Evidence that wild-type TP53, and not genes on either chromosome 1 or 11, controls the tumorigenic phenotype of the human fibrosarcoma HT1080

Homologous recombination promotes non-immunogenic mitotic cell death upon DNA damage

Double-strand breaks (DSBs) can initiate mitotic catastrophe, a complex oncosuppressive phenomenon characterized by cell death during or after cell division. Here we unveil how cell cycle-regulated DSB repair guides disparate cell death outcomes through single-cell analysis of extended live imaging. Following DSB induction in S or G2, passage of unresolved homologous recombination intermediates into mitosis promotes non-immunogenic intrinsic apoptosis in the immediate attempt at cell division. Conversely, non-homologous end joining, microhomology-mediated end joining and single-strand annealing cooperate to enable damaged G1 cells to complete the first cell cycle with an aberrant cell division at the cost of delayed extrinsic lethality and interferon production. Targeting non-homologous end joining, microhomology-mediated end joining or single-strand annealing promotes mitotic death, while suppressing mitotic death enhances interferon production. Together the data indicate that a temporal repair hierarchy, coupled with cumulative DSB load, serves as a reliable predictor of mitotic catastrophe outcomes following genome damage. In this pathway, homologous recombination suppresses interferon production by promoting mitotic lethality.

TP53 mutations predict poor response to immunotherapy in patients with metastatic solid tumors

BACKGROUND

TP53 is the most commonly mutated gene across all cancer types. R175H mutation was considered structural mutation where the mutation causes misfolding of the protein and leads to a significant conformational alterations within p53's DNA binding domain. The aim of this study was to explain the reason why R175H worse the response to immunotherapy by analyzing tumor immune microenvironment through the expression of immune cells and PD-1.

MATERIALS AND METHODS

Patients diagnosed with metastatic carcinoma, including colorectal cancer (CRC), breast cancer (BRCA), gastric cancer (GC), non-small cell lung cancer (NSCLC), and 20 other cancer types, treated in a palliative setting at Samsung Medical Center between October 2019 and April 2021, were enrolled. Of these patients, those who underwent TDS analysis (TruSight™ Oncology 500 assay [TSO 500]) were finally analyzed.

RESULTS

Of 1770 patients, 1012 (57.2%) harbored genetic alterations in TP53. All mutations were single nucleotide variants (SNVs), and the most frequent SNV was R175H (n = 84, 7.5%) which was known as one of the most common hotspot TP53 mutation. The overall survival of patients with TP53 R175H mutations was significantly worse following chemotherapy (606 vs. 456 days, p < 0.001) or immunotherapy (822 vs. 350 days, p  < 0.001) compared to those with TP53 mutation in other loci. RNA sequencing indicated that the immune response-related pathways were downregulated in tumors harboring TP53 R175H mutation. Moreover, the expression of CD8(+) T cells PD-1 were lowered in R175H mutation tumors. In the analysis of TP53 structural domain, compared to those having TP53 mutation in other domain, patients with mutations occurring in the nuclear exporter signal (NES) and E4F1-binding domains had significantly worse overall survival following chemotherapy (NES: 606 vs. 451 days, p = 0.043; E4F1: 606 vs. 469 days, p = 0.046) and immunotherapy (NES: 822 vs. 403 days, p  < 0.001; E4F1: 822 vs. 413 days, p  < 0.001). In addition, tumors with TP53 mutation and co-existing copy number amplification of CCND1, FGF4, and FGF19 in chromosome 11 conferred worse prognosis than those with only TP53 mutation (p  < 0.050).

DISCUSSION

Each TP53 mutations indicated differential treatment outcomes following chemotherapy or immunotherapy in patients with metastatic cancer. Functional analysis including RNASeq suggested that TP53 mutation downregulated immune response.

CONCLUSION

Overall, we found each TP53 mutation to indicate different prognoses in patients with metastatic tumors undergoing chemotherapy and ICI treatment. Further validations, including a prospective cohort study or a functional study, would be particularly valuable in advancing the knowledge on this aspect and developing improved prognostic parameters.

Newcastle disease virus degrades HIF-1α through proteasomal pathways independent of VHL and p53

Newcastle disease virus (NDV) is a candidate agent for oncolytic virotherapy. Despite its potential, the exact mechanism of its oncolysis is still not known. Recently, we reported that NDV exhibited an increased oncolytic activity in hypoxic cancer cells. These types of cells negatively affect therapeutic outcome by overexpressing pro-survival genes under the control of the hypoxia-inducible factor (HIF). HIF-1 is a heterodimeric transcriptional factor consisting of a regulated α (HIF-1α) and a constitutive β subunit (HIF-1β). To investigate the effects of NDV infection on HIF-1α in cancer cells, the osteosarcoma (Saos-2), breast carcinoma (MCF-7), colon carcinoma (HCT116) and fibrosarcoma (HT1080) cell lines were used in the present study. Data obtained showed that a velogenic NDV infection diminished hypoxia-induced HIF-1α accumulation, leading to a decreased activation of its downstream target gene, carbonic anhydrase 9. This NDV-induced downregulation of HIF-1α occurred post-translationally and was partially abrogated by proteasomal inhibition. The process appeared to be independent of the tumour suppressor protein p53. These data revealed a correlation between NDV infection and HIF-1α downregulation, which highlights NDV as a promising agent to eliminate hypoxic cancer cells.

Variant repeats are interspersed throughout the telomeres and recruit nuclear receptors in ALT cells

Variant repeats interspersed throughout ALT telomeres recruit nuclear receptors, leading to the destabilized telomere architecture and enhanced telomeric recombination.

Telomeres in cells that use the recombination-mediated alternative lengthening of telomeres (ALT) pathway elicit a DNA damage response that is partly independent of telomere length. We therefore investigated whether ALT telomeres contain structural abnormalities that contribute to ALT activity. Here we used next generation sequencing to analyze the DNA content of ALT telomeres. We discovered that variant repeats were interspersed throughout the telomeres of ALT cells. We found that the C-type (TCAGGG) variant repeat predominated and created a high-affinity binding site for the nuclear receptors COUP-TF2 and TR4. Nuclear receptors were directly recruited to telomeres and ALT-associated characteristics were induced after incorporation of the C-type variant repeat by a mutant telomerase. We propose that the presence of variant repeats throughout ALT telomeres results from recombination-mediated telomere replication and spreading of variant repeats from the proximal regions of the telomeres and that the consequent binding of nuclear receptors alters the architecture of telomeres to facilitate further recombination.

Spontaneous occurrence of telomeric DNA damage response in the absence of chromosome fusions

Telomere dysfunction is typically studied under conditions in which a component of the six-subunit shelterin complex that protects chromosome ends is disrupted. The nature of spontaneous telomere dysfunction is less well understood. Here we report that immortalized human cell lines lacking wild-type p53 function spontaneously show many telomeres with a DNA damage response (DDR), commonly affecting only one sister chromatid and not associated with increased chromosome end-joining. DDR(+) telomeres represent an intermediate configuration between the fully capped and uncapped (fusogenic) states. In telomerase activity-positive (TA(+)) cells, DDR is associated with low TA and short telomeres. In cells using the alternative lengthening of telomeres mechanism (ALT(+)), DDR is partly independent of telomere length, mostly affects leading strand-replicated telomeres, and can be partly suppressed by TRF2 overexpression. In ALT(+) (but not TA(+)) cells, DDR(+) telomeres preferentially associate with large foci of extrachromosomal telomeric DNA and recombination proteins. DDR(+) telomeres therefore arise through different mechanisms in TA(+) and ALT(+) cells and have different consequences.

Characterization of NVX-207, a novel betulinic acid-derived anti-cancer compound

BACKGROUND

Development of betulinic acid derivatives for clinical use has been hampered by adverse pharmacological and physico-chemical characteristics of this class of compounds. We here present a novel semi-synthetic betulinic acid-derived drug candidate well suited for further clinical development.

MATERIALS AND METHODS

In vitro activity and mode of action of NVX-207 were determined using normal as well as cancer cell lines. Gene expression profiling was performed with Affymetrix U133 microarrays. NVX-207 binding partners were identified using a heterobifunctional chemical crosslinker system. Potential binding proteins were identified by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) analysis. Clinical studies were conducted in canine cancer patients suffering from spontaneously arising pre-treated tumours.

RESULTS

NVX-207 showed anti-tumour activity (mean IC(50) = 3.5 microM) against various human and canine cell lines. NVX-207-induced apoptosis was associated with activation of the intrinsic apoptotic pathway via cleavage of caspases -9, -3, -7 and of poly (ADP-ribose) polymerase (PARP). Global gene expression profiling demonstrated regulation of genes associated with lipid metabolism, most notably an upregulation of genes coding for insulin-induced gene 1 (Insig-1), low-density lipoprotein receptor (LDL-R) and of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA). NVX-207 bound to apolipoprotein A-I, a major regulator of lipid metabolism and cholesterol transport. A phase I/II study in dogs suffering from naturally occurring cancer receiving local treatment of NVX-207 (10 mg mL(-1)) showed excellent clinical responses including a complete remission in so far 5/5 treated animals.

CONCLUSIONS

NVX-207 is well tolerated and has significant anti-cancer activity in vitro and in vivo in dogs with treatment-resistant malignancies.

Activated MEK suppresses activation of PKR and enables efficient replication and in vivo oncolysis by Deltagamma(1)34.5 mutants of herpes simplex virus 1

Herpes simplex virus mutants lacking the gamma(1)34.5 gene are not destructive to normal tissues but are potent cytolytic agents in human tumor cells in which the activation of double-stranded RNA-dependent protein kinase (PKR) is suppressed. Thus, replication of a Deltagamma(1)34.5 mutant (R3616) in 12 genetically defined cancer cell lines correlates with suppression of PKR but not with the genotype of RAS. Extensive analyses of two cell lines transduced with either dominant negative MEK (dnMEK) or constitutively active MEK (caMEK) indicated that in R3616 mutant-infected cells dnMEK enabled PKR activation and decreased virus yields, whereas caMEK suppressed PKR and enabled better viral replication and cell destruction in transduced cells in vitro or in mouse xenografts. The results indicate that activated MEK mediates the suppression of PKR and that the status of MEK predicts the ability of Deltagamma(1)34.5 mutant viruses to replicate in and destroy tumor cells.

Retrotransposition-Competent Human LINE-1 Induces Apoptosis in Cancer Cells With Intact p53

Retrotransposition of human LINE-1 (L1) element, a major representative non-LTR retrotransposon in the human genome, is known to be a source of insertional mutagenesis. However, nothing is known about effects of L1 retrotransposition on cell growth and differentiation. To investigate the potential for such biological effects and the impact that human L1 retrotransposition has upon cancer cell growth, we examined a panel of human L1 transformed cell lines following a complete retrotransposition process. The results demonstrated that transposition of L1 leads to the activation of the p53-mediated apoptotic pathway in human cancer cells that possess a wild-type p53. In addition, we found that inactivation of p53 in cells, where L1 was undergoing retrotransposition, inhibited the induction of apoptosis. This suggests an association between active retrotransposition and a competent p53 response in which induction of apoptosis is a major outcome. These data are consistent with a model in which human retrotransposition is sensed by the cell as a "genetic damaging event" and that massive retrotransposition triggers signaling pathways resulting in apoptosis.

Regulation of p53 expression by the RAS-MAP kinase pathway

Activation of MAP kinase leads to the activation of p53-dependent pathways, and vice-versa. Although the amount of p53 protein increases in response to MAP kinase-dependent signaling, the basis of this increase is not yet fully understood. We have isolated the mutant cell line AP14, defective in p53 expression, from human HT1080 fibrosarcoma cells, which have an activated ras allele. The expression of p53 mRNA and protein is approximately 10-fold lower in AP14 cells than in the parental cells. The high constitutive phosphorylation and activities of the MAP kinases ERK1 and ERK2 in HT1080 cells are greatly reduced in AP14 cells, although the levels of these proteins are unchanged, suggesting that the defect in the mutant cells affects the steady-state phosphorylation of ERK1 and ERK2. Overexpression of ERK2 in AP14 cells restored both MAP kinase activity and p53 expression, and incubation of the mutant cells with the phosphatase inhibitor orthovanadate resulted in strong coordinate elevation of MAP kinase activity and p53 expression. The levels of expression of the p53-regulated gene p21 parallel those of p53 throughout, showing that basal p21 expression depends on p53. The levels of p53 mRNA increased by 5-8-fold when activated ras was introduced into wild-type cells, and the levels of the p53 and p21 proteins decreased substantially in wild-type cells treated with the MEK inhibitor U0216. We conclude that MAP kinase-dependent pathways help to regulate p53 levels by regulating the expression of p53 mRNA.

A novel host cell reactivation assay to assess homologous recombination capacity in human cancer cell lines

Repair of DNA double-strand breaks (DSB) is essential for cell viability and genome stability. Homologous recombination repair plays an important role in DSB repair and impairment of this repair mechanism may lead to loss of genomic integrity, which is one of the hallmarks of cancer. Recent research has shown that the tumor suppressor genes p53 and BRCA1 and -2 are involved in the proper control of homologous recombination, suggesting a role of this type of repair in human cancer. We developed a novel assay based on recombination between two Green Fluorescent Protein (GFP) sequences in transiently transfected plasmid DNA. The plasmid construct contains an intact, emission-shifted, "blue" variant of GFP (BFP), with a 300 nucleotide stretch of homology to a nonfunctional copy of GFP. In the absence of homologous recombination only BFP is present, but homologous recombination can create a functional GFP. The homologous regions in the plasmid were constructed in both the direct and the inverted orientation of transcription to detect possible differences in the recombination mechanisms involved. A panel of human tumor cell lines was chosen on the basis of genetic background and chromosome integrity and tested for homologous recombination using this assay. The panel included cell lines with varying levels of karyotypic abnormalities, isogenic cell lines with normal and mutant p53, isogenic cell lines with or without DNA mismatch repair, BRCA1 and -2 mutant cell lines, and the lymphoma cell line DT40. With this assay, the observed differences between cell lines with the lowest and highest levels of recombination were about 100-fold. Increased levels of recombination were associated with mutant p53, whereas a low level of recombination was found in the BRCA1 mutant cell line. In the cell line HT1080TG, a mutagenized derivative of HT1080 with two mutant alleles of p53, high levels of recombination were found with the direct orientation but not with the inverted orientation plasmid. No difference in recombination was detected between two isogenic cell lines that only differed in DNA mismatch repair capability. We conclude that this assay can detect differences in homologous recombination capacity in cultured cell lines and that these differences follow the patterns that would be expected from the different genotypes of these cell lines. Future application in normal cells may be useful to identify genetic determinants controlling genomic integrity or to detect differences in DNA repair capacity in individuals.

p53-mediated negative regulation of stathmin/Op18 expression is associated with G(2)/M cell-cycle arrest

Utilizing the technique of differential display of mRNA, we have identified p53-responsive genes that are transcriptionally up- or down-regulated as cells enter growth arrest. One gene that was down-regulated, pong16, was found to be identical to stathmin/Op18, a protein involved in the regulation of microtubule dynamics. Evidence that p53 is directly or indirectly involved in negative regulation of stathmin/Op18 expression includes the following: (i) p53-mediated growth inhibition is associated with repression of stathmin/Op18 expression following serum stimulation, (ii) reporter gene assays revealed p53-mediated repression of stathmin/Op18 promoter activity and (iii) constitutive over-expression of stathmin/Op18 bypasses a p53-mediated G(2)/M arrest in the cell cycle. These results suggest that p53-mediated negative regulation of stathmin/Op18 plays an important role in cell-cycle control.

A functional investigation of tumor suppressor gene activities in a nasopharyngeal carcinoma cell line HONE1 using a monochromosome transfer approach

Monochromosome transfers of selected chromosomes into a nasopharyngeal carcinoma (NPC) cell line were performed to determine if tumor suppressing activity for NPC mapped to chromosomes 9, 11, and 17. Current information from cytogenetic and molecular allelotyping studies indicate that these chromosomes may harbor potential tumor suppressor genes vital to NPC. The present results show the importance of CDKN2A on chromosome 9 in NPC development. There was no functional suppression of tumor development in nude mice with microcell hybrids harboring the newly transferred chromosome 9 containing an interstitial deletion at 9p21, whereas transfection of CDKN2A into the NPC HONE1 cells resulted in obvious growth suppression. Whereas intact chromosome 17 transfers into HONE1 cells showed no functional suppression of tumor formation, chromosome 11 was able to do so. Molecular analysis of chromosome 11 tumor segregants indicated that at least two tumor suppressive regions mapping to 11q13 and 11q22-23 may be critical for the development of NPC.

A new functional classification of tumor-suppressing genes and its therapeutic implications

Cell fusion studies have demonstrated that malignancy can be suppressed by a single dose of malignancy suppressor genes (MSGs), indicating that malignancy is a recessive phenotype. Correspondingly, it is widely believed that mutational inactivation of both alleles of tumor suppressor genes (TSGs), in familial and sporadic tumors, is the formal proof of the recessive nature of malignancy. Evidence presented here, however, shows that unlike MSGs, identified solely through cell fusion studies with no gene of this class yet cloned, many well-known TSGs have gene dosage effects and inhibit cellular growth in vitro. Moreover, homozygous inactivation of a growth-inhibitory TSG (GITSG) is not directly correlated with malignancy. An alternative interpretation is provided for the loss of wild-type alleles of these genes in the tumors. It is concluded that the MSGs and the GITSGs do not belong to the same class of genes. The functional classification of tumor-suppressing genes has important implications for developing effective cancer therapies.

Lentiviral delivery of HIV-1 Vpr protein induces apoptosis in transformed cells

Most current anticancer therapies act by inducing tumor cell stasis followed by apoptosis. HIV-1 Vpr effectively induces apoptosis of T cells after arrest of cells at a G(2)/M checkpoint. Here, we investigated whether this property of Vpr could be exploited for use as a potential anticancer agent. As a potentially safer alternative to transfer of genes encoding Vpr, we developed a method to efficiently introduce Vpr protein directly into cells. Vpr packaged into HIV-1 virions lacking a genome induced efficient cell cycle arrest and apoptosis. Introduction of Vpr into tumor cell lines of various tissue origin, including those bearing predisposing mutations in p53, XPA, and hMLH1, induced cell cycle arrest and apoptosis with high efficiency. Significantly, apoptosis mediated by virion-associated Vpr was more effective on rapidly dividing cells compared with slow-growing cells, thus, in concept, providing a potential differential effect between some types of tumor cells and surrounding normal cells. This model system provides a rationale and proof of concept for the development of potential cancer therapeutic agents based on the growth-arresting and apoptotic properties of Vpr.

Differential contribution of the ERK and JNK mitogen-activated protein kinase cascades to Ras transformation of HT1080 fibrosarcoma and DLD-1 colon carcinoma cells

Although an important contribution of ERK and JNK mitogen-activated protein kinase (MAPK) activation in Ras transformation of rodent fibroblasts has been determined, their role in mediating oncogenic Ras transformation of human tumor cells remains to be established. We have utilized the human HT1080 fibrosarcoma and DLD-1 colon carcinoma cell lines, which contain endogenous mutated and oncogenic N- and K-ras alleles, respectively, to address this role. Study of these cells is advantageous over Ras-transformed rodent model cell systems for two key reasons. First, the ras mutations occurred naturally in the progression of the tumors from which the cell lines were derived, rather than due to overexpression of an exogenously introduced gene. Second, although these tumor cells possess defects in multiple genetic loci, it has been established that mutated Ras contributes significantly to the transformed phenotype of these cells. Clonal variant lines of HT1080 and DLD-1 have been isolated which have lost the oncogenic ras allele and exhibit a corresponding impairment in growth transformation in vitro and in vivo. We found that upregulation of Raf/MEK/ERK and JNK correlated with expression of oncogenic Ras in HT1080, but not DLD-1 cells. Furthermore, inhibition of ERK activation in parental HT1080 cells caused the same changes in cell morphology and actin stress fiber organization seen with loss of expression of activated N-Ras(61K). Thus, we suggest that constitutive activation of the Raf/MEK/ERK and JNK pathways is necessary for Ras-induced transformation of HT1080 but not DLD-1 cells. These results emphasize that cell type differences exist in the signaling pathways by which oncogenic Ras causes transformation.

p53-dependent cell death/apoptosis is required for a productive adenovirus infection

The p53 tumor suppressor protein binds to both cellular and viral proteins, which influence its biological activity. One such protein is the large E1b tumor antigen (E1b58kDa) from adenoviruses (Ads), which abrogates the ability of p53 to transactivate various promoters. This inactivation of p53 function is believed to be the mechanism by which E1b58kDa contributes to the cell transformation process. Although the p53-E1b58kDa complex occurs during infection and is conserved among different serotypes, there are limited data demonstrating that it has a role in virus replication. However, loss of p53 expression occurs after adenovirus infection of human cells and an E1b58kDa deletion mutant (Onyx-015, also called dl 1520) selectively replicates in p53-defective cells. These (and other) data indicate a plausible hypothesis is that loss of p53 function may be conducive to efficient adenovirus replication. However, wild-type (wt) Ad5 grows more efficiently in cells expressing a wt p53 protein. These studies indicate that the hypothesis may be an oversimplification. Here, we show that cells expressing wt p53, as well as p53-defective cells, allow adenovirus replication, but only cells expressing wt p53 show evidence of virus-induced cytopathic effect. This correlates with the ability of adenovirus to induce cell death. Our data indicate that p53 plays a necessary part in mediating cellular destruction to allow a productive adenovirus infection. In contrast, p53-deficient cells are less sensitive to the cytolytic effects of adenovirus and as such raise questions about the use of E1b58kDa-deficient adenoviruses in tumor therapy.

Adenovirus type 12-induced fragility of the human RNU2 locus requires p53 function

Adenovirus type 12 (Ad12) infection of human cells induces four chromosomal fragile sites corresponding to the U1 small nuclear RNA (snRNA) genes (the RNU1 locus), the U2 snRNA genes (RNU2), the U1 snRNA pseudogenes (PSU1), and the 5S rRNA genes (RN5S). Ad12-induced fragility of the RNU2 locus requires U2 snRNA transcriptional regulatory elements and viral early functions but not viral replication or integration, or chromosomal sequences flanking the RNU2 locus. We now show that Ad12 cannot induce the RNU1, RNU2, or PSU1 fragile sites in Saos-2 cells lacking the p53 and retinoblastoma (Rb) proteins but that viral induction of fragility is rescued in these cells when the expression of wild-type p53 or selected hot-spot mutants (i.e., V143A, R175H, R248W, and R273H) is restored by transient expression or stable retroviral transduction. We also observed weak constitutive fragility of the RNU1 and RNU2 loci in cells belonging to xeroderma pigmentosum complementation groups B and D (XPB and XPD) which are partially defective in the ERCC2 (XPD) and ERCC3 (XPB) helicase activities shared between the repairosome and the RNA polymerase H basal transcription factor TFIIH. We propose a model for Ad12-induced chromosome fragility in which interaction of p53 with the Ad12 E1B 55-kDa transforming protein (and possibly E4orf6) induces a p53 gain of function which ultimately perturbs the RNA polymerase II basal transcription apparatus. The p53 gain of function could interfere with chromatin condensation either by blocking mitotic shutdown of U1 and U2 snRNA transcription or by phenocopying global or local DNA damage. Specific fragilization of the RNU1, RNU2, and PSU1 loci could reflect the unusually high local concentration of strong transcription units or the specialized nature of the U1 and U2 snRNA transcription apparatus.

Tumor-suppressive activity of the growth arrest-specific gene GAS1 in human tumor cell lines

The GAS1 gene product induces growth arrest through a p53-dependent mechanism. To investigate whether GAS1 is a tumor suppressor gene, we transfected GAS1-negative human tumor cells with GAS1 plasmids and analyzed growth characteristics of stable transfectants. When a constitutively expressing GAS1 plasmid was transfected into A549 cells, no stable colonies expressing GAS1 were isolated. When A549 cells were transfected with a dexamethasone-inducible GAS1 plasmid, expression of GAS1 inhibited growth in vitro, and fewer slow-growing tumors arose in nude mice. GAS1 also inhibited proliferation of an HT1080 subline with wild-type (wt) p53 and normal MDM2. However, when the HT1080 subline HTD114 was transfected with the constitutive GAS1 plasmid, there was no reduction in colony number. GAS1-transfectant clones had unaltered growth in vitro, were morphologically unchanged and showed no difference in their ability to form tumors in nude mice. Although HTD114 cells contain wt p53, levels of MDM2 were elevated by 10-15 fold. The HT1080-6TGc5 subline with mutant p53 and normal MDM2 was also refractory to GAS1. Our results show that GAS1 suppresses the growth and tumorigenicity of human tumor cells and overexpression of MDM2 or p53 mutation inhibits the GAS1-mediated growth-suppressing pathway.

DNA damage and p53-mediated cell cycle arrest: a reevaluation

Most mammalian cells exhibit transient delays in the G1 and G2 phases of the cell cycle after treatment with radiation or radiomimetic compounds. p53 is required for the arrest in G1, which provides time for DNA repair. Recently, a role of p53 in the G2/M transition has also been suggested. However, it has been reported that the presence of functional p53 does not always correlate with the induction of these checkpoints. To precisely assess the role of p53 in activating cell cycle checkpoints and in cell survival after radiation, we studied the response of two isogenic human fibrosarcoma cell lines differing in their p53 status (wild type or mutant). We found that when irradiated cells undergo a wild-type p53-dependent G1 arrest, they do not subsequently arrest in G2. Moreover, wild-type p53 cells irradiated past the G1 checkpoint arrest in G2 but do not delay in the subsequent G1 phase. Furthermore, in these cell lines, which do not undergo radiation-induced apoptosis, the wild-type p53 cell line exhibited a greater radioresistance in terms of clonogenic survival. These results suggest that the two checkpoints may be interrelated, perhaps through a control system that determines, depending on the extent of the damage, whether the cell needs to arrest cell cycle progression at the subsequent checkpoint for further repair. p53 could be a crucial component of this control system.

Tumor suppressor p53 gene forms multiple isoforms: evidence for single locus origin and cytoplasmic complex formation with heat shock proteins

The tumor suppressor protein p53 is a major cell cycle control factor, and mutations in p53 are the most common genetic lesion found in human tumors, resulting in loss of function and contributing to malignant transformation. This report reviews several studies which show that p53 protein appears as at least eleven isoforms having the same amino acid backbone but varying in charge by level of phosphorylation. All isoforms are derived from a single locus, which indicates that p53 activity is modulated by post-translational modification. In addition, mutant p53 forms hetero-oligomers with two families of proteins: HSP70 and a 90 kDa group similar to HSP90. Cytoplasmic complexes are most likely formed to protect p53 from proteolysis and are probably involved in translocation of activated p53 from the cytoplasm to the nucleus for transactivation of other cell cycle control genes.

HSP binding and mitochondrial localization of p53 protein in human HT1080 and mouse C3H10T1/2 cell lines

In normal cells, the tumor suppressor actions of p53 protein are mediated by specific DNA binding and protein-protein interactions within the nucleus. Mutant p53 proteins, however, often assume an aberrant conformation devoid of tumor suppressor activity and newly capable of binding to the cognate or inducible HSP70. Recent reports from our laboratory and others show that additional unknown proteins may also complex with mutant p53. In this study, we characterize p53:HSP complexes and their subcellular location in the transformed cell lines, human HT1080 and murine C3H10T1/2, which both contain aberrant p53 conformers. Immunoprecipitation and SDS-PAGE of p53 from whole cell lysates revealed the additional presence of a broad 70 kDa band and a 90 kDa band in both lines, while p53 isolated from nuclear lysates was free from other proteins. 2D-PAGE was used to isolate and identify HSP members from cytoplasmic and nuclear lysates by immunoprecipitation, Western blotting and protein sequencing. Anti-p53 immune complexes from cytoplasmic lysates contained not only HSC70 but also GRP75, GRP78 and a weakly basic 90 kDa protein, which may be related to HSP90. The inducible form of HSP70 was not complexed to p53 protein, even though expressed in these cells. Analysis of anti-HSP70, anti-GRP75 and anti-HSP90 immune complexes suggests that HSP members exist as performed complexes in the cytoplasm, but not the nucleus. The presence of the mitochondrial and endoplasmic reticular chaperones, GRP75 and GRP78, in p53:HSP complexes suggested that p53 might be found in these cytoplasmic organelles which was confirmed in mitochondria by biochemical and immunoelectron microscopic evidence. These studies suggest that newly identified members of p53:HSP complexes represent components of a chaperone program which affects the subcellular distribution of p53 protein in these transformed lines.

Loss of oncogenic ras expression does not correlate with loss of tumorigenicity in human cells

ras oncogenes are mutated in at variety of human tumors, which suggests that they play an important role in human carcinogenesis. To determine whether continued oncogenic ras expression is necessary to maintain the malignant phenotype, we studied the human fibrosarcoma cell line, HT1080, which contains one mutated and one wild-type N-ras allele. We isolated a variant of this cell line that no longer contained the mutated copy of the N-ras gene. Loss of mutant N-ras resulted in cells that displayed a less transformed phenotype characterized by a flat morphology, decreased growth rate, organized actin stress fibers, and loss of anchorage-independent growth. The transformed phenotype was restored following reintroduction of mutant N-ras. Although loss of the oncogenic N-ras drastically affected in vitro growth parameters, the variant remained tumorigenic in nude mice indicating that mutated N-ras expression is not necessary for maintenance of the tumorigenic phenotype. We confirmed this latter observation in colon carcinoma cell lines that have lost activated K-ras expression via targeted knockout of the mutant K-ras gene.

Characterisation of a human chromosome 1 somatic cell hybrid mapping panel and regional assignment of 6 novel STS

A somatic cell hybrid mapping panel has been constructed which allows subdivision of human chromosome 1 into 8 distinct subregions. All of the hybrids carry copies of chromosome 1 with specific deletions and the position of the breakpoints has been determined relative to the location of microsatellite markers in the genetic linkage map produced by Genethon. The majority of the breakpoints can be positioned between adjacent loci on the map. The usefulness of this hybrid panel for physical mapping has been demonstrated by the regional assignment of 6 novel STS markers made from Alu-PCR clones generated from a hybrid which contains the short arm of chromosome 1.

Monochromosome transfers to Syrian hamster BHK cells via microcell fusion provide functional evidence for suppressor genes on human chromosome 9 both for anchorage independence and for tumorigenicity

We previously identified an anchorage independence-suppressor gene, SAII, on rat chromosome (RNO) 5. RNO5 is homologous to human chromosomes (HSA) 1 and 9. In order to find the human homolog of the SAII gene, we transferred HSA1 and HSA9 to an anchorage-independent and tumorigenic Syrian hamster BHK 191-5C cell line by microcell fusion. For HSA9, we used a t(X;9)-derivative chromosome to force the retention of this chromosome in hybrids by hypoxanthine-aminopterin-thymidine (HAT) selection. To study the possible effect of the X portion of the der(9)t(X;9), we also transferred a normal X to 191-5C cells. For HSA1, a neo-tagged chromosome was introduced. Following the transfer of der(9)t(X;9) to 191-5C cells, the hybrid cells became anchorage dependent and nontumorigenic, and, upon the loss of this chromosome, the cells regained their tumorigenic and anchorage-independent phenotypes. The transfer of HSAX or HSA1, on the other hand, affected neither of these phenotypes. These results provide functional proof of suppressor genes on HSA9 involving both anchorage independence and tumorigenicity. In addition, our data suggest the presence of another gene on HSA9 that causes a negative growth effect and whose phenotypic expression, contrary to the suppressor genes, is dosage dependent.

DNA damage triggers a prolonged p53-dependent G1 arrest and long-term induction of Cip1 in normal human fibroblasts

The tumor suppressor p53 is a cell cycle checkpoint protein that contributes to the preservation of genetic stability by mediating either a G1 arrest or apoptosis in response to DNA damage. Recent reports suggest that p53 causes growth arrest through transcriptional activation of the cyclin-dependent kinase (Cdk)-inhibitor Cip1. Here, we characterize the p53-dependent G1 arrest in several normal human diploid fibroblast (NDF) strains and p53-deficient cell lines treated with 0.1-6 Gy gamma radiation. DNA damage and cell cycle progression analyses showed that NDF entered a prolonged arrest state resembling senescence, even at low doses of radiation. This contrasts with the view that p53 ensures genetic stability by inducing a transient arrest to enable repair of DNA damage, as reported for some myeloid leukemia lines. Gamma radiation administered in early to mid-, but not late, G1 induced the arrest, suggesting that the p53 checkpoint is only active in G1 until cells commit to enter S phase at the G1 restriction point. A log-linear plot of the fraction of irradiated G0 cells able to enter S phase as a function of dose is consistent with single-hit kinetics. Cytogenetic analyses combined with radiation dosage data indicate that only one or a small number of unrepaired DNA breaks may be sufficient to cause arrest. The arrest also correlated with long-term elevations of p53 protein, Cip1 mRNA, and Cip1 protein. We propose that p53 helps maintain genetic stability in NDF by mediating a permanent cell cycle arrest through long-term induction of Cip1 when low amounts of unrepaired DNA damage are present in G1 before the restriction point.