Post-transcriptional control of c-myc proto-oncogene expression by glucocorticoid hormones in human T lymphoblastic leukemic cells

Dexamethasone Is Not Sufficient to Facilitate Tenogenic Differentiation of Dermal Fibroblasts in a 3D Organoid Model

Self-assembling three-dimensional organoids that do not rely on an exogenous scaffold but maintain their native cell-to-cell and cell-to-matrix interactions represent a promising model in the field of tendon tissue engineering. We have identified dermal fibroblasts (DFs) as a potential cell type for generating functional tendon-like tissue. The glucocorticoid dexamethasone (DEX) has been shown to regulate cell proliferation and facilitate differentiation towards other mesenchymal lineages. Therefore, we hypothesized that the administration of DEX could reduce excessive DF proliferation and thus, facilitate the tenogenic differentiation of DFs using a previously established 3D organoid model combined with dose-dependent application of DEX. Interestingly, the results demonstrated that DEX, in all tested concentrations, was not sufficient to notably induce the tenogenic differentiation of human DFs and DEX-treated organoids did not have clear advantages over untreated control organoids. Moreover, high concentrations of DEX exerted a negative impact on the organoid phenotype. Nevertheless, the expression profile of tendon-related genes of untreated and 10 nM DEX-treated DF organoids was largely comparable to organoids formed by tendon-derived cells, which is encouraging for further investigations on utilizing DFs for tendon tissue engineering.

Multiplicity of Glucocorticoid Action in Inhibiting Allograft Rejection

Glucocorticoids (GCs) are used as immunosuppressive and antiinflammatory agents in organ transplantation and in treating autoimmune diseases and inflammatory disorders. GCs were shown to exert their antiproliferative effects directly through blockade of certain elements of an early membrane-associated signal transduction pathway, modulation of the expression of select adhesion molecules, and by suppression of cytokine synthesis and action. GCs may act indirectly by inducing lipocortin synthesis, which in turn, inhibits arachidonic acid release from membrane-bound stores, and also by inducing transforming growth factor (TGF)-β expression that subsequently blocks cytokine synthesis and T cell activation. Furthermore, by preferentially inhibiting the production of Th1 cytokines, GCs may enhance Th2 cell activity and, hence, precipitate a long-lasting state of tolerance through a preferential promotion of a Th2 cytokine-secreting profile. In exerting their antiproliferative effects, GCs influence both transcriptional and posttranscriptional events by binding their cytosolic receptor (GR), which subsequently binds the promoter region of cytokine genes on select DNA sites compatible with the GCs responsible elements (GRE) motif. In addition to direct DNA binding, GCs may also directly bind to, and hence antagonize, nuclear factors required for efficient gene expression, thereby markedly reducing transcriptional rate. The pleiotrophy of the GCs action, coupled with the diverse experimental conditions employed in assessing the GCs effects, indicate that GCs may utilize more than one mechanism in inhibiting T cell activation, and warrant careful scrutiny in assigning a mechanism by which GCs exert their antiproliferative effects. © 1998 Elsevier Science Inc.

Pharmacological strategies for improving the efficacy and therapeutic ratio of glucocorticoids in inflammatory lung diseases

Glucocorticoids are widely used to treat various inflammatory lung diseases. Acting via the glucocorticoid receptor (GR), they exert clinical effects predominantly by modulating gene transcription. This may be to either induce (transactivate) or repress (transrepress) gene transcription. However, certain individuals, including those who smoke, have certain asthma phenotypes, chronic obstructive pulmonary disease (COPD) or some interstitial diseases may respond poorly to the beneficial effects of glucocorticoids. In these cases, high dose, often oral or parental, glucocorticoids are typically prescribed. This generally leads to adverse effects that compromise clinical utility. There is, therefore, a need to enhance the clinical efficacy of glucocorticoids while minimizing adverse effects. In this context, a long-acting beta(2)-adrenoceptor agonist (LABA) can enhance the clinical efficacy of an inhaled corticosteroid (ICS) in asthma and COPD. Furthermore, LABAs can augment glucocorticoid-dependent gene expression and this action may account for some of the benefits of LABA/ICS combination therapies when compared to ICS given as a monotherapy. In addition to metabolic genes and other adverse effects that are induced by glucocorticoids, there are many other glucocorticoid-inducible genes that have significant anti-inflammatory potential. We therefore advocate a move away from the search for ligands of GR that dissociate transactivation from transrepression. Instead, we submit that ligands should be functionally screened by virtue of their ability to induce or repress biologically-relevant genes in target tissues. In this review, we discuss pharmacological methods by which selective GR modulators and "add-on" therapies may be exploited to improve the clinical efficacy of glucocorticoids while reducing potential adverse effects.

Cyclin D3 and c-MYC control glucocorticoid-induced cell cycle arrest but not apoptosis in lymphoblastic leukemia cells

Glucocorticoids (GC) induce cell cycle arrest and apoptosis in lymphoblastic leukemia cells. To investigate cell cycle effects of GC in the absence of obscuring apoptotic events, we used human CCRF-CEM leukemia cells protected from cell death by transgenic bcl-2. GC treatment arrested these cells in the G1 phase of the cell cycle due to repression of cyclin D3 and c-myc. Cyclin E and Cdk2 protein levels remained high, but the kinase complex was inactive due to increased levels of bound p27(Kip1). Conditional expression of cyclin D3 and/or c-myc was sufficient to prevent GC-induced G1 arrest and p27(Kip1) accumulation but, importantly, did not interfere with the induction of apoptosis. The combined data suggest that repression of both, c-myc and cyclin D3, is necessary to arrest human leukemia cells in the G1 phase of the cell division cycle, but that neither one is required for GC-induced apoptosis.

mRNA degradation machines in eukaryotic cells

The steady-state levels of mRNAs depend upon their combined rates of synthesis and processing, transport from the nucleus to cytoplasm, and decay in the cytoplasm. In eukaryotic cells, the degradation of mRNA is an essential determinant in the regulation of gene expression, and it can be modulated in response to developmental, environmental, and metabolic signals. This level of regulation is particularly important for proteins that are active for a brief period, such as growth factors, transcription factors, and proteins that control cell cycle progression. The mechanisms by which mRNAs are degraded and the sequence elements within the mRNAs that affect their stability are the subject of this review. We will summarize the current state of knowledge regarding cis-acting elements in mRNA and trans-acting factors that contribute to mRNA regulation decay. We will then consider the mechanisms by which specific signaling proteins seem to contribute to a dynamic organization of the mRNA degradation machinery in response to physiological stimuli.

Transcriptional and post-transcriptional mechanisms of glucocorticoid antiproliferative effects

Glucocorticoids (GCs) are used as immunosuppressive and anti-inflammatory agents in treating organ transplantation rejection, autoimmune diseases, (hematological) cancers, and inflammatory disorders. GCs exert their effects through a multitude of mechanisms, the most significant of which is inhibition of cytokine production, and for some cytokines their effects on target cells. Paradoxically, GCs also upregulate the expression of (pro-inflammatory) high-affinity cytokine receptors on target cells in the face of lost ligand (cytokine) stimulation. GC inhibition of cytokine expression occurs at both transcriptional and post-transcriptional levels. GCs acted transcriptionally by binding their cytosolic receptor (GR), thereby facilitating its nuclear translocation and subsequent binding to the promoter region of cytokine genes on sites compatible with GC response element (GRE) motifs, which in turn directly or indirectly regulated gene expression. In addition to direct DNA binding, GCs acted post-transcriptionally by: (1) antagonism of nuclear factors required for efficient gene expression either directly or through induction of the expression of specific transcription factor antagonists, (2) altered Th lineage development by favouring the generation of (anti-inflammatory) Th2 cells and suppressing the induction or the activity of established (pro-inflammatory) Th1 cells, and (3) stimulating the expression of transforming growth factor (TGF)-beta, an immunosuppressive cytokine which inhibited cytokine production. However, these mechanisms are not mutually exclusive, since GCs may utilize more than one mechanism in exerting their anti-proliferative effect.

Understanding nuclear receptor function: from DNA to chromatin to the interphase nucleus

The regulation of gene expression by steroid receptors is the fundamental mechanism by which these important bioregulatory molecules exert their action. As such, mechanisms utilized by receptors in the modulation of genetic expression have been intensively studied since the first identification of hormone-binding proteins. Although these mechanisms include both posttranscriptional (1) and posttranslational (2) components, the primary level of control involves direct modulation of the rate of transcription, and it is this process that has been the major focus of research in the field.

Glucocorticoid mediated transcriptional repression of c-myc in apoptotic human leukemic CEM cells

Suppression of c-myc has been implicated as a critical event in some glucocorticoid-evoked apoptotic systems. It is therefore of interest to understand the mechanism of glucocorticoid-regulation of the c-myc gene. In the present study, a detailed analysis of dexamethasone (Dex)-evoked regulation of the human c-myc gene in human leukemic CEM-C7 cells has been performed. Dex suppresses c-myc mRNA and immunoreactive protein expression in clone CEM-C7 and subclone CEM-C7-14 cells. Nuclear run-on assays suggested that the regulation occurred at the level of transcription initiation. The half-life of c-myc mRNA was approximately 30 min and its stability was not affected by Dex treatment. In addition, Dex suppressed luciferase gene expression driven by -2052 to +34 bp c-myc promoter in transfected CEM-C7-14 cells. This result further supports that c-myc gene is suppressed by Dex at the transcriptional level in apoptotic human leukemic cells.

Identification of a novel dexamethasone-sensitive RNA-destabilizing region on rat monocyte chemoattractant protein 1 mRNA

Glucocorticoids are potent anti-inflammatory agents widely used in the treatment of human disease. We have previously shown that the inflammatory cytokine monocyte chemoattractant protein 1 (MCP-1) is regulated posttranscriptionally by glucocorticoids in arterial smooth muscle cells (SMC). To elucidate the mechanism mediating this effect, in vitro-transcribed radiolabeled MCP-1 mRNA was incubated with cytoplasmic extracts from SMC and analyzed by gel electrophoresis. Extracts from SMC treated with platelet-derived growth factor (PDGF) did not degrade the transcripts for up to 3 h. In contrast, extracts from cells treated with 1 microM dexamethasone (Dex) alone or in combination with PDGF degraded the probe with a half-life of approximately 15 min. Dex had maximal effect at concentrations above 0.01 microM and was effective on both rat and human MCP-1 transcripts. By deletion analysis, the Dex-sensitive region of the MCP-1 mRNA was localized to the initial 224 nucleotides (nt) at the 5' end and did not involve an AU-rich sequence in the 3' untranslated end. The 224-nt region conferred Dex sensitivity to heterologous mRNA. These studies provide new insights into the molecular mechanisms underlying the effect of glucocorticoids on gene expression.

Radioprotective and antileukemic effects of growth factors in regenerating hematopoietic tissue

We have examined the role of growth factors present in regenerating hematopoietic tissue in the prevention of some radiation-induced effects. These factors, extracted from calf spleen undergoing reparative regeneration, increased the 30-day survival of irradiated mice and partially decreased the incidence of leukemia in surviving mice. The growth factors modified the properties of leukemia cells in vitro, and could suppress residual leukemia cells in vivo. The antileukemic activity of regenerating hematopoietic tissue can be purified to a homogeneous state. The radioprotective activity is associated with the production of regulatory molecules that have been partially characterized. These findings provide evidence that the natural resistance of regenerating hematopoietic tissue (which has an increased number of cell targets for radiation and other damaging agents) results from concurrent local production of a battery of defensive regulatory molecules.

Regulation of gene expression of a binding protein for fibroblast growth factors by retinoic acid

Retinoids are potent regulators of growth and differentiation and have shown promise as chemotherapeutic agents against selected cancers in particular squamous cell carcinoma (SCC). Earlier studies from our laboratory showed that a secreted binding protein for fibroblast growth factors (BP) is expressed at high levels in SCC cell lines and tissue samples. Here we investigate whether retinoids affect BP gene expression in SCC. In six different human SCC cell lines, we found that all-trans-retinoic acid (tRA) down-regulated BP mRNA by 39-89% within 24 h. From this group of cell lines, we selected the ME-180 cell line for more detailed studies of the mechanisms of this regulation. tRA down-regulated BP mRNA in a time- and dose-dependent manner. The effect of tRA was reversible, and BP mRNA returned to control levels within 24 h after removal of tRA. We also measured BP mRNA half-life and performed nuclear run-on experiments to study if tRA down-regulates BP by destabilizing the mRNA and/or by decreasing the rate of transcription. BP mRNA in ME-180 cells is very stable with a half-life of >16 h, and tRA decreased BP mRNA with a half-time of 5 h. Actinomycin D and cycloheximide blocked the tRA effect, suggesting that transcriptional regulation as well as de novo protein synthesis contribute to this post-transcriptional regulation of BP mRNA levels. In addition, tRA decreased the rate of BP gene transcription by 2- to 3-fold within 1 h. We conclude that retinoids down-regulate BP gene expression by post-transcriptional as well as by transcriptional mechanisms.

The effect of cyclic-AMP on the regulation of c-myc expression in T lymphoma cells

Myc is implicated in the control of growth in a variety of cell types. I investigated c-myc gene expression in several lymphoid cell lines to determine the response to cyclic AMP. Cyclic AMP causes a precipitous decline in c-myc message concentration that precedes G1 cell cycle arrest in wild type S49 cells but not in KIN- cells that lack cAMP dependent PKA activity. In wild-type S49 cells washout of cyclic AMP restores c-myc message levels within 2 h but does not relieve the G1 arrest until 10 h later. Transcription runoff studies demonstrate inhibition of both transcriptional initiation and prolongation of initiated transcripts. However, the degree of inhibition is insufficient to explain the absence of detectable myc message suggesting that the predominant effect of cyclic AMP is to destabilize the c-myc message. In contrast to wild-type cells, the "Deathless" mutant S49 cell line is viable when arrested in G1 by exposure to cyclic AMP and has preserved c-myc expression. Thus, in S49 cells down regulation of c-myc expression appears to be associated with loss of viability rather than G1 cell cycle arrest. Interestingly, CEM human T lymphoma cells do not arrest in G1 phase when exposed to cyclic AMP in spite of losing detectable c-myc gene expression. This suggests that in some T lymphoma cells c-myc gene expression may not be necessary for cell cycle progression and proliferation.

Expression of c-Myc in glucocorticoid-treated fibroblastic cells

Glucocorticoids inhibit proliferation of L929 fibroblastic cells in culture. Inhibition of proliferation is reversible and is not associated with changes in the plating efficiency of the cells. Flow cytometric analysis indicates that glucocorticoid-treated cells exhibit a decrease in the percentage of cells with DNA content > 2 N. Thymidine kinase expression is inhibited as cells with 2 N DNA content accumulate. These observations indicate that glucocorticoids arrest proliferation of L929 cells in the G1 phase of the cell cycle. The abundance of c-Myc mRNA does not decrease in glucocorticoid-treated cells, and c-Myc protein content in dexamethasone-treated cells is approximately the same as that detected in mid-log phase cells. Nuclear run-on transcription of c-Myc is not inhibited by glucocorticoids. These observations indicate that glucocorticoid regulation of fibroblastic cell proliferation does not involve inhibition of c-Myc transcription. Although regulation of c-Myc expression is central to the mechanism whereby glucocorticoids regulate proliferation of lymphoid cells, it is clear that different mechanisms must be involved in glucocorticoid regulation of fibroblastic cell proliferation.

Corticosterone-responsive mRNAs in primary rat astrocytes

Glucocorticoids are important in neuronal development, regulation of the hypothalamic-pituitary-adrenal axis, adaptive behavior, and neuronal survival. Glia have receptors for glucocorticoid hormones and thus represent targets for hormone action in the brain. To identify mRNAs that are regulated by corticosterone in primary type 1 rat astrocytes, we have utilized ultra-high resolution giant two-dimensional gel electrophoresis of in vitro translated proteins. Our results reveal 12 in vitro translation products likely representing 10 mRNA species that are regulated by corticosterone. Eleven products are significantly increased and one decreased, most within 3 h of hormone treatment. Inclusion of cycloheximide does not prevent these changes, suggesting that they represent alterations in transcription; however, other mechanisms, such as changes in mRNA stability, cannot be excluded. Two corticosterone-regulated proteins were identified as glucocortin and glutamine synthetase. These two proteins are glucocorticoid-regulated in a variety of cell types, whereas the others appear to be astrocyte-specific. Future identification of these hormone-responsive mRNAs and proteins will help elucidate the molecular basis for glucocorticoid action in the CNS.

Dexamethasone decreases urokinase plasminogen activator mRNA stability in MAT 13762 rat mammary carcinoma cells

The glucocorticoid dexamethasone was observed to decrease urokinase plasminogen activator (uPA) RNA levels from within 1 h of treatment of MAT 13762 mammary adenocarcinoma cells. The drug did not alter the rate of uPA gene transcription in these cells, but decreased the stability of cytoplasmic uPA mRNA transcripts. Results from cycloheximide and actinomycin D experiments indicated that the dexamethasone-mediated reduction in uPA RNA required both new protein and RNA synthesis. Based on these results, we propose that dexamethasone induces a short-lived protein(s) which down-regulates uPA RNA levels post-transcriptionally in these metastatic tumour cells.

Transcriptional down-regulation of c-myc in human prostate carcinoma cells by the synthetic androgen mibolerone

The mechanism of down-regulation of c-myc RNA associated with androgen-induced suppression of the transformed phenotype in the human prostate carcinoma cell line LNCaP was investigated. The synthetic androgen mibolerone (7 alpha-17 alpha-Dimethyl-19-nortestosterone) reversibly inhibits the proliferation of LNCaP cells and, from 12-72 h after hormone addition reduces the level of c-myc transcripts to a few per cent of controls. P1, P2, and P0 c-myc transcripts decline at the same rate, whereas P3 transcripts are much less hormone sensitive. Nuclear run-on analysis revealed that c-myc is down-regulated at the level of transcription initiation in LNCaP cells. The level of c-myc transcripts prevailing in untreated control cells can be restored in androgen-induced cells by excess antiandrogen, indicating the involvement of the androgen receptor in c-myc down-regulation.

Glucocorticoid receptor-mediated suppression of the interleukin 2 gene expression through impairment of the cooperativity between nuclear factor of activated T cells and AP-1 enhancer elements

The immunosuppressant hormone dexamethasone (Dex) interferes with T cell-specific signals activating the enhancer sequences directing interleukin 2 (IL-2) transcription. We report that the Dex-dependent downregulation of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and calcium ionophore-induced activity of the IL-2 enhancer are mediated by glucocorticoid receptor (GR) via a process that requires intact NH2- and COOH-terminal and DNA-binding domains. Functional analysis of chloramphenicol acetyltransferase (CAT) vectors containing internal deletions of the -317 to +47 bp IL-2 enhancer showed that the GR-responsive elements mapped to regions containing nuclear factor of activated T cells protein (NFAT) (-279 to -263 bp) and AP-1 (-160 to -150 bp) motifs. The AP-1 motif binds TPA and calcium ionophore-induced nuclear factor(s) containing fos protein. TPA and calcium ionophore-induced transcriptional activation of homo-oligomers of the NFAT element were not inhibited by Dex, while AP-1 motif concatemers were not stimulated by TPA and calcium ionophore. When combined, NFAT and AP-1 motifs significantly synergized in directing CAT transcription. Such a synergism was impaired by specific mutations affecting the trans-acting factor binding to either NFAT or AP-1 motifs. In spite of the lack of hormone regulation of isolated cis elements, TPA/calcium ionophore-mediated activation of CAT vectors containing a combination of the NFAT and the AP-1 motifs became suppressible by Dex. Our results show that the IL-2-AP-1 motif confers GR sensitivity to a flanking region containing a NFAT element and suggest that synergistic cooperativity between the NFAT and AP-1 sites allows GR to mediate the Dex inhibition of IL-2 gene transcription. Therefore, a Dex-modulated second level of IL-2 enhancer regulation, based on a combinatorial modular interplay, appears to be present.

1,25-Dihydroxyvitamin D3 stimulates both alkaline phosphatase gene transcription and mRNA stability in human bone cells

We have previously reported that 1,25(OH)2D3 stimulated the cellular alkaline phosphatase (ALP) activity and increased the steady-state level of ALP mRNA in a human osteosarcoma cell line (TE-85), under serum-free conditions. To define the molecular mechanism by which 1,25(OH)2D3 acts to stimulate ALP activity, the time courses of the increases in ALP activity and in the steady-state ALP mRNA level in response to 1,25(OH)2D3 were evaluated. 1,25(OH)2D3 progressively increased the steady-state level of ALP mRNA from 5 to 24 h of treatment, at which time a plateau was observed. In contrast, no significant increase in ALP-specific activity was detected until after 10 h of treatment, at which time the activity increased linearly with time up to 72 h. These time courses are consistent with the premise that the increased ALP activity was the result of increased gene expression. Nuclear runoff analysis indicated that the transcription rate of the ALP gene was more than five-fold higher in the 1,25(OH)2D3-treated cells than in the control cells. In addition, it was found that 1,25(OH)2D3 treatment increased ALP mRNA stability. The 1,25(OH)2D3-induced increase in ALP mRNA stability was not due to an interaction of the 1,25(OH)2D3-receptor complex with the ALP mRNA, since the removal of 1,25(OH)2D3 did not abolish its stabilizing effect. In the presence of cycloheximide, the stabilizing effect of 1,25(OH)2D3 was abolished, suggesting that a 1,25(OH)2D3-inducible protein factor was involved. Based on these findings, we have proposed a model in which 1,25(OH)2D3 stimulated ALP activity in human bone cells through mechanisms involving both (1) increased transcription of the ALP gene and (2) increased stability of ALP mRNA, an effect which requires the de novo synthesis of a protein, a putative ALP mRNA "stabilizing factor."

Glucocorticoid receptors in human malignancies: a review

The present knowledge of the human glucocorticoid receptor (hGCR) in primary malignancies is reviewed. It is concluded that hGCR is present in a large number of these tissues; in all tissue specimens of lymphoid malignancies and in varying fractions of the different solid tumors. The hGCR functions as a hormone dependent, specific enhancer interacting protein in mediating the considerable effects of glucocorticoids on growth regulation, both through stimulation and inhibition of expression of the target genes, including other transcription regulation systems. The processes of receptor activation and regulation, as well as the effects of glucocorticoids, are tissue-specific. Subjects for future research are proposed: Establishment of more cell lines and animal models to extend investigation beyond the present concentration on only a few cell lines, especially CEM-C7, application of 'dynamic' assays to cells obtained from patients, in an attempt to predict development of glucocorticoid resistance, and further investigation of the relationships among GCR and growth factors and oncogenes.

A secreted Mr approximately 40,000 glycoprotein specifically induced by glucocorticoids in a rat colon carcinoma cell line

Glucocorticoid hormones are thought to play a role in carcinogenesis, as they regulate cell differentiation and proliferation. We have previously shown that dexamethasone inhibits the growth of a rat colon carcinoma cell line, and induces the secretion of an Mr approximately 40,000 protein. We now report that the synthesis and the release in the culture medium of this protein is stimulated specifically by glucocorticoid agonists, and not by other steroid hormones. The anti-glucocorticoid RU 38486 is inefficient and suppresses the induction of this protein by dexamethasone. Induction is sensitive to actinomycin D, suggesting that regulation may be exerted by altering the rate of mRNA synthesis. Characterization of culture medium from dexamethasone-treated cells revealed that the Mr approximately 40,000 protein is glycosylated, and can be further separated from other secreted proteins by high-performance anion-exchange chromatography.

A negative regulatory element in the human papillomavirus type 16 genome acts at the level of late mRNA stability

A negative regulatory element present in the human papillomavirus type 16 genome has been characterized. Deletion analysis has localized the 5' end of the element to the late region of the genome at the extreme 3' end of the coding region of the L1 open reading frame, around the L1 stop codon, with the element extending into the L1 3' untranslated region. For the cell lines used, the element's function was independent of cell type, tissue, or species of origin, unlike papillomavirus infection, which is very dependent on such factors. By using an mRNA decay assay, we have determined that polyadenylated RNA containing the element is much less stable than polyadenylated RNA lacking the element. This indicates that the element acts as an mRNA instability element. The significance of A-rich, GU-rich, and AUG-rich sequences for the functioning of this human papillomavirus type 16 instability element is discussed.