapeutic approaches.

The polycystic kidney disease-1 gene is a target for p53-mediated transcriptional repression

This study provides evidence that the tumor suppressor protein, p53, is a transcriptional repressor of PKD1. Kidneys of p53-null mice expressed higher Pkd1 mRNA levels than wild-type littermates; gamma-irradiation suppressed PKD1 gene expression in p53+/+ but not p53-/- cells; and chromatin immunoprecipitation assays demonstrated the binding of p53 to the PKD1 promoter in vivo. In transient transfection assays, p53 repressed PKD1 promoter activity independently of endogenous p21. Deletion analysis mapped p53-mediated repression to the proximal promoter region of PKD1. Mutations of the DNA binding or C-terminal minimal repression domains of p53 abolished its ability to repress PKD1. Moreover, trichostatin A, an inhibitor of histone deacetylase activity, attenuated p53-induced repression of the PKD1 promoter. These findings, together with previous reports showing that dedifferentiated Pkd1-deficient cells express lower p53 and p21 levels, suggest a model whereby PKD1 signaling activates the p53-p21 differentiation pathway. In turn, p53 cooperates with histone deacetylases to repress PKD1 gene transcription. Loss of a p53-mediated negative feedback loop in PKD1 mutant cells may therefore contribute to deregulated PKD1 expression and cystogenesis.