Cisplatin resistance in human cancers

Cisplatin Resistance and Metabolism: Simplification of Complexity

Cisplatin is one of the most well-known anti-cancer drugs and has demonstrated efficacy against numerous tumor types for many decades. However, a key challenge with cisplatin, as with any chemotherapeutic agent, is the development of resistance with a resultant loss of efficacy. This resistance is often associated with metabolic alterations that allow insensitive cells to divide and survive under treatment. These adaptations could vary greatly among different tumor types and may seem questionable and incomprehensible at first glance. Here we discuss the disturbances in glucose, lipid, and amino acid metabolism in cisplatin-resistant cells as well as the roles of ferroptosis and autophagy in acquiring this type of drug intolerance.

Novel Silver Complexes Based on Phosphanes and Ester Derivatives of Bis(pyrazol-1-yl)acetate Ligands Targeting TrxR: New Promising Chemotherapeutic Tools Relevant to SCLC Management

Bis(pyrazol-1-yl)acetic acid (HC(pz)₂COOH) and bis(3,5-dimethyl-pyrazol-1-yl)acetic acid (HC(pz^Me2)₂COOH) were converted into the methyl ester derivatives 1 (L^OMe) and 2 (L^2OMe), respectively, and were used for the preparation of silver(I) complexes 3-5. The Ag(I) complexes were prepared by the reaction of AgNO₃ and 1,3,5-triaza-7-phosphaadamantane (PTA) or triphenylphosphine (PPh₃) with L^OMe and L^2OMe in methanol solution. All Ag(I) complexes showed a significant in vitro antitumor activity, proving to be more effective than the reference drug cisplatin in the in-house human cancer cell line panel containing examples of different solid tumors. Compounds were particularly effective against the highly aggressive and intrinsically resistant human small-cell lung carcinoma (SCLC) cells, either in 2D and 3D cancer cell models. Mechanistic studies revealed their ability to accumulate into cancer cells and to selectively target Thioredoxin (TrxR), thus leading to redox homeostasis unbalance and ultimately inducing cancer cell death through apoptosis.

Guedes da Silva MF, Marzano C. Synthesis, Characterization and Biological Activity of Novel Cu(II) Complexes of 6-Methyl-2-Oxo-1,2-Dihydroquinoline-3-Carbaldehyde-4n-Substituted Thiosemicarbazones

Three new 6-methyl-2-oxo-1,2-dihydroquinoline-3-carbaldehyde-thiosemicarbazones-N-4-substituted pro-ligands and their Cu(II) complexes (1, -NH2; 2, -NHMe; 3, -NHEt) have been prepared and characterized. In both the X-ray structures of 1 and 3, two crystallographically independent complex molecules were found that differ either in the nature of weakly metal-binding species (water in 1a and nitrate in 1b) or in the co-ligand (water in 3a and methanol in 3b). Electron Paramagnetic Resonance (EPR) measurements carried out on complexes 1 and 3 confirmed the presence of such different species in the solution. The electrochemical behavior of the pro-ligands and of the complexes was investigated, as well as their biological activity. Complexes 2 and 3 exhibited a high cytotoxicity against human tumor cells and 3D spheroids derived from solid tumors, related to the high cellular uptake. Complexes 2 and 3 also showed a high selectivity towards cancerous cell lines with respect to non-cancerous cell lines and were able to circumvent cisplatin resistance. Via the Transmission Electron Microscopy (TEM) imaging technique, preliminary insights into the biological activity of copper complexes were obtained.

Screening of Pleural Mesothelioma Cell Lines for Kinase Activity May Identify New Mechanisms of Therapy Resistance in Patients Receiving Platin-Based Chemotherapy

Background

Malignant pleural mesothelioma (MPM) is a rare, predominantly asbestos-related and biologically highly aggressive tumor associated with a dismal prognosis. Multimodal therapy consisting of platinum-based chemotherapy is the treatment of choice. The reasons underlying the rather poor efficacy of platinum compounds remain largely unknown. Kinase activity might influence cellular response to these regimens.

Materials and Methods

For this exploratory study, we screened MPM cell lines (NCI-H2452, NCI-H2052, and MSTO-211H) differing in response to cisplatin and benign control fibroblasts (MRC-5) for overall phosphorylation patterns as well as kinase activity with respect to cellular response to cisplatin-based therapeutics. We analysed the cell lines for cellular kinases in a high-throughput manner using the highly innovative technique PamGene. Cell state analysis including apoptosis, necrosis, and cell viability was performed by using enzyme activity and fluorescent-based assays.

Results

Cisplatin alters cellular phosphorylation patterns affecting cell cycle, migration, adhesion, signal transduction, immune modulation, and apoptosis. In cisplatin-responsive cell lines, phosphorylation of AKT1 and GSK3B was decreased but could not be influenced in cisplatin-resistant NCI-H2452 cells. Cisplatin-responsive cell lines showed increased phosphorylation levels of JNK1/2/3 but decreased phosphorylation in cisplatin-resistant NCI-H2452 cells.

Conclusion

Kinase phosphorylation and activity might play a crucial role in cellular response to cytostatic agents. Cisplatin influences phosphorylation patterns with distinct features in cisplatin-resistant cells. These alterations exert a significant impact on cell cycle, migration, adhesion, signal transduction, immune modulation, and apoptosis of the respective tumor cells. Based on our results, the induction of p38 or JNK1/3, or inhibition of AKT1 by, for example, BIA-6, might offer a positive synergistic effect by induction of an apoptotic response to cisplatin-based treatment, thus potentially enhancing the clinical outcome of MPM patients.

Randomized phase 2 study of gemcitabine and cisplatin with or without vitamin supplementation in patients with advanced esophagogastric cancer

Purpose

Preclinical research and prior clinical observations demonstrated reduced toxicity and suggested enhanced efficacy of cisplatin due to folic acid and vitamin B12 suppletion. In this randomized phase 2 trial, we evaluated the addition of folic acid and vitamin B12 to first-line palliative cisplatin and gemcitabine in patients with advanced esophagogastric cancer (AEGC).

Methods

Patients with AEGC were randomized to gemcitabine 1250 mg/m² (i.v. days 1, 8) and cisplatin 80 mg/m² (i.v. day 1) q 3 weeks with or without folic acid (450 µg/day p.o.) and vitamin B12 (1000 µg i.m. q 9 weeks). The primary endpoint was response rate (RR). Secondary endpoints included overall survival (OS), time to progression (TTP), toxicity, and exploratory biomarker analyses. Cisplatin sensitivity and intracellular platinum levels were determined in adenocarcinoma cell lines cultured under high and low folate conditions in vitro.

Results

Adenocarcinoma cells cultured in medium with high folate levels were more sensitive to cisplatin and this was associated with increased intracellular platinum levels. In the randomized phase 2 clinical trial, which ran from October 2004 to September 2013, treatment was initiated in 78 of 82 randomized pts, 39 in each study arm. The RR was similar; 42.1% for supplemented patients vs. 32.4% for unsupplemented patients; p = 0.4. Median OS and TTP were 10.0 and 5.9 months for supplemented vs. 7.7 and 5.4 months for unsupplemented patients (OS, p = 0.9; TTP, p = 0.9). Plasma homocysteine was lower in the supplemented group [n = 20, 6.9 ± 1.6 (mean ± standard error of mean, SEM) µM; vs. 12.5 ± 4.0 µM; p < 0.001]. There was no significant difference in the C_max of gemcitabine and cisplatin in the two treatment groups.

Conclusion

Folic acid and vitamin B12 supplementation do not improve the RR, PFS, or OS of cisplatin and gemcitabine in patients with AEGC.

Dual-acting antitumor Pt(iv) prodrugs of kiteplatin with dichloroacetate axial ligands

DNA and mitochondria of tumor cells are the targets of Pt(iv) complexes of kiteplatin with biologically active dichloroacetate as axial ligands.

An insight into the complex roles of metallothioneins in malignant diseases with emphasis on (sub)isoforms/isoforms and epigenetics phenomena

Metallothioneins (MTs) belong to a group of small cysteine-rich proteins that are ubiquitous throughout all kingdoms. The main function of MTs is scavenging of free radicals and detoxification and homeostating of heavy metals. In humans, 16 genes localized on chromosome 16 have been identified to encode four MT isoforms labelled by numbers (MT-1-MT-4). MT-2, MT-3 and MT-4 proteins are encoded by a single gene. MT-1 comprises many (sub)isoforms. The known active MT-1 genes are MT-1A, -1B, -1E, -1F, -1G, -1H, -1M and -1X. The rest of the MT-1 genes (MT-1C, -1D, -1I, -1J and -1L) are pseudogenes. The expression and localization of individual MT (sub)isoforms and pseudogenes vary at intra-cellular level and in individual tissues. Changes in MT expression are associated with the process of carcinogenesis of various types of human malignancies, or with a more aggressive phenotype and therapeutic resistance. Hence, MT (sub)isoform profiling status could be utilized for diagnostics and therapy of tumour diseases. This review aims on a comprehensive summary of methods for analysis of MTs at (sub)isoforms levels, their expression in single tumour diseases and strategies how this knowledge can be utilized in anticancer therapy.

Synthesis, characterization, and biological activity of platinum II, III, and IV pivaloamidine complexes

Imino ligands have proven to be able to activate the trans geometry of platinum(II) complexes towards antitumor activity. These ligands, like aromatic N-donor heterocycles, have a planar shape but, different from the latter, have still an H atom on the coordinating nitrogen which can be involved in H-bond formation. Three classes of imino ligands have been extensively investigated: iminoethers (HN=C(R)OR'), ketimines (HN=CRR'), and amidines (HN=C(R)NR'R″). The promising efficacy of the platinum compounds with amidines (activity comparable to that of cisplatin for cis complexes and much greater than that of transplatin for trans complexes) prompted us to extend the investigation to amidine complexes with a bulkier organic residue (R = t-Bu). The tert-butyl group can confer greater affinity for lipophilic environments, thus potentiating the cellular uptake of the compound. In the present study we describe the synthesis and characterization of pivaloamidine complexes of platinum(II), (cis and trans-[PtCl2(NH3){Z-HN=C(t-Bu)NH2}] and cis and trans-[PtCl2{Z-HN=C(t-Bu)NH2}2]), platinum(III) ([Pt2Cl4{HN=C(t-Bu)NH}2(NH3)2]), and platinum(IV) (trans-[PtCl4(NH3){Z-HN=C(t-Bu)NH2}] and trans-[PtCl4{Z-HN=C(t-Bu)NH2}2]). The cytotoxicity of all new Pt complexes was tested toward a panel of cultured cancer cell lines, including cisplatin and multidrug resistant variants. In addition, cellular uptake and DNA binding, perturbations of cell cycle progression, induction of apoptosis, and p53 activation were investigated for the most promising compound trans-[PtCl2(NH3){Z-HN=C(t-Bu)NH2}]. Remarkably, the latter complex was able to overcome both acquired and intrinsic cisplatin resistance.

Metallothioneins and zinc in cancer diagnosis and therapy

Metallothioneins (MTs) are involved in protection against oxidative stress (OS) and toxic metals and they participate in zinc metabolism and its homeostasis. Disturbing of zinc homeostasis can lead to formation of reactive oxygen species, which can result in OS causing alterations in immunity, aging, and civilization diseases, but also in cancer development. It is not surprising that altered zinc metabolism and expression of MTs are of great interest in the case of studying of oncogenesis and cancer prognosis. The role of MTs and zinc in cancer development is tightly connected, and the structure and function of MTs are strongly dependent on Zn²⁺ redox state and its binding to proteins. Antiapoptic effects of MTs and their interactions with proteins nuclear factor kappa B, protein kinase C, esophageal cancer-related gene, and p53 as well as the role of MTs in their proliferation, immunomodulation, enzyme activation, and interaction with nitric oxide are reviewed. Utilization of MTs in cancer diagnosis and therapy is summarized and their importance for chemoresistance is also mentioned.

The kinase RSK2 modulates the sensitivity of ovarian cancer cells to cisplatin

Platinum-based chemotherapy (e.g. cisplatin, carboplatin) is standard of care for many types of cancer including ovarian cancer, however, the efficacy of treatment is hampered by the development of therapy resistance. The mechanisms behind platinum resistance are not completely understood. Here, we have investigated the role of the family of p90 Ribosomal S6 kinases (RSK), important downstream mediators of ERK1/2, in the response to cisplatin chemotherapy. Strikingly, whereas treatment with cisplatin did not alter the levels of RSK1 in response to cisplatin treatment, the structurally related RSK2 protein was downregulated in an ovarian cancer cell line (A2780). Furthermore, we found that knockdown of RSK2, in contrast to knockdown of RSK1, gave rise to enhanced cisplatin sensitivity in a cisplatin sensitive as well as a cisplatin-resistant A2780 cell line. These results indicate that RSK2 is regulated in response to cisplatin treatment, and this downregulation may contribute to the cytotoxic action of cisplatin. Since RSK2 is frequently amplified in a growing number of cancers, this may have implications for the sensitivity of these tumours to platinum-based cytotoxics.

A new class of antitumor trans-amine-amidine-Pt(II) cationic complexes: influence of chemical structure and solvent on in vitro and in vivo tumor cell proliferation

The reactions of cyclopropylamine, cyclopentylamine, and cyclohexylamine with trans-[PtCl2(NCMe)2] afforded the bis-cationic complexes trans-[Pt(amine)2(Z-amidine)2]2+[Cl-]2, 1-3. The solution behavior and biological activity have been studied in different solvents (DMSO, water, polyethylene glycol (PEG 400), and polyethylene glycol dimethyl ether (PEG-DME 500)). The biological activity was strongly influenced by the cycloaliphatic amine ring size, with trans-[Pt(NH2CH(CH2)4CH2)2{N(H) horizontal lineC(CH3)N(H)CH(CH2)4CH2}2]2+[Cl-]2 (3) being the most active compound. Complex 3 overcame both cisplatin and MDR resistance, inducing cancer cell death through p53-mediated apoptosis. Alkaline single-cell gel electrophoresis experiments indicated direct DNA damage, reasonably attributable to DNA adducts of trans-[PtCl(amine)(Z-amidine)2][Cl] species, which can evolve to produce disruptive and nonrepairable lesions on DNA, thus leading to the drug-induced programmed cancer cell death. Preliminary in vivo antitumor studies on C57BL mice bearing Lewis lung carcinoma highlighted that complex 3 promoted a significant and dose-dependent tumor growth inhibition without adverse side effects.

Distinct mechanisms for Ctr1-mediated copper and cisplatin transport

The Ctr1 family of integral membrane proteins is necessary for high affinity copper uptake in eukaryotes. Ctr1 is also involved in cellular accumulation of cisplatin, a platinum-based anticancer drug. Although the physiological role of Ctr1 has been revealed, the mechanism of action of Ctr1 remains to be elucidated. To gain a better understanding of Ctr1-mediated copper and cisplatin transport, we have monitored molecular dynamics and transport activities of yeast Saccharomyces cerevisiae Ctr1 and its mutant alleles. Co-expression of functional Ctr1 monomers fused with either cyan or yellow fluorescent protein resulted in fluorescence resonance energy transfer (FRET), which is consistent with multimer assembly of Ctr1. Copper near the K(m) value of Ctr1 enhanced FRET in a manner that correlated with cellular copper transport. In vitro cross-linking of Ctr1 confirmed that copper-induced FRET reflects conformational changes within pre-existing Ctr1 complexes. FRET assays in membrane-disrupted cells and protein extracts showed that intact cell structure is necessary for Ctr1 activity. Despite Ctr1-dependent cellular accumulation, cisplatin did not change Ctr1 FRET nor did it attenuate copper-induced FRET. A Ctr1 allele defective in copper transport enhanced cellular cisplatin accumulation. N-terminal methionine-rich motifs that are dispensable for copper transport play a critical role for cisplatin uptake. Taken together, our data reveal functional roles for structural remodeling of the Ctr1 multimeric complex in copper transport and suggest distinct mechanisms employed by Ctr1 for copper and cisplatin transport.

Inhibition of thioredoxin reductase by auranofin induces apoptosis in cisplatin-resistant human ovarian cancer cells

Cisplatin is an effective antitumor agent for the treatment of several carcinomas. However, the development of resistance to cisplatin represents a serious clinical problem. The effects of auranofin, a gold(I) compound clinically used as an antirheumatic agent, on cisplatin-sensitive (2008) and-resistant (C13*) cancer cells were studied. Auranofin is more effective than cisplatin in decreasing cell viability and its action is particularly marked in C13* cells, indicating that no cross-resistance occurs. Furthermore, auranofin is able to permeate C13* cells more efficiently than 2008 cells. Treatment with auranofin determines a consistent release of cytochrome c in both cell lines, while cisplatin is effective only in sensitive cells. Both auranofin and cisplatin induce apoptosis in 2008 cells, while in C13* cells only auranofin is effective. Apoptosis is accompanied by an increased production of hydrogen peroxide that, however, is inhibited by N-acetyl-L-cysteine. In resistant cells, H(2)O(2) production is counteracted by a large overexpression of thioredoxin reductase that constitutes the preferred target of the inhibitory action of auranofin. This specific effect of auranofin might rationalize its ability in overcoming cisplatin resistance in human ovarian cancer cells.

The role of tamoxifen in combination with cisplatin on oral squamous cell carcinoma cell lines

The purpose of this study was to evaluate the effect of tamoxifen (TAM) when used in combination with cisplatin on oral squamous cell carcinoma (OSCC). For this, the relation between estrogen receptor (ER) expression level and the cytotoxic effect of TAM, the apoptotic effect and its molecular mechanisms of TAM were investigated using OSCC cell lines. Combination treatment demonstrated a superior cytotoxic and apoptotic effect on OSCC cell lines. Considerable amount of ER was detected in some OSCC cell lines, but there were no significant differences of cytotoxic effect of TAM. TAM inhibited PKC activity and up-regulated TGF-beta1 secretion.

Human cells deficient in transcription-coupled repair show prolonged activation of the Jun N-terminal kinase and increased sensitivity following cisplatin treatment

PURPOSE

The Jun N-terminal kinases (JNKs) are activated by many biological, physical, and chemical stimuli, including the chemotherapeutic agent cisplatin. The primary pathway that repairs cisplatin-DNA adducts is nucleotide excision repair (NER). Xeroderma pigmentosum (XP) cells from complementation group C (XP-C) are competent in the transcription-coupled repair (TCR) pathway of NER but deficient in global genomic repair (GGR), Cockayne's syndrome (CS) cells are deficient in TCR but have normal GGR, and XP-A cells are deficient in both TCR and GGR. We used NER-deficient human fibroblasts to study the role of DNA damage in the activation of JNK and cell death following cisplatin treatment.

MATERIALS AND METHODS

JNK-1 activity and clonogenic survival were examined in normal and NER-deficient human fibroblasts following cisplatin treatment.

RESULTS

Cisplatin induced a transient increase in JNK-1 activity of about tenfold in normal and XP-C fibroblasts, which declined to about two- to threefold 24 h after treatment. In contrast, the activation of JNK-1 was persistent in XP-A and CS fibroblasts at 24 h after treatment and CS cells and XP-A cells, but not XP-C cells, were more sensitive to cisplatin than normal cells.

CONCLUSIONS

These results suggest that a deficiency in the TCR pathway of NER results in amplified and prolonged JNK activation due to persistent damage within the transcribed strand of active genes. Further, it is this amplified and prolonged JNK activation that correlates with cisplatin-induced cell death.

Synthesis, characterization and cytotoxicity evaluation of some new platinum(II) complexes of tetrazolo[1,5-a]quinolines

Several new platinum(II) complexes of the general formulae cis-[PtCl(2)(DMSO)L], where L is a Schiff base or hydrazone derived from tetrazolo[1,5-a]quinoline-4-carboxaldehyde as carrier ligands, have been synthesized and characterized physicochemically and spectroscopically. These platinum complexes which are structurally analogues to what so called cisplatin [cis-[PtCl2(NH3)2]; the first generation anticancer agent] were evaluated for their cytotoxicity on HL-60 (human promyelocytic leukemia) cells. Two of the platinum complexes were almost similar in their activity to cisplatin, while the remaining three complexes have demonstrated higher efficacy than that of cisplatin. Based on our findings, these novel platinum complexes appear to be valuable leading compounds with high efficacy.

Enhancement of sensitivity to cisplatin by orobol is associated with increased mitochondrial cytochrome c release in human ovarian carcinoma cells

OBJECTIVES

Based on our previous report showing that orobol, a potent phosphatidylinositol 4-kinase (PI4K) inhibitor, produced cisplatin (DDP) sensitivity, we have determined the mechanism of orobol-sensitization effect.

METHODS AND RESULTS

Orobol produced >2-fold DDP sensitivity in human ovarian carcinoma 2008 cells and its DDP-resistant variant 2008/C13*5.25 cells (C13). Because orobol had no effect on conventional mechanisms such as DDP accumulation or cellular metallothionein and glutathione content, we have focused on the apoptotic signaling pathway. Orobol induced a significant increase in apoptosis in DDP-treated cells, as estimated by frequency of condensed nuclear chromatin with Hoechst 33342 stain, although orobol alone did not have any effect on apoptotic potential. The caspase-3-inhibiting peptide Ac-DEVD-CHO completely inhibited the orobol sensitization effect but did not block DDP cell cytotoxicity per se. Orobol rendered both of these cells resistant to rhodamine 123 (Rh) by more than 2.5-fold, indicating significant decrease of mitochondrial membrane potential (DeltaPsim). Confocal laser microscopy of cells stained with the mitochondria (MT)-specific dye Rh revealed that orobol decreased Rh-fluorescent intensity. Electron microscopy of these cells showed that orobol induced swelling and condensation of MT. Orobol suppressed both naturally expressed and the DDP-induced Bcl-2 expression significantly. Orobol and DDP treatment reduced cytochrome c level in MT determined by Western blot analysis, indicating increased amount of cytochrome c release from MT, whereas orobol alone did not alter the amount of cytochrome c in MT.

CONCLUSIONS

These results indicate that orobol produced DDP sensitivity in human ovarian carcinoma cells by inducing apoptosis through the MT-dependent signaling pathway.

Modulation of multidrug resistance in human ovarian cancer cell lines by inhibition of P-glycoprotein 170 and PKC isoenzymes with antisense oligonucleotides

Multidrug resistance in human ovarian carcinoma cell lines is caused by the expression of several related proteins, namely P-glycoprotein 170 (Pgp-170), glutathione S-transferase-pi GST-pi), and thymidylate synthase (TS). These proteins seem to be regulated by a common mechanism in which the expression of protein kinase C (PKC) is involved. Additionally, the function of Pgp-170 is dependent on PKC phosphorylation. However, in ovarian carcinoma cell lines the role of different PKC enzymes responsible for resistance is not quite clear. In the present study we circumvented resistance in taxol resistant human ovarian carcinoma cell lines with antisense oligonucleotides to PKC alpha and PKC beta mRNA and compared the effects with those obtained by Pgp-170 antisense oligonucleotides. We found a significant inhibition of cell number after treatment with Pgp-170 antisense oligonucleotides in combination with taxol. Additionally, resistance could be reversed by treatment with taxol and antisense oligomers to PKC alpha and PKC beta. This shows that regulatory correlations between these proteins exist and that inhibition of the mRNA of PKC alpha and PKC beta isoforms and Pgp-170 can reverse multidrug resistance.

Potential new antitumor agents from an innovative combination of demethylcantharidin, a modified traditional Chinese medicine, with a platinum moiety

A combination of demethylcantharidin, a modified component of a traditional Chinese medicine (TCM), with a platinum moiety has produced a series of TCM-based platinum compounds [Pt(C(8)H(8)O(5))(NH(2)R)(2)] 1-5, which demonstrate selective cytotoxicity toward SK-Hep-1 (human liver) cell line, and circumvention of cross-resistance. The inclusion of demethylcantharidin rendered the compounds highly active as protein phosphatase (PP2A) inhibitors. The new TCM-Pt compounds may possess a novel dual mechanism of antitumor action: inhibition of PP2A and platination of DNA.

Mechanisms of action of flavopiridol

Flavopiridol inhibits phosphokinases. Its activity is strongest on cyclin dependent kinases (cdk-1, -2, -4, -6, -7) and less on receptor tyrosine kinases (EGFR), receptor associates tyrosine kinases (pp60 Src) and on signal transducing kinases (PKC and Erk-1). Although the inhibiting activity of flavopiridol is strongest for cdk, the cytotoxic activity of flavopiridol is not limited to cycling cells. Resting cells are also killed. This fact suggests that inhibition of cdks involved in the control of cell cycle is not the only mechanism of action. Inhibition of cdk's with additional functions (i.e. involved in the control of transcription or function of proteins that do not control cell cycle) may contribute to the antitumoral effect. Moreover, direct and indirect inhibition of receptor activation (EGFR) and/or a direct inhibition of kinases (pp60 Src, PKC, Erk-1) involved in the signal transduction pathway could play a role in the antiproliferative activity of flavopiridol. From pharmacokinetic data in patients it can be concluded that the inhibitory activity (IC50) of flavopiridol on these kinases is in the range of concentrations that might be achieved intracellularly after systemic application of non-toxic doses of flavopiridol. However, no in situ data from flavopiridol treated cells have been published yet that prove that by inhibition of EGFR, pp60 Src, PKC and/or Erk-1 (in addition to inhibition of cdk's) flavopiridol is able to induce apoptosis. Thus many questions regarding the detailed mechanism of antitumoral action of flavopiridol are still open. For the design of protocols for future clinical studies this review covers the essential information available on the mechanism of antitumoral activity of flavopiridol. The characteristics of this antitumoral activity include: High rate of apoptosis, especially in leukemic cells; synergy with the antitumoral activity of many cytostatics; independence of its efficacy on pRb, p53 and Bcl-2 expression; lack of interference with the most frequent multidrug resistance proteins (P-glycoprotein and MRP-190); and a strong antiangiogenic activity. Based on these pharmacological data it can be concluded that flavopiridol could be therapeutically active in tumor patients: independent on the genetic status of their tumors or leukemias (i.e. mutations of the pRb and/or p53, amplification of bcl-2); in spite of drug resistance of their tumors induced by first line treatment (and caused by enhanced expression of multidrug resistance proteins); in combination with conventional chemotherapeutics preferentially given prior to flavopiridol; and due to a complex mechanism involving cytotoxicity on cycling and on resting tumor cells, apoptosis and antiangiogenic activity. In consequence, flavopiridol is a highly attractive, new antitumoral compound and deserves further elucidation of its clinical potency.

Modulation of protein kinase C in antitumor treatment

PKC isoenzymes were found to be involved in proliferation, antitumor drug resistance and apoptosis. Therefore, it has been tried to exploit PKC as a target for antitumor treatment. PKC alpha activity was found to be elevated, for example, in breast cancers and malignant gliomas, whereas it seems to be underexpressed in many colon cancers. So it can be expected that inhibition of PKC activity will not show similar antitumor activity in all tumors. In some tumors it seems to be essential to inhibit PKC to reduce growth. However, for inhibition of tumor proliferation it may be an advantage to induce apoptosis. In this case an activation of PKC delta should be achieved. The situation is complicated by the facts that bryostatin leads to the activation of PKC and later to a downmodulation and that the PKC inhibitors available to date are not specific for one PKC isoenzyme. For these reasons, PKC modulation led to many contradicting results. Despite these problems, PKC modulators such as miltefosine, bryostatin, safingol, CGP41251 and UCN-01 are used in the clinic or are in clinical evaluation. The question is whether PKC is the major or the only target of these compounds, because they also interfere with other targets. PKC may also be involved in apoptosis. Oncogenes and growth factors can induce cell proliferation and cell survival, however, they can also induce apoptosis, depending on the cell type or conditions in which the cells or grown. PKC participates in these signalling pathways and cross-talks. Induction of apoptosis is also dependent on many additional factors, such as p53, bcl-2, mdm2, etc. Therefore, there are also many contradicting results on PKC modulation of apoptosis. Similar controversial data have been reported about MDR1-mediated multidrug resistance. At present it seems that PKC inhibition alone without direct interaction with PGP will not lead to successful reversal of PGP-mediated drug efflux. One possibility to improve chemotherapy would be to combine established antitumor drugs with modulators of PKC. However, here also very contrasting results were obtained. Many indicate that inhibition, others, that activation of PKC enhances the antiproliferative activity of anticancer drugs. The problem is that the exact functions of the different PKC isoenzymes are not clear at present. So further investigations into the role of PKC isoenzymes in the complex and interacting signalling pathways are essential. It is a major challenge in the future to reveal whether modulation of PKC can be used for the improvement of cancer therapy.

Cisplatin resistance and oncogenes--a review

Cisplatin is among the most widely used broadly active cytotoxic anticancer drugs; however, its clinical efficacy is often limited by primary or the development of secondary resistance. Several mechanisms have been implicated in cisplatin resistance, including reduced drug uptake, increased cellular thiol/folate levels and increased DNA repair. More recently, additional pathways have been characterized indicating that altered expression of oncogenes that subsequently limit the formation of cisplatin-DNA adducts and activate anti-apoptotic pathways may also contribute to the resistance phenotype. Several lines of evidence suggest that expression of ras oncogenes can confer resistance to cisplatin by reducing drug uptake and increasing DNA repair; however, this is not a uniform finding. Tumor cells, in contrast to normal cells, respond to cisplatin exposure with transient gene expression to protect or repair their chromosomes. The c-fos/AP-1 complex, a master switch for turning on other genes in response to DNA-damaging agents, has been shown to play a major role in cisplatin resistance. In addition, AP-2 transcription factors, modulated by protein kinase A, are also implicated in cisplatin resistance by regulating genes encoding for DNA polymerase beta and metallothionines. Furthermore, considerable evidence indicates that mutated p53 plays a significant role in the development of cisplatin resistance since several genes implicated in drug resistance and apoptosis (e.g. mismatch repair, bcl-2, high mobility group proteins, DNA polymerases alpha and beta, PCNA, and insulin-like growth factor) are known to be regulated by the p53 oncoprotein. Improved understanding of molecular factors for the development of cisplatin resistance may allow the prediction of clinical response to cisplatin-based treatment. Furthermore, the identification of oncogenes involved in cisplatin resistance has already led to in vitro approaches which successfully inactivated these genes using ribozymes or antisense oligodeoxynucleotides, thus restoring cisplatin sensitivity. It is conceivable that these strategies, once transferred to a clinical setting, may have the potential to enhance the efficacy of cisplatin against a great variety of malignancies and thus more fully exploit the antineoplastic and curative potential of this drug.

Regulation of apoptosis reduction in the cisplatin-resistant A431 cell line by Bcl-2 and CPP32

Cisplatin (cis-diamminedichloroplatinum(II), CDDP) is one of the most important chemotherapeutic agents; however, the mechanisms of resistance to this drug are still unknown. Recent reports have demonstrated that chemotherapy can induce apoptosis in some cancer cells, indicating that apoptosis may play a very important role in cancer therapy. Therefore, we used a CDDP-resistant cell line from the human epidermoid carcinoma cell line A431 to investigate whether the modulation of apoptosis influences CDDP resistance. In the CDDP-resistant cell, the cell cycle was not perturbed after CDDP treatment. DNA gel electrophoresis and ELISA of the CDDP-resistant cell showed reduced apoptosis when compared with A431 cells treated with CDDP. We determined the p53, Bcl-2, Bax and CPP32 protein levels by Western blotting. This analysis demonstrated a marked increase in Bcl-2 protein levels and a reduction in CPP32 protein levels in CDDP-resistant cells. Our results indicate that the reduction of apoptosis was one of the CDDP-resistant mechanisms, and that reduced apoptosis in CDDP-resistant cells was influenced by Bcl-2 and CPP32 proteins.

Estrogen inducibility of c-Ha-ras transcription in breast cancer cells. Identification of functional estrogen-responsive transcriptional regulatory elements in exon 1/intron 1 of the c-Ha-ras gene

Although mutation of ras gene is rare in human breast cancer, overexpression of normal c-Ha-ras gene is frequently observed. Using a mouse mammary metastasis model consisting of genetically related mammary tumor sublines with variant metastatic potential, we have previously (i) demonstrated a direct correlation between c-Ha-ras mRNA and protein levels and metastatic potential and (ii) identified a novel hormone-responsive transcriptional regulatory element in intron 1 of the mouse c-Ha-ras gene that contains the consensus half-site of a glucocorticoid response element and flanking consensus half-sites for estrogen response element. Here, we have examined the functionality of intron 1 sequence in context of upstream sequences by using transient transfection assays with plasmids expressing chloramphenicol acetyltransferase. Intron 1 sequence and sequences similar to intron 1 element located in exon 1 function as transcriptional regulatory elements that confer hormonal inducibility to chloramphenicol acetyltransferase gene expression both independently and in context of 5'-flanking sequences. Measurement of c-Ha-ras transcription rates and protein expression by nuclear run-on and metabolic labeling assays showed a 5-12-fold enhancement, respectively, following treatment with 17beta-estradiol that was blunted by ICI 182,780 in the nonmetastatic variant. In contrast, constitutive overexpression of c-Ha-ras transcripts and protein in the metastatic subline was unaffected by estrogen and ICI 182,780. Gel shift assays demonstrated specific interaction of c-Ha-ras exon 1 sequence with nuclear proteins of human breast cancer MCF-7 cells with formation of two complexes, one of which contains estrogen receptor. Our data demonstrate a direct (i) interaction of c-Ha-ras sequence with estrogen receptor and (ii) stimulatory effect of estrogen on c-Ha-ras gene transcription and suggest that alteration in transcriptional regulation of c-Ha-ras gene by estrogen may play an important role in progression of breast cancer.

Rapid fluorescence-based reporter-gene assays to evaluate the cytotoxicity and antitumor drug potential of platinum complexes

BACKGROUND

The need for new platinum antitumor drugs is underscored by the usefulness of cisplatin and carboplatin in chemotherapy and the resistance of many tumors to these compounds. Combinatorial chemistry could aid in the search for cisplatin analogs if fast, high-throughput assays were available. Our goal was to develop rapid cell-based assays suitable for high-throughput screening that accurately predict the cytotoxicity of platinum complexes. We examined the effects of platinum complexes and other agents on reporter-gene expression in cancer cells.

RESULTS

HeLa Tet-On cells with inducible enhanced green fluorescent protein (EGFP) were prepared. Cisplatin and other cis-disubstituted platinum complexes inhibited EGFP expression, with a strong positive correlation between EGFP inhibition and cytotoxicity. By contrast, trans-[Pt(NH(3))(2)Cl(2)], other trans-platinum complexes, methyl methanesulfonate or heat shock stimulated EGFP expression. Northern and nuclear run-on analyses revealed that the changes in EGFP expression were at the level of transcription. In another reporter-gene assay in Jurkat cells, cisplatin, but not trans-[Pt(NH(3))(2)Cl(2)] or K(2)[PtCl(4)], inhibited beta-lactamase expression, as measured by hydrolysis of the fluorescent substrate CCF2.

CONCLUSIONS

The EGFP results indicate that cytotoxic stress enhances transcription from the inducible promoter, whereas compounds able to form the 1,2-intrastrand platinum-DNA cross-links repress transcription. Both fluorescence-based reporter-gene assays afford promising new approaches to platinum anticancer drug discovery.

Mechanisms of synergism between cisplatin and gemcitabine in ovarian and non-small-cell lung cancer cell lines

2',2'-Difluorodeoxycytidine (gemcitabine, dFdC) and cis-diammine-dichloroplatinum (cisplatin, CDDP) are active agents against ovarian cancer and non-small-cell lung cancer (NSCLC). CDDP acts by formation of platinum (Pt)-DNA adducts; dFdC by dFdCTP incorporation into DNA, subsequently leading to inhibition of exonuclease and DNA repair. Previously, synergism between both compounds was found in several human and murine cancer cell lines when cells were treated with these drugs in a constant ratio. In the present study we used different combinations of both drugs (one drug at its IC25 and the other in a concentration range) in the human ovarian cancer cell line A2780, its CDDP-resistant variant ADDP, its dFdC-resistant variant AG6000 and two NSCLC cell lines, H322 (human) and Lewis lung (LL) (murine). Cells were exposed for 4, 24 and 72 h with a total culture time of 96 h, and possible synergism was evaluated by median drug effect analysis by calculating a combination index (CI; CI < 1 indicates synergism). With CDDP at its IC25, the average CIs calculated at the IC50, IC75 IC90 and IC95 after 4, 24 and 72 h of exposure were < 1 for all cell lines, indicating synergism, except for the CI after 4 h exposure in the LL cell line which showed an additive effect. With dFdC at its IC25, the CIs for the combination with CDDP after 24 h were < 1 in all cell lines, except for the CIs after 4 h exposure in the LL and H322 cell lines which showed an additive effect. At 72 h exposure all CIs were < 1. CDDP did not significantly affect dFdCTP accumulation in all cell lines. CDDP increased dFdC incorporation into both DNA and RNA of the A2780 cell lines 33- and 79-fold (P < 0.01) respectively, and tended to increase the dFdC incorporation into RNA in all cell lines. In the AG6000 and LL cell lines, CDDP and dFdC induced > 25% more DNA strand breaks (DSB) than each drug alone; however, in the other cell lines no effect, or even a decrease in DSB, was observed. dFdC increased the cellular Pt accumulation after 24 h incubation only in the ADDP cell line. However, dFdC did enhance the Pt-DNA adduct formation in the A2780, AG6000, ADDP and LL cell lines (1.6-, 1.4-, 2.9- and 1.6-fold respectively). This increase in Pt-DNA adduct formation seems to be related to the incorporation of dFdC into DNA (r = 0.91). No increase in DNA platination was found in the H322 cell line. dFdC only increased Pt-DNA adduct retention in the A2780 and LL cell lines, but decreased the Pt-DNA adduct retention in the AG6000 cell line. In conclusion, the synergism between dFdC and CDDP appears to be mainly due to an increase in Pt-DNA adduct formation possibly related to changes in DNA due to dFdC incorporation into DNA.

Cisplatin resistant glioblastoma cells may have increased concentration of urokinase plasminogen activator and plasminogen activator inhibitor type 1

Gliomas are the most common form of intrinsic primary brain tumors, that extensively invade the surrounding normal brain tissue. The failure of chemotherapy treatment of these tumors is chiefly attributed to drug-resistance. From human glioblastoma we developed two cell sublines resistant to cisplatin due to acute (AT cells) or continuous (CT cells) treatment with clinically relevant doses of cisplatin. We examined their sensitivity to different cytostatics by colorimetric MTT assay. The concentrations of urokinase plasminogen activator (uPA) and plasminogen activator inhibitor type 1 (PAI-1) were determined by the ELISA assay. The results reveal that both AT and CT cells became resistant to cisplatin and vincristine; AT cells became resistant also to etoposide. Both AT and CT cells did not significantly change their sensitivity to doxorubicin, 5-fluorouracil and chlorambucil. Concentrations of uPA and PAI-1 were increased in CT cells, with no change in AT cells. In the conditioned medium of both, AT and CT cells, the level of uPA were increased. No differences in concentrations of PAI-1 in the conditioned medium of these cells were found. Thus, our results show that drug-resistance of glioblastoma cells may be accompanied with the increased levels of markers for tumor invasion.

Pharmacokinetic schedule finding study of the combination of gemcitabine and cisplatin in patients with solid tumors

PURPOSE

To determine possible schedule dependent pharmacokinetic and pharmacodynamic interactions between gemcitabine (2',2'-difluorodeoxycytidine, dFdC) and cisplatin (cis-diammine-dichloroplatinum, CDDP) in patients with advanced stage solid tumors in a phase I trial.

PATIENTS AND METHODS

A total of 33 patients with advanced stage solid tumors were treated with gemcitabine (30-min infusion, 800 mg/m2) and cisplatin (one-hour infusion, 50 mg/m2). Sixteen patients had a four-hour interval between gemcitabine (days 1, 8, 15) and cisplatin (days 1 and 8), followed by the reverse schedule and seventeen patients had a 24-hour interval between gemcitabine (days 1, 8, 15) and cisplatin (days 2 and 9), followed by the reverse schedule. Gemcitabine and cisplatin pharmacokinetics were measured in plasma and white blood cells (WBC), isolated from blood samples taken at several time points after the start of treatment.

RESULTS

A four-hour time interval between both agents did not reveal major differences in plasma pharmacokinetics of gemcitabine, dFdU (deaminated gemcitabine) and platinum (Pt), and of gemcitabine-triphosphate (dFdCTP) accumulation and Pt-DNA adduct formation in WBC between the two different sequences of gemcitabine and cisplatin. In the patients treated with the 24-hour interval, cisplatin before gemcitabine did not significantly change peak gemcitabine levels and the AUC of plasma dFdU, but tended to increase dFdCTP AUC in WBC 1.5-fold (P < 0.06). Gemcitabine before cisplatin decreased the plasma AUC of Pt 2.1-fold (P = 0.03). No significant differences in Pt-DNA adduct levels in WBC were found, although gemcitabine before cisplatin tended to increase the 24-hour retention of Pt-DNA adducts. Creatinine clearance on day 28 was related to the peak plasma levels of total Pt (linear regression coefficient (r) = 0.47, P = 0.02, n = 26). Furthermore, the increase in the Pt-GG to Pt-AG ratio 24 hours after cisplatin treatment was related to the overall response of patients (r = 0.89, P < 0.01, n = 8).

CONCLUSIONS

Of all schedules the treatment of patients with cisplatin 24 hours before gemcitabine led to the highest dFdCTP accumulation in WBC and total Pt levels in plasma. These characteristics formed the basis for further investigation of this schedule in a phase II clinical study.

Establishment and characterization of cisplatin-resistant human epidermoid carcinoma cell line, A431 cell

Cisplatin, cis-diamminedichloroplatinum(II) (CDDP) is one of the most important anticancer agents, initially producing good responses in various tumors. However, resistance to this drug often develops in various tumors, and additional administration decreases its chemotherapeutic efficacy. The precise mechanism of acquisition of resistance to this drug is still uncertain. However in the present study, we established two CDDP-resistant sublines A431/CDDP1 and A431/CDDP2 from human epidermoid carcinoma cell line A431. These resistant sublines were constituted by exposing A431 cells to a gradually increasing dose of CDDP (A431/CDDP1), and by mutagenic induction with mutagen (A431/CDDP2). A431/CDDP1 and A431/CDDP2 have developed 3.1 and 2.7 times more resistance to CDDP than the original A431 cell in terms of IC50. The two CDDP-resistant sublines showed cross-resistance to the CDDP analogue, carboplatin (CBDCA), but not to other chemotherapeutic drugs such as Adriamycin (ADR) and 5-fluorouracil (5-FU). These CDDP-resistant sublines were transplanted into nude mice to demonstrate the resistance to CDDP treatment in vivo. According to the in vitro assay, the mechanism of resistance in A431/CDDP1 and A431/CDDP2 seems to be based on a reduction of intracellular accumulation of CDDP, because their platinum concentration, which is the major component of CDDP, significantly declined. The established CDDP-resistant sublines may be used in further trials to improve the understanding of the mechanisms of resistance to CDDP.

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.

p53-Dependent and -independent responses to cisplatin in mouse testicular teratocarcinoma cells

Testicular cancers respond favorably to chemotherapy with the platinum-containing drug cis-diamminedichloroplatinum(II) (cisplatin). One factor that could explain the efficacy of cisplatin is the low frequency of p53 mutations observed in this tumor type. The present study examines the p53-mediated responses in murine testicular teratocarcinoma cells exposed to the drug. Cisplatin treatment of teratocarcinoma cells with a wild-type p53 gene resulted in accumulation of the p53 protein through posttranscriptional mechanisms; induction of p53-target genes was also observed. Drug treatment resulted in rapid apoptosis in p53-wild-type cells but not in p53(-/-) teratocarcinoma cells. In the latter cells, cisplatin exposure caused prolonged cell cycle arrest accompanied by induction of the p21 gene. Clonogenic assays demonstrated that the p53 mutation did not confer resistance to cisplatin. These experiments suggest that cisplatin inhibits cellular proliferation of testicular teratocarcinoma cells by two possible mechanisms, p53-dependent apoptosis and p53-independent cell cycle arrest.

Cisplatin resistance in cyclic AMP-dependent protein kinase mutants

The emergence of cisplatin resistance poses a major problem to the successful treatment of a variety of human malignancies. Therefore, understanding the molecular mechanisms that underlie cisplatin resistance could significantly improve the clinical efficacy of this cytotoxic agent. Various studies have described that cellular sensitivity to cisplatin can be influenced by several signal transduction pathways. In this review, we examine the role of the cyclic AMP-dependent protein kinase (PKA) in the modulation of drug resistance in cancer. By a somatic mutant genetic approach, the role of PKA in the development of resistance to chemotherapeutic agents has been investigated. A series of mutants with decreased PKA activity was examined for their sensitivity to cisplatin. PKA mutants with defective regulatory (RIalpha) subunits, but not altered catalytic (C) subunits, exhibit increased resistance to cisplatin, as well as other DNA-damaging agents. Furthermore, since RIalpha subunit mutants show enhanced DNA repair we, therefore, hypothesize that functional inactivation of PKA may result in increased recognition and repair of cisplatin lesions. Alternatively, it seems likely that mutation of the RIalpha subunit may affect cellular sensitivity to various anticancer drugs, suggesting that the RIalpha subunit may have other physiological functions in addition to inhibiting the kinase activity of the C subunit. Therefore, exploitation of cyclic AMP levels or functional alteration of the R subunit may potentiate the cytotoxicity of chemotherapeutic agents and circumvent drug resistance in cancer. More importantly, the altered pattern and mechanism of drug resistance may offer the opportunity to investigate novel regulatory functions of the RIalpha subunit of PKA.

Human canalicular multispecific organic anion transporter (cMOAT) is expressed in human lung, gastric, and colorectal cancer cells

Human canalicular multispecific organic anion transporter (cMOAT), a glutathione conjugate membrane transporter, has been isolated from cisplatin-resistant cancer cells and is distributed mainly in normal liver. We analyzed the expression of human cMOAT in 14 lung, 11 gastric, and 9 colorectal non-drug-selected human cancer cells, two multidrug-resistant cells, and one cisplatin-resistant cells, using quantitative RT-PCR and newly developed anti-human cMOAT antibody. All cell lines analyzed here expressed human cMOAT at the level of mRNA and protein, and some of them expressed higher levels of human cMOAT than the cisplatin-resistant cells. The two multidrug-resistant cell lines co-expressed human cMOAT gene and both or either of MRP and MDR1 genes. Immunostaining showed that human cMOAT was predominantly localized to the cytoplasm of these single cells. Our results indicate that human cMOAT is expressed in various human cancer cells including drug-resistant cells.

Irinotecan hydrochloride (CPT-11) resistance identified by K-ras mutation in patients with progressive colon cancer after treatment with 5-fluorouracil (5-FU)

OBJECTIVE

To determine the prognostic role of a K-ras mutation in tumor tissue of patients with refractory colon cancer who received irinotecan hydrochloride (CPT-11).

METHODS

DNA was extracted from paraffin-stored tumor tissue of 35 patients with progressive colon cancer failing treatment with 5-fluorouracil who subsequently received CPT-11 (100 mg/m2 i.v. per week x 4 weeks with 2 weeks off per course). The first exon of the K-ras gene was amplified by polymerase chain reaction by using K-ras-specific primers followed by mutant enrichment sequencing. Survival differences of patients with a K-ras mutation were compared with those of patients with a normal K-ras status.

RESULTS

A total of 21 patients had a normal K-ras sequence and 14 patients had a K-ras mutation [GAT, n = 7; TGT, n = 3; and GCT, AGT, GTT, GAC (codon 13), n = 1 each]. Median survival of patients with a normal ras sequence from time of treatment with CPT-11 was 332 days compared with 169 days for patients with a K-ras mutation (p = 0.0036). No differences in age, sex, cancer stage, surgical treatment, or chemotherapy treatment were observed.

CONCLUSION

Determination of the presence of a K-ras mutation may predict survival in patients with progressive colon cancer after treatment with 5-fluorouracil who receive CPT-11.

Characterization of a cAMP-dependent protein kinase mutant resistant to cisplatin

The signal transduction pathway of cAMP, mediated by the cAMP-dependent protein kinase (PKA), is involved in the regulation of metabolisms, cell growth and differentiation and gene expression. Isolated PKA mutants from Chinese hamster ovary (CHO) cells were used in our laboratory to study the role of cAMP in the development of drug resistance in cancer. We have found that PKA mutants harboring a defective regulatory (RI alpha) subunit, but not the catalytic (C) subunit, are more resistant to the chemotherapeutic drug cisplatin. To clarify the role of PKA in cisplatin resistance, we have performed a step-wise selection with a CHO RI alpha subunit mutant cell line, 10248, for further resistance to cisplatin. A representative clone (10248/CDDP(R)-5) was used for further characterization. These cisplatin-resistant PKA mutant cells remained refractory to cAMP-induced growth inhibition and had decreased PKA activity comparable to the parental 10248 mutant cells. Furthermore, 10248/CDDP(R)-5 also exhibited cross-resistance to the nitrogen mustard melphalan but maintained the same sensitivity as wild-type cells to non-DNA-damaging agents such as methotrexate. The mechanism of resistance may be due to increased DNA repair as assessed by the host cell reactivation assay. We speculate that mutation and functional inactivation of PKA may result in deregulated growth response to cAMP, as well as the acquisition of resistance to cisplatin and other DNA-damaging agents in cancer.

Cisplatin and adriamycin resistance are associated with MutLalpha and mismatch repair deficiency in an ovarian tumor cell line

In contrast to parental A2780 ovarian tumor cells, extracts of one doxorubicin-resistant and two independent cis-diamminedichloroplatinum(II)-resistant derivatives are defective in strand-specific mismatch repair. The repair defect of the three hypermutable, drug-resistant cell lines is only evident when the strand break that directs the reaction is located 3' to the mismatch, and in each case repair is restored to extracts by addition of purified MutLalpha heterodimer. As judged by immunological assay, drug resistance is associated with the virtual absence of the MutLalpha MLH1 subunit and greatly reduced levels of the PMS2 subunit. These findings implicate a functional mismatch repair system in the cytotoxic effects of these antitumor drugs and may have ramifications for their clinical application.

Cross-resistance and collateral sensitivity to natural product drugs in cisplatin-sensitive and -resistant rat lymphoma and human ovarian carcinoma cells

The cytotoxicity of mitotic spindle poisons, vinca alkaloids and the anthracycline, adriamycin, against cisplatin-sensitive and -resistant rat lymphoma and human ovarian carcinoma cell lines was investigated. Interestingly, it was found that all cell lines were more sensitive to the mitotic spindle poisons, vincristine and vinblastine. Adriamycin was the least effective and taxol had intermediate activity. The Walker rat lymphoma cell line resistant to cisplatin (WR) exhibited the multiple drug resistance phenotype since it showed collateral resistance to all drugs (ranging from twofold to taxol, colcemid and colchicine and sixfold to the vinca alkaloids). Verapamil potentiated the cytotoxic activity of adriamycin and vincristine in a striking fashion with the Walker cells. P-glycoprotein was found to be present in the plasma membranes of the Walker cells with approximately a 2.5-fold increase in the WR as compared to the sensitive (WS) cells. Glutathione levels were elevated in all of the cisplatin-resistant cell lines when compared to the cisplatin-sensitive parental cell lines. A profound effect of buthionine sulfoximine pretreatment on adriamycin cytotoxicity was observed. Glutathione S-transferase (pi) was present in all the human cell lines but the WS cells had markedly lower levels (almost negligible) when compared to the WR cells. These observations imply that cisplatin-resistant cells may be more sensitive to mitotic spindle poisons and vinca alkaloids, irrespective of the mechanism of platinum resistance, and that the cytotoxicity of vinca alkaloids could be further modulated by verapamil, irrespective of the presence or absence of P-glycoprotein.