Glucocorticoid-resistant lymphoma cell variants that contain functional glucocorticoid receptors

Pathophysiology of Glucocorticoid Signaling

Glucocorticoids (GC), such as cortisol or dexamethasone, control various physiological functions, notably those involved in development, metabolism, inflammatory processes and stress, and exert most of their effects upon binding to the glucocorticoid receptor (GR, encoded by NR3C1 gene). GC signaling follows several consecutive steps leading to target gene transactivation, including ligand binding, nuclear translocation of ligand-activated GR complexes, DNA binding, coactivator interaction and recruitment of functional transcriptional machinery. Any step may be impaired and may account for altered GC signaling. Partial or generalized glucocorticoid resistance syndrome may result in a reduced level of functional GR, a decreased hormone affinity and binding, a defect in nuclear GR translocation, a decrease or lack of DNA binding and/or post-transcriptional GR modifications. To date, 26 loss-of-function NR3C1 mutations have been reported in the context of hypertension, hirsutism, adrenal hyperplasia or metabolic disorders. These clinical signs are generally associated with biological features including hypercortisolism without negative regulatory feedback loop on the hypothalamic-pituitary-adrenal axis. Patients had often low plasma aldosterone and renin levels despite hypertension. Only one GR gain-of-function mutation has been described associating Cushing's syndrome phenotype with normal urinary-free cortisol. Some GR polymorphisms (ER22/23EK, GR-9β) have been linked to glucocorticoid resistance and a healthier metabolic profile whereas some others seemed to be associated with GC hypersensitivity (N363S, BclI), increasing cardiovascular risk (diabetes type 2, visceral obesity). This review focuses on the earlier findings on the pathophysiology of GR signaling and presents criteria facilitating identification of novel NR3C1 mutations in selected patients.

Phosphodiesterase 4 inhibitors augment levels of glucocorticoid receptor in B cell chronic lymphocytic leukemia but not in normal circulating hematopoietic cells

Type 4 cyclic AMP (cAMP) phosphodiesterase (PDE4) inhibitors, a class of compounds in clinical development that activate cAMP-mediated signaling by inhibiting cAMP catabolism, offer a feasible means by which to potentiate glucocorticoid-mediated apoptosis in lymphoid malignancies such as B-cell chronic lymphocytic leukemia (B-CLL). In this study, we show that PDE4 inhibitors up-regulate glucocorticoid receptor (GRalpha) transcript levels in B-CLL cells but not T-CLL cells or Sezary cells or normal circulating T cells, B cells, monocytes, or neutrophils. Because GRalpha transcript half-life does not vary in CLL cells treated with the prototypic PDE4 inhibitor rolipram, the 4-fold increase in GRalpha mRNA levels observed within 4 h of rolipram treatment seems to result from an increase in GRalpha transcription. Rolipram treatment increases levels of transcripts derived from the 1A3 promoter to a greater extent than the 1B promoter. Treatment of B-CLL cells with two other PDE4 inhibitors currently in clinical development also augments GR transcript levels and glucocorticoid-mediated apoptosis. Washout studies show that simultaneous treatment with both drug classes irreversibly augments apoptosis over the same time frame that GR up-regulation occurs. Although treatment of B-CLL cells with glucocorticoids reduces basal GRalpha transcript levels in a dose-related manner, cotreatment with rolipram maintained GRalpha transcript levels above baseline. Our results suggest that as a result of their unusual sensitivity to PDE4 inhibitor-mediated up-regulation of GRalpha expression, treatment of B-CLL patients with combined PDE4 inhibitor/glucocorticoid therapy may be of therapeutic benefit in this disease.

Characterization of the MM.1 human multiple myeloma (MM) cell lines: a model system to elucidate the characteristics, behavior, and signaling of steroid-sensitive and -resistant MM cells

Multiple myeloma (MM) is a clonal B-lymphocyte malignancy, which is characterized by the accumulation of terminally differentiated antibody-producing cells in the bone marrow. Because current treatments offer only a median survival of 3 years, investigators continue to search for novel therapeutic strategies to combat the disease. Rational drug design is enhanced by understanding MM cell proliferation and key signaling pathways employed. In addition, a model system for preclinical evaluation of novel therapeutics is critical. Our laboratory has developed MM cell lines to study drug action and resistance, cell proliferation, and apoptosis. These cell lines are widely used in MM research. From a single MM patient, three separate cell lines were established that parallel the progression of the disease. These three cell lines, designated MM1.S, MM1.R(E), and MM1.R(L), can be distinguished on the basis of their sensitivity to steroid hormones such as glucocorticoids (GCs). Utilization of these cell lines to study the etiology of MM, effects of chemotherapeutic agents, and development of clinical resistance, will provide us with vital information for the evolution of new and more efficacious therapeutics. The aim of this review is to summarize the morphological, biochemical, and growth characteristics of these cells, and to review the results from investigations of the MM.1 signaling pathways. This information will enhance the study, treatment, and eventual eradication of MM.

Correlation of membrane glucocorticoid receptor levels with glucocorticoid-induced apoptotic competence using mutant leukemic and lymphoma cells lines

We have studied the presence and functional implications of membrane glucocorticoid receptor (mGR) in several wild type (WT) and mutant mouse lymphoid cell lines (nuclear transfer decrease, NT(-); nuclear transfer increase, NT(i); and receptorless, R(-)). Direct fluorescent antibody staining revealed large aggregates of mGR-specific fluorescing antigens in the plasma membrane of the WT and mGR-enriched (mGR(++)) S-49 cells. While R(-) cells totally lacked mGR, this receptor level was low in NT(-) and NT(i) groups. FACS analysis corroborated these results, showing a approximately 4-10-fold difference between the highest mGR levels (mGR(++)) and the R(-) and NT(i) cells. Membrane extracts were analyzed for mGR by immunoblotting. Multiple receptor forms, ranging in M(r) from 94,000 to > 200,000, were observed in the WT cells, while only smaller peptides (85,000-94,000) were found in NT(-) cells. No detectable immunoreactive bands were identified in either membrane or cytosol immunoprecipitates of NT(i) and R(-) cell groups. Within 48 h post dexamethasone exposure > 98% of WT and mGR(++) S-49 cells underwent apoptosis, compared to 0-30% in the mutant cells, albeit the total receptor number is two to three times higher in NT(i) compared to WT. These results suggest a better correlation between the quantity and quality of mGRs (rather than total cellular GRs) and the ability of glucocorticoids (GCs) to lyse lymphoid cells.

Dysfunctional glucocorticoid receptor with a single point mutation ablates the CCAAT/enhancer binding protein-dependent growth suppression response in a steroid-resistant rat hepatoma cell variant

We used glucocorticoid-resistant and -sensitive hepatoma cell variants to characterize the mechanism of hepatoma cell resistance to the growth inhibitory effects of glucocorticoids. BDS1 hepatoma cells express transcriptionally active glucocorticoid receptors and undergo a stringent G1 cell cycle arrest in response to glucocorticoids that is dependent on the induced expression of the CCAAT/enhancer binding protein alpha (C/EBPalpha) transcription factor. In contrast, EDR1 hepatoma cells, which express normal levels of glucocorticoid receptors, fail to growth arrest or express C/EBPalpha when treated with glucocorticoids. Ectopic expression of wild-type rat glucocorticoid receptors into EDR1 cells restored the growth suppression response, suggesting a defect in the EDR1 receptor. DNA sequence analysis revealed a single point mutation causing a cysteine-to-tyrosine substitution at amino acid position 457 (C457Y-GR) in the zinc finger region of the glucocorticoid receptor that mediates both receptor-DNA and receptor-protein interactions. Glucocorticoid activation of the alpha1-acid glycoprotein (AGP) promoter, a liver acute-phase response gene, requires receptor-DNA binding as well as an interaction with C/EBPalpha. In contrast to the wild-type glucocorticoid receptor, ectopic expression of C/EBPalpha in EDR1 cells, or coexpression of C/EBPalpha along with the C457Y-GR into receptor-deficient EDR3 cells was required to partially restore glucocorticoid responsiveness of the AGP promoter by the EDR1 glucocorticoid receptor. Constitutive expression of the wild-type glucocorticoid receptor, but not the C457Y-GR mutant, was sufficient to restore the glucocorticoid growth suppression response to receptor-deficient EDR3 cells. Thus, we have identified a glucocorticoid-resistant hepatoma cell variant with a single point mutation in the zinc finger region of the glucocorticoid receptor gene that ablates the glucocorticoid growth suppression response and attenuates transcriptional activation of the AGP promoter.

Identification and localization of steroid-binding and nonsteroid-binding forms of the glucocorticoid receptor in the mouse P1798 lymphosarcoma

Glucocorticoid receptors (GCRs) were characterized in sublines of the mouse P1798 lymphosarcoma that are sensitive (S) or resistant (R) to glucocorticoid-mediated apoptosis. Previous work had identified two steroid-binding GCRs in S and R cells: a 97 kDa wild-type GCR in S cells (WT-GCR), and a 45 kDa truncated GCR in R cells (TR-GCR). A third GCR, a 97 kDa nonsteroid-binding GCR (NSB-GCR), was also identified in R cells. Using subcellular fractionation and Western blotting, we now show that in contrast to the WT-GCR which is localized in both the cytoplasm and nucleus of S cells, the NSB-GCR is localized predominantly in R cell nuclei. Moreover, gel filtration chromatography revealed that treatment with 400 mM NaCl and heat did not significantly alter the Stokes radius of the NSB-GCR suggesting that this receptor is not present in a heterooligomeric complex with other proteins. The TR-GCR was localized predominantly in the soluble cytoplasmic fraction but also in the crude membrane fractions of R cell nuclei, suggesting that this receptor is tightly associated with nuclear structures. It was not detected in the soluble nuclear fraction. Unexpectedly, a 45 kDa nonsteroid-binding immunoreactive protein was detected in crude membrane fractions of S cells. These studies describe a complex GCR system in the P1798 lymphosarcoma that necessitates a further consideration of glucocorticoid signaling in S and R cells.

Elevated glutathione S-transferase gene expression is an early event during steroid-induced lymphocyte apoptosis

Based on the finding that glutathione S-transferase Yb1 (GST) gene expression is elevated in the regressing prostate of androgen-ablated rats, we analyzed GST transcript levels during steroid-induced lymphocyte cell death. It was found that GST gene expression was induced in steroid-sensitive cells within 4 hr of dexamethasone treatment, required functional glucocorticoid receptor, and was dose-dependent with regard to hormone. GST expression was not induced in an apoptosis-defective variant that contained normal levels of functional receptor, indicating that GST up-regulation was the result of secondary events that occur during steroid-mediated apoptosis. Using the calcium ionophore A23817 to induce lymphocyte cell death, GST RNA levels were increased in both steroid-sensitive and steroid-resistant cell lines, supporting the conclusion that elevated GST expression was the result of cellular processes associated with apoptosis, rather than a direct consequence of steroid-mediated transcriptional control. The cells were also treated with dibutyryl cAMP to cause cell death; however, this mode of killing did not result in GST up-regulation. Taken together, these results suggest that GST induction in dexamethasone-treated T-lymphocytes occurs early in the steroid-regulated apoptotic pathway and that this may be a marker of calcium-stimulated cell death. Based on the known function of GST as an antioxidant defense enzyme and its transcriptional regulation by reactive oxygen intermediates, we propose that the gene product of a primary GR target gene(s) directly or indirectly effects the redox state of the cell. Thus activation of GST gene expression in apoptotic lymphocytes is likely a indicator of oxidative stress, rather than a required step in the pathway.

William L. McGuire Memorial Symposium. Estrogen and progestin effects in human breast carcinogenesis

The influences of estrogen and progestin on human mammary neoplasia are reviewed with a view to identifying what is known about their effects. Estrogens promote growth of established cancer and pharmacological levels of progestins induce remission. In vivo, highest proliferation of histologically normal mammary epithelium occurs in the progestogenic phase of the menstrual cycle or under the progestogenic influence of oral contraceptives. Little additional hard data exist to indicate whether progestins promote or inhibit human mammary carcinogenesis. Effects on proliferation, steroid receptor content and development are discussed together with interpretation of epidemiological data on risk factors that have hormonal components. Progestins may not be the benign or beneficial agents previously supposed, and there are virtually no data to suggest that they are antiestrogenic. It is hypothesized that carcinogenesis may be accompanied by increased sensitivity to estrogen, which provides a growth advantage to the tumor by maximizing use of the low estrogen concentrations encountered in the postmenopausal state.

A discussion of the roles of oestrogen and progestin in human mammary carcinogenesis

Oestrogens and progestins are important for both the genesis of human breast cancer and growth of those tumours once formed. Their role at different stages of the neoplastic process are reviewed and discussed within the context of a change in sensitivity of epithelial cells during either initiation or promotion stages. Evidence favours, but does not conclusively prove, the view that progestins are the predominant mitogen for normal breast epithelium whilst oestrogen assumes that function in neoplastic epithelium. Alterations in oestrogen receptor levels could provide the key for such a change. There are insufficient data on physiological progestin concentrations to judge their effect on established cancer. Models for steroidal effects on cell proliferation and oestrogen and progestin receptor regulation that are based on endometrial data are not appropriate for breast.

The mechanism for glucocorticoid-resistance in a rat hepatoma cell variant that contains functional glucocorticoid receptor

The mechanism of glucocorticoid resistance has been studied in a rat hepatoma cell variant (6.10.2), which contains low levels of glucocorticoid receptor (GR). These cells seem to have lost all the glucocorticoid-induced transcriptional responses as measured by the lack of induction of expression of stably integrated mouse mammary tumor virus (MMTV) and the endogenous gene tyrosine aminotransferase (TAT), as well as the transcriptional suppression of GR gene expression. Physico-chemical characterization of the GR in the glucocorticoid resistant 6.10.2 cells revealed that the receptor is indistinguishable from the wild-type receptor with regard to size, hormone- and DNA-binding. The levels of the receptor mRNA and the total immunoreactive protein found in 6.10.2 cells were about 20% of those found in wild-type cells. Further analysis of 6.10.2 cells demonstrated that the receptor was indeed biologically functional. Treatment of 6.10.2 cells with 8-bromo-cAMP, which induced the endogenous GR level two-fold, restored responsiveness to glucocorticoids. Secondly, pretreatment of the cells with cycloheximide also led to reacquisition of cellular responsiveness to glucocorticoids. We propose that there exists a "threshold" level of GR, which is required for responsiveness and that under normal culture conditions, the level of GR in 6.10.2 cells is below this threshold. Glucocorticoid responsiveness can be restored by raising the GR level above the threshold with 8-bromo-cAMP or, alternatively, by removing the threshold barrier (repressor protein) with cycloheximide. Finally, the existence of such a repressor protein for MMTV induction was shown by in vivo titration with an isolated negative cis-element from the MMTV promoter.

Growth and development of the axr1 mutants of Arabidopsis

We have recovered eight new auxin-resistant lines of Arabidopsis that carry mutations in the AXR1 gene. These eight lines, together with the 12 lines described in a previous report, define at least five different axr1 alleles. All of the mutant lines have a similar phenotype. Defects include decreases in plant height, root gravitropism, hypocotyl elongation, and fertility. Mutant line axr1-3 is less resistant to auxin than the other mutant lines and has less severe morphological abnormalities. This correlation suggests that the morphological defects are a consequence of a defect in auxin action. To determine whether the altered morphology of mutant plants is associated with changes in cell size or tissue organization, tissue sections were examined using scanning electron microscopy. No clear differences in cell size were observed between wild-type and mutant tissues. However, the vascular bundles of mutant stems were found to be less well differentiated than those in wild-type stems. The auxin sensitivity of rosette-stage plants was determined by spraying plants with auxin solutions. Mutant rosettes were found to be significantly less sensitive to exogenously applied auxin than wild-type rosettes, indicating that the AXR1 gene functions in aerial portions of the plant. Our studies suggest that the AXR1 gene is required for auxin action in most, if not all, tissues of the plant and plays an important role in plant development. Linkage studies indicate that the gene is located on chromosome 1 approximately 2 centiMorgans from the closest restriction fragment length polymorphism.

Growth and development of the axr1 mutants of Arabidopsis

We have recovered eight new auxin-resistant lines of Arabidopsis that carry mutations in the AXR1 gene. These eight lines, together with the 12 lines described in a previous report, define at least five different axr1 alleles. All of the mutant lines have a similar phenotype. Defects include decreases in plant height, root gravitropism, hypocotyl elongation, and fertility. Mutant line axr1-3 is less resistant to auxin than the other mutant lines and has less severe morphological abnormalities. This correlation suggests that the morphological defects are a consequence of a defect in auxin action. To determine whether the altered morphology of mutant plants is associated with changes in cell size or tissue organization, tissue sections were examined using scanning electron microscopy. No clear differences in cell size were observed between wild-type and mutant tissues. However, the vascular bundles of mutant stems were found to be less well differentiated than those in wild-type stems. The auxin sensitivity of rosette-stage plants was determined by spraying plants with auxin solutions. Mutant rosettes were found to be significantly less sensitive to exogenously applied auxin than wild-type rosettes, indicating that the AXR1 gene functions in aerial portions of the plant. Our studies suggest that the AXR1 gene is required for auxin action in most, if not all, tissues of the plant and plays an important role in plant development. Linkage studies indicate that the gene is located on chromosome 1 approximately 2 centiMorgans from the closest restriction fragment length polymorphism.

Isolation and characterization of a mouse L cell variant deficient in glucocorticoid receptors

The growth of mouse L cell fibroblasts is inhibited by glucocorticoids, and we have selected spontaneous glucocorticoid-resistant L cells in culture. One cloned variant exhibits a stable phenotype in the absence of selective conditions. This variant contains no specific glucocorticoid-binding capacity, no immunoreactive glucocorticoid receptor protein, and no detectable glucocorticoid receptor messenger RNA. A glucocorticoid-dependent reporter gene requires exogenous glucocorticoid receptor cDNA and steroid in order to be expressed in this variant. Genomic DNA analysis of the variant cell line indicates that there has been no gross alteration in receptor gene structure. These results suggest that the variant may be deficient in transcription of the glucocorticoid receptor gene.