Constitutive nuclear factor-kappa B mRNA, protein overexpression and enhanced DNA-binding activity in thymidylate synthase inhibitor-resistant tumour cells

In this study, the gene copy number, mRNA and protein expression levels and nuclear DNA-binding activity of nuclear factor kappa B (NF-kappa B) were compared in a panel of five pairs of thymidylate synthase (TS) inhibitor-resistant and wild-type parent cancer cell lines. High constitutive NF-kappa B DNA-binding activity was detected in all chemoresistant cell lines. The upregulated NF-kappa B activity was composed of NF-kappa B subunits p50 and p65. Four out of five resistant cell lines constitutively overexpressed NF-kappa B p50 and p65 mRNA and protein. One resistant cell line with the highest NF-kappa B DNA-binding activity showed normal p50 and p65 protein expression. No NF-kappa B gene amplification was detected in resistant cell lines. Transient exposure of wild-type RKO(WT) and H630(WT) cells to 5-FU induced NF-kappa B DNA-binding activity but had no effect on NF-kappa B protein expression in these cells. Our results indicate that high constitutive NF-kappa B activity caused by its gene overexpression is an intrinsic character of TS inhibitor-resistant cells. NF-kappa B can antagonise anticancer drug-induced apoptosis. High NF-kappa B expression and nuclear activity in TS inhibitor-resistant cancer cells may play an important role in the chemoresistance.

Crystal structure of a deletion mutant of human thymidylate synthase Delta (7-29) and its ternary complex with Tomudex and dUMP

The crystal structures of a deletion mutant of human thymidylate synthase (TS) and its ternary complex with dUMP and Tomudex have been determined at 2.0 A and 2.5 A resolution, respectively. The mutant TS, which lacks 23 residues near the amino terminus, is as active as the wild-type enzyme. The ternary complex is observed in the open conformation, similar to that of the free enzyme and to that of the ternary complex of rat TS with the same ligands. This is in contrast to Escherichia coli TS, where the ternary complex with Tomudex and dUMP is observed in the closed conformation. While the ligands interact with each other in identical fashion regardless of the enzyme conformation, they are displaced by about 1.0 A away from the catalytic cysteine in the open conformation. As a result, the covalent bond between the catalytic cysteine sulfhydryl and the base of dUMP, which is the first step in the reaction mechanism of TS and is observed in all ternary complexes of the E. coli enzyme, is not formed. This displacement results from differences in the interactions between Tomudex and the protein that are caused by differences in the environment of the glutamyl tail of the Tomudex molecule. Despite the absence of the closed conformation, Tomudex inhibits human TS ten-fold more strongly than E. coli TS. These results suggest that formation of a covalent bond between the catalytic cysteine and the substrate dUMP is not required for effective inhibition of human TS by cofactor analogs and could have implications for drug design by eliminating this as a condition for lead compounds.

A dose-finding study of raltitrexed (tomudex) with cisplatin and epirubicin in advanced gastro-oesophageal adenocarcinoma

The standard treatment for advanced gastro-oesophageal cancer in the UK is epirubicin, cisplatin and continuous infusion 5-fluoruracil by an indwelling central venous catheter (ECF), which has significant morbidity. Raltitrexed (tomudex), a specific inhibitor of thymidylate synthase with a long plasma terminal half-life (50-100 h) has activity in gastro-intestinal tract malignancy. To reduce the Hickman line-associated morbidity of ECF; we have conducted a dose-finding study of tomudex combined with epirubicin and cisplatin. Twenty-four patients (22 males, two female), median age 63 years (range 21-75), ECOG performance status < or =2 with histologically proven, unresectable or metastatic gastric (14 patients), gastro-oesophageal junction (nine patients) or oesophageal (one patient) adenocarcinoma received treatment with 3-weekly cisplatin 60 mg m(-2), epirubicin 50 mg m(-2) and tomudex at doses of 2 mg m(-2), 2.5 mg m(-2) or 3 mg m(-2) in successive cohorts. Six patients were treated per dose level with no intra-patient dose escalation. Dose escalation occurred after six patients had completed at least one cycle of chemotherapy at the previous dose level. After defining the maximum tolerated dose a further six patients were treated at the preceding dose level to assess toxicity at the proposed phase II dose. A total of 102 cycles (50% completed 6 cycles) were administered. The dose-limiting toxicities are neutropenia and diarrhoea occurring in 2/6 patients at the 3 mg m(-2) dose level. Of those patients evaluable for response, there were eight partial and one complete response (overall response rate 38%). The median survival was 9.9 months. ECT is an active regimen in oesophagogastric adenocarcinoma. The recommended dose of tomudex for further study in combination with epirubicin and cisplatin is 2.5 mg m(-2).

Involvement of bcl-2 and p21waf1 proteins in response of human breast cancer cell clones to Tomudex

Mechanisms of resistance to Tomudex include increased thymidylate synthase activity, as well as reduced intracellular drug uptake and polyglutamation. However, little is known about other mechanisms of resistance, such as a possible protection against Tomudex-induced apoptosis mediated by bcl-2. We transfected the MDA-MB-435 human breast cancer cell line, which is characterized by a mutated p53 gene, with cDNA of the bcl-2 gene and generated two clones (MDA-bcl4 and MDA-bcl7) characterized by bcl-2 expression twofold and fourfold that observed in the control cell clone (MDAneo). A concomitant overexpression of p21wafl was also detected in the MDA-bcl7 clone. The MDA-bcl4 clone was three times more resistant to a 24-h Tomudex exposure than the MDAneo clone, whereas the MDA-bcl7 clone was as sensitive to Tomudex as the control cell clone. A lower sensitivity of the MDA-bcl4 clone than MDAneo and MDA-bcl7 clones to 5-fluorouracil and gemcitabine was also observed. No significant difference was noted in the susceptibility of clones to fludarabine and methothrexate. Basal levels of thymidylate synthase activity were superimposable in the three clones. Tomudex induced a marked accumulation of cells in the S phase in all the clones. However, an apoptotic hypodiploid DNA peak and the characteristic nuclear morphology of apoptosis were observed only in the MDA-bcl7 clone after exposure to Tomudex. No difference in the treatment-induced modulation of proteins involved in cell cycle progression (cyclin A, cdk2, pRB, E2F-1) and apoptosis (bcl-2, bax) was observed in the three clones. The only exception was that the expression of p21wafl in the MDA-bcl4 clone was inducible at a Tomudex concentration much higher than that required to induce the protein in the other clones. Overall, the results indicate that bcl-2 and p21wafl proteins concur in determining the cellular profile of sensitivity/resistance to Tomudex.

Antisense down-regulation of thymidylate synthase to suppress growth and enhance cytotoxicity of 5-FUdR, 5-FU and Tomudex in HeLa cells

1. Thymidylate synthase (TS), the key enzyme in de novo synthesis of thymidine, is an important target for antitumour chemotherapy. It was hypothesized that antisense oligonucleotide down-regulation of TS mRNA would decrease TS levels and enhance the cytotoxicity of inhibitors of TS, including the pyrimidine analogues 5-fluorouracil (5-FU) and 5-fluorodeoxyuridine (5-FUdR), and the folate analogue Tomudex (ICI D1694; N-(5-[N-(3, 4-dihydro-2-methyl-4-oxoquinazolin-6-ylmethyl)-N-methylamino ]-2-theon yl-L-glutamic acid). 2. 2'-Methoxyethoxylated, phosphorothioated 20-mer oligodeoxynucleotides (ODNs), complementary to various sequences in TS mRNA, were synthesized, along with control oligomers consisting of the same, respective bases in randomized order, against which all the biological effects were compared. Following a 6-h transfection of HeLa cells using polycationic liposome at 3 microg ml(-1), ODN 83 (50 nM), complementary to a region in the 3'-untranslated region of the TS mRNA, decreased TS mRNA levels by approximately 70% within 24 h. ODN 83 also decreased TS enzyme activity, as measured by binding of TS to radiolabelled 5-fluorodeoxyuridine monophosphate. In addition to inhibiting proliferation by up to approximately 40%, ODN 83 enhanced the cytotoxicity of Tomudex or 5-FU, added 1 day following transfection, by 50 - 60%. ODN 83 also enhanced sensitivity to 5-FUdR by 70%, but did not affect the toxicity of cisplatin, chlorambucil, melphalan, doxorubicin, ionizing radiation, paclitaxel, or irinotecan. 3. These data indicate that antisense ODN down-regulation of TS can inhibit human tumour cell proliferation and enhance the efficacy of TS-targeted drugs.