Immunological selection of variant mouse lymphoid cells with altered glucocorticoid responsiveness

Overcoming apoptotic resistance afforded by Bcl-2 in lymphoid tumor cells: a critical role for dexamethasone

Bcl-2 is an anti-apoptotic protein that promotes cell survival and resistance to cell death. Predictably, Bcl-2 as well as other anti-apoptotic Bcl-2 family members have been found to be overexpressed in a variety of human cancers. Approaches to overcome apoptotic resistance afforded by Bcl-2 in cells include anti-sense oligonucleotides, drugs that inhibit Bcl-2 function, and BH3 mimics have not been universally effective; thus, the need to understand the underlying mechanism of this resistance is vital. Glucocorticoids are stress hormones that act through their cognate receptors to control the transcription of numerous target genes, and in turn regulate a diverse array of biological processes. Synthetic glucocorticoids, such as dexamethasone, are prescribed in many chemotherapy protocols for neoplasms of lymphoid origin based on their ability to inhibit lymphocyte proliferation and promote apoptosis. However, lymphoid cells expressing Bcl-2 are resistant to glucocorticoid-induced cell death. We observed both pro- and anti-apoptotic characteristics in lymphoid cells expressing Bcl-2 following glucocorticoid treatment. These cells exhibited a profound change in their intracellular ionic composition, but a limited apoptotic ion flux and the absence of cell death. Provocatively, mimicking the loss of intracellular potassium using a low dose of a microbial toxin that acts as a potassium ionophore in combination with dexamethasone overcame the resistance afforded by Bcl-2 and killed the cells. Extending our study using other potassium ionophores revealed that direct depolarization of the mitochondria membrane potential coupled with prior treatment with glucocorticoids is the key mechanism for activating the cell death program and bypassing the resistance afforded by Bcl-2 in lymphoid cells. Finally, we show that the duration of dexamethasone pre-treatment is critical for regulating distinct genes and signaling pathways that sensitize the cells to die.

GAS5, a non-protein-coding RNA, controls apoptosis and is downregulated in breast cancer

Effective control of both cell survival and cell proliferation is critical to the prevention of oncogenesis and to successful cancer therapy. Using functional expression cloning, we have identified GAS5 (growth arrest-specific transcript 5) as critical to the control of mammalian apoptosis and cell population growth. GAS5 transcripts are subject to complex post-transcriptional processing and some, but not all, GAS5 transcripts sensitize mammalian cells to apoptosis inducers. We have found that, in some cell lines, GAS5 expression induces growth arrest and apoptosis independently of other stimuli. GAS5 transcript levels were significantly reduced in breast cancer samples relative to adjacent unaffected normal breast epithelial tissues. The GAS5 gene has no significant protein-coding potential but expression encodes small nucleolar RNAs (snoRNAs) in its introns. Taken together with the recent demonstration of tumor suppressor characteristics in the related snoRNA U50, our observations suggest that such snoRNAs form a novel family of genes controlling oncogenesis and sensitivity to therapy in cancer.

Isolation of genes controlling apoptosis through their effects on cell survival

The identification of the most suitable molecular targets for gene and drug therapy is the crucial first step in the development of new disease treatments. The rational identification of such targets depends on a detailed understanding of the pathological changes occuring at the molecular level. We have applied forward genetics approaches to the identification of the critical genes involved in the control of apoptosis in mammalian cells, since defective control of apoptosis underlies many diseases, including cancer and neurodegenerative diseases. We have identified two groups of genes by their effects on cell survival using retroviral cDNA functional expression cloning and retroviral insertional mutagenesis. The identification of these novel genes opens up new areas for apoptosis research and subsequently for the development of new gene and drug therapies.

Functional expression cloning reveals a central role for the receptor for activated protein kinase C 1 (RACK1) in T cell apoptosis

Mammalian cDNA expression cloning was used to identify novel genes that regulate apoptosis. Using a functional screen, we identified a partial cDNA for the receptor for activated protein kinase C 1 (RACK1) through selection for resistance to phytohemagglutinin and gamma-irradiation. Expression of this partial cDNA in T cell lines using a mammalian expression vector produced an increase in RACK1 expression and resulted in resistance to dexamethasone- and ultraviolet-induced apoptosis. Down-regulation of RACK1 using RNA interference abolished the resistance of the transfected cells to apoptosis. Overexpression of full-length RACK1 also resulted in the suppression of apoptosis mediated by several apoptotic stimuli, and this effect was quantitatively consistent with the effects of the original cDNA isolated on endogenous RACK1 levels. Together, these findings suggest that RACK1 plays an important role in the intracellular signaling pathways that lead to apoptosis in T cells.

Regulation of apoptosis by fau revealed by functional expression cloning and antisense expression

Functional expression cloning is a powerful strategy for identifying critical steps in biological pathways independently of prior assumptions. It is particularly suitable for the identification of molecules crucial to the control of apoptosis. Our screen for sequences suppressing T-cell apoptosis isolated a sequence antisense to fau (Finkel-Biskis-Reilly murine sarcoma virus (FBR-MuSV)-associated ubiquitously expressed gene). The fox gene in FBR murine osteosarcoma virus is also antisense to fau and several reports have indicated that fau displays tumour suppressor and oncogenic properties in different contexts. Our observations indicate that the fau antisense sequence suppresses expression of endogenous fau mRNA and produces resistance to apoptosis induced both by the glucocorticoid analogue dexamethasone' by ultraviolet radiation, and by the anticancer drug cisplatin. In all cases, colony-forming ability is protected, indicating that fau affects the critical events prior to commitment to cell death. Overexpression of fau in the sense orientation induces cell death, which is inhibited both by Bcl-2 and by inhibition of caspases, in line with its proposed role in apoptosis.

Functional expression cloning reveals proapoptotic role for protein phosphatase 4

Functional expression cloning strategies are highly suitable for the analysis of the molecular control of apoptosis. This approach has two critical advantages. Firstly, it eliminates prior assumptions about the properties of the proteins involved, and, secondly, it selectively targets proteins that are causally involved in apoptosis control and which affect the crucial cellular decision between survival and death. The application of this strategy to the isolation of cDNAs conferring resistance to dexamethasone and gamma-irradiation resulted in the isolation of a partial cDNA for the catalytic subunit of protein phosphatase 4 (PP4). Cells transfected with this partial cDNA in an expression vector downregulated PP4 and were resistant to both dexamethasone and UV radiation, as demonstrated by both membrane integrity and colony-forming assays. These observations suggest that PP4 plays an important proapoptotic role in T lymphocytes.

Molecular failure of apoptosis: inappropriate cell survival and mutagenesis?

Since cell death by apoptosis is achieved through complex interactions between numerous molecular components, cells may fail to die when stimulated because of molecular abnormalities in the apoptosis pathway or in its control mechanisms. Such inappropriate cell survival is well established when apoptosis is suppressed by elevated expression of bcl-2, at least for some cell types. Many cells undergo apoptosis at moderate levels of DNA damage and suppression of such apoptosis might be expected to increase the rate of mutation because of the persistence of cells with damaged DNA. We and others have now confirmed this prediction in bcl-2 transfected cells. Suppression of the apoptosis pathway can only lead to inappropriate cell survival if it relates to events before the cell becomes committed to die. We have analyzed this question for agents that inhibit the caspases, the site-specific proteases which form the biochemical core of the process of apoptosis. We have shown that inhibition of certain caspases does lead to the survival of Jurkat human T-cells induced to undergo Fas-mediated apoptosis.

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.

A neuroectoderm-associated ganglioside participates in fibronectin receptor-mediated adhesion of germinal cells to fibronectin

Previous studies with a rat neural cell line have shown that the D1.1 ganglioside, an O-acetylated derivative of GD3, is involved in cellular adhesion to fibronectin. In vivo, D1.1 is present on germinal cells of the developing rat central nervous system, but not on postmitotic cells that migrate away from the germinal zones. These observations suggest that D1.1 could participate in adhesive interactions with germinal zones and that the loss of D1.1 could be involved in the decision to being migration. In support of this hypothesis, immunofluorescence histochemistry shows that both fibronectin and fibronectin receptor are colocalized with D1.1 in the ventricular zones of the embryonic rat brain and in the external granule cell layer of the postnatal cerebellum. Dishes coated with monoclonal antibody against D1.1 were used to isolate D1.1-positive germinal cells from Embryonic Day 14 cerebrum and from Postnatal Day 6 cerebellum. These cells are able to adhere to fibronectin-coated dishes by a mechanism that is inhibitable by a synthetic hexapeptide containing the arg-gly-asp cell recognition sequence of fibronectin. Adhesion is also partially inhibited by antibody against fibronectin receptor and is slowed by anti-D1.1 antibody, implicating both the receptor and the ganglioside in the adhesion process. During 3 days in culture these D1.1-positive, fibronectin receptor-positive cells exhibit a neuronal phenotype, as judged by morphology and staining with tetanus toxin. This further confirms the neuroepithelial origin of the cells. The cells do not synthesize detectable amounts of fibronectin, thus leaving unidentified the source of the fibronectin seen in the germinal zones in tissue sections. Immunoprecipitation experiments show that the fibronectin receptors present on these cells are heterodimers. Under nonreducing conditions, the immunoprecipitates contain an alpha-subunit of 150-160 kDa and a beta-subunit of 115-125 kDa.

Involvement of gangliosides and glycoprotein fibronectin receptors in cellular adhesion to fibronectin

We have used a rat neural cell line, B65, to investigate the relative contributions of gangliosides and glycoprotein receptors in adhesion to fibronectin. Monoclonal antibodies against two neuroectoderm-associated gangliosides, D1.1 and GD3, inhibit the rate of B65 attachment to fibronectin, suggesting that these gangliosides are involved in the adhesion process. Adhesion to fibronectin is not affected by a third monoclonal antibody against a separate, unidentified cell-surface component of B65 cells. Furthermore, B65 cells lacking D1.1 adhere to fibronectin at a slower rate than B65 cells that express D1.1. The involvement of glycoprotein receptors in adhesion is demonstrated by the ability of antibodies against human fibronectin receptor to inhibit B65 attachment to fibronectin. In addition, adhesion is blocked by a hexapeptide containing the Arg-Gly-Asp fibronectin sequence which is necessary for binding to the receptor. Trypsin treatment of B65 cells in the absence of divalent cations results in proteolysis of the fibronectin receptor with an accompanying loss of ability of the cells to attach to fibronectin. D1.1 and GD3 expression is not affected by this trypsinization, indicating that the gangliosides alone are incapable of mediating attachment. The glycoprotein receptors must be primarily responsible for adhesion to fibronectin with the gangliosides playing a secondary role as enhancers or modulators.

Glucocorticoid-dependent maturation of viral proteins in mouse lymphoma cells: isolation of defective and hormone-independent cell variants

Maturation of mouse mammary tumor virus proteins is dependent on glucocorticoid hormones in W7MG1, a stably infected mouse T-lymphosarcoma cell line derived from WEH17. We used an immunological procedure to select variant cell lines with altered levels of viral glycoproteins on the cell surface. One variant, W7M329a, expressed lower-than-normal levels of the major viral glycoprotein, gp52env, on the cell surface before and after exposure to hormone. Two other variants, W7M302b and W7M326.4, expressed elevated levels of gp52env on the cell surface even in the absence of hormone. Analysis of the levels and/or rates of synthesis of viral RNA and glycoproteins before and after hormone treatment indicated that the variant phenotypes resulted from changes in posttranslational steps of viral gene expression. The hormone-independent maturation of MMTV proteins is a novel variant phenotype that has not previously been reported.