The kinetics of ER fusion protein activation in vivo.

Reversibly switchable proteins are powerful tools with which to explore protein function in vitro and in vivo. For example, the activity of many proteins fused to the hormone-binding domain of the modified oestrogen receptor (ER(TAM)) can be regulated by provision or removal of 4-hydroxytamoxifen (4-OHT). Despite the widespread use of ER(TAM) fusions in vivo, inadequate data are available as to the most efficacious routes for systemic tamoxifen delivery. In this study, we have used two well-characterized ER(TAM) fusion proteins, both reversibly activated by 4-OHT, to compare the effectiveness and kinetics of 4-OHT delivery in mice in vivo by either tamoxifen in food or by intraperitoneal injection. Our data indicate that dietary tamoxifen offers an effective, facile and ethically preferable means for long-term activation of ER(TAM) fusion proteins in vivo.

Chromosomal and extrachromosomal instability of the cyclin D2 gene is induced by Myc overexpression.

We examined the expression of cyclins D1, D2, D3, and E in mouse B-lymphocytic tumors. Cyclin D2 mRNA was consistently elevated in plasmacytomas, which characteristically contain Myc-activating chromosome translocations and constitutive c-Myc mRNA and protein expression. We examined the nature of cyclin D2 overexpression in plasmacytomas and other tumors. Human and mouse tumor cell lines that exhibited c-Myc dysregulation displayed instability of the cyclin D2 gene, detected by Southern blot, fluorescent in situ hybridization (FISH), and in extrachromosomal preparations (Hirt extracts). Cyclin D2 instability was not seen in cells with low levels of c-Myc protein. To unequivocally demonstrate a role of c-Myc in the instability of the cyclin D2 gene, a Myc-estrogen receptor chimera was activated in two mouse cell lines. After 3 to 4 days of Myc-ER activation, instability at the cyclin D2 locus was seen in the form of extrachromosomal elements, determined by FISH of metaphase and interphase nuclei and of purified extrachromosomal elements. At the same time points, Northern and Western blot analyses detected increased cyclin D2 mRNA and protein levels. These data suggest that Myc-induced genomic instability may contribute to neoplasia by increasing the levels of a cell cycle-regulating protein, cyclin D2, via intrachromosomal amplification of its gene or generation of extrachromosomal copies.

Use of retroviruses to express exogenous genes in vascular smooth muscle cells.

The ability to express cloned genes in mammalian cells has proved invaluable in the study of gene expression and function and in clinical applications for the correction of functional gene loss by gene therapy. Despite the wide use of DNA-mediated transfection of genes into eukaryotic cells, viruses possess several advantages for the transfer and expression of exogenous genes. Several types of relatively small viruses including the papovavirus SV40, papillomaviruses, adenoviruses, and retroviruses have been successfully employed. Vectors based on larger viruses such as Epstein-Barr, herpes simplex, and vaccinia are generally able to maintain infectivity in a wide range of cell types and have a greater capacity for foreign DNA. However, because most introduced cDNA sequences are relatively small these vectors have not been widely used.

Suppression of c-Myc-induced apoptosis by the Epstein-Barr virus gene product BHRF1.

Constitutive expression of the c-myc proto-oncogene in growth factor-deprived fibroblasts promotes proliferation and induces apoptosis. In these cells, apoptosis can be inhibited by survival factors such as insulin-like growth factor I or the bcl-2 proto-oncogene product. Deregulated c-Myc expression is a common feature in Epstein-Barr virus-positive Burkitt's lymphoma in which the c-myc gene is reciprocally translocated and placed under the control of one of the immunoglobulin loci. BHRF1 is an Epstein-Barr virus protein expressed early in the lytic cycle. BHRF1 is a member of the Bcl-2 family and has been shown to suppress apoptosis and to increase cell survival in different settings. In the present study, we report that BHRF1 inhibits c-Myc-induced apoptosis which occurs in the absence of survival factors. It does not, however, affect the capacity of c-Myc to promote cell growth. These findings demonstrate that BHRF1 has not only structural but also functional similarities to Bcl-2.

Increased sensitivity of human vascular smooth muscle cells from atherosclerotic plaques to p53-mediated apoptosis.

The recent demonstration that apoptosis of vascular smooth muscle cells (VSMCs) occurs in human atherosclerotic plaques suggests that VSMC apoptosis may promote plaque rupture and subsequent myocardial infarction. In culture, human plaque VSMCs show higher rates of apoptosis than VSMCs from normal vessels, although the mechanism of this effect is unknown. In earlier studies, we have shown that the tumor suppressor gene p53 regulates apoptosis of rat VSMCs after deregulated cell cycle control. We therefore analyzed p53 function in cultured VSMCs derived from human coronary plaques or the media of normal coronary arteries. VSMCs with reduced or increased p53 activity were created by infecting VSMCs with retroviruses containing a dominant-negative p53 minigene or a chimeric p53 protein (p53TMER), which could be activated pharmacologically. Basal p53 protein expression and transcriptional activity were similar in plaque and normal VSMCs, and suppression of p53 activity blocked growth arrest in response to DNA damage in both VSMC types. In contrast, suppression of p53 activity failed to block apoptosis of plaque or normal VSMCs in low- or high-serum conditions or after DNA damage. Furthermore, in plaque VSMCs, p53 overexpression induced apoptosis in all conditions tested and also induced growth arrest. p53-mediated apoptosis was independent of new gene transcription or protein synthesis but was suppressed by prior growth arrest of cells, indicating that growth status can regulate sensitivity to p53-mediated apoptosis. No effect of increased p53 activity was seen in normal VSMCs. We conclude that VSMCs from human plaques have an increased sensitivity to p53-mediated apoptosis compared with normal VSMCs. Our data also suggest that the mechanism of p53-mediated apoptosis of plaque VSMCs may be distinct from that inducing growth arrest.

Large induction of c-Myc is not essential for the mitogenic response of Swiss 3T3 fibroblasts.

Quiescent Swiss 3T3 fibroblasts can be stimulated to reenter the cell cycle following stimulation with growth factors. Among these, bombesin is a potent mitogen for Swiss 3T3 cells and can act synergistically with insulin to stimulate DNA synthesis through protein kinase C-independent pathways. One of the earliest nuclear responses of quiescent cells treated with a combination of bombesin and insulin is a dramatic increase in c-Myc expression, and it has been suggested that this proto-oncogene plays a central role in the mitogenic response. In the present study, we have taken two approaches to study the relationship between c-Myc expression and the reinitiation of DNA synthesis. First, low concentrations of bombesin, in the presence of insulin, stimulated DNA synthesis in Swiss 3T3 fibroblasts in the absence of a large increase in c-myc mRNA or protein levels. Second, selective down-regulation of phorbol ester-inducible protein kinase C in Swiss 3T3 cells resulted in a 90% decrease in the induction of c-myc mRNA and an 80% reduction in Myc protein expression but did not affect the mitogenic response to bombesin and insulin. These observations were confirmed in detailed dose-response and time-course experiments. We conclude that the large induction of c-Myc is not an essential event for the entry of Swiss 3T3 fibroblasts into S phase. Quantitation of Myc protein levels using a sensitive ELISA indicated that quiescent cells could enter S phase with only 450 c-Myc molecules per cell. These results indicate that cells in the G0 phase of the cell cycle can be stimulated to reinitiate DNA synthesis with only marginal increases in Myc protein expression.

Induction of apoptosis by tamoxifen-activation of a p53-estrogen receptor fusion protein expressed in E1A and T24 H-ras transformed p53-/- mouse embryo fibroblasts.

A fusion gene consisting of wild-type p53 linked to a modified ligand binding domain of the murine estrogen receptor has been constructed and should be a useful tool for studying controlled activation of wild-type p53 function in a variety of experimental cell systems. The protein product of this gene, p53ERTM, is expressed in cells constitutively but is not functional unless associated with tamoxifen or 4-hydroxytamoxifen. p53ERTM was introduced into p53-deficient mouse embryo fibroblasts (MEFs) expressing the E1A and T24 H-ras oncogenes. Activation of p53 in these transformed cells by the addition of tamoxifen or 4-hydroxytamoxifen resulted in apoptosis. In addition to engaging the apoptotic machinery, the tamoxifen-activated fusion protein exhibited other functions characteristic of wild-type p53, such as induction of WAF1 and MDM2 gene expression and activation of the p53-dependent spindle checkpoint in cells treated with nocodazole. Activation of p53ERTM expressed in p53-positive MEFs coexpressing E1A and ras had, at most, only a small cytotoxic effect. When three cell lines of transformed p53+/+ fibroblasts not expressing p53ERTM were tested for sensitivity to the DNA-damaging drug doxorubicin, the p53+/+ clones displayed either comparable sensitivity, or at most an increase in drug sensitivity of less than fourfold, as compared to several p53-/- cell lines. Our data show that restoration of wild-type p53 activity is sufficient to trigger apoptosis in p53-/- MEFs transformed with E1A and T24 H-ras and suggest that rare propagable clones of p53-normal MEFs expressing the E1A and T24 H-ras oncogenes have suffered compensatory alterations that compromise the ability to undergo p53-dependent apoptosis.

Inducible site-directed recombination in mouse embryonic stem cells.

The site-directed recombinase Cre can be employed to delete or express genes in cell lines or animals. Clearly, the ability to control remotely the activity of this enzyme would be highly desirable. To this end we have constructed expression vectors for fusion proteins consisting of the Cre recombinase and a mutated hormone-binding domain of the murine oestrogen receptor. The latter still binds the anti-oestrogen drug tamoxifen but no longer 17 beta-oestradiol. We show here that in embryonic stem cells expressing such fusion proteins, tamoxifen can efficiently induce Cre-mediated recombination, thereby activating a stably integrated LacZ reporter gene. In the presence of either 10 microM tamoxifen or 800 nM 4-hydroxy-tamoxifen, recombination of the LacZ gene is complete within 3-4 days. By placing a tamoxifen-binding domain on both ends of the Cre protein, the enzymatic activity of Cre can be even more tightly controlled. Transgenic mice expressing such an tamoxifen-inducible Cre enzyme may thus provide a new and useful genetic tool to mutate or delete genes at specific times during development or in adult animals.

A modified oestrogen receptor ligand-binding domain as an improved switch for the regulation of heterologous proteins.

A number of proteins have been rendered functionally oestrogen-dependent by fusion with the hormone-binding domain of the oestrogen receptor. There are, however, several significant disadvantages with such fusion proteins. First, their use in cells in vitro requires phenol red-free medium and laborious stripping of steroid hormones from serum in order to avoid constitutive activation. Secondly, control of oestrogen receptor fusion proteins in vivo is precluded by high endogenous levels of circulating oestrogens. Thirdly, the hormone-binding domain of the oestrogen receptor functions as a hormone-dependent transcriptional activation domain making interpretation of fusions with transcription factors problematical. In order to overcome these drawbacks we have used a transcriptionally inactive mutant of the murine oestrogen receptor which is unable to bind oestrogen yet retains normal affinity for the synthetic ligand, 4-hydroxytamoxifen. When the hormone-binding domain of this mutant oestrogen receptor is fused to the C-terminus of the c-Myc protein, Myc-induced proliferation and apoptosis in fibroblasts becomes dependent on 4-hydroxytamoxifen, but remains refractory to 17 beta-oestradiol.

Down-regulation of the c-myc proto-oncogene in inhibition of vascular smooth-muscle cell proliferation: a signal for growth arrest?

Vascular smooth muscle (VSM) cell proliferation contributes to the pathogenesis of atherosclerosis, restenosis after angioplasty and vein graft disease. The regulation of genes involved in VSM cell proliferation, particularly by naturally occurring inhibitors, is therefore of some importance. We have investigated the role of the c-myc proto-oncogene in growth arrest of exponentially proliferating rat VSM cells, following mitogen withdrawal, treatment with heparin (50 micrograms/ml), interferon-gamma (IFN-gamma) (100 i.u./ml), or the cyclic nucleotide analogues, 8-bromo-adenosine-3'5'-cyclic monophosphate (8-Br-cAMP; 0.1 mM) and 8-bromoguanosine-3'5'-cyclic monophosphate (8-Br-cGMP; 0.1 mM). Growth arrest was accompanied by down-regulation of c-Myc protein and mRNA following treatment with all inhibitors. Serum withdrawal or IFN-gamma treatment suppressed c-myc expression by more than 50% within 2 h, and this occurred throughout the cell cycle. Platelet-derived growth factor, epidermal growth factor and basic fibroblast growth factor all contributed independently to the maintenance of c-myc expression. Heparin, 8-Br-cAMP or 8-Br-cGMP also suppressed c-myc, but this occurred later, after 24-48 h, and was also observed following arrest by metabolic block. We conclude that c-myc expression is linked to VSM cell growth arrest in response to endogenous regulators and metabolic block. Down-regulation of c-myc expression may thus be an essential part of the arrest programme in VSM cells induced by many pharmacological agents.

The c-Myc protein induces cell cycle progression and apoptosis through dimerization with Max.

The c-Myc protein (Myc) is involved in cellular transformation and mitogenesis, but is also a potent inducer of programmed cell death, or apoptosis. Whether these apparently opposite functions are mediated through common or distinct molecular mechanisms remains unclear. Myc and its partner protein, Max, dimerize and bind DNA in vitro and in vivo through basic/helix-loop-helix/leucine zipper motifs (bHLH-LZ). By using complementary leucine zipper mutants (termed MycEG and MaxEG), which dimerize efficiently with each other but not with their wild-type partners, we demonstrate that both cell cycle progression and apoptosis in nontransformed rodent fibroblasts are induced by Myc-Max dimers. MycEG or MaxEG alone are inactive, but co-expression restores ability to prevent withdrawal from the cell cycle and to induce cell death upon removal of growth factors. Thus, Myc can control two alternative cell fates through dimerization with a single partner, Max.

The role of c-myc in cell growth.

Recent experiments have established that the c-myc oncogene encodes a sequence-specific DNA-binding protein that interacts with a specific intracellular partner, Max, and probably manifests its effects through transcriptional modulation. In addition, the range of biological functions attributed to expression of c-myc has grown to include not only transformation and mitogenesis but also cell death.

Oncogenic activity of the c-Myc protein requires dimerization with Max.

c-Myc (Myc) and Max proteins dimerize and bind DNA through basic-helix-loop-helix-leucine zipper motifs (b-HLH-LZ). Using a genetic approach, we demonstrate that binding to Max is essential for Myc transforming activity and that Myc homodimers are inactive. Mutants of Myc and Max that bind efficiently to each other but not to their wild-type partners were generated by either exchanging the HLH-LZ domains or reciprocally modifying LZ dimerization specificities. While transformation defective on their own, complementary mutants restore Myc transforming activity when coexpressed in cells. The HLH-LZ exchange mutants also have dominant negative activity on wild-type Myc function. In addition, wild-type max antagonizes myc function in a dose-dependent manner, presumably through competition of Max-Max and Myc-Max dimers for common target DNA sites. Therefore, Max can function as both suppressor and activator of Myc. A general model for the role of Myc and Max in growth control is discussed.

Transcriptional activation by the human c-Myc oncoprotein in yeast requires interaction with Max.

The c-myc protein (Myc) contains an amino-terminal transcriptional activation domain and a carboxy-terminal basic helix-loop-helix-leucine zipper (bHLH-Z) domain that directs dimerization of Myc with its partner, the max protein (Max), and promotes DNA binding to sites containing a CACGTG core consensus sequence. Despite these characteristics and the observation that Myc can modulate gene expression, a direct role for Myc or Max as transcription factors has never been demonstrated. Here we use Saccharomyces cerevisiae as an in vivo model system to show that the Myc protein is a sequence-specific transcriptional activator whose DNA binding is strictly dependent on dimerization with Max. Transactivation is mediated by the amino-terminal domain of Myc. We find that Max homodimers bind to the same DNA sequence as Myc+Max but that they fail to transactivate and thus can antagonize Myc+Max function. We also show that the Max HLH-Z domain has a higher affinity for the Myc HLH-Z domain than for itself, and suggest that the heterodimeric Myc+Max activator forms preferentially at equilibrium.

Max and c-Myc/Max DNA-binding activities in cell extracts.

We have examined the interactions and DNA-binding activities of the c-Myc oncoprotein and its partner Max. In cell extracts virtually all c-Myc molecules are associated with Max in heterodimeric complexes. Moreover, DNA-binding studies with in vitro-translated protein and cell extracts show that both Max alone and c-Myc/Max bind the same DNA sequence. Conversely, c-Myc is unable to bind this sequence in the absence of Max. These findings suggest that c-Myc may function via obligate complex formation with Max.

Induction of apoptosis in fibroblasts by c-myc protein.

Although Rat-1 fibroblasts expressing c-myc constitutively are unable to arrest growth in low serum, their numbers do not increase in culture because of substantial cell death. We show this cell death to be dependent upon expression of c-myc protein and to occur by apoptosis. Regions of the c-myc protein required for induction of apoptosis overlap with regions necessary for cotransformation, autoregulation, and inhibition of differentiation, suggesting that the apoptotic function of c-myc protein is related to its other functions. Moreover, cells with higher levels of c-myc protein are more prone to cell death upon serum deprivation. Finally, we demonstrate that deregulated c-myc expression induces apoptosis in cells growth arrested by a variety of means and at various points in the cell cycle.

c-myc protein expression in untransformed fibroblasts.

We have examined and quantitated the expression of c-myc protein in two untransformed fibroblast cell lines, murine Swiss 3T3 and human MRC-5, c-myc protein is not detectable in quiescent cells, but it is rapidly induced upon mitogenic stimulation. Peak expression is seen about 3-5 h after serum stimulation, and corresponds to about 3-6000 molecules per cell (mpc). Thereafter, levels fall back to a quiescent level in confluent fibroblasts, but remain elevated at 1-3000 mpc in subconfluent cells. The c-myc protein is phosphorylated and has the same size and short half-life as seen in tumour cells. Removal of serum growth factors from the culture medium causes very rapid loss of the c-myc protein from all cells, irrespective of their positions in the cell cycle. Thus, c-myc expression is continuously dependent upon the presence of mitogens. However, no single tested mitogen is obligatory for maintenance of expression in proliferating cells. Growth arrest of cells, either by metabolite starvation or by drugs which inhibit DNA synthesis, does not affect expression of the c-myc protein, which remains completely dependent upon the presence of mitogens. These data are consistent with the c-myc protein's having a continuous role in proliferating cells as an intracellular integrator of growth regulatory signalling pathways.

Domains of human c-myc protein required for autosuppression and cooperation with ras oncogenes are overlapping.

Amino acids 106 to 143 and 354 to 433 of the human c-myc protein (439 amino acids) were shown to be required for the protein to suppress c-myc gene transcription and were found to exactly overlap with those necessary for c-myc to cooperate with ras oncogenes in the transformation of rat embryo fibroblasts. The essential carboxyl-terminal region harbors structural motifs (a basic region, a helix-loop-helix motif, and a "leucine zipper"), which, in other proteins, can mediate dimerization and sequence-specific DNA binding.