Activity of antifolates against Pneumocystis carinii dihydrofolate reductase and identification of a potent new agent.

The therapy of Pneumocystis carinii (PC) pneumonia is often unsuccessful, particularly in patients with acquired immune deficiency syndrome (AIDS). Because of difficulties in growing the organism in vitro or obtaining purified organisms, current treatment choices have been made with little information on the metabolic effects of therapeutic agents on PC. This report quantitates the effects of the commonly used antifolates as well as the classic antineoplastic antifolate methotrexate and a lipid-soluble analogue, trimetrexate, on the target enzyme, dihydrofolate reductase (DHFR), in the PC organisms. Trimethoprim and pyrimethamine were found to be weak inhibitors (ID50 = 39,600 and 2,800 nM, respectively), while methotrexate and trimetrexate were potent reductase inhibitors (ID50 = 1.4 and 26.1 nM, respectively). transport studies with radiolabeled compounds showed that compounds with the classic folate structure (methotrexate and leucovorin) were not taken up by the intact PC organisms. In contrast, trimetrexate exhibited rapid uptake. These results suggest a major therapeutic advantage may be gained by combining a potent, readily transported PC DHFR inhibitor such as trimetrexate with the reduced folate leucovorin to achieve a highly potent antiprotozoan effect while preventing toxicity to mammalian cells.

Effect of methotrexate on intracellular folate pools in purified myeloid precursor cells from normal human bone marrow.

We investigated the effects of the antifolate methotrexate on intracellular folate pools of human myeloid precursor cells (MPCs). Immature MPCs, representing 3.2% of the original marrow population, were selected from normal human bone marrow by immune rosetting. The intracellular folate pools were labeled by incubation with 5 X 10(-8) M [3H]5-formyl-FH4 and were quantitated by high performance liquid chromatography. The predominant folates were 5-methyl-tetrahydrofolate (5-methyl-FH4) (36%), 10-formyl-FH4 (41.4%), 5-formyl-FH4 (12.3%), and FH4 (10.3%). A 12-h exposure to 1 microM methotrexate (MTX) resulted in a 34% reduction in the intracellular concentration of 10-formyl-FH4, a 61% decrease in 5-formyl-FH4, and a 62% decrease in 5-methyl-FH4, as well as the appearance and progressive expansion of the FH2 and 10-formyl-FH2 pools. These changes were maximal after 4 h of incubation with MTX. Paralleling the changes in folates, particularly the increase in FH2, were a 64% reduction in myeloid colony formation and a 77% depression of de novo purine synthesis after 4 h of MTX. We conclude that MTX does not produce quantitative depletion of 10-formyl-FH4 and that its antipurine effect may be mediated by direct inhibition of de novo purine synthesis by FH2 and, at later time points, by MTX polyglutamates.

Interaction of sulfonamide and sulfone compounds with Toxoplasma gondii dihydropteroate synthase.

Toxoplasma gondii is a common protozoan disease that often causes life-threatening disease, particularly among patients with the acquired immunodeficiency syndrome. This study demonstrates that the dihydropteroate synthase in T. gondii is kinetically distinct from the enzyme characterized from other sources and can be highly purified with a high yield using sequential dye-affinity chromatography. Conditions have been identified that allow for stabilization of the purified enzyme, and its physical characteristics have been elucidated. The molecular weight of the native protein was 125,000 and the protein appeared to contain both dihydropteroate synthase and 6-hydroxymethyl-dihydropterin pyrophosphokinase activities. The sulfonamide class of compounds vary in inhibitory potency by more than three orders of magnitude. Sulfathiazole, sulfamethoxazole, and sulfamethazine, with 50% inhibitory concentrations (IC50's) of 1.7, 2.7, and 5.7 microM, respectively, represent the most potent of this class of inhibitors. Several sulfone analogues, including dapsone, were identified as highly potent inhibitors with IC50's less than 1 microM. The results of these cell-free experiments were corroborated by investigating the metabolic inhibition produced by the various inhibitors in intact organisms. The qualitative and quantitative relations among the inhibitors were preserved in both the cell-free and intact cell assay systems. These studies suggest that the sulfones may be important therapeutic agents for the treatment of toxoplasmosis.

Potent in vitro and in vivo antitoxoplasma activity of the lipid-soluble antifolate trimetrexate.

Trimetrexate, a highly lipid-soluble quinazoline antifolate now undergoing trials as an anticancer agent, was found to be a potent inhibitor of the dihydrofolate reductase (DHFR) isolated from Toxoplasma gondii. The concentration required for 50% inhibition of protozoal DHFR was 1.4 nM. As an inhibitor of this enzyme, trimetrexate was almost 600-fold (amount of antifolate required to inhibit catalytic reaction by 50%) and 750-fold (inhibition constant) more potent than pyrimethamine, the DHFR inhibitor currently used to treat toxoplasma infection. When the protozoan was incubated with 1 microM trimetrexate, the drug rapidly reached high intracellular concentrations. Since toxoplasma organisms lack a transmembrane transport system for physiologic folates, host toxicity can be prevented by co-administration of the reduced folate, leucovorin, without reversing the antiprotozoal effect. The effectiveness of trimetrexate against toxoplasma was demonstrated both in vitro and vivo. Proliferation of toxoplasma in murine macrophages in vitro was completely inhibited by exposure of these cells to 10(-7) M trimetrexate for 18 h. When used alone, trimetrexate was able to extend the survival of T. gondii-infected mice.

Identification of a thymidylate synthase ribonucleoprotein complex in human colon cancer cells.

Translation of thymidylate synthase (TS) mRNA is controlled by its own protein product, TS, in an autoregulatory manner. Direct binding of TS protein to two different cis-acting elements on the TS mRNA is associated with this translational regulation. In this study, an immunoprecipitation-reverse transcription-PCR technique was used to identify a TS ribonucleoprotein (RNP) complex in cultured human colon cancer cells. Using antibodies specific for TS protein, we show that TS is complexed in vivo with its own TS RNA. Furthermore, evidence demonstrating a direct interaction between the mRNA of the nuclear oncogene c-myc and TS protein is presented.

Thymidylate synthase protein and p53 mRNA form an in vivo ribonucleoprotein complex.

A thymidylate synthase (TS)-ribonucleoprotein (RNP) complex composed of TS protein and the mRNA of the tumor suppressor gene p53 was isolated from cultured human colon cancer cells. RNA gel shift assays confirmed a specific interaction between TS protein and the protein-coding region of p53 mRNA, and in vitro translation studies demonstrated that this interaction resulted in the specific repression of p53 mRNA translation. To demonstrate the potential biological role of the TS protein-p53 mRNA interaction, Western immunoblot analysis revealed nearly undetectable levels of p53 protein in TS-overexpressing human colon cancer H630-R10 and rat hepatoma H35(F/F) cell lines compared to the levels in their respective parent H630 and H35 cell lines. Polysome analysis revealed that the p53 mRNA was associated with higher-molecular-weight polysomes in H35 cells compared to H35(F/F) cells. While the level of p53 mRNA expression was identical in parent and TS-overexpressing cell lines, the level of p53 RNA bound to TS in the form of RNP complexes was significantly higher in TS-overexpressing cells. The effect of TS on p53 expression was also investigated with human colon cancer RKO cells by use of a tetracycline-inducible system. Treatment of RKO cells with a tetracycline derivative, doxycycline, resulted in 15-fold-induced expression of TS protein and nearly complete suppression of p53 protein expression. However, p53 mRNA levels were identical in transfected RKO cells in the absence and presence of doxycycline. Taken together, these findings suggest that TS regulates the expression of p53 at the translational level. This study identifies a novel pathway for regulating p53 gene expression and expands current understanding of the potential role of TS as a regulator of cellular gene expression.

Disruption of Myc-tubulin interaction by hyperphosphorylation of c-Myc during mitosis or by constitutive hyperphosphorylation of mutant c-Myc in Burkitt's lymphoma.

Somatic mutations at Thr-58 of c-Myc have been detected in Burkitt's lymphoma (BL) tumors and have been shown to affect the transforming potential of the Myc oncoprotein. In addition, the N-terminal domain of c-Myc has been shown to interact with microtubules in vivo, and the binding of c-Myc to alpha-tubulin was localized to amino acids 48 to 135 within the c-Myc protein. We demonstrate that c-Myc proteins harboring a naturally occurring mutation at Thr-58 from BL cell lines have increased stability and are constitutively hyperphosphorylated, which disrupts the in vivo interaction of c-Myc with alpha-tubulin. In addition, we show that wild-type c-Myc-alpha-tubulin interactions are also disrupted during a transient mitosis-specific hyperphosphorylation of c-Myc, which resembles the constitutive hyperphosphorylation pattern of Thr-58 in BL cells.

Thymidylate synthase binds to c-myc RNA in human colon cancer cells and in vitro.

Using an immunoprecipitation-reverse transcription-PCR technique, we characterized a thymidylate synthase (TS) ribonucleoprotein complex in cultured human colon cancer cells that consists of TS protein and the mRNA of the nuclear oncogene c-myc. TS protein is complexed in intact cells with the C-terminal coding region of c-myc mRNA that includes nucleotide positions 1625 to 1790. RNA electrophoretic gel mobility shift assays confirm a specific interaction between TS protein and c-myc mRNA and provide additional evidence that the C-terminal coding region represents an important cis-acting regulatory element. Further evidence demonstrates that the in vitro translational efficiency of c-myc mRNA is inhibited as a result of its direct interaction with TS protein. In addition, the presence of exogenous c-myc mRNA specifically relieves the inhibitory effects of TS protein on TS mRNA translation.

Characterization of a cis-acting regulatory element in the protein coding region of thymidylate synthase mRNA.

Thymidylate synthase (TS) functions as an RNA-binding protein by interacting with two different sequences on its own mRNA. One site is located in the 5'-upstream region of human TS mRNA while the second site is located within the protein coding region corresponding to nt 434-634. In this paper, a 70 nt RNA sequence, corresponding to nt 480-550, was identified that binds TS protein with an affinity similar to that of full-length TS mRNA and TS434-634 RNA. In vitro translation studies confirmed that this sequence is critical for the translational autoregulatory effects of TS. To document in vivo biological significance, TS sequences contained within this region were cloned onto the 5'-end of a luciferase reporter plasmid and transient transfection experiments were performed using H630 human colon cancer cells. In cells transfected with p644/TS434-634 or p644/TS480-550, luciferase activity was decreased 2.5-fold when compared to cells transfected with p644 plasmid alone. Luciferase mRNA levels were identical for each of these conditions as determined by RNase protection and RT-PCR analysis. Immunoprecipitation of TS ribonucleoprotein complexes revealed a direct interaction between TS protein and TS480-550 RNA in transfected H630 cells. Treatment with 5-fluorouridine resulted in a nearly 2-fold increase in luciferase activity only in cells transfected with p644/TS434-634 and p644/TS480-550. This study identifies a 70 nt TS response element in the protein coding region of TS mRNA with in vitro and in vivo translational regulatory activity.

Toxicity of levamisole and 5-fluorouracil in human colon carcinoma cells.

The combination of 5-fluorouracil (5-FU) with the immunomodulator levamisole (Lev) has been clinically tested in patients with metastatic colorectal carcinoma and as adjuvant therapy following primary tumor surgery. In some studies in advanced disease, the addition of Lev to 5-FU improved the median duration of response; in the adjuvant setting, the combination was associated with improvement in the disease-free survival. We studied whether Lev was directly toxic to three human colorectal carcinoma cell lines (HCT 116, SNU-C4, and NCI-H630). We also evaluated the toxicity of Lev in combination with 5-FU in these three cell lines. Lev inhibited the growth of all three colorectal cell lines, but only at concentrations two logs above that achieved with a standard 150-mg oral dose of Lev. In cell growth studies, 500 and 1,000 microM Lev increased the toxicity of 5-FU in HCT 116 cells in an additive fashion. In clonogenic assays, continuous exposure to 10 or 100 microM Lev was minimally toxic and did not enhance the lethality associated with a 24-hour exposure to 5-FU in any of the cell lines. Lev alone at 1,000 microM decreased colony formation by 45% in HCT 116 cells. A combination of 1,000 microM Lev with 10 microM 5-FU resulted in a decrease in HCT 116 colony formation from 54% to 6% of control levels. Continuous exposure of NCI-H630 cells to 500 microM Lev decreased colony formation to 76.5% of control levels; when Lev was combined with 50 microM 5-FU, colony formation was decreased from 59.5% to 27.5% of control levels. We conclude that at concentrations achievable with conventional doses of Lev, there was no evidence of direct toxicity in these colorectal cell lines. Furthermore, an additive interaction with 5-FU was evident only at suprapharmacologic doses of Lev.

Decreased catabolism of fluorouracil in peripheral blood mononuclear cells during combination therapy with fluorouracil, leucovorin, and interferon alpha-2a.

BACKGROUND: We previously reported that recombinant interferon alpha-2a (IFN alpha-2a) therapy was associated with a dose-dependent decrease in fluorouracil (5-FU) clearance. PURPOSE: In this study, we used peripheral blood mononuclear cells (PBMCs), which are responsive to IFNs, as surrogate tissue to determine whether the change in clearance might be explained by decrease in 5-FU catabolism during IFN alpha-2a therapy. METHODS: The study population consisted of 45 patients with adenocarcinoma arising in the gastrointestinal tract. Thirty-seven patients received therapy containing IFN alpha-2a at a median dose of 5 million U/m2 per day (range, 1.7-7.5 million U/m2 per day) starting on day 1 and continuing through either day 7 or day 14 in conjunction with intravenous high-dose leucovorin (LV) followed by bolus 5-FU on days 2-6. Eight patients received the same schedule of 5-FU and LV daily for 5 days without IFN alpha-2a but with granulocyte-macrophage colony-stimulating factor starting on day 6 and ending at least 3 days prior to the start of the next cycle. Peripheral blood was collected during 70 cycles on days 1, 2, and 4 prior to the daily treatment with IFN alpha-2a + 5-FU+LV and during 19 cycles on days 1 and 4 prior to the daily treatment with 5-FU+LV without IFN alpha-2a. In a given patient cycle, matched samples were drawn at approximately the same time of day. PBMCs were isolated, and the intact cells were exposed to 4 microM [3H]5-FU, and the formation of [3H]dihydrofluorouracil was determined by reverse-phase high-performance liquid chromatography. RESULTS: In 47 matched patient cycles from IFN alpha-2a + 5-FU+LV-treated patients in which samples were available on days 1, 2, and 4, 5-FU catabolism decreased by 20% (P2 = .03) and 41% (P2 = .0001) from the baseline catabolic rate (2.5 +/- 0.2 pmol/min per 10(6) cells [mean +/- SE]) on days 2 and 4, respectively. Using information from all paired samples, the mean change from baseline on day 2 was -0.4 +/- 0.2 pmol/min per 10(6) cells (n = 54; P2 = .05), and the change from baseline on day 4 was -1.3 +/- 0.3 pmol/min per 10(6) cells (n = 63; P2 = .0001). In contrast, changes in 5-FU catabolism were not evident in the PBMCs of the reference population receiving 5-FU+LV without IFN alpha-2a. CONCLUSIONS: The magnitude of the change in 5-FU catabolism is similar to the magnitude of the decrease in 5-FU clearance in our previous study. These observations suggest that changes in 5-FU catabolism during therapy with IFN alpha-2a, 5-FU, and LV may account for the decreased 5-FU clearance.

Enhancement of fluorinated pyrimidine-induced cytotoxicity by leucovorin in human colorectal carcinoma cell lines.

Reduced folates have been shown to increase the cytotoxicity of 5-fluorouracil (5-FU) by stabilizing the 5-fluoro-2'-deoxyuridine-5'-monophosphate-thymidylate synthase complex, thus increasing the block in the DNA synthetic pathway. Using an in vitro colorimetric [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] cytotoxicity assay, we tested the effects of 5-FU and 5-fluoro-2'-deoxyuridine (FdUrd) with and without leucovorin (LV) on a panel of 11 human colorectal carcinoma cell lines. The effect of LV on 5-FU and FdUrd was quantitatively similar. A clinically achievable level of LV (20 microM) increased the cytotoxicity in all three replicate experiments in 10 of the 11 cell lines (P less than .05, binomial test). LV alone at a concentration of 20 microM had no effect on cell survival. In three cell lines, 50% inhibition of growth occurred at a clinically achievable area under the curve of 5-FU alone. With the addition of LV, one additional cell line showed 50% growth inhibition at a clinically achievable level of 5-FU. Hence large clinical trials may be necessary to detect a significant improvement in survival as a result of adding LV to the fluorinated pyrimidines.

Thymidylate synthase expression and response to neoadjuvant chemotherapy in patients with advanced head and neck cancer.

BACKGROUND: Thymidylate synthase (TS), an essential enzyme in DNA synthesis, is a target for the fluoropyrimidines, an important group of antineoplastic agents used widely in the treatment of head and neck cancer. PURPOSE: We evaluated relationships between the level and/or pattern of tumor TS expression and response to fluorouracil (5-FU)-based neoadjuvant chemotherapy in patients with advanced head and neck cancer. METHODS: Tumor specimens from 86 patients were available for this retrospective analysis. The patients were enrolled in four consecutive phase II studies that tested combinations of 5-FU, leucovorin, and cisplatin with or without added methotrexate plus piritrexim or interferon alfa-2b (IFN alpha-2b). TS protein expression in the tumors was assessed by use of the TS 106 monoclonal antibody and standard immunohistochemical staining techniques. TS immunostaining was classified according to its level of intensity (TS 0-1 = low, TS 2 = intermediate, and TS 3 = high) and according to its extent (focal pattern = less than 25% of tumor cells positive; diffuse pattern = greater than or equal to 25% of tumor cells positive). Data from 79 patients were available for an analysis of tumor TS expression and patient/tumor characteristics; 70 patients were assessable for their response to neoadjuvant chemotherapy. RESULTS: There was a statistically significant association between the level of tumor TS expression and the degree of tumor differentiation; a higher proportion of patients whose tumors exhibited TS 0-1 immunostaining had undifferentiated or poorly differentiated tumors than patients whose tumors exhibited TS 2 or TS 3 immunostaining (P = .04, Jonckheere-Terpstra trend test). Among the 70 patients who were assessable for response to neoadjuvant chemotherapy, TS 3 tumor immunostaining was associated with a lower rate of complete response (i.e., complete disappearance of clinically detectable disease for a minimum of 4 weeks from time of initial determination) than was TS 2 or TS 0-1 immunostaining, but this association was not statistically significant (P = .09, exact trend test); among the 39 patients who were treated with regimens that included 5-FU, leucovorin, cisplatin, and IFN alpha-2b, this inverse association between TS immunostaining intensity and response was statistically significant (P = .02, exact trend test). Tumor TS immunostaining intensity and overall survival were not found to be associated. Patients with tumors exhibiting a focal pattern of TS immunostaining have experienced significantly longer survival than patients with tumors exhibiting a diffuse pattern; for the 53 patients with diffuse tumor TS immunostaining, the median survival was 24.7 months, whereas the median survival has not yet been reached for the 22 patients with focal tumor TS immunostaining (P = .04, two-tailed logrank test). However, the survival advantage for the focal versus diffuse TS immunostaining pattern was limited to patients whose tumors also exhibited a TS 3 level of immunostaining intensity. CONCLUSIONS AND IMPLICATIONS: Characterization of tumor TS expression may be of value in identifying patients with advanced head and neck cancer who would benefit from fluoropyrimidine-based neoadjuvant chemotherapy.

Interaction of methotrexate polyglutamates and dihydrofolate during leucovorin rescue in a human breast cancer cell line (MCF-7).

Previous investigations have suggested that high-dose methotrexate with leucovorin rescue is a potentially useful strategy for overcoming antifolate resistance. Interactions between methotrexate (MTX) and leucovorin and their respective metabolites appear to occur at multiple intracellular sites, including dihydrofolate reductase (MTX/MTX polyglutamates versus dihydrofolate) and other folate-dependent enzymes (MTX polyglutamates versus reduced folate substrates). The present studies were designed to test the ability of dihydrofolate to compete with methotrexate and methotrexate polyglutamates for dihydrofolate reductase activity using an intact human breast carcinoma cell line (MCF-7) as the model system. Exposure of the breast cells to methotrexate for 24 h resulted in a concentration-dependent formation of methotrexate polyglutamates that markedly exceeded the dihydrofolate reductase-binding capacity for up to 24 h after the removal of drug from the growth media. Under these conditions of dihydrofolate reductase inhibition, we found that tritium-labeled dihydrofolate was capable of competing with methotrexate and its metabolites for dihydrofolate reductase activity as evidenced by the appearance of tritium-labeled reduced folates in the treated cells. We found the interaction between dihydrofolate and methotrexate to be dependent on the exposure concentrations of both methotrexate and dihydrofolate. These studies provide direct evidence that competition during leucovorin rescue occurs at the level of dihydrofolate reductase between methotrexate polyglutamates and dihydrofolate polyglutamates in intact human cells.

Intracellular metabolism of 5-formyl tetrahydrofolate in human breast and colon cell lines.

This report describes the intracellular metabolism of 5-formyltetrahydrofolate into the various one-carbon substituted folate and polyglutamate pools in a human breast (MCF-7) and colon (HCT 116) carcinoma cell line. Metabolism into the one-carbon substituted pools was found to be time and dose dependent over a concentration range up to 50 microM. A 3-fold increase in total intracellular folate was noted over a 50-fold concentration range (1-50 microM) of 5-formyltetrahydrofolate tested in the colon cell line, while in the breast line, a 6-fold increase was detected over a 500-fold concentration range (0.1-50 microM). The level of 5, 10-methylenetetrahydrofolate, which was detectable only in the breast cell line, was found to increase by a factor of 10 (1.8 pmol/mg to 17.9 pmol/mg) over the concentration range studied. The majority of metabolism was into the 10-formyltetrahydrofolate and tetrahydrofolate pools in the breast cells and into the 5-methyltetrahydrofolate pool in the colon cells. Polyglutamation was also time and dose dependent, with a significant proportion of the total pool represented by the higher polyglutamate forms (Glu3-Glu5) after 24 h of continuous exposure to 5-formyl tetrahydrofolate. Pentaglutamate was the highest level noted in both cell lines. The intracellular half-life of the polyglutamate forms was inversely related to the length of the polyglutamate tail with half-lives of 71, 131, 143, 441, and 1167 min for the mono- through pentaglutamate, respectively. Finally, up to a 20:1 ratio of the biologically inactive (6R) isomer to active (6S) isomer of 5-formyltetrahydrofolate resulted in no effect on metabolism into the one-carbon substituted folate pools and only minimal decreases in metabolism to the polyglutamate forms. These studies suggest that prolonged exposure to even relatively low doses of 5-formyltetrahydrofolate may be optimal for intracellular metabolism to the most biologically relevant forms for ternary complex formation with thymidylate synthase and fluorodeoxyuradylate, since longer exposures result in a greater accumulation of the higher polyglutamates.

Enhancement of the toxicity and DNA incorporation of arabinosyl-5-azacytosine and 1-beta-D-arabinofuranosylcytosine by cyclopentenyl cytosine.

We evaluated the interaction of a biochemically active concentration of cyclopentenyl cytosine (CPE-C), an investigational antimetabolite which inhibits CTP synthetase, on the cytotoxicity of arabinosyl-5-azacytosine (Ara-AC) and 1-beta-D-arabinofuranosylcytosine (Ara-C) in HCT 116 colon carcinoma cells. A 3-h exposure to 0.5 microM CPE-C depleted CTP pools by over 90% and decreased dCTP pools by 57%; the effect on CTP pools persisted for up to 24 h following washout of CPE-C. A 3-h pre-exposure to 0.5 microM CPE-C augmented the growth inhibition resulting from a 24-h exposure to Ara-AC. The combination of 1 microM cytidine and deoxycytidine fully reversed the enhancement associated with CPE-C pretreatment, to a level of growth inhibition expected from either CPE-C or Ara-AC alone. A striking enhancement of toxicity was observed in clonogenic studies with pre-exposure to CPE-C at a nonlethal dose followed by either Ara-AC or Ara-C. CPE-C increased the formation of Ara-AC and Ara-C nucleotides by as much as 3-fold, and this was accompanied by increased incorporation of the arabinosyl nucleotides into methanol-precipitable material. Analysis of purified RNase-treated nucleic acids by cesium sulfate density centrifugation confirmed that a 3-h pre-exposure to CPE-C increased [3H]-Ara-C incorporation into DNA at 4 and 24 h by 2.4- and 2.7-fold, respectively. Thus, these studies indicate that CPE-C can function as a biochemical modulator. Following a brief exposure to a nonlethal concentration, CPE-C is capable of augmenting the cytotoxicity and intracellular metabolism of Ara-AC and Ara-C.

Epitope mapping of a series of human thymidylate synthase monoclonal antibodies.

We have reported previously the development and application of several monoclonal antibodies to thymidylate synthase (TS). In this study, we used a series of overlapping 17-mer peptides that spanned the entire TS protein to map the epitope recognized by three TS monoclonal antibodies (TS 106, TS 109, and TS 110). Using an ELISA, we identified two peptides (R126-F142 and L131-R147) that bound all three antibodies, which suggests that each antibody recognized a similar epitope on TS. A second set of peptides, representing sequential single-residue truncations from either the amino terminus or the carboxyl terminus starting with a G129-E145 17-mer, was synthesized. A 10-amino acid sequence P133-F142 (PVYGFQWRHF) was identified as the binding epitope for all three antibodies. Further investigation via substitution mutational analysis of each residue within this epitope revealed that residues F137, W139, R140, H141, and F142 were critical for maximal binding of TS 106 and TS 110. TS 109 showed a similar pattern except in regard to R140, with which there was no apparent loss of binding. In addition to the utility of the three antibodies in detecting and measuring TS levels, identification of the binding locus permits the potential application of these antibodies in the investigation of TS enzymatic and regulatory function.

Interaction of gamma interferon and 5-fluorouracil in the H630 human colon carcinoma cell line.

The antiproliferative effects and pharmacological interactions of 5-fluorouracil (5-FU) in combination with gamma interferon (IFN-gamma) were determined against the human colon carcinoma H630 cell line in vitro. H630 was 9-fold more resistant to 5-FU, as compared to a relatively sensitive human colon line (C1). IFN-gamma showed modest antiproliferative activity against the H630 line, with a 50% inhibitory concentration of 440 units/ml. Simultaneous treatment of H630 with subinhibitory concentrations of IFN-gamma and 5-FU produced a significant enhancement of the 5-FU-associated growth inhibition. The growth-inhibitory activity of the combination against H630 was prevented by the addition of 20 microM thymidine. Thymidylate synthase (TS) activity was measured by both the 5-fluoro-2'-deoxyuridine-5'-monophosphate binding and catalytic assays, using cytosolic extracts. A 24-h exposure to 1 microM 5-FU in the H630 line resulted in a 3.1-fold increase in the total amount of TS, while in the 5-FU/IFN-gamma-treated cells TS remained unchanged from non-drug-treated control levels. Moreover, we found that free thymidylate synthase in the 5-FU/IFN-gamma-treated cells was significantly decreased, as compared to the cells treated with 5-FU alone. Incorporation of 5-FU into both the RNA and DNA fractions did not change with the addition of IFN-gamma. Accumulation of the fluoropyrimidine metabolites 5-fluoro-2'-deoxyuridine-5'-monophosphate and 5-fluorouridine-5'-triphosphate remained the same for 5-FU alone and the combination treatment. These findings suggest that acute TS induction by 5-FU may provide an important mechanism by which human colon carcinoma cells express decreased sensitivity to 5-FU and that IFN-gamma can reverse the development of resistance to 5-FU in the H630 line by inhibiting the overexpression of TS that results from 5-FU exposure. These studies contribute to a growing understanding of the complex interaction between 5-FU and IFN-gamma.

Immunological quantitation of thymidylate synthase using the monoclonal antibody TS 106 in 5-fluorouracil-sensitive and -resistant human cancer cell lines.

Thymidylate synthase (TS) (EC 2.1.1.45) is an important cellular target for the fluoropyrimidine cytotoxic drugs that are widely used in the treatment of solid tumors. Using the TS 106 monoclonal antibody to human TS, we have compared the immunological quantitation of TS by Western immunoblot and immunofluorescent techniques to the conventional biochemical 5-fluorodeoxyuridine monophosphate binding assay in a panel of 5-fluorouracil (5-FU)-sensitive and -resistant human cancer cell lines. Densitometric quantitation of TS 106-labeled Western immunoblot analysis of cell lysates from two 5-FU-resistant colon carcinoma cell lines, NCI H630R1 and NCI H630R10, revealed 12.8- and 16-fold increases in TS, respectively, compared to the parent 5-FU-sensitive NCI H630 colon cell line. By biochemical analysis, the TS level was 15- and 23-fold higher, respectively, in these resistant cell lines. Similarly, immunoblot analysis of cell lysates from two 5-FU-resistant breast cancer cell lines, MCF-Ad5 and MCF-Ad10, detected a 2.3- and 6.3-fold increase in TS, respectively, compared to the parent MCF-7 cell line. By biochemical assay the TS activity was 1.8- and 7.0-fold higher in these resistant breast cell lines. Western immunoblotting analysis revealed a 35-fold range of TS protein concentrations among the 10 cell lines examined, compared to a 38-fold range demonstrated by the biochemical assay. Direct comparison of Western blotting and the biochemical assay revealed a highly significant correlation (r2 = 0.93) between the two assays. Moreover, using the monoclonal antibody TS 106, the Western blotting technique was capable of detecting TS protein levels as low as 0.3 fmol in cellular lysates. Quantitation of TS in intact cells by immunofluorescent TS labeling and flow cytometric analysis was performed using three of the cell lines, NCI H630, NCI H630R10, and MCF-Ad10. This revealed a 26-fold increase in TS in the 5-FU-resistant NCI H630R10 line compared to the parent NCI H630 line and a 3.5-fold increase in TS compared to the 5-FU-resistant MCF-Ad10 breast cell line. The 5-FU-resistant MCF-Ad10 breast cell line, in turn, displayed a 7.7-fold increase in TS, compared to the 5-FU-sensitive NCI H630 cell lines. TS immunofluorescent analysis was capable of measuring TS within individual cells. The development of these immunological assays using an anti-TS monoclonal antibody will facilitate the quantitation of TS in cell lines and tissue samples.

Identification of a protein factor secreted by 3T3-L1 preadipocytes inhibitory for the human MCF-7 breast cancer cell line.

The 3T3-L1 cell line is a preadipocyte cell line derived from the Swiss 3T3 mouse fibroblast cell line. We have compared the effect of 3T3-L1 conditioned medium (3T3-L1 CM) and Swiss 3T3 conditioned medium (3T3 CM) on the growth of normal mouse mammary cells (NMMG) and the human MCF-7 breast carcinoma cell line. 3T3 CM increased the growth of both NMMG and MCF-7 cells by 19 +/- 2% (SD) and 24 +/- 3%, respectively, and increased thymidine incorporation by 74 +/- 4% and 104 +/- 8%, respectively. Conditioned medium from 3T3-L1 cells stimulated the growth of NMMG cells by 64 +/- 2%; in contrast, 3T3-L1 CM inhibited the growth of MCF-7 cells by 36 +/- 1%. In parallel with these growth studies, thymidine incorporation increased by 20 +/- 4% in NMMG cells and decreased by 72 +/- 5% in the MCF-7 cells. Moreover, a similar effect was also noted in NCI H630 colon cancer cells, where 3T3-L1 CM produced a 58 +/- 4% decrease in growth and a 82 +/- 6% decrease in thymidine incorporation. Heating the 3T3-L1 CM at 100 degrees C for 30 min destroyed all inhibitory activity. Several known inhibitory growth factors (fibroblast growth factor, 20 ng/ml; interleukin 6, 1000 units/ml; tumor necrosis factor alpha, 15 ng/ml; transforming growth factor beta, 1 ng/ml) were tested for activity in the MCF-7 cells. Tumor necrosis factor alpha and transforming growth factor beta produced a 97% and 67% inhibition of thymidine uptake, respectively, whereas interleukin 6 and fibroblast growth factor had no effect. Neither transforming growth factor beta nor tumor necrosis factor alpha activity was detectable in 3T3-L1 CM using an enzyme-linked immunosorbent assay. High-performance liquid chromatography fractionation of the 3T3-L1 CM revealed that the inhibitory activity eluted at a molecular weight of 67,000; moreover, silver staining of these eluates on a denaturing polyacrylamide gel revealed that M(r) 69,000 peptide was the predominant protein band in the inhibitory fractions. Thus 3T3-L1 CM stimulates the growth of normal breast epithelial cells and inhibits the growth of MCF-7 breast cancer cells. This inhibitory activity appears to be due to a protein secreted by 3T3-L1 preadipocytes.