Differential oncogene amplification in tumor cells from a patient treated with cisplatin and 5-fluorouracil

The inhibitory effect of Cordycepin on the proliferation of cisplatin-resistant A549 lung cancer cells

The goal of this study is to determine the anti-cancer mechanism of Cordycepin in A549 Cisplatin-Resistance (CR) lung cancer cells. Cordycepin inhibited the viability of A549CR cells in a dose-dependent manner. The cell inhibition was due to induction of apoptosis in the cells treated with Cordycepin by activation of caspase -3, -8 and -9 activities. The cell cycle analysis showed that accumulation of Sub G1 was observed in Cordycepin-treated with A549CR lung cancer cells. Based on the data of expression profile analysis of cell signaling proteins using IPS-FPAA, H-Ras was down-regulated in Cordycepin-treated A549CR cells. Collectively, anti-proliferative function of Cordycepin was due to stimulation of the cell apoptosis and the cell cycle arrest via caspases activation and down-regulation of H-Ras.

Inauhzin(c) inactivates c-Myc independently of p53

Oncogene MYC is deregulated in many human cancers, especially in lymphoma. Previously, we showed that inauhzin (INZ) activates p53 and inhibits tumor growth. However, whether INZ could suppress cancer cell growth independently of p53 activity is still elusive. Here, we report that INZ(c), a second generation of INZ, suppresses c-Myc activity and thus inhibits growth of human lymphoma cells in a p53-independent manner. INZ(c) treatment decreased c-Myc expression at both mRNA and protein level, and suppressed c-Myc transcriptional activity in human Burkitt's lymphoma Raji cells with mutant p53. Also, we showed that overexpressing ectopic c-Myc rescues the inhibition of cell proliferation by INZ(c) in Raji cells, implicating c-Myc activity is targeted by INZ(c). Interestingly, the effect of INZ(c) on c-Myc expression was impaired by disrupting the targeting of c-Myc mRNA by miRNAs via knockdown of ribosomal protein (RP) L5, RPL11, or Ago2, a subunit of RISC complex, indicating that INZ(c) targets c-Myc via miRNA pathways. These results reveal a new mechanism that INZ

Inhibition of c-Myc by let-7b mimic reverses mutidrug resistance in gastric cancer cells

Chemotherapy is one of the few effective choices for patients with advanced or recurrent gastric cancer (GC). However, the development of mutidrug resistance (MDR) to cancer chemotherapy is a major obstacle to the effective treatment of advanced GC. Additionally, the mechanism of MDR remains to be determined. In the present study, we tested IC50 of cisplatin (DDP), vincristine (VCR) and 5-fluorouracil (5-FU) in SGC7901, SGC7901/DDP and SGC7901/VCR gastric cancer cells using an MTT assay. The expression of let-7b and c-Myc in these cells was detected by qPCR and western blot analysis. The relationship between let-7b and c-Myc was explored using a luciferase reporter assay. Transfection of let-7b mimic or inhibitor was used to confirm the effect of let-7b on drug sensitivity in chemotherapy via the regulation of c-Myc expression. We found that the expression of let-7b was lower in chemotherapy-resistant SGC7901/DDP and SGC7901/VCR gastric cancer cells than that in chemotherapy-sensitive SGC7901 cells. By contrast, the expression of c-Myc was higher in SGC7901/DDP and SGC7901/VCR cells than that in SGC7901 cells. Furthermore, we confirmed that let-7b suppresses c-Myc gene expression at the mRNA and protein levels in a sequence-specific manner, while transfection of let-7b mimic increases drug sensitivity in chemotherapy-resistant SGC7901/DDP and SGC7901/VCR cells by targeting downregulation of c-Myc. In SGC7901 drug-sensitive cells, however, the sensitivity of chemotherapy was significantly decreased following let-7b inhibitor transfection. The present study results demonstrated that let-7b increases drug sensitivity in chemotherapy‑resistant SGC7901/DDP and SGC7901/VCR gastric cancer cells by targeting the downregulation of c-Myc and that, let-7b mimic reverses MDR by promoting cancer stem cell differentiation controlled by double-negative autoregulatory loops (Lin28/let-7 and Myc/let-7) and a double-positive autoregulatory loop (Lin28/Lin28B/Myc) existing in GC cells, which remains to be confirmed.

Use of ribozymes and antisense oligodeoxynucleotides to investigate mechanisms of drug resistance

Chemotherapy can cure a number of human cancers but resistance (either intrinsic or acquired) remains a significant problem in many patients and in many types of solid tumour. Combination chemotherapy (using drugs with different cellular targets/mechanisms) was introduced in order to kill cells which had developed resistance to a specific drug, and to allow delivery of a greater total dose of anti-cancer chemicals by combining drugs with different side-effects (Pratt et al., 1994). Nearly all anti-cancer drugs kill tumour cells by activating an endogenous bio-chemical pathway for cell suicide, known as programmed cell death or apoptosis.

Prognostic significance of numeric aberrations of genes for thymidylate synthase, thymidine phosphorylase and dihydrofolate reductase in colorectal cancer

BACKGROUND

Most human cancer cells have structural aberrations of chromosomal regions leading to loss or gain of gene specific alleles. This study aimed to assess the range of gene copies per nucleus of thymidylate synthase (TYMS), thymidine phosphorylase (TP) and dihydrofolate reductase (DHFR) in colorectal cancer, and to evaluate its prognostic significance following adjuvant chemotherapy, since these enzymes are closely related to efficacy of 5-fluorouracil (5FU).

PATIENTS AND METHODS

Consecutive patients (n = 314), who were completely resected for colorectal cancer stages II-IV and adjuvantly treated with 5-FU were retrospectively evaluated. Paraffin embedded tumor specimens were assessed for gene copies per nucleus of TYMS, TP and DHFR by fluorescence in situ hybridisation (FISH) using specific peptide nucleic acid probes. Outcome according to gene copies per nucleus above and below the median were compared. Also TYMS expression, assessed by immunohistochemistry, was associated with TYMS copies per nucleus.

RESULTS

The number of gene copies per nucleus were 1.7 (0.7-2.8), 1.8 (0.9-3.1) and 1.8 (1.1-2.7) median (range) for TYMS, TP and DHFR, respectively. TYMS expression was directly associated with TYMS genes per nucleus (p = 0.05). Cox multivariate analysis, adjusted for the prognostic impact of disease stage, vascular tumor invasion, and bowel obstruction at resection, revealed that high TYMS gene copy number was associated with significantly higher risk of recurrence (HR = 1.6; 95%CI 1.1-2.2; p = 0.02) and death (HR = 1.6; 95%CI 1.1-2.3; p = 0.01). No significant differences in outcome appeared according to TP and DHFR gene ratios.

CONCLUSION

Aberration of TYMS gene is of significance to expression of TYMS, which may influence the biology and 5-FU sensitivity of colorectal cancer. This may be utilized in the allocation of patients for treatment approaches and for decision on follow-up programs.

Vitamin C enhances chemosensitization of esophageal cancer cells in vitro

Chemotherapy is increasingly utilised in multimodal protocols to try and improve outcomes. Cisplatin and 5-fluorouracil (5-Fu) are the mainstay of chemotherapeutic regimens, and an understanding of sensitivity and resistance of esophageal cancer to these agents is of considerable clinical importance. Antioxidants may modulate the response to chemotherapy, and in this study we examined the effect of vitamin C on 5-Fu and cisplatin cytotoxicity and related pathways in the esophageal cancer cell lines OE33 and SKGT-4. The antiproliferative effect of antitumor agents was measured by the MTT assay, and the transcription factors NF-kappaB and AP-1 pathways were assessed by electrophoretic mobility gel shift assay. 5-Fu and cisplatin demonstrated marked morphological changes and decreased cell proliferation. A combination of vitamin C with 5-Fu or cisplatin exerted a significantly enhanced cytotoxic effect compared to both drugs individually. Treatment of esophageal cancer cells with 5-Fu and cisplatin induced NF-kappaB and AP-1 activation. Pretreatment with vitamin C inhibited 5-Fu or cisplatin induced NF-kappaB nuclear translocation and DNA binding activity, but vitamin C had no effect on IkappaB-alpha protein levels. Vitamin C also inhibited 5-Fu- and cisplatin-induced AP-1 activation. Our data demonstrate that vitamin C enhances the antitumor activity of 5-Fu and cisplatin, in part by inhibiting translocation of NF-kappaB and AP-1, and sensitizes cancer cells to drug-induced cell death. The data suggest that vitamin C supplementation may improve the efficacy of chemotherapy for esophageal cancer.

Efficiency of extension of mismatched primer termini across from cisplatin and oxaliplatin adducts by human DNA polymerases beta and eta in vitro

DNA polymerases beta and eta are among the few eukaryotic polymerases known to efficiently bypass cisplatin and oxaliplatin adducts in vitro. Our laboratory has previously established that both polymerases misincorporated dTTP with high frequency across from cisplatin- and oxaliplatin-GG adducts. This decrease in polymerase fidelity on platinum-damaged DNA could lead to in vivo mutations, if this base substitution were efficiently elongated. In this study, we performed a steady-state kinetic analysis of the steps required for fixation of dTTP misinsertion during translesion synthesis past cisplatin- and oxaliplatin-GG adducts by pol beta and pol eta. The efficiency of translesion synthesis by pol eta past Pt-GG adducts was very similar to that observed for this polymerase when the template contains thymine-thymine dimers. This finding suggested that pol eta could play a role in translesion synthesis past platinum-GG adducts in vivo. On the other hand, translesion synthesis past platinum-GG adducts by pol beta was much less efficient. Translesion synthesis by pol eta is likely to be predominantly error-free, since the probability of correct insertion and extension by pol eta was 1000-2000-fold greater than the probability of incorrect insertion and extension. Our results also indicated that for pol eta the frequency of misincorporation is the same across from the 3'G and the 5'G of the platinum-GG adducts for both cisplatin and oxaliplatin adducts. On the other hand, pol beta is more likely to misinsert at the 3'G of the adducts and misinsertion occurs at higher frequency for oxaliplatin-GG than for cisplatin-GG adducts.

cis-Diamminedichloroplatinum-resistant cell lines derived from human epithelial ovarian carcinoma express increased susceptibility to angiogenesis inhibitor TNP-470

OBJECTIVE

To elucidate the efficacy of TNP-470 on ovarian carcinomas by using cis-diamminedichloroplatinum (CDDP)-resistant cell lines.

METHODS

The susceptibility of human ovarian carcinoma-derived cell lines and its resistant cell lines against cis-diamminedichloroplatinum (CDDP) to angiogenesis inhibitor, TNP-470, were analyzed using three human cultured cell lines derived from ovarian carcinoma (TYK, KF-92, and Nakajima) and each CDDP-resistant cell line (TYK-R, TYK-R', KF/ra, KF/rb, Nakajima-S1, and Nakajima-S2).

RESULTS

TNP-470 revealed suppression of thymidine incorporation by all of the nine cell lines linearly dependent on the concentration of TNP-470. Significant suppression was not observed for either uridine or leucine incorporation by all nine cell lines. To elucidate the site of each cell line, in which TNP-470 revealed the antitumor effect, the incorporation of (3)H-TNP-470 by cultured cells or by DNA extracted from cultured cells was examined in the cell lines, and the ratio of (3)H-TNP-470 incorporation by DNA to (3)H-TNP-470 incorporation by cultured cells ranged from 2.3% to 4.4% in three parent cell lines. The ratio in the CDDP-resistant cell lines ranged from 11.0% to 46.7%. The ability of TNP-470 to inhibit neoplastic growth in vivo was evaluated using KF-92, KF/ra, KF/rb, Nakajima, Nakajima-S1, and Nakajima-S2. Concerning KF-92, KF/ra, and KF/rb, 30 mg/kg of TNP-470 significantly suppressed the tumorigenicity of KF-92, 10 and 30 mg/kg of TNP-470 suppressed the tumorigenicity of KF/ra, and 30 mg/kg of TNP-470 suppressed the tumorigenicity of KF/rb. Concerning Nakajima, Nakajima-S1, and Nakajima-S2, TNP-470 revealed no inhibitory effect on the tumorigenicity of Nakajima. Contrary, significant inhibition was observed when 30 mg/kg of TNP-470 was used to the CDDP-resistant cell lines Nakajima-S1 and Nakajima-S2.

CONCLUSION

These results suggest that the clinical application of TNP-470 may be one of the possible treatments for the CDDP-resistant ovarian carcinomas.

Cellular determinants of oxaliplatin sensitivity in colon cancer cell lines

Oxaliplatin (L-OHP) is a new platinum analogue that has shown antitumour activity against colon cancer both in vitro and in vivo and is now used in the chemotherapeutic treatment of metastatic colon and rectal cancer. L-OHP like cisplatin (CDDP), is detoxified by glutathione (GSH)-related enzymes and forms platinum (Pt)-DNA adducts lesions that are repaired by the nucleotide excision repair system (NER). We investigated the cytotoxicity and the pharmacology of L-OHP and CDDP on a panel of six colon cell lines in vitro. We showed that GSH and glutathione S-transferase (GST) activity were not correlated to oxaliplatin cytotoxicity. Pt-DNA adducts formation and repair were correlated with CDDP, but not with L-OHP cytotoxicity. The determination of ERCC1 and XPA expression, two enzymes of the NER pathway, by reverse transcriptase-polymerase chain reaction (RT-PCR), demonstrated that ERCC1 expression was predictive of L-OHP sensitivity (r(2)=0.67, P=0.02) and XPA level after oxaliplatin exposure was also correlated to L-OHP IC(50) (r(2)=0.5; P=0.04). The knowledge of such correlations could help predict the sensitivity of patients with colon cancer to L-OHP.

Frameshifts and deletions during in vitro translesion synthesis past Pt-DNA adducts by DNA polymerases beta and eta

DNA polymerases beta (pol beta ) and eta (pol eta ) are the only two eukaryotic polymerases known to efficiently bypass cisplatin and oxaliplatin adducts in vitro. Frameshift errors are an important aspect of mutagenesis. We have compared the types of frameshifts that occur during translesion synthesis past cisplatin and oxaliplatin adducts in vitro by pol beta and pol eta on a template containing multiple runs of nucleotides flanking a single platinum-GG adduct. Translesion synthesis past platinum adducts by pol beta resulted in approximately 50% replication products containing single-base deletions. For both adducts the majority of -1 frameshifts occurred in a TTT sequence 3-5 bp upstream of the DNA lesion. For pol eta, all of the bypass products for both cisplatin and oxaliplatin adducts contained -1 frameshifts in the upstream TTT sequence and most of the products of replication on oxaliplatin-damaged templates had multiple replication errors, both frameshifts and misinsertions. In addition, on platinated templates both polymerases generated replication products 4-8 bp shorter than the full-length products. The majority of short cisplatin-induced products contained an internal deletion which included the adduct. In contrast, the majority of oxaliplatin-induced short products contained a 3' terminal deletion. The implications of these in vitro results for in vivo mutagenesis are discussed.

Mechanisms of resistance to cisplatin

The use of cisplatin in cancer chemotherapy is limited by acquired or intrinsic resistance of cells to the drug. Cisplatin enters the cells and its chloride ligands are replaced by water, forming aquated species that react with nucleophilic sites in cellular macromolecules. The presence of the cisplatin adducts in DNA is thought to trigger cell cycle arrest and apoptosis. Knowledge of the mechanism of action of cisplatin has improved our understanding of resistance. Decreased intracellular concentration due to decreased drug uptake, increased reflux or increased inactivation by sulfhydryl molecules such as glutathione can cause resistance to cisplatin. Increased excision of the adducts from DNA by repair pathways or increased lesion bypass can also result in resistance. Finally, altered expression of regulatory proteins involved in signal transduction pathways that control the apoptotic pathway can also affect sensitivity to the drug. An improved understanding of the mechanisms of resistance operative in vivo has identified targets for intervention and may increase the utility of cisplatin for the treatment of cancer.

Prognostic role of K-ras in patients with progressive colon cancer who received treatment with Marimastat (BB2516)

We determined the prognostic role of K-ras mutation in tumor tissue of patients with refractory colon cancer who received Marimastat (BB2516). DNA was extracted from paraffin-stored tumor tissue of 27 patients who previously failed 5-fluorouracil and were treated with BB2516. The presence of K-ras mutation was characterized by Polymerase Chain Reaction using ras- and p53-specific primers. ras and p53 oncoprotein expression was analyzed by an automated biotin-avidin immunoproxidase technique. Seventeen patients had a normal K-ras sequence and 10 patients had a K-ras mutation. Median survival of patients with a normal ras sequence was 330 days from the time of BB2516 treatment compared with 160 days for patients with a K-ras mutation (p = 0.0442, Wilcoxon; 0.0130 Log-Rank). No differences in age, sex, cancer stage, surgical treatment, or chemotherapy treatment were observed. Abnormalities involving ras expression did not affect survival. By comparison, median survival for patients with p53 mutation or p53 overexpression was both 158 days after BB2516 treatment. Patients having both K-ras and p53 mutations had the poorest median survival of 113 days (p = 0.035). There is a suggestion by univariate analysis that the presence of a K-ras mutation may predict survival in patients with progressive colon cancer. Further assessment with larger patient numbers and multivariate analysis is indicated.

The efficiency and fidelity of translesion synthesis past cisplatin and oxaliplatin GpG adducts by human DNA polymerase beta

DNA polymerase beta (pol beta) is the only mammalian DNA polymerase identified to date that can catalyze extensive bypass of platinum-DNA adducts in vitro. Previous studies suggest that DNA synthesis by pol beta is distributive on primed single-stranded DNA and processive on gapped DNA. The data presented in this paper provide an analysis of translesion synthesis past cisplatin- and oxaliplatin-DNA adducts by pol beta functioning in both distributive and processive modes using primer extension and steady-state kinetic experiments. Translesion synthesis past Pt-DNA adducts was greater with gapped DNA templates than with single-stranded DNA templates. In the processive mode pol beta did not discriminate between cisplatin and oxaliplatin adducts, while in the distributive mode it displayed about 2-fold increased ability for translesion synthesis past oxaliplatin compared with cisplatin adducts. The differentiation between cisplatin and oxaliplatin adducts resulted from a K(m)-mediated increase in the efficiency of dCTP incorporation across from the 3'-G of oxaliplatin-GG adducts. Rates of misincorporation across platinated guanines determined by the steady-state kinetic assay were higher in reactions with primed single-stranded templates than with gapped DNA and a slight increase in the misincorporation of dTTP across from the 3'-G was found for oxaliplatin compared with cisplatin adducts.

Genomic imbalances associated with acquired resistance to platinum analogues

During the past several years, a panel of human tumor cell lines (predominantly ovarian) with acquired resistance to cisplatin, the orally bioavailable analogue JM216, and the structurally hindered analogue AMD473, has been established and characterized for underlying mechanisms of resistance. We have examined these resistant cell lines for gains and losses of DNA associated with the acquisition of resistance using the molecular cytogenetic technique of comparative genomic hybridization. Our comparison of three analogues has shown the most frequently observed changes to include amplification of 4q (5/7) and 6q (5/7), followed by amplification of 5q (3/7). We have defined four minimal common overrepresented regions, two each on 4q and 6q, which are potential loci of genes associated with platinum analogue resistance. Additional consistent abnormalities appear to be associated with cell lines sharing specific resistance mechanisms. For example, amplification of 12q was observed in the CH1 lines made respectively resistant to JM216 and AMD473 in which increased DNA repair appears to be a major mechanism of resistance for both agents. Hence, these comparative genomic hybridization studies have identified distinct chromosomal aberrations which may correlate with defined mechanisms of resistance and contain hitherto unrecognized genes that may provide targets for future therapeutic intervention.

Cisplatin induction of ERCC-1 mRNA expression in A2780/CP70 human ovarian cancer cells

ERCC-1 is a critical gene within the nucleotide excision repair pathway, and cells without a functional ERCC-1 do not perform cisplatin-DNA adduct repair. We therefore investigated the cisplatin effect on ERCC-1 mRNA expression in vitro. In response to a 1-h cisplatin exposure, A2780/CP70 human ovarian cancer cells showed a 6-fold increase in steady-state level of ERCC-1 mRNA. This rise was attributable to increased transcription as measured by nuclear run-on assays and a 60% increase in ERCC-1 mRNA half-life. The increase in ERCC-1 mRNA was preceded by a 4-5-fold rise in mRNA expressions of c-fos and c-jun, a 14-fold increase in c-Jun protein phosphorylation, and an increase in in vitro nuclear extract binding activity to the AP-1-like site of ERCC-1. These data suggest that the induction of ERCC-1 expression in A2780/CP70 cells exposed to cisplatin results from two major factors: (a) an increase in the expression of transactivating factors that bind the AP-1-like site in the 5'-flanking region of ERCC-1 and (b) an increase in the level of c-Jun phosphorylation that enhances its transactivation property.

Tumour cells surviving in vivo cisplatin chemotherapy display elevated c-myc expression

The c-myc oncogene has been extensively implicated in cell proliferation, cell differentiation and programmed cell death. Aberrant expression of the c-myc gene product has been observed in a range of tumours and has also been implicated in cisplatin (cis-dichlorodiammineplatinum)-mediated chemoresistance. A solid transplantable tumour model in syngeneic DA rats was subjected to treatment with cisplatin to determine the impact of such therapy on endogenous c-myc gene expression. Serially transplanted tumours were intravenously treated with a single cisplatin dose (1 mg/kg) and c-myc expression analysed 2 and 7 days after treatment. The surviving tumour cells display a significant 2-fold elevation in c-myc expression at 48 h and 7 days after treatment. Primary cell cultures have been derived from untreated in vivo tumours of the same model and subjected to treatment with a c-myc phosphorothioate antisense oligomer. Administration of 5 microM c-myc antisense oligomer directed at the initiation codon and first four codons of c-myc mRNA results in total inhibition of c-myc expression and coincident suspension of cell growth for a period of 4 days in culture. Antisense therapies directed at the c-myc gene may well prove an effective tool for treating tumours in conjunction with cisplatin as these findings show that tumour cells surviving cisplatin chemotherapy display elevated c-myc expression.

Expression of apoptosis-related oncoproteins and modulation of apoptosis by caffeine in human leukemic cells

We investigated the modulation of radio- and chemoresistance by caffeine and mechanisms of resistance in human leukemic cell lines and mononuclear cells from 18 leukemic patients. Caffeine synergistically potentiated cytotoxicity and apoptosis induced by ionizing radiation or carboplatin (CPt), but attenuated induction of apoptosis by daunorubicin (DNR) in KG-1a cells. Since caffeine released irradiated as well as DNR-treated KG-1a cells from G2M cell cycle arrest and CPt-treated cells from S-phase arrest, this release does not fully explain the different effects of caffeine. Caffeine synergistically reduced the level of the apoptosis inhibitor glutathione after irradiation or CPt treatment. In contrast, treatment with DNR plus caffeine diminished glutathione levels to a lesser extent than DNR alone. We conclude that the effect of caffeine on glutathione depletion represents a mechanism of action by which caffeine can modulate apoptosis. Caffeine increased CPt cytotoxicity in K562 cells and its doxorubicin-resistant subline (K562/ADM), but little effect was seen in HL-60 cells or mononuclear cells from leukemic patients. Multivariate cluster analysis revealed an association of CPt resistance with the expression of c-Fos, c-N-Ras, and p53 oncoproteins and with proliferative activity (S-phase of cell cycle), but not with Bcl-2 expression.

BCNU-resistant human glioma cells with over-representation of chromosomes 7 and 22 demonstrate increased copy number and expression of platelet-derived growth factor genes

We used standard karyotypic analyses of first-division cells to identify a subpopulation of cells in primary malignant gliomas with over-representation of chromosomes 7 and 22. These cells are a minor subpopulation in the primary tumor but become the dominant population after treatment in vitro of the cells with the chemotherapeutic agent 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). The selection for a cell with this specific karyotypic abnormality suggests that these chromosomes contain genes important to the growth of BCNU-resistant cells. Southern blot hybridization analyses demonstrate an increased copy number of the genes encoding platelet-derived growth factor (PDGF) A-chain and B-chain, which have been mapped to chromosomes 7 and 22, respectively. Reverse transcription followed by polymerase chain reaction (RT-PCR) analysis demonstrates increased expression of these genes. In addition, these cells secrete a mitogenic factor that stimulates 3H-thymidine uptake in NIH 3T3 cells. This factor is sensitive to anti-PDGF antibodies and beta-mercaptoethanol, but not to anti-EGF antibodies. These data suggest that autocrine and/or paracrine mechanisms occur in human malignant gliomas, and that over-expression of PDGF may play a role in the growth of BCNU-resistant cells in these tumors.

The effect of retinoic acid on chemosensitivity of PA-1 human teratocarcinoma cells and its modulation by an activated N-ras oncogene

Combination of chemotherapeutic drugs with agents that induce cell differentiation is a possible means of improving cancer chemotherapy. To explore this approach we used 4 cell lines established from the human teratocarcinoma-derived cell line PA-1; 2 retinoic acid (RA)-sensitive lines compared to 2 RA-resistant lines transformed by an activated N-ras oncogene. Equal numbers of colony-forming cells were exposed for 72 hr to 10(-6)M RA and subsequently to a range of concentrations of cisplatinum, etoposide or bleomycin. Enhanced cytotoxicity of cisplatin and etoposide (3- to 5-fold) was observed in the N-ras-transformed cell lines compared to the non-transformed lines. Treatment with RA caused an increase in the cytotoxicity of all 3 drugs to the 2 RA-sensitive cell lines. In contrast, a reduction of cytotoxicity was observed in the 2 N-ras-transformed lines. Our results indicate that sensitivity to cytotoxic agents can be increased by RA in RA-sensitive cells, but the opposite effect is seen in N-ras transformed, RA-resistant cells. Therefore, a general rationale for combination therapy with RA and cytotoxic drugs cannot be inferred.

Monofunctional DNA-platinum(II) adducts block frequently DNA polymerases

The question of whether monofunctional DNA platinum(II) adducts block synthesis of DNA by purified DNA polymerases of different types and origin has been investigated by comparing the time dependence of synthesis arrest and of DNA adduct formation. Activated salmon testis DNA is used as a suitable substrate for DNA synthesis allowing to probe inhibition by platinum(II) monoadducts for the variety of inherent template-primers. Reaction amplitudes are related to defined mixtures of dichloro and chloroaqua platinum(II) complexes. It is found that (i) all investigated DNA polymerases seem arrested (100% efficiency) at bifunctional DNA adducts. (ii) human DNA polymerase beta bypasses most of the monofunctional lesions of the three platinum(II) complexes investigated. (iii) Klenow fragment is blocked by monoadducts with increasing efficiency in the order cis-diamminechloroaquaplatinum(II) (0%) less than meso-[1,2-bis(2,6- dichloro-4-hydroxyphenyl)ethylenediamine] chloroaquaplatinum(II) (50%) less than trans-diamminechloro-aquaplatinum(II) (75%). (iv) Escherichia coli DNA polymerase I, Thermus aquaticus DNA polymerase, Physarum polycephalum DNA polymerase alpha, and calf thymus DNA polymerase alpha appear to be arrested by monoadducts. According to these examples, blocking efficiencies depend on the cis/trans-stereogeometry of fixation of the carrier ligands at platinum(II) residues, on the size/chemical nature of the platin(II) carrier ligand and on the type/origin of DNA polymerase.

The utility of an anti-fos ribozyme in reversing cisplatin resistance in human carcinomas

The results presented here demonstrate that expression of a fos ribozyme limits Fos protein synthesis and enhances sensitivity of A2780DDP cells to antineoplastic agents, including cisplatin. Moreover, the reversal of this resistance is associated with down-regulation of dTMP synthase, DNA polymerase beta, topoisomerase I and hMTII-A, genes previously linked to DNA synthesis and repair. Thus these studies further implicate the role of the c-fos gene in DNA synthesis through modulation of expression of dTMP syntase, DNA polymerase beta and topoisomerase I. Finally, the use of ribozymes to circumvent drug resistance suggests their potential utility as agents to inhibit tumor cell growth.

Ribozyme-mediated cleavage of c-fos mRNA reduces gene expression of DNA synthesis enzymes and metallothionein

The c-fos gene product Fos has been implicated in many cellular processes, including signal transduction, DNA synthesis, and resistance to antineoplastic agents. A fos ribozyme (catalytic RNA) was designed to evaluate the effects of suppressing Fos protein synthesis on expression of enzymes involved in DNA synthesis, DNA repair, and drug resistance. DNA encoding the fos ribozyme (fosRb) was cloned into the pMAMneo expression plasmid, and the resultant vector was transfected into A2780DDP cells resistant to the chemotherapeutic agent cisplatin. The parental drug-sensitive A2780S cells were transfected with the pMMV vector containing the c-fos gene. Morphological alterations were accompanied by significant changes in pharmacological sensitivity in both c-fos- and fosRb-transfected cells. pMAMneo fosRb transfectants revealed decreased c-fos gene expression, concomitant with reduced thymidylate (dTMP) synthase, DNA polymerase beta, topoisomerase I, and metallothionein IIA mRNAs. In contrast, c-myc expression was elevated after fos ribozyme action. Insertion of a mutant ribozyme, mainly capable of antisense activity, into A2780DDP cells resulted in smaller reductions in c-fos gene expression and in cisplatin resistance than the active ribozyme. These studies establish a role for c-fos in drug resistance and in mediating DNA synthesis and repair processes by modulating expression of genes such as dTMP synthase, DNA polymerase beta, and topoisomerase I. These studies also suggest the utility of ribozymes in the analysis of cellular gene expression.

Cisplatin resistance in human cancers

Cancer chemotherapeutic agents primarily act by damaging cellular DNA directly or indirectly. Tumor cells, in contrast to normal cells, respond to cisplatin with transient gene expression to protect and/or repair their chromosomes. Repeated cisplatin treatments results in a stable resistant cell line with enhanced gene expression but lacking gene amplification for the proteins that will limit cisplatin cytotoxicity. Recently, several new human cell lines have been characterized for cisplatin resistance. These cell lines have led to a better understanding of the molecular and biochemical basis of cisplatin resistance. The c-fos proto-oncogene, a master switch for turning on other genes in response to a wide range of stimuli, has been shown to play an important role in cisplatin resistance both in vitro and in patients. Based on these studies, new strategies have been developed to circumvent and/or exploit clinical cisplatin resistance.

Cyclosporin A suppresses cisplatin-induced oncogene expression in human cancer cells

Most cancer chemotherapeutic agents are designed to damage DNA directly or indirectly. One mechanism of cellular resistance to these agents is enhanced DNA repair. Human ovarian carcinoma cells resistant to cisplatin (A2780DDP) respond to cisplatin treatment with a 3-6-fold increase in gene expression for oncogenes, DNA repair enzymes and enzymes necessary for the synthesis of thymidine. Cyclosporin A has been shown to reverse drug resistance, but its mechanism of action is unclear. In this study, weekly exposures of A2780DDP cells to cyclosporin A resulted in the evolution of a revertant cell line A2780DDP/CSA that was sensitive to cisplatin again and suppressed the induction of genes necessary for the repair of drug-induced DNA damage.