A novel form of Epstein-Barr virus latency in normal B cells in vivo

The novel heterodinucleoside dimer 5-FdU-NOAC is a potent cytotoxic drug and a p53-independent inducer of apoptosis in the androgen-independent human prostate cancer cell lines PC-3 and DU-145

BACKGROUND

We analyzed the cytotoxic properties of the new heterodinucleoside phosphate dimer 5-FdU-NOAC, which is composed of the cytotoxic drugs 5-FdU and N(4)-octadecyl-1-beta-D-arabinofuranosylcytosine (NOAC) against human prostate tumor cells.

METHODS

5-FdU-NOAC effects on cell proliferation, cell cycle distribution, thymidylate synthase activity, and apoptosis were investigated in vitro in the two human prostate carcinoma cell lines DU-145 and PC-3 and compared to cells treated with the corresponding single drugs 5-FdU and NOAC.

RESULTS

Treatment of the cells with 5-FdU-NOAC resulted in IC(50) values of 3.9-5 microM and in a complete inhibition of cell proliferation at 200 microM after 96 hr compared to 5-FdU, where 10% of the cells remained resistant. Flow cytometric analysis revealed cell cycle perturbations in S-phase only in the DU-145 cells. 5-FdU-NOAC caused 50% inhibition of thymidylate synthase after 90 min at 0.6 microM in both cell lines. Apoptotic cell fractions in DU-145 (66%) and in PC-3 (34%) cells were found after treatment with 5-FdU-NOAC for 96 hr. DNA fragmentation further confirmed the induction of apoptosis.

CONCLUSIONS

5-FdU-NOAC inhibits thymidylate synthase and cell cycle progression causing proliferation arrest and apoptosis in DU-145 and PC-3 cells, suggesting a potential role of 5-FdU-NOAC for the treatment of prostate cancer.

Hdmx stabilizes Mdm2 and p53

The Mdm2 protein is a key regulator of p53 activity and stability. Upon binding, Mdm2 inhibits the transcription regulatory activity of p53 and promotes its rapid degradation. In this study we investigated the effect of the human Mdm2 homologue Hdmx on p53 stability. We found that Hdmx does not target p53 for degradation, although, like Mdm2, it inhibits p53-mediated transcription activation. On the contrary, Hdmx was found to counteract the degradation of p53 by Mdm2, and to stabilize both p53 and Mdm2. The RING finger of Hdmx was found to be necessary and sufficient for this stabilization, and it probably involves hetero-oligomerization with the RING finger of Mdm2, which may lead to inhibition of Mdm2's ubiquitin ligase activity. However, Hdmx does not relieve the inhibition by Mdm2 of transcription activation by p53, probably due to the formation of a trimeric complex consisting of Hdmx, Mdm2, and p53. We propose a model in which Hdmx secures a pool of largely inactive p53, which, upon the induction of stress, can be quickly activated.

Gustatory innervation and bax-dependent caspase-2: participants in the life and death pathways of mouse taste receptor cells

In the adult mouse tongue, an average of 11% of the gustatory receptor cells are replaced each day. In investigating homeostatic cell death mechanisms in gustatory renewing epithelium, we observed that taste receptor cells were selectively immunopositive for the bcl-2 family death factor, Bax, and for the protease Caspase-2 (Nedd2/Ich1). We determined that 8-10% of the taste receptor cells of the vallate papilla were Bax positive and that 11% were Caspase-2 positive. Some of these immunopositive taste cells had apoptotic morphological defects. Within the subset of vallate taste cells immunopositive for either Caspase-2 or Bax, up to 79% coexpressed both death factors. Bax and Caspase-2 first appeared in occasional vallate taste receptor cells on the same postnatal day-the day after birth. bax null mutation markedly reduced gustatory Caspase-2 immunoexpression. These observations suggest that taste cell death pathways utilize p53, Bax, and Caspase-2 to dispose of aged receptor cells. Apart from reducing Caspase-2 expression, Bax deficiency also altered taste organ development. bax(-/-) mice had a more profusely innervated vallate papilla, which grew to be 25% longer and taller, with the mean taste bud containing more than twice the normal number of taste cells. This augmentation of taste organ development with increased innervation is complementary to the well-documented reduction in taste organ development with sparse innervation. We propose that additional taste neurons survived programmed cell death in Bax-deficient mice, thereby providing an inductive boost to vallate gustatory development.

Regulation of the Epstein-Barr virus C promoter by AUF1 and the cyclic AMP/protein kinase A signaling pathway

EBNA2 is an Epstein-Barr virus (EBV)-encoded protein that regulates the expression of viral and cellular genes required for EBV-driven B-cell immortalization. Elucidating the mechanisms by which EBNA2 regulates viral and cellular gene expression is necessary to understand EBV-induced B-cell immortalization and viral latency in humans. EBNA2 targets to the latency C promoter (Cp) through an interaction with the cellular DNA binding protein CBF1 (RBPJk). The EBNA2 enhancer in Cp also binds another cellular factor, C promoter binding factor 2 (CBF2), whose protein product(s) has not yet been identified. Within the EBNA2 enhancer in Cp, we have previously identified the DNA sequence required for CBF2 binding and also determined that this element is required for efficient activation of Cp by EBNA2. In this study, the CBF2 activity was biochemically purified and microsequenced. The peptides sequenced were identical to the hnRNP protein AUF1. Antibodies against AUF1 but not antibodies to related hnRNP proteins reacted with CBF2 in gel mobility shift assays. In addition, stimulation of the cellular cyclic AMP (cAMP)/protein kinase A (PKA) signal transduction pathway results in an increase in detectable CBF2/AUF1 binding activity extracted from stimulated cells. Furthermore, the CBF2 binding site was able to confer EBNA2 responsiveness to a heterologous promoter when transfected cells were treated with compounds that activate PKA or by cotransfection of plasmids expressing a constitutively active catalytic subunit of PKA. EBNA2-mediated stimulation of the latency Cp is also increased in similar cotransfection assays. These results further support an important role for CBF2 in mediating EBNA2 transactivation; they identify the hnRNP protein AUF1 as a major component of CBF2 and are also the first evidence of a cis-acting sequence other than a CBF1 binding element that is able to confer responsiveness to EBNA2.

The apoptotic pathway: a target for therapy in chronic lymphocytic leukemia

Cell division and apoptosis (programmed cell death) are the two major physiological processes which control the size of cell populations. Chronic lymphocytic leukaemia arises as a result of the clonal expansion of, usually B-, lymphocytes in which a dysregulation of apoptosis leads to prolonged cell survival. The same process becomes exaggerated with increasing drug resistance, the usual cause of treatment failure in this condition. The identification of points in the apoptotic pathway at which dysregulation occurs is beginning to open up new therapeutic opportunities where the conventional cytotoxic chemotherapy approach is found to fail. Although these strategies are still in their infancy they may increase understanding of the pathogenesis of the disorder and overcome the problem of drug resistance.

DNA replication of first-generation adenovirus vectors in tumor cells

A major role of the early gene 1A and 1B products (E1A and E1B) in adenovirus infection is to create a cellular environment appropriate for viral DNA replication. This is, in part, achieved by inactivation of tumor suppressor gene products such as pRb or p53. The functions of these same cellular proteins are also frequently lost in tumor cells. Therefore, we hypothesized that tumor cell lines with deregulated p53 and/or pRb pathways might support replication of E1A/E1B-deleted, first-generation adenovirus vectors (AdE1(-)). Here, we analyzed the impact of virus uptake, cell cycling, and the status of cell cycle regulators on AdE1(-) DNA synthesis. Cellular internalization of AdE1(-) vectors varied significantly among different tumor cell lines, whereas nuclear import of incoming viral DNA appeared to be less variable. Replication assays performed under equalized infection conditions demonstrated that all analyzed tumor cell lines supported AdE1(-) synthesis to varying degrees. There was no obvious correlation between the efficiency of viral DNA replication and the status of p53, pRb, and p16. However, the amount of virus attached and internalized changed with the cell cycle, affecting the intracellular concentration of viral DNA and thereby the replication efficacy. Furthermore, infection with AdE1 - vectors caused a partial G(2)/M arrest or delay in cell cycle progression, which became more pronounced in consecutive cell cycles. Correspondingly, vector DNA replication was found to be enhanced in cells artificially arrested in G(2)/M. Our findings suggest that cell cycling and thus passing through G(2)/M supports AdE1(-) DNA replication in the absence of E1A/E1B. This has potential implications for the use of first-generation adenovirus vectors in tumor gene therapy.

Apoptosis induced by the nuclear death domain protein p84N5 is associated with caspase-6 and NF-kappa B activation

Although the mechanisms involved in responses to extracellular or mitochondrial apoptotic signals have received considerable attention, the mechanisms utilized within the nucleus to transduce apoptotic signals are not well understood. We have characterized apoptosis induced by the nuclear death domain-containing protein p84N5. Adenovirus-mediated N5 gene transfer or transfection of p84N5 expression vectors induces apoptosis in tumor cell lines with nearly 100% efficiency as indicated by cellular morphology, DNA fragmentation, and annexin V staining. Using peptide substrates and Western blotting, we have determined that N5-induced apoptosis is initially accompanied by activation of caspase-6. Activation of caspases-3 and -9 does not peak until 3 days after the peak of caspase-6 activity. Expression of p84N5 also leads to activation of NF-kappaB as indicated by nuclear translocation of p65RelA and transcriptional activation of a NF-kappaB-dependent reporter promoter. Changes in the relative expression level of Bcl-2 family proteins, including Bak and Bcl-Xs, are also observed during p84N5-induced apoptosis. Finally, we demonstrate that p84N5-induced apoptosis does not require p53 and is not inhibited by p53 coexpression. We propose that p84N5 is involved in an apoptotic pathway distinct from those triggered by death domain-containing receptors or by p53.

Impaired BAX protein expression in breast cancer: mutational analysis of the BAX and the p53 gene

We have previously shown that the pro-apoptotic BAX protein is differentially expressed in breast cancer and in other epithelial tumors. In this line, a reduced BAX protein expression is a negative prognostic factor in various carcinomas including breast cancer. For p53, a trancriptional activator of BAX in apoptosis, mutations in the coding sequence were shown to modulate BAX protein expression in cell line models on the transcriptional level. We therefore investigated the BAX gene in 68 breast cancer specimens for the presence of mutations in the coding sequence by single-strand conformation polymorphism (SSCP)-PCR and direct sequencing. The expression of BAX protein was assessed by immunohistochemistry. In addition, we screened for mutations in the exons 5-8 of the p53 gene by SSCP-PCR to assess whether mutations in the DNA-binding domain of this upstream regulator of BAX gene transcription are responsible for differences in BAX protein expression. As previously observed, BAX was differentially expressed in the breast cancer samples, but no mutations in the coding sequence of the BAX gene were found besides a polymorphism in exon 6 at the position 552 (G->A) and additional intronic polymorphisms. In contrast, we identified 16 of 68 (23.5%) tumors to bear mutations in the p53 gene. In the subset of BAX-expressing tumors, the mutational inactivation of p53 did result in a reduced BAX protein expression (Fisher exact test, p = 0. 047). Nevertheless, we identified a subset of BAX-negative tumors lacking BAX or p53 mutations. Thus, additional, not yet identified regulators, apart from p53, appear to be involved in the regulation of BAX protein expression.

Epstein-Barr virus latency: LMP2, a regulator or means for Epstein-Barr virus persistence?

Like other herpesviruses, Epstein-Barr virus (EBV) persists in its host through an ability to establish a latent infection that periodically reactivates, producing infectious virus that infects naïve hosts. Disease syndromes in humans caused by EBV reflect the cell types that EBV infects, being primarily of lymphoid or epithelial origin. The most notable lymphoid disease, infectious mononucleosis, is a self-limiting lymphoproliferative disease that occurs in normal adolescents on primary infection. Children are normally able to resolve primary EBV infection with few or no symptoms. By the age of 25 most individuals are EBV seropositive. EBV is associated with a variety of hematopoietic cancers such as African Burkitt's lymphoma, Hodgkin's, and adult T-cell leukemia. EBV-associated lymphoproliferative disease occurs in individuals with congenital or acquired cellular immune deficiencies. The two notable epithelial diseases associated with EBV infection are nasopharyngeal cancer, a malignancy endemic to southern China, and oral hairy leukoplakia, an epithelial hyperplasia of the lingual squamous epithelium in AIDS patients. Latent membrane protein 2 (LMP2) is expressed both in normal EBV latency and EBV-associated pathologies. LMP2 may regulate reactivation from latency by interfering with normal B-cell signal transduction processes and in doing so may also provide a survival signal that could be important for viral persistence. Current knowledge about the function of LMP2 is described, defining a new class of regulators of herpesvirus latency.

Death and decoy receptors and p53-mediated apoptosis

Recently, several tumor necrosis factor receptor 1 (TNF-R1) and Fas-related death receptors have been discovered and include DR3, DR4, DR5 and DR6. These receptors contain an extracellular region containing varying numbers of cysteine-rich domains and an intracellular region that contains the death domain. The death receptors are activated in a ligand-dependent or independent manner and transduce apoptotic signals via their respective intracellular death domains. In addition to death receptors, several decoy molecules have also been identified and include DcR1/TRID, DcR2/TRUNDD, DcR3 and osteoprotegrin (OPG). The decoy molecules do not transduce apoptotic signals but rather compete with the death receptors for ligand binding and thereby inhibit ligand-induced apoptosis. Recent evidence suggests that p53 upregulates the expression of death receptors Fas and DR5, and thus, may mediate apoptosis in part via Fas and/or DR5. However, p53 also regulates the expression of TRAIL decoy receptors DcR1/TRID and DR2/TRUNDD. Although the significance of p53-dependent regulation of decoy receptors remains unclear, evidence suggests that DcR1/TRUNDD appears to inhibit 53-mediated apoptosis. It is, therefore, possible that p53 may blunt its DR5-dependent apoptotic effects by controlling the levels of decoy receptors.

Apoptosis and apoptotic-related factors in mucoepidermoid carcinoma of the oral minor salivary glands

The oncoproteins Bcl-2 and Bax, the tumor suppressor gene product p53, TUNEL (TdT [terminal deoxynucleotidyl transferase] dUTP nick end-labeling) and the cell-cycle antigen Ki-67 were studied in 71 cases of mucoepidermoid carcinoma originating in the oral minor salivary glands. Grade I tumors had higher expression of Bcl-2 than Grade II and III tumors (chi2 test, 0.01<P<0.025) and the Bcl-2-positive group had a higher survival rate than the Bcl-2-negative group (generalized Wilcoxon, P = 0.00051). Patients with strong TUNEL positivity had a higher survival rate than those with either weak positivity or negativity (generalized Wilcoxon, P = 0.047). The expression of p53 and TUNEL had a positive correlation (P = 0.0315). Grade II and III tumors had a higher frequency of positive Ki-67 expression than Grade I tumors (chi2 test, 0.01<P<0.025) and patients with Ki-67-negative tumors had better survival than patients with Ki-67-positive tumors (generalized Wilcoxon, P = 0.000099). This study showed that Bcl-2 proteins, p53 protein, TUNEL and Ki-67 are potentially useful prognostic markers for survival in patients with mucoepidermoid carcinoma of the oral minor salivary glands.

Radiation-induced apoptosis in the adult central nervous system is p53-dependent

Oligodendrocytes and subependymal cells in the adult CNS have been shown to undergo radiation-induced apoptosis. Here, we examined the role of p53 in radiation-induced apoptosis in the adult mouse CNS. In the spinal cord of p53+/+ mice, apoptotic glial cells were observed within 24 h after irradiation, and the apoptotic response peaked at 8 h. These apoptotic cells demonstrated the immunohistochemical phenotype of oligodendrocytes, and decreased oligodendrocyte density was observed at 24 h after 22 Gy. A similar time course of radiation-induced apoptosis was seen in subependymal cells in the adult mouse brain. Radiation-induced apoptosis was preceded by an increase in nuclear p53 expression in glial cells of the spinal cord and subependymal cells of the brain. There was no evidence of radiation-induced apoptosis in the spinal cord and subependymal region of p53-/- animals. We conclude that the p53 pathway may be a mechanism through which DNA damage induces apoptosis in the adult CNS.

Mechanisms of cell-cycle checkpoints: at the crossroads of carcinogenesis and drug discovery

Human tumors arise from multiple genetic changes that gradually transform growth-limited cells into highly invasive cells that are unresponsive to growth controls. The genetic evolution of normal cells into cancer cells is largely determined by the fidelity of DNA replication, repair, and division. Cell-cycle arrest in response to stress is integral to the maintenance of genomic integrity. The control mechanisms that restrain cell-cycle transition or induce apoptotic signaling pathways after cell stress are known as cell-cycle checkpoints. This review will focus on the mechanisms of cell-cycle checkpoint pathways and how different components of these pathways are frequently altered in the genesis of human tumors. As our knowledge of cell-cycle regulation and checkpoints increases, so will our understanding of how xenobiotic agents can affect these processes to either initiate or inhibit tumorigenesis.

Apoptosis: implications for inflammatory bowel disease

Cells of the intestinal mucosa live in a harsh environment and therefore rely heavily on the highly regulated process of cell death, apoptosis, to maintain tissue integrity. Imbalance in the intracellular events that modulate apoptosis may contribute to the pathogenesis of inflammatory bowel disease.

Bax-mediated apoptosis in the livers of rats after partial hepatectomy in the retrorsine model of hepatocellular injury

Retrorsine is a member of the pyrrolizidine alkaloid family of compounds whose toxic effects on the liver include a long-lasting inhibition of the proliferative capacity of hepatocytes. Despite the retrorsine-induced blockade of hepatocyte proliferation, retrorsine-exposed rats are able to reconstitute completely their liver mass after surgical partial hepatectomy (PH) via the sustained proliferation of a population of small, incompletely differentiated hepatocyte-like progenitor cells (SHPCs). The extensive proliferation of SHPCs in retrorsine-injured livers is accompanied by the progressive loss of irreversibly injured megalocytes. To study the mechanism by which retrorsine-damaged hepatocytes are removed after PH, we performed TUNEL analysis to establish apoptotic indices for hepatocytes in the livers of retrorsine-exposed and control rats up to 14 days post-PH. Apoptotic indices are highest (approximately 6.0%) in the livers of retrorsine-exposed rats at 1 day post-PH, gradually declining thereafter, yet remaining significantly elevated (approximately 1%) over control rats (<0.1%) at 14 days post-PH (P <.05). After PH, levels of the proapoptotic protein Bax are increased in livers from retrorsine-exposed rats relative to the levels observed in control livers. Similarly, levels of the antiapoptotic protein Bcl-x(L) are significantly decreased (P <.05) compared with controls at t = 0 resulting in an increased (approximately 3.5-fold) Bax/Bcl-x protein ratio that is significantly elevated (P <.05) compared with controls. Finally, increased levels of Bax protein are localized to the mitochondria of retrorsine-exposed rat livers after PH during the same time that cytochrome c is released. These observations combine to suggest that retrorsine-injured hepatocytes are removed after PH via apoptotic pathways dependent on relative levels and localization of Bax and Bcl-x(L) protein.

KILLER/DR5, a novel DNA-damage inducible death receptor gene, links the p53-tumor suppressor to caspase activation and apoptotic death

TRAIL and its emerging receptors are the newest members of the TNF receptor super-family. The activation of TRAIL receptors by ligand binding leads to apoptosis through caspase activation through an as yet unclear signaling pathway that does not require the FADD adaptor. The TRAIL receptor KILLER/DR5, is induced by DNA damage and appears to be regulated by the tumor suppressor gene p53. Both the Fas receptor and KILLER/DR5 provide potential links between DNA damage-mediated activation of the p53 tumor suppressor and caspase activation. While further evaluation of the role of TRAIL receptors in human cancer is ongoing, initial studies suggest that both KILLER/DR5 and DR4 may be targets for inactivation and that these pro-apooptotic receptors may be tumor suppressor genes. Understanding the regulation of TRAIL and its receptors may thus be beneficial for the development of novel approaches for cancer treatment. TRAIL appears to be a cancer-specific cytotoxic agent and thus offers promise as a novel therapy for cancer either through replacement of the cytokine or potentially via gene replacement. Preliminary studies suggest the potential to combine TRAIL with classical cytotoxic chemotherapeutic drugs to achieve synergistic cell killing.

Induction of wild-type p53 activity in human cancer cells by ribozymes that repair mutant p53 transcripts

Several groups have attempted to develop gene therapy strategies to treat cancer via introduction of the wild-type (wt) p53 cDNA into cancer cells. Unfortunately, these approaches do not result in regulated expression of the p53 gene and do not reduce expression of the mutant p53 that is overexpressed in cancerous cells. These shortcomings may greatly limit the utility of this gene replacement approach. We describe an alternative strategy with trans-splicing ribozymes that can simultaneously reduce mutant p53 expression and restore wt p53 activity in various human cancers. The ribozyme accomplished such conversion by repairing defective p53 mRNAs with high fidelity and specificity. The corrected transcripts were translated to produce functional p53 that can transactivate p53-responsive promoters and down-modulate expression of the multidrug resistance (MDR1) gene promoter. The level of wt p53 activity generated was significant, resulting in a 23-fold induction of a p53-responsive promoter and a 3-fold reduction in MDR1 promoter expression in transfected cancer cells. Once efficient delivery systems are developed, this strategy should prove useful for making human cancers more responsive to p53 activity and more sensitive to chemotherapeutic agents.

p38 mitogen-activated protein kinase regulates a novel, caspase-independent pathway for the mitochondrial cytochrome c release in ultraviolet B radiation-induced apoptosis

The mechanisms of UVB-induced apoptosis and the role of p38 mitogen-activated protein kinase (MAPK) were investigated in HaCaT cells. UVB doses that induced apoptosis also produced a sustained activation of p38 MAPK and mitochondrial cytochrome c release, leading to pro-caspase-3 activation. Late into the apoptotic process, UVB also induced a caspase-mediated cleavage of Bid. Caspase inhibitors benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone and benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone substantially blocked the UVB-induced apoptosis without preventing the release of mitochondrial cytochrome c and the p38 MAPK activation. The inhibition of p38 MAPK counteracted both apoptosis and cytochrome c release as well as the DEVD-amino-4-methylcoumarin cleavage activity without affecting the processing of pro-caspase-8. These results indicate that UVB induces multiple and independent apoptotic pathways, which culminate in pro-caspase-3 activation, and that the initial cytochrome c release is independent of caspase activity. Importantly, we show that a sustained p38 MAPK activation contributes to the UVB-induced apoptosis by mediating the release of mitochondrial cytochrome c into the cytosol.

The central region of Gadd45 is required for its interaction with p21/WAF1

Cell cycle arrest represents an important response to genotoxic stress and the tumor suppressor p53 has been described to act as a critical effector in this biological event. Upon stress, p53 becomes transcriptionally active and up-regulates the transcription of downstream effector genes, which contain p53 recognition sites in their regulatory regions. Among the genes activated are p21 and GADD45, each of which independently exhibits growth-suppressive activity. The Gadd45 protein has been described to form a complex with p21, and thus, work was undertaken to map the regions of Gadd45 involved in this interaction and to examine the roles of those two proteins in growth suppression. In this report, a Gadd45 overlapping peptide library and a series of Gadd45 deletion mutants were used to define the domains of Gadd45 involved in the association with p21. Results using both in vitro and in vivo methods have shown that the interaction of Gadd45 with p21 involves a central region of Gadd45. Interestingly, the p21-binding domain of Gadd45 also encodes the Cdc2-binding activity, indicating that the central region of Gadd45 may serve as an important "core," through which Gadd45 protein is able to present cross-talk with other cell cycle regulators. In addition, GADD45 inhibition of Cdc2 kinase activity was compared with Myd118 and CR6, two other members of the GADD45 family. GADD45 was shown to generate the strongest inhibitory effect on Cdc2 activity. Finally, results from short-term survival assays further demonstrated that p21 and GADD45 act upon different cellular pathways to exert their growth-suppressive function.

Histone deacetylases specifically down-regulate p53-dependent gene activation

p53, the most commonly mutated gene in cancer cells, directs cell cycle arrest or induces programmed cell death (apoptosis) in response to stress. It has been demonstrated that p53 activity is up-regulated in part by posttranslational acetylation. In agreement with these observations, here we show that mammalian histone deacetylase (HDAC)-1, -2, and -3 are all capable of down-regulating p53 function. Down-regulation of p53 activity by HDACs is HDAC dosage-dependent, requires the deacetylase activity of HDACs, and depends on the region of p53 that is acetylated by p300/CREB-binding protein (CBP). These results suggest that interactions of p53 and HDACs likely result in p53 deacetylation, thereby reducing its transcriptional activity. In support of this idea, GST pull-down and immunoprecipitation assays show that p53 interacts with HDAC1 both in vitro and in vivo. Furthermore, a pre-acetylated p53 peptide was significantly deacetylated by immunoprecipitated wild type HDAC1 but not deacetylase mutant. Also, co-expression of HDAC1 greatly reduced the in vivo acetylation level of p53. Finally, we report that the activation potential of p53 on the BAX promoter, a natural p53-responsive system, is reduced in the presence of HDACs. Taken together, our findings indicate that deacetylation of p53 by histone deacetylases is likely to be part of the mechanisms that control the physiological activity of p53.

A mathematical model of caspase function in apoptosis

Caspases (cysteine-containing aspartate-specific proteases) are at the core of the cell's suicide machinery. These enzymes, once activated, dismantle the cell by selectively cleaving key proteins after aspartate residues. The events culminating in caspase activation are the subject of intense study because of their role in cancer, and neurodegenerative and autoimmune disorders. Here we present a mechanistic mathematical model, formulated on the basis of newly emerging information, describing key elements of receptor-mediated and stress-induced caspase activation. We have used mass-conservation principles in conjunction with kinetic rate laws to formulate ordinary differential equations that describe the temporal evolution of caspase activation. Qualitative strategies for the prevention of caspase activation are simulated and compared with experimental data. We show that model predictions are consistent with available information. Thus, the model could aid in better understanding caspase activation and identifying therapeutic approaches promoting or retarding apoptotic cell death.

Induction of IW32 erythroleukemia cell differentiation by p53 is dependent on protein tyrosine phosphatase

The biological activity of p53 in IW32 erythroleukemia cells was investigated. IW32 cells had no detectable levels of p53 mRNA and protein expression. By transfecting a temperature-sensitive mutant p53 cDNA, tsp53val135, into the cells, we have established several clones stably expressing the mutant p53 allele. At permissive temperature, these p53 transfectants were arrested in G1 phase and underwent apoptosis. Moreover, differentiation along the erythroid pathway was observed as evidenced by increased benzidine staining and mRNA expression of beta-globin and the erythroid-specific delta-aminolevulinic acid synthase (ALAS-E). Treatment of cells with protein tyrosine phosphatase inhibitor vanadate blocked the p53-induced differentiation, but not that of cell death or growth arrest. Increased protein tyrosine phosphatase activity as well as mRNA levels of PTPbeta2 and PTPepsilon could be observed by wildtype p53 overexpression. These results indicate that p53 induced multiple phenotypic consequences through separate signal pathways in IW32 erythroleukemia cells, and protein tyrosine phosphatase is required for the induced differentiation.

Apoptosis regulation in the testis: involvement of Bcl-2 family members

Using immunohistochemical techniques and Western blot analysis, the possible role of Bcl-2 family members Bax, Bcl-2, Bcl-x(s), and Bcl-x(l) in male germ cell density-related apoptosis and DNA damage induced apoptosis was studied. The apoptosis inducer Bax was localized in all mouse and human testicular cell types, but despite the fact that irradiation induces its transcriptional activator, p53 in the human, Bax expression did not change after irradiation. The apoptosis inhibitor Bcl-2 appeared to be present in late spermatocytes and spermatids and was up-regulated in these cells after a dose of 4 Gy of X-rays. Finally, Bcl-x was expressed in both the mouse and human testis. The apoptosis inhibiting long transcripts of Bcl-x, Bcl-x(l), were expressed in spermatogonia and spermatocytes and were up-regulated after X-irradiation. The apoptosis inducing shorter form of Bcl-x, Bcl-x(s), was found to be expressed only in somatic cells, like peritubular and Leydig cells. While Bax is important in germ cell density regulation, Bax expression did not change after DNA damage inflicted by X-radiation. Hence, spermatogonial apoptosis after X-irradiation may not be induced via the apoptosis inducer Bax. Furthermore, as Bcl-x(l), but not Bcl-2, is present in spermatogonia and spermatocytes, Bcl-x(l) may regulate germ cell density, possibly in cooperation with Bax. As Bcl-x(l) expression is enhanced after irradiation, this protein may also have a role in the response of spermatogonia and spermatocytes to irradiation.

Placental transforming growth factor-beta is a downstream mediator of the growth arrest and apoptotic response of tumor cells to DNA damage and p53 overexpression

The p53 tumor suppressor gene and members of the transforming growth factor-beta (TGF-beta) superfamily play central roles in signaling cell cycle arrest and apoptosis (programmed cell death) in normal development and differentiation, as well as in carcinogenesis. Here we describe a distantly related member of the TGF-beta superfamily, designated placental TGF-beta (PTGF-beta), that is up-regulated in response to both p53-dependent and -independent apoptotic signaling events arising from DNA damage in human breast cancer cells. PTGF-beta is normally expressed in placenta and at lower levels in kidney, lung, pancreas, and muscle but could not be detected in any tumor cell line studied. The PTGF-beta promoter is activated by p53 and contains two p53 binding site motifs. Functional studies demonstrated that one of these p53 binding sites is essential for p53-mediated PTGF-beta promoter induction and specifically binds recombinant p53 in gel mobility shift assays. PTGF-beta overexpression from a recombinant adenoviral vector (AdPTGF-beta) led to an 80% reduction in MDA-MB-468 breast cancer cell viability and a 50-60% reduction in other human breast cancer cell lines studied, including MCF-7 cells, which are resistant to growth inhibition by recombinant wild-type p53. Like p53, PTGF-beta overexpression was seen to induce both G(1) cell cycle arrest and apoptosis in breast tumor cells. These results provide the first evidence for a direct functional link between p53 and the TGF-beta superfamily and implicate PTGF-beta as an important intercellular mediator of p53 function and the cytostatic effects of radiation and chemotherapeutic cancer agents.

Prolonged survival of heart allografts from p53-deficient mice

BACKGROUND

Acute rejection of the heart allograft is the major cause of heart failure in the first month after transplantation. Most studies on the prevention of acute rejection have concentrated on immune suppression of the recipients, whereas little is known about the effects of genetically manipulated donor organs on heart allograft survival. Herein, we describe a mouse model of heart allografts donated by p53-/- mice that can prolong the survival time of the grafts.

METHODS

Hearts of p53-/- or p53+/+ C57BL/6J mice were grafted to the neck carotid artery and jugular vein of BALB/c mice using a cuff technique. The graft survival was observed daily. The hearts were analyzed using several techniques, including histology, immunofluorescence, terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL), and Western blot analysis.

RESULTS

p53+/+ allografts ceased beating at 7.6+/-0.5 days, whereas p53-/- hearts were beating at 10.5+/-1.1 days after transplantation (P<0.01). Mean histological rejection scores were significantly lower in allografts donated by p53-deficient mice. Furthermore, apoptotic cells, determined by TUNEL and a reagent kit for detection of cardiac apoptosis, were of high numbers in the allograft sections from wild-type hearts but rare in p53-/- allografts (4.2+/-1.3 vs. 0.7+/-0.5/250x field). Immunofluorescence staining and Western blot analysis revealed that high levels of p53 and proapoptotic protein Bax were expressed in wild-type grafts but not p53-/- allografts. Interestingly, Bcl-2, an antiapoptotic protein, was abundant in cardiac allografts from p53-/- mice and almost undetectable in grafts from wild-type mice.

CONCLUSIONS

Thus, p53 is involved in cardiac apoptosis induced by alloimmune reaction, and prolonged survival of heart allografts can be achieved when p53 is lacking.

Inhibition of cell death by lens-specific overexpression of bcl-2 in transgenic mice

Previous studies on cell cycle regulation in the ocular lens using transgenic mice have shown that inactivation of the retinoblastoma tumor suppressor protein (pRb) can cause postmitotic lens fiber cells to enter the cell cycle. However, when the p53 gene and protein are intact, inactivation of pRb in this terminally differentiated cell type results in cell death, rather than continued proliferation. Since bcl-2 has been shown to act as a cell death repressor, the ability of this gene to block p53-dependent apoptosis in lenses was examined. Transgenic mice were generated that overexpress bcl-2 in a lens-specific fashion. Surprisingly, overexpression of bcl-2 was sufficient to interfere with normal fiber cell differentiation, inducing cataracts, microphakia, vacuolization, fiber cell disorganization, and inhibition of fiber cell denucleation. The bcl-2 mice were mated to mice exhibiting lens-specific expression of the N-terminal region of simian virus 40 large T antigen (termed truncT). The resulting double transgenic mice showed a marked reduction in the truncT-induced fiber cell death. Apoptosis in the truncT mice could also be suppressed by crossing these mice into a p53-deficient background. Either overexpression of bcl-2 or loss of p53 in truncT mice resulted in proliferation of fiber cells around the cortex of the lens. These proliferating fiber cells continue to express beta- and gamma-crystallin proteins, which are normally only expressed following withdrawal from the cell cycle. The p53 protein is known to upregulate expression of certain target genes, including p21, a protein that can block cell cycle progression by inhibition of cyclin-dependent kinases. In order to assess whether bcl-2 interferes with the transcriptional activation activity of p53, transgenic lenses were assayed by in situ hybridization for levels of p21 expression. Lenses that expressed both truncT and bcl-2 showed elevated p21, implying that bcl-2 does not inhibit apoptosis by directly inhibiting p53, but instead may block a later step in the apoptosis pathway. In addition, overexpression of p21 is not sufficient to cause apoptosis. These experiments show that the lenses of transgenic mice represent a valuable in vivo setting for studies of both induction and inhibition of programmed cell death.

Alternative product of the p16/CKDN2A locus connects the Rb and p53 tumor suppressors

Two distinct products are specified by the CDKN2A locus, the p16INK4a cyclin dependent kinase inhibitor and a protein termed ARF. ARF has been shown to bind to the Mdm2-p53 complex, resulting in stabilisation of both proteins, and a feedback loop exists through which ARF levels are negatively regulated by p53. Significantly, ARF expression is positively regulated by members of the E2F family of transcription factors. This provides a link between the Rb and p53 pathways and a mechanism whereby inactivation of Rb and release of E2F will lead to the stabilisation and functional activation of p53. The alternative exon encoding the functional amino terminal portion of ARF presumably represents an independent gene that has become co-localized with p16INK4a in order to exploit a common regulatory mechanism or purpose.

Transduction of Apaf-1 or caspase-9 induces apoptosis in A-172 cells that are resistant to p53-mediated apoptosis

p53 replacement gene therapy has been carried out clinically for cancers with p53 mutations; however, some cancers are resistant to p53 gene therapy. In this study, we transduced A-172 and U251 cells harboring p53 mutations with wild-type p53 using adenovirus vectors to induce wild-type p53 protein at similar expression levels. A-172 cells did not undergo apoptosis after p53 transduction, whereas U251 cells were markedly sensitive to p53-mediated apoptosis. A-172 cells showed higher endogenous expression of Bcl-X(L) than U251, and transduction of Bcl-X(L) repressed p53-mediated apoptosis in U251 cells, suggesting that high endogenous expression of Bcl-X(L) renders A-172 cells, at least in part, resistant to p53-mediated apoptosis. We transduced A-172 cells and U251 cells with the Apaf-1 or caspase-9 genes; both are downstream components of p53-mediated apoptosis. We found that A-172 cells were highly sensitive to Apaf-1- and caspase-9-mediated apoptosis. The results indicate that A-172 cells harboring mutant p53 were not susceptible to p53-mediated apoptosis, possibly due to high endogenous expression of Bcl-X(L). Transduction of Apaf-1 or caspase-9 would override the resistance mechanism of apoptosis in A-172 cells. These findings provide potentially a novel approach in killing cancers that are resistant to p53 replacement gene therapy.

Susceptibility to drug-induced apoptosis correlates with differential modulation of Bad, Bcl-2 and Bcl-xL protein levels

To define the responses of apoptotic regulatory proteins to different chemotherapeutic agents, we investigated the expression of Bcl-2 family gene products, the release of cytochrome c, and the activation of pro-caspase-3 during apoptosis induced by Taxol and Thiotepa, in the MCF-7 breast carcinoma and the HL-60 leukemia cell lines. The earliest event induced by drug exposure was increase in Bad protein levels, followed by Bcl-2 down-regulation, cytochrome c release, and Bcl-xL and Bax up-regulation. Bak accumulation was a late event. Activation of pro-caspase-3 and cleavage of Bcl-2 protein occurred in the HL-60 cells only, and followed the cytochrome c release. The overall responses were qualitatively similar in both cell types, but MCF-7 cells treated with Taxol showed a significant delay in apoptosis, correlating with early up-regulation of Bcl-2 and delayed release of cytochrome c. We conclude that Bad up-regulation is an early indicator of a cellular response that will lead to cell death, but may be modulated by survival mechanisms, which cumulatively govern the ultimate susceptibility to apoptosis.

Normal breast epithelial cells induce apoptosis of MCF-7 breast cancer cells through a p53-mediated pathway

Cancer development depends not only on the nature of the tumor cells themselves but also on the regulatory effects of various normal cells. The present study was performed to better understand the mechanism by which normal breast epithelial cells (NBEC) can control the growth of MCF-7 breast cancer cells. When MCF-7 cells were treated with NBEC conditioned medium, cell growth was inhibited in a concentration-dependent manner. This inhibition was due to an induction of apoptosis without any change in cell cycle progression. The induction of apoptosis was correlated with increased levels of p53, p21(waf1) and decreased levels of bcl-2. Transient transfections of MCF-7 cells with two p53 cDNA constructs demonstrated the induction of apoptosis was mediated by endogenous p53. Taken together, our results indicate that NBEC inhibit the growth of MCF-7 breast cancer cells by inducing apoptosis in them via endogenous p53.

Increase in ultraviolet sensitivity by overexpression of calpastatin in ultraviolet-resistant UVr-1 cells derived from ultraviolet-sensitive human RSa cells

Human RSa cells are highly sensitive to apoptotic-like cell death by ultraviolet irradiation (UV) while UVr-1 cells are their variant with an increased resistance to UV. Three days after UV at 10 J/m2, the viability of RSa cells was approximately 17% while that of UVr-1 cells was 65%. This different survival might reflect apoptotic cell death since apoptosis-specific DNA ladder was more clearly observed in RSa cells than in UVr-1 cells after UV. Addition of ALLN/calpain inhibitor I to the culture medium after UV resulted in similar survival (14 - 18%) between RSa and UVr-1 cells. Immunoblot analysis showed down-regulation of protein kinase CTheta, Src, Bax and mu-calpain after UV was more prominent in UVr-1 than in RSa cells. Activated mu-calpain appeared within 1 h post-UV only in UVr-1 cells. The expression of calpastatin, a specific endogenous inhibitor of calpain, was higher in RSa than in UVr-1 cells. To further examine the role of calpain in UV-induced cell death, cDNA of human calpastatin was transfected into UVr-1 cells. The results showed that overexpression of calpastatin suppressed down-regulation of Src, mu-calpain and Bax. Concomitantly, colony survival after UV was reduced in calpastatin-transfected cells as compared to vector control cells. Our results suggest that activation of calpain might account for, at least in part, the lower susceptibility to UV-induced cell death in UVr-1 cells.

CREM: a master-switch regulating the balance between differentiation and apoptosis in male germ cells

Cyclic AMP-responsive element modulator (CREM) is a transcription factor highly expressed in the post-meiotic germ cells of the testis. Its pivotal role is to regulate the expression of several germ cell-specific genes, a crucial function as demonstrated by the severe phenotype of mice whose CREM gene was mutated by homologous recombination. CREM-deficient male animals are sterile and display a ten-fold increase in the apoptosis of germ cells. Recent results have shown that CREM needs a tissue-specific co-activator, ACT (activator of CREM in testis) to elicit its regulatory function in testis.

Pathways through which a regimen of melatonin and retinoic acid induces apoptosis in MCF-7 human breast cancer cells

It has been established that melatonin (Mlt) and retinoic acid, individually, inhibit the proliferation of the estrogen receptor-alpha (ER alpha)-positive MCF-7 breast cancer cell line. Our laboratory has previously demonstrated that Mlt and all-trans-retinoic acid (atRA) not only inhibit the proliferation, but also induce apoptosis of MCF-7 cells when used in a sequential regimen of Mlt followed 24 h later by atRA. Using this same MCF-7 breast cancer cell line, we investigated the potential pathways through which apoptosis is being induced. We found that treatment of MCF-7 cells with Mlt for 24 h before the addition of atRA decreased the protein levels of the death suppressor, Bcl-2, and increased, although with different time courses, the levels of the death promoters, Bax and Bak; however, there was no change in the levels of the tumor suppressor gene, p53. MCF-7 cells treated sequentially with Mlt and atRA also demonstrated an enhanced sensitivity to the apoptotic effects of atRA, which did not appear to be due to increased expression of the retinoic acid receptors, RAR alpha or RXR alpha, but rather to enhanced transcriptional activity of the RAR alpha. These data suggest that the sequential treatment regimen of Mlt and atRA may induce apoptosis by modulation of members of the Bcl-2 family of proteins. Thus, this combinatorial regimen, which reduces the concentration of atRA needed for clinical efficacy while enhancing its anti-tumorigenic activity, could be of great therapeutic benefit, and may, in fact, specifically induce the regression of established breast tumors due to its apoptosis-promoting effects.

Chromium(VI) induces p53-dependent apoptosis in diploid human lung and mouse dermal fibroblasts

Some forms of hexavalent chromium [Cr(VI)] are known to cause damage to respiratory-tract tissue and DNA and are thought to be human lung carcinogens. In general, Cr(VI) is mutagenic and carcinogenic at doses that also evoke some cell death, and we previously showed that the predominant mode of death is apoptosis. Because p53 has been shown to initiate apoptosis after genotoxic insults, the objective of these experiments was to determine whether p53 is activated in and necessary for apoptosis of normal diploid human lung fibroblasts (HLF cells) after chromium exposure. By using annexin(V) staining and fluorescent microscopy, we found that Cr(VI) caused up to 14% of HLF cells to undergo apoptosis within 24 h after exposure. In addition, by using western blotting, we found that p53 protein levels increased fourfold to sixfold after exposure to sodium chromate. Because the major function of p53 is as a transcription factor, it must be translocated from the cytoplasm to the nucleus after chromate exposure to be active. Immunofluorescence studies using an antibody against p53 showed that, after chromate exposure, p53 was located in the nucleus of the treated HLF cells. The necessity of p53 for chromium-induced apoptosis was examined in two ways. One approach used dermal fibroblasts from p53 wild-type, heterozygous, and null mice, and the other approach used HLF cells that were transiently transfected with the human papilloma virus E6 gene, which targets p53 for degradation and creates a functional p53-null cell. These studies showed that chromium-induced apoptosis was p53 dependent. Mol. Carcinog. 28:111-118, 2000.

Death signal-induced localization of p53 protein to mitochondria. A potential role in apoptotic signaling

The mechanism of p53-mediated apoptosis after cellular stress remains poorly understood. Evidence suggests that p53 induces cell death by a multitude of molecular pathways involving activation of target genes and transcriptionally independent direct signaling. Mitochondria play a key role in apoptosis. We show here that a fraction of p53 protein localizes to mitochondria at the onset of p53-dependent apoptosis but not during p53-independent apoptosis or p53-mediated cell cycle arrest. The accumulation of p53 to mitochondria is rapid (within 1 h after p53 activation) and precedes changes in mitochondrial membrane potential, cytochrome c release, and procaspase-3 activation. Immunoelectron microscopy and immuno-fluorescence-activated cell sorter analysis of isolated mitochondria show that the majority of mitochondrial p53 localizes to the membranous compartment, whereas a fraction is found in a complex with the mitochondrial import motor mt hsp70. After induction of ectopic p53 without additional DNA damage in p53-deficient cells, p53 again partially localizes to mitochondria, preceding the onset of apoptosis. Overexpression of anti-apoptotic Bcl-2 or Bcl-xL abrogates stress signal-mediated mitochondrial p53 accumulation and apoptosis but not cell cycle arrest, suggesting a feedback signaling loop between p53 and mitochondrial apoptotic regulators. Importantly, bypassing the nucleus by targeting p53 to mitochondria using import leader fusions is sufficient to induce apoptosis in p53-deficient cells. We propose a model where p53 can contribute to apoptosis by direct signaling at the mitochondria, thereby amplifying the transcription-dependent apoptosis of p53.

The bradykinin type 2 receptor is a target for p53-mediated transcriptional activation

The bradykinin type 2 receptor (BK2) is a developmentally regulated G protein-coupled receptor that mediates diverse actions such as vascular reactivity, salt and water excretion, inflammatory responses, and cell growth. However, little is known regarding regulation of the BK2 gene. We report here that the rat BK2 receptor is transcriptionally regulated by the tumor suppressor protein p53. The 5'-flanking region of the rat BK2 gene contains two p53-like binding sites: a sequence at -70 base pairs (P1 site) that is conserved in the murine and human BK2 genes; and a sequence at -707 (P2) that is not. The P1 and P2 motifs bind specifically to p53, as assessed by gel mobility shift assays. Transient transfection into HeLa cells of a CAT reporter construct driven by 1.2-kilobases of the BK2 gene 5'-flanking region demonstrated that the BK2 promoter is dose dependently activated by co-expression of wild-type p53. Co-expression of a dominant negative mutant p53 suppresses the activation of BK2 by wild-type p53. Promoter truncation localized the p53-responsive element to the region between -38 and -94 base pairs encompassing the p53-binding P1 sequence. Furthermore, p53-mediated activation of the BK2 promoter is augmented by the transcriptional co-activators, CBP/p300. Interestingly, removal of the P2 site by truncation or site-directed deletion amplifies p53-mediated activation of the BK2 promoter. These results demonstrate that the rat BK2 promoter is a target for p53-mediated activation and suggest a new physiological role for p53 in the regulation of G protein-coupled receptor gene expression.

p53 suppresses the c-Myb-induced activation of heat shock transcription factor 3

Expression of heat shock proteins (HSPs) is controlled by heat shock transcription factors (HSFs). Vertebrates express multiple HSFs whose activities may be regulated by distinct signals. HSF3 is specifically activated in unstressed proliferating cells by direct binding to the c-myb proto-oncogene product (c-Myb), which plays an important role in cellular proliferation. This suggests that the c-Myb-induced HSF3 activation may contribute to the growth-regulated expression of HSPs. Here we report that the p53 tumor suppressor protein directly binds to HSF3 and blocks the interaction between c-Myb and HSF3. In addition, p53 stimulates the degradation of c-Myb through a proteasome-dependent mechanism, which is, at least partly, mediated by induction of Siah in certain types of cells. Induction of p53 by a genotoxic reagent in DT40 cells disrupts the HSF3-c-Myb interaction and down-regulates the expression of certain HSPs. Mutated forms of p53 found in certain tumors did not inhibit c-Myb-induced HSF3 activation. The regulation of HSF3 activity by c-Myb and p53 sheds light on the molecular events that govern HSP expression during cellular proliferation and apoptosis.

Noxa, a BH3-only member of the Bcl-2 family and candidate mediator of p53-induced apoptosis

A critical function of tumor suppressor p53 is the induction of apoptosis in cells exposed to noxious stresses. We report a previously unidentified pro-apoptotic gene, Noxa. Expression of Noxa induction in primary mouse cells exposed to x-ray irradiation was dependent on p53. Noxa encodes a Bcl-2 homology 3 (BH3)-only member of the Bcl-2 family of proteins; this member contains the BH3 region but not other BH domains. When ectopically expressed, Noxa underwent BH3 motif-dependent localization to mitochondria and interacted with anti-apoptotic Bcl-2 family members, resulting in the activation of caspase-9. We also demonstrate that blocking the endogenous Noxa induction results in the suppression of apoptosis. Noxa may thus represent a mediator of p53-dependent apoptosis.

Bleomycin-mediated pulmonary toxicity: evidence for a p53-mediated response

Bleomycin damages DNA and causes lung injury and fibrosis. To determine whether bleomycin is associated with the appearance of DNA damage-inducible proteins, C3H mice received either 0.4 mg bleomycin or normal saline intratracheally and were killed 1 to 14 d later. The lungs were examined for expression of p53, p21(WAF1/PiCl), and proliferating cell nuclear antigen (PCNA) using immunohistochemistry and Western blotting. p53-positive cells first appeared at 5 d after treatment and peaked at 7 d; PCNA-positive cells appeared at 1 d after treatment and peaked at 7 d; and p21-positive cells appeared at 5 d and peaked at 9 d. Western blot analysis confirmed that bleomycin upregulated the DNA damage-inducible proteins in a similar fashion. This is the first evidence that bleomycin causes a p53-dependent response associated with acute injury in the lung.

The Drosophila tumor necrosis factor receptor-associated factor-1 (DTRAF1) interacts with Pelle and regulates NFkappaB activity

A member of the tumor necrosis factor (TNF) receptor-associated factor (TRAF) family was identified in Drosophila. DTRAF1 contains 7 zinc finger domains followed by a TRAF domain, similar to mammalian TRAFs and other members of the family identified in data bases from Caenorhabditis elegans, Arabidopsis, and Dictyostelium. Analysis of DTRAF1 binding to different members of the human TNF receptor family showed that this protein can interact through its TRAF domain with the p75 neurotrophin receptor and weakly with the lymphotoxin-beta receptor. DTRAF1 can also self-associate and binds to human TRAF1, TRAF2, and TRAF4. Interestingly, DTRAF1 interacts with human cIAP-1 and cIAP-2 but not with Drosophila DIAP-1 and -2. By itself, DTRAF1 did not induce significant NFkappaB activation when overexpressed in mammalian cells, although it specifically increased NFkappaB induction by TRAF6. In contrast, TRAF2-mediated NFkappaB induction was partially inhibited by DTRAF1. Mutants of DTRAF1 lacking the N-terminal region inhibited NFkappaB induction by either TRAF2 or TRAF6. DTRAF1 specifically associated with the regulatory N-terminal domain of Pelle, a Drosophila homolog of the human kinase interleukin-1 receptor-associated kinase (IRAK). Interestingly, though Pelle and DTRAF1 individually were unable to induce NFkappaB in a human cell line, co-expression of Pelle and DTRAF1 resulted in significant NFkappaB activity. Interactions of DTRAF1 with human TRAF-, TNF receptor-, and IAP-family proteins imply strong evolutionary conservation of TRAF protein structure and function throughout Metazoan evolution.

Neuronal cell death in Down's syndrome

Down's syndrome (DS), occurring in 0.8 out of 1,000 live births, is a genetic disorder in which an extra portion of chromosome 21 leads to several abnormalities. With respect to the nervous system, it causes mental retardation. It is conceived that abnormal neuronal cell death in development is involved, but there is no direct evidence yet. In addition to developmental brain abnormalities, almost all DS brains over 40 years old manifest a similar pathology to Alzheimer's disease (AD), including the presence of senile plaques (SP) and neurofibrillary tangles (NFT). Although there was a debate to segregate dementia from underlying mental retardation, at least some portion of DS patients exhibit deteriorated mental status with aging. The mechanism underlying these abnormalities at the molecular level remains to be elucidated. Recently there have been several reports suggesting abnormalities reflecting increased risk to apoptosis in DS brains. Increased expression of several apoptosis-related genes (p53, fas, ratio of bax to bcl-2, GAPDH) in DS brains has been reported. Cultured neurons from both patients and model animals are reportedly more vulnerable to apoptosis. Overproduction of reactive oxygen species and its causative roles for increased apoptosis in DS tissues are suggested. One possible hypothesis is an increased susceptibility to apoptosis due to p53 overactivation in DS brains. A beta 42, a critical peptide for AD pathology from amyloid precursor protein (APP), can be detected in DS brains. A beta 42 is deposited in SP from an early stage, suggesting common molecular mechanisms in DS and AD. Animal models for DS are important in the search of molecular mechanisms. Several types of models are now available. Future DS studies are expected to integrate information from animal models and human tissues.

Wild type and mutant p53 differentially regulate the gene expression of human collagenase-3 (hMMP-13)

Matrix metalloproteinases (MMPs) are a family of secreted or transmembrane proteins that can degrade all the proteins of the extracellular matrix and have been implicated in many abnormal physiological conditions including arthritis and cancer metastasis. Recently we have shown for the first time that the human MMP-1 gene is a p53 target gene subject to repression by wild type p53 (Sun, Y., Sun, Y. I., Wenger, L., Rutter, J. L., Brinckerhoff, C. E., and Cheung, H. S. (1999) J. Biol. Chem. 274, 11535-11540). Here, we report that cotransfection of fibroblast-like synoviocytes with p53 expression and hMMP13CAT reporter plasmids revealed that (i) hMMP13, another member of the human MMP family, was down-regulated by wild type p53, whereas all six of the p53 mutants tested lost the wild type p53 repressor activity in fibroblast-like synoviocytes; (ii) this repression of hMMP-13 gene expression by wild type p53 could be reversed by overexpression of p53 mutants p53-143A, p53-248W, p53-273H, and p53-281G; (iii) the dominant effect of p53 mutants over wild type p53 appears to be a promoter- and mutant-specific effect. An intriguing finding was that p53 mutant p53-281G could conversely stimulate the promoter activity of hMMP13 up to 2-4-fold and that it was dominant over wild type p53. Northern analysis confirmed these findings. Although the significance of these findings is currently unknown, they suggest that in addition to the effect of cytokines activation, the gene expression of hMMP13 could be dysregulated during the disease progression of rheumatoid arthritis (or cancer) associated with p53 inactivation. Since hMMP13 is 5-10 times as active as hMMP1 in its ability to digest type II collagen, the dysregulation or up-modulation of MMP13 gene expression due to the inactivation of p53 may contribute to the joint degeneration in rheumatoid arthritis.

Physical interaction between Wilms tumor 1 and p73 proteins modulates their functions

The WT1 gene, which is heterozygously mutated or deleted in congenital anomaly syndromes and homozygously mutated in about 15% of all Wilms tumors, encodes tissue-specific developmental regulators. Through alternative mRNA splicing, four main WT1 protein isoforms are synthesized. All isoforms can bind to DNA via their zinc fingers, albeit with different affinities and specificities, and thereby modulate the transcriptional activity of their target genes. Several proteins bind to and alter the transcription regulatory properties of the WT1 proteins, including the product of the tumor suppressor gene p53. Interaction between WT1 and p53 was shown to modulate their ability to regulate the transcription of their respective target genes. Here, we report that all four isoforms of WT1 bind to p73, a recently cloned homologue of p53. p73 binds to the zinc finger region of WT1 and thereby inhibits DNA binding and transcription activation by WT1. Similarly, WT1 inhibits p73-induced transcription activation in reporter assays and counteracts p73-induced expression of endogenous Mdm2. This, taken together with our finding that WT1 also interacts with p63/KET, another p53 homologue, suggests that association between WT1 and the members of the p53 family of proteins may be an important determinant of their functions in cell growth and differentiation.

Ischemia-induced STAT-1 expression and activation play a critical role in cardiomyocyte apoptosis

We show here that exposure of cardiac cells to simulated ischemia results in apoptosis and is accompanied by phosphorylation and increased expression and transcriptional activity of STAT-1. Similarly, interferon-gamma, which is known to induce STAT-1 activation, also induced apoptosis in cardiac cells. STAT-1-transfected cells were more susceptible to ischemia-induced cell death than cells transfected with a control plasmid lacking the STAT-1 coding sequence. Furthermore, an antisense STAT-1 vector reduced both ischemia- and overexpressed STAT-1-induced cell death in cardiac cells. Both STAT-1 overexpression and interferon-gamma treatment or exposure to ischemia activated the promoter of the pro-apoptotic caspase-1 gene in cardiomyocytes. Finally, ischemia/reperfusion also induced STAT-1 activation and caspase-1 processing in ventricular myocytes in the intact heart ex vivo. Immunofluorescent staining demonstrated an increase in STAT-1-positive staining in cardiomyocytes in response to ischemia/reperfusion that co-localized with terminal deoxynucleotidyl transferase dVTP nick end-labeling-positive apoptotic cells. These results suggest that STAT-1 plays a critical role in the regulation of ischemia/reperfusion-induced apoptosis in cardiac cells, acting at least in part via a caspase-1 activation-dependent pathway.

Apoptosis, anoikis and their relevance to the pathobiology of colon cancer

The maintenance of a constant number of cells in an adult organism is a tightly regulated process. This is particularly important in organs where cells are in a constant rate of renewal during the entire lifespan. In these organs, cell number homeostasis is the direct consequence of a balance between cell proliferation and apoptosis. The colonic epithelium is an example of such a site and the high prevalence of colon cancer makes the understanding of cell number homeostasis more important to define. Normal colonic epithelium is organized in crypts where cell proliferation, migration, differentiation and apoptosis are topographically organized in a linear fashion along the crypt axis. Normal colonic crypts are composed of stem cells at the base, a proliferation and a differentiation zone in the lower third of the crypt, a migration zone in the upper two-thirds, and the surface epithelium where senescent cells are eliminated by apoptosis. Globally, apoptosis can be defined as a normal process of cell suicide, critical for development and tissue homeostasis. Colonic epithelial cells migrate from the base of the crypt to the surface epithelium in 6-7 days. The normal architecture of the crypt is maintained by a balance between cell proliferation at the base and apoptosis at the top of the crypt and surface epithelium.

Immunohistochemical profile of endometrial adenocarcinoma: a study of 61 cases and review of the literature

The differences in immunohistochemical expression of p53, bcl-2, bax, estrogen receptor (ER), and progesterone receptor (PR) were evaluated in 40 endometrioid and 21 papillary serous carcinomas of endometrium and correlated with known predictors of survival, such as grade and stage. Uterine papillary serous adenocarcinomas (UPSA) showed significantly higher p53 expression than did uterine endometrioid adenocarcinomas (UEA) (76.2% versus 35%), whereas both ER and PR were more often positive in endometrioid than in serous tumors (p = .005 and .0005). No significant difference was found in bcl-2 and bax expression between both histologic types. However, there was definite decrease in intensity of bcl-2 in UPSA compared with UEA. In endometrioid carcinoma, p53 overexpression was associated with high-grade and advanced-stage tumors (p = .0006 and .006), whereas ER and PR expression was associated with low-grade and early-stage tumors (p = .0006 and .0001; p = .003 and .0006). Bcl-2 immunopositivity was more common in low-grade, early-stage rather than in high-grade, advanced-stage adenocarcinomas, but the difference was not statistically significant (p = .24 and .07). Bax immunopositivity was associated with well-differentiated (p = .04) and early-stage tumors. Furthermore, a significant inverse relationship between bax and p53 reactivity was defined (p = .05), especially in tumors of endometrioid type. Bax and PR immunoexpression correlated near the limit of statistical significance (p = .08), whereas no relationship was found among bax, bcl-2, and ER immunopositivity. Our results indicate that the differences in immunohistochemical profiles of endometrioid and serous carcinomas support the existence of different molecular pathways of their development. The correlation of immunohistochemical findings with histologic grade and clinical stage could help in predicting biologic behavior and planning treatment in patients who are diagnosed as having these tumors.

Isolation and characterization of a novel TP53-inducible gene, TP53TG5, which suppresses growth and shows cell cycle-dependent transition of expression

Through a strategy of direct cloning of TP53-binding DNA sequences from human genome DNA, we have identified a novel TP53-target gene, termed TP53TG5 (TP53-target gene 5). This gene, localized to chromosome band 20q13.1 by fluorescence in situ hybridization, encodes a 290-amino-acid peptide with no significant homology with any known proteins in the public database. A colony-formation assay using human glioblastoma cell line T98G, which lacks wild-type TP53 and expresses no endogenous TP53TG5, revealed a growth-suppressive effect of the TP53TG5 gene product. Furthermore, immunohistochemical studies, following transfection of T98G with plasmid designed to express green fluorescent protein-fused TP53TG5, revealed cell cycle-dependent intracellular localization of this protein. Our results suggest that functional studies of TP53TG5 may provide new insights into the complex physiological activities of TP53.