Biochemical characterization of a mitomycin C resistant colon cancer cell line variant

NAD(P)H:quinone oxidoreductase 1 (NQO1) in the sensitivity and resistance to antitumor quinones

Quinones represent a large and diverse class of antitumor drugs and many quinones are approved for clinical use or are currently undergoing evaluation in clinical trials. For many quinones reduction to the hydroquinone has been shown to play a key role in their antitumor activity. The two-electron reduction of quinones by NQO1 has been shown to be an efficient pathway to hydroquinone formation. NQO1 is expressed at high levels in many human solid tumors making this enzyme ideally suited for intracellular drug activation. Cellular levels of NQO1 are influenced by the NQO1*2 polymorphism. Individuals homozygous for the NQO1*2 allele are NQO1 null and homozygous NQO1*2*2 cell lines have been shown to be more resistant to antitumor quinones when compared to isogenic cell lines overexpressing NQO1. In this review we will discuss the role of NQO1 in the sensitivity and resistance of human cancers to the quinone antitumor drugs mitomycin C, β-lapachone and the benzoquinone ansamycin class of Hsp90 inhibitors including 17-AAG. The role of NQO1 in the bioreductive activation of mitomycin C remains controversial but pre-clinical data strongly suggests a role for NQO1 in the activation of β-lapachone and the benzoquinone ansamycin class of Hsp90 inhibitors. Despite a large volume of preclinical data demonstrating that NQO1 is an important determinant of sensitivity to these antitumor quinones there is little information on whether the clinical response to these agents is influenced by the NQO1*2 polymorphism. The availability of simple assays for the determination of the NQO1*2 polymorphism should facilitate clinical testing of this hypothesis.

Relationship between colorectal cancer glutathione levels and patient survival: early results

PURPOSE

Elevated glutathione is a cause of resistance to anticancer agents and x-rays. The purpose of this study was to determine the frequency and clinical significance of glutathione elevation in human colorectal cancer.

METHODS

Glutathione levels were measured in 41 colon cancers, 24 rectal cancers, and corresponding normal tissues. The patients were then followed up prospectively for tumor recurrence and survival. Survival was analyzed by the Kaplan-Meir method and Cox proportional hazards regression.

RESULTS

Glutathione levels in primary colorectal cancers were significantly higher than in the corresponding normal tissues. Elevated glutathione levels had a significant negative effect on survival in patients with colorectal cancer, whether based on the mean (P = 0.02) or median (P = 0.04) normal tissue levels. A negative effect of glutathione levels on survival was apparent in patients with colorectal cancer, whether or not they were treated with postoperative therapy. The larger the ratio of tumor glutathione to normal tissue glutathione, the poorer the prognosis. When adjusted for other covariates, glutathione was still a significant predictor of survival.

CONCLUSIONS

An elevated tumor glutathione level at the time of diagnosis appears to confer a poor prognosis in patients with colorectal cancer. Longer-term study using a larger number of patients will be required to confirm these findings. Knowledge of tumor glutathione content may help identify patients requiring more intensive therapy.

Quinone toxicity in DT-diaphorase-efficient and -deficient colon carcinoma cell lines

The human colon carcinoma cell lines Caco-2 and HT-29 were exposed to three structurally related naphthoquinones. Menadione (MEN), 1,4-naphthoquinone (NQ), and 2,3-dimethoxy-1,4-naphthoquinone (DIM) redoxcycle at similar rates, NQ is a stronger arylator than MEN, and DIM does not arylate thiols. The Caco-2 cell line was particularly vulnerable to NQ and MEN and displayed moderate toxic effects of DIM. The HT-29 cell line was only vulnerable to NQ and MEN after inhibition of DT-diaphorase (DTD) with dicoumarol, whereas dicoumarol did not affect the toxicity of quinones to Caco-2 cells. DTD activity in the HT-29 and Caco-2 cell lines, as estimated by the dicoumarol-sensitive reduction of 2,6-dichlorophenolindophenol, was 393.7 +/- 46.9 and 6.4 +/- 2.2 nmol NADPH x min(-1) x mg protein(-1), respectively. MEN depleted glutathione to a small extent in the HT-29 cell line, but a rapid depletion similar to Caco-2 cells was achieved when dicoumarol was added. The data demonstrated that the DTD-deficient Caco-2 cell line was more vulnerable to arylating or redoxcycling quinones than DTD-expressing cell lines. Exposure of the Caco-2 cell line to quinones produced a rapid rise in protein disulphides and oxidised glutathione. In contrast to NQ and DIM, no intracellular GSSG was observed with MEN. The relatively higher levels of ATP in MEN-exposed cells may account for the efficient extrusion of intracellular GSSG. The reductive potential of the cell as measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide reduction was only increased by MEN and not with NQ and DIM. We conclude that arylation is a major contributing factor in the toxicity of quinones. For this reason, NQ was the most toxic quinone, followed by MEN, and the pure redoxcycler DIM elicited modest toxicity in Caco-2 cells.

The importance of DT-diaphorase in mitomycin C resistance in human colon cancer cell lines

BACKGROUND

Prior studies have suggested the multifactorial nature of mitomycin C (MMC) resistance. However, the relative importance of the different resistance mechanisms is unknown.

MATERIALS AND METHODS

A panel of colon cancer cell lines with levels of MMC resistance from 2- to 15-fold compared to the parent line HT-29 was produced by repeated MMC exposure. Cell survival was measured using clonogenic assay. Glutathione and related enzymes and DT-diaphorase were measured using biochemical assays. P-glycoprotein expression was measured using flow cytometry. Topoisomerase II activity was measured using the pBR322 DNA relaxation assay.

RESULTS

Multiple drug resistance mechanisms were altered in the resistant cell lines (glutathione reductase, glutathione peroxidase, topoisomerase II). However, the level of DT-diaphorase correlated best with the degree of MMC resistance. The importance of DT-diaphorase was confirmed by using BMY 25282, an MMC analogue which is less dependent on DT-diaphorase for activation. Resistance in the HT-29R54 cell line was 15-fold with MMC compared to 5-fold with BMY 25282. P-glycoprotein-mediated resistance does not appear important in this model.

CONCLUSIONS

Although MMC resistance appears to be multifactorial, the results of this study strongly suggest that DT-diaphorase is the major contributor to MMC resistance under aerobic conditions. Strategies to enhance drug activation may therefore be useful for reversing MMC resistance.

Growth factors, glia and gliomas

The abilities of growth factors to cause normal cells to express the properties associated with transformed cells is discussed in specific reference to the oligodendrocyte-type-2 astrocyte (O-2A) progenitor cell. In the O-2A lineage, it has been possible to use growth factors and other defined molecules to induce or promote in normal cells all of the main properties of tumor cells, these being continued cell division in the absence of differentiation, more subtle modulations of self-renewal probabilities, promotion of cell migration and inhibition of programmed cell death. In addition to our studies on primary cells, our application to the growth of human tumor specimens of techniques utilized to study primary glial progenitor cells has allowed us to isolate a human glioblastoma multiforme (GBM)-derived population that expresses many properties otherwise uniquely expressed by oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells. Hu-O-2A/Gb1 (for Human O-2A lineage Glioblastoma number 1) cells responded to similar mitogens and differentiation modulators as rodent O-2A progenitors, and generated cells with features of precursor cells, oligodendrocytes and astrocytes. Moreover, 1H-NMR analysis of amino acid composition demonstrated a striking conservation of types and quantities of free amino acids between the human tumour cells and the rodent primary cells. Hu-O-2A/Gb1 cells represent the first human glioma-derived population for which unambiguous lineage assignment has been possible. Our results thus demonstrate that the human O-2A lineage can contribute to one of the most malignant of glial tumours. Our analyses further indicate that at least two distinct glial lineages can generate glioblastomas. In addition, the highly diagnostic 1H-NMR spectrum expressed by Hu-O-2A/Gb1 cells raises the possibility of eventual non-invasive identification of tumors of this lineage.

Modulation of mitomycin C mutagenicity on Saccharomyces cerevisiae by glutathione, cytochrome P-450, and mitochondria interactions

It is well established that most anticancer drugs also have mutagenic effects and require metabolic activation before exerting their mutagenic/antiblastic activity. Antitumoral compound effects strongly depend on the biochemical/physiological conditions of the tumoral cells, and especially on the activation of specific drugs metabolizing enzymes and on respiration. We examined the mitomycin C-induced mutagenic effects on the D7 strain of Saccharomyces cerevisiae and on its derivative mitochondrial mutant p degrees at different contents of glutathione and cytochrome P-450, molecules able to activate/detoxicate xenobiotics. The mutagenic activity of the drug was evaluated as frequency of mitotic gene conversion and reversion in different physiological conditions. The highest frequencies of reversion and especially of gene conversion were observed at the highest cytochrome P-450 contents in the D7 strain with a further increase at high glutathione level. In the respiratory-deficient strain, the highest frequency of convertants was shown at low glutathione level and lack of cytochrome P-450. These results suggest the relevance of mitochondrial functionality for the expression of genotoxic activity of this anticancer drug.

Effect of acute and chronic intermittent hypoxia on DNA topoisomerase II alpha expression and mitomycin C-induced DNA damage and cytotoxicity in human colon cancer cells

Recently, we reported that alterations in topoisomerase II (topo II) activity appear to contribute to mitomycin C (MMC) resistance in HT-29R13 human colon cancer cells under aerobic conditions. In this study, the expression of topo II alpha and topo II beta in parent HT-29 and MMC resistant variant HT-29R13 cells was investigated under aerobic, acute hypoxic (after 4 hr in 95% N2, 5% CO2 < 0.01% O2), and chronic intermittent hypoxic (after 4 hr hypoxia/day x 7 days) conditions. Acute hypoxia induced topo II alpha mRNA and protein, effects that were more pronounced in HT-29 cells. Chronic intermittent hypoxia caused a decrease in topo alpha mRNA and protein, changes that were again more pronounced in HT-29 cells. The observed changes in topo II alpha protein were associated with parallel changes in topo II activity under all conditions tested. Topo II beta mRNA was expressed at a very low level in both cell lines under aerobic and hypoxic conditions. Compared with cells under aerobic conditions, HT-29 cells were more sensitive to MMC under acute hypoxia but more resistant under chronic intermittent hypoxia. In contrast, the senstivity of HT-29R13 cells was unchanged under acute hypoxia, but the cells were more resistant under chronic intermittent hypoxia. Under all conditions tested, the degree of cytotoxicity corresponded to the frequency of MMC-induced DNA cross-links and topo II alpha protein levels and activity. Our results demonstrated that MMC cytotoxicity in hypoxic cells is highly dependent upon the type of hypoxia and the cell type. Hypoxia has significant effects on topo II alpha expression in HT-29 and HT-29R13 cells which correlate with MMC cytotoxicity.

Loss of heterozygosity at the NAD(P)H: quinone oxidoreductase locus associated with increased resistance against mitomycin C in a human bladder carcinoma cell line

The human bladder carcinoma cell line RT112 and the mitomycin C-resistant cell line RT112MMC, derived from RT112 cells, were examined for their expression of NAD(P)H:quinone oxidoreductase (NQOR) and glutathione S-transferases (GSTs). RT112 cells were 40-fold more sensitive towards mitomycin C than RT112MMC cells. The NQOR mRNA level in RT112MMC cells was decreased to 15% as compared to RT112 cells. NQOR enzyme activity was 391 +/- 140 mU/mg protein in RT112 cells, whereas NQOR activity in RT112MMC cells was not measurable. As shown by a fast PCR-based assay and DNA-sequencing, the cell line RT112 is heterozygous, whereas RT112MMC is homozygous for a null allele of the NQOR gene without enzymatic activity. Accordingly, both wild-type and null allele mRNAs were present in RT112 cells, whereas only null allele mRNA was found in RT112MMC. The lack of NQOR enzyme activity in RT112MMC cells was thus associated with loss of heterozygosity at the NQOR locus. By a PCR-RFLP assay, three kidney carcinoma patients without measurable NQOR enzyme activity were shown to be homozygous for the null allele. The PCR assay described here is useful for examination of large numbers of samples. The relative amount of GST-Pi mRNA was decreased by 30% in RT112MMC as compared to RT112, contributing to a diminished level of GST enzyme activity, using CDNB as a substrate, from 95 +/- 62 mU/mg protein in RT112 to 26 +/- 6 mU/mg protein in RT112MMC.