US 5,484,710 AGrant
Method of Down-Regulating a Gene Linked to a P-53 Responsive Element
Issue Date:1996-01-16
•1 Claims
•5 Drawing Sheets
Abstract
The present invention provides regulatory elements that are linked to genes involved in cell death and are regulated by p53 tumor suppressor protein. Examples of such p53 responsive elements (p53-RE) include p53-RE.sup.D, which is involved in p53-mediated down-regulation of the Bcl-2 gene, and p53-RE.sup.U, which is involved in p53-mediated up-regulation of the Bax gene. The invention also provides screening assays for identifying agents such as drugs that effectively modulate expression of a gene that is involved in cell death and contains a p53-RE.
Metadata
Assignee
- La Jolla Cancer Research Foundation
Inventors
- John C. Reed
- Toshiyuki Miyashita
- Masayoshi Harigai
- Motoi Hanada
Application Information
Application Number:US 1826190
Filing Date:1994-01-14
Priority Date:1994-01-14
Art Unit:185
Classifications
IPC:
C12P 2100C12N 1509
Field of Search:
53643523.1;24.169.1
Patent Drawings (5 sheets)
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the fields of molecular biology and gene regulation and more specifically to regulatory elements that are present in genes involved in cell death.
2. Background Information
Cell death occurs by a variety of processes including, for example, programmed cell death and necrosis. The term "apoptosis" describes the morphological features of cells undergoing the process of programmed cell death, which is responsible for maintaining a steady-state level of cells in a self-renewing tissues. Under normal conditions, apoptosis assures that the number of dying cells in a tissue is roughly equivalent to the number of newly produced cells. However, in various disease states or as a result of an insult to a tissue, dysregulation of the process of apoptosis can occur. Similarly, various diseases states are associated with increased levels of cell death due to processes other than apoptosis.
In Alzheimer's disease, Parkinson's disease, Huntingtons' chorea, epilepsy, amyotrophic lateral sclerosis, stroke, ischemic heart disease, spinal cord injury and many viral infections, for example, abnormally high levels of cell death occur. In at least some of these diseases, there is evidence that the excessive cell death occurs through mechanisms consistent with apoptosis. Among these are 1) spinal cord injury, where the severing of axons deprives neurons of neurotrophic factors necessary to sustain cellular viability; 2) stroke, where after an initial phase of necrotic cell death due to ischemia, the rupture of dead cells releases excitatory neurotransmitters such as glutamate and oxygen free radicals that stimulate apoptosis in neighboring healthy neurons; and 3) Human Immunodeficiency Virus (HIV), which induces apoptosis of T-lymphocytes.
In contrast, the level of apoptosis is decreased in cancer cells, which allows the cancer cells to survive longer than their normal cell counterparts. As a result of the increased number of surviving cancer cells, the mass of a tumor can increase even if the doubling time of the cancer cells does not increase. Furthermore, the high level of expression in a cancer cell of the Bcl-2 gene, which is involved in regulating apoptosis and, in some cases, necrotic cell death, renders the cancer cell relatively resistant to chemotherapeutic agents and to radiation therapy.
The molecular mechanisms that regulate cell death are not well understood. It is now becoming clear, however, that several proteins such as Bcl-2 and a Bcl-2-related protein, termed "Bax," have a central role in apoptosis. Specifically, the expression of Bcl-2 in a cell blocks apoptosis, whereas the expression of Bax in a cell induces apoptosis. Thus, when Bcl-2 levels in a cell are decreased and/or when Bax levels are elevated, the rate of cell death is accelerated. Conversely, when Bcl-2 levels in a cell are increased and/or when Bax levels are decreased, apoptosis is inhibited. In addition, Bcl-2 also may be involved in the process of necrotic cell death.
The p53 tumor suppressor protein (p53) is another example of a protein that is involved in the process of apoptosis. The wild type p53 protein induces apoptosis in a cell, whereas mutant p53 proteins do not induce apoptosis. Many cancers have mutations in the genes encoding p53 and, therefore, either do not express any p53 protein or express a mutant p53 protein. Thus, the absence of a wild type p53 tumor suppressor in a cancer cell also can contribute to the low level of apoptosis that occurs in cancer cells.
The ability to manipulate the mechanism by which the genes involved in cell death are regulated would provide physicians with a potential target for therapies aimed at ameliorating the effects of diseases that are characterized by abnormal levels of cell death and also would allow for the development of methods to identify agents that can effectively regulate, for example, apoptosis in a cell. However, the mechanisms by which these genes are regulated in a cell have not yet been described. Thus, there exists a need to identify methods to manipulate the regulatory elements for genes involved in apoptosis. The present invention satisfies this need and provides related advantages as well.
SUMMARY OF THE INVENTION
The present invention provides a gene regulatory element, the p53 responsive element (p53-RE), which is required for p53-mediated regulation of genes involved in cell death. The invention provides, for example, the p53-RE.sup.D, which down-regulates expression of the Bcl-2 gene, and the p53-RE.sup.U, which up-regulates expression of the Bax gene. The invention also provides screening assays for identifying agents such as drugs that effectively modulate expression of a gene that contains a p53-RE and is involved in cell death.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the sequence for the p53-RE.sup.D element, which consists of sequence positions -273 to -84 of the Bcl-2 gene (SEQ ID NO: 1).
FIG. 2 demonstrates the effect of wild type p53 tumor suppressor protein on cell survival. "p53" indicates cells transfected with a plasmid encoding a temperature sensitive p53 protein and "Neo" indicates cells transfected with a control plasmid. Cell viability was determined using trypan blue and was examined at various times after shifting the cells to the permissive temperature (32.5.degree. C.) or after incubating the cells at 37.degree. C. Each point represents the mean +/-standard deviation for three experiments.
FIG. 3 shows the relative levels of Bcl-2 mRNA and .beta.2-microglobulin (control) mRNA in cells cultured for 4 hr and 19 hr at 37.degree. C. or 32.5.degree. C. Cell lines are as described in FIG. 2. The mRNA levels were determined using the reverse transcriptase-polymerase chain reaction method.
FIG. 4 shows a northern blot analysis of Bax mRNA and .beta.2-microglobulin (control) mRNA obtained from cells incubated for 4 hr and 19 hr at 37.degree. C. or 32.5.degree. C. Cell lines are as described in FIG. 2.
FIG. 5 shows an immunoblot analysis of the levels of Bcl-2 protein in cells incubated for 4 hr and 19 hr at 32.5.degree. C. Cell lines are as described in FIG. 2.
FIG. 6 shows an immunoblot analysis of the levels of Bax protein in cells incubated for 19 hr at 32.5.degree. C. or 37.degree. C. Cell lines are as described in FIG. 2.
FIG. 7 provides the results of CAT assays used to identify the Bcl-2 p53-RE.sup.D. The various plasmids are described in detail in Example II.A. CAT activity was determined 48 hr after transfection and the results are expressed relative to the activity produced by the control pSVCAT construct, which does not contain a p53-RE.sup.D.
FIG. 8 provides the results of CAT assays used to determine the position specificity of the Bcl-2 p53-RE.sup.D. The plasmids are described in Example II.A. and relative CAT activity was determined as described in FIG. 7.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides regulatory elements (also called "responsive elements"), which are DNA sequences that control the expression of genes such as Bcl-2 and Bax, which are involved in the process of cell death (see Vaux et al., Nature 335:440-442 (1988); Oltvai et al., Cell 74:609-619 (1993); Kane et al., Science 262:1274-1277 (1993)). The regulatory elements, which are characterized by the requirement that the p53 tumor suppressor protein is required for their activity, are referred to as p53 responsive elements (p53-RE). Examples of p53-RE's include p53-RE.sup.D which is required for p53-mediated down-regulation of the Bcl-2 gene, and p53-RE.sup.U, which is required for p53-mediated up-regulation of the Bax gene.
FIG. 1 shows the DNA sequence of positions -276 to -84 of the Bcl-2 gene, which has the characteristics of a p53-RE.sup.D as defined herein (SEQ ID NO: 1). One skilled in the art would recognize that the point mutations and deletions can be made to the sequence shown in FIG. 1 (SEQ ID NO: 1) without altering the ability of the sequence to act as a p53-RE.sup.D. In addition, active fragments of the DNA sequence shown in FIG. 1 can be obtained. The method for obtaining active fragment of the p53-RE.sup.D shown in FIG. 1 (SEQ ID NO: 1) are routine and well known in the art. Thus, for example, the overlapping fragments of the sequence shown in FIG. 1 can be synthesized and cloned into the vectors described in Example II to determine active p53-RE.sup.D fragments. Similarly, point mutations can be introduced into the sequence shown in FIG. 1 (SEQ ID NO: 1) using, for example, site directed mutagenesis or simply by synthesizing sequences having random nucleotides at one or more predetermined positions.
The p53 tumor suppressor is a 53 kilodalton (kDa) nuclear phosphoprotein that can induce the transcription of various genes by binding to a consensus DNA binding site on the gene or can inhibit transcription by interacting, for example, with other transcription factors required for expression of a gene (Zhan et al., Mol. Cell. Biol. 13:4242-4250 (1993), which is incorporated herein by reference). The p53 protein contributes to tumor suppression through at least two mechanisms, arrest of cell proliferation and induction of apoptosis (see Hartwell, Cell 71:543-546 (1992); Yonish-Rouach et al., Nature 352:345-347 (1991); Baker et al., Science 249:912-915 (1990)). Recently, potential pathways to explain the mechanism by which p53 arrests cell proliferation have been suggested (Harper et al., Cell 75:805-816 (1993); El-Deiry et al., Cell 75:817-825 (1993)). However, the mechanism by which p53 induces apoptosis has not yet been described.
various genes encoding proteins such as Bcl-2 and Bcl-2-related proteins such as Bax are involved in the process of cell death. As used herein, the term "cell death" is used in its broadest sense to include cell death resulting from various processes such as apoptosis and necrosis. Reference to "a gene involved in cell death" is meant to include a gene that encodes a protein required for the initiation or continuation of the process of cell death such as apoptosis, which occurs in many cell types as a result of development, damage or disease. In addition, a gene involved in cell death is further characterized by containing a p53-RE having the characteristics described herein. Genes involved in apoptosis are well known in the art and include, for example, the Bcl-2 gene and the Bax gene. Genes encoding members of the Bcl-2-related family of proteins such as Bcl-X and genes encoding members of the family of Bcl-2-associated proteins such as R-ras (Fernandez-Sarabla and Bischoff, Nature 366:274-275 (1993), which is incorporated herein by reference) also are considered genes involved in apoptosis, provided that such genes contain a p53-RE.
The terms "p53-responsive element" and "p53-RE" specifically refer to a cis-acting, position-independent DNA sequence that is required for regulation of a gene by the p53 tumor suppressor. As described below, the p53-RE is considered a cis-acting regulatory element because it must be linked to a gene in order to confer p53-mediated regulation upon the gene. In addition, the p53-RE is considered a position-independent element because it is active regardless of whether it is located upstream or downstream of the promotor of a gene involved in cell death (see Example II.C.). It follows from the characteristics of the p53-RE that the term "linked" is used in its broadest sense and indicates that the p53-RE and the gene are located within a continuous DNA sequence that usually does not exceed about ten to fifty kilobases.
Regulation of a gene that is involved in cell death and contains a p53-RE is mediated by the p53 tumor suppressor. As used herein, the term "p53-mediated regulation" means that the wild type p53 tumor suppressor protein is required for the regulation of gene expression by the p53-RE. As shown in Example I, expression of wild type p53 tumor suppressor in a cell results, for example, in the down-regulation of Bcl-2 in a cell, which, in turn, results in cell death (see, also, FIGS. 1-5).
p53-mediated regulation can be due to direct binding of the p53 tumor suppressor to the p53-RE or can be due to p53 interacting with one or more other proteins, any or all of which can bind to the p53-RE to cause regulation of the gene (see Example II.D.). A p53 binding site has been described in several genes that are induced by p53 (El-Deiry et al., 1993). However, inspection of the DNA sequence of the p53-RE.sup.D does not reveal a consensus p53 binding site (FIG. 1 (SEQ ID NO: 1); Young and Korsmeyer, Mol. Cell. Biol. 13:3686-3697 (1993), which is incorporated herein by reference).
p53 also has been reported to interact with other transcription factors such as the TATA-binding factor to cause down-regulation of a gene (see Zhan et al., 1993). A common feature of genes that are down-regulated by p53 is that the gene is expressed from a TATA-containing promotor (Id.). Although the Bcl-2 gene contains a TATAA sequence, only minimal Bcl-2 gene transcription occurs from this promotor. Instead, transcription of Bcl-2 is initiated predominantly at a second, non-TATA-containing promoter. Thus, down-regulation of the Bcl-2 gene by p53 is not likely the result of p53 binding to a TATA-binding transcription factor.
p53-mediated regulation results in activation of gene expression by a p53-RE. As used herein, the term "activation" indicates that the p53-RE is performing its normal function of regulating the expression of the gene to which the p53-RE is linked. For example, the function of a p53-RE such as the p53-RE.sup.D in the Bcl-2 gene can be to down-regulate the expression of the gene. The term "down-regulated" indicates that expression of the gene is reduced or inhibited. A p53-RE having such characteristics is referred to as a "negative regulatory element," which, when activated, decreases the level of transcription of the gene to which it is linked.
Young and Korsmeyer (1993) have described a negative regulatory element that is contained within the same DNA sequence as the p53-RE.sup.D described herein (SEQ ID NO: 1). However, the element described by Young and Korsmeyer is a position-dependent regulatory element and, therefore, is readily distinguishable from the p53-RE.sup.D, which is a position-independent element that can regulate expression of a gene whether the p53-RE.sup.D is located upstream or downstream of the gene (see Example II.B.).
Another p53-RE is the p53-RE.sup.U element, which is present in the Bax gene. In contrast to p53-RE.sup.D, the p53-RE.sup.U element functions as a positive regulatory element, which is a DNA sequence that, when activated by p53, increases the level of transcription of the gene to which it is linked.
The identification of p53-RE's allows manipulation of the mechanism by which cell death is regulated in diseases that are characterized by abnormal levels of cell death. As used herein, the term "disease characterized by an abnormal level of cell death" refers to diseases such as cancer, in which the level of apoptosis is less than normal for the particular cell-type from which the tumor cell is derived.
It is well known that the p53 tumor suppressor often is absent or mutated in cancer cells. As a result, Bcl-2 gene expression can be improperly regulated in the p53 deficient cells, resulting in higher than normal levels of Bcl-2 protein, which decreases apoptosis in these cells. As used herein, the term "p53-deficient cell" is meant to include any cell that does not express a wild type p53 tumor suppressor protein. Such a cell, for example, can be a p53-null cell, which is a cell that does not contain any p53 protein, or a cell that contains a mutant p53 protein. A cancer cell that has point mutations, allelic loss, rearrangements or deletions of both p53 genes and, therefore, either does not express a p53 protein or expresses a mutant p53 protein is an example of a p53-deficient cell. Other p53-deficient cells include, for example, H358 lung cancer-derived cells, which are described in Example II.B. Such cells are particularly useful for identifying agents that can effectively regulate cell death as described in detail below.
Other diseases such as stroke are characterized by abnormally high levels of cell death due to necrosis and apoptosis. In stroke, oxygen deprivation leads to necrotic cell death. Subsequent destruction of the necrotic neuronal cells results in the release of agents such as glutamate, which can induce apoptosis in surrounding cells presumably, in part, by allowing intracellular levels of active oxygen species to increase (Behl et al., Biochem. Biophys. Res. Comm. 197:949-956 (1993), which is incorporated herein by reference). Increasing the level of Bcl-2 protein in neuronal cells exposed to glutamate or to agents that induce high intracellular active oxygen concentrations greatly increases cell survival (Behl et al., 1993; Kane et al., 1993).
Alzheimer's disease, ataxia telangiectasia, Bloom's syndrome and progeria also are characterized by having abnormally high levels of cell death. Patients having these diseases are characterized by an accumulation of DNA damage due, for example, to oxidative damage or to defects in DNA repair. Cells from ataxia telangiectasia patients are highly susceptible to ultraviolet- and X-radiation, which damages DNA in the cells and induces apoptosis. Exposure of ataxia telangiectasia cells to radiation has been reported to result in increased expression of genes containing the consensus p53 binding site (Kastan et al., Cell 71:587-597 (1992), which is incorporated herein by reference). Similarly, the abnormalities in cell death regulation in ataxia telangiectasia patients can be due to a defect in p53-mediated regulation of a gene such as Bcl-2 or Bax through a p53-RE.
The ability to manipulate the regulatory elements involved in the abnormal regulation of cell death in various diseases and the availability of a variety of cell types from patients having such diseases allows for the identification of agents that can be used to effectively treat patients having these diseases. Thus, the invention also provides screening assays for identifying agents such as drugs that effectively modulate expression of a gene that is involved in cell death and contains a p53-RE.
As used herein, the term "agent" means a biological or chemical compound such as a simple or complex organic molecule, a peptide, a protein or an oligonucleotide. The screening assays described herein are particularly useful in that it can be automated, which allows for high through-put screening of randomly designed agents in order to identify those agents that effectively modulate the level of expression of gene that is involved in cell death and that contains a p53-RE. Thus, the screening assays provide a method for identifying an "effective agent," which can be used to modulate cell death in a cell in vitro or in a patient.
As used herein, the term "modulate" means that the effective agent can increase or decrease the level of expression of a gene that is involved in cell death and that contains a p53-RE. For example, an effective agent for treating a cancer cell would allow a p53-deficient cell to behave as if it expressed a wild type p53 tumor suppressor and, therefore, would down-regulate a Bcl-2 gene and up-regulate a Bax gene, which, in turn, would increase the level of apoptosis in the cancer cell. Such an effective agent can act in various ways. For example, an effective agent that is a peptide or a protein can modulate Bcl-2 gene expression by binding to the p53-RE and down-regulating expression of the gene. Alternatively, the effective can agent can be a small organic molecule that affects the structure or binding ability of a mutant p53 protein such that the p53 binds to a p53-RE and modulates the expression of the gene involved in cell death.
The screening assays described in Example III allow one skilled in the art to identify an effective agent. The cotransfection assay described in Example IV.A. is particularly useful for screening various agents because it provides a well defined system containing, for example, a temperature sensitive p53 tumor suppressor and a well defined p53-RE. Screening of agents at the non-permissive temperature can be used to identify an effective agent, which will modulate transcription from the p53-RE to the same extent as the p53 protein does when expressed at the permissive temperature. Thus, this system contains a well defined standard with which to compare the screening results. The assay described in Example IV.B. provides the further advantage of using a cell line obtained, for example, from a cancer patient. Thus, the assay allows the identification of agents that are particularly effective at modulating the p53-mediated regulation of apoptosis for a specific patient.
An effective agent also can modulate the expression of a gene involved in cell death by reducing or inhibiting the p53-mediated regulation of the gene, thereby up-regulating, for example, a Bcl-2 gene and down-regulating a Bax gene. An example of such an effective agent is an oligonucleotide that has the structure of a p53-RE and, therefore, can bind free p53 in a cell, thus preventing p53 from regulating expression of the gene. Alternatively, an effective agent can bind to the binding site on either p53 or on the p53-RE and sterically inhibit binding of p53 to the p53-RE.
Example V provides various assays for identifying an agent that effectively inhibits the p53-mediated regulation of a gene involved in cell death. The gel shift assay is particularly useful because it does not require the use of living cells. Thus, this assay can be used as a primary method of screening for potentially effective agents, which can then be examined using the transfection methods described in Examples V.B. and V.C. Again, these transfection assays allow the identification of effective agents that, in this case, decrease cell death.
Agents that effectively decrease the level of cell death are particularly useful for treating a patient having a disease characterized by abnormally high levels of apoptosis. In this case, the effective agent can be administered to the patient. The route of administration of an effective agent to a patient will depend on the location of the diseased cells or tissue. For example, a patient with a neurodegenerative disorder, viral encephalitis, stroke, spinal cord injury or hereditary disorders that involve neuronal cell death such as ataxia telangiectasia can be treated by intrathecal administration of an agent. In contrast, it may be advantageous to administer an effective agent intravenously to a cancer patient with metastatic disease. These and other methods of administration are well known in the art and are selected based on the requirements for a particular patient.
In addition to being suitable for developing high through-put assays for screening agents, the assays provide the additional advantage of allowing the identification of agents that effectively regulate various different p53-RE's, which can readily be inserted into the vectors described herein. Also, if desired, any of several different reporter genes can be linked to a p53-RE. As used herein, a "reporter gene" is a gene that encodes a gene product that can be identified using simple, inexpensive methods. As described, below, the chloramphenicol acetyltransferase reporter gene can be used to determine the level of transcriptional activity due to p53-mediated regulation of a p53-RE. Other reporter genes such as luciferase also can be used in the disclosed assays. Such reporter genes are well known in the art and described, for example, by Sambrook et al. (Molecular Cloning: A laboratory manual (Cold Spring Harbor Laboratory Press 1989), which is incorporated herein by reference). The following examples are intended to illustrate but not limit the present invention.
EXAMPLE I
The p53 Tumor Suppressor Induces Apoptosis by Decreasing the Level of Expression of the Bcl-2 Gene
This example demonstrates that the p53 tumor suppressor acts as a transcription factor that decreases Bcl-2 gene expression and, thereby, induces apoptosis in a cell.
A. Transfection of M1 cells with a plasmid encoding a temperature-sensitive p53 tumor suppressor
Cells from the murine myeloid leukemic cell line, M1, which do not express either the p53 tumor suppressor protein or mRNA encoding p53 (Yonish-Rouach et al., 1991), were maintained in Dulbecco's Modified Eagle's Medium supplemented with 10% (vol/vol) heat-inactivated horse serum, 2 mM glutamine, 50 U/ml penicillin and 0.1 mg/ml of streptomycin. Cells were either transfected with pSV2-Neo, alone, or were cotransfected with pSV2-Neo and pLTRp53cGval135, which encodes the murine Val-135 mutant p53, (see Michalovitz et al., Cell 62:671-680 (1990), which is incorporated herein by reference). The p53 tumor suppressor encoded by the plasmid, pLTRp53cGval135, is a temperature sensitive mutant that has normal p53 activity at the permissive temperature, 32.5.degree. C., but is inactive at the non-permissive temperature, 37.degree. C. Transfection was by electroporation using the BioRad "GENE PULSER" electroporator. A pulse (1.5 KV, 1 mF) was delivered to a 0.7 ml suspension containing 1.5.times.10.sup.7 cells and 50 .mu.g of linearized plasmid DNA.
Following transfection, the cells were transferred into 24 well tissue culture plates at a concentration of 5.times.10.sup.4 cells per well and allowed to adjust to the culture medium. After 48 hr incubation, the medium was removed and replaced with fresh medium containing 400 .mu.g/ml of geneticin (GIBCO BRL) (1 ml per well). In the cotransfected cells, one drug resistant clone, designated M1-p53, that expressed the highest levels of p53 was selected and used for further experiments. In the parallel transfection, one cell line, designated M1-Neo, was selected and used as a control in further studies.
B. Effect of p53 tumor suppressor on cell survival
M1-p53 and M1-Neo cells were incubated at either 32.5.degree. C. or 37.5.degree. C. Cell samples were taken at various times and viability was determined by trypan-blue exclusion (mean +/-standard deviation for three experiments). As shown in FIG. 2, shifting the M1-p53 cells to 32.5.degree. C. (permissive temperature) resulted in cell death. In contrast, cell survival was unchanged in M1-p53 cells maintained at 37.degree. C. (non-permissive temperature) and in M1-Neo cells incubated at either 32.5.degree. C. or at 37.degree. C.
These results indicate that expression of the p53 tumor suppressor in a cell results in cell death. Since a large number of M1-p53 cells die following incubation for 19 hr at 32.5.degree. C., prior to some experiments dead cells were removed by centrifugation in "HISTOPAQUE" density gradient solution (SIGMA) as described by manufacturer.
C. Effect of p53 tumor suppressor on Bcl-2 mRNA levels
Bcl-2 is known to be involved in the process of apoptosis in a cell. Since p53 can induce cell death, the effect of the p53 tumor suppressor on Bcl-2 mRNA expression was determined using the reverse transcriptase-polymerase chain reaction (RT-PCR) method. As a control, the level of .beta.B2-microglobulin mRNA was determined in each sample.
M1-p53 and M1-Neo were incubated at 32.5.degree. C. or 37.degree. C. for various times, then collected. In samples of M1-p53 cells harvested 19 hr after being shifted to 32.5.degree. C., dead cells were removed by centrifugation in "HISTOPAQUE" density gradient solution (SIGMA). Total RNA was isolated using "TRIZOL" solubilizing reagent (GIBCO BRL) as described by the manufacturer. Reverse transcription was used to produce cDNA from the total RNA. Five .mu.g RNA was incubated in a 50 .mu.l reaction volume containing 200 U Moloney Leukemia Virus Reverse Transcriptase (GIBCO BRL), 4 .mu.l random hexamer (62.5 A.sub.260 U/ml), 10 mM dithiothreitol, 20 U "RNASIN" RNAsa inhibitor (Promega) and 1 mM each of dGTP, dATP, dTTP, dCTP in the reaction buffer provided by the manufacturer. Reactions were incubated at 37.degree. C. and allowed to proceed for 1 hr.
The cDNA product was amplified using the PCR method. Ten .mu.l of the cDNA product was resuspended in a 100 .mu.l reaction volume containing 2.5 U Taq DNA polymerase (Perkin-Elmer-Cetus), 60 mM each of dGTP, dATP, dTTP and dCTP and 50 pmol each of the forward and reverse primers in the reaction buffer provided by the manufacturer (primers are described below). Amplification cycles consisted of 94.degree. C. for 30 sec, 57.degree. C. for 30 sec, 72.degree. C. for 3 min. In order to obtain data within the linear range of the assay, each cDNA was amplified in serial cycles from 27 to 39 and 18 to 27 with increments of 3 cycles for Bcl-2 and .beta.2-microglobulin, respectively.
Following the PCR reaction, RT-PCR products were size-fractionated by electrophoresis in 3% agarose gels, then transferred to "GENESCREEN PLUS" nylon filters (NEN Research Products) using 0.4N NaOH and hybridized with labelled internal probes (described below). Approximately 0.1 .mu.g internal probe was end-labelled using .gamma.-.sup.32 P-ATP (3000 Ci/mmol; NEN) and 10 U T4 polynucleotide kinase. Radiolabelled oligomers were purified by "BIO-SPIN" chromatography columns (BIO RAD). Hybridization was performed at 57.degree. C. in a buffer containing 5X SSPE (1X SSPE=0.15M NaCl/0.01M NaH.sub.2 PO.sub.4, pH 7.0/1 mM EDTA), 0.6% SDS, 50 mg/ml denatured salmon sperm DNA. Following hybridization, the filters were washed in 5X SSPE/0.1% SDS at 57.degree. C. and exposed to Kodak XAR films (Eastman Kodak) at -70.degree. C. with an intensifying screen.
Primers and internal probes used were as follows. Murine Bcl-2 forward primer: 5'-TGCACCTGAGCGCCTTCAC-3' (SEQ ID NO: 2); Bcl-2 reverse primer: 5'-TAGCTGATTCGACCATTTGCCTGA-3' (SEQ ID NO: 3); Bcl-2 internal probe: 5'-CCAGGAGAAATCAAACAAAGG-3' (SEQ ID NO: 4); murine .beta.2-microglobulin forward primer: 5'-ATGGCTCGCTCGGTGACCCTAG-3' (SEQ ID NO: 5); .beta.2-microglobulin reverse primer: 5'-TCATGATGCTTGATCACATGTCTCG-3' (SEQ ID NO: 6), .beta.2-microglobulin internal probe: 5'-GCTACGTAACACAGTTCCAC-3+ (SEQ ID NO: 7). Forward and reverse primers were designed to span exon-intron junctions and, therefore, do not amplify any genomic DNA that may contaminate the RNA samples. Expected sizes of the amplified products of Bcl-2 and .beta.2-microglobulin were 575 bp and 373 bp, respectively.
As shown in FIG. 3, Bcl-2 mRNA levels decreased with time after M1-p53 cells were shifted to the permissive temperature (32.5.degree. C.) and were less than 90% of the control level after 19 hr. In contrast, .beta.2-microglobulin mRNA remained at the steady-state level during incubation at 32.5.degree. C. In addition, Bcl-2 (and .beta.2-microglobulin) mRNA levels remained at steady-state levels in M1-p53 cells maintained at 37.degree. C. and in M1-Neo cells incubated at either 32.5.degree. C. or at 37.degree. C. These results indicate that the decrease in Bcl-2 mRNA in M1-p53 cells incubated at 32.5.degree. C. is correlated to expression of the p53 tumor suppressor and is not due, for example, to the temperature of incubation.
D. Effect of p53 tumor suppressor on Bax mRNA levels
The product of the Bax gene also is involved in the process of apoptosis in a cell. Therefore, the level of Bax mRNA expression was determined by northern blot analysis and compared to the level of expression of .beta.2-microglobulin mRNA.
M1-p53 and M1-Neo were incubated at 32.5.degree. C. or 37.degree. C. for various times, then collected. In samples of M1-p53 cells harvested 19 hr after being shifted to 32.5.degree. C. dead cells were removed by centrifugation in "HISTOPAQUE" density gradient solution (SIGMA). Total RNA was isolated as described above and 15 .mu.g total RNA was size-fractionated in 1.2% agarose gels containing 2.2M formaldehyde (Sambrook et al., 1989). RNA was transferred to "GENESCREEN PLUS" nylon filters (NEN) using 10X SSC (1X SSC=0.15M NaCl/0.015M sodium citrate) and covalently bound to the membrane using UV irradiation. Probes, as described below, were labelled using .alpha.-.sup.32 P-dCTP by the random primer method (Sambrook et al., 1989; specific activity=1.times.10.sup.9 cpm/.mu.g).
.sup.32 P-labelled probes were added to the hybridization solution and hybridization was performed for 16 hr at 42.degree. C. (hybridization solution is 50% formamide, 10% dextran sulfate, 1M NaCl, 1% SDS, 1X Denhardt's solution, 25 mMTris (pH 7.4) and 50 mg/ml denatured salmon sperm DNA) (50X Denhardt's solution contains 5 g Ficoll (Type 400; Pharmacia), 5 g polyvinylpyrrolidone, 5 g bovine serum albumin (Fraction V; Sigma) and water to 500 ml). Following hybridization, the filters were washed with 2X SSC/0.1% SDS at room temperature, then with the same solution at 68.degree. C. and exposed to X-ray film as described above.
A murine Bax-specific probe was prepared by amplifying the entire open reading frame of the Bax cDNA using RT-PCR as described above. The forward primer was 5'-GGAATTCGCGGTGATGGACGGGTCCGG-3' (SEQ ID NO: 8) and the reverse primer was 5'-GGAATTCTCAGCCCATCTTCTTCCAGA-3' (SEQ ID NO: 9). An EcoRI linker sequence was included at the 5'-end of each primer (underlined). Amplified cDNA products were gel-purified using a "GENECLEAN II" affinity gel (Bio 101, Inc.), cleaved using EcoRI (10 U/.mu.g DNA) and subcloned into a "BLUESCRIPT" plasmid pSK-II (Stratagene). An EcoRI restriction fragment of approximately 600 base pairs was excised from the cloned insert, gel-purified and used as a hybridization probe. The murine .beta.2-microglobulin cDNA probe is described by Parnes et al., Proc. Natl. Acad. Sci., USA 78:2253-2257 (1081), which is incorporated herein by reference.
As shown in FIG. 4, shifting M1-p53 cells to the permissive temperature resulted in a rapid increase in the level of Bax mRNA. No change in Bax mRNA levels were observed in M1-p53 cells maintained at 37.degree. C. or in M1-Neo cells incubated at either 32.5.degree. C. or at 37.degree. C. Levels of .beta.2-microglobulinmRNA did not change in any of the samples. These results indicate that the p53 tumor suppressor increases the expression of Bax mRNA. Thus, the p53 tumor suppressor regulates the expression of at least two genes involved in cell death, i.e., Bcl-2 and Bax.
E. Effect of p53 tumor suppressor on Bcl-2 and Bax protein levels
Decreased levels of Bcl-2 protein and increased levels of Bax protein induce apoptosis in a cell. In order to confirm that the mRNA changes described above are responsible for the Bcl-2 and Bax protein levels associated with apoptosis, the level of these proteins was determined.
M1-p53 and M1-Neo were incubated at 32.5.degree. C. or 37.degree. C. for various times, then collected. In samples of M1-p53 cells harvested 19 hr after being shifted to 32.5.degree. C. dead cells were removed by centrifugation in "HISTOPAQUE" density gradient solution (SIGMA). Cells were washed in ice cold phosphate-buffered saline (PBS; pH 7.4) and collected by centrifugation. The cell pellets were resuspended in ice-cold lysis buffer containing the protease inhibitors, 0.7 mg/ml pepstatin, 1 mM phenylmethylsulfonylfluoride (PMSF), 0.23 U/ml aprotinin, 10 mM leupeptin and 1 mM benzamidine (lysis buffer is 10 mM Tris (pH 7.4), 0.15M NaCl, 5 mM EDTA, 1% (v/v) Triton X-100). Following incubation on ice for 30 min, samples were centrifuged at 16,000 X g for 10 min and the postnuclear supernatants were collected. Protein concentrations were determined using the bicinchoninic acid protein assay kit (Pierce, Inc.).
Twenty .mu.g protein were size-fractionated under reducing conditions by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) using 12% gels, as described by Laemmli et al. (Nature 227:680-685 (1970), which is incorporated herein by reference), then electrophoretically transferred to nitrocellulose filters. The filters were incubated for 2 hr at room temperature with preblocking solution (10 mM Tris (pH 7.6), 0.15M NaCl, 5% skim milk, 2% BSA and 0.1% Tween 20), then the solution was removed, fresh solution containing 0.1-0.2% (vol:vol) of the appropriate antibody (described below) was added and incubation was continued for 16 hr at 4.degree. C.
Following incubation with the first antibodies, the filters were washed 3 x (5 min each) in a solution containing 0.12M NaCl, 8.7 mM NaH.sub.2 P.sub.O.sub.4, 31 mM K.sub.2 HPO.sub.4 (pH 7.6), then incubated with preblocking solution for 30 min, followed by fresh preblocking solution containing biotinylated goat anti-rabbit IgG (H+L) antibody (Vector Laboratories, Inc.). After washing as above, antibody binding was detected using an avidin-biotin-complex method, which employed the VectastainABC kit (Vector Laboratories Inc.) followed by the addition of 0.4 mg/mlAEC (3-amino-9-ethyl carbazole) containing 0.01% H.sub.2 O.sub.2 for color development.
Rabbit anti-mouse Bcl-2 antibody was raised against synthetic peptides corresponding to amino acids 68 to 86 of the mouse Bcl-2 protein, including an additional cysteine at the C-terminus (RTSPLRPLVATAGPALSPVC; SEQ ID NO: 10), conjugated to maleimide-activated Keyhole Limpet Hemocyanin (Pierce, Inc., Rockford Ill.). Rabbit anti-mouse Bax antibody was raised against synthetic peptides corresponding to amino acids 43 to 61 of the mouse Bax protein, including an additional cysteine at the N-terminus (CPELTLEQPPQDASTKKLSE; SEQ ID NO: 11), conjugated to maleimide-activated Keyhole Limpet Hemocyanin.
As shown in FIG. 5, the steady-state level of p25-Bcl-2 protein declined in M1-p53 cells following 19 hr incubation at 32.5.degree. C. but not in any of the control cells Although Bcl-2 mRNA levels rapidly decline following transfer to the permissive temperature, no change in the p25-Bcl-2 protein was observed after 4 hr. However, this slower decline in the level of Bcl-2 protein likely reflects a relatively long half-life for the murine Bcl-2 protein. For comparison, the half-life of human Bcl-2 protein is about 10-12 hr (not shown).
In contrast to the result observed for the p25-Bcl-2 protein, the steady-state level of the p20-Bax protein increased following the shift of M1-p53 cells to the permissive temperature (FIG. 6). These results indicate that the induction of apoptosis by the p53 tumor suppressor is achieved by p53-regulation of the transcriptional activity of at least two genes involved in apoptosis. Furthermore, the magnitude of the effects shown here likely are an underestimate of the actual changes in the level of expression of Bcl-2 and Bax, since cells with low Bcl-2 and high Bax levels undergo apoptosis and, therefore, would have been lost from the population of cells analyzed in these experiments.
EXAMPLE II
Identification of the p53-Responsive Element that Down-Regulates Bcl-2 Gene Expression (p53-RE.sup.D)
This example demonstrates that a nucleic acid sequence located in the 5'-untranslated region of the Bcl-2 gene is required for transcriptional regulation of the Bcl-2 gene by the p53 tumor suppressor.
A. Plasmid constructions
The nucleic acid sequence corresponding to positions -273 and -84 (p53-RE.sup.D) of the Bcl-2 gene was used in these studies (FIG. 1; SEQ ID NO: 1). This sequence does not contain a consensus p53 binding site (see El-Deiry et al., supra, (1993), which is incorporated herein by reference), but does contain a position-specific negative regulatory element that blocks Bcl-2 gene expression (Young and Korsmeyer, 1993).
The p53-RE.sup.D sequence (SEQ ID NO: 1) was amplified with Pfu DNA polymerase (Stratagene) using the forward primer, 5'-GCGAAGCTTGTAGACTGATATTAAC-3' (SEQ ID NO: 12), and the reverse primer, 5'-GCGAAGCTTATAATCCAGCTATTTT-3' (SEQ ID NO: 13). A HindIII linker sequence was added to 5' end of each primer (underlined). Plasmid p18-21H was used as the template (Tsujimoto et al., Proc. Natl. Acad. Sci., USA 84:1329-1331 (1987), which is incorporated herein by reference).
The amplified p53-RE.sup.D product was gel-purified, then digested with HindIII and subcloned into the unique HindIII site of either pUCSVOCAT, which contains a chloramphenicol acetyltransferase (CAT) reporter gene, or pUCSV3CAT, which contains the CAT reporter gene and an SV40 promotor (Fukamizu et al., Biomed. Biochim. Acta 50:659-663 (1991), which is incorporated herein by reference). The HindIII site is located upstream of the CAT gene in pUCSV0CAT and between the SV40 promotor and the CAT gene in pUCSV3CAT; the constructed plasmids, therefore, were designated pUCSV40(p53-RE.sup.D)CAT and pUC(p53-RE.sup.D)CAT, respectively. Proper construction of the plasmids was confirmed by DNA sequencing.
The p53-RE.sup.D fragment (SEQ ID NO: 1) also was subcloned in a position upstream or downstream of the SV40-CAT transcriptional unit in pUCSV3CAT. Essentially, the unique HindIII site was destroyed by digesting the plasmid with HindIII, then blunting the ends with the Klenow fragment of DNA polymerase I and self-ligating the plasmids. The BamHI site, which is located downstream of the CAT gene, and a BglII site, which is located upstream of the SV40 promotor then were converted to HindIII sites using the appropriate linkers. The p53-RE.sup.D fragment (SEQ ID NO: 1) was subcloned into each of the newly created HindIII sites to generate the plasmids, pSV40CAT(p53-RE.sup.D) and p(p53-RE.sup.D)SV40CAT, respectively.
The p53 expression plasmids, CMV-p53.sub.wt and CMV-p53.sub.179, and the reporter plasmid, pFSVCAT, which contains six copies of the consensus p53 binding sequence and is regulated by p53 tumor suppressor, are described by Mietz et al., EMBO J. 11:5013-5020 (1992) and by Unger et al., EMBO J. 11:1383-1390 (1992), each of which is incorporated herein by reference. The plasmid, pCMV.beta.-Gal, which expresses .beta.-galactosidase and is not regulated by p53, was used as a control for transfection (MacGregor and Caskey, Nucl. Acids Res. 17:2365 (1989), which is incorporated herein by reference).
B. The p53-RE.sup.D is a position-independent negative regulatory element
Transient transfection assays were used to determine that the p53-RE.sup.D is a position-independent cis-acting regulatory element that can mediate p53-dependent inhibition of Bcl-2 gene expression. Transient transfection assays were performed using human lung cancer-derived H358 cells, which have a homozygous deletion of p53 genomic DNA sequences and do not produce any detectable p53 mRNA or protein (p53-null) (Takahashi et al., Science 248:491-494 (1989), which is incorporated herein by reference).
H358 cells were maintained in RPMI 1640 medium supplemented with 10% (vol/vol) heat-inactivated fetal bovine serum, 2 mM glutamine, 50 U/ml penicillin and 0.1 mg/ml of streptomycin. Cells were plated in 6-well plates and were transfected when they reached about 70% confluency. Each well received 3 .mu.g of either CMV-p53.sub.wt or CMV-p53.sub.179, 3 .mu.g pFSVCAT, 1 .mu.g pCMV.beta.-Gal and 30 .mu.g of "LIPOFECTIN" transfecting reagent (GIBCO BRL) in 2.5 ml of "HL-1" tissue culture medium (Ventrex).
Sixteen hr after transfection, the medium was replaced with fresh medium as described above and 32 hr later (48 hr after transfection) cells were collected by scraping with a rubber policeman into Hank's balanced salt solution (HBSS). The cells were washed 2x with HBSS, then the cell pellets were resuspended in 100 ml of ice cold 0.25M Tris (pH 7.8) and frozen and thawed 3x in ethanol-dry ice. Samples were centrifuged at 16,000 x g for 5 min and the supernatants were collected.
CAT assays were performed by adding 30 .mu.l of the cell lysate supernatant to a 70 .mu.l reaction mixture containing 10 mM HCl, 43 mM acetyl-coenzyme A, 0.2 mCi (Acetyl-.sup.3 H)-coenzyme A (4.48 Ci/mmol NEN) and 0.7 mM chloramphenicol in a 96-well plate and incubating at 37.degree. C. for 1 hour. The reaction was stopped by adding 100 .mu.l of 7M urea, the reaction mixtures were transferred to scintillation vials containing 2 ml of toluene scintillator and radioactivity was measured using a scintillation counter as described by Phahl et al., Meth. Enzymol. 189:267-270 (1990), which is incorporated herein by reference.
.beta.-galactosidase (.beta.-gal) assays were performed by adding 10 .mu.l of the cell lysate supernatant to a 190 .mu.l reaction mixture containing 0.35% of 2-mercaptoethanol and 0.7 mg/ml of o-nitrophenyl-b-D-galactopyranoside in a 96-well plate and incubating at 37.degree. C. for 30-60 min. .beta.-gal activity was measured using an ELISA plate reader at a wavelength of 405 nm as described by Phahl et al. (1990).
For CAT assays and .beta.-gal assays, serial dilutions of standard enzyme were included to verify that the results were within the linear phase of the reactions. In some experiments, the volume of the cell lysate supernatant added to the reaction was adjusted to obtain results within the linear range. Since p53 does not affect .beta.-gal expression from pCMV.beta.-gal, CAT activity was normalized for .beta.-gal activity. Normalization for protein concentration yielded comparable results (not shown).
As shown in FIG. 7, wild type p53 tumor suppressor inhibited the expression of CAT activity from the pUCSV40(p53-RE.sup.D)CAT construct by about 50%, whereas the mutant p53.sup.179 did not affect the level of expression of CAT from this plasmid. In contrast, wild type p53 had no effect on the expression of CAT from pSV40CAT. No CAT activity was expressed from the plasmid, pUC(p53-RE.sup.D)CAT, indicating that the p53-RE.sup.D does not have transcriptional activity, itself. CAT activity was present in cells that expressed wild type p53 and contained the plasmid, pFSVCAT, indicating that p53 does not cause a general decrease in gene expression in H358 cells. These results indicate that expression of CAT activity in a cell can be inhibited by linking the p53-RE.sup.D (SEQ ID NO: 1) to the CAT coding sequence and expressing wild type p53 in the p53-null cells.
Expression from the pSV40(p53-RE.sup.D)CAT plasmid produces a p53-RE.sup.D -CAT fusion transcript. As a result, some type of post-translational mechanism of regulation could account for the decreased level of CAT activity in cells containing that plasmid. However, inhibition of CAT activity also was observed when wild type p53 (as compared to mutant p53.sup.179) was expressed in cells containing either pSV40CAT(p53-RE.sup.D) or p(p53-RE.sup.D)SV40CAT, which contain the p53-RE.sup.D (SEQ ID NO: 1) in the upstream or downstream, respectively, of the SV40CAT transcription unit (FIG. 8).
These results demonstrate that the p53-RE.sup.D (SEQ ID NO: 1) is a negative regulatory element that, in the presence of p53 tumor suppressor, acts at the transcriptional level to decrease the level of expression of a gene. In addition, the p53-RE.sup.D (SEQ ID NO: 1) acts in a position-independent manner and, therefore, can be distinguished from the negative regulatory element described by Young and Korsmeyer (1993). Remarkably, the p53-RE.sup.D (SEQ ID NO: 1) does not contain a consensus p53 binding site (El-Deiry et al., 1993).
C. The p53 tumor suppressor binds the p53-RE.sup.D
H358 cells were grown in T75 flasks and transfected with pCMV-p53.sub.wt or pCMV-p53.sub.179 as described above. Untransfected cells were used as a negative control. Flasks were washed 3x with Tris-buffered saline, then 2.5 ml lysis buffer was added to each flask (lysis buffer is 20 mM HEPES (pH 7.6), 20% glycerol, 10 mM NaCl, 1.5 mM MgCl.sub.2, 0.2 mM EDTA, 0.1% Triton X-100, 1 mM dithiothreitol (DTT), 1 mM PMSF, 10 .mu.g/ml leupeptin, 10 g/ml pepstatin and 100 .mu.g/ml aprotinin). Cells were dislodged by scraping and were pelleted by centrifugation at 2000 rpm at 4.degree. C. Nuclei were resuspended at 2.5.times.10.sup.7 nuclei per ml in nuclear extraction buffer, which is lysis buffer containing 500 mM NaCl. Nuclei were rocked gently for 1 hr at 4.degree. C. then centrifuged for 10 min at 10,000 rpm. The supernatant was aliquoted into cryotubes, then quick frozen in liquid nitrogen and stored at -80.degree. C.
Binding reactions were performed by adding 2 .mu.l poly dI-dC (1 mg/ml; Pharmacia), 16 .mu.l binding buffer, 1 l (.alpha.-.sup.32 P)dCTP-labelled p53-RE.sup.D probe (1.times.10.sup.4 cpm) and 1 .mu.l unlabelled competitor p53-RE.sup.D DNA (3 pmol) to 2 .mu.l of nuclear extract and incubating the reaction for 30 min at room temperature (binding buffer is 25 mM HEPES (pH 7.9), 0.5 mM EDTA (pH 8.0), 50 mM KCl, 10% glycerol, 0.5 mM DTT and 0.5 mMPMSF). In some experiments, 1 .mu.g of an anti-p53 monoclonal antibody was included in the reaction mixture.
The p53-RE.sup.D probe was isolated by HindIII digestion of pUCSV40(p53-RE.sup.D)CAT, followed by gel purification of the p53-RE.sup.D (SEQ ID NO: 1) fragment. The probe was labelled using (.alpha.-.sup.32 P)dCTP and the Klenow fragment of DNA polymerase I (Sambrook et al., 1989). Following the binding reaction, samples were subjected to polyacrylamide gel electrophoresis using a 4% gel and a buffer containing 50 mM Tris (pH 8.5), 0.4M glycine, 2 mM EDTA and 3% glycerol. The gels then were dried and exposed to X-ray film as described above.
The use of cell lysates prepared from p53.sub.wt -producing cells results in a shift in the mobility of the .sup.32 P-labelled p53-RE.sup.D, whereas cell lysates prepared from cells producing p53.sup.179 do not shift the mobility of the probe. Additionally, the anti-p53 monoclonal antibody "super-shifts" the probe. These results indicate that wild type, but not mutant, p53 binds to the p53-RE.sup.D (SEQ ID NO: 1).
EXAMPLE III
Identification of the p53-RE.sup.U in the Bax Gene
This example describes a method for identifying the p53-RE.sup.U, which is a DNA regulatory element that, in the presence of p53, up-regulates the expression of the Bax gene and, thereby, increases the level of apoptosis in a cell.
The expression of Bax in a cell is associated with increased levels of apoptosis (Oltvai et al., supra, (1993), which is incorporated herein by reference; see, also, FIG. 2). p53 induces the expression of Bax mRNA and, consequently, Bax protein (see FIGS. 4 and 6). Since the Bax gene is up-regulated in response to p53, the assays are designed to identify a positive regulatory element. The assays for identifying and characterizing the p53-RE.sup.U are well known in the art and described in detail in Example II.
A human genomic clones representing the Bax gene was cloned from a cosmid library using standard methods as described, for example, by Sambrook et al. (1989). Briefly, the cosmid library was screened using the mouse Bax cDNA probe described in Example I.D. Restriction fragments representing non-coding portions of the Bax gene located upstream of the open reading frame are subcloned into the HindIII site of the promotor-less CAT-containing plasmid pUCSV0CAT, as described above. These Bax-CAT constructs produce significant levels of CAT activity when transfected into cells that contain p53.
p53-RE.sup.U elements are identified by cotransfecting the Bax-CAT plasmids with CMV-p53.sub.wt or with CMV-p53.sub.179 into a cell line such as H358, as described above. Segments of the Bax gene that confer wild type p53-mediated up-regulation on the promotor-less pUCSV-CAT plasmid are identified and the region of interest is reduced to a minimal DNA fragment by analysis of 5' and 3' deletion mutants.
Once the p53-RE.sup.U has been identified, it can be sequenced and compared to the known consensus p53 binding site. In addition, the p53-RE.sup.U can be examined to determine whether it, like the p53-RE.sup.D (SEQ ID NO: 1), is position independent In addition, the ability of the p53-RE.sup.U to bind p53 can be determined. The p53-RE.sup.U can be used in the screening assays described below.
EXAMPLE IV
Screening Assays for Identifying an Agent that Effectively Increases the Level of Cell Death
This example describes screening assays that are useful for identifying an agent that can act as a p53 analog and, therefore, can induce apoptosis in a cell.
A. Cotransfection assay using a p53-null cell line
Cotransfection of a p53-null cell line such as H358 cells with a plasmid expressing a mutant p53 protein such as a temperature sensitive p53 protein (Yonish-Rouach et al., 1991) and a plasmid containing a p53-RE linked to a reporter gene such as CAT is useful for obtaining a cell line that can be used to screen for an agent that effectively increases the level of cell death in a population of cells. The cell line obtained following transfection provides a well defined system having a known p53 protein and a known p53-RE.
A screening assay using cells such as those described above is particularly useful for identifying an effective agent that either 1) confers upon a mutant p53 protein the ability to act like a wild type p53 tumor suppressor or 2) itself acts as a p53-analog in regulating expression of a gene that is involved in cell death and contains a p53-RE. These different mechanisms of action can be distinguished using the assay described in Example IV.C. An additional advantage of these cells is that a sample of the cells can be incubated at the permissive temperature, which allows for expression of the wild type p53 tumor suppressor. Thus, the level of regulation achieved by the wild type p53 and can be compared to the level of regulation obtained by various potentially effective agents.
Typically, the assay is performed in 96-well plates, which allows screening of a large number of agents in parallel. Various reporter genes can be used in the assay. However, a gene such as the luciferase gene provides the advantage that, following addition of the agent, the 96-well plates can be automatically scanned by a luminescence detector to identify those agents that desirably modulate the level of expression of the reporter gene. For example, if the p53-RE.sup.D (SEQ ID NO: 1) is linked to the reporter gene, an agent that decreased the luciferase activity would be considered an effective agent. Alternatively, if the p53-RE.sup.U is linked to the reporter gene, an effective agent would increase the level of expression of the reporter gene.
In some cases, an agent that is determined to be an effective agent in this assay can be further examined using the following assay, which provides the advantage that the effective agent can be examined in a cell obtained from a patient.
B. Transfection assay using a cell line that expresses a mutant p53 protein
This assay uses a cell such as a tumor cell that expresses a mutant p53 gene and is obtained from a cancer patient. In this case, the cell is transfected only with a plasmid containing p53-RE linked to a reporter gene and various agents are screened as described above. This assay is particularly advantageous in that an effective agent obtained using the assay of Example III.A. can be screened for its ability to induce apoptosis in a particular tumor cell obtained from a patient. Thus, the assay allows the selection of the most effective agent for a particular patient.
In addition, the assay can be used to select, from a pre-selected panel of effective agents, a effective agent that most effectively induces apoptosis in the particular patient's cancer cells. In this case, it is desirable to have previously screened agents using, for example, the cotransfection assay described above and to have preselected potentially effective agents.
C. Transfection assay using a p53-null cell line
Transfection of a p53-null cell line with a plasmid containing the p53-RE linked to a reporter gene provides a cell line that is useful for screening to identify an agent that act as a p53 analog in a cell. The assay is performed as described in Example IV.A. and can be used to identify an effective agent that can independently provide the function of a wild type p53 in regulating expression of a gene containing a p53-RE. This assay can be particularly useful when the p53-null cells are obtained from a cancer patient and the effective agent that is identified can be used to treat the patient.
EXAMPLE V
Screening Assays for Identifying an Agent that Effectively Inhibits p53-Mediated Regulation of the p53-RE
This example describes screening assays that are useful for identifying an agent that can prevent the p53-mediated regulation of a gene containing the p53-RE and, therefore, can reduce or inhibit apoptosis in a cell.
A. Gel shift assay
The gel shift assay described above provides a simple and efficient method of screening various agents to identify an agent that effectively inhibits the ability of p53 to regulate expression of a gene containing the p53-RE. The method can be automated and, therefore, allows for the rapid screening of a large number of potentially effective agents.
This binding reactions are performed essentially as described in Example II.E., except that the reactions are performed in 96-well plates and various agents are added to the well as appropriate. Following incubation of the samples can be transferred in parallel to precast gels for separation of the reaction products. Effective agents are selected by identifying those agents that inhibit the binding of p53 to a p53-RE. If desired, the selected effective agents can be further examined using the assays described below.
B. Cotransfection assay using a p53-null cell line
Cotransfection of a p53-null cell line such as H358 cells with a plasmid expressing a wild type p53 tumor suppressor and a plasmid containing the p53-RE linked to a reporter gene such as CAT can provide cells that are useful for identifying agents that effectively inhibit the ability of p53 to regulate expression of a gene containing a p53-RE. These assays are performed as described in Example IV.A.
C. Transfection of a cell expressing a wild type p53 tumor suppressor
Transfection of a cell that expresses wild type p53 with a plasmid containing the p53-RE linked to a reporter gene can be used to identify an agent that inhibits apoptosis in a cell. This assay is particularly useful when the cell that is transfected is particularly susceptible to cell death. An example of such a cell is a neuronal cell obtained, for example, from a patient with amyotrophic lateral sclerosis patient. In this case, an effective agent that is selected based on the particular cell type to be treated.
Alternatively, the cell can be either 1) a cell that is obtained, for example, from the American Tissue Type Culture and is known to exhibit the characteristics of a cell obtained from a patient having a particular disease such as ataxia telangiectasia or 2) a neuronal cell line such as the cell lines described by Behl et al. (1993) that is exposed, for example, to amyloid beta protein (ABP) or to glutamate and, therefore, is a model for the type of cell death that occurs in Alzheimer's disease or in stroke, respectively. In this case, the assay provides the advantage that the cell lines that are used in the assay are adapted for tissue culture.
Although the invention has been described with reference to the disclosed examples, it should be understood that various modifications can be made without departing from the spirit of the invention. Accordingly, the invention is limited only by the following claims.
Claims
We claim:
1. A method of down-regulating the expression of a gene, comprising operatively linking a p53-responsive element having the sequence shown in FIG. 1 (SEQ ID NO: 1) or an active fragment thereof to said gene and expressing said gene.
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