Decreased cytotoxic effects of doxorubicin in a human ovarian cancer-cell line expressing wild-type p53 and WAF1/CIP1 genes

Model-based understanding of single-cell CRISPR screening

The recently developed single-cell CRISPR screening techniques, independently termed Perturb-Seq, CRISP-seq, or CROP-seq, combine pooled CRISPR screening with single-cell RNA-seq to investigate functional CRISPR screening in a single-cell granularity. Here, we present MUSIC, an integrated pipeline for model-based understanding of single-cell CRISPR screening data. Comprehensive tests applied to all the publicly available data revealed that MUSIC accurately quantifies and prioritizes the individual gene perturbation effect on cell phenotypes with tolerance for the substantial noise that exists in such data analysis. MUSIC facilitates the single-cell CRISPR screening from three perspectives, i.e., prioritizing the gene perturbation effect as an overall perturbation effect, in a functional topic-specific way, and quantifying the relationships between different perturbations. In summary, MUSIC provides an effective and applicable solution to elucidate perturbation function and biologic circuits by a model-based quantitative analysis of single-cell-based CRISPR screening data.

New insights into the kinetic resistance to anticancer agents

Kinetic resistance plays a major role in the failure of chemotherapy towards many solid tumors. Kinetic resistance to cytotoxic drugs can be reproduced in vitro by growing the cells as multicellular spheroids (Multicellular Resistance) or as hyperconfluent cultures (Confluence-Dependent Resistance). Recent findings on the cell cycle regulation have permitted a better understanding why cancer cells which arrest in long quiescent phases are poorly sensitive to cell-cycle specific anticancer drugs. Two cyclin-dependent kinase inhibitors (CDKI) seem particularly involved in the cell cycle arrest at the G1 to S transition checkpoint: the p53-dependent p21(cip1) protein which is activated by DNA damage and the p27(kip1) which is a mediator of the contact inhibition signal. Cell quiescence could alter drug-induced apoptosis which is partly dependent on an active progression in the cell cycle and which is facilitated by overexpression of oncogenes such as c-Myc or cyclins. Investigations are yet necessary to determine the influence of the cell cycle on the balance between antagonizing (bcl-2, bcl-X(L)...) or stimulating (Bax, Bcl-X(S), Fas...) factors in chemotherapy-induced apoptosis. Quiescent cells could also be protected from toxic agents by an enhanced expression of stress proteins, such as HSP27 which is induced by confluence. New strategies are required to circumvent kinetic resistance of solid tumors: adequate choice of anticancer agents whose activity is not altered by quiescence (radiation, cisplatin), recruitment from G1 to S/G2 phases by cell pretreatment with alkylating drugs or attenuation of CDKI activity by specific inhibitors.

Increase in CIP2A expression is associated with doxorubicin resistance

The cancerous inhibitor of protein phosphatase 2A (CIP2A) increases the migration and metastasis of various cancer cells. Overexpression of CIP2A has been shown to increase the proliferation of MDA-MB-231 cells. We thus assessed whether CIP2A expression is associated with sensitivity to doxorubicin. MDA-MB-231 cells showed an increase in CIP2A expression after treatment with doxorubicin, while MCF-7 cells showed a decrease in CIP2A expression. The overexpression of CIP2A in MCF-7 cells overcame the inhibition of cell proliferation in response to doxorubicin treatment. CIP2A expression was not affected by wild-type or mutant p53. However, mutant p53 blocked doxorubicin-mediated CIP2A down-regulation in HCT116 cells. As a regulation mechanism of doxorubicin-mediated CIP2A expression, we showed that phosphorylated Akt was involved in the suppression of CIP2A expression.

Cellular mechanisms of resistance to anthracyclines and taxanes in cancer: intrinsic and acquired

Taxanes and anthracyclines are two of the most potent and broadly effective classes of chemotherapeutic agents. However, resistance to these agents is common and significantly limits their potential. As such, there is a great need to understand the mechanisms underlying de novo and acquired resistance to these agents. Beyond the resistance barrier lies even greater potential to significantly alter the natural course of human cancer. This review discusses what we currently understand about the mechanisms of resistance to taxanes and anthracyclines. Preclinical models suggest a role for ATP-binding cassette transporters, tubulin isoforms, microtubule-associated proteins, tubulin gene mutations, and mitotic checkpoint signaling proteins in resistance to taxanes. Preclinical models also suggest that drug transport proteins, antioxidant defenses, apoptotic signaling, and topoisomerase modulation may mediate anthracycline resistance. Many of these hypotheses remain untested in appropriately designed clinical studies, but limited clinical evidence will be reviewed. Epothilones represent a novel class of non-taxane microtubule stabilizing agents with distinct drug-resistance profiles. Potential mechanisms behind these differences and their potential role in the treatment of both taxane- and anthracycline-refractory patients are discussed.

Improving the selectivity of cancer treatments by interfering with cell response pathways

The cellular response to the stress induced by treatment with anticancer agents is a key determinant of drug activity. A pivotal role in this response is played by checkpoint proteins that control the normal passage of cells through the cell cycle. There is evidence that cancer cells often have defects in one checkpoint control that makes them more vulnerable to inhibition of a second checkpoint, thereby enhancing the overall response to treatment. The G1 and G2 checkpoints are particularly crucial for the decision of a cell to arrest in the cell cycle after damage. The checkpoints are used to try to allow the repair of any damage, or to activate the apoptotic (programmed cell death) machinery. Inhibition of both G1 and G2 checkpoints in cancer cells is therefore likely to result in an induction of the death response in cancer cells. Similarly, an increasing knowledge of the molecular mechanisms that form the basis of apoptotic pathways has helped to define why cancer cells have a reduced propensity to undergo apoptosis following the activation of apoptotic inhibitory pathways or the inhibition of pro-apoptotic pathways. Therefore, the possibility to modulate these pathways is likely to result not only in the increased activity of anticancer agents, but also in an increase in their specificity.

Decision making by p53: life, death and cancer

The p53 tumor-suppressor plays a critical role in the prevention of human cancer. In the absence of cellular stress, the p53 protein is maintained at low steady-state levels and exerts very little, if any, effect on cell fate. However, in response to various types of stress, p53 becomes activated; this is reflected in elevated protein levels, as well as augmented biochemical capabilities. As a consequence of p53 activation, cells can undergo marked phenotypic changes, ranging from increased DNA repair to senescence and apoptosis. This review deals with the mechanisms that underlie the apoptotic activities of p53, as well as the complex interactions between p53 and central regulatory signaling networks. In p53-mediated apoptosis, the major role is played by the ability of p53 to transactivate specific target genes. The choice of particular subsets of target genes, dictated by covalent p53 modifications and protein-protein interactions, can make the difference between life and apoptotic death of a cell. In addition, transcriptional repression of antiapoptotic genes, as well as transcription-independent activities of p53, can also contribute to the apoptotic effects of p53. Regarding the crosstalk between p53 and signaling networks, this review focuses on the interplay between p53 and two pivotal regulatory proteins: beta-catenin and Akt/PKB. Both proteins can regulate p53 as well as be regulated by it. In addition, p53 interacts with the GSK-3beta kinase, which serves as a link between Akt and beta-catenin. This review discusses how the functional balance between these different interactions might dictate the likelihood of a given cell to become cancerous or be eliminated from the replicative pool, resulting in suppression of cancer.

Genetic alterations in ovarian cancer cells that might account for sensitivity to chemotherapy in patients

The cellular response to anticancer agent treatments is determined by many factors that could be altered in tumor cells. The induction of apoptosis of cancer cells is thought to be important for the overall response of these cells. Despite the introduction of new and potent anticancer agents, the survival rate for patients with ovarian cancer remains poor. In general ovarian cancer cells present a poor propensity to undergo apoptosis, which could be one of the reasons for this relatively poor response observed in the clinic. Induction of apoptosis is the result of activation and repression of pro- and antiapoptotic genes, which are regulated by complex mechanisms. Many cancer cells activate a "survival" program to escape disruption and allow propagation. In this review we have analyzed the role of genetic alterations observed in ovarian cancer cells in determining cellular response to drug treatment.

Resistance of p53 knockout cells to doxorubicin is related to reduced formation of DNA strand breaks rather than impaired apoptotic signaling

The anthracycline doxorubicin (adriamycin) is an important chemotherapeutic agent used in the treatment of solid epithelial and mesenchymal tumors as well as leukemias. A variety of mechanisms has been proposed to be involved in doxorubicin-induced cytotoxicity such as DNA intercalation, oxidative stress, DNA strand breakage by inhibition of topoisomerase II, activation of death receptors, and altered p53 expression. Concerning doxorubicin resistance and p53 status data reported are contradictory. Here, we show that mouse fibroblasts deficient in p53 (p53(-/-)) are more resistant to doxorubicin than p53 wild-type (p53 wt) cells. This is in contrast to other genotoxic agents (UV-light, alkylating drugs) for which p53(-/-) fibroblasts proved to be more sensitive. Resistance of p53(-/-) cells to doxorubicin is related to reduced induction of apoptosis. This is not likely to be due to altered apoptotic signaling since the expression of Bax and Bcl-2 was unchanged and the induction of Fas/CD95/APO-1 receptor and caspase-8 was the same in p53(-/-) and p53 wt cells on treatment with doxorubicin. However, we observed a clearly lower level of doxorubicin-induced DNA strand breaks in p53(-/-) cells compared to the wt. P170 glycoprotein was equally expressed and the accumulation and elimination of the drug occurred with identical kinetics in both cell types. p53 deficient cells were cross-resistant to another topoisomerase II inhibitor etoposide, which also provoked increased DNA strand breakage in p53 wt cells. Based on the data we conclude that the p53 status significantly impacts the generation of DNA strand breaks because of drug-induced topoisomerase inhibition rather than death receptor signaling. Since human tumors are frequently mutated in p53 the findings bear clinical implications.

Correlation of p53 immunostaining in primary and residual ovarian cancer at the time of positive second-look laparotomy and its prognostic role: a Southwest Oncology Group ancillary study

OBJECTIVE

The objective of this study was to verify the correlation between p53 immunostaining at initial diagnosis and at positive reassessment after completing platinum-based chemotherapy and to assess prognostic differences between patients whose tumors display positive immunostaining versus those that have negative immunostaining at such reassessment.

METHODS

This study made use of samples from patients entered into a prospective randomized study of the Southwest Oncology Group (SWOG 8835) that treated patients with minimal residual disease at second-look laparotomy with either intraperitoneal (ip) mitoxantrone or fluorodeoxyuridine (FUDR). Unstained slides from tumor obtained at the initial diagnosis and at reassessment were retrospectively requested from individual institutions. The degree of nuclear staining was determined using the anti-p53 mouse monoclonal antibody Pab1801 and previously published techniques, with a cutoff of 10% or more staining of tumor cell nuclei for a positive result. Cox model regression analysis was performed for overall survival and progression-free survival, with p53 status, ip treatment, and baseline CA125 as independent variables.

RESULTS

p53 determination was feasible in 22 patients both at diagnosis and at the second-look samples; 9 additional patients had only either sample available. Since concordance between the 10 negative and 12 positive immunostained samples was 100%, all 31 patients were considered in the Cox model. The death hazard ratio of p53-positive versus p53-negative patients was 4.18 (two-sided P value of 0.006).

CONCLUSION

p53 immunostaining at second-look laparotomy correlates with the immunostaining at diagnosis. In this series confined to patients with minimal residual disease after initial therapy subjected to second-line intraperitoneal treatment, it appears to identify a poor prognostic (positive) subset for survival.

Differential regulation of phosphatidylserine externalization and DNA fragmentation by caspases in anticancer drug-induced apoptosis of rat mammary adenocarcinoma MTLn3 cells

Caspase activation is a central event in the execution phase of apoptosis and is associated with phosphatidylserine (PS) externalization and DNA fragmentation. We investigated the role of caspase activity in anticancer drug-induced PS externalization and DNA fragmentation in MTLn3 cells. Caspase activation (DEVD-AMC cleavage) occurred in a time- and concentration-dependent manner after exposure to doxorubicin, in association with cleavage of poly(ADP) ribose polymerase and protein kinase C delta, two caspase-3 substrates. Caspase activation was closely followed by oligonucleosomal DNA fragmentation and PS externalization as determined by flow cytometric analysis. Similar observations were made for etoposide and cisplatin. Inhibition of caspases with zVAD-fmk inhibited almost completely doxorubicin-induced DNA fragmentation as well as proteolysis of protein kinase C delta. In contrast, PS externalization induced by doxorubicin was only partly affected by caspase inhibition. Flow cytometric cell sorting demonstrated that DNA fragmentation in the remaining PS positive cells after doxorubicin treatment in the presence of zVAD-fmk was fully blocked. In conclusion, these data indicate that while DNA fragmentation in anticancer drug-induced apoptosis of MTLn3 cells is fully dependent on caspase activity, PS externalization is controlled by both caspase-dependent and caspase-independent pathways.

Signaling pathways activated by daunorubicin

The anthracycline daunorubicin is widely used in the treatment of acute nonlymphocytic leukemia. The drug has, of course, been the object of intense basic research, as well as preclinical and clinical study. As reviewed in this article, evidence stemming from this research clearly demonstrates that cell response to daunorubicin is highly regulated by multiple signaling events, including a sphingomyelinase-initiated sphingomyelin-ceramide pathway, mitogen-activated kinase and stress-activated protein/c-Jun N-terminal kinase activation, transcription factors such as nuclear factor kappa B, as well as the Fas/Fas-ligand system. These pathways are themselves influenced by a number of lipid products (diacylglycerol, sphingosine-1 phosphate, and glucosyl ceramide), reactive oxygen species, oncogenes (such as the tumor suppressor gene p53), protein kinases (protein kinase C and phosphoinositide-3 kinase), and external stimuli (hematopoietic growth factors and the extracellular matrix). In light of the complexity and diversity of these observations, a comprehensive review has been attempted toward the understanding of their individual implication (and regulation) in daunorubicin-induced signaling. (Blood. 2001;98:913-924)

Apoptosis induced by doxorubicin in neurotumor cells is divorced from drug effects on ceramide accumulation and may involve cell cycle-dependent caspase activation

Doxorubicin (0.5 microgram/ml) induced caspase-dependent apoptosis in SH-SY5Y neuroblastoma and CHP-100 neuroepithelioma cells. The apoptotic response started to be evident approximately 15 h after drug administration and, as monitored over a 48-h period, was more pronounced in CHP-100 than in SH-SY5Y cells. In both systems, apoptosis was accompanied by elevation of intracellular ceramide levels. Ceramide accumulation was blocked by the ceramide synthase inhibitor fumonisin B(1) (25 microM); this compound, however, did not prevent drug-induced apoptosis. Untreated cells from both lines expressed negligible p53 levels; on the other hand, whereas p53 and p21(Cip1/Waf1) were rapidly up-regulated in doxorubicin-treated SH-SY5Y cells, such a response was not observed in CHP-100 cells. Doxorubicin induced a G(2)/M phase block in both cell lines, but whereas the G(1) phase was markedly depleted in CHP-100 cells, it was substantially retained in SH-SY5Y cells. In the latter system, double G(1) and G(2)/M block largely preceded cell death; however, as apoptosis underwent completion, it selectively targeted late S and G(2)/M cells. Moreover, apoptosis suppression by caspase inhibition did not result in a recovery of the G(1) cell population. These results support the notion that doxorubicin-induced apoptosis and ceramide elevation are divorced events in neuroectodermal tumors and that p53 function is at least dispensable for apoptosis completion. Indeed, as G(1) cells appear to be refractory to doxorubicin-induced apoptosis, p53 up-regulation and p21(Cip1/Waf1) expression may provide an unfavorable setting for the apoptotic action of the drug.

Diverse effects of 9-hydroxyellipticine on the chemosensitivity of human pancreatic cancer cells harboring p53 mutations

Recently, it has been shown that 9-hydroxyellipticine (9-HE), an antitumor alkaloid has a unique property of restoring functional wild-type (wt) p53 activity via inhibition of mutant (mt) p53 protein phosphorylation. In the present study, we investigated the effect of 9-HE on the drug sensitivity of human pancreatic cancer cells. Exposure of cells to 9-HE at a relatively low concentration of 1 microM induced almost no cell death but was sufficient to restore wt p53 activity, as evidenced by an induction of endogenous p21WAF1/CIP1 concomitant with G1 and G2/M arrests in cell-cycle progression. Pretreatment with 1 microM 9-HE markedly enhanced cell killing when combined with cisplatin or mitomycin C. In contrast, 9-HE pretreatment protected cells from killing by 5-fluorouracil, VP-16, or vincristine. These effects of 9-HE were specific for several cell lines containing mt p53 and were not observed in p53-negative or wt p53 expressing cells. Taken together, these findings suggest that 9-HE may exert different effects on the drug sensitivity of pancreatic cancer cells displaying p53 mutations possibly through restoration of wt p53.

p73 competes with p53 and attenuates its response in a human ovarian cancer cell line

The transcriptional activity of the p53 tumor suppressor protein is crucial for the regulation of cell growth, apoptosis and tumor progression. The first identified p53 relative, p73, was reported to be monoallelically expressed in normal tissues. In some tumors, loss of heterozygosity was associated with overexpression of the silent allele. Human p73alpha was transfected into the wild-type p53-expressing human ovarian carcinoma cell line A2780. Unlike human osteosarcoma Saos-2 cells, A2780 cells could tolerate hyperexpression of p73alpha and clones over-expressing p73alpha could be isolated. No p53-p73 protein-protein interaction was found in these clones in co-immunoprecipitation experiments. Endogenous p53 transcriptional activity was markedly decreased both when p73 was integrated into the genome and in transient transfections using a reporter plasmid containing the p53 binding site linked to luciferase. Transient transfection of p73 with a mutation in the DNA-binding domain did not show these effects. The competition for p53 DNA binding by p73alpha was also evident in gel shift experiments. The results suggest that p73 can modulate p53 function by inhibiting its DNA binding and that overexpression of p73 in tumors might be a novel mechanism of inactivation of p53.

p53 and chemosensitivity

BACKGROUND

Although hematologic malignancies and some solid tumors such as germ cell tumors and pediatric malignancies can be cured by cytotoxic treatment, the most prevalent solid tumors are relatively resistant to these interventions. Apoptosis is involved in the cell kill of anticancer drugs and p53 is believed to be of principal importance in this process. However p53 also plays a role in cell cycle arrest and DNA repair, cellular processes that can decrease the sensitivity to chemotherapy. Therefore, p53 may play a dual role after exposure to cytotoxic treatment, activating either mechanisms that lead to apoptosis or launching processes directing to DNA repair and survival of the cell.

DESIGN

In this article, we review in details the p53 functions involved in the mediation of chemosensitivity. The preclinical and clinical data published in the recent years about the relation between p53 and chemosensitivity are discussed and the potential pitfalls associated to most of these studies, and that may account for the contradictory results produced so far are also mentioned.

p53-Independent activation of the gadd45 promoter by Delta12-prostaglandin J2

A p53-inducible gene, the growth arrest and DNA damage-inducible gene 45 (gadd45), is associated with cell growth inhibition, DNA damage response and DNA repair. Here we report that Delta12-prostaglandin J2 (Delta12-PGJ2), an in vivo metabolite of arachidonic acid, which inhibits cell proliferation, induces gadd45 mRNA in HeLa cells. Because the p53 protein in HeLa cells is inactivated by papilloma virus E6, this type of gadd45 induction appears to be p53-independent. The induction was dose-dependent, and the maximum induction was observed at a concentration of 7.5 microgram/ml. In a time course study, gadd45 mRNA was induced 3 h after the addition of 7.5 microgram/ml Delta12-PGJ2. To investigate the transcriptional mechanism of gadd45 mRNA induction, we cloned a human genomic DNA fragment containing the gadd45 promoter region, and investigated the effect of Delta12-PGJ2 on the gadd45 promoter activity. In HeLa cells, 7.5 microgram/ml Delta12-PGJ2 markedly stimulated the gadd45 gene promoter about 20-fold or more. On the other hand, Delta12-PGJ2 did not stimulate the promoter activity of a reporter plasmid containing only p53 binding sites in HeLa cells, indicating that the gadd45 promoter activation by Delta12-PGJ2 was not mediated by p53. These results suggest that one of the mechanisms of cell growth arrest by Delta12-PGJ2 is mediated through a p53-independent induction of gadd45.

Contribution of the cyclin-dependent kinase inhibitor p27KIP1 to the confluence-dependent resistance of HT29 human colon carcinoma cells

We have previously shown that growth of HT29 human colorectal cancer cells at confluence increased their resistance to the cytotoxic agent cisplatin. This study further explores the mechanisms of this resistance phenotype. DNA platination induced by cisplatin exposure is slightly reduced by confluence. However, at an equivalent DNA platination level, non-confluent cells accumulate in the G2/M phase of the cell cycle, demonstrate aberrant mitotic figures and die by apoptosis, while confluent cells progress slowly through the cell cycle, do not reach mitosis and are more resistant to drug-induced cell death. At a molecular level, cisplatin enhances cyclin B and p34cdc2 levels and histone H1 kinase activity in non-confluent, but not in confluent, cells. Furthermore, when HT29 cells reach confluence, expression of the cyclin-dependent kinase inhibitor p27Kip1 increases and cells accumulate in the G0/G1 phase of the cell cycle. Transfection-mediated over-expression of p27Kip1 in non-confluent HT29 cells decreases the cytotoxic activity of cisplatin as well as its ability to trigger apoptosis. Non-confluent HT29 cells over-expressing p27Kip1 are also more resistant to doxorubicin, etoposide and 5-fluorouracil. Our results suggest that p27Kip1 contributes to the confluence-dependent resistance phenotype.

Chemosensitivity of human malignant glioma: modulation by p53 gene transfer

Loss of wild-type p53 activity is one of the most common molecular abnormalities in human cancers including malignant gliomas. The p53 status is also thought to modulate sensitivity to irradiation and chemotherapy. Here, we studied the effect of a p53 gene transfer on the chemosensitivity of three human glioma cell lines with different endogenous p53 status (LN-229, wild-type; LN-18, mutant; LN-308, deleted), using the murine temperature-sensitive p53 val135 mutant. Expression of mutant p53 enhanced proliferation of LN-308 cells but reduced proliferation in the other cell lines. Expression of wild-type p53 caused reversible growth arrest of all cell lines but failed to induce apoptosis. Growth arrest induced by wild-type p53 was associated with strong induction of p21 expression. Strong induction of BAX expression and loss of BCL-2 expression, which are associated with p53-dependent apoptosis rather than growth arrest, were not observed. Wild-type p53 failed to sensitize glioma cells to cytotoxic drugs including BCNU, cytarabine, doxorubicin, teniposide and vincristine. The combined effects of wild-type p53 gene transfer and drug treatment were less than additive rather than synergistic, suggesting that the intracellular cascades activated by p53 and chemotherapy are redundant. Unexpectedly, forced expression of mutant p53 modulated drug sensitivity in that it enhanced the toxicity of some drugs but attenuated the effects of others. These effects may represent a dominant negative effect of mutant p53 in LN-229 cells which have wild-type p53 activity but must be considered a gain of function-type effect in the other two cell lines which have no wild-type p53 activity. Importantly, no clear-cut pattern emerged among the three cell lines studied. We conclude that somatic gene therapy based on the reintroduction of p53 will limit the proliferation of human malignant glioma cells but is unlikely to induce clinically relevant sensitization to chemotherapy in these tumors.

Profiling expression patterns and isolating differentially expressed genes by cDNA microarray system with colorimetry detection

A high-density cDNA microarray with colorimetry detection system to simultaneously monitor the expression of many genes on nylon membrane is described and characterized. To quantify the expression of genes and to isolate differentially expressed genes, the southern hybridization process on filter membranes was employed. The levels of gene expression were represented by color intensities generated by colorimetric reactions in place of hazardous radioisotopes or costly laser-induced fluorescence detection. The gene expression patterns on nylon membranes were digitized by devices such as an economical flatbed scanner or a digital camera. The quantitative information of gene expression was retrieved by image analysis software. Quantitative comparison of the northern dot-blotting method with the microarray system is described. Applications employing single-color detection as well as dual-color detection to isolate differentially expressed genes among thousands of genes are demonstrated.

Chromatin relaxation by overexpression of mutant p53, HPV16-E6, or cyclin G transgenes

In this study, using a cell line that carries endogenous wild-type p53 genes, we show that transfection of cells with mutant p53, HPV16-E6, or cyclin G transgenes results in the disruption of higher-order chromatin structure, as evidenced by enhanced sensitivity to micrococcal nuclease. Multiple mechanisms may contribute to this phenotype, including histone H1 phosphorylation, direct binding of oncoproteins to nuclear matrix attachment sites, and altered expression of component genes of the p53 pathway, whose products may function in maintenance of chromatin structure.

Inactivation of p53 in a human ovarian cancer cell line increases the sensitivity to paclitaxel by inducing G2/M arrest and apoptosis

Paclitaxel-induced cytotoxicity, cell cycle perturbation, and apoptosis were determined in a human ovarian cancer cell line expressing wt p53 (A2780) and in a subclone (A2780/E6) obtained upon transfection with the product of the E6 gene of the human papilloma virus HPV16. The inactivation of wt p53 in A2780/E6 was verified by measuring the inability of the clone to induce p53 and p21 expression after paclitaxel treatment. The p53-negative clone (A2780/E6) was approximately 50-fold more sensitive to paclitaxel than wt p53-expressing A2780 cells. This increased sensitivity was related to the ability of paclitaxel to induce a strong arrest of cells in the G2/M phase of the cell cycle in A2780/E6 but not in A2780 cells. This different cell cycle arrest was accompanied by increased frequency of paclitaxel-induced p53-independent apoptosis. Initial studies on proteases activation tend to exclude a direct role of ICE and CPP32 in the induction of apoptosis in these cells and show a paclitaxel-dependent increase in FLICE levels, whose biological relevance is however at present not defined.

Acute overexpression of wt p53 facilitates anticancer drug-induced death of cancer and normal cells

The relationship between chemosensitivity and p53 is currently considered from two mutually exclusive points of view: (1) wt p53 increases chemosensitivity due to apoptosis and (2) wt p53 decreases chemosensitivity due to growth arrest and DNA repair. We used p53-expressing adenovirus (Ad-p53) to directly evaluate effect of p53 on sensitivity to anticancer drugs. When p53 was expressed at sublethal levels, it sensitized cells to the DNA-damaging drugs Adriamycin, mitomycin C, actinomycin D, etoposide (VP16), cisplatin and CPT11. This sensitization was observed in cancer cell lines (N=10) regardless of endogenous p53 status and also in normal human lung and skin fibroblasts. The degree of sensitization appeared to be greater in cancer cells with mutant p53. Normal fibroblasts required significantly higher doses of Ad-p53 to affect a drug's sensitivity partly because of their lower infectivity by adenovirus. Wt p53 not only decreased IC50 but also accelerated cell death induced by DNA-damaging drugs. In contrast, sensitization to microtubule-active drugs by p53 was shown only in a few cell lines. We conclude that exogenous wt p53 accelerates cell death induced by DNA damaging agents in both normal and cancer cells and offers no protection from anticancer drugs.

Mechanism of resistance to cisplatin in a human ovarian-carcinoma cell line selected for resistance to doxorubicin: possible role of p53

A possible novel mechanism of cross-resistance to cisplatin (CDDP) in the doxorubicin-resistant ovarian-cancer cell line A2780-DX3, which displays atypical multidrug resistance, is presented. A2780-DX3 is found to be more resistant than the parental line A2780 in terms of CDDP-induced cytotoxicity and apoptosis. Resistance is not related to the amount of cross-links. Topoisomerase-II (topII) protein levels were similar in both cell lines, with lower cleavage activity in A2780-DX3 cells. The parental and the doxorubicin-resistant cells expressed the same level of c-erb2, which could be implicated in CDDP resistance. bcl2 was almost undetectable in both cell lines. At the same time, we found strong induction of p53, waf-1 and bax protein levels after CDDP treatment in the A2780, but not in the A2780-DX3, cell line. Treatment of both cell lines with mitomycin C (MMC), which acts with a mechanism different from CDDP, caused equal accumulation of p53 and induction of bax. We found that A2780-DX3 cells exhibit altered cellular localization of p53 protein in comparison with A2780. A significant proportion of p53 in A2780-DX3 cells was found in the cytoplasmic compartment, and CDDP treatment induced a functional p53 protein in the nucleus of A2780 much more strongly than in A2780-DX3, which coincides with an increase of transcriptional activity of p53 in treated A2780 cells. We propose that the cross-resistance to CDDP in the A2780-DX3 cell line may be due to inactivation of a CDDP-dependent p53-accumulation pathway.

DDP-induced cytotoxicity is not influenced by p53 in nine human ovarian cancer cell lines with different p53 status

Nine human ovarian cancer cell lines that express wild-type (wt) or mutated (mut) p53 were used to evaluate the cytotoxicity induced by cisplatin (DDP). The concentrations inhibiting the growth by 50% (IC50) were calculated for each cell line, and no differences were found between cells expressing wt p53 and mut p53. Using, for each cell line, the DDP IC50, we found that these concentrations were able to induce an increase in p53 levels in all four wt-p53-expressing cell lines and in one out of five mut-p53-expressing cell lines. WAF1 and GADD45 mRNAs were also increased by DDP treatment, independently of the presence of a wt p53. Bax levels were only marginally affected by DDP, and this was observed in both wt-p53- and mut-p53-expressing cells. DDP-induced apoptosis was evident 72 h after treatment, and the percentage of cells undergoing apoptosis was slightly higher for wt-p53-expressing cells. However, at doses near the IC50, the percentage of apoptotic cells was less than 20% in all the cell lines investigated. We conclude that the presence of wt p53 is not a determinant for the cytotoxicity induced by DDP in human ovarian cancer cell lines.

Sensitivity and cellular response to different anticancer agents of a human ovarian cancer cell line expressing wild-type, mutated or no p53

BACKGROUND

The cytotoxicity and gene expression induced by anticancer drugs with different mechanisms of action was tested in clones from a human ovarian cancer cell line expressing no p53, mutated p53 or wild type (wt) p53.

MATERIALS AND METHODS

We used clones from SKOV3 cells transfected with a temperature-sensitive mutant p53 which expresses mutated p53 at 37 degrees C and a wild type-like p53 at 32 degrees C. Cytotoxicity and expression of p53-related genes (WAF1 and GADD45) were tested after 24 hours of treatment with different drugs.

RESULTS

All of the drugs were equally active in the different systems, independently of the presence of p53, with the exception of doxorubicin which was less cytotoxic in cells expressing a wtp53. An increase in the transcription of WAF1 and GADD45 genes was found in cells expressing p53 and treated with the drugs. GADD45 and WAF1 expression was also found in cells not expressing p53 but treated with the drugs, suggesting that these genes can also be activated by DNA damage through a pathway independent of p53. A highly DNA-sequence-specific alkylator, tallimustine (FCE 24517), which causes a very small number of DNA lesions, does not increase the expression of these genes. Cyclin D1 gene expression was not changed after treatment with the drugs tested in cells both expressing and not expressing wtp53.

CONCLUSIONS

Our data suggest that p53 expression does not play a role in increasing the susceptibility of cells not undergoing apoptosis after DNA damage, but that, at least in the case of doxorubicin, it can enhance the repair systems and reduce the cytotoxicity.