Benzamide riboside induced mitochondrial mediated apoptosis in human lung cancer H520 cells

Structural, biological and computational study of oxamide derivative

A dicarboxylato-diamide-type compound 2,2'-[(1,2-dioxoethane-1,2- -diyl)diimino]dibenzoic acid (H4obbz) (1) was synthesized and characterized. The crystal structure of K2H2obbz?2H2O (2) was determined by X-ray diffracttion analysis. The cytotoxic activities of the compounds were tested against four different cancer cell lines MCF-7, A549, HT-29, HeLa and a human normal cell line MRC-5. The results indicate reasonable dose-dependent cytotoxicity of the ligands that show selectivity against the tested carcinoma and healthy cell lines. Flow cytometric analysis and fluorescence microscopy showed that the most active compound, H4obbz, induced apoptosis and G0/G1 cell cycle arrest, indicating blockage of DNA synthesis as a possible mechanism that triggers apoptosis. Docking and molecular dynamics simulations gave similar responses regarding interactions (binding) between their ligands and chaperon Grp78. The MMGBSA determined ?G binding energies were in the range from -104 to -140 kJ mol-1.

Purine-Metabolising Enzymes and Apoptosis in Cancer

The enzymes of both de novo and salvage pathways for purine nucleotide synthesis are regulated to meet the demand of nucleic acid precursors during proliferation. Among them, the salvage pathway enzymes seem to play the key role in replenishing the purine pool in dividing and tumour cells that require a greater amount of nucleotides. An imbalance in the purine pools is fundamental not only for preventing cell proliferation, but also, in many cases, to promote apoptosis. It is known that tumour cells harbour several mutations that might lead to defective apoptosis-inducing pathways, and this is probably at the basis of the initial expansion of the population of neoplastic cells. Therefore, knowledge of the molecular mechanisms that lead to apoptosis of tumoural cells is key to predicting the possible success of a drug treatment and planning more effective and focused therapies. In this review, we describe how the modulation of enzymes involved in purine metabolism in tumour cells may affect the apoptotic programme. The enzymes discussed are: ectosolic and cytosolic 5'-nucleotidases, purine nucleoside phosphorylase, adenosine deaminase, hypoxanthine-guanine phosphoribosyltransferase, and inosine-5'-monophosphate dehydrogenase, as well as recently described enzymes particularly expressed in tumour cells, such as deoxynucleoside triphosphate triphosphohydrolase and 7,8-dihydro-8-oxoguanine triphosphatase.

Selective ATP hydrolysis inhibition in F1Fo ATP synthase enhances radiosensitivity in non-small-cell lung cancer cells (A549)

Background

F1Fo-ATP synthase (F1Fo-ATPase) is a reversibly rotary molecular machine whose dual functions of synthesizing or hydrolyzing ATP switch upon the condition of cell physiology. The robust ATP-hydrolyzing activity occurs in ischemia for maintaining the transmembrane proton motive force of mitochondria inner membrane, but the effect of F1Fo-ATPase on X-ray response of non-small-cell lung cancer (NSCLC) cells is unknown.

Methods and Findings

We studied whether ATP hydrolysis affected X-ray radiation induced cell death. NSCLC cells (A549) were pretreated with BTB06584 (BTB), an elective ATP hydrolysis inhibitor, followed by X-ray radiation. Cell viability and clonogenic survival were markedly decreased, clear indications of enhanced radiosensitivity through BTB incubation. Additionally, ATP5α1 was upregulated in parallel with elevated ATP hydrolytic activity after X-ray radiation, showing an increased mitochondrial membrane potential (ΔΨm). ATP hydrolysis inhibition led to collapse of ΔΨm suggesting ATP hydrolytic activity could enhance ΔΨm after X-ray radiation. Furthermore, we also demonstrated that apoptosis was pronounced with the prolonged collapse of ΔΨm due to hydrolysis inhibition by BTB incubation.

Conclusion

Overall, these findings supported that ATP hydrolysis inhibition could enhance the radiosensitivity in NSCLC cells (A549) after X-ray radiation, which was due to the collapse of ΔΨm.

Evodiamine induces G2/M arrest and apoptosis via mitochondrial and endoplasmic reticulum pathways in H446 and H1688 human small-cell lung cancer cells

The goal of this study was to evaluate the ability of EVO to decrease cell viability and promote cell cycle arrest and apoptosis in small cell lung cancer (SCLC) cells. Lung cancer has the highest incidence and mortality rates among all cancers. Chemotherapy is the primary treatment for SCLC; however, the drugs that are currently used for SCLC are less effective than those used for non-small cell lung cancer (NSCLC). Therefore, it is necessary to develop new drugs to treat SCLC. In this study, the effects of evodiamine (EVO) on cell growth, cell cycle arrest and apoptosis were investigated in the human SCLC cell lines NCI-H446 and NCI-H1688. The results represent the first report that EVO can significantly inhibit the viability of both H446 and H1688 cells in dose- and time-dependent manners. EVO induced cell cycle arrest at G2/M phase, induced apoptosis by up-regulating the expression of caspase-12 and cytochrome C protein, and induced the expression of Bax mRNA and by down-regulating of the expression of Bcl-2 mRNA in both H446 and H1688 cells. However, there was no effect on the protein expression of caspase-8. Taken together, the inhibitory effects of EVO on the growth of H446 and H1688 cells might be attributable to G2/M arrest and subsequent apoptosis, through mitochondria-dependent and endoplasmic reticulum stress-induced pathways (intrinsic caspase-dependent pathways) but not through the death receptor-induced pathway (extrinsic caspase-dependent pathway). Our findings suggest that EVO is a promising novel and potent antitumor drug candidate for SCLC. Furthermore, the cell cycle, the mitochondria and the ER stress pathways are rational targets for the future development of an EVO delivery system to treat SCLC.

A Review of the Potential Utility of Mycophenolate Mofetil as a Cancer Therapeutic

Tumor cells adapt to their high metabolic state by increasing energy production. To this end, current efforts in molecular cancer therapeutics have been focused on signaling pathways that modulate cellular metabolism. However, targeting such signaling pathways is challenging due to heterogeneity of tumors and recurrent oncogenic mutations. A critical need remains to develop antitumor drugs that target tumor specific pathways. Here, we discuss an energy metabolic pathway that is preferentially activated in several cancers as a potential target for molecular cancer therapy. In vitro studies have revealed that many cancer cells synthesize guanosine triphosphate (GTP), via the de novo purine nucleotide synthesis pathway by upregulating the rate limiting enzyme of this pathway, inosine monophosphate dehydrogenase (IMPDH). Non-proliferating cells use an alternative purine nucleotide synthesis pathway, the salvage pathway, to synthesize GTP. These observations pose IMPDH as a potential target to suppress tumor cell growth. The IMPDH inhibitor, mycophenolate mofetil (MMF), is an FDA-approved immunosuppressive drug. Accumulating evidence shows that, in addition to its immunosuppressive effects, MMF also has antitumor effects via IMPDH inhibition in vitro and in vivo. Here, we review the literature on IMPDH as related to tumorigenesis and the use of MMF as a potential antitumor drug.

Overexpression of CK20, MAP3K8 and EIF5A correlates with poor prognosis in early-onset colorectal cancer patients

PURPOSE

Due to ethnic, genetic and environmental factors, the clinical and molecular characteristics of Turkish colorectal cancer (CRC) patients are different from those of Western populations. The aim of this study was to clarify the relevant alterations of gene expression associated with colorectal carcinogenesis in early-onset patients and to identify specific biomarkers that could provide novel therapeutic molecular targets in this population.

METHODS

The expression profiles of 114 different genes were evaluated using mRNA PCR arrays in 39 tumors and 20 surgical margin tissue samples from 39 sporadic CRC patients diagnosed at less than 50 years of age.

RESULTS

The expression levels of IMPDH2, CK20, MAP3K8 and EIF5A were strongly up-regulated in CRC tissues compared with normal colorectal tissues (p < 0.05). The highly significant expression ratios of CK20, MAP3K8 and EIF5A observed in the colorectal tumors of patients predicted recurrence (p < 0.05). The expression of IMPDH2, CK20, MAP3K8 and EIF5A was significantly higher in the tumors of patients with short median survival (log-rank p value < 0.05). Progression-free survival was also significantly increased in patients with low expression of the EIF5A gene compared with those who exhibited high expression of this gene (log-rank p value < 0.05).

CONCLUSION

We demonstrated that high CK20, MAP3K8 and EIF5A expression levels were significant prognostic factors for poor overall survival in CRC patients. Further studies and validations are required; these genes may provide novel therapeutic molecular targets for CRC treatment, as well as new directions for the development of anticancer drugs.

MicroRNA-128 downregulates Bax and induces apoptosis in human embryonic kidney cells

MicroRNAs (miRNAs) are short ~21-nt non-coding RNA molecules that have been shown to regulate a number of biological processes. Previous reports have shown that overexpression of miR-128 in glioma cells inhibited cell proliferation. Literature also suggests that miR-128 negatively regulates prostate cancer cell invasion. Here, we show that overexpression of hsa-miR-128, a brain-enriched microRNA, induces apoptosis in HEK293T cells as elucidated by apoptosis assay, cell cycle changes, loss of mitochondrial membrane potential and multicaspase assay. By in silico analysis, we identified a putative target site within the 3' untranslated region (UTR) of Bax, a proapoptotic member of the apoptosis pathway. We found that ectopic expression of hsa-miR-128 suppressed a luciferase reporter containing the Bax-3' UTR and reduced the levels of Bax in HEK293T cells. Taken together, our study demonstrates that overexpression of hsa-miR-128 not only induces apoptosis in HEK293T cells but also is an endogenous regulator of Bax protein.

Overexpression of inosine 5'-monophosphate dehydrogenase type II mediates chemoresistance to human osteosarcoma cells

Background

Chemoresistance is the principal reason for poor survival and disease recurrence in osteosarcoma patients. Inosine 5′-monophosphate dehydrogenase type II (IMPDH2) encodes the rate-limiting enzyme in the de novo guanine nucleotide biosynthesis and has been linked to cell growth, differentiation, and malignant transformation. In a previous study we identified IMPDH2 as an independent prognostic factor and observed frequent IMPDH2 overexpression in osteosarcoma patients with poor response to chemotherapy. The aim of this study was to provide evidence for direct involvement of IMPDH2 in the development of chemoresistance.

Methodology/Principal Findings

Stable cell lines overexpressing IMPDH2 and IMPDH2 knock-down cells were generated using the osteosarcoma cell line Saos-2 as parental cell line. Chemosensitivity, proliferation, and the expression of apoptosis-related proteins were analyzed by flow cytometry, WST-1-assay, and western blot analysis. Overexpression of IMPDH2 in Saos-2 cells induced strong chemoresistance against cisplatin and methotrexate. The observed chemoresistance was mediated at least in part by increased expression of the anti-apoptotic proteins Bcl-2, Mcl-1, and XIAP, reduced activation of caspase-9, and, consequently, reduced cleavage of the caspase substrate PARP. Pharmacological inhibition of IMPDH induced a moderate reduction of cell viability and a strong decrease of cell proliferation, but no increase in chemosensitivity. However, chemoresistant IMPDH2-overexpressing cells could be resensitized by RNA interference-mediated downregulation of IMPDH2.

Conclusions

IMPDH2 is directly involved in the development of chemoresistance in osteosarcoma cells, suggesting that targeting of IMPDH2 by RNAi or more effective pharmacological inhibitors in combination with chemotherapy might be a promising means of overcoming chemoresistance in osteosarcomas with high IMPDH2 expression.

Identification of IMPDH2 as a tumor-associated antigen in colorectal cancer using immunoproteomics analysis

BACKGROUND AND AIMS

Sera from cancer patients contain tumor-specific autoantibodies directly against antigenic proteins. The identification of tumor autoantigens may have utility in cancer diagnosis, prognosis, and therapy. In this study, we used immunoproteomics analysis to identify tumor proteins that elicit humoral response in colorectal cancer (CRC).

MATERIALS AND METHODS

The CRC cell line HCT116 was used as a source of proteins for two-dimensional polyacrylamide gel electrophoresis and subsequent Western blot analysis in which individual serum from patients with CRC was analyzed for autoantibodies. Proteins that specifically react with sera from cancer patients were identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometric analysis. In addition, the selected protein expression in tumor tissues collected from 40 patients with CRC were assessed by immunohistochemistry.

RESULTS

An autoantibody against inosine monophosphate dehydrogenase II (IMPDH2) identified by mass spectrometry was detected in eight out of 25 patients with CRC. However, none of the 15 healthy controls demonstrated autoantibody to IMPDH2.The expression of IMPDH2 in tumor tissue was significantly higher in patients with CRC than that in healthy subjects.

CONCLUSIONS

The result confirmed that the immunoproteomics analysis holds considerable promise for the discovery of tumor-associated antigens. IMPDH2 may be a protein biomarker and novel therapeutic target in CRC.

Upregulation of miR-23a-27a-24-2 cluster induces caspase-dependent and -independent apoptosis in human embryonic kidney cells

miRNAs have emerged as important players in the regulation of gene expression and their deregulation is a common feature in a variety of diseases, especially cancer. Currently, many efforts are focused on studying miRNA expression patterns, as well as miRNA target validation. Here, we show that the over expression of miR-23a approximately 27a approximately 24-2 cluster in HEK293T cells induces apoptosis by caspase-dependent as well as caspase-independent pathway as proved by the annexin assay, caspase activation, release of cytochrome-c and AIF (apoptosis inducing factor) from mitochondria. Furthermore, the over expressed cluster modulates the expression of a number of genes involved in apoptosis including FADD (Fas Associated protein with Death Domain). Bioinformatically, FADD is predicted to be the target of hsa-miR-27a and interestingly, FADD protein was found to be up regulated consistent with very less expression of hsa-miR-27a in HEK293T cells. This effect was direct, as hsa-miR-27a negatively regulated the expression of FADD 3'UTR based reporter construct. Moreover, we also showed that over expression of miR-23a approximately 27a approximately 24-2 sensitized HEK293T cells to TNF-alpha cytotoxicity. Taken together, our study demonstrates that enhanced TNF-alpha induced apoptosis in HEK293T cells by over expression of miR-23a approximately 27a approximately 24-2 cluster provides new insights in the development of novel therapeutics for cancer.

Apoptotic signaling induced by benzamide riboside: an in vitro study

Benzamide riboside (BR) is a novel anticancer agent exhibiting potent cytotoxic activity in malignant cell lines. However, the mechanism of induction of apoptosis is not clear. The purpose of this study was to elucidate the apoptotic signaling induced by BR on different human cancer cell lines. Our results revealed that BR at a dose of 50 microM induces apoptosis in SiHa, Hep2, and Ca Ski cells as studied by morphology and flow cytometry. A downregulation of anti-apoptotic proteins Bcl-2 and Bcl-xL was observed, whereas the expression level of the pro-apoptotic protein Bax remained unaffected. An upregulation of p53 was observed while no change was seen on the level of apoptosis inducing factor (AIF). A significant increase in caspase-3 and -9 activities was seen, which was accompanied by PARP cleavage. Release of cytochrome c from the mitochondria to the cytosol was also observed. Taken together, the findings suggest that BR induces apoptosis in SiHa, Hep2, and Ca Ski cells via the intrinsic mitochondrial pathway.

Apoptogenic effect of 7,8-diacetoxy-4-methylcoumarin and 7,8-diacetoxy-4-methylthiocoumarin in human lung adenocarcinoma cell line: role of NF-kappaB, Akt, ROS and MAP kinase pathway

Coumarin (1,2-benzopyrone) is a naturally occurring fragrant compound found in a variety of plants and spices. Coumarins have attracted intense interest in recent years because of their diverse pharmacological activities. This study examines the antioxidant coumarin 7,8-diacetoxy-4-methylcoumarin (DAMC) and its thiocoumarin derivative 7,8-diacetoxy-4-methylthiocoumarin (DAMTC) for their effect on human non-small cell lung cancer A549 cells. Here we show that both DAMC and DAMTC not only inhibited cell proliferation, but also induced apoptosis with an IC(50) of 160 microg/ml as confirmed by morphological examination, annexin-V assay and flow cytometric analysis. Interestingly, it was observed that these two coumarin compounds exhibited little cytotoxicity towards peripheral blood mononuclear cells but induced apoptosis in malignant cells. DAMC/DAMTC treatment also resulted in pronounced release of apoptogenic cytochrome c from mitochondria to cytosol, alteration of mitochondrial membrane potential (DeltaPsi(m)), and activation of caspase-9 and caspase-3. Although an increase in the levels of reactive oxygen species (ROS) was observed, pre-treatment with antioxidant showed no protective effect against DAMC/DAMTC-induced apoptosis. Results of present study suggest that downregulation of Bcl-xl, Cox-2 and mitogen activated protein kinase pathway and upregulation of p53, Akt and NF-kappaB pathway are involved in the underlying molecular mechanism of apoptosis induction by DAMC and DAMTC in A549 cells.

Antiproliferative effects of AVN944, a novel inosine 5-monophosphate dehydrogenase inhibitor, in prostate cancer cells

Inosine 5-monophosphate dehydrogenase II, a key enzyme in the de novo synthesis of purine nucleotides, is expressed in prostate tumors and prostate cancer cells. AVN944 is a new, specific, noncompetitive IMPDH inhibitor. In this study, we investigated the effects of IMPDH inhibitor AVN944 on LNCaP, CWR22Rv1, DU145 and PC-3 human prostate cancer cells. AVN944 inhibited proliferation of these 4 prostate cancer cell lines and was associated with cell cycle G1 arrest of LNCaP cells and S-phase block of androgen-independent CWR22Rv1, DU145 and PC-3 cells. AVN944 induced caspase-dependentand caspase-independent cell death in LNCaP, CWR22Rv1, and DU145 cells. AVN944 induced expression of p53-target proteins Bok, Bax and Noxa in androgen-responsive cell lines and suppressed expression of survivin in prostate cancer cells regardless of their androgen sensitivity. AVN944 also induced differentiation of androgen-independent prostate cancer cells as indicated by morphological changes and increased expression of genes coding for prostasomal proteins, keratins and other proteins, including tumor suppressor genes MIG-6 and NDRG1. AVN944-differentiated androgen-independent DU145 and PC-3 cells are sensitized to TRAIL-induced apoptosis as demonstrated by induction of caspases and PARP cleavage. In summary, AVN944 inhibited the growth of human prostate cancer cells by inducing cell cycle arrest, cell death as well as differentiation. AVN944 is a novel, promising therapeutic agent that might be combined with other agents for treatment of human prostate cancer.

Efficient synthesis of benzamide riboside, a potential anticancer agent

An efficient five step synthesis of benzamide riboside (BR) amenable for a large scale synthesis has been developed. It allows for extensive pre-clinical studies of BR as a potential anticancer agent.

7,8-Dihydroxy-4-methylcoumarin induces apoptosis of human lung adenocarcinoma cells by ROS-independent mitochondrial pathway through partial inhibition of ERK/MAPK signaling

Coumarins have attracted intense interest in recent years because they have been identified from natural sources, especially green plants and have diverse pharmacological properties. In this study, we investigated whether 7,8-dihydroxy-4-methylcoumarin (DHMC) caused apoptosis in A549 human non-small cell lung carcinoma cells (NSCLC) and, if so, by what mechanisms. Here, we show that, in A549 human NSCLC cells, DHMC induces apoptosis through mitochondria-mediated caspase-dependent pathway. Although an increase in the levels of reactive oxygen species (ROS) was observed, pre-treatment with antioxidant showed no protective effect against DHMC-induced apoptosis. In addition, our immunoblot data revealed that DHMC treatment led to down-regulation of Bcl-xl, Bax, p21, Cox-2, p53 and upregulation of c-Myc. Results in the present study for the first time suggest that DHMC induces apoptosis in human lung A549 cells through partial inhibition of ERK/MAPK signaling.

Enhancement of the infectivity of SARS-CoV in BALB/c mice by IMP dehydrogenase inhibitors, including ribavirin

Because of the conflicting data concerning the SARS-CoV inhibitory efficacy of ribavirin, an inosine monophosphate (IMP) dehydrogenase inhibitor, studies were done to evaluate the efficacy of ribavirin and other IMP dehydrogenase inhibitors (5-ethynyl-1-beta-D-ribofuranosylimidazole-4-carboxamide (EICAR), mizoribine, and mycophenolic acid) in preventing viral replication in the lungs of BALB/c mice, a replication model for severe acute respiratory syndrome (SARS) infections (Subbarao, K., McAuliffe, J., Vogel, L., Fahle, G., Fischer, S., Tatti, K., Packard, M., Shieh, W.J., Zaki, S., Murphy, B., 2004. Prior infection and passive transfer of neutralizing antibody prevent replication of severe acute respiratory syndrome coronavirus (SARS-CoV) in the respiratory tract of mice. J. Virol. 78, 3572-3577). Ribavirin given at 75 mg/kg 4 h prior to virus exposure and then given twice daily for 3 days beginning at day 0 was found to increase virus lung titers and extend the length of time that virus could be detected in the lungs of mice. Other IMP dehydrogenase inhibitors administered near maximum tolerated doses using the same dosing regimen as for ribavirin were found to slightly enhance virus replication in the lungs. In addition, ribavirin treatment seemed also to promote the production of pro-inflammatory cytokines 4 days after cessation of treatment, although after 3 days of treatment ribavirin inhibited pro-inflammatory cytokine production in infected mice, significantly reducing the levels of the cytokines IL-1alpha, interleukin-5 (IL-5), monocyte chemotactic protein-1 (MCP-1), and granulocyte-macrophage colony stimulating factor (GM-CSF). These findings suggest that ribavirin may actually contribute to the pathogenesis of SARS-CoV by prolonging and/or enhancing viral replication in the lungs. By not inhibiting viral replication in the lungs of infected mice, ribavirin treatment may have provided a continual source of stimulation for the inflammatory response thought to contribute to the pathogenesis of the infection. Our data do not support the use of ribavirin or other IMP dehydrogenase inhibitors for treating SARS infections in humans.

Progesterone receptor modulator CDB-2914 down-regulates proliferative cell nuclear antigen and Bcl-2 protein expression and up-regulates caspase-3 and poly(adenosine 5'-diphosphate-ribose) polymerase expression in cultured human uterine leiomyoma cells

The present study was conducted to evaluate the effects of the progesterone receptor modulator CDB-2914 on proliferative activity and apoptosis in cultured human uterine leiomyoma cells. Isolated leiomyoma cells were subcultured in phenol red-free DMEM supplemented with 10% fetal bovine serum for 120 h and then stepped down to serum-free conditions for 12, 24, 48, and 96 h in the absence or presence of graded concentrations of CDB-2914 (10(-9), 10(-8), 10(-7), and 10(-6) M). The number of viable cultured leiomyoma cells was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazodium bromide assay. Proliferating cell nuclear antigen (PCNA) expression was evaluated by immunocytochemistry and Western blot analysis. Apoptosis was examined by terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate nick end labeling (TUNEL) assay. Caspase-3, cleaved poly(ADP-ribose) polymerase (PARP), and Bcl-2 expression were assessed by Western blot analysis. Compared with untreated control cultures, treatment with CDB-2914 decreased the number of viable cultured leiomyoma cells and the PCNA-positive rate in those cells and increased the TUNEL-positive rate in cultured leiomyoma cells in a dose-dependent manner. Western blot analysis revealed that treatment with CDB-2914 significantly decreased the expression of PCNA and Bcl-2 protein and increased the expression of cleaved caspase-3 and cleaved PARP in a dose-dependent manner compared with untreated control cultures. These results suggest that CDB-2914 inhibits the proliferation of cultured leiomyoma cells by down-regulating PCNA expression and induces apoptosis by up-regulating cleaved caspase-3 and PARP expression and down-regulating Bcl-2 protein expression in those cells.

Role of human nucleoside transporters in the cellular uptake of two inhibitors of IMP dehydrogenase, tiazofurin and benzamide riboside

Benzamide riboside (BR) and tiazofurin (TR) are converted to analogs of NAD that inhibit IMP dehydrogenase (IMPDH), resulting in cellular depletion of GTP and dGTP and inhibition of proliferation. The current work was undertaken to identify the human nucleoside transporters involved in cellular uptake of BR and TR and to evaluate their role in cytotoxicity. Transportability was examined in Xenopus laevis oocytes and Saccharomyces cerevisiae that produced individual recombinant human concentrative nucleoside transporter (CNT) and equilibrative nucleoside transporter (ENT) types (hENT1, hENT2, hCNT1, hCNT2, or hCNT3). TR was a better permeant than BR with a rank order of transportability in oocytes of hCNT3 >> hENT1 > hENT2 > hCNT2 >> hCNT1. The concentration dependence of inhibition of [(3)H]uridine transport in S. cerevisiae by TR exhibited lower K(i) values than BR: hCNT3 (5.4 versus 226 microM), hENT2 (16 versus 271 microM), hENT1 (57 versus 168 microM), and hCNT1 (221 versus 220 microM). In cytotoxicity experiments, BR was more cytotoxic than TR to cells that were either nucleoside transport-defective or -competent, and transport-competent cells were more sensitive to both drugs. Exposure to nitrobenzylmercaptopurine ribonucleoside conferred resistance to BR and TR cytotoxicity to hENT1-containing CEM cells, thereby demonstrating the importance of transport capacity for manifestation of cytoxicity. A breast cancer cell line with mutant p53 exhibited 9-fold higher sensitivity to BR than the otherwise similar cell line with wild-type p53, suggesting that cells with mutant p53 may be potential targets for IMPDH inhibitors. Further studies are warranted to determine whether this finding can be generalized to other cell types.