Regulation of epidermal growth factor receptors by glucocorticoids during the cell cycle in HeLa S3 cells

Ribociclib, a Cdk4/Cdk6 kinase inhibitor, enhances glucocorticoid sensitivity in B-acute lymphoblastic leukemia (B-All)

Dysregulation of the cyclin D1-CDK4/CDK6 complex is frequently observed in almost all human cancer and contributes to aberrant cell proliferation and consequent tumorigenesis. Although many reports described the importance of CDK4/CDK6 in different set of human tumors, only few studies have been performed on leukemia. By gene expression analysis performed in a cohort of childhood patients affected by B-acute lymphoblastic leukemia (B-ALL) we found that both CDK4 and CDK6 are highly expressed. Moreover, reverse phase protein array (RPPA) analysis showed that cyclin D1 levels are higher in patients undergoing relapse. Starting from these considerations, we evaluated the effect of dual inhibition of CDK4/CDK6 in B-ALL and if this inhibition could enhance cytotoxic killing of leukemia cells after combination treatment with dexamethasone. We treated B-ALL cell lines with ribociclib, a highly specific CDK4/6 inhibitor. As expected, treatment with ribociclib induced growth inhibition of B-ALL cell lines, accompanied by strong cell cycle arrest in G1 phase, along with a dose-dependent decrease in phosphorylated retinoblastoma protein. Ribociclib exposure strongly synergizes with dexamethasone in SEM and RCH-ACV, two dexamethasone-resistant cell lines, along with a strong decrease in proliferation and a significant increase in apoptotic cell death. These results were also confirmed on primary cultures derived from bone marrow of pediatric patients affected by B-ALL. Immunoblot analysis showed a significant increase in glucocorticoid receptor (GR) along with some of its target genes, after combined treatment with ribociclib and dexamethasone. Altogether our findings support the concept that pharmacologic inhibition of CDK4/CDK6 may represent a useful therapeutic strategy to control cell proliferation in B-ALL and provide new insight in understanding potential mechanism of glucocorticoid resistance.

Cell cycle phase regulates glucocorticoid receptor function

The glucocorticoid receptor (GR) is a member of the nuclear hormone receptor superfamily of ligand-activated transcription factors. In contrast to many other nuclear receptors, GR is thought to be exclusively cytoplasmic in quiescent cells, and only translocate to the nucleus on ligand binding. We now demonstrate significant nuclear GR in the absence of ligand, which requires nuclear localisation signal 1 (NLS1). Live cell imaging reveals dramatic GR import into the nucleus through interphase and rapid exclusion of the GR from the nucleus at the onset of mitosis, which persists into early G(1). This suggests that the heterogeneity in GR distribution is reflective of cell cycle phase. The impact of cell cycle-driven GR trafficking on a panel of glucocorticoid actions was profiled. In G2/M-enriched cells there was marked prolongation of glucocorticoid-induced ERK activation. This was accompanied by DNA template-specific, ligand-independent GR transactivation. Using chimeric and domain-deleted receptors we demonstrate that this transactivation effect is mediated by the AF1 transactivation domain. AF-1 harbours multiple phosphorylation sites, which are consensus sequences for kinases including CDKs, whose activity changes during the cell cycle. In G2/M there was clear ligand independent induction of GR phosphorylation on residues 203 and 211, both of which are phosphorylated after ligand activation. Ligand-independent transactivation required induction of phospho-S211GR but not S203GR, thereby directly linking cell cycle driven GR modification with altered GR function. Cell cycle phase therefore regulates GR localisation and post-translational modification which selectively impacts GR activity. This suggests that cell cycle phase is an important determinant in the cellular response to Gc, and that mitotic index contributes to tissue Gc sensitivity.

Schlafen-1 causes a cell cycle arrest by inhibiting induction of cyclin D1

Schlafen-1 (Slfn-1), the prototypic member of the Schlafen family of proteins, was described as an inducer of growth arrest in T-lymphocytes and causes a cell cycle arrest in NIH3T3 fibroblasts prior to the G1/S transition. How Slfn-1 exerts its effects on the cell cycle is not currently known. We report that synchronized murine fibroblasts expressing Slfn-1 do not exit G1 when stimulated with fetal calf serum, platelet-derived growth factor BB (PDGF-BB) or epidermal growth factor (EGF). The induction of cyclin D1 by these stimuli was blocked in the presence of Slfn-1 as were all downstream cell cycle processes. Overexpression of cyclin D1 in growth-arrested, Slfn-1-expressing cells induced an increase in cell growth consistent with this protein being the biological target of Slfn-1. Activation of the mitogen-activated protein kinase pathway by EGF or phorbol 12-myristate 13-acetate was unaffected by Slfn-1 expression. PDGF signaling was, however, almost completely blocked. This was due to a lack of PDGF receptor expression in Slfn-1-expressing cells consistent with Slfn-1 blocking the cell cycle in G1 where PDGF receptor expression is normally down-regulated. Finally, overexpression of Slfn-1 inhibited the activation of the cyclin D1 promoter. Slfn-1 therefore causes a cell cycle arrest during G1 by inhibiting induction of cyclin D1 by mitogens.

The origin and functions of multiple human glucocorticoid receptor isoforms

Glucocorticoid hormones are necessary for life and are essential in all aspects of human health and disease. The actions of glucocorticoids are mediated by the glucocorticoid receptor (GR), which binds glucocorticoid hormones and regulates gene expression, cell signaling, and homeostasis. Decades of research have focused on the mechanisms of action of one isoform of GR, GRa. However, in recent years, increasing numbers of human GR (hGR) isoforms have been reported. Evidence obtained from this and other laboratories indicates that multiple hGR isoforms are generated from one single hGR gene via mutations and/or polymorphisms, transcript alternative splicing, and alternative translation initiation. Each hGR protein, in turn, is subject to a variety of posttranslational modifications, and the nature and degree of posttranslational modification affect receptor function. We summarize here the processes that generate and modify various hGR isoforms with a focus on those that impact the ability of hGR to regulate target genes. We speculate that unique receptor compositions and relative receptor proportions within a cell determine the specific response to glucocorticoids. Unchecked expression of some isoforms, for example hGRbeta, has been implicated in various diseases.

Modulation of glucocorticoid receptor function via phosphorylation

The glucocorticoid receptor (GR) is phosphorylated at multiple serine residues in a hormone-dependent manner. It has been suggested that GR phosphorylation affects turnover, subcellular trafficking, or the transcriptional regulatory functions of the receptor, yet the contribution of individual GR phosphorylation sites to the modulation of GR activity remains enigmatic. This review critically evaluates the literature on GR phosphorylation and presents more recent work on the mechanism of GR phosphorylation from studies using antibodies that recognize GR only when it is phosphorylated. In addition, we present support for the notion that GR phosphorylation modifies protein-protein interactions, which can stabilize the hypophosphorylated form of the receptor in the absence of ligand, as well as facilitate transcriptional activation by the hyperphosphorylation of GR via cofactor recruitment upon ligand binding. Finally, we propose that GR phosphorylation also participates in the nongenomic activation of cytoplasmic signaling pathways evoked by GR. Thus, GR phosphorylation is a versatile mechanism for modulating and integrating multiple receptor functions.

Loss of androgen receptor transcriptional activity at the G(1)/S transition

Androgens are essential for the differentiation, growth, and maintenance of male-specific organs. The effects of androgens in cells are mediated by the androgen receptor (AR), a member of the nuclear receptor superfamily of transcription factors. Recently, transient transfection studies have shown that overexpression of cell cycle regulatory proteins affects the transcriptional activity of the AR. In this report, we characterize the transcriptional activity of endogenous AR through the cell cycle. We demonstrate that in G0, AR enhances transcription from an integrated steroid-responsive mouse mammary tumor virus promoter and also from an integrated androgen-specific probasin promoter. This activity is strongly reduced or abolished at the G(1)/S boundary. In S phase, the receptor regains activity, indicating that there is a transient regulatory event that inactivates the AR at the G(1)/S transition. This regulation is specific for the AR, since the related glucocorticoid receptor is transcriptionally active at the G(1)/S boundary. Not all of the effects of androgens are blocked, however, since androgens retain the ability to increase AR protein levels. The transcriptional inactivity of the AR at the G(1)/S junction coincides with a decrease in AR protein level, although activity can be partly rescued without an increase in receptor. Inhibition of histone deacetylases brings about this partial restoration of AR activity at the G(1)/S boundary, demonstrating the involvement of acetylation pathways in the cell cycle regulation of AR transcriptional activity. Finally, a model is proposed that explains the inactivity of the AR at the G(1)/S transition by integrating receptor levels, the action of cell cycle regulators, and the contribution of histone acetyltransferase-containing coactivators.

Activity of the GR in G2 and mitosis

To elucidate the mechanisms mediating the reported transient physiological glucocorticoid resistance in G2/M cell cycle phase, we sought to establish a model system of glucocorticoid-resistant cells in G2. We synchronized various cell lines in G2 to measure dexamethasone (DEX)-induced transactivation of either two endogenous promoters (rat tyrosine aminotransferase and mouse metallothionein I) or the mouse mammary tumor virus (MMTV) promoter stably or transiently transfected. To circumvent the need for synchronization drugs, we stably transfected an MMTV-driven green fluorescent protein to directly correlate DEX-induced transactivation with the cell cycle position for each cell of an asynchronous population using flow cytometry. Surprisingly, all promoters tested were DEX-inducible in G2. Even in mitotic cells, only the stably transfected MMTV promoter was repressed, whereas the same promoter transiently transfected was inducible. The use of Hoechst 33342 for synchronization in previous studies probably caused a misinterpretation, because we detected interference of this drug with GR-dependent transcription independent of the cell cycle. Finally, GR activated a simple promoter in G2, excluding a functional effect of cell cycle-dependent phosphorylation of GR, as implied previously. We conclude that GR itself is fully functional throughout the entire cell cycle, but GR responsiveness is repressed in mitosis due to chromatin condensation rather than to specific modification of GR.

In-vivo particle mediated delivery of mRNA to mammalian tissues: ballistic and biologic effects

Biolistic transmission of mRNA provides transient gene therapy to in vivo organs. This study documents particle mediated mRNA transmission to a solid organ and wound healing model using the mRNA of Green Fluorescent Protein to determine optimal delivery parameters. Renal function, bullet penetration, cellular injury, and Green Fluorescent Protein synthesis were quantified. Chimeric human epidermal growth factor-FLAG epitope cDNA or mRNA was transmitted to wounds in normal or steroid treated animals. Wound bursting strength, human epidermal growth factor-FLAG, and collagen synthesis were determined. Injury and bullet penetration correlated with the delivery velocity and bullet size. Optimal delivery parameters were established which provided widespread Green Fluorescent Protein synthesis. Human epidermal growth factor-FLAG treatment significantly increased collagen content and wound breaking strength in normal and steroid treated animals. FLAG protein synthesis was evident in mRNA treated fascia following treatment. We found the gene gun provides a novel method for efficient, in vivo delivery of mRNA-based therapeutic strategies to mammalian organs with minimal histologic damage allowing transient expression of protein in in vivo target tissues. Co-delivery of Green Fluorescent Protein mRNA may provide a useful positive control to determine effective transmission. Biolistic transmission of human epidermal growth factor-FLAG mRNA provides increased tissue epidermal growth factor levels and accelerates wound healing in normal and steroid exposed animals.

Glucocorticoid receptor phosphorylation: overview, function and cell cycle-dependence

All steroid hormone receptors are phosphorylated and undergo hormone-induced hyperphosphorylation. Most phosphorylated residues identified so far are serines in the N-terminal domain. Other residues and domains may also be phosphorylated, e.g. the estrogen receptor is phosphorylated on tyrosine in the hormone-binding domain. Many sites lie in consensus sequences for proline-directed, cell cycle-associated kinases. In some receptors hyperphosphorylation is induced by hormone antagonists as well as agonists, and leads to new phosphorylated sites. With glucocorticoid receptors, hyperphosphorylation is specific for glucocorticoid agonists, follows receptor activation and produces no new sites. Rate studies suggest that hyperphosphorylation is due to accelerated phosphorylation rather than delayed dephosphorylation. Evidence to date indicates that steroid hormone receptor phosphorylation serves not as an on-off switch but modulates function more subtly. Mutations of phosphorylated sites to alanine have been found to decrease activity by 0 to 90%, depending on mutated site, cell type, reporter gene and hormone concentration. With glucocorticoid receptors, some alanine mutants are up to 75% less active in hormone-induced transactivation of certain reporter genes. They are also inactive in hormone-induced repression of transcription of their own gene and down regulation of the receptor protein. Furthermore, they are much less sensitive to degradation. Both basal phosphorylation and hormone-dependent hyperphosphorylation of these receptors are cell cycle-dependent, basal phosphorylation being low in S phase and high in G2/M and hyperphosphorylation the reverse, suggesting a causal relation to the cell cycle-dependence of glucocorticoid activity reported with several cell lines. Hyperphosphorylation appears to be regulated by basal phosphorylation through negative charge in the N-terminal domain, which in S phase is relatively low and permits hyperphosphorylation, but in G2/M is relatively high and blocks hyperphosphorylation.

Expression and subcellular distribution of the beta-isoform of the human glucocorticoid receptor

Alternative splicing of the human glucocorticoid receptor (hGR) primary transcript produces two highly homologous protein isoforms, termed hGR alpha and hGRbeta, that differ at their carboxy-termini. In contrast to the well characterized hGR alpha isoform, which modulates gene expression in a hormone-dependent fashion, the biological significance of hGRbeta has only recently begun to emerge. We and others have shown that the hGRbeta messenger RNA transcript is widely expressed in human tissues and that the hGRbeta protein functions as a dominant negative inhibitor of hGR alpha in transfected cells. Unfortunately, these initial studies did not determine whether the hGRbeta protein was made in vivo. Such analyses are hindered because available anti-hGR antibodies cannot discriminate between the similarly sized hGR alpha and hGRbeta proteins. Therefore, to investigate the expression of the hGRbeta protein, we have produced an antipeptide, hGRbeta-specific antibody termed BShGR. This antibody was made against the unique 15-amino acid peptide at the carboxy-terminus of hGRbeta and recognizes both the native and denatured conformations of hGRbeta, but does not cross-react with hGR alpha. Using BShGR on Western blots and in immunoprecipitation experiments, we detected the hGRbeta protein in a variety of human cell lines and tissues. Immunocytochemistry was then performed with BShGR on HeLa S3 and CEM-C7 cells and on tissue sections prepared from lung, thymus, and liver to assess the cellular and subcellular distribution of hGRbeta. In all immunopositive cells, hGRbeta was found in the nucleus independent of glucocorticoid treatment. Within tissues, the hGRbeta protein was expressed most abundantly in the epithelial cells lining the terminal bronchiole of the lung, forming the outer layer of Hassall's corpuscle in the thymus, and lining the bile duct in the liver. As a potential in vivo inhibitor of hGR alpha activity, expression of hGRbeta may be an important factor regulating target cell responsiveness to glucocorticoids.

Cell cycle regulation of membrane glucocorticoid receptor in CCRF-CEM human ALL cells: correlation to apoptosis

The human leukemic cell line (CCRF-CEM) and a subline enriched for the plasma membrane-resident glucocorticoid receptor (mGR) were studied for the influence of the cell cycle on the expression and function of mGR. Three synchronization procedures (double thymidine, colcemid, and combined thymidine-colcemid blocks) were used. Fluorescent microscopy and flow cytometry simultaneously assessed antibody-tagged mGR and DNA. In addition, mGR was quantitated and characterized by immunoprecipitation and immunoblotting. Apoptosis was assayed by DNA fragmentation (TUNEL assay) and by cell survival (trypan blue exclusion). All synchronization procedures demonstrated that progression from DNA replication (S) to the second growth phase and mitosis (G2/M) leads to cells having the highest levels of mGR expression and being highly glucocorticoid sensitive in the apoptosis assays: 32 and 80% sensitivity of wild type and mGR-enriched cells, respectively, compared with 12 and 30% sensitivity in asynchronous cells. Therefore, mGR expression appears to be cell cycle regulated, with its highest expression at late S-G2/M, when the cells are most sensitive to the lymphocytolytic effects of glucocorticoids.

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.

Selectivity of cell cycle regulation of glucocorticoid receptor function

The restricted expression of some genes to distinct stages of the cell cycle is often brought about through alterations in the activity and/or abundance of specific transcription factors. Many cells have been shown to be unresponsive to glucocorticoid hormone action during the G2 phase of the mammalian cell cycle, suggesting that some activities of the glucocorticoid receptor (GR), a ligand-activated transcription factor, are subjected to cell cycle control. We show here that GR insensitivity in G2 is selective, affecting receptor-mediated transactivation from a simple glucocorticoid response element, but not repression from a composite glucocorticoid response element. Since glucocorticoid-dependent down-regulation of GR protein levels is also unaffected in G2, distinct activities of the receptor that participate in this homologous down-regulation must be operating as effectively in G2-synchronized cells as in asynchronous cells. Finally, the phosphorylation state of the GR is altered in G2-synchronized cells reflecting, in part, both site-specific phosphorylation and dephosphorylation events. These results suggest that, while GR may be a target for cell cycle regulated kinases and phosphatases, the resulting changes in receptor phosphorylation have an impact only on selected GR functions.

Charge heterogeneity in wildtype and variant glucocorticoid receptors

Two-dimensional electrophoresis was used to examine charge heterogeneity in glucocorticoid receptors (GCRs) from sublines of the thymic-derived, mouse P1798 lymphosarcoma which were sensitive (S) or resistant (R) to glucocorticoid-mediated apoptosis. Previous work had identified the 97 kDa wildtype GCR (WT-GCR) in S cells and two variant GCRs in R cells: a 45 kDa, steroid-binding truncated GCR (TR-GCR), and a 97 kDa non steroid-binding GCR (NSB-GCR). Using denaturing isoelectric focusing, we now show that S cells as well as adult mouse thymus gland also express the NSB-GCR at pI 5.6 in addition to the WT-GCR which resolves between pH 5.9-7.1. Thus, the NSB-GCR is detected in steroid-sensitive cells and is not unique to R cells. Separation of receptors by native isoelectric focusing suggested that the TR-GCR in R cells resolved at a single, high pI (8.1) relative to the WT-GCR which resolved in a broad range (pI 5.8-8.0). The high pI of the TR-GCR may alter its functional activity thereby contributing to the resistance phenotype.

Glucocorticoids enhance the potency of Schwann cell mitogens

Previous studies have documented that cultured Schwann cells require serum-containing medium to respond maximally to mitogens. We now report that Schwann cells are able to proliferate to a mitogenic response in a serum-free defined medium termed oligodendrocyte defined media (ODM). Glucocorticoids are the essential component of ODM which allow Schwann cell proliferation in the serum-free medium. Charcoal treatment of the fetal calf serum decreases the mitogenic potency of the axolemma-enriched fraction (AEF) by 50%. The addition of 2 microM hydrocortisone to charcoal-treated fetal calf serum restores 75% of the lost mitogenicity. These observations are consistent with the view that glucocorticoids present in fetal calf serum are potent co-mitogens essential for AEF-induced Schwann cell proliferation. The synthetic glucocorticoid, dexamethasone, is a more potent co-mitogen than hydrocortisone, with a maximal effect at concentrations less than 10 nM. In contrast, other steroids including aldosterone, progesterone, testosterone, and 17 beta-estradiol have no effect on enhancing the mitogenic response of Schwann cells to the AEF. The glucocorticoid antagonists RU 486 and dehydroepiandrosterone (DHEA), but not the antiestrogenic compound tamoxifen, block AEF-induced Schwann cell proliferation. These results suggest that glucocorticoid-induced Schwann cell proliferation is mediated through a glucocorticoid receptor (GR) mechanism. We detected immunoreactivity to the GR in the cytoplasm, but not in the nuclei of Schwann cells grown in ODM lacking dexamethasone. The addition of 100 nM dexamethasone to these cultures resulted in immunoreactivity in the nucleus. This data suggests that glucocorticoids working through the GR are potent co-mitogens for Schwann cell proliferation.

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.

Effects of dexamethasone on the growth and epidermal growth factor receptor expression of the OVCA 433 ovarian cancer cells

We studied the correlation between dexamethasone (Dex) induced growth effects and modulation of epidermal growth factor receptor (EGFR) expression in OVCA 433 ovarian cancer cells. These cells express specific high and low affinity 125I-EGF binding sites and are growth stimulated by EGF. Dex exhibits mitoinhibitory effects by recruiting OVCA 433 cells in the G0-G1 phase of the cycle, but increases the number of both the high and the low affinity EGFR in a dose dependent manner. The maximal EGFR expression increase occurs after 24 h of Dex treatment consistently with Northern blot studies. The mitogenic activity of EGF in OVCA 433 cells is not affected by the presence of Dex. Moreover Dex growth inhibition occurs in JA1 cells, an ovarian cancer cell line which expresses unfunctional EGFR and which is unresponsive to EGF. Our results indicate that the Dex induced growth effects occur independently of EGFR expression.

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.

Effect of glucocorticoid on epidermal growth factor receptor in human salivary gland adenocarcinoma cell line HSG

Human salivary gland adenocarcinoma (HSG) cells treated with 10(-6) M triamcinolone acetonide for 48 h exhibited a 1.7- to 2.0-fold increase in [125I]human epidermal growth factor (hEGF) binding capacity as compared with untreated HSG cells. Scatchard analysis of [125I]EGF binding data revealed that the number of binding sites was 83,700 (+/- 29,200) receptors/cell in untreated cells and 160,500 (+/- 35,500) receptors/cell in treated cells. No substantial change in receptor affinity was detected. The dissociation constant of the EGF receptor was 0.78 (+/- 0.26).10(-9) M for untreated cells, whereas it was 0.93 (+/- 0.31).10(-9)M for treated cells. The triamcinolone acetonide-induced increase in [125I]EGF binding capacity was dose-dependent between 10(-9) and 10(-6)M, and maximal binding was observed at 10(-6)M. EGF receptors on HSG cells were affinity-labeled with [125I]EGF by use of the cross-linking reagent disuccinimidyl suberate (DSS). The cross-linked [125I]EGF was 3-4% of the total [125I]EGF bound to HSG cells. The affinity-labeled EGF receptor was detected as a specific 170 kDa band in the autoradiograph after SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Densitometric analysis revealed that triamcinolone acetonide amplified the intensity of this band 2.0-fold over that of the band of untreated cells. EGF receptor synthesis was also measured by immunoprecipitation of [3H]leucine-labeled EGF receptor protein with anti-hEGF receptor monoclonal antibody. Receptor synthesis was increased 1.7- to 1.8-fold when HSG cells were treated with 10(-8)-10(-6)M triamcinolone acetonide for 48 h. When the immunoprecipitated, [35S]methionine-pulse-labeled EGF receptor was analyzed by SDS-PAGE and fluorography, the newly synthesized EGF receptor was detected at the position of 170 kDa; and treatment of HSG cells with triamcinolone acetonide resulted in a 2.0-fold amplification of this 170 kDa band. There was no significant difference in turnover rate of EGF receptor between treated and untreated HSG cells. These results demonstrate that the triamcinolone acetonide-induced increase in [125I]EGF binding capacity is due to the increased synthesis of EGF receptor protein in HSG cells.

Dexamethasone-mediated potentiation of P450IA1 induction in H4IIEC3/T hepatoma cells is dependent on a time-consuming process and associated with induction of the Ah receptor

The synergistic effect of dexamethasone (DEX) and polycyclic aromatic hydrocarbons on the induction of cytochrome P450IA1 (P450IA1) was examined in H4IIEC3/T Reuber hepatoma cells. P450IA1 activity was determined by the hydroxylation of benzo[a]pyrene (AHH) and deethylation of 7-ethoxyresorufin (EROD). The amount of Ah receptor, i.e. the specific cytosolic binding protein of 3-methylcholanthrene or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in H4IIEC3/T cells was characterized and quantitated by high performance gel filtration. Benz[a]anthracene and TCDD induced AHH and EROD activities, respectively, about 20-fold within 4 h. The increase was about 100-fold when cells were pretreated with DEX. The glucocorticoid alone induced P450IA1 activities 3-4 fold. DEX elicited half maximum AHH induction at a concentration of 20 nM in the presence or absence of benz[a]anthracene. Maximal potentiation of AHH induction required treatment with DEX for at least 32 h prior to the exposure to benz[a]anthracene. Treatment of H4IIEC3/T cells with DEX for 20 h caused a 2-3-fold increase in the amount of Ah receptor. The results suggest that the synergistic effect of DEX and polycyclic aromatic hydrocarbons on P450IA1 induction involves a time-consuming process which may consist of the synthesis or modification of a factor, possibly the Ah receptor.

Epidermal growth factor (EGF) prevents methylprednisolone-induced inhibition of wound healing

Subcutaneously implanted cylindrical hollow viscose cellulose sponges were used to study the effect of locally applied epidermal growth factor (EGF) on methylprednisolone-induced inhibition of granulation tissue formation in rats. In in vivo studies the sponges were treated immediately after implantation with a single injection of 2 mg (approximately 1.7 x 10(-3) M) of depot methylprednisolone or with its carrier solution only. Thereafter the implants were injected daily with 5 micrograms of EGF or with its carrier solution 0.1% albumin for 7 days. Methylprednisolone treatment decreased the accumulation of nucleic acids, collagen, and glycosaminoglycans in the developing granulation tissue. After daily injections of EGF the concentrations of these tissue components returned close to the control values. In cultures of rat granulation tissue fibroblasts, 10(-4) M and 10(-3)M methylprednisolone decreased collagen synthesis by 41 and 81% from the control level, respectively. In the presence of methylprednisolone EGF treatment could not increase collagen synthesis of fibroblasts. Methylprednisolone treatment resulted in a dose-dependent reduction in pro alpha 1(I) collagen mRNA levels, which was partially inhibited by low EGF concentrations (1 and 10 ng/ml). In the presence of methylprednisolone all concentrations of EGF increased fibronectin mRNA levels in a dose-dependent manner. It is concluded that EGF treatment can prevent the inhibitory effect of methylprednisolone on wound healing by stimulating fibroblast proliferation but does not stimulate collagen synthesis per cell.

A simplified solid-phase assay for the quantitation of native membrane proteins. Application to the measurement of EGF receptor induction by dexamethasone

Direct application of membrane fractions to a nitrocellulose support without previous solubilization, denaturation or fixation of the antigen permits the quantitation of antibodies binding to denaturation sensitive epitopes. In experiments reported here, this method was used to determine the rate of EGF receptor expression on squamous carcinoma cell lines. The assay revealed that dexamethasone treatment leads to an eight-fold increase of EGF receptor protein expression on C4-I cervical carcinoma cells. The data, when compared to the results obtained in immunoprecipitation experiments, suggest that this simple protocol yields reliable and precise quantitative data. The simplicity of the method permits simultaneous testing of large sample numbers with various antibodies.

Glucocorticoids increase the length of the G2 and M phases of the HeLa S3 cell cycle

Glucocorticoids increase the size and content of RNA and protein in randomly growing HeLa S3 cells; however, the rates of growth and macromolecule synthesis are not effected. In synchronized populations of HeLa S3 cells, addition of 10(-7) M dexamethasone at the G1/S border causes a 1 h delay in cell division. The length of the S phase, determined in synchronized cells by incorporation of [3H] thymidine into acid precipitable material, is unaffected by glucocorticoids. Thus, the 1 h prolongation occurs during the G2/M phases, when the cells are largest, and accounts for the increase in the size and content of RNA and protein. As with the induction of specific proteins, the events producing the G2 effects are initiated during the S phase of the cell cycle. Thus, the HeLa S3 cells contain a G2 regulatory point, which is influenced by glucocorticoids.

Analysis of the glucocorticoid antagonist action of dexamethasone 21-mesylate in HeLa S3 cells

Several properties of human glucocorticoid receptors complexed to the synthetic glucocorticoid agonists dexamethasone (DEX) and triamcinolone acetonide (TA) and the antagonist dexamethasone 21-mesylate (DM) are compared in an attempt to define the mode of action of DM. Both DEX and TA induce an increase in alkaline phosphatase activity in HeLa S3 cells. Not only is DM without effect on alkaline phosphatase activity at concentrations as great as 10(-7) M, it blocks the action of DEX and TA on enzyme induction, thus acting as a pure antagonist in this system. DM-receptor complexes, like agonist-receptor complexes, are recovered in the cytosol when cells are incubated with ligand at 0 degrees C but are recovered from the nucleus when incubation is shifted to 37 degrees C, suggesting that activation of the antagonist-receptor complex occurs in vivo. The molecular species that undergoes this temperature-dependent shift from the cytosolic compartment to the nuclear compartment exhibits saturable binding to the antagonist. Both the cytosolic and nuclear species exhibit a relative molecular mass of approximately equal to 94,000 Daltons when analysed by SDS-polyacrylamide gel electrophoresis. Receptors labeled in intact cells with [3H]DM at 0 degrees C sediment at approximately 8S in sucrose gradients, shifting to 4S when the gradients contain 0.4 M KCl. DEX- and TA-labeled receptors show the same sedimentation behavior, which has been accepted as one criterion of receptor subunit dissociation, or activation.