In vitro investigations on induction and reversal of cisplatin resistance in a rat ovarian tumor cell line

Revisiting the BRCA-pathway through the lens of replication gap suppression: "Gaps determine therapy response in BRCA mutant cancer"

The toxic lesion emanating from chemotherapy that targets the DNA was initially debated, but eventually the DNA double strand break (DSB) ultimately prevailed. The reasoning was in part based on the perception that repairing a fractured chromosome necessitated intricate processing or condemned the cell to death. Genetic evidence for the DSB model was also provided by the extreme sensitivity of cells that were deficient in DSB repair. In particular, sensitivity characterized cells harboring mutations in the hereditary breast/ovarian cancer genes, BRCA1 or BRCA2, that function in the repair of DSBs by homologous recombination (HR). Along with functions in HR, BRCA proteins were found to prevent DSBs by protecting stalled replication forks from nuclease degradation. Coming full-circle, BRCA mutant cancer cells that gained resistance to genotoxic chemotherapy often displayed restored DNA repair by HR and/or restored fork protection (FP) implicating that the therapy was tolerated when DSB repair was intact or DSBs were prevented. Despite this well-supported paradigm that has been the impetus for targeted cancer therapy, here we argue that the toxic DNA lesion conferring response is instead single stranded DNA (ssDNA) gaps. We discuss the evidence that persistent ssDNA gaps formed in the wake of DNA replication rather than DSBs are responsible for cell killing following treatment with genotoxic chemotherapeutic agents. We also highlight that proteins, such as BRCA1, BRCA2, and RAD51 known for canonical DSB repair also have critical roles in normal replication as well as replication gap suppression (RGS) and repair. We review the literature that supports the idea that widespread gap induction proximal to treatment triggers apoptosis in a process that does not need or stem from DSB induction. Lastly, we discuss the clinical evidence for gaps and how to exploit them to enhance genotoxic chemotherapy response.

Anticancer Alkaloids from Trees: Development into Drugs

Trees have made an enormous phytochemical contribution in anticancer drugs' development more than any other life form. The contributions include alkaloids that are biosynthesized in various ways and yield. Lead alkaloids isolated from the trees are taxol and camptothecins that currently have annual sales in billion dollars. Other important alkaloids isolated from these life forms include rohitukine, harringtonine, acronycine, thalicarpine, usambarensine, ellipticine, and matrines. Studies on their mechanism of action and target on the DNA and protein of cancerous cells aided the development of potent hemisynthesized congeners. The molecules and their congeners passed/are passing a long period of historical development before approved as antineoplastic drugs for cancer chemotherapy. Some of them did not find the application as anticancer drugs due to ineffectiveness in clinical trials; others are generating research interest in the antineoplastic activity at the present and have reached clinical trial stages. Potentials in antineoplastic molecules from trees are high and are hoped to be commensurate with cancer types afflicting human society in the future.

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.

An active efflux system for heavy metals in cisplatin-resistant human KB carcinoma cells

The mechanism for cisplatin resistance in cisplatin-resistant KCP-4 cells was studied. Although multidrug resistance-associated protein (MRP) was not detected in KCP-4 cells, the cells were more resistant to heavy metals than multidrug-resistant C-A120 cells that overexpressed MRP. KCP-4 cells expressed metallothionein, but it was scarcely involved in cisplatin resistance in these cells. KCP-4 cells did not express canalicular multispecific organic anion transporter (cMOAT). The glutathione (GSH) level was 4.7-fold higher in KCP-4 cells than in KB-3-1 cells. When the GSH level in KCP-4 cells was decreased by treating the cells with buthionine sulfoximine and nitrofurantoin, the accumulation of and sensitivity to cisplatin in the cells were increased. C-A120 cells were only 3.0-fold more resistant to cisplatin than KB-3-1 cells and this resistance was not affected by the increased glutathione level. The accumulation of platinum in C-A120 and KCP-4 cells was 68.5 and 20.4% of that in KB-3-1 cells, respectively, while the intracellular levels of antimony potassium tartrate in C-A120 and KCP-4 cells were 13.2 and 9.9% of that in KB-3-1 cells, respectively. The ATP-dependent efflux of antimony was enhanced in both C-A120 and KCP-4 cells. These results, taken together, suggest an efflux pump for heavy metals different from MRP and cMOAT is involved in cisplatin resistance in KCP-4 cells.

Altered expression of gamma-glutamylcysteine synthetase, metallothionein and topoisomerase I or II during acquisition of drug resistance to cisplatin in human ovarian cancer cells

This study was designed to elucidate the mechanisms of cisplatin (CDDP) resistance using two human ovarian cancer cell lines, KF and TYK, and two CDDP-resistant lines, KFr and TYK/R, derived from the former lines. KFr and TYK/R showed about 3-fold higher resistance to the cytotoxic effects of CDDP than their parental lines. They also showed a significant increase in sensitivity to not only etoposide, but also (+)-(4S)-4, 11-diethyl-4-hydroxy-9-[(4-piperidino -piperidino)carbonyloxy]-1H -pyrano[3',4':6,7]inodolizino[1,2-b]quinoline-3,14(4H, 12H)-dione hydrochloride trihydrate (CPT-11). Cellular CDDP accumulation levels in KFr and TYK/R were decreased from those of the parental cells. By contrast, the cellular glutathione (GSH) content in KFr cells was 1.7-fold higher than that in KF, whereas TYK/R cells had a 40% lower content than TYK cells. Cellular mRNA levels of drug-resistance-related genes, such as DNA topoisomerase (topo) I and topo II, glutathione S-transferase-pi (GST-pi), gamma-glutamylcysteine synthetase (gamma-GCS), and metallothionein (hMT) genes, were compared between drug-sensitive KF or TYK and KFr or TYK/R. KFr cells had 8.5- and 24.7-fold higher mRNA levels of gamma-GCS and topo II genes than KF cells while KFr had only a slight increase in GST-pi mRNA level as compared with KF. By contrast, TYK/R cells had 2.9- and 1.7-fold higher hMT and topo I mRNA levels than TYK cells. Acquisition of CDDP resistance in human ovarian cancer cells thus appeared to be related mainly to expression of gamma-GCS, topo II and hMT genes, and partly to that of topo I and GST-pi genes, in addition to a decrease in CDDP accumulation.

Cell cycle effects of thaliblastine

The non-myelotoxic antitumor drug thaliblastine (thalicarpine, NSC-68075, CAS-5373-42-21) has a novel chemical structure; it is a complex dimeric-type aporphine benzylisoquinoline alkaloid possessing antiproliferative and antitumor activities in experimental and clinical studies. In this study the effect of this drug on the cell cycle progression of ovarian tumor line O-342 and its cisplatin-resistant subline O-342/DDP was evaluated. As assessed by flow cytometric analysis, thaliblastine affected the cell cycle progression. In both lines, a comparable pattern of cell cycle arrest was found. Within the first 5 h of thaliblastine exposure, a G2/M block was observed; thereafter cell-cycle arrest in G1 became prominent, while S-phase cells finished DNA replication.

Positive correlation between cellular glutathione and acquired cisplatin resistance in human ovarian cancer cells

While multiple changes are frequently found to be associated with cisplatin resistance in a variety of tumor cell lines, a cause-effect relationship of these alterations with the resistant phenotype has not been established. In order to identify the resistance-relevant determinants, a series of cisplatin-resistant sublines with different degrees of resistance to cisplatin was developed in a human ovarian carcinoma cell line (O-129). Three derived resistant cell lines displayed 2.1-fold (O-129/DDP4, low), 4.1-fold (O-129/DDP8, moderate) and 6.3-fold (O-129/DDP16, high) resistance, respectively, to cisplatin, compared with the sensitive parental line O-129. While the activity of poly(ADP-ribose) polymerase, an enzyme proposed to be involved in DNA repair, was elevated in all three resistant lines, a significant karyotypic change was observed only in the high-resistance line with the karyotype alteration from near diploidy to heteroploidy. The moderate (4.1-fold) and high (6.3-fold) DDP resistance was associated with a slow proliferation rate in drug-free medium, but cellular glutathione level was highly correlated with DDP sensitivity in all four cell lines. Taken together, the present studies establish that while many changes at cellular level can occur with development of cisplatin resistance, only elevation of intracellular glutathione concentration appears to be related to the resistance phenotype in these human ovarian cancer cells.

Role of cellular glutathione and glutathione S-transferase in the expression of alkylating agent cytotoxicity in human breast cancer cells

Glutathione (GSH) and glutathione S-transferases (GSTs) play an important role in the protection of cells against toxic effects of many electrophilic drugs and chemicals. Modulation of cellular GSH and/or GST activity levels provides a potentially useful approach to sensitizing tumor cells to electrophilic anti-cancer drugs. In this study, we describe the interactions of four representative alkylating agents (AAs), melphalan, 4-hydroperoxy-cyclophosphamide (4HC), an an activated form of cyclophosphamide, 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), and cisplatin, with GSH and GST in the human breast cancer cell line MCF-7. Depletion of cellular GSH pools by approximately 80% by treatment of the cells with the GSH synthesis inhibitor buthionine sulfoximine (BSO) sensitized the tumor cells to each AA to a different extent, with dose-modifying factors of 2.39, 2.21, 1.64, and 1.27 observed for melphalan, 4HC, cisplatin, and BCNU, respectively. Treatment of the cells with the GST inhibitor ethacrynic acid (EA) failed to show any significant effects on the cytotoxicity of these AAs. However, EA did potentiate the cytotoxicity of melphalan when given in combination with BSO, an effect that may be due to a more complete depletion of cellular GSH levels by the combined modulator treatment. Following a 1-hr exposure to cytotoxic-equivalent concentrations of these AAs, GSH levels decreased substantially in the case of 4HC and BCNU, but increased by 30-50% in the case of cisplatin and melphalan. BSO pretreatment largely blocked this effect of cisplatin and melphalan on cellular GSH, while it further enhanced the GSH-depleting activity of both 4HC and BCNU. On the basis of these results, it is concluded that (a) GSH affects the cytotoxicity of different AAs to different extents, (b) basal GST expression in MCF-7 cells does not play a major role in AA metabolism, (c) EA can potentiate the enhancing effect of BSO on melphalan cytotoxicity in MCF-7 cells, and (d) depletion of cellular GSH by pretreatment with BCNU or cyclophosphamide may correspond to a useful strategy for enhancing the anti-tumor activity of other AAs given in a sequential combination.

Reversal of acquired cisplatin resistance by nicotinamide in vitro and in vivo

At a concentration of 2.5 mM, nicotinamide (NA), an inhibitor of poly(ADP-ribose) polymerase (PARP), significantly potentiated the cytotoxicity of cisplatin (DDP) in a DDP-resistant rat ovarian tumor cell line (O-342/DDP) in vitro, whereas the same treatment had no substantial effect on DDP's cytotoxic activity against the DDP-sensitive parental line (O-342). Furthermore, in a nude mouse model where the O-342/DDP tumor grew intraperitoneally, whereas DDP given alone at 1 mg/kg x 3 exhibited no antitumor activity as compared with control values due to the resistance, NA given at a nontoxic dose (5 mmol/kg x3) significantly increased the mean survival time (MST) of the tumor-bearing NMRI nude mice from 20.7 days in the DDP-treated group to 29.0 days in the combination group. Mechanism studies showed that endogenous PARP activity (incorporation of tritiated nicotinamide adenine dinucleotide, [3H]-NAD) was 2.6 times higher in O-342/DDP than in O-342 cells and that the presence of 2.5 mM NA during the incubation with the isotope resulted in 73.3% inhibition of the enzyme activity in O-342/DDP cells but in only about 30% inhibition in the sensitive line. However, treatment with NA during and after DDP exposure failed to produce any significant effect on the formation of DNA single-strand breaks (SSB) but decreased the induction of DNA interstrand cross-links (ISCL) by DDP in the sensitive and resistant cell lines. These results suggest that NA might have some clinical potential in reversing DDP resistance, and further studies are therefore warranted to confirm the resistance-reversing effect of NA in other DDP-resistant cell lines.

Multiple effects of 3-aminobenzamide on DNA damage induced by cisplatin (DDP) in DDP-sensitive and -resistant rat ovarian tumor cell lines

To investigate the mechanisms by which 3-aminobenzamide (3AB) reverses cisplatin (DDP) resistance in a rat ovarian tumor cell line, the effects of 3AB on DDP-induced DNA damage and repair were kinetically determined over a post-exposure period of 48 h. DNA single strand breaks (SSB) occurred maximally 12 h and 24 h following DDP exposure in DDP-resistant (O-342/DDP) and -sensitive (O-342) rat ovarian tumor cells, respectively. 3AB, present during and after the exposure, significantly increased SSB formation by DDP at 24 h (P < 0.02) and 48 h (P < 0.01) in O-342/DDP cells. To a lesser extent (P > 0.05), a similar tendency was also observed in O-342 cells. Formation of DNA interstrand cross-links (ISCL) by DDP reached a maximum by 12 h in either O-342 or O-342/DDP cells, but in the resistant cells they were both much lower and more rapidly removed. 3AB decreased ISCL in the sensitive cells at 12 h and thereafter with a maximum at 24 h (P < 0.05), while in the resistant cells the same treatment decreased ISCL at 12 h, had no effect at 24 h and increased ISCL at 48 h following DDP treatment. Therefore, it is concluded that 3AB has multiple effects on DNA damage and repair induced by DDP in both cell lines and increase of DNA-ISCL by 3AB at 48 h after the exposure in O-342/DDP cells might be related to its chemosensitizing effect in this line.

Modulation of cis-diamminedichloroplatinum(II) resistance: a review

In this review an inventory is made of agents used to circumvent cis-diamminedichloroplatinum(II) (CDDP) resistance in vitro and in vivo. Agents that affect CDDP accumulation and membrane related systems, cytoplasmic defense mechanisms, as well as DNA accessibility and repair are reviewed. In resistant cell lines that have decreased accumulation, this can be restored by hyperthermic treatment. With or without effects on accumulation compounds that affect cell signal transduction often increase CDDP cytotoxicity. Calcium channel blockers and calmodulin inhibitors do not seem to be uniformly good modulators of CDDP resistance. For transduction modulators as well as cellular calcium affecting agents mechanisms are mainly unclear or controversial. Glutathione appears, with the now available agents, to be the most promising target for modulation of cytoplasmic defense mechanisms. At the nuclear level the inhibition of DNA repair related enzymes as well as the use of modified nucleosides to interfere with repair is studied in various cell lines. Results with these agents suggest opportunities for clinically feasible cytotoxicity modulation. DNA accessibility could in vitro be affected, but seems to be an unreliable target for modulation. Whenever possible the resistance mechanism affected and the mode of action of the modulator are discussed. As an alternative for modulation another method of overcoming CDDP resistance namely the application of CDDP analogues is considered.

Relationships between the aqueous chemistry and the in vitro cytotoxic activities of mixed-amine cisplatin analogues

The possibility that variations in the cytotoxic activities of cisplatin analogues could be a result of differences in the aqueous chemistry of the compounds was investigated. A series of structurally related mixed-amine dichloroplatinum complexes (cis-coordinated with amine and various diphenylmethylamines and 1,2-diphenylethylamines) was prepared and selected physicochemical properties of the new compounds were characterized. Cytotoxicity was determined in two human breast cancer cell lines (MDA-MB-231 and MCF-7) and one human ovarian cancer cell line (SK-OV-3) by means of a microtiter assay. There is no apparent relationship between the hydrophobicities of the compounds and their cytotoxic potencies. There is no evidence for an inverse relationship between the aqueous stability of the dichloroplatinum complexes and cytotoxic potency, as has been reported for nitrogen mustards and some nitrosoureas. The differences in cytotoxic activity cannot be explained by inter-compound variations in the area under the concentration-time curves (AUC) of the dichloroplatinum complexes in culture medium. Thus, it appears that the differences in the cytotoxic potencies of this series of cisplatin analogues are related to factors other than dissimilarities in these physiochemical properties. Nevertheless, a relationship was found between the AUC of a dichloroplatinum complex in medium and the efficacy of the compound in the MCF-7 cell line. However, the AUC-efficacy relationship does not always hold in the MDA-MB-231 and SK-OV-3 cell lines. In these cells, treatment with a "high" bolus dose of platinum complex over finite exposure times is often less cytotoxic than treatment with lower doses of the same compound but over a continuous exposure time, although the cells are subjected to the same AUC of dichloroplatinum complex.

Antitumor activity of thaliblastine (NSC-68075) in experimental ovarian tumor cell lines sensitive and resistant to cisplatin

Cytotoxicity of thaliblastine (thalicarpine, TBL; NSC-68075) and/or cisplatin (DDP) in DDP-sensitive (O-342) and-resistant (O-342/DDP) rat ovarian tumor cell lines was comparatively determined using the MTT assay. The 50% inhibitory dose (ID50) of DDP was found to be 6.2 microM in O-342 cells and 23.4 microM in O-342/DDP cells, while, vice versa, the ID50 of TBL was 39.3 micrograms/ml in the sensitive line and 27.3 micrograms/ml in the resistant line. Furthermore, simultaneous exposure of cells to DDP and TBL showed a significant superiority over DDP alone in O-342 cells, as evaluated with variance analysis (P less than 0.001). This enhancing effect of TBL on DDP cytotoxicity, however, was not observed in the resistant cells.

Karyotypic change from heteroploidy to near diploidy associated with development of cisplatin resistance in a rat ovarian tumour cell line

In a rat ovarian tumour cell line a 33-fold resistance to cisplatin (O-342/DDP) was developed in vitro by continuous exposure of the parental cell line (O-342) to stepwise increase cisplatin concentration in the culture medium. Both cell lines had a similar growth rate in vitro. Development of resistance was accompanied by a change of the karyotype from heteroploidy in chemosensitive O-342 cells to near diploidy in resistant O-342/DDP cells as shown by chromosome number distribution. This finding was confirmed by measuring cellular DNA content using flow-cytometry analysis. Flow karyotyping showed significant differences in chromosomal DNA contents between both cell lines. Our results suggest that the parent line O-342 consists of at least two subpopulations, a cisplatin-sensitive and a cisplatin-resistant one, corresponding to hyperploidy and near diploidy, respectively. Continuous cisplatin exposure of O-342 cells selectively killed the sensitive fraction, resulting in the karyotypic change observed.