Binding activities of cis-platin-damage-recognition proteins in human tumour cell lines

Recognition and processing of cisplatin- and oxaliplatin-DNA adducts

The cytotoxicity of platinum compounds is thought to be determined primarily by their DNA adducts. Cisplatin and oxaliplatin are structurally distinct, but form the same types of adducts at the same sites on DNA. However, the DNA adducts are differentially recognized by a number of cellular proteins. For example, mismatch repair proteins and some damage-recognition proteins bind to cisplatin-GG adducts with higher affinity than to oxaliplatin-GG adducts, and this differential recognition of cisplatin- and oxaliplatin-GG adducts is thought to contribute to the differences in cytotoxicity and tumor range of cisplatin and oxaliplatin. A detailed kinetic analysis of the insertion and extension steps of dNTP incorporation in the vicinity of the adduct shows that both DNA polymerase beta (pol beta) and DNA polymerase eta (pol eta) catalyze translesion synthesis past oxaliplatin-GG adducts with greater efficiency than past cisplatin-GG adducts. In the case of pol eta, the efficiency and fidelity of translesion synthesis in vitro is very similar to that previously observed with cyclobutane TT dimers, suggesting that pol eta is likely to be involved in error-free bypass of Pt adducts in vivo. This has been confirmed for cisplatin by comparing the cisplatin-induced mutation frequency in human fibroblast cell lines with and without pol eta. Thus, the greater efficiency of bypass of oxaliplatin-GG adducts by pol eta may explain the lower mutagenicity of oxaliplatin compared to cisplatin. The ability of these cellular proteins to discriminate between cisplatin and oxaliplatin adducts suggest that there exist significant conformational differences between the adducts, yet the crystal structures of the cisplatin- and oxaliplatin-GG adducts were very similar. We have recently solved the solution structure of the oxaliplatin-GG adduct and have shown that it is significantly different from the previously published solution structures of the cisplatin-GG adducts. Furthermore, the observed differences in conformation provide a logical explanation for the differential recognition of cisplatin and oxaliplatin adducts by mismatch repair and damage-recognition proteins.

Enhancement of excision repair of cisplatin-DNA adducts by cell-free extract from a cisplatin-resistant rat cell line

To characterize the enhanced repair synthesis of defined DNA lesions, oligodeoxyribonucleotides were synthesized and inserted into plasmid DNA. The inserted plasmid DNA was treated with cis-diamminedichloroplatinum(II) (cisplatin) and subjected to in vitro DNA repair assay with soluble extract from the rat liver cell line Ac2F. All cisplatin adducts tested stimulated DNA repair synthesis. Moreover, two cisplatin-resistant cell lines, Ac2F-CR4 and Ac2F-CR10, were established by stepwise exposure of Ac2F cells to this drug. The DNA repair synthesis was enhanced 3- to 4-fold in the extract from cisplatin-resistant Ac2F cells relative to that from Ac2F cells. Such repair synthesis was suppressed by the specific DNA polymerase inhibitor aphidicolin. The results of the present study suggested that the enhanced repair activity induced by a cisplatin adduct can be detected by in vitro DNA repair assay with soluble cell extract.

Cisplatin-induced apoptosis and p53 gene status in a cisplatin-resistant human ovarian carcinoma cell line

Cisplatin-induced apoptosis and p53 gene status were analyzed in human ovarian carcinoma using a parental IGR-OV1 line and a derived cisplatin-resistant IGR-OV1/DDP subline. Compared with parental cells, cisplatin-resistant cells exhibited a 5-fold higher resistance index and a 2-fold longer doubling time. Cisplatin induced apoptosis in both cell lines, as assessed by cell morphology and the presence of a DNA ladder. However, high concentrations were necessary to induce apoptosis in resistant cells. These cells elicited a 5-fold decrease in the number of platinum atoms bound per nucleotide. IGR-OV1/DDP cells also exhibited enhanced drug efflux and a higher glutathione content. Our data suggest that the levels of cisplatin-DNA lesions are critical for drug sensitivity and apoptosis induction in this in vitro ovarian carcinoma model. Comparative analysis of the p53 gene in sensitive and resistant cells revealed the presence of the same heterozygous mutation in exon 5. A 2-fold increase in p53 mRNA and protein amounts was observed in resistant cells as assessed by Northern and Western blots, respectively. Immunocytochemical staining revealed a higher percentage of p53 stained nuclei in resistant cells. RT-PCR analysis of p53 transcripts showed that both wild-type and mutated alleles were transcribed in sensitive as well as in resistant cells. However, mutated transcripts were 1.5-fold more abundant than wild-type transcripts in sensitive cells, whereas they were 2-fold higher in resistant cells. In addition, mdm-2 protein was over-expressed in resistant cells. Our results address the question of the functionality of p53 protein and its possible role in apoptosis induction in this model. In resistant cells, p53 protein might be inactivated by 2 mechanisms: mutation and complexation with mdm-2 protein. Therefore, the presence of non-functional p53 in resistant cells might be involved in the relative failure of cisplatin-induced apoptosis in these cells.

Sensitivity of testis tumour cells to chemotherapeutic drugs: role of detoxifying pathways

In contrast to most other types of cancer, metastatic testicular germ cell tumours (TGCT) are cured in most patients using cisplatin-based combination chemotherapy. The biochemical mechanisms underlying this sensitivity have not been defined. Drug detoxification can modulate response to chemotherapy in vivo and in vitro, and therefore we measured levels of glutathione (GSH), glutathione-S-transferase (GST) and both constitutive and cisplatin- and dexamethasone-induced levels of metallothionein (MT) in five human testis tumour cell lines. The levels were compared with those in five human bladder cancer cell lines and two cell lines with cisplatin resistance acquired in vitro. GSH levels were relatively low in the testis tumour cell lines, as might be expected in drug-sensitive cells, and there was a 77-fold increase in GSH levels in the cisplatin-resistant testis tumour cell line. GST levels were similar in the two cell types, while metallothionein levels were relatively high in the testis tumour cell lines. These data indicate that GSH may contribute to the sensitivity of TGCT to chemotherapy, and that GSH expression may be involved in the acquisition of cisplatin resistance in these tumours.

Cisplatin-DNA binding specificity of calf high-mobility group 1 protein

We have identified a series of proteins with an affinity for cisplatin -damaged DNA using damaged DNA affinity chromatography. We have purified one of these proteins to homogeneity on the basis of a mobility shift assay detecting binding to cisplatin-damaged DNA. The protein was identified as high-mobility group 1 protein (HMG-1) by N-terminal protein sequence analysis. Analysis of a variety of DNA structures revealed that fully duplex DNAs were the best substrates for HMG-1 binding, while partial duplexes were less avidly bound. The decreased levels of binding are attributed to the length of the duplex region of the DNA substrates. A 3-fold increase in binding was observed when a cisplatin-damaged DNA substrate containing a single break in the phosphodiester backbone was joined by DNA ligase. The strict DNA size dependence of binding was also assessed, and a 10-fold increase in binding was observed when the length of the DNA duplex was increased from 44 to 180 base pairs (bp) at the same level of cisplatin damage. HMG-1 binding also was correlated with the degree of cisplatin-DNA damage, suggesting a higher affinity for DNA containing multiple cisplatin adducts. Nuclease degradation of the cisplatin-damaged DNA demonstrated that at the lowest levels of cisplatin damage all of the substrates contained at least one cisplatin adduct. The potential role of HMG-1 in the repair of cisplatin-DNA adducts is discussed.

Genetic basis of drug sensitivity in human testis tumour cells

Testicular germ cell tumours (TGCT) are cured in over 80% of patients by using combination chemotherapy. However, the mechanism regulating this sensitivity has not been defined. Because cells derived from patients with DNA repair syndromes are similar to TCGT in their sensitivity to certain DNA-damaging agents and some of the genes involved have been cloned by functional complementation, the purpose of our study was to determine whether drug sensitivity in TGCT also has a genetic basis. Three testis tumour cell lines (cisplatin-sensitive) and 3 bladder cancer cell lines (cisplatin-resistant) were fused with a cisplatin-sensitive cell line (D98orC1). The authenticity of the hybrids was confirmed by karyotyping and PCR analysis of locus-specific sites, and sensitivities to cisplatin were measured by colony forming assays. The hybrids between sensitive cell lines were more resistant to cisplatin than the parental cells, indicating that functional complementation had occurred. The hybrids between the cisplatin-resistant and sensitive cells were intermediate in their cisplatin sensitivity, indicating that resistance is incompletely dominant. We conclude that cisplatin sensitivity has a genetic basis in TGCT and that resistance to cisplatin can be conferred by somatic cell fusion. Our data indicate that gene(s) controlling sensitivity to chemotherapy in TGCT might be identified by expression cloning.

Cellular basis for differential sensitivity to cisplatin in human germ cell tumour and colon carcinoma cell lines

Cisplatin (CDDP) resistance mechanisms were studied in a model of three germ cell tumour and three colon carcinoma cell lines representing intrinsically CDDP-sensitive and -resistant tumours respectively. The CDDP sensitivity of the cell lines mimicked the clinical situation. The glutathione levels of the cell lines correlated with CDDP concentrations inhibiting cell survival by 50% (IC50); total cellular sulphydryl content (TSH) was unexpectedly inversely correlated with IC50. IC50 correlated neither with glutathione S-transferase (GST) nor with GST pi expression, topoisomerase I or II activity. Immediately after 4 h incubation with CDDP, platinum (Pt) accumulation and Pt bound to DNA were not correlated, but after another 24 h drug-free culture, Pt binding to DNA in germ cell tumour but not in colon carcinoma cell lines correlated with IC50. With the exception of in vitro sensitivity and TSH, none of the parameters studied discriminated between the two groups of cell lines. Correction of CDDP sensitivity parameters for phenotypical differences did not influence statistical correlations. Analysis of variance revealed a correlation between IC50 and the combination of glutathione, GST activity and Pt bound to DNA. But at other CDDP cytotoxicity levels sensitivity was also correlated with Pt accumulation, topoisomerase II activity and TSH in various combinations. This model of intrinsic CDDP resistance showed that multiple parameters ought to be studied to explain CDDP resistance, but did not elucidate the cause of the unique sensitivity of germ cell carcinoma, although the unexpected values of TSH deserve further attention.

Constitutive over-production of DNA-damage recognition proteins and acquired UV resistance in prolonged culture of F9 teratocarcinoma stem cells

An ultraviolet (UV)-resistant F9 variant cell line, termed F9Vc, was established from a prolonged culture of murine F9 embryonic stem cells. A 6-fold UV resistance was detected in F9V2 cells compared to the F9 parental cells, as determined by ID50 (36 J/m2 vs. 6 J/m2), the UV dose causing 50% growth inhibition. Using a DNA mobility-shift assay, a nuclear protein (termed UVDRP) that preferentially binds to UV-damaged DNA was detected in F9 and F9Vc cell extracts. The UVDRP in F9Vc cells was present at a 7-fold higher concentration than that of F9 cells. Interestingly, the F9 UVDRP was transiently induced following cellular differentiation by retinoic acid (RA)/cAMP, with optimum induction (15-fold) at 6 days. Although constitutively over-produced, UVDRP also remained inducible in F9Vc cells in response to RA/cAMP. Indirect DNA repair measurement by host cell reactivation of UV-damaged plasmid DNA demonstrated that F9Vc cells exhibited a slight increase or a similarity in repair ability compared to the F9 cells. Parallel experiments using the repair-defective xeroderma pigmentosum (XP) group A fibroblasts and the normal VA13 fibroblasts also indicated that over-production of UVDRP binding activity was associated with enhanced DNA repair and UV resistance. The findings indicate that prolonged culture of F9 cells can establish a condition sufficient to cause constitutive over-production of UVDRP binding activity and UV resistance. The results also suggest that the RA/cAMP-inducible UVDRP in F9 stem cells may be important for the sensitivity or resistance of the cells to UV damage.