Kinetic Pulse-Chase Labeling Study of the Glucocorticoid Receptor in Mouse Lymphoma Cells

Inhibition of proteasome-mediated glucocorticoid receptor degradation restores nitric oxide bioavailability in myocardial endothelial cells in vitro

BACKGROUND INFORMATION

Glucocorticoids (GCs), including the synthetic GC derivate dexamethasone, are widely used as immunomodulators. One of the numerous side effects of dexamethasone therapy is hypertension arising from reduced release of the endothelium-derived vasodilator nitric oxide (NO).

RESULTS

Herein, we described the role of dexamethasone and its glucocorticoid receptor (GR) in the regulation of NO synthesis in vitro using the mouse myocardial microvascular endothelial cell line, MyEND. GC treatment caused a firm decrease of extracellular NO levels, whereas the expression of endothelial NO synthase (eNOS) was not affected. However, GC application induced an impairment of tetrahydrobiopterin (BH4 ) concentrations as well as GTP cyclohydrolase-1 (GTPCH-1) expression, both essential factors for NO production upstream of eNOS. Moreover, dexamethasone stimulation resulted in a substantially decreased GR gene and protein expression in MyEND cells. Importantly, inhibition of proteasome-mediated proteolysis of the GR or overexpression of an ubiquitination-defective GR construct improved the bioavailability of BH4 and strengthened GTPCH-1 expression and eNOS activity.

CONCLUSIONS

Summarising our results, we propose a new mechanism involved in the regulation of NO signalling by GCs in myocardial endothelial cells. We suggest that a sufficient GR protein expression plays a crucial role for the management of GC-induced harmful adverse effects, including deregulations of vasorelaxation arising from disturbed NO biosynthesis.

Three mechanisms are involved in glucocorticoid receptor autoregulation in a human T-lymphoblast cell line

Glucocorticoids up-regulate the glucocorticoid receptor (GR) in the human T-lymphoblast cell line CEM-C7. One mechanism for the up-regulation of the GR protein is the well-known up-regulation of GR transcripts. We have investigated the effect of other factors on the up-regulation. At least three promoters (1A, 1B, and 1C) exist, which give rise to GR transcripts with different exon 1 sequences. Transcripts with different exon 1 sequences have similar stabilities. Glucocorticoids have little, if any, effect on mRNA stability. In transfection experiments of the GR-deficient mouse fibroblast cell line E8.2, different exon 1 sequences furthermore caused no significant differences in the translational efficiencies of GR transcripts. However, the ratio between the concentrations of the glucocorticoid receptor B (GR-B) isoform and the glucocorticoid receptor A (GR-A) isoform was higher for transcripts containing the exon 1A3 sequence arising from promoter 1A than in transcripts containing exon 1 sequences from promoters 1B and 1C. Because the GR-B isoform is more active in transactivation then GR-A, this would tend to fine-tune glucocorticoid responsiveness of CEM-C7 cells, which express exon 1A3-containing transcripts. We also found that glucocorticoids do not decrease the stability of the GR protein in CEM-C7 cells. In contrast to other cell lines that downregulate GR expression in response to glucocorticoids, CEM-C7 lymphoblasts possess three mechanisms ensuring high glucocorticoid responsiveness: an up-regulation of GR mRNA by glucocorticoids, no destabilization of GR protein by glucocorticoids, and a high activity of promoter 1A with concomitant high expression of the GR-B isoform.

Protein stability and domain topology determine the transcriptional activity of the mammalian glial cells missing homolog, GCMb

The glial cells missing (GCM) family of transcription factors consists of Drosophila GCM and the mammalian proteins GCMa and GCMb. They are expressed in a highly restricted manner during development and are known or assumed to be important regulators of developmental fate decisions. As the biochemical properties of GCMb have not been studied so far, we have undertaken a detailed structure-function analysis of the mouse GCMb (mGCMb) protein. DNA-binding specificity was very similar to that of other GCM proteins. Nevertheless, mGCMb was only a weak transcriptional activator in a number of different tissue culture systems. Interestingly, this was not due to an intrinsic absence of transactivation potential. In effect, we were able to identify two separate transactivation domains within mGCMb, one carboxyl-terminally adjacent to the DNA-binding domain and the second within the extreme carboxyl terminus. Activity of both transactivation domains was, however, modulated by an inhibitory region unique to mGCMb and located between the two transactivation domains. Furthermore, pulse-chase experiments proved that the mGCMb protein has a half-life approximately four times shorter than mGCMa. Introduction of the above mentioned inhibitory domain of mGCMb into mGCMa shortened the half-life of mGCMa to a value typical of mGCMb with a concomitant reduction in transactivation potential. Given the strong correlation between protein stability and transactivation potential, functional differences between the two mammalian GCM homologs are likely due to differences in stability with a single inhibitory region in mGCMb being involved in the reduction of both.

Surface expression and metabolic half-life of AMPA receptors in cultured rat cerebellar granule cells

The surface expression and metabolic turnover time of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors are important factors in determining the involvement of these proteins in synaptic function. We used the membrane-impermeant chemical crosslinking reagent BS3 and subsequent immunoblot analysis to show that a large proportion of each of the AMPA receptor subunits GluR1-4 is surface expressed in cultured rat cerebellar granule cells. The apparent molecular mass of the individual subunits was 105 kDa and the surface expressed crosslinked complex was 530 kDa for each of the antibodies tested. These results are consistent with functional ionotropic glutamate receptors being pentameric subunit assemblies. We investigated the time course of AMPA receptor surface expression using the membrane-impermeant biotinylating agent NHS-SS-biotin. The half-life of surface expressed AMPA receptors was found to be 30 h. To estimate the mean degradation rate of AMPA receptors and total trichloracetic acid (TCA)-precipitable protein we used [35S]methionine/cysteine pulse-chase labelling. The half-life of AMPA receptors immunoprecipitated with anti-GluR1 antibody was approximately 48 h and the half-life of total TCA-precipitable protein from the same samples was 37 h.

Differential effects of agonist and antagonists on autoregulation of glucocorticoid receptors in a rat colonic adenocarcinoma cell line

The relative abilities of a potent glucocorticoid receptor (GR) agonist (RU 28362), a weak GR agonist (aldosterone) and a potent GR antagonist (RU 38486) to promote in vivo activation/transformation and subsequent down-regulation of GR mRNA and protein levels were compared using the PROb rat colonic adenocarcinoma cell line. In vivo activation, which is followed immediately by nuclear translocation, by these ligands (1 microM) was evaluated in terms of their abilities to deplete cytoplasmic GR protein levels after a 30 min incubation period. Western blotting experiments with the anti-GR monoclonal antibody BuGR2 demonstrated that a brief incubation with RU 28362 resulted in nearly complete depletion of cytoplasmic GR, whereas incubation with aldosterone resulted in a 50% depletion of the cytoplasmic GR protein. Incubation with RU 38486 was even more effective than aldosterone in promoting this key step in the GR pathway. Prolonged treatment (18 h) with RU 28362 resulted in significant down-regulation of GR mRNA and total cellular GR protein levels. Similar incubation with aldosterone resulted in a transient decrease in the GR mRNA level and also down-regulated the total GR protein level. Although prolonged incubation with RU 38486 did not result in detectable down-regulation of the GR mRNA level, this antagonist very effectively down-regulated total cellular GR protein levels. Taken collectively, these data demonstrate that agonists capable of promoting in vivo activation (and subsequent nuclear translocation) of GR are also effective at down-regulating GR at both the mRNA and protein levels. Although the antagonist RU 38486 is also capable of down-regulating GR protein levels by shortening the half-life of the receptor, it appears to be incapable of altering the rate of transcription of the GR gene. Glucocorticoid target tissue sensitivity may thus be decreased via two independent mechanisms: agonist-induced repression of GR gene transcription; and/or ligand-induced degradation of total cellular GR protein levels.

Glucocorticoid receptors: ATP-dependent cycling and hormone-dependent hyperphosphorylation

The dependence of hormone binding to glucocorticoid receptors (GRs) on cellular ATP levels led us to propose that GRs normally traverse an ATP-dependent cycle, possibly involving receptor phosphorylation, and that without ATP they accumulate in a form that cannot bind hormone. We identified such a form, the null receptor, in ATP-depleted cells. GRs are basally phosphorylated, and become hyperphosphorylated after treatment with hormone (but not RU486). In mouse receptors we have identified 7 phosphorylated sites, all in the N-terminal domain. Most are on serines and lie within a transactivation region. The time-course of hormone-induced hyperphosphorylation indicates that the primary substrates for hyperphosphorylation are the activated receptors; unliganded and hormone-liganded nonactivated receptors become hyperphosphorylated more slowly. After dissociation of substrates for hyperphosphorylation are the activated receptors; unliganded and hormone-liganded nonactivated receptors become hyperphosphorylated more slowly. After dissociation of hormone, most receptors appear to be recycled and reutilized in hyperphosphorylated form. From these and related observations, we have concluded that the postulated ATP-dependent cycle can be accounted for by hormone-induced or spontaneous dissociation of receptor-Hsp90 complexes, followed by reassociation of unliganded receptors with Hsp90 via an ATP-dependent reaction like that demonstrated in cell-free systems. Other steroid hormone receptors might traverse a similar cycle. Four of the 7 phosphorylated sites in the N-terminal domain are in consensus sequences for p34cdc2 kinases important in cell cycle regulation. This observation, along with the known cell cycle-dependence of sensitivity to glucocorticoids and other evidence, point to a role for receptor phosphorylation in controlling responses to glucocorticoids through the cell cycle.

Autoregulation of glucocorticoid receptor gene expression

Glucocorticoid receptors are members of a highly conserved family of steroid receptor proteins, which are ligand-dependent transcription factors. Previous studies have shown that the presence of functional glucocorticoid receptors is a prerequisite for manifestation of cellular responses to hormone. Glucocorticoid receptors undergo down-regulation following treatment with glucocorticoids. To define the molecular mechanisms that are involved in this process we have analyzed the down-regulation of glucocorticoid receptors both in HeLa cells, which contain endogenous receptors, and in cells containing receptors that have been introduced by DNA transfection. Our results show that cells that contain glucocorticoid receptors--either endogenous or transfected--undergo down-regulation of steroid-binding capabilities, as well as reductions in receptor protein and mRNA levels, in a remarkably similar fashion. DNA sequences in the coding region of the human glucocorticoid receptor cDNA appear to be sufficient to account for down-regulation of receptor. This novel finding suggests that unique mechanisms are involved in controlling glucocorticoid receptor homeostasis.