A new determinant of glucocorticoid sensitivity in lymphoid cell lines

Epigenetic alteration by DNA-demethylating treatment restores apoptotic response to glucocorticoids in dexamethasone-resistant human malignant lymphoid cells

Background

Glucocorticoids (GCs) are often included in the therapy of lymphoid malignancies because they kill several types of malignant lymphoid cells. GCs activate the glucocorticoid receptor (GR), to regulate a complex genetic network, culminating in apoptosis. Normal lymphoblasts and many lymphoid malignancies are sensitive to GC-driven apoptosis. Resistance to GCs can be a significant clinical problem, however, and correlates with resistance to several other major chemotherapeutic agents.

Methods

We analyzed the effect of treatment with the cytosine analogue 5 aza-2’ deoxycytidine (AZA) on GC resistance in two acute lymphoblastic leukemia (T or pre-T ALL) cell lines- CEM and Molt-4- and a (B-cell) myeloma cell line, RPMI 8226. Methods employed included tissue culture, flow cytometry, and assays for clonogenicity, cytosine extension, immunochemical identification of proteins, and gene transactivation. High throughput DNA sequencing was used to confirm DNA methylation status.

Conclusions

Treatment of these cells with AZA resulted in altered DNA methylation and restored GC-evoked apoptosis in all 3 cell lines. In CEM cells the altered epigenetic state resulted in site-specific phosphorylation of the GR, increased GR potency, and GC-driven induction of the GR from promoters that lie in CpG islands. In RPMI 8226 cells, expression of relevant coregulators of GR function was altered. Activation of p38 mitogen-activated protein kinase (MAPK), which is central to a feed-forward mechanism of site-specific GR phosphorylation and ultimately, apoptosis, occurred in all 3 cell lines. These data show that in certain malignant hematologic B- and T-cell types, epigenetically controlled GC resistance can be reversed by cell exposure to a compound that causes DNA demethylation. The results encourage studies of application to in vivo systems, looking towards eventual clinical applications.

Genomic screening for genes silenced by DNA methylation revealed an association between RASD1 inactivation and dexamethasone resistance in multiple myeloma

PURPOSE

Epigenetic changes such as DNA methylation play a key role in the development and progression of multiple myeloma. Our aim in the present study was to use genomic screening to identify genes targeted for epigenetic inactivation in multiple myeloma and assess their role in the development of resistance to dexamethasone.

EXPERIMENTAL DESIGN

Gene expression was examined using microarray screening, reverse transcription-PCR, and real-time quantitative PCR. DNA methylation was examined using bisulfite PCR, bisulfite sequencing, and bisulfite pyrosequencing in 14 multiple myeloma cell lines, 87 multiple myeloma specimens, and 12 control bone marrow samples. WST-8 assays were used to assess cell viability after treatment with 5-aza-2'-deoxycytidine and/or dexamethasone.

RESULTS

Microarray analysis was done to screen for genes up-regulated by 5-aza-2'-deoxycytidine. In RPMI8226 cells, 128 genes were up-regulated, whereas 83 genes were up-regulated in KMS12PE cells. Methylation of 22 genes with CpG islands in their 5' regions, including RASD1, was confirmed. Methylation of RASD1 was associated with its inactivation, which correlated with resistance to dexamethasone. Treating multiple myeloma cells with 5-aza-2'-deoxycytidine restored sensitivity to dexamethasone. Methylation of RASD1 was also detected in a subset of primary multiple myeloma specimens, and the levels of methylation were increased after repeated antitumor treatments. Gene signature analysis revealed various genes to be synergistically induced by treatment with a combination of 5-aza-2'-deoxycytidine plus dexamethasone.

CONCLUSION

Our findings indicate that epigenetic inactivation of genes, including RASD1, plays a key role in the development of dexamethasone resistance in multiple myeloma. Moreover, they show the utility of demethylation therapy in cases of advanced multiple myeloma.

Glucocorticoid-induced apoptosis in lymphocytes

Although the effects of glucocorticoids on lymphocytes have been scrutinized for years, researchers have yet to determine how these hormones induce apoptosis in susceptible cells. Compelling evidence indicates that DNA binding of the GR and subsequent transcriptional regulation of specific genes is required for this process. However, it is not clear whether the activation or repression of responsive genes is essential and more importantly, which of these genes, if any, are responsible for the induction of apoptosis. This review will focus on how glucocorticoid-induced apoptosisin lymphocytes is mediated by the glucocorticoid receptor, including a discussion of GR structure, function, and recent data implicating its role in the apoptotic process.

Glucocorticoid-induced apoptosis and regulation of NF-kappaB activity in human leukemic T cells

Glucocorticoid-induced apoptosis was investigated in glucocorticoid-sensitive 6TG1.1 and resistant ICR27TK.3 human leukemic T cells. Following glucocorticoid treatment of 6TG1.1 cells, chromatin fragmentation was observed after a delay of 24 h. Fragmentation was not observed in ICR27TK.3 cells containing mutant glucocorticoid receptors (L753F) that are activation-deficient but retain the ability to repress AP-1 activity. Nor was fragmentation observed after treatment with RU38486, indicating that repression of AP-1 activity is not involved. As described in other systems, fragmentation required ongoing protein synthesis. However, inhibition of protein synthesis with cycloheximide anytime during the first 18 h of steroid treatment was as effective in blocking chromatin fragmentation as inhibition for the entire period, suggesting that synthesis of a component with a rapid turnover rate is required. Dexamethasone treatment completely blocked 12-O-tetradecanoylphorbol 13-acetate induction of nuclear factor-kappaB (NF-kappaB) activity and elicited an increase in the amount of immunoreactive IkappaB alpha in sensitive 6TG1.1 cells but not in resistant ICR27TK.3 cells. In addition, mild detergent treatment of cell extracts indicated that a substantial amount of cytoplasmic NF-kappaB is complexed with IkappaB alpha or some other inhibitory factor. These results suggest that induction of a labile inhibitory factor such as IkappaB alpha may contribute to glucocorticoid-induced apoptosis.

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.

Glucocorticoid-mediated immunomodulation: hydrocortisone enhances immunosuppressive endogenous retroviral protein (p15E) expression in mouse immune cells

To define glucocorticoid (GC)-regulated genes contributing to the anti-inflammatory and immunosuppressive effects of GC, previous work from our laboratory revealed up-regulation of transcripts from endogenous type B mouse mammary tumour virus (Mtv) and type C murine leukaemia virus (Emv) loci by high dose GC treatment of P388D1 macrophage-like cells. This study demonstrates enhancement of expression from Mtv and Emv loci in P388D1 cells by more physiological hydrocortisone concentrations (1 microM), and shows direct transcriptional mode of regulation by blocking GC-mediated signal transduction at different levels. Furthermore, we found up-regulation of Emv mRNA steady-state levels in murine lymphoid lineage cells (T-like EL4 and BW5147 cells; B-like X63 cells) upon GC treatment. The Emv transcripts shown by us to be GC-up-regulated encode for the transmembrane envelope protein TM/p15E which is highly conserved in several retroviruses. TM/p15E and the p15E-like products found in humans exert immunosuppressive effects in different test systems. Thus, our findings raise the possibility that immunomodulation by GC might be mediated in part by enhanced expression of p15E(-like) products.

Evidence for trans regulation of apoptosis in intertypic somatic cell hybrids

The genetic components required for glucocorticoid induction of apoptosis were studied by using somatic cell hybridization. Intertypic whole-cell hybrids were generated by crossing the glucocorticoid-resistant rat liver cell line Fado-2 with the glucocorticoid-sensitive mouse thymoma cell line BW5147.3. Morphological and biochemical criteria were used to assess sensitivity or resistance to glucocorticoid-induced cell death. Both phenotypes were observed, and all of the hybrids retained a functional glucocorticoid receptor as judged by their abilities to induce the metallothionein gene in response to dexamethasone (Dex). Sensitivity to apoptosis did not correlate with morphological phenotype in that not all suspension cells were sensitive. The effect of glucocorticoids on the expression of apoptosis-linked genes was analyzed in a subset of Dex-sensitive and Dex-resistant hybrids. p53 and c-myc mRNAs were present in parental cells as well as sensitive and resistant hybrid cells, and their levels were not affected by glucocorticoid treatment. bcl-2 expression was restricted to the thymoma cell line and was also not affected by glucocorticoids. We did not detect any bcl-2 mRNA in the hepatoma cell line and the hybrids, suggesting that, as with most tissue-specific genes, bcl-2 is regulated in trans. Furthermore, while the majority of hybrids analyzed retained a full complement of mouse chromosomes, sensitive hybrids were missing some rat chromosomes (preferentially chromosomes 16 and 19), indicating that apoptosis is subject to trans repression. Resistant cells thus appear to repress the activity or synthesis of a nuclear factor that interacts with a glucocorticoid-dependent gene(s) to activate the cell death pathway.

Evidence for trans regulation of apoptosis in intertypic somatic cell hybrids

The genetic components required for glucocorticoid induction of apoptosis were studied by using somatic cell hybridization. Intertypic whole-cell hybrids were generated by crossing the glucocorticoid-resistant rat liver cell line Fado-2 with the glucocorticoid-sensitive mouse thymoma cell line BW5147.3. Morphological and biochemical criteria were used to assess sensitivity or resistance to glucocorticoid-induced cell death. Both phenotypes were observed, and all of the hybrids retained a functional glucocorticoid receptor as judged by their abilities to induce the metallothionein gene in response to dexamethasone (Dex). Sensitivity to apoptosis did not correlate with morphological phenotype in that not all suspension cells were sensitive. The effect of glucocorticoids on the expression of apoptosis-linked genes was analyzed in a subset of Dex-sensitive and Dex-resistant hybrids. p53 and c-myc mRNAs were present in parental cells as well as sensitive and resistant hybrid cells, and their levels were not affected by glucocorticoid treatment. bcl-2 expression was restricted to the thymoma cell line and was also not affected by glucocorticoids. We did not detect any bcl-2 mRNA in the hepatoma cell line and the hybrids, suggesting that, as with most tissue-specific genes, bcl-2 is regulated in trans. Furthermore, while the majority of hybrids analyzed retained a full complement of mouse chromosomes, sensitive hybrids were missing some rat chromosomes (preferentially chromosomes 16 and 19), indicating that apoptosis is subject to trans repression. Resistant cells thus appear to repress the activity or synthesis of a nuclear factor that interacts with a glucocorticoid-dependent gene(s) to activate the cell death pathway.

Evidence that glucocorticoid- and cyclic AMP-induced apoptotic pathways in lymphocytes share distal events

WEHI7.2 murine lymphocytes undergo apoptotic death when exposed to glucocorticoids or elevated levels of intracellular cyclic AMP (cAMP), and these pathways are initiated by the glucocorticoid receptor (GR) and protein kinase A, respectively. We report the isolation and characterization of a novel WEHI7.2 variant cell line, WR256, which was selected in a single step for growth in the presence of dexamethasone and arose at a frequency of approximately 10(-10). The defect was not GR-related, as WR256 expressed functional GR and underwent GR-dependent events associated with apoptosis, such as hormone-dependent gene transcription and inhibition of cell proliferation. Moreover, the glucocorticoid-resistant phenotype was stable in culture and did not revert after treatment with 5-azacytidine or upon stable expression of GR cDNA. In addition, WR256 did not exhibit the diminished mitochondrial activity commonly associated with apoptosis. Interestingly, WR256 was also found to be resistant to 8-bromo-cAMP and forskolin despite having normal levels of protein kinase A activity and the ability to induce cAMP-dependent transcription. We examined the steady-state transcript levels of bcl-2, a gene whose protein product acts dominantly to inhibit thymocyte apoptosis, to determine whether elevated bcl-2 expression could account for the resistant phenotype. Our data showed that bcl-2 RNA levels were similar in the two cell lines and not altered by either dexamethasone or 8-bromo-cAMP treatment. These results suggest that WR256 exhibits a "deathless" phenotype and has a unique defect in a step of the apoptotic cascade that may be common to the glucocorticoid- and cAMP-mediated cell death pathways.

Evidence that glucocorticoid- and cyclic AMP-induced apoptotic pathways in lymphocytes share distal events

WEHI7.2 murine lymphocytes undergo apoptotic death when exposed to glucocorticoids or elevated levels of intracellular cyclic AMP (cAMP), and these pathways are initiated by the glucocorticoid receptor (GR) and protein kinase A, respectively. We report the isolation and characterization of a novel WEHI7.2 variant cell line, WR256, which was selected in a single step for growth in the presence of dexamethasone and arose at a frequency of approximately 10(-10). The defect was not GR-related, as WR256 expressed functional GR and underwent GR-dependent events associated with apoptosis, such as hormone-dependent gene transcription and inhibition of cell proliferation. Moreover, the glucocorticoid-resistant phenotype was stable in culture and did not revert after treatment with 5-azacytidine or upon stable expression of GR cDNA. In addition, WR256 did not exhibit the diminished mitochondrial activity commonly associated with apoptosis. Interestingly, WR256 was also found to be resistant to 8-bromo-cAMP and forskolin despite having normal levels of protein kinase A activity and the ability to induce cAMP-dependent transcription. We examined the steady-state transcript levels of bcl-2, a gene whose protein product acts dominantly to inhibit thymocyte apoptosis, to determine whether elevated bcl-2 expression could account for the resistant phenotype. Our data showed that bcl-2 RNA levels were similar in the two cell lines and not altered by either dexamethasone or 8-bromo-cAMP treatment. These results suggest that WR256 exhibits a "deathless" phenotype and has a unique defect in a step of the apoptotic cascade that may be common to the glucocorticoid- and cAMP-mediated cell death pathways.

Programmed cell death: concept, mechanism and control

Programmed cell death or apoptosis occurs under physiological conditions as a result of physiological effectors. It is a relatively slower process and requires active participation of the cell in the suicidal mechanism. Apoptosis is controlled by precise intrinsic genetic programme and may be induced by almost all those stimuli causing necrosis. The role played by the intensity in determining the death process and the underlying mechanism is imperfectly understood. Morphologically apoptotic cells appear as small condensed body. The chromatin is dense and fragmented, packed into compact membrane-bound bodies together with randomly distributed cell organelles. The plasma membrane loses its characteristic architecture and shows extensive blebbing. It buds off projections so that the whole cell may split into several membrane-bound apoptotic bodies. Significant chemical changes take place in the plasma membrane. This helps in recognition of the apoptotic bodies by phagocytes. At this moment it is unclear if all cells can undergo apoptosis or it is a characteristic of only some tissues which are predisposed to apoptotic death being directly under the control of hormones or growth factors. Experimental studies aimed at comparison of induction of apoptosis in cells of different origin are warranted to elucidate this point. Biochemically a pre-commitment step for induction of death programmation through macromolecular synthesis is essential for most systems. The double-stranded linker DNA between nucleosomes is cleaved at regular inter-nucleosomal sites through the action of a Ca2+, Mg(2+)-sensitive neutral endonuclease. Zinc is a potent inhibitor of the enzyme. Calcium probably plays a key controlling role in activation of the enzyme since prevention of Ca2+ increase prevents endonuclease activation. It is becoming evident that signal transduction through appropriate receptors control the Ca2+ flux in the cells. Most apoptotic cells require synthesis of RNA and proteins. Delay or abrogation of apoptosis by inhibition of macromolecular synthesis is well known. The dying cells show high mRNA levels for several enzymes. Several degradative enzymes become active. Regulatory proteins maintain control over the apoptotic cascade. At the molecular level, search has been initiated for the mammalian equivalents of the cell death (ced) gene. Activation of several specific genes is indicated. Specific expression of cell death-associated gene products (e.g. TRPM-2/SGP-2) has been reported in several unrelated apoptotic cell systems. Sequential induction of c-fos, c-myc and 70 kDa heat shock protein is reported. Studies demonstrate that certain genes must remain in a transcriptionally active demethylated state during programmed cell death. Recent evidences clearly indicate that apoptosis may be positively or negatively modulated by certain genes.(ABSTRACT TRUNCATED AT 400 WORDS)

Apoptosis. Biochemical events and relevance to cancer chemotherapy

Two distinct pathways for cell death exist. Compared to necrotic death, physiological or apoptotic cell death is an active suicidal process that consists of a cascade of well-regulated synthetic events. Participation of specific genes in apoptosis, and its possible molecular regulation, are considered in order to investigate the mechanism of cell death induced by some cancer chemotherapeutic agents.

Coordinate change in phenotype in a mouse cell line selected for CD8 expression

A CD4+, CD8+ derivative of the CD4+, CD8- cell line SAKRTLS 12.1 was isolated by fluorescence activated cell sorting for CD8+ cells. This derivative showed a co-ordinate change in a number of independent characters: The parental cell line was CD4+, CD8-, CD3+, CD5hi, HSA+, DEXR, CD44hi, while the derivative was CD4+, CD8+, CD3-, CD5(10), HSA+, DEXS, CD44(10). The derivative expressed the Thy-1.1, Ly-2.1, and Ly-3.1 surface antigens, consistent with origin from the SAKRTLS 12.1 parental cell line, and showed a drug resistance profile identical to that of the parent. It was not possible to isolate revertants with a phenotype identical to that of the parental cell line. Activation of the structural gene coding for CD8 alpha chain was correlated with demethylation at several sites. We interpret these results to mean that this CD8+ derivative of SAKRTLS 12.1 arose as a result of an alteration of a gene that coordinately regulates multiple genes whose expression changes during thymocyte differentiation. Gene methylation may contribute, directly or indirectly, to some or all of the changes in gene expression observed.

Cortivazol mediated induction of glucocorticoid receptor messenger ribonucleic acid in wild-type and dexamethasone-resistant human leukemic (CEM) cells

Cortivazol is a phenylpyrazolo glucocorticoid of high potency and unusual structure. In both wild-type and highly dexamethasone(dex)-resistant clones of the human leukemic cell line CEM, exposure to cortivazol leads to cell death. It has been shown recently that in wild-type CEM cells but not in a dex-resistant, glucocorticoid receptor(GR)-defective clone ICR-27 TK-3, dex induces GR mRNA. To test the hypothesis that cortivazol acts in dex-resistant cells by making use of the residual GR found there, wild-type and dex-resistant clones were treated with various concentrations of cortivazol and induction of GR mRNA was studied. Cortivazol significantly induced GR mRNA in the normal CEM-C7 as well as in two classes of dex-resistant clones, although the dex-resistant clones needed at least 10 times more cortivazol than the normal cells for significant GR mRNA induction. Increased levels of GR mRNA were noticed as early as 3 h after treatment. A general correlation between induction of GR mRNA and lysis of the normal and dex-resistant cells was found. Positive induction of GR mRNA might be one of the earliest crucial steps in the lysis of normal and dex-resistant CEM cells, or might serve as a marker for the process. However, the lysis pathway in the dex-resistant cells is defective in that dex-resistant clones needed significantly more cortivazol than the normal cells for lysis of the cells.

A model system for T-lymphocyte differentiation: regulation of CD4 and CD8 gene expression in SL12.4 T-lymphoma cell clones

The T-cell surface proteins CD4 (L3T4) and CD8 (Lyt2) are first expressed on thymocytes as they undergo maturation in the thymus. Two immature T-lymphoma cell clones SL12.4 and RS4.2 which constitutively express low or undetectable levels of CD4 and CD8 were used to investigate the activation of CD4 and CD8 gene expression. The protein synthesis inhibitors cycloheximide (CHX) and pactamycin rapidly and reversibly increased CD4 and CD8 mRNA in the cloned cell lines, suggesting that a labile inhibitor protein(s) may regulate the expression of these transcripts. Cell surface CD4 and CD8 proteins were transiently detectable following a pulse of CHX. Thymic epithelial cell lines also induced CD4 and CD8 mRNA and cell surface protein, as well as TCR-alpha mRNA when co-cultivated with SL12.4 T lymphoma cells. The increase in CD4 and CD8 was modest, but stable for at least 22 cell generations after the thymic epithelial inducer cells were removed. Epithelial cells of non-thymic origin did not cause induction of these T-cell differentiation markers in SL12.4 T-lymphoma cells. Since the induction elicited by thymic epithelial cells and protein synthesis inhibitors differed dramatically in kinetics and reversibility, it is likely that these inducers act, at least in part, via different mechanisms. This lymphoma model system may be useful for analysis of molecular events which occur in immature thymocytes undergoing differentiation.

Glucocorticoid activation of deoxyribonucleic acid degradation in bursal lymphocytes

Treatment of animals with exogenous adrenal steroids or elevation of endogenous glucocorticoids results in a profound involution of lymphoid tissue. In rodent species, this lymphoinvolution is accompanied by lymphocyte cell death and extensive degradation of the genome prior to lymphocytolysis. In the present study, this process was investigated in the bursa of Fabricius of domestic fowl. Four-wk-old chicks were treated with a single injection of dexamethasone, and bursal regression and cell viability were monitored over a 72-h period. Following hormone treatment, DNA was extracted from bursal lymphocytes and analyzed by agarose gel electrophoresis. Dexamethasone treatment resulted in a rapid regression of bursal tissue that could be detected as soon as 6 h posttreatment, but lymphocyte viability was not altered until 24 h afterward. The DNA isolated from bursal lymphocytes of glucocorticoid-treated birds appeared to be degraded at internucleosomal sites and generated a "ladder" of discrete DNA fragments when analyzed by agarose gel electrophoresis. This form of hormone-induced cell death, referred to as programmed cell death, may play a key role in glucocorticoid-mediated immunosuppression.

Isolation and characterization of glucocorticoid- and cyclic AMP-induced genes in T lymphocytes

Glucocorticoids and cyclic AMP exert dramatic effects on the proliferation and viability of murine T lymphocytes through unknown mechanisms. To identify gene products which might be involved in glucocorticoid-induced responses in lymphoid cells, we constructed a lambda cDNA library prepared from murine thymoma WEHI-7TG cells treated for 5 h with glucocorticoids and forskolin. The library was screened with a subtracted cDNA probe enriched for sequences induced by the two drugs, and cDNA clones representing 11 different inducible genes were isolated. The pattern of expression in BALB/c mouse tissues was examined for each cDNA clone. We have identified two clones that hybridized to mRNAs detected exclusively in the thymus. Other clones were identified that demonstrated tissue-specific gene expression in heart, brain, brain and thymus, or lymphoid tissue (spleen and thymus). The kinetics of induction by dexamethasone and forskolin were examined for each gene. The majority of the cDNA clones hybridized to mRNAs that were regulated by glucocorticoids and forskolin, two were regulated only by glucocorticoids, and three hybridized to mRNAs that required both drugs for induction. Inhibition of protein synthesis by cycloheximide resulted in the induction of all mRNAs that were inducible by glucocorticoids. Preliminary sequence analysis of four of the 11 cDNAs suggests that two cDNAs represent previously undescribed genes while two others correspond to the mouse VL30 retrovirus-like element and the mouse homolog of chondroitin sulfate proteoglycan core protein.

Early pre-B-cell transformation induced by the v-fms oncogene in long-term mouse bone marrow cultures

Murine long-term bone marrow cultures that support B-lymphoid-cell development were infected with a helper-free retrovirus containing the v-fms oncogene. Infection of B-lymphoid cultures resulted in the rapid clonal outgrowth of early pre-B cells, which grew to high cell densities on stromal cell feeder layers, expressed v-fms-coded glycoproteins, and underwent immunoglobulin heavy-chain gene rearrangements. Late-passage cultures gave rise to factor-independent variants that proliferated in the absence of feeder layers, developed resistance to hydrocortisone, and became tumorigenic in syngeneic mice. The v-fms oncogene therefore recapitulates known effects of the v-abl and bcr-abl oncogenes on B-lineage cells. The ability of v-fms to induce transformation of early pre-B cells in vitro underscores the capacity of oncogenic mutants of the colony-stimulating factor-1 receptor to function outside the mononuclear phagocyte lineage.

Early pre-B-cell transformation induced by the v-fms oncogene in long-term mouse bone marrow cultures

Murine long-term bone marrow cultures that support B-lymphoid-cell development were infected with a helper-free retrovirus containing the v-fms oncogene. Infection of B-lymphoid cultures resulted in the rapid clonal outgrowth of early pre-B cells, which grew to high cell densities on stromal cell feeder layers, expressed v-fms-coded glycoproteins, and underwent immunoglobulin heavy-chain gene rearrangements. Late-passage cultures gave rise to factor-independent variants that proliferated in the absence of feeder layers, developed resistance to hydrocortisone, and became tumorigenic in syngeneic mice. The v-fms oncogene therefore recapitulates known effects of the v-abl and bcr-abl oncogenes on B-lineage cells. The ability of v-fms to induce transformation of early pre-B cells in vitro underscores the capacity of oncogenic mutants of the colony-stimulating factor-1 receptor to function outside the mononuclear phagocyte lineage.

Isolation and characterization of glucocorticoid- and cyclic AMP-induced genes in T lymphocytes

Glucocorticoids and cyclic AMP exert dramatic effects on the proliferation and viability of murine T lymphocytes through unknown mechanisms. To identify gene products which might be involved in glucocorticoid-induced responses in lymphoid cells, we constructed a lambda cDNA library prepared from murine thymoma WEHI-7TG cells treated for 5 h with glucocorticoids and forskolin. The library was screened with a subtracted cDNA probe enriched for sequences induced by the two drugs, and cDNA clones representing 11 different inducible genes were isolated. The pattern of expression in BALB/c mouse tissues was examined for each cDNA clone. We have identified two clones that hybridized to mRNAs detected exclusively in the thymus. Other clones were identified that demonstrated tissue-specific gene expression in heart, brain, brain and thymus, or lymphoid tissue (spleen and thymus). The kinetics of induction by dexamethasone and forskolin were examined for each gene. The majority of the cDNA clones hybridized to mRNAs that were regulated by glucocorticoids and forskolin, two were regulated only by glucocorticoids, and three hybridized to mRNAs that required both drugs for induction. Inhibition of protein synthesis by cycloheximide resulted in the induction of all mRNAs that were inducible by glucocorticoids. Preliminary sequence analysis of four of the 11 cDNAs suggests that two cDNAs represent previously undescribed genes while two others correspond to the mouse VL30 retrovirus-like element and the mouse homolog of chondroitin sulfate proteoglycan core protein.

Isolation and characterization of cortisol-sensitive and -resistant P1798 mouse lymphosarcoma cell lines

Three cell lines have been isolated and characterized from the P1798 mouse lymphosarcoma. One line, derived from a glucocorticoid-resistant tumor, was glucocorticoid-resistant in vitro. The other two cell lines, derived from glucocorticoid-sensitive and -resistant parental tumors, respectively, were shown to be glucocorticoid-sensitive in vitro. The glucocorticoid receptor from all three cell lines bound glucocorticoid with similar affinity and capacity. However, based on Sephacryl S-300 gel filtration, the glucocorticoid receptor from the resistant cell line was smaller than that of the two sensitive cell lines. Moreover, the glucocorticoid receptor from the resistant cell line accumulated to a greater extent in the nucleus. This resistant cell line thus resembles the nti variant of the S49 lymphoma cell line. All three cell lines were tumorigenic and metastatic when reimplanted into mice, contained the normal mouse diploid complement of 40 chromosomes and exhibited the same responsiveness to cortisol in vivo as they did in vitro. It is concluded that the ready passage of these cell lines in vitro or in vivo and the presence of the small receptor in the resistant line should make them excellent model systems for the study of glucocorticoid resistance.

Selective transformation of primitive lymphoid cells by the BCR/ABL oncogene expressed in long-term lymphoid or myeloid cultures

The BCR/ABL gene, formed by the Philadelphia chromosome translocation (Ph1) of human chronic myelogenous leukemia, encodes an altered ABL gene product, P210. P210 is strongly implicated in the malignant process of chronic myelogenous leukemia, but it precise role is unknown. Infection of long-term bone marrow cultures enriched for B-lymphoid cell types with a Moloney murine leukemia virus retroviral vector containing the BCR/ABL cDNA resulted in clonal outgrowths of immature B-lymphoid cells which expressed abundant P210 kinase activity. Surprisingly, infection of long-term myeloid lineage-enriched cultures also resulted in clonal outgrowths of immature B-lymphoid cells. The P210-expressing lymphoid cell lines resulting from either type of culture were resistant to the lethal effects of corticosteroids. These findings indicate that high levels of P210 expressed from a Moloney murine leukemia virus long terminal repeat preferentially stimulate the growth of immature B-lineage cells, and this effect is apparent even in myeloid lineage-enriched cultures, in which few if any lymphoid cells can be detected prior to infection.

Selective transformation of primitive lymphoid cells by the BCR/ABL oncogene expressed in long-term lymphoid or myeloid cultures

The BCR/ABL gene, formed by the Philadelphia chromosome translocation (Ph1) of human chronic myelogenous leukemia, encodes an altered ABL gene product, P210. P210 is strongly implicated in the malignant process of chronic myelogenous leukemia, but it precise role is unknown. Infection of long-term bone marrow cultures enriched for B-lymphoid cell types with a Moloney murine leukemia virus retroviral vector containing the BCR/ABL cDNA resulted in clonal outgrowths of immature B-lymphoid cells which expressed abundant P210 kinase activity. Surprisingly, infection of long-term myeloid lineage-enriched cultures also resulted in clonal outgrowths of immature B-lymphoid cells. The P210-expressing lymphoid cell lines resulting from either type of culture were resistant to the lethal effects of corticosteroids. These findings indicate that high levels of P210 expressed from a Moloney murine leukemia virus long terminal repeat preferentially stimulate the growth of immature B-lineage cells, and this effect is apparent even in myeloid lineage-enriched cultures, in which few if any lymphoid cells can be detected prior to infection.

Progression to steroid insensitivity can occur irrespective of the presence of functional steroid receptors

A major problem in treatment of cancers arising in steroid-sensitive cells is their inevitable progression to a steroid-insensitive state; current therapies are based on the assumption that hormone insensitivity is associated with loss of receptor. We demonstrate for the first time that breast tumor cells can progress to steroid insensitivity in spite of functional steroid receptors. Transfection of the steroid-inducible LTR-C3 gene into unresponsive S115 mouse mammary tumor cells results in full inducibility of that gene with both androgen and glucocorticoid. Thus, although all known endogenous inducible parameters are lost, the steroid sensitivity of a transfected exogenous gene demonstrates that the machinery for steroid responsiveness is still fully functional. Furthermore, these transfected genes retain steroid sensitivity only while steroid is present; on prolonged withdrawal of steroid, they lose responsiveness, implying an epigenetic mechanism is involved.

Glucocorticoid receptor-like antigen in lymphoma cell membranes: correlation to cell lysis

S-49 mouse lymphoma cells undergo lysis when treated with glucocorticoids; the mechanism of this effect is not understood. A protein was detected in the plasma membrane of these cells by means of direct immunofluorescent labeling with a monoclonal antibody to the soluble glucocorticoid receptor. Cellular heterogeneity in the content of this glucocorticoid receptor-like molecule was evident. By immunoadsorption to antibody-coated tissue culture plates, the cells were separated into populations positive (100%) and depleted (38%) for this membrane antigen. Gel electrophoresis, specific immunoblot, and autoradiographic (binding of [3H]dexamethasone mesylate) analysis of the membrane proteins from the membrane antigen-positive group revealed multiple protein bands ranging in size from 85 to 145 kilodaltons. Furthermore, comparison of the glucocorticoid sensitivity of these groups of cells showed complete lysis of the membrane antigen-positive cells and only partial lysis of the antigen-deficient group, which suggests that the lysis response of cells to glucocorticoids is mediated by a glucocorticoid receptor-like molecule located in the plasma membrane.

Lymphotoxic activity of methyl prednisolone in vitro--I. Comparative toxicity of methyl prednisolone in human cell lines of B and T origin

The cytotoxic activity of methyl prednisolone was compared in EB-3(B), NALM-6(B), CCRF-CEM(T) and RPMI-8226 (plasma cell) cell lines derived from human lymphoid malignancies. Whereas EB-3 cells were steroid-sensitive, NALM-6 cells were partially sensitive and CCRF-CEM and RPMI-8226 were steroid resistant at concentrations of methyl prednisolone up to 10(-4) M. A high concentration of methyl prednisolone, 2.5 X 10(-3) M was toxic to all cell lines. Steroid-sensitivity did not correlate with the incorporation of [3H] dexamethasone and could not be mimicked by flurbiprofen, a non-steroidal anti-inflammatory agent. Both theophylline and di-butyryl cAMP were toxic towards NALM-6, EB-3 and CCRF-CEM cells; however, this toxicity was reversible and did not reflect the cells' sensitivities towards methyl prednisolone. Furthermore, elevated levels of cAMP in theophylline-treated cells, were not demonstrable in cells treated with methyl prednisolone at toxic or non-toxic concentrations of the steroid. Steroid-sensitive EB-3 cells exposed to 10(-5) M methyl prednisolone, produced a soluble factor which was toxic CCRF-CEM cells.

Glucocorticoid regulation of murine leukemia virus transcription elements is specified by determinants within the viral enhancer region

The transcriptional control region (the long terminal repeat, LTR) of the leukemogenic murine retrovirus SL3-3 contains a glucocorticoid-responsive consensus sequence, as does the corresponding region of the nonleukemogenic virus Akv. Dexamethasone increases gene expression directed by both LTR sequences. However, the responses of the LTRs of the two viruses to dexamethasone differ according to the cell line in which the response is measured. The results of these studies provide insights regarding differences in response to glucocorticoids dependent upon cell line and indicate that the glucocorticoid-responsive elements may compose one of the determinants of tissue specificity and pathogenicity of the murine retroviruses.

Glucocorticoid action on the immune system

Glucocorticoids have profound effects on immune function that are mediated, in part, by steroid-induced cell death. Our studies have been aimed at identifying the mechanism of this lymphocytolytic process using the rat thymocyte as a model system. Administration of glucocorticoids in vivo resulted in internucleosomal cleavage of the lymphocyte genome that was detectable within 2 h of treatment and increased with time after hormone administration. Six h after steroid treatment greater than 50% of the genome was degraded, yet cell viability remained greater than 90% indicating that this event preceded cell death. Furthermore, this process appeared to be mediated by the glucocorticoid receptor since the antagonist RU 486 blocked glucocorticoid-mediated DNA degradation. To further characterize this lymphocytolysis we have analyzed glucocorticoid-treated thymocytes for nucleases. Two families of nuclear proteins have been identified, a 30-32 kDa doublet and a series of 3-4 proteins that are 12-19 kDa, both of which are induced by glucocorticoid treatment (137 +/- 6% and 342 +/- 24%, respectively) and have prominent nuclease activity. These nucleases can also be induced in vitro indicating that glucocorticoids act directly on thymocytes to mediate this response. Moreover, this nuclease induction, like glucocorticoid-mediated DNA degradation, could be blocked by RU 486. Based on these findings we propose a working model of glucocorticoid-mediated lymphocytolysis in which these steroids, acting via a receptor mediated process, induce the expression of a lysis gene product (nuclease) which degrades the genome and results in cell death.

Complementation between glucocorticoid receptor and lymphocytolysis in somatic cell hybrids of two glucocorticoid-resistant human leukemic clonal cell lines

Somatic hybrids between two glucocorticoid-resistant clonal cell lines, CEM C1 and ICR-27, isolated independently from the CCRF-CEM human lymphoblastoid cell line, were constructed to investigate the complementation effect between the glucocorticoid receptor and the gene product(s) for inducing receptor-mediated lymphocytolysis. CEM C1 (r+ly-) has a normal amount of functional glucocorticoid receptor as compared to the steroid-sensitive clonal line CEM C7. Clone ICR-27 (r-ly?), which was originally isolated following mutagenesis of CEM C7 with the mutagen ICR 191, has few glucocorticoid receptors as determined by whole-cell receptor assay. The eight randomly selected CEM C1 X ICR-27 hybrid clones all showed sensitivity to 10(-6) M dexamethasone (ly+). The receptor site content of two near-tetraploid hybrids chosen for analysis was close to that of CEM C1 (r+). Hybrids constructed between CEM C1 and the receptor-bearing, steroid-sensitive clone, CEM C7 (r+ly+) also showed glucocorticoid sensitivity, and their receptor sites corresponded to the sum of those of CEM C1 and CEM C7 (r+r+ly+). These results indicate: that CEM C1 has no trans-active inhibitor of lysis; that CEM C1 has intact glucocorticoid receptor; and that ICR-27 and CEM C1 complement one another to restore lymphocytolysis. Therefore, CEM C1 cells can serve as a donor of human glucocorticoid receptors.

Negative trans-regulation of T-cell antigen receptor/T3 complex mRNA expression in murine T-lymphoma somatic cell hybrids

The antigen-specific T-cell receptor (TCR) is composed of variable antigen-recognition chains TCR-alpha and TCR-beta in noncovalent association with the invariant T3 multimer. The TCR-alpha and TCR-beta chains are encoded by gene segments that must be juxtaposed by rearrangement in order to be expressed. To examine whether mechanisms other than gene rearrangement might regulate TCR/T3 gene expression, somatic cell hybrids were formed among closely related murine SL12 T-lymphoma clones that differ in TCR/T3 mRNA levels. In hybrid cells formed between cell clones in which one parent is TCR-beta+ and the other is TCR-beta-, the resultant hybrid cells lack detectable TCR-beta transcripts. Since the protein synthesis inhibitor cycloheximide partially reverses TCR-beta repression in the hybrid cells, we postulate that a labile repressor protein is involved. The amount of mRNA encoding one of the T3 polypeptide chains, T3-delta, is also strongly negatively transregulated in the same hybrid cells in which TCR-beta mRNA expression is repressed. The negative trans-regulation of TCR-beta and T3-delta mRNA expression is relatively specific, since the levels of TCR-alpha mRNA and several thymocyte surface antigens are not repressed in somatic cell hybrids. Our results indicate that rearrangement of the TCR genes alone is not sufficient for TCR-beta expression and that trans-acting factors regulate the amounts of both TCR-beta and T3-delta mRNA in this system.

Glucocorticoid receptors and glucocorticoid resistance in human leukemia in vivo and in vitro

Clinical measurements quantitating glucocorticoid receptor sites in leukemic blasts may give useful prognostic information. In childhood ALL, where the most data is available, "high" receptor content in peripheral or marrow blasts correlated with likelihood of remission on therapy, longer durations of remission and better prognosis generally. In lymphomas and CLL as well, high receptor content correlated with likelihood of response to steroid therapy, though the number of studies is less. In AML the correlation with receptor site content is moot, and in other leukemias the reports are less complete. A model system for childhood ALL is provided by CEM cells, a glucocorticoid sensitive human cell line from a patient with the disease. These cells have glucocorticoid receptors which must be filled by hormone for greater than 24 hr for cell lysis to begin. Four types of glucocorticoid resistance have been identified thus far in clones of these cells. Their distinctive properties are described and their relevance to clinical situations briefly discussed.

Glucocorticoid resistance is a dominant trait in hybrids between cytolytic T-lymphocyte lines and AKR thymomas

By screening several cytolytic T-lymphocyte lines, AKR thymomas, and CTL X AKR thymoma hybrids from two different crosses for their sensitivity to the glucocorticoid (GC) analog dexamethasone (dex), we have found that CTL lines and cytolytically active, IL-2-dependent (CTL-like) hybrids are resistant to the cytostatic or cytolytic effects of dex; AKR thymomas and thymoma-like hybrids (cytolytically inactive, IL-2-independent), however, are sensitive to these effects of the drug. The GC resistance behaves like a dominant trait in these crosses. Although they are resistant to GC, the CTL lines and the CTL-like hybrids do contain functional hormone receptors and macrophage-activating factor (MAF) release by the CTL lines and CTL-like hybrids is inhibited by dex.

Glucocorticoid receptors in human leukemias and related diseases

The evidence to date is compelling that steroid initiated cell lysis involves participation of the glucocorticoid receptor. Not only do the concentrations and specificity of hormones for cell lysis and receptor occupancy correspond, but also steroid resistant cells selected with or without prior mutagenesis often have altered receptors. The glucocorticoid receptor protein from humans and other species is a approximately 95,000 d, thiol group-containing monomer, prone to aggregation when "unactivated." After having bound steroid and been "activated," the monomeric steroid-receptor complex is altered in charge and shape so that its binding to chromatin and DNA is greatly enhanced. Simple measurement of numbers of receptor sites in cells from patients with various blood dyscrasias has given, in some disease, good correlations between high numbers of receptor sites and good therapeutic response. These correlations are strongest for childhood acute lymphoblastic leukemia (ALL) and for non Hodgkins' lymphoma. In other diseases, notably acute myelogenous leukemia, such correlations have not been found. The CEM human ALL line has been used in vitro to study mechanisms of glucocorticoid action and resistance. The requirement for "activated" steroid-receptor complex for cell lysis is shown in these cells by the spontaneous occurrence of steroid resistant, activation-labile receptor mutants. A second category of resistant cells with normal receptors has been defined. Treatment of these "lysis defective" resistant cells with compounds which result in DNA demethylation can render them steroid sensitive. Since DNA demethylation can allow formerly silent genes to become transcribed, it is possible that one or more genes specific for lysis has been "opened" in such cells. Alternatively, DNA demethylation may produce a general biochemical effect on the cell which renders it susceptible to lysis. Mutagenized CEM cells selected for steroid resistance give rise to a third class of mutants, which are deficient in receptor quantity. Each of these classes of steroid resistant cells contains information pertinent to understanding the use of glucocorticoids and the role of glucocorticoid receptors in human leukopathic disease.

Assignment of the human gene for the glucocorticoid receptor to chromosome 5

Human lymphoblastic leukemia cells of line CEM-C7 are glucocroticoid-sensitive and contain glucocorticoid receptors of wild-type characteristics. EL4 mouse lymphoma cells are resistant to lysis by glucocorticoids due to mutant receptors that exhibit abnormal DNA binding. Hybrids between the two cell lines were prepared and analyzed with respect to glucocorticoid responsiveness and to receptor types by DNA-cellulose chromatrography. Sensitive hybrid cell clones contained the CEM-C7-specific receptor in addition to the EL4 type of receptor. Several sensitive hybrid cell clones were used for selection of resistant segregants by growth in the presence of high concentrations of glucocorticoid. These segregants had lost the wild-type CEM-C7 receptor, while the EL4-specific receptor was retained. To identify the human chromosome that was lost concordantly with the CEM-C7 receptor the chromosomes of hybrid cells were studied by alkaline Giemsa (G-11) staining and trypsin/Giemsa banding. All hybrids contained human chromosomes in addition to one to two sets of EL4 chromosomes. Human chromosome 5 was present in all hybrid cell clones that expressed the CEM-C7 receptor and it was absent from those that did not. This absolute correlation was not observed for any other human chromosome. We conclude that the human gene for the glucocorticoid receptor is located on chromosome 5.

Regressive events in neurogenesis

The development of most regions of the vertebrate nervous system includes a distinct phase of neuronal degeneration during which a substantial proportion of the neurons initially generated die. This degeneration primarily adjusts the magnitude of each neuronal population to the size or functional needs of its projection field, but in the process it seems also to eliminate many neurons whose axons have grown to either the wrong target or an inappropriate region within the target area. In addition, many connections that are initially formed are later eliminated without the death of the parent cell. In most cases such process elimination results in the removal of terminal axonal branches and hence serves as a mechanism to "fine-tune" neuronal wiring. However, there are now also several examples of the large-scale elimination of early-formed pathways as a result of the selective degeneration of long axon collaterals. Thus, far from being relatively minor aspects of neural development, these regressive phenomena are now recognized as playing a major role in determining the form of the mature nervous system.

Isolation and characterization of lymphosarcoma P1798 variants selected for resistance to the cytolytic effects of glucocorticoids in vivo and in culture

Clonal subpopulations of lymphosarcoma P1798 have been subjected to glucocorticoid selection in vivo and in culture and the glucocorticoid binding and responsiveness of the resistant variants have been compared with those of the parental lines. Cell populations that are resistant to the cytolytic effects of glucocorticoids in vivo exhibit a slightly reduced level of glucocorticoid binding, although nuclear translocation of the hormone-receptor complex is not reduced. Sensitive and resistant tumors exhibit similar kinetics of hormone uptake and dissociation following a single injection of dexamethasone. Selection for glucocorticoid resistance in vivo does not result in an increase in the modal number of chromosomes. Cells that are resistant to the cytolytic effects of glucocorticoids in vivo are completely sensitive to the antiproliferative effects of glucocorticoids in culture. Moreover, dexamethasone increases the expression of mouse mammary tumor provirus in cytolysis-resistant and sensitive cells both in vivo and in culture. Selection for glucocorticoid resistance in culture yields variants with decreased glucocorticoid binding and/or nuclear translocation of the hormone-receptor complex. These cells appear to express classical receptor-defective phenotypes. Nevertheless, cells that are resistant to glucocorticoids in culture undergo cytolysis when treated with glucocorticoids in vivo. These data indicate that, under certain circumstances, different mechanisms may be involved in loss of glucocorticoid responsiveness in vivo and in culture.

The 46,000-dalton tyrosine protein kinase substrate is widespread, whereas the 36,000-dalton substrate is only expressed at high levels in certain rodent tissues

Proteins of molecular mass 46,000 (p46) and 34,000-39,000 (p36) daltons are phosphorylated at tyrosine in Rous sarcoma virus-transformed chicken and mouse fibroblasts. p46 has recently been identified as an isozyme of enolase but the function of p36 is unknown. The expression of these proteins in various mouse and rat tissues has been examined. In most tissues, except muscle, p46 is found at relatively constant levels. In muscle, a more basic, related protein is present. In contrast, the abundance of p36 varies more widely from tissue to tissue, suggesting that it has a function in some but not all differentiated cells. By SDS gel electrophoresis and immunoblotting, high levels of p36 (60-120% of its relative abundance in fibroblasts) were found in small intestine, lung, and thymus, and intermediate levels (20-50%) were found in spleen, lymph nodes, and testes. No p36 was detectable in brain and muscle. Where studied, p36 mRNA expression paralleled protein levels. The cell types within each tissue expressing p36 were identified by immunofluorescence and immunoperoxidase staining. These cell types include all endothelial cells and fibroblastic cells examined, as well as various epithelial cells, cardiac muscle cells, macrophages, and testicular interstitial cells. We were unable to detect p36 in skeletal or smooth muscle cells, erythrocytes, nerve cells, or lymphocytes in any of the examined tissues. p36 appears to be concentrated in the terminal web region of intestinal columnar epithelial cells.

Genetic analysis of dexamethasone resistance in L cells by somatic cell hybridization

Stable dexamethasone resistant and receptor-containing (R+) variants of L cells have been characterized by somatic cell hybridization. Neither of the variants had a clearly dominant phenotype in hybrids with dexamethasone-sensitive fibroblast lines, i.e. the resistance of the variants was not due to transdominant factors. Somatic cell hybrids formed between one of the R+-resistant clones and an independent resistant fibroblast cell line showed complementation--the hybrid clones were as sensitive to the steroid as the sensitive parental lines. Complementation, however, disappeared after continued culture of the clones. The return of the dexamethasone-sensitive phenotype was not always linked with similar changes in the responsiveness to another steroid, e.g. progesterone. Our clones can be considered to be resistant variants, designated death-less (d-), where the cells are defective in a non-receptor component involved in the hormone response. The fact that complementation can occur indicates the existence of at least two such steps in the pathway.

Role of de novo DNA methylation in the glucocorticoid resistance of a T-lymphoid cell line

A correlation has been shown between changes in the methylation pattern of cytosine residues in DNA and the expression of specific genes in differentiated tissues. The pattern of DNA methylation is conserved, through cell division, by a maintenance methylase but the mechanism by which a given pattern of methylation is established is unknown. De novo methylation of foreign DNA molecules has been shown to occur in several systems, and may serve as a signal to arrest gene expression. Conversely, treatment of cultured cell lines with 5-azacytidine results in DNA hypomethylation and leads to transcriptional activation of previously unexpressed genes. The results described here demonstrate spontaneous de novo methylation of DNA in a T-lymphoid cell line previously treated with 5-azacytidine to generate glucocorticoid sensitivity. This de novo methylation is accompanied by the acquisition of the glucocorticoid-resistant phenotype.