Early growth response-1-dependent apoptosis is mediated by p53

The early growth response-1 (EGR-1) protein is an anti-proliferative signal for certain tumor cells and is required for apoptosis induced by stimuli that elevate intracellular Ca2+. We present evidence that EGR-1 transactivates the promoter of the p53 gene and up-regulates p53 RNA and protein levels. Inhibition of p53 function with dominant-negative p53 mutants abrogates EGR-1-dependent apoptosis. These findings establish a direct functional link between EGR-1 and the p53-mediated cell death pathway and suggest that mutant forms of p53 in tumor cells may provide resistance to the anti-proliferative effects of EGR-1.     

Suramin increases p53 protein levels but does not activate the p53-dependent G1 checkpoint

Dental somatosensory evoked potentials (SEPs) corresponding to the stimulus intensity levels were recorded at 6 different levels of intensity presented in a randomized order. The relationships between the amplitude of the late SEP component with latency between 150 and 300 msec and each stimulus intensity level were also compared in conditions of randomized intensity and constant intensity. The amplitude of the late component increased significantly with the increased stimulus intensity both in the randomized and constant intensity stimulation. The amplitude of the late component in the randomized stimulation with a 1-sec interstimulus interval (ISI) increased in the same manner as that in the constant intensity condition with a 1-sec ISI. The randomized stimulation with the prolonged ISI increased the amplitude of the late component. The latency of the late positive component significantly increased with the randomized stimulation with a 3-sec ISI. This phenomenon might be attributable to the psychological contamination. SEP recording in the randomized dental stimulation with a 1-sec ISI may have applications in neuropharmacological research or physiological research on pain and evaluation of the effects of analgesics, anesthetics, acupuncture and transcutaneous electrical nerve stimulation (TENS).     

p21WAF1/CIP1/SDI1 is elevated through a p53-independent pathway by mimosine

Tumor suppressor p53 is a nuclear protein that is induced by DNA damage and is involved in G1 and G2 phase control of the cell cycle. p21WAF1/CIP1/SDI1 (p21), a cyclin-dependent kinase inhibitor, is a downstream target and effector of p53 to induce G1 arrest. Mimosine is a potent reversible late G1 phase blocker of the cell cycle. In this study, we showed that mimosine can increase both p21 mRNA and protein levels, indirectly inhibit cyclin E-associated kinase activity without affecting the cyclin E protein level, block human breast cancer cells (21PT) in the late G1 phase of the cell cycle, and induce a p53-independent p21 pathway in these cells. These results support the possibility of restoring a G1 checkpoint by use of mimosine. They also suggest that the mechanism of the effect of mimosine is complex and may have more than one target in the cell.     

Involvement of p85 in p53-dependent apoptotic response to oxidative stress

Reactive oxygen species have damaging effects on cellular components and so trigger defensive responses by the cell and even programmed cell death, although the mechanisms by which mammalian cells transmit signals in response to oxidative damage are unknown. We report here that the protein p85, a regulator of the signalling protein phosphatidyl-3-OH kinase (PI(3)K), participates in the cell death process that is induced in response to oxidative stress and that this role of p85 in apoptosis does not involve PI(3)K. We show that disruption of p85 by homologous recombination impairs the cellular apoptotic response to oxidative stress. Because the protein p53 is required for cell death induced by oxidative damage, we examined the relation between p85 and p53. Using a chimaeric p53 fusion protein with the oestrogen receptor (p53ER) to supply p53 (p53 is induced upon binding of p53ER to oestradiol) in a p53-deficient cell line, we found that p85 is upregulated by p53 and that its involvement in p53-mediated apoptosis is independent of PI(3)K. We propose that p85 acts as a signal transducer in the cellular response to oxidative stress, mediating cell death regulated by p53.     

Human papillomavirus type 16 E6 and E7 oncogenes abrogate radiation-induced DNA damage responses in vivo through p53-dependent and p53-independent pathways

E6 and E7 oncoproteins from high risk human papillomaviruses (HPVs) transform cells in tissue culture and induce tumors in vivo. Both E6, which inhibits p53 functions, and E7, which inhibits pRb, can also abrogate growth arrest induced by DNA-damaging agents in cultured cells. In this study, we have used transgenic mice that express HPV-16 E6 or E7 in the epidermis to determine how these two proteins modulate DNA damage responses in vivo. Our results demonstrate that both E6 and E7 abrogate the inhibition of DNA synthesis in the epidermis after treatment with ionizing radiation. Increases in the levels of p53 and p21 proteins after irradiation were suppressed by E6 but not by E7. Through the study of p53-null mice, we found that radiation-induced growth arrest in the epidermis is mediated through both p53-dependent and p53-independent pathways. The abrogation of radiation responses in both E6 and E7 transgenic mice was more complete than was seen in the p53-null epidermis. We conclude that E6 and E7 each have the capacity to modulate p53-dependent as well as p53-independent cellular responses to radiation. Additionally, we found that the conserved region (CR) 1 and CR2 domains in E7 protein, which are involved in the inactivation of pRb function and required for E7's transforming function, were also required for E7 to modulate DNA damage responses in vivo. Thus pRb and/or pRb-like proteins likely mediate both p53-dependent and p53-independent responses to radiation.     

KARP-1 is induced by DNA damage in a p53- and ataxia telangiectasia mutated-dependent fashion

The KARP-1 (Ku86 Autoantigen Related Protein-1) gene, which is expressed from the human Ku86 autoantigen locus, appears to play a role in mammalian DNA double-strand break repair as a regulator of the DNA-dependent protein kinase complex. Here we demonstrate that KARP-1 gene expression is significantly up-regulated following exposure of cells to DNA damage. KARP-1 mRNA induction was completely dependent on the ataxia telangiectasia and p53 gene products, consistent with the presence of a p53 binding site within the second intron of the KARP-1 locus. These observations link ataxia telangiectasia, p53, and KARP-1 in a common pathway.     

G1 phase accumulation induced by UCN-01 is associated with dephosphorylation of Rb and CDK2 proteins as well as induction of CDK inhibitor p21/Cip1/WAF1/Sdi1 in p53-mutated human epidermoid carcinoma A431 cells

UCN-01 (7-hydroxyl-staurosporine) was originally isolated as a Ca2+- and phospholipid-dependent protein kinase C selective inhibitor and now is being developed as an anticancer agent. Results from our and other laboratories have suggested that UCN-01 induces preferential G1-phase accumulation in several human tumor cell lines tested. To elucidate this mechanism, we examined the effects of UCN-01 on several cell cycle-regulatory proteins critical for G1-S-phase transition in p53-mutated human epidermoid carcinoma A431 cells. After 24 h exposure at around 50% growth-inhibitory concentrations (IC50s), 260 and 520 nM, UCN-01 induced the accumulation of pRb (the dephosphorylated retinoblastoma protein form). The protein expression of cyclin A but not cyclin E was markedly reduced and that of cyclin D1 was partially reduced under the same condition. UCN-01 also showed the concentration-dependent inhibitions of the activity of cyclin-dependent kinase 2 (CDK2) using histone H1 and pRb as substrates in vitro (IC50, 530 and 640 nM, respectively). In addition, CDK2 activities of the cells pretreated with UCN-01 for 24 h at 260 and 520 nM were markedly inhibited, giving IC50s of far less than 260 nM. When the same cell lysates were analyzed by Western blotting for CDK2, the lower band (e.g., active and phosphorylated CDK2) was remarkably reduced, in accordance with the reduced activity. Furthermore, UCN-01 induced the expression of the CDK inhibitor p21 protein and its complex formation with CDK2 after 24 h exposure at 260 and 520 nM, whereas the expression level was very low or undetectable in untreated or DNA-damaged cells. The increase of p21 mRNA levels was also induced under the same condition. UCN-01 further increased luciferase activities in A431 cells transiently transfected with p21 promoter-luciferase reporter plasmid after 24 h exposure at 260 and 520 nM. UCN-01 also increased the expression of the CDK inhibitor p27 protein after 24 h exposure at 260 and 520 nM. These results suggest that G1-phase accumulation induced by UCN-01 is associated with dephosphorylation of Rb and CDK2 proteins as well as induction of CDK inhibitors p21 and p27.     

Monoallelically expressed gene related to p53 at 1p36, a region frequently deleted in neuroblastoma and other human cancers

We describe a gene encoding p73, a protein that shares considerable homology with the tumor suppressor p53. p73 maps to 1p36, a region frequently deleted in neuroblastoma and other tumors and thought to contain multiple tumor suppressor genes. Our analysis of neuroblastoma cell lines with 1p and p73 loss of heterozygosity failed to detect coding sequence mutations in remaining p73 alleles. However, the demonstration that p73 is monoallelically expressed supports the notion that it is a candidate gene in neuroblastoma. p73 also has the potential to activate p53 target genes and to interact with p53. We propose that the disregulation of p73 contributes to tumorigenesis and that p53-related proteins operate in a network of developmental and cell cycle controls.     

Hus1p, a conserved fission yeast checkpoint protein, interacts with Rad1p and is phosphorylated in response to DNA damage

The hus1+ gene is one of six fission yeast genes, termed the checkpoint rad genes, which are essential for both the S-M and DNA damage checkpoints. Classical genetics suggests that these genes are required for activation of the PI-3 kinase-related (PIK-R) protein, Rad3p. Using a dominant negative allele of hus1+, we have demonstrated a genetic interaction between hus1+ and another checkpoint rad gene, rad1+. Hus1p and Rad1p form a stable complex in wild-type fission yeast, and the formation of this complex is dependent on a third checkpoint rad gene, rad9+, suggesting that these three proteins may exist in a discrete complex in the absence of checkpoint activation. Hus1p is phosphorylated in response to DNA damage, and this requires rad3+ and each of the other checkpoint rad genes. Although there is no gene related to hus1+ in the Saccharomyces cerevisiae genome, we have identified closely related mouse and human genes, suggesting that aspects of the checkpoint control mechanism are conserved between fission yeast and higher eukaryotes.     

Heat shock induces transient p53-dependent cell cycle arrest at G1/S

The HER-2/neu proto-oncogene is frequently amplified or overexpressed in human breast and ovarian cancers, and is significantly correlated with shorter survival. We have previously reported that the adenovirus type 5 early region 1A (E1A) gene product can repress HER-2/neu overexpression by repressing HER-2/neu promoter activity, and suppress the tumorigenic potential of HER-2/neu-overexpressing ovarian cancer cells. To examine E1A tumor suppressor function in breast cancer, we transduced E1A in vitro by adenovirus into both HER-2/neu-overexpressing and low expressing human breast cancer cell lines. In HER-2/neu-overexpressing cells, E1A greatly inhibited tumor cell growth in vitro. However, in HER-2/neu low expressing cancer cell lines, E1A had no significant effect on cell growth in culture medium. To test the therapeutic efficacy of E1A, we used both adenovirus-mediated and cationic liposome-mediated E1A gene delivery systems in an orthotopic breast cancer animal model. An advanced breast cancer model was established by inoculation of HER-2/neu-overexpressing human breast cancer cells in mammary fat pad and treated by local injections of either replication-deficient adenovirus expressing E1A, Ad.E1A(+) or a liposome-E1A DNA complex. As controls, mice bearing tumors were also treated with Ad.E1A(-) which is virtually the same adenovirus as Ad.E1A(+) except that E1A is deleted, a liposome-E1A frame-shift mutant DNA complex, or just PBS. In mice bearing a HER-2/neu-overexpressing breast cancer cell line, E1A delivered either by adenovirus or liposome significantly inhibited tumor growth and prolonged mouse survival compared with the controls. In fact, 60-80% of E1A-treated mice lived longer than 2 years versus only 0-20% of control mice (P<0.05). Western blot analysis showed that E1A protein was expressed in tumor tissue and immunohistochemical analysis showed that HER-2/neu p185 protein expression was suppressed. Taken together, our results indicated that both adenovirus and cationic liposome delivery systems were effective in transfering E1A gene for tumor suppression in a HER-2/neu-overexpressing breast cancer model.