Glucocorticoid receptors in human malignancies: a review

Therapeutic targeting of pancreatic cancer stem cells by dexamethasone modulation of the MKP-1-JNK axis

Postoperative recurrence from microscopic residual disease must be prevented to cure intractable cancers, including pancreatic cancer. Key to this goal is the elimination of cancer stem cells (CSCs) endowed with tumor-initiating capacity and drug resistance. However, current therapeutic strategies capable of accomplishing this are insufficient. Using in vitro models of CSCs and in vivo models of tumor initiation in which CSCs give rise to xenograft tumors, we show that dexamethasone induces expression of MKP-1, a MAPK phosphatase, via glucocorticoid receptor activation, thereby inactivating JNK, which is required for self-renewal and tumor initiation by pancreatic CSCs as well as for their expression of survivin, an anti-apoptotic protein implicated in multidrug resistance. We also demonstrate that systemic administration of clinically relevant doses of dexamethasone together with gemcitabine prevents tumor formation by CSCs in a pancreatic cancer xenograft model. Our study thus provides preclinical evidence for the efficacy of dexamethasone as an adjuvant therapy to prevent postoperative recurrence in patients with pancreatic cancer.

Influence of short-term glucocorticoid therapy on regulatory T cells in vivo

BACKGROUND

Pre- and early clinical studies on patients with autoimmune diseases suggested that induction of regulatory T(T(reg)) cells may contribute to the immunosuppressive effects of glucocorticoids (GCs).

OBJECTIVE

We readdressed the influence of GC therapy on T(reg) cells in immunocompetent human subjects and naïve mice.

METHODS

Mice were treated with increasing doses of intravenous dexamethasone followed by oral taper, and T(reg) cells in spleen and blood were analyzed by FACS. Sixteen patients with sudden hearing loss but without an inflammatory disease received high-dose intravenous prednisolone followed by stepwise dose reduction to low oral prednisolone. Peripheral blood T(reg) cells were analyzed prior and after a 14 day GC therapy based on different markers.

RESULTS

Repeated GC administration to mice for three days dose-dependently decreased the absolute numbers of T(reg) cells in blood (100 mg dexamethasone/kg body weight: 2.8±1.8×10(4) cells/ml vs. 33±11×10(4) in control mice) and spleen (dexamethasone: 2.8±1.9×10(5)/spleen vs. 95±22×10(5)/spleen in control mice), which slowly recovered after 14 days taper in spleen but not in blood. The relative frequency of FOXP3(+) T(reg) cells amongst the CD4(+) T cells also decreased in a dose dependent manner with the effect being more pronounced in blood than in spleen. The suppressive capacity of T(reg) cells was unaltered by GC treatment in vitro. In immunocompetent humans, GCs induced mild T cell lymphocytosis. However, it did not change the relative frequency of circulating T(reg) cells in a relevant manner, although there was some variation depending on the definition of the T(reg) cells (FOXP3(+): 4.0±1.5% vs 3.4±1.5%*; AITR(+): 0.6±0.4 vs 0.5±0.3%, CD127(low): 4.0±1.3 vs 5.0±3.0%* and CTLA4+: 13.8±11.5 vs 15.6±12.5%; * p<0.05).

CONCLUSION

Short-term GC therapy does not induce the hitherto supposed increase in circulating T(reg) cell frequency, neither in immunocompetent humans nor in mice. Thus, it is questionable that the clinical efficacy of GCs is achieved by modulating T(reg) cell numbers.

Gastrin-releasing peptide links stressor to cancer progression

INTRODUCTION

Gastrin-releasing peptide (GRP) plays an important role in cancer growth and metastasis; however, the mechanisms of how GRP affects cancer progression are not well understood. Recent studies revealed that chronic stress is a major risk factor for cancer progression, and this effect may be mediated by GRP. In this review, we will discuss the mechanisms and implications of GRP linking stressor to cancer progression.

MATERIALS AND METHODS

We retrieved the studies of the relationship between GRP, stress and cancers through PubMed using systematic methods to search, select, and evaluate the findings.

RESULTS

The results suggested that GRP can mediate the effects of stress on cancers at systemic, tissue and cellular levels: Stress elicits the secretion of GRP in the brain and GRP in turn activates the stress response pathways resulting in an elevation of stress hormones and GRP in the plasma and tissues. GRP in synergy with stress hormones stimulates the growth and invasion of cancer cells by suppressing the anti-tumor immune function and directly activating the pro-proliferative and pro-migratory signaling pathways in cancer cells.

CONCLUSION

GRP is a multi-functional peptide, which acts as a stress mediator as well as a growth factor linking stressor to cancer progression. GRP and its high-affinity receptor are useful targets for the diagnosis and treatment of cancers.

Psychological aspect of cancer: From stressor to cancer progression

Substantial evidence indicates that psychological stress can influence the incidence and progression of cancers, and adequate psychotherapies are beneficial to cancer patients. Recently, the mechanisms responsible for the effects of psychological stress on cancer cells have been extensively investigated at the systemic, biochemical and molecular levels. Accumulating data indicate that the effects of psychological stress on cancer cells are mainly mediated by key stress-related mediators and their corresponding receptors in multi-fold pathways: chronic stressors act on the paraventricular nucleus and the suprachiasmatic nuclei. The effects are then transmitted through the sympathetic nervous system and the hypothalamic-pituitary-adrenal axis, amplified by the unchecked release of stress-related mediators and altered behaviors. These mediators act as immunosuppressors or mitogens in the tumor microenvironment. The converging effects of psychological stress on cancer cells finally signal through receptors of the stress mediators and cytokines to activate the intracellular pro-proliferative and pro-migratory signaling pathways, and reset the molecular clock in tumor cells. Understanding these action mechanisms of psychological stress in promoting the growth and invasion of cancer cells is crucial for devising effective interventions.

Glucocorticoid-mediated apoptosis resistance of solid tumors

More than a quarter of a century ago, the phenomenon of glucocorticoid-induced apoptosis in the majority of hematological cells was first recognized. More recently, glucocorticoid-induced antiapoptotic signaling associated with apoptosis resistance towards cytotoxic therapy has been identified in cells of epithelial origin, most of malignant solid tumors and some other tissues. Despite these huge amounts of data demonstrating differential pro- and anti-apoptotic effects of glucocorticoids, the underlying mechanisms of cell type-specific glucocorticoid signaling are just beginning to be described. This review summarizes our present understanding of cell type-specific pro- and anti-apoptotic signaling induced by glucocorticoids. We shortly introduce mechanisms of glucocorticoid resistance of hematological cells. We highlight and discuss the emerging molecular evidence of a general induction of survival signaling in epithelial cells and carcinoma cells by glucocorticoids. We give a summary of our current knowledge of decreased proliferation rates in response to glucocorticoid pre- and combination treatment, which are suspicious to be involved not only in protection of normal tissues, but also in protection of solid tumors from cytotoxic effects of anticancer agents.

Dexamethasone induces rapid tyrosine-phosphorylation of ZAP-70 in Jurkat cells

Steroid hormones are known to mediate rapid non-genomic effects occurring within minutes, besides the classical genomic actions mediated by the nuclear translocation of the cytoplasmic glucocorticoid receptor (GR). The glucocorticoid hormone (GC) has significant role in the regulation of T-cell activation; however, the cross-talk between the GC and T-cell receptor (TcR) signal transducing pathways are still to be elucidated. We examined the rapid effects of GC exposure on in vitro cultured human T-cells. Our results showed that Dexamethasone (DX), a GC analogue, when applied at high dose (10 microM), induced rapid (within 5 min) tyrosine-phosphorylation events in Jurkat cells. Short DX pre-treatment strongly inhibited the tyrosine-phosphorylation stimulated by CD3 cross-linking. Furthermore, we also investigated the phosphorylation status of ZAP-70, an important member of tyrosine kinase mediated signalling pathway of TcR-elicited T-cell activation. Here, we demonstrate that high dose DX induced a rapid ZAP-70 tyrosine-phosphorylation in Jurkat T-cells. DX-induced ZAP-70 phosphorylation could be inhibited by RU486 (GR antagonist), suggesting that this process was GR mediated. DX-induced ZAP-70 phosphorylation did not occur in the absence of active p56-lck as examined in the p56-lck kinase-deficient Jurkat cell line JCaM1.6. Our results show that DX, at a high dose, can rapidly influence the initial tyrosine-phosphorylation events of the CD3 signalling pathway in Jurkat cells, thereby modifying TcR-derived signals. Lck and ZAP-70 represent an important molecular link between the TcR and GC signalling pathways.

Isolation and characterization of genes associated with the anti-tumor activity of glucocorticoids

Treatment of ST1 rat glioma cells with glucocorticoid hormones leads to complete reversion of their transformed phenotype and loss of their tumorigenic potential. In order to study the molecular basis of the anti-tumor activity of these hormones, we isolated glucocorticoid-regulated cDNA sequences associated with ST1 cells' phenotypic reversion, using suppression subtractive hybridization (SSH). DNA sequencing of the subtracted cDNA pool, cloned into the pBluescript vector, revealed three widely expressed, well known negative growth regulators, namely, thrombospondin 1, cyclin G and tyrosine phosphatase CL100, as primary targets of glucocorticoid hormones. Additionally, a gene recently described in human brain, NRP/B (nuclear restricted protein in brain) that associates with p110Rb in induction of neuronal differentiation and a new truncated transcript of the tenascin-X gene family, are also shown to be up-regulated by glucocorticoids. The products of these genes are strong candidates to be important players in glucocorticoids anti-tumor activity.

Glucocorticosteroid therapy in childhood acute lymphoblastic leukemia

Treatment of childhood acute lymphoblastic leukemia has included glucocorticosteroids for almost 50 years. Glucocorticoids are the subject of renewed interest. In one randomized trial, deferral of glucocorticosteroids from the initial month of induction therapy to the second month of therapy decreased event free survival despite preservation of remission induction rate. Dexamethasone in induction and maintenance provides a better event free survival than prednisone for standard risk patients in an isotoxic comparison even though all patients received dexamethasone in Delayed Intensification (protocol II). In a third report, patients with prior glucocorticosteroid therapy who achieved remission with subsequent multiagent therapy had a relapse rate similar to that of patients in second remission after failure of multiagent therapy. In vitro and in vivo response of leukemic cells to glucocorticosteroids is highly predictive of outcome. At relapse, loss of in vitro sensitivity to glucocorticosteroids is common and out of proportion to the loss of sensitivity to other agents. Glucocorticoid induced cell kill does not require p53 function. Investigation of leukemic cell lines finds that glucocorticosteroid resistance is most commonly linked to altered receptor number or function. Not all ligands are equivalent. Cortivazol, a pyrazolosteroid, may bind to altered receptor in some cases and induce apoptosis in dexamethasone resistant leukemic cells. Host response to exogenous glucocorticosteroid also varies. Associations between host sensitivity, disease sensitivity, and glucocorticosteroid side effects like avascular necrosis of bone remain to be investigated.

Glucocorticoid receptors and growth inhibitory effects of dexamethasone in human lung cancer cell lines

Expression of glucocorticoid receptors (GR) and growth effects of dexamethasone and the antiglucocorticoid RU-486 were investigated in six cell lines originating from small cell lung cancer (SCLC) and 13 cell lines from non-small cell lung cancer (NSCLC; four adenocarcinoma, four squamous cell carcinoma, four large cell carcinoma and one mesothelioma). All cell lines contained specific and saturable binding sites for the synthetic glucocorticoid dexamethasone, as determined by whole cell assays and by cytosolic receptor assays. The presence of GRs in the carcinoma cells was confirmed by immunocytochemistry. In NSCLC cell lines, GRs were present in large amounts (37-638 fmol/mg cytosolic protein). In SCLC cell lines, GRs were also detectable but in considerably lower concentrations. Growth inhibitory effects of dexamethasone were seen in the cultures of two squamous cell carcinoma lines (EPLC-32M1 and NCI-H157), one adenocarcinoma line (A-549), one large cell carcinoma cell line (LCLC-97TM1) and the cell line of a mesothelioma (MSTO-211H). All cell lines responsive to dexamethasone had high GR concentrations (> or = 164 fmol/mg cytosolic protein). The antiglucocorticoid RU-486 was virtually inactive when administered alone but was able to block the growth-inhibitory effect of dexamethasone. The results indicate that glucocorticoids may inhibit the progression of individual non-small cell lung carcinoma.

Human small cell lung cancer cell lines expressing the proopiomelanocortin gene have aberrant glucocorticoid receptor function

Some human small cell lung carcinomas (SCLC) secrete proopiomelanocortin (POMC) derived peptides, but in contrast to the pituitary, glucocorticoids fail to inhibit this hormone production. We have previously described an in vitro model using human SCLC cell lines that express POMC and are resistant to glucocorticoids. We have now identified the glucocorticoid receptor (GR) in the SCLC cell line COR L24 using a whole cell ligand binding assay (Kd = 5.7 nM; Bmax = 11 fmol/million cells), while another cell line, DMS 79, lacked significant glucocorticoid binding. To analyze GR function both positive (GMCO) and negative (TRE)3-tkCAT), glucocorticoid-regulated reporter gene constructs were transfected into COR L24 cells. In the SCLC cell line, neither hydrocortisone nor dexamethasone (500-2,000 nM) significantly induced chloramphenicol acetyltransferase expression from GMCO; in addition, they did not suppress chloramphenicol acetyltransferase expression from (TRE)3-tkCAT. Similar results were obtained with two other POMC-expressing SCLC cell lines. Expression of wild type GR in COR L24 cells restored glucocorticoid signaling, with marked induction of GMCO reporter gene expression by dexamethasone (9,100 +/- 910%; n = 3), and an estimated EC50 of 10 nM. This failure of the GR explains the resistance of the POMC gene to glucocorticoid inhibition and may have implications for cell growth in SCLC.

Glucocorticoid resistance in childhood leukemia

Glucocorticoids (GC) are being used in the treatment of childhood leukemia for several decades, most successfully in newly diagnosed acute lymphoblastic leukemia (ALL). However, GC resistance is seen in 10-30% of untreated ALL patients, and is much more frequent in relapsed ALL and in acute nonlymphoblastic leukemia (ANLL). Sensitivity or resistance to GC can be measured using a cell culture drug resistance assay. For this purpose, we use the colorimetric methyl-thiazol-tetrazolium (MTT) assay. We have shown that GC resistance in childhood leukemia is related to clinical and cell biological features, and to the clinical outcome after multi-drug chemotherapy. These results are summarized in this review. In addition, we describe the apoptotic 'cell-lysis pathway' by which GC exert their antileukemic activity. This description provides a model to discuss the mechanisms of GC resistance, and to summarize the relevant literature. Possible levels of resistance relate to the diffusion of GC through the cell membrane, binding to the GC receptor (GCR), activation of the GC-GCR complex, translocation of the complex into the nucleus, binding to DNA, endonuclease-mediated DNA fragmentation, and DNA repair. A low number of GCR has been shown to be the cause of resistance in some children with ALL. However, GC resistance is likely to be caused at the post-receptor level in most leukemias. Unfortunately, there is still a lack of knowledge relating to the clinical relevance of mechanisms of GC resistance at the post-receptor level. Studies on the mechanisms of GC resistance other than those directly related to the GCR should be initiated, especially if patient material is used, as the results might indicate ways to circumvent or modulate GC resistance. A further increase in our knowledge regarding the relation between GC resistance and patient and cell biological features, the clinical relevance of GC resistance, and the mechanisms of GC resistance in leukemia patients, may contribute to further improvement in the results of GC therapy in leukemia.

Glucocorticoid responsive ACTH secreting bronchial carcinoid tumours contain high concentrations of glucocorticoid receptors

Cushing's syndrome due to a bronchial ACTH secreting carcinoid tumour may be difficult to distinguish from a pituitary microadenoma (corticotrophinoma) causing Cushing's disease, since in both disorders ACTH secretion may be responsive to glucocorticoids. Why some bronchial carcinoid tumours are responsive is unknown but it could be because of co-secretion of corticotrophin releasing factor (CRF) and/or expression of glucocorticoid receptors. We report two patients with glucocorticoid responsive ACTH secreting bronchial carcinoid tumours, neither of whom produced or responded to CRF. Significant glucocorticoid receptor binding capacity (92 and 102 pmol/g protein), compared with control lung tissue, was found in extracts from both tumours. These findings suggest that corticotrophinoma-like responses to glucocorticoids observed in some ACTH secreting bronchial carcinoids result from expression of glucocorticoid receptors and are not necessarily related to the production of CRF.