Synergistic action of tiazofurin with hypoxanthine and allopurinol in human neuroectodermal tumor cell lines

5-FdUrd-araC heterodinucleoside re-establishes sensitivity in 5-FdUrd- and AraC-resistant MCF-7 breast cancer cells overexpressing ErbB2

ErbB2 overexpressing breast tumors have a poor prognosis and a high risk to develop chemoresistance to therapeutic treatment. "Chemoresistance" is a response of cells to toxic stress, and, although it is a common phenomenon, it is still poorly defined. However, a detailed understanding is required to target desensitized pathways and mechanisms for successful reactivation as part of a tailored therapy. To gain insight, which malfunctions contribute to chemoresistance, two mechanisms relevant for tissue homeostasis, the regulation of the cell cycle and of apoptosis, were investigated. Maternal MCF-7- and ErbB2-overexpressing MCF-7(erbB2) breast cancer cells were long term pretreated with 2'-deoxy-5-fluorodeoxyuridine (5-FdUrd) or 1-beta-d-arabinofuranosylcytosine (AraC) and the acquisition of drug-insensitivity was analyzed. A phosphate-conjugated heterodinucleoside consisting of one 5-FdUrd- and one AraC-moiety (5-fluoro-2'-desoxyuridylyl-(3'-->5')-Arabinocytidine) was utilized as a tool to assess the type of acquired resistances. ErbB2-overexpression disrupted proper cell cycle regulation and furthermore facilitated the development of an apoptosis-refractory phenotype upon exposure to 5-FdUrd. Experiments with dimer 5-FdUrd-araC in ErbB2-overexpressing MCF-7(erbB2) cells, and also with nucleoside 5-FdUrd in maternal MCF-7 cells, evidenced that the phenotypes of resistance to cell cycle inhibition and to apoptosis induction were differently affected. The expression profile of cyclin D1 (but not that of p53, p21, or p27) correlated with the proliferative phenotypes and nuclear accumulation of apoptosis inducing factor (but not activation of caspase 7) with apoptotic phenotypes. Dimer 5-FdUrd-araC overrode acquired chemoresistances, whereas combined application of 5-FdUrd and AraC exhibited significantly less activity. Dimer 5-FdUrd-araC remained active in MCF-7 clones most likely by circumventing the prerequisite of first-step phosphorylation. The acquisition of chemoresistance encompassed the affection of apoptosis- and cell-cycle regulation to, respectively, different extents. Thus, drug-induced cell cycle arrest and apoptosis induction are independent of each other.

Analysis of mechanisms contributing to AraC-mediated chemoresistance and re-establishment of drug sensitivity by the novel heterodinucleoside phosphate 5-FdUrd-araC

Chemoresistance is a biological response of cells to survive toxic stress. During cancer treatment the development of chemoresistance is a major problem. The mechanisms how cells become insensitive, and which downstream pathways are affected are not completely understood. Since it has not been well analysed which and how many regulative disorders are subsummised under the term "chemoresistance", we examined and measured arabinosylcytosine (AraC)-mediated desensitation of two mechanisms relevant for tissue homeostasis, cell cycle inhibition and apoptosis induction. MCF-7 cells harbouring ectopic mutated p53 were suitable for this investigation because they activated these mechanisms subsequently and became insensitive to AraC with regard to cell cycle inhibition and apoptosis induction. The major causal mechanism of acquired resistance against AraC was most likely through the inhibition of the first step of AraC phosphorylation within the cell, which is rate limiting for its activation. With regard to cell cycle inhibition AraC-resistant cells were also resistant against 5-fluorodeoxyuridine (5-FdUrd), but fully responsive to 5-FdUrd-induced apoptosis, evidencing that cell cycle and apoptosis are independent of each other. Apoptosis correlated with AIF-activation and was independent of Caspase 7, whereas cell cycle inhibition correlated with cyclinD1 expression but not with induction of p21 or p27. The phosphate conjugated 5-FdUrd-araC heterodimer (5-Fluoro-2'-desoxyuridylyl-(3'-->5')-Arabinocytidine), which is a prodrug of AraC-monophosphate, reactivated AIF and down-regulated cyclin D1 in AraC-resistant cells and circumvented resistance to apoptosis and to cell cycle inhibition. Also, cells which were resistant to 5-FdUrd or doxorubicin were sensitive to 5-FdUrd-araC. This investigation demonstrates that chemoresistance affects apoptosis induction and cell cycle inhibition independently and that detailed knowledge about the affected downstream pathways would enable the design of targeted intervention with small molecules to restore chemosensitivity.

Tiazofurin enhances the anabolism and toxicity of 5-fluorouracil

Tiazofurin, a clinically active anticancer agent, is undergoing additional clinical testing in combination with other agents. We found that tiazofurin is an effective biochemical modulator of 5-fluorouracil anabolism. Pretreatment of cultured L1210 cells with tiazofurin at concentrations of 1-100 microM results in an increase in the rate of conversion of 5-fluorouracil to phosphorylated metabolites. Concentrations of tiazofurin effective in increasing 5-fluorouracil anabolism cause a corresponding increase in the 5-phosphoribosyl-1-pyrophosphate pool. Studies in mice show that tiazofurin increases the lethal toxicity of 5-fluorouracil and increases the antitumor effectiveness of low doses of 5-fluorouracil: however, the combination is not more effective than an optimal dose of 5-fluorouracil given alone. These results indicate that caution should be exercised in the concurrent use of tiazofurin with other drugs, particularly 5-fluorouracil, that require 5-phosphoribosyl-1-pyrophosphate for activation or that are affected by a decrease in pyrimidine nucleotide synthesis.

Purine deoxynucleoside metabolism in human melanoma cells with a high spontaneous mutation rate

A human melanoma cell line (MM96L) had a spontaneous mutation rate at the HGPRT locus of approx. 7 times normal. The cells had elevated dATP and dGTP pools, lacked purine nucleoside phosphorylase (PNP) and were sensitive to killing by deoxyadenosine, deoxyinosine and related purines but not to inosine or hypoxanthine. Four other melanoma cell lines exhibited a range of nucleoside sensitivities and dNTP pool sizes. Failure of intact MM96L cells to degrade exogenous deoxyadenosine and deoxyinosine to hypoxanthine was confirmed by NMR of culture medium. Normal melanocytes were PNP+ and were insensitive to deoxyinosine. Comparison of the metabolites of [14C]deoxyinosine from MM96L and a PNP+ cell line of similar doubling time (HeLa) showed that both cell types produced 14C-labelled guanine and adenine nucleotides, with [14C]dATP and [14C]dADP being found in MM96L. This indicates that human sAMP synthetase or a similar enzyme catalyses the conversion of dIMP to dAMP, the resultant elevation of dATP causing base misincorporation and a mutator phenotype.