Cellular commitment to oncogene-induced transformation or apoptosis is dependent on the transcription factor IRF-1

Expression, purification, and co-crystallization of IRF-I bound to the interferon-beta element PRDI

Interferon regulatory factor 1 (IRF-1) is an essential factor involved in the regulation of type I interferon (IFN) and IFN-inducible genes. The protein consists of 329 amino acids that are highly conserved from mouse to human. Similar to other transcription factors, the protein is modular in nature with a basic N-terminal region involved in DNA binding and an acidic C-terminal region required for activation. We report here the expression, purification and co-crystallization of the minimal N-terminal region of IRF-1 involved in DNA binding (amino acids 1-113) with a 13 bp DNA fragment from the IFN-beta promoter. The crystals diffract to at least 3.0 A in resolution and belong to space group R3 with unit cell parameters of a = b = 84.8 A, c = 203.7 A.

How does interferon exert its cell growth inhibitory effect?

The interferons (IFNs) have become accepted therapy in a range of haematological and non-haematological malignancies. The mechanism behind IFN's antitumour action is, however, unclear. Interferons (IFNs) are capable of modulating a variety of cellular responses. One prominent effect of IFNs is their cell growth inhibitory activity, which has also been suggested to be of major importance in their antitumour action. In the present review we will discuss the cellular events leading to a decreased number of cells following IFN treatment, the molecular mechanisms underlying these phenomena, and the importance of these effects in a clinical perspective.

Interferon-independent and -induced regulation of Epstein-Barr virus EBNA-1 gene transcription in Burkitt lymphoma

Replication of the Epstein-Barr virus (EBV) genome within latently infected cells is dependent on the EBV EBNA-1 protein. The objective of this study was to identify transcriptional regulatory proteins that mediate EBNA-1 expression via the viral promoter Qp, which is active in EBV-associated tumors such as Burkitt lymphoma and nasopharyngeal carcinoma. Results of a yeast one-hybrid screen suggested that a subset of the interferon regulatory factor (IRF) family may regulate EBNA-1 transcription by targeting an essential cis-regulatory element of Qp, QRE-2. Further investigation indicated that the transcriptional activator IRF-1 and the closely related IRF-2, a repressor of interferon-induced gene expression, are both capable of activating Qp. However, the major QRE-2-specific binding activity detected within extracts of Burkitt lymphoma cells was attributed to IRF-2, suggesting that interferon-independent activation of Qp is largely mediated by IRF-2 in these cells. We observed no effect of gamma interferon on Qp activity in transfection assays, whereas we observed a moderate but significant repression of Qp activity in response to alpha interferon, possibly mediated by either the interferon consensus sequence binding protein or IRF-7, a novel alpha interferon-inducible factor identified in this study. Since expression of IRF-1 and IRF-2 is increased in response to interferons, the Qp activity observed in the presence of interferon likely represented an equilibrium between IRF factors that activate and those that repress gene expression in response to interferon. Thus, by usurping both IRF-1 and its transcriptional antagonist IRF-2 to activate Qp, EBV has evolved not only a mechanism to constitutively express EBNA-1 but also one which may sustain EBNA-1 expression in the face of the antiviral effects of interferon.

Fas- or ceramide-induced apoptosis is mediated by a Rac1-regulated activation of Jun N-terminal kinase/p38 kinases and GADD153

In the present study, we show that Fas receptor ligation or cellular treatment with synthetic C6-ceramide results in activation or phosphorylation, respectively, of the small G-protein Rac1, Jun N-terminal kinase (JNK)/p38 kinases (p38-K), and the transcription factor GADD153. A signaling cascade from the Fas receptor via ceramide, Ras, Rac1, and JNK/p38-K to GADD153 is demonstrated employing transfection of transdominant inhibitory N17Ras, N17Rac1, c-Jun, or treatment with a specific p38-K inhibitor. The critical function of this signaling cascade is indicated by prevention of Fas- or C6-ceramide-induced apoptosis after inhibition of Ras, Rac1, or JNK/p38-K.

Bi-transgenic mice reveal that K-rasVal12 augments a p53-independent apoptosis when small intestinal villus enterocytes reenter the cell cycle

Studies in cell culture systems have indicated that oncogenic forms of Ras can affect apoptosis. Activating mutations of Ras occur in approximately 30% of all human tumors and 50% of colorectal carcinomas. Since these mutations appear at early or intermediate stages in multistep journeys to neoplasia, an effect on apoptosis may help determine whether initiated cells progress towards a more neoplastic state. We have tested the effects of K-rasVal12 on apoptosis in transgenic mice. A lineage-specific promoter was used to direct expression of human K-rasVal12, with or without wild-type (wt) or mutant SV-40 T antigens (TAg), in postmitotic villus enterocytes, the principal cell type of the small intestinal epithelium. Enterocytes can be induced to reenter the cell cycle by TAgWt. Reentry is dependent upon the ability of TAg to bind pRB and is associated with a p53-independent apoptosis. Analyses of K-rasVal12 x TAgWt bi-transgenic animals indicated that K-rasVal12 can enhance this apoptosis threefold but only in cycling cells; increased apoptosis does not occur when K-rasVal12 is expressed alone or with a TAg containing Glu107,108two head right arrow Lys107, 108 substitutions that block its ability to bind pRB. Analysis of bi-transgenic K-rasVal12 x TAgWt mice homozygous for wild-type or null p53 alleles established that the enhancement of apoptosis occurs through a p53-independent mechanism, is not attributable to augmented proliferation or to an increase in abortive cell cycle reentry (compared to TAgWt mice), and is not associated with detectable changes in the crypt-villus patterns of expression of apoptotic regulators (Bcl-2, Bcl-xL, Bak, and Bax) or mediators of epithelial cell-matrix interactions and survival (e.g., alpha5beta1 integrin and its ligand, fibronectin). Coexpression of K-rasVal12 and TAgWt produces dysplasia. The K-rasVal12-augmented apoptosis is unrelated to this dysplasia; enhanced apoptosis is also observed in cycling nondysplastic enterocytes that produce K-rasVal12 and a TAg with a COOH-terminal truncation. The dysplastic epithelium of K-rasVal12 x TAgWt mice does not develop neoplasms. Our results are consistent with this finding: (a) When expressed in initiated enterocytes with a proliferative abnormality, K-rasVal12 facilitates progression to a dysplastic phenotype; (b) by diminishing cell survival on the villus, the oncoprotein may impede further progression; and (c) additional mutations may be needed to suppress this proapoptotic response to K-rasVal12.

The interferon-inducible murine p48 (ISGF3gamma) gene is regulated by protooncogene c-myc

p48 protein is an integral component of the multimeric interferon (IFN)-regulated transcription factor, ISGF3. We have shown earlier that this gene is regulated by a novel IFN-gamma-regulated element. In addition to the IFN-regulated element, a myc-max binding site is also present in this promoter. In this investigation we have studied the role of this site in the regulation of the p48 gene. In serum-induced quiescent cells Myc up-regulated the expression of p48 mRNA. We show that the protooncogene Myc regulates the expression of p48 through the element CACGTG. Mutations in this motif abolish Myc-inducibility of the reporter genes carrying p48 promoter elements. Purified Myc and Max proteins interact with the Myc-stimulated element of the p48 promoter. We also show that cells lacking p48 expression are highly susceptible to the cytocidal action of anticancer drugs. Taken together these data suggest that p48 may function as an anti-stress cell survival factor.

Interferon-regulatory factors during development of CD4 and CD8 thymocytes

Selection events in the thymus occur at the double-positive CD4+ CD8+ (DP) developmental stage leading either to further differentiation of the CD4+ and CD8+ lineages or to deletion. The interferon-regulatory factor IRF-1 has been implicated in signalling for T-cell death and also in CD8+ thymic differentiation. IRF-1 is an activator and IRF-2 a repressor of gene transcription regulated by type 1 interferons (IFN). To evaluate the role of IRF-1 and IRF-2 in the differentiation of CD4 and CD8 thymocytes, we analysed their DNA-binding activity before and after antigenic stimulation at different stages of thymic development and in peripheral T cells. Unseparated, double-positive and single-positive thymocytes as well as peripheral T lymphocytes from mice transgenic (tg) for a T-cell receptor (TCR), restricted either by major histocompatibility complex class I or class II, were stimulated by their nominal antigen. Our results demonstrate that the DNA-binding activity of IRF-2 and, weakly, that of IRF-1 are inducible in total thymocytes in response to antigen. There is no induction of IRF-1/IRF-2 binding activity at the double-positive stage of thymic development in the MHC class II-restricted model whereas in the MHC class I-restricted model IRF-1/IRF-2 activity is induced weakly. At the single-positive stage, antigen induces the IRF-1/IRF-2 DNA binding in both CD4+ and CD8+ thymocytes, but not in mature lymphocytes from the periphery. This pattern of expression suggests that IRF-1/IRF-2 binding activities resulting from antigen stimulation are developmentally regulated. No evidence for a selective role of IRF-1 in the development of the CD8+ lineage was found, however.

Endothelial apoptosis in Braf-deficient mice

Tyrosine kinase growth factor receptors and Ras/Raf/MEK/MAPK signalling have been implicated in the suppression as well as augmentation of programmed cell death. In addition, a Ras-independent role for Raf as a suppressor of programmed cell death has been suggested by the recent finding that Craf1 interacts with members of the Bcl-2 family at mitochondrial membranes. However, genetic studies of C. elegans and Drosophila, as well as the targeted mutagenesis of the murine Araf gene, have failed to support such a role. Here we show that mice with a targeted disruption in the Braf gene die of vascular defects during mid-gestation. Braf -/- embryos, unlike Araf -/- or Craf1 -/- embryos (L.W. et al., unpublished), show an increased number of endothelial precursor cells, dramatically enlarged blood vessels and apoptotic death of differentiated endothelial cells. These results establish Braf as a critical signalling factor in the formation of the vascular system and provide the first genetic evidence for an essential role of Raf gene in the regulation of programmed cell death.

Ras activation leads to cell proliferation or apoptotic cell death upon interleukin-2 stimulation or lymphokine deprivation, respectively

Lymphokine-dependent cells undergo apoptosis upon lymphokine withdrawal. We describe that lymphokine deprivation of the interleukin (IL)-2- or IL-4-dependent mouse T cell line TS1 alpha beta induces Ras activation which plays a role in programmed cell death, since blocking Ras activity reduces the induction of apoptosis. Induction of apoptosis by lymphokine deprivation can be prevented by expression of the Bcl-2 protein. Rescue from cell death by IL-2 also promotes Ras activation, but, in contrast to lymphokine withdrawal, stimulates Bcl-2 expression. IL-4-induced cell survival is Ras- and Bcl-2 independent. These results are compatible with a model in which cell proliferation requires the simultaneous induction of at least two pathways which act in combination to prevent cell death.

Attenuation of nitric oxide synthase induction in IRF-1-deficient glial cells

Nitric oxide (NO) produced by inducible nitric oxide synthase (iNOS) exerts inhibitory and cytotoxic effects on various cells including neuronal cells. Glial NO production, mediated via induction of iNOS, is thought to facilitate neuronal damage during cerebral ischemia. Recently, interferon regulatory factor-1 (IRF-1) has been reported to be an essential transcription factor for iNOS mRNA induction in murine macrophages. However, expression of IRF-1 and its role in the central nervous system have not been examined. In the present study, by using primary glial cell cultures from mice with targeted disruption of the IRF-1 gene, we investigated whether IRF-1 is involved in iNOS mRNA induction in glial cells. After stimulation with lipopolysaccharide and interferon-gamma, IRF-1 mRNA was strongly induced in wild-type (IRF-1 +/+) glial cells. iNOS mRNA induction and nitrite production in IRF-1 -/- glial cells were reduced as compared with those observed in IRF-1 +/+ glial cells. Diethyldithiocarbamate, a selective inhibitor of nuclear transcription factor kappa B (NF-kappa B), completely inhibited iNOS mRNA induction. These results suggest that not only NF-kappa B but also IRF-1 play important roles in iNOS mRNA induction in the central nervous system.

Interferon regulatory factor-1 up-regulates angiotensin II type 2 receptor and induces apoptosis

The expression of the angiotensin II type 2 (AT2) receptor is developmentally and growth regulated. In cultured R3T3 cells, expression of this receptor is markedly induced at the confluent state and with serum deprivation. In this study we demonstrated that the removal of serum from culture media resulted in the induction of apoptosis in these cells and the addition of angiotensin II further enhanced apoptosis. We have previously identified an interferon regulatory factor (IRF) binding motif in the mouse AT2 receptor gene promoter region. In this report, we observed that serum removal increased IRF-1 expression, with a rapid and transient decrease of IRF-2. To prove that the changes in IRFs after serum removal mediated apoptosis and up-regulated AT2 receptor, we transfected antisense oligonucleotides for IRF-1 or IRF-2 into R3T3 cells and observed that IRF-1 antisense oligonucleotide attenuated apoptosis and abolished the up-regulation of AT2 receptor. IRF-2 antisense oligonucleotide pretreatment did not affect the onset of apoptosis after serum removal; instead, it increased AT2 receptor binding and enhanced angiotensin II-mediated apoptosis. Taken together, these results suggest that increased IRF-1 after serum starvation contributes to the induction of apoptosis and that increased IRF-1 up-regulates the AT2 receptor expression after serum starvation, resulting in enhanced angiotensin II-mediated apoptosis.

Phosphorylation events modulate the ability of interferon consensus sequence binding protein to interact with interferon regulatory factors and to bind DNA

Two families of transcription factors mediate interferon (IFN) signaling. The first family, signal transducers and activators of transcription (STATs), is activated within minutes of IFN treatment. Specific phosphorylation events lead to their translocation to the nucleus, formation of transcriptional complexes, and the induction of the second family of transcription factors termed interferon regulatory factors (IRFs). Interferon consensus sequence binding protein (ICSBP) is a member of IRF family that is expressed only in cells of the immune system and acts as a transcriptional repressor. ICSBP binds DNA through the association with other transcription factors such as IRF-1 or IRF-2. In this communication, the domain that is involved in protein-protein interactions was mapped to the carboxyl terminus of ICSBP. This domain is also important for mediating ICSBP-repressing activity. In vitro studies demonstrated that direct binding of ICSBP to DNA is prevented by tyrosine (Tyr) phosphorylation. Yet, Tyr-phosphorylated ICSBP can bind target DNA only through the association with IRF-2 and IRF-1. This type of phosphorylation is essential for the formation of heterocomplexes. Tyr-phosphorylated ICSBP and IRF-2 are detected in expressing cells constitutively, and Tyr-phosphorylated IRF-1 is induced by IFN-gamma. These results strongly suggest that like the STATs, the IRFs are also modulated by Tyr phosphorylation that affects their biological activities.

Abrogation of G1 arrest after DNA damage is associated with constitutive overexpression of Mdm2, Cdk4, and Irf1 mRNAs in the BALB/c 3T3 A31 variant 1-1 clone

A search of cell lines showing aberrant cell-cycle checkpoints revealed the lack of gamma-irradiation-induced G1 arrest in the BALB/c 3T3 A31 variant 1-1 (A31-1-1) clone. This clone is well-known for its hypersensitivity to transformation by DNA damaging agents. p53 stabilization and p21 mRNA induction after 8 Gy irradiation were observed, suggesting that the abrogation of G1 arrest occurred in spite of normal p53 and p21 responses by abnormal regulation of other cellular factors. Constitutive overexpression of Mdm2 and Cdk4 mRNAs was found, which might have contributed to the loss of G1 arrest. In addition, overexpression of a growth-suppressive transcription factor, Irf1, a putative key molecule in the p53-independent pathway after DNA damage, was also observed, although the relation to the loss of G1 arrest could not be elucidated.

A double-stranded RNA-activated protein kinase-dependent pathway mediating stress-induced apoptosis

Apoptosis occurs in response to different cellular stresses, including viral infection, inflammatory cytokines, growth factor deprivation, and UV light, but it is unclear whether these inducers share a common mechanism of induction. The interferon-induced, double-stranded RNA-activated protein kinase (PKR) has been implicated in processes that rely on apoptosis as control mechanisms in vivo, including antiviral activities, cell growth regulation, and tumorigenesis. Here we report that mouse embryo fibroblasts from mutant mice containing homozygous deletions in the PKR gene (Pkr(0/0) mice) were resistant to apoptotic cell death in response to double-stranded RNA, tumor necrosis factor-alpha, or lipopolysaccharide. The mechanism underlying the suppression of apoptosis in the Pkr(0/0) cells could be attributed to defects in the activation of DNA-binding activity for the transcription factor interferon regulatory factor-1 and in Fas mRNA induction. Thus, these results provide genetic evidence implicating a requirement for PKR in mediating different forms of stress-related apoptosis.

Cytokines, molecular biological abnormalities, and acute myelogenous leukemia

Leukemia cell proliferation is dependent upon cytokines produced by the leukemia cells or by the microenvironment under stimulation by leukemia cells. Abnormal expression of the p53, rb, and ras genes is associated with cytokine production, suggesting that abnormal expression can affect leukemia cells by affecting intracellular growth controls and by stimulating cytokine production. While these observations suggest that cytokines can be used to stimulate leukemia proliferation during cytotoxic therapy increasing the sensitivity to treatment, they also suggest that inhibition of cytokine affects could increase clinical responses by reducing leukemia cell regrowth between courses of therapy.

Oncogenic ras provokes premature cell senescence associated with accumulation of p53 and p16INK4a

Oncogenic ras can transform most immortal rodent cells to a tumorigenic state. However, transformation of primary cells by ras requires either a cooperating oncogene or the inactivation of tumor suppressors such as p53 or p16. Here we show that expression of oncogenic ras in primary human or rodent cells results in a permanent G1 arrest. The arrest induced by ras is accompanied by accumulation of p53 and p16, and is phenotypically indistinguishable from cellular senescence. Inactivation of either p53 or p16 prevents ras-induced arrest in rodent cells, and E1A achieves a similar effect in human cells. These observations suggest that the onset of cellular senescence does not simply reflect the accumulation of cell divisions, but can be prematurely activated in response to an oncogenic stimulus. Negation of ras-induced senescence may be relevant during multistep tumorigenesis.

Expression of the 90K immunostimulator gene is controlled by a promoter with unique features

90K is a secreted glycoprotein with tumor suppressive functions, which is up-regulated in various types of cancer and in AIDS. In order to understand the regulation of its expression, the mouse 90K gene was isolated and analyzed. The gene spans about 8.8-kilobase pairs and consists of 6 exons and was localized on chromosome 11, region E. RNase protection identified one major transcription start site (+1) and three minor ones (-3, +32, +34). The mouse 90K gene was found to have a TATA-less promoter of unusual structure. The 2. 3-kilobase pair 5'-flanking region exhibited strong promoter activity in NIH 3T3 cells; however, it contained neither a TATA-box nor a SP1 site and was not GC-rich. No known initiator motif was found around the transcription start site. 5'- and 3'-deletions defined a minimal promoter of 51 base pairs (-66 --> -16), not including the start site, essential and sufficient for promoter activity. This minimal promoter showed increased activity after stimulation with interferon-gamma or poly(I.C), a substance mimicking viral infection. Essential for both inductions was the integrity of an interferon regulatory factor element within this sequence, a potential binding site for the anti-oncogenic transcription factor interferon regulatory factor-1.

p53 protein accumulation and p53 gene mutation in esophageal carcinoma. A molecular and immunohistochemical study with clinicopathologic correlations

BACKGROUND

p53 gene mutation and p53 protein accumulation are common in human cancer. However, their clinical significance is controversial and p53 accumulation may not correlate with gene mutation. The current study investigates the occurrence of p53 alterations in esophageal carcinoma, the correlation between the analyses at the gene and protein level, and their prognostic significance.

METHODS

A series of 74 esophageal carcinomas (46 squamous cell carcinomas, 21 Barrett's adenocarcinomas, and 7 undifferentiated carcinomas) was studied by single strand conformation polymorphism (SSCP) analysis and immunohistochemistry (IHC) to detect p53 mutation and accumulation, respectively.

RESULTS

p53 mutations in exons 5-8 were detected in 53% of the carcinomas whereas p53 accumulation was observed in 57% of cases. Comparing SSCP and IHC, there were 27 discordant cases (38%). Overall, only 20 tumors (27%) did not display p53 mutation and/or p53 accumulation. No associations were found between p53 aberrations and clinicopathologic parameters, including patients age and gender tumor type, stage, and grade. p53 protein accumulation and p53 gene mutation were not related to patient survival by univariate or multivariate analysis in esophageal carcinomas.

CONCLUSIONS

p53 aberrations are very common in esophageal carcinomas. However, p53 gene mutation and p53 protein accumulation have a significant discordance, suggesting that p53 function may be inactivated by mechanisms other than mutation. p53 aberrations do not independently predict prognosis in esophageal tumors.

Constitutive activation of Epstein-Barr virus (EBV) nuclear antigen 1 gene transcription by IRF1 and IRF2 during restricted EBV latency

The Epstein-Barr virus (EBV) EBNA1 gene promoter active in the type I program of restricted viral latency was recently identified and shown to reside in the viral BamHI Q fragment. This promoter, Qp, is active in a wide variety of cell lines and has an architecture reminiscent of eukaryotic housekeeping gene promoters (B. C. Schaefer, J. L. Strominger, and S. H. Speck, Proc. Natl. Acad. Sci. USA 92:10565-10569, 1995; B. C. Schaefer, J. L. Strominger, and S. H. Speck, Mol. Cell. Biol. 17:364-377, 1997). Here we demonstrate by deletion analysis that the important cis-acting elements regulating Qp are clustered in a relatively small region (ca. 80 bp) surrounding the site of transcription initiation. Immediately upstream of the site of initiation is a region which is protected from DNase I digestion by crude nuclear extracts. Electrophoretic mobility shift analyses (EMSA) employing probes spanning this region demonstrated the presence of two major protein complexes. Deletion analysis of Qp demonstrated that at least one of these complexes plays an important role in Qp activity. Evidence that interferon response factor 2 (IRF2) is a major constituent of the most prominent EMSA complex and that IRF1 may be a minor component of this complex is presented. Transfections into IRF1-/-, IRF2-/-, and IRF1,2-/- fibroblasts demonstrated that absence of both IRF1 and IRF2 reduced Qp activity to approximately the same extent as mutation of the IRF-binding site in Qp, strongly implicating IRF2, and perhaps IRF1, in the regulation of Qp activity. Notably, transcription from Qp was not inducible by either alpha or gamma interferon in EBV-negative B cells but rather was shown to be constitutively activated by IRF1 and IRF2. This observation suggests that IRF1 and IRF2 have a previously unrecognized role as constitutive activators of specific genes. Additionally, data presented indicate that a protein complex containing the nonhistone architectural protein HMG-I(Y) binds to the region identified as the major transcription initiation site for Qp. This observation raises the possibility that HMG-I(Y)-induced DNA bending plays a role in the initiation of transcription from Qp.

Differential human multiple myeloma cell line responsiveness to interferon-alpha. Analysis of transcription factor activation and interleukin 6 receptor expression

Although IFN-alpha is commonly used as maintenance treatment for multiple myeloma patients, its effectiveness is varied. In this study, we have used a panel of IL-6 responsive myeloma cell lines that vary remarkably in responsiveness to IFN-alpha. Three cell lines were growth arrested by IFN-alpha; however, IFN-alpha significantly stimulated growth of the fourth cell line, KAS-6/1. Our studies have focused on elucidating the mechanism of differential IFN-alpha responsiveness. First, we have shown that IFN-alpha-stimulated growth of the KAS-6/1 cells did not result from induction of autocrine IL-6 expression. Second, analysis of Stats 1, 2, and 3 and IFN regulatory factor-1 (IRF-1) and IRF-2 activation failed to reveal differences between the IFN-alpha growth-arrested or growth-stimulated cells. Third, although IFN-alpha treatment of the IFN-alpha growth-inhibited cell lines reduced IL-6 receptor (IL-6R) expression, IFN-alpha also reduced KAS-6/1 IL-6R expression. Finally, although IFN-alpha treatment reduced IL-6R numbers on each cell line, analysis of Stat protein activation revealed that the receptors were still functional. We conclude that myeloma cell responsiveness to IFN-alpha is heterogeneous and that mechanisms of IFN-alpha-mediated growth inhibition other than IL-6R downregulation must exist in myeloma. Identification of these mechanisms may allow development of agents that are more universally effective than IFN-alpha.

Requirement for the transcription factor LSIRF/IRF4 for mature B and T lymphocyte function

Lymphocyte-specific interferon regulatory factor (LSIRF) (now called IRF4) is a transcription factor expressed only in lymphocytes. Mice deficient in IRF4 showed normal distribution of B and T lymphocyes at 4 to 5 weeks of age but developed progressive generalized lymphadenopathy. IRF4-deficient mice exhibited a profound reduction in serum immunoglobulin concentrations and did not mount detectable antibody responses. T lymphocyte function was also impaired in vivo; these mice could not generate cytotoxic or antitumor responses. Thus, IRF4 is essential for the function and homeostasis of both mature B and mature T lymphocytes.

Reduced incidence and severity of antigen-induced autoimmune diseases in mice lacking interferon regulatory factor-1

Interferon regulatory factor-1 (IRF-1) is a transcription factor that regulates interferon-induced genes and type I interferons. Recently, studies of IRF-l-deficient mice have revealed that IRF-I regulates the induction of molecules that play important roles in inflammation, such as inducible nitric oxide synthase (iNOS) and interleukin-l beta-converting enzyme (ICE). To study the role of IRF-1 in autoimmunity, we investigated type II collagen-induced arthritis (CIA), and experimental allergic encephalomyelitis (EAE), in mice lacking IRF-1. The incidence and severity of CIA were significantly decreased in IRF-1-/- mice compared with IRF-l +/- mice, as was the production of interferon (IFN)-gamma in lymph node cells. Both IRF-l+/- and IRF-1-/- mice exhibited mild and transient disease after adoptive transfer of a type II collagen (CII)-specific T cell line together with sera from arthritic mice, but the IRF-1-/- mice were less severely affected than the IRF-1+/- mice. In addition, the incidence of EAE in IRF-1-/- mice was decreased as compared with IRF-1 +/- mice. Reverse transcription polymerase chain reaction showed that IRF-1 mRNA was constitutively expressed in the spinal cords of IRF-1+/- mice, and was upregulated in mice with clinical EAE. Expression of iNOS was also detected in inflamed spinal cords. These results suggest that IRF-I plays a key role in promoting inflammation and autoimmunity in CIA and EAE animal models.

Deficient cytokine signaling in mouse embryo fibroblasts with a targeted deletion in the PKR gene: role of IRF-1 and NF-kappaB

The interferon (IFN)-induced double-stranded RNA (dsRNA)-activated Ser/Thr protein kinase (PKR) plays a role in the antiviral and antiproliferative effects of IFN. PKR phosphorylates initiation factor eIF2alpha, thereby inhibiting protein synthesis, and also activates the transcription factor, nuclear factor-kappaB (NF-kappaB), by phosphorylating the inhibitor of NF-kappaB, IkappaB. Mice devoid of functional PKR (Pkr(o/o)) derived by targeted gene disruption exhibit a diminished response to IFN-gamma and poly(rI:rC) (pIC). In embryo fibroblasts derived from Pkr(o/o) mice, interferon regulatory factor 1 (IRF-1) or guanylate binding protein (Gbp) promoter-reporter constructs were unresponsive to IFN-gamma or pIC but response could be restored by co-transfection with PKR. The lack of responsiveness could be attributed to a diminished activation of IRF-1 and/or NF-kappaB in response to IFN-gamma or pIC. Thus, PKR acts as a signal transducer for IFN-stimulated genes dependent on the transcription factors IRF-1 and NF-kappaB.

Interferon gamma-induced transcription of the murine ISGF3gamma (p48) gene is mediated by novel factors

In this investigation, we show that the gene encoding p48, a subunit of transcription factor ISGF3, is transcriptionally induced by interferon gamma (IFN-gamma). We have identified a novel IFN-gamma-activated response element in the p48 gene promoter. This motif, notated as gamma-activated transcriptional element (GATE), has no significant resemblance to either pIRE (palindromic IFN-response element) or GAS (the IFN-gamma-activated sequence) but has partial homology to ISRE (IFN-stimulated response element). When fused to a neutral promoter, GATE, a 24-bp element, induced the expression of reporter genes following IFN-gamma treatment. In murine RAW cells, two IFN-gamma-inducible factors (GIF) bind to GATE. Binding of these factors to GATE is inhibited by cycloheximide and staurosporine. Although p48 gene induction is dependent on STAT1 and JAK1, activated STAT1 does not bind to GATE. Thus, GIFs appear to be novel trans-acting factors in the IFN-signaling pathway.

Functional aspects of apoptosis in hematopoiesis and consequences of failure

Apoptosis is an internally directed, physiological method of cell destruction. Cellular components are dismantled within the confines of an intact cell membrane, and rapid ingestion by phagocytic cells prevents local inflammation. A variety of genes have now been identified as positive or negative regulators of apoptosis. Transfection experiments and studies of gene cooperation in viral transformation suggest that full cellular transformation requires not only the deregulation of proliferation, but also the inhibition of concomitant apoptosis programs. The regulation of apoptosis is fundamental to hematopoietic homeostasis. Stem cell renewal is continuously counterbalanced by apoptosis in functionally inactive or terminally differentiated cells. Extensive cell death in developing lymphocyte populations ensures that only cells recognizing non-self antigens are released into the periphery, and the finite lifespan of terminally differentiated cells enables the extensive cell turnover demanded by functional aspects of the hematopoietic system. The requirement of each hematopoietic sub-population for a specific sub-set of survival factors, provides a flexible mechanism for dictating the cellular composition of the mature population and for controlling population size. Surplus cell production and apoptosis are therefore normal features of hematopoiesis. The consequences of deregulated apoptosis are severe. Excessive apoptosis in lymphocyte populations plays a major role in the pathogenesis of acquired immunodeficiency syndrome (AIDS), whereas ineffective apoptosis has been associated with the development of inflammation, autoimmunity and hematological malignancies. The identification of various genetic abnormalities which influence apoptosis in leukaemic cells (e.g., mutant p53, Bcr-Abl and over-expression of Bcl-2), suggests that the acquisition of an anti-apoptotic lesions is an important event in the multi-step evolution of hematological malignancies. In addition, the nature of some leukaemias particularly the chronic leukemias, in which the leukemic cells are nonproliferative and long lived, suggests that anti-apoptotic lesions are early events in the pathogenesis of these diseases. It is likely that the utilization of mechanisms to evade apoptosis would facilitate disease progression in all leukemias and contribute to the development of multi-drug resistance. A better understanding of apoptosis mechanisms in hematopoietic cells, and their exploitation by leukemic cells should be useful in the development of improved cytotoxic regimes.

Fas-induced apoptosis is mediated by activation of a Ras and Rac protein-regulated signaling pathway

Fas induces apoptosis in lymphocytes via a poorly defined intracellular signaling mechanism. We and others have previously demonstrated the involvement and significance of a signaling cascade from the Fas receptor via sphingomyelinases and ceramide to Ras in apoptosis (Gulbins, E., Bissonette, R., Mahboubi, A., Nishioka, W., Brunner, T., Baier G., Baier-Bitterlich, G., Byrd, C., Lang, F., Kolesnick, R., Altman, A., and Green, D. (1995) Immunity 2, 341; Cifone, M. G., DeMaria, R., Roncali, P., Rippo, M. R., Azuma, M., Lanier, L. L., Santoni, A., and Testi, R. (1994) J. Exp. Med. 180, 1547-1552; Gill, B. M., Nishikata, H., Chan, G., Delovitch, T. L., and Ochi, A. (1994) Immunol. Rev. 142, 113-126). Here, we demonstrate an activation of the small G-proteins Rac 1 and Rac 2 after Fas receptor triggering. Expression of a transdominant inhibitory Ras mutant (N17Ras) prevents Rac 1 and Rac 2 stimulation, suggesting a signaling cascade from the Fas receptor via Ras to Rac 1 and Rac 2. Genetic and pharmacological inhibition of Ras or Rac 1 and Rac 2 stimulation blocks Fas-induced apoptosis, pointing to an important function of a Ras and Rac protein-regulated signaling pathway in Fas-mediated programmed cell death.

Interdependent transcription control elements regulate the expression of the SPRR2A gene during keratinocyte terminal differentiation

Expression of the SPRR2A gene, a member of the small proline-rich family of cornified cell envelope precursor proteins, is strictly linked to keratinocyte terminal differentiation both in vivo and in vitro. In this study, we explored the molecular mechanisms underlying this regulation in transiently transfected primary keratinocytes induced to differentiate in vitro. Deletion mapping and site-directed mutagenesis of SPRR2A promoter-chloramphenicol acetyltransferase constructs indicate that four transcription control elements are essential and sufficient for promoter activity. These elements were further characterized by electrophoretic mobility shift and identified as (i) an inverted octamer doublet, bound by the POU domain factor Oct-11 (Skn-1a/i, Epoc-1), (ii) an interferon-stimulated response element recognized by interferon regulatory factors 1 and 2, (iii) an Ets binding site partially overlapping the interferon-stimulated response element, and (iv) a TG box recognized by the Sp1 family of zinc finger transcription factors. Destruction of a single terminal differentiation element is sufficient to completely abolish transcription from the SPRR2A promoter, indicating that these transcription control elements function in concert in an interdependent manner. Apparently, integration of signals transmitted by the above-mentioned transcription factors is necessary and sufficient to promote gene expression during keratinocyte terminal differentiation.

Fas-induced programmed cell death is mediated by a Ras-regulated O2- synthesis

Fas induces apoptosis in lymphocytes via a poorly defined intracellular signalling cascade. Previously, we have demonstrated the involvement and significance of a signalling cascade from the Fas receptor via sphingomyelinases and ceramide to Ras in Fas-induced apoptosis. Here we demonstrate rapid and transient synthesis of reactive oxygen intermediates (ROI) via activation of Ras after Fas. Genetic inhibition of Ras by transfection of transdominant inhibitory N17Ras blocked Fas-mediated ROI synthesis and programmed cell death. Likewise, the antioxidants N-acetyl-cysteine and N-t-butyl-phenylnitrone abolished Fas-induced cell death, pointing to an important role for Ras-triggered ROI synthesis in Fas-mediated programmed cell death.

Interferon regulatory factor-2 directs transcription from the gp91phox promoter

Repressor elements in the gp91(phox) promoter are necessary to restrict tissue-specific transcription to mature phagocytes. Deletion of these elements leads to significant promoter activity in cell lines such as HEL and K562 that do not normally express gp91(phox). The -100 to +12 base pair gp91(phox) promoter region is sufficient to direct maximal de-repressed transcription in these cells. However, promoter activity is dramatically decreased following a 16-base pair truncation that deletes an interferon-stimulated response element. This element interacts with IRF-1 and IRF-2, members of the interferon regulatory factor family of transcription factors. In addition, this promoter region is bound by a factor with properties similar to BID, a DNA-binding protein that also interacts with three upstream sites within the gp91(phox) promoter. Transient transfection studies using mutated promoters indicate that both the IRF and BID binding sites are required for maximal gp91(phox) promoter activity. Overexpression of IRF-1 or IRF-2 in K562 cells leads to transactivation of gp91(phox) promoter constructs, which is dependent on the presence of an intact IRF binding site. IRF-2 predominates in macrophages that express the gp91(phox) gene as well as in HEL and K562 cells. We conclude that IRF-2 and BID activate gp91(phox) promoter activity in the absence of transcriptional repression.

Structure, mapping and expression of a human NOR-1 gene, the third member of the Nur77/NGFI-B family

We identified a human homologue of NOR-1 (neuron-derived orphan receptor) from the fetal brain. There are two transcripts for human NOR-1, encoding 626 amino acid residues with a calculated molecular mass of 68 kDa. The high homology between hNOR-1, mNur77/rNGFI-B/hTR3, and mNurr1/rRNR-1/hNOT indicated that these three orphan receptors form a distinct subfamily within the steroid/thyroid receptor superfamily. Human NOR-1 mRNA was detected in the adult heart and skeletal muscle as well as in the fetal brain, indicating that its expression is not restricted to events that occur during neural development. The hNOR-1 gene is more than 35 kilobases long and interrupted by seven introns. The exon-intron structure of the gene is generally conserved when compared with the steroid/thyroid receptor superfamily and is remarkably similar to that of the Nur77/NGFI-B genes. This suggests that the Nur77/NGFI-B family has evolved from a common ancestral gene. Fluorescence in situ hybridization (FISH) revealed that the gene is located on chromosome 9q.

Treatment of ME180 cells with interferon-gamma causes apoptosis as a result of tryptophan starvation

Indoleamine 2'3 dioxygenase (INDO), the rate-limiting enzyme in the catabolism of the essential amino acid L-tryptophan, is induced in many cell lines following interferon gamma (IFN-gamma) treatment. The induction of this enzyme has been associated with the antiparasitic and cytotoxic activities of human IFN-gamma. DNA analysis coupled to morphologic studies indicated that ME180 cells underwent apoptosis within 48 h of treatment with IFN-gamma. We hypothesized that apoptosis results from L-tryptophan starvation following INDO induction. This was confirmed by the prevention of apoptosis on adding back tryptophan to IFN-gamma-treated cells and the induction of apoptosis by removing tryptophan from the medium in the absence of IFN-gamma.

Interferons and the tumor cell

Optimal use of interferons (IFNs) for the treatment of tumor disease requires experimental work in order to precisely define IFN actions. We have pointed out three modes of such actions relevant for the antitumor efficacy exerted by IFNs: effects on apoptosis, effects on genes involved in malignant transformation and effects on angiogenesis. These are but three selected areas forming a basis for the development of optimal IFN therapy. Further experimental work, undertaken in these and additional IFN areas, is mandatory for the most effective clinical use of IFNs for the treatment of tumor disease.

Cooperation of the tumour suppressors IRF-1 and p53 in response to DNA damage

Normally growing cells promptly cease DNA synthesis when exposed to genotoxic stresses, such as radiation, and this cell-cycle arrest prevents the accumulation of mutations. The transcription factor interferon regulatory factor (IRF)-1 is essential for the regulation of the interferon system, inhibits cell growth, and manifests tumour-suppressor activities. Here we show that mouse embryonic fibroblasts (EFs) lacking IRF-1 are deficient in their ability to undergo DNA-damage-induced cell-cycle arrest. A similar phenotype has been observed in EFs lacking the tumour suppressor p53 (refs 8, 9), although the expression of IRF-1 and p53 are independent of one another. Furthermore, we show that transcriptional induction of the gene encoding p21 (WAF1, CIP1), a cell-cycle inhibitor, by gamma-irradiation is dependent on both p53 and IRF-1, and that the p21 promoter is activated, either directly or indirectly, by both in a transient cotransfection assay. These two tumour-suppressor transcription factors therefore converge functionally to regulate the cell cycle through the activation of a common target gene.

Mycobacterium tuberculosis heat shock protein 10 increases both proliferation and death in mouse P19 teratocarcinoma cells

Exogenously added Mycobacterium tuberculosis Hsp10, either synthetic or recombinant, but not other related heat shock proteins (GroES from Escherichia coli or bovine Ubiquitin), increases apoptosis in serum-deprived P19 mouse teratocarcinoma cells. The effect is dose-dependent, with a bell-shaped curve and peak activity at 10(-9) M (maximal effect: 62.9 +/- 17.7% increase, mean +/- SD, n = 10) and is specifically inhibited by a polyclonal antibody raised against the synthetic protein. On the other hand, when the same cells are exponentially growing, M. tuberculosis Hsp10 increases cell proliferation with a bell-shaped dose-response curve and a moderate decrease in potency (peak-activity at 10(-8)-10(-7) M, with a 43.7 +/- 8.1% increase, mean +/- SD, n = 3). Therefore, it appears that this bacterial protein can exert two opposite effects, behaving either as a death- or as a growth-promoting factor, depending on the conditions of the target.

ISGF3 gamma p48, a specificity switch for interferon activated transcription factors

Interferon (IFN) induces gene expression by phosphorylating latent transcription factors of the STAT family. Two different STAT multimeric complexes that bind distinct enhancer elements are activated by IFN alpha and IFN gamma, dictated by the DNA-binding protein ISGF3 gamma p48. This protein, a member of the IFN regulatory factor (IFR) family, acts as an adaptor protein to redirect STAT multimers from their intrinsic palindromic sequence specificity to interactions with a composite element composed of an IRF site juxtaposed with a STAT half-site. Sequence similarity within the IRF family suggests that other members could serve as adaptor proteins for transcriptional activators. Recent evidence suggests that PIP (LSIRF) sequesters the Ets protein PU.1 at a composite DNA element lends support to this adaptor hypothesis.

Lessons from the p53 mutant mouse

The use of the mouse as a model organism in cancer research has a long and productive history, from the earliest studies of chemical carcinogenesis to the recent advances in gene targeting. Many of the basic principles of tumorigenesis have been formed in whole or in part through the study of tumor development in the mouse. Over the past decade, the major experimental approach has been to generate cancer-prone strains, either through transgenic technologies or, more recently, gene targeting. Here, I will review the state of the field of gene targeting of tumor-suppressor genes and concentrate on the p53 mutant strains and the lessons learned from the p53 mutant mouse.

Human chromosome 3 mediates growth arrest and suppression of apoptosis in microcell hybrids

Chemotherapeutic treatment of tumor cells leads either to tumor cell death (usually by apoptosis) or to the formation of drug-resistant subpopulations. Known mechanisms of cancer cell drug resistance include gene amplification and increased expression of drug transporters. On the other hand, normal cells survive many forms of chemotherapy with minimal damage probably because of their capacity for growth arrest and stringent control of apoptosis. Microcell hybrids between B78 (murine melanoma) and HSF5 (normal human fibroblasts) were analyzed to identify a new human chromosomal region involved in the promotion of drug-induced growth arrest and suppression of apoptosis. In these hybrids, the presence of human chromosome 3 was strongly associated with suppression of apoptosis via G1 and G2 growth arrest during exposure to the antimetabolite N-phosphonoacetyl-L-aspartate (PALA), suggesting that a gene(s) on chromosome 3 serves an antiproliferative role in a drug-responsive growth arrest pathway.

Why do so many cancer patients fail to respond to interferon therapy?

Since their first use in the clinic some 25 years ago, interferons (IFNs) have become accepted therapy in a range of cancer forms. However, although in some patients they induce remission, in the great majority they are of no benefit or, at best, lead only to minor improvements. This review considers possible reasons for these failures.

Autocrine activation by interferon-gamma of STAT factors following T cell activation

The activation of T cells requires engagement of the T cell receptor/CD3 complex and co-stimulatory molecules, and results in the triggering of several signaling pathways which lead rapidly to the nuclear translocation of several transcription factors, such as nuclear factor (NF)-kappa B and NF-AT. A result of this activation process is the induction of a number of genes, including those encoding cytokines such as interleukin-2, tumor necrosis factor-alpha, and interferon (IFN)-gamma which have important immunoregulatory effects. We report here that a DNA-binding factor containing STAT1 also becomes activated in human peripheral blood T lymphocytes or Jurkat cells, although not until 1-2 h after stimulation. Activation is delayed a further 1-2 hr when mononuclear cell cultures are stimulated by an antigen which requires processing. Appearance of the STAT1 factor is significantly reduced in the presence of cyclosporin A, and blocked by cycloheximide, indicating that its activation is dependent upon a protein(s) synthesized in response to initial signaling events. Neutralizing antiserum against IFN-gamma, but not other cytokines tested, blocked activation of the factor almost completely, and IFN-gamma was found in the culture supernatants of stimulated cells at levels at which recombinant IFN-gamma could activate the factor in naive cells. Therefore, a STAT1 transcription factor is activated by IFN-gamma synthesized and released upon stimulation of T lymphocyte populations. While Jurkat cells both secrete and respond to IFN-gamma in an autocrine loop, it seems likely that the responding cells may differ from those synthesizing this cytokine in the mononuclear cell cultures in the light of the recent report that Th1 cells lack the IFN-gamma receptor chain necessary for activation of STAT1 (Pernis, A., Gupta, S., Gollob, K.J., Garfein, E., Coffman, R.L., Schindler, C., and Rothman, P., Science 1995. 269:245).

Wild-type p53 induces diverse effects in 32D cells expressing different oncogenes

Expression of exogenous wild-type (wt) p53 in different leukemia cell lines can induce growth arrest, apoptotic cell death, or cell differentiation. The hematopoietic cell lines that have been used so far to study wt p53 functions have in common the characteristic of not expressing endogenous p53. However, the mechanisms involved in the transformation of these cells are different, and the cells are at different stages of tumor progression. It can be postulated that each type of neoplastic cell offers a particular environment in which p53 might generate different effects. To test this hypothesis, we introduced individual oncogenes into untransformed, interleukin-3 (IL-3)-dependent myeloid precursor 32D cells to have a single transforming agent at a time. The effects induced by wt p53 overexpression were subsequently evaluated in each oncogene-expressing 32D derivative. We found that in not fully transformed, v-ras-expressing 32D cells, as already shown for the parental 32D cells, overexpression of the wt p53 gene caused no phenotypic changes and no reduction of the proliferative rate as long as the cells were maintained in their normal culture conditions (presence of IL-3 and serum). An accelerated rate of apoptosis was observed after IL-3 withdrawal. In contrast, in transformed, IL-3-independent 32D cells, wt p53 overexpression induced different effects. The v-abl-transformed cells manifested a reduction in growth rate, while the v-src-transformed cells underwent monocytic differentiation. These results show that the phenotype effects of wt p53 action(s) can vary as a function of the cellular environment.

Curcumin induces apoptosis in immortalized NIH 3T3 and malignant cancer cell lines

Curcumin, which is a widely used dietary pigment and spice, has been demonstrated to be an effective inhibitor of tumor promotion in mouse skin carcinogenesis. We report that curcumin induces cell shrinkage, chromatin condensation, and DNA fragmentation, characteristics of apoptosis, in immortalized mouse embryo fibroblast NIH 3T3 erb B2 oncogene-transformed NIH 3T3, mouse sarcoma S180, human colon cancer cell HT-29, human kidney cancer cell 293, and human hepatocellular carcinoma Hep G2 cells, but not in primary culture of mouse embryonic fibroblast C3H 10T1/2, rat embryonic fibroblast, and human foreskin fibroblast cells in a concentration- and time-dependent manner. Many cellular and biochemical effects of curcumin in mouse fibroblast cells have been reported, such as inhibition of protein kinase C (PKC) activity induced by phorbol 12-myristate 13-acetate treatment, inhibition of tyrosine protein kinase activity, and inhibition of arachidonic acid (AA) metabolism. Treatment of NIH 3T3 cells with the PKC inhibitor staurosporine, the tyrosine kinase inhibitor herbimycin A, and the AA metabolism inhibitor quinacrine induces apoptotic cell death. These results suggest that, in some immortalized and transformed cells, blocking the cellular signal transduction might trigger the induction of apoptosis.

Deficient signaling in mice devoid of double-stranded RNA-dependent protein kinase

Double-stranded RNA-dependent protein kinase (PKR) has been implicated in interferon (IFN) induction, antiviral response and tumor suppression. We have generated mice devoid of functional PKR (Pkr%). Although the mice are physically normal and the induction of type I IFN genes by poly(I).poly(C) (pIC) and virus is unimpaired, the antiviral response induced by IFN-gamma and pIC was diminished. However, in embryo fibroblasts from Pkr knockout mice, the induction of type I IFN as well as the activation of NF-kappa B by pIC, were strongly impaired but restored by priming with IFN. Thus, PKR is not directly essential for responses to pIC, and a pIC-responsive system independent of PKR is induced by IFN. No evidence of the tumor suppressor activity of PKR was demonstrated.

Massive programmed cell death in intestinal epithelial cells induced by three-dimensional growth conditions: suppression by mutant c-H-ras oncogene expression

Deregulation of molecular pathways controlling cell survival and death, including programmed cell death, are thought to be important factors in tumor formation, disease progression, and response to therapy. Studies devoted to analyzing the role of programmed cell death in cancer have been carried out primarily using conventional monolayer cell culture systems. However the majority of cancers grow as three-dimensional solid tumors. Because gene expression, and possibly function, can be significantly altered under such conditions, we decided to analyze the control and characteristics of cell death using a compatible three-dimensional tissue culture system (multicellular spheroids) and compare the results obtained to those using two-dimensional monolayer cell culture. To do so we selected for study an immortalized, but nontumorigenic line of rat intestinal epithelial cells, called IEC-18, and several tumorigenic variants of IEC-18 obtained by transfection with a mutant (activated) c-H-ras oncogene. The rationale for choosing these cell lines was based in part on the fact that intestinal epithelial cells grow in vivo in a monolayer-like manner and form solid tumors only after sustaining certain genetic mutations, including those involving the ras gene family. We found that the IEC-18 cells, which grow readily and survive in monolayer cell culture, undergo massive cell death within 48-72 h when cultured as multicellular spheroids on a nonadhesive surface. This process was accompanied by a number of features associated with programmed cell death including chromatin condensation (Hoechst 33258 staining) apoptotic morphology, DNA degradation, and a virtual complete loss of colony forming (clonogenic) ability in the absence of apparent membrane damage as well as accumulation of lipid containing vacuoles in the cytoplasm. Moreover, enforced over-expression of a transfected bcl-2 gene could prevent this cell death process from taking place. In marked contrast, three different stably transfected ras clones of IEC-18 survived when grown as multicellular spheroids. In addition, an IEC cell line (called clone 25) carrying its mutant transfected ras under a glucocorticoid inducible promoter survived in three-dimensional culture only when the cells were exposed to dexamethasone. If exposure to dexamethasone was delayed for as long as 48 h the cells nevertheless survived, whereas the cells became irreversibly committed to programmed cell death (PCD) if exposed to dexamethasone after 72 h. These results suggest that intestinal epithelial cells may be programmed to activate a PCD pathway upon detachment from a physiologic two-dimensional monolayer configuration, and that this process of adhesion regulated programmed cell death (ARPCD) can be substantially suppressed by expression of a mutant ras oncogene.(ABSTRACT TRUNCATED AT 400 WORDS)