Long-lived crowded-litter mice have an age-dependent increase in protein synthesis to DNA synthesis ratio and mTORC1 substrate phosphorylation

Increasing mouse litter size [crowded litter (CL)] presumably imposes a transient nutrient stress during suckling and extends lifespan through unknown mechanisms. Chronic calorically restricted and rapamycin-treated mice have decreased DNA synthesis and mTOR complex 1 (mTORC1) signaling but maintained protein synthesis, suggesting maintenance of existing cellular structures. We hypothesized that CL would exhibit similar synthetic and signaling responses to other long-lived models and, by comparing synthesis of new protein to new DNA, that insight may be gained into the potential preservation of existing cellular structures in the CL model. Protein and DNA synthesis was assessed in gastroc complex, heart, and liver of 4- and 7-mo CL mice. We also examined mTORC1 signaling in 3- and 7-mo aged animals. Compared with controls, 4-mo CL had greater DNA synthesis in gastroc complex with no differences in protein synthesis or mTORC1 substrate phosphorylation across tissues. Seven-month CL had less DNA synthesis than controls in heart and greater protein synthesis and mTORC1 substrate phosphorylation across tissues. The increased new protein-to-new DNA synthesis ratio suggests that new proteins are synthesized more so in existing cells at 7 mo, differing from 4 mo, in CL vs. controls. We propose that, in CL, protein synthesis shifts from being directed toward new cells (4 mo) to maintenance of existing cellular structures (7 mo), independently of decreased mTORC1.

Participation in a 1,000-mile race increases the oxidation of carbohydrate in Alaskan sled dogs

The Alaskan Husky has been specifically bred for endurance performance and is capable of extreme endurance performance. We examined sled dogs in the trained state at the beginning of the race season and after a 1,600-km race (Iditarod). Our hypothesis was that lipids would be the predominant substrate during submaximal exercise in long-distance racing sled dogs, and a 1,600-km race would increase the reliance on lipids during an exercise bout at the same absolute exercise intensity. The experiments were completed over three testing periods, which were completed in January of two different years before participation in a 1,600-km race, or in March shortly after completion of a 1,600-km race. After determination of H(13)CO3 (-) recovery, the dogs were tested with primed continuous infusions of [1,1,2,3,3-(2)H]glycerol, [3-(13)C]lactate, or [6,6-(2)H2]glucose. During exercise, respiratory exchange ratio was significantly higher in raced (0.92 ± 0.01) compared with nonraced (0.87 ± 0.01) dogs. During exercise, glucose rate of appearance was potentially sustained by a large glycerol rate of disappearance with an increase in lactate rates of oxidation after a 1,600-km race. Therefore, contrary to our hypothesis, the sled dogs were dependent on carbohydrate energy sources, a reliance that increased further after participation in a 1,600-km race.

Assessment of mitochondrial biogenesis and mTORC1 signaling during chronic rapamycin feeding in male and female mice

Chronic inhibition of the protein synthesis regulator mTORC1 through rapamycin extends life span in mice, with longer extension in females than in males. Whether rapamycin treatment inhibits protein synthesis or whether it does so differently between sexes has not been examined. UM-HET3 mice were fed a control or rapamycin-supplemented (Rap) diet for 12 weeks. Protein synthesis in mixed, cytosolic (cyto), and mitochondrial (mito) fractions and DNA synthesis and mTORC1 signaling were determined in skeletal muscle, heart, and liver. In both sexes, mito protein synthesis was maintained in skeletal muscle from Rap despite decreases in mixed and cyto fractions, DNA synthesis, and rpS6 phosphorylation. In the heart, no change in protein synthesis occurred despite the decreased DNA synthesis. In the heart and liver, Rap males were more sensitive to mTORC1 inhibition than Rap females. In conclusion, we show changes in protein synthesis and mTORC1 signaling that differ by sex and tissue.

Calorie restriction does not increase short-term or long-term protein synthesis

Increased protein synthesis is proposed as a mechanism of life-span extension during caloric restriction (CR). We hypothesized that CR does not increase protein synthesis in all tissues and protein fractions and that any increased protein synthesis with CR would be due to an increased anabolic effect of feeding. We used short- (4 hours) and long-term (6 weeks) methods to measure in vivo protein synthesis in lifelong ad libitum (AL) and CR mice. We did not detect an acute effect of feeding on protein synthesis while liver mitochondrial protein synthesis was lower in CR mice versus AL mice. Mammalian target of rapamycin (mTOR) signaling was repressed in liver and heart from CR mice indicative of energetic stress and suppression of growth. Our main findings were that CR did not increase rates of mixed protein synthesis over the long term or in response to acute feeding, and protein synthesis was maintained despite decreased mTOR signaling.

AICAR treatment for 14 days normalizes obesity-induced dysregulation of TORC1 signaling and translational capacity in fasted skeletal muscle

The aim of this study was to determine the effect of 14 days of 5-aminoimidazole-4-carboxamide-1β-4-ribofuranoside (AICAR) treatment on mammalian target of rapamycin (mTOR) signaling and mTOR-regulated processes (i.e., translation initiation) in obese mouse skeletal muscle. Our hypothesis was that daily treatment (14 days) with AICAR would normalize obesity-induced alterations in skeletal muscle mTOR signaling and mTOR-regulated processes to lean levels and positively affect muscle mass. Fourteen-week-old male, lean (L; 31.3 g body wt) wild-type and ob/ob (O; 59.6 g body wt) mice were injected with the AMP-activated kinase (AMPK) activator AICAR (A) at 0.5 mg·g body wt(-1)·day(-1) or saline control (C) for 14 days. At 24 h after the last injection (including a 12-h fast), all mice were killed, and the plantar flexor complex muscle (gastrocnemius, soleus, and plantaris) was excised for analysis. Muscle mass was lower in OC (159 ± 12 mg) than LC, LA, and OA (176 ± 10, 178 ± 9, and 166 ± 16 mg, respectively) mice, independent of a body weight change. A decrease in obese muscle mass corresponded with higher muscle cross section staining intensity for lipid and glycogen, higher blood glucose and insulin levels, and lower nuclear-enriched fractions for peroxisome proliferator-activated receptor-γ coactivator-1α protein expression in OC skeletal muscle, which was normalized with AICAR treatment. AMPK and acetyl-cocarboxylase phosphorylation was reduced in OC mice and augmented by AICAR treatment in OA mice. Conversely, OC mice displayed higher activation of downstream targets (S6 kinase-1 and ribosomal protein S6) of mTOR and lower raptor-associated mTOR than LC mice, which were reciprocally altered after 14 days of AICAR treatment. Dysregulation of translational capacity was improved in OA mice, as assessed by sucrose density gradient fractionation of ribosomes, total and ribosome-associated RNA content, eukaryotic initiation factor 4F complex formation, and eukaryotic initiation factor 4G phosphorylation. These data show that short-term (14 days) AMPK agonist treatment augments regulatory processes in atrophic obese mouse skeletal muscle through the normalization of mTOR signaling and mRNA translation closer to lean levels.

Cell cycle regulation of the G0/G1 transition in 5-fluorouracil-sensitive and -resistant human colon cancer cell lines

PURPOSE

Resistance to 5-fluorouracil (5-FU) has been associated with thymidylate synthase (TS) gene amplification and increased TS protein levels. Increased TS protein expression has also been found to be a significant independent prognostic factor for disease-free survival and overall survival in patients treated with adjuvant 5-FU-based chemotherapy. In these studies and in our prior preclinical studies, TS has been considered a marker of proliferative capacity. The purpose of the current study was to further evaluate the association between TS levels and cell cycle regulation, by investigating cell cycle kinetics in a 5-FU-resistant cell line with constitutive overexpression of TS. The influence of increased TS levels on cell cycle progression may provide insight into methods to overcome 5-FU resistance.

MATERIALS

5-FU-sensitive NCI H630(WT) and 5-FU-resistant NCI H630(R1) (with 15- to 20-fold higher TS protein levels) were utilized in this investigation to determine the influence of constitutive overexpression of TS on cell cycle kinetics.

RESULTS

There was no apparent influence of increased TS levels on cell cycle distribution during asynchronous growth, and both cell lines reach plateau growth phase in 120 hours, arresting in G0/G1 as determined by flow cytometry. In the H630(WT) cells, this G0/ G1 arrest was associated with a 14- to 17-fold reduction in TS activity and protein levels (using the TS-106 monoclonal antibody), whereas in the H630(R1) cells, only a two- to fivefold reduction was noted. Flow cytometry analysis utilizing Ki-67 indicated that there was no evidence of a G0 population in the confluent H630(R1), whereas 26% +/- 7% of confluent H630(WT) cells were Ki-67 negative (G0) and the remainder had low Ki-67 signal intensity. Analysis of pRb phosphorylation and p16 and p21 expression suggested that the arrest point for both cell lines was before the point at which Rb phosphorylation takes place, yet the confluent H630(R1) cells had threefold higher p21 than confluent H630(WT) cells.

DISCUSSION

These data suggest that the 5-FU-resistant H630(R1) cell lines arrest at a later point in G0/G1 and have a potentially greater capacity for proliferation.

5-fluorouracil-mediated thymidylate synthase induction in malignant and nonmalignant human cells

Thymidylate synthase (TS, EC 2.1.1.45) is an important target enzyme for the fluoropyrimidines used in cancer chemotherapy. Studies have documented a 2- to 4-fold induction of TS protein following 5-fluorouracil (5-FU) treatment of malignant cells. We measured the effect that 5-FU exposure had on TS protein expression in nonmalignant human breast (MCF-10 and HBL-100), colorectal (ATCC Co18, Co112, and Co33), and bone marrow cells along with malignant breast (MCF-7) and colon (NCI-H630) cells. Twenty-four hours after plating, cells were treated with 0.01 to 10 microM of 5-FU for a period of 24 hr. TS was quantitated by Western immunoblot using monoclonal antibody TS106. Absolute levels of TS in nonmalignant cells were substantially lower than in the malignant lines, ranging from approximately 40% in HBL-100 cells to less than 10% in the colon lines. An approximately two-fold induction in the level of TS was found for all cell lines examined, and there was a strong dependence on 5-FU exposure concentration in free TS levels of MCF-WT, and total TS levels of H630-WT, normal bone marrow, and MCF-10 cells. The induction of TS following 5-FU exposure is a generally observed phenomenon in both malignant and nonmalignant cells, suggesting that a selective means for inhibiting this induction may be critical for the development of alternative therapeutic strategies using 5-FU and the antifolate TS inhibitors.

Resistance to tomudex (ZD1694): multifactorial in human breast and colon carcinoma cell lines

ZD1694 (Tomudex; TDX) is a quinazoline antifolate that, when polyglutamated, is a potent inhibitor of thymidylate synthase (TS), the enzyme that converts dUMP to dTMP. Continuous exposure of MCF-7 breast and NCI H630 colon cells to TDX, with stepwise increases in TDX up to 2.0 microM, resulted in stably resistant cell lines (MCFTDX and H630TDX) that were highly resistant to TDX. Initial studies revealed 34-fold increase in TS protein levels in MCFTDX and a 52-fold increase in TS levels in H630TDX cell lines. Despite continued exposure of these cells to 2.0 microM TDX, TS protein and TS mRNA expression decreased to parental levels in H630TDX cells, whereas in MCFTDX cells TS mRNA expression and TS protein levels remained elevated. Southern blot analysis revealed a 20-fold TS gene amplification in the MCFTDX cell line. TDX uptake was 2-fold higher in resistant MCFTDX cells than in parental MCF-7 cells, whereas in H630TDX cells TDX uptake was 50-fold less than that observed in parental H630 cells. In contrast, no change in the transport of either leucovorin or methotrexate into H630TDX cells was noted when compared with the H630 parental cells. In H630TDX cells, folylpolyglutamate synthetase (FPGS) activity was 48-fold less compared to parent H630 cells; however, FPGS mRNA expression was similar in both lines. H630TDX cells were also highly resistant to ZD9331, a novel quinazoline TS inhibitor that does not require polyglutamation, suggesting that defective transport by the reduced folate carrier was also an important mechanism of resistance in these cells. In MCFTDX and H630TDX resistant cells, several mechanisms of resistance are apparent: one increased TS expression; the others evolved over time from increased TS expression to decreased FPGS levels and decreased TDX transport.

Increased thymidylate synthase protein levels are principally associated with proliferation but not cell cycle phase in asynchronous human cancer cells

We have analysed cell cycle variations in thymidylate synthase (TS) protein in asynchronously growing NCl H630 and HT 29 colon cancer and MCF-7 breast cancer cell lines. Western immunoblot analysis using the TS 106 monoclonal antibody revealed a 14- to 24-fold variation in TS levels between the peak exponential and confluent growth phase in the three cell lines. Similar variations in TS levels and TS activity were detected using the 5-fluorodeoxyuridine monophosphate and deoxyuridine monophosphate biochemical assays. The percentage of cells in S-phase, which paralleled changes in TS levels, reached a maximum of 38-60% in asynchronous exponentially growing cells compared with 5-10% in confluent cells. In asynchronous exponential cells, analysis of TS levels in each cell cycle phase using two-parameter flow cytometric analysis revealed that TS protein levels were 1.3- to 1.5-fold higher in S than in G0/G1 phase cells, and 1.5- to 1.8-fold higher in G2/M than G0/G1 cells. Similar differences of 1.1- to 1.5-fold between G0/G1 and S-phase and 1.6- to 1.9-fold between G0/G1 and G2/M-phase were detected by Western immunoblot and biochemical assays. TS protein was not detectable by Western blot analysis, flow cytometry or biochemical analysis in the G0/G1 population of confluent cells. Twenty-six per cent of cells in this population were G0 cells compared with 2% in exponentially growing cells. In contrast to TS, a 4-fold difference in thymidine kinase (TK) was detected between G0/G1 and S-phase cells in exponentially growing MCF-7 cells. The level of TS enzyme is associated with cellular proliferation and the percentage of cells in S-phase; however, TS protein is not exclusively associated with S-phase in asynchronously growing cells. The variation in TS levels between exponentially growing and confluent cell population appears to be due to differences in TS levels between G0 and G1 cells.

The effect of dose and interval between 5-fluorouracil and leucovorin on the formation of thymidylate synthase ternary complex in human cancer cells

We examined the importance of dosing interval between leucovorin (LCV) and 5-fluorouracil (5-FU) on intracellular thymidylate synthase (TS) ternary complex, free TS and total TS protein levels in human MCF-7 breast and NCI H630 colon cancer cell lines. A 2- to 3-fold increase in total TS was noted when either cell line was exposed to 5-FU 10 microM plus LCV (0.01-10 microM) compared with a 1.4- to 1.6-fold increase in total TS due to 5-FU 10 microM alone. The amount of TS ternary complex formed was 2- to 3-fold higher in both cell lines treated with the combination of 5-FU and LCV compared with 5-FU alone. TS complex formation and total TS protein increased with LCV dose (0.1-10 microM). In MCF-7 cells, the maximal increase in total TS protein and TS ternary complex formation was observed when 5-FU was delayed for 4 h after the start of LCV exposure. In NCI H630 cells, maximal total TS protein and ternary complex formation occurred when 5-FU was delayed for 18 h after the start of LCV exposure. The amount of free TS did not change in either cell line whether 5-FU was given concurrently with LCV or delayed for up to 24 h. The accumulation rate of intracellular folates in the form of higher glutamates Glu3-Glu5 was rapid in MCF-7 cells (maximal formation after 4 h), whereas in H630 cells accumulation of higher polyglutamates continued to increase up to 18 h. The time of peak folate polyglutamate (Glu3-Glu5) formation coincided with the time of peak TS complex formation and total TS protein in each cell line. In these human carcinoma cell lines, the LCV dose and interval between 5-FU and LCV play a role in increased TS total protein and TS ternary complex; however, the amount of free TS is independent of the interval between 5-FU and LCV. The time-and dose-dependent increases in TS ternary complex and TS total protein are associated with differences in the accumulation of folate polyglutamates in these cell lines.

Thymidylate synthase gene amplification in human colon cancer cell lines resistant to 5-fluorouracil

A series of 5-fluorouracil (5-FU)-resistant human colon H630 cancer cell lines were established by continuous exposure of cells to 5-FU. The concentration of 5-FU required to inhibit cell proliferation by 50% (IC50) in the parent colon line (H630) was 5.5 microM. The 5-FU IC50 values for the resistant H630-R1, H630-R10, and H630-R cell lines were 11-, 29-, and 27-fold higher than that for the parent H630 cell line. Using both the radioenzymatic 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP) binding and catalytic assays for measurement of thymidylate synthase (TS) enzyme activity, there was significantly increased TS activity in resistant H630-R1 (13- and 23-fold), H630-R10 (37- and 40-fold), and H630-R (24- and 34-fold) lines, for binding and catalytic assays, respectively, compared with the parent H630 line. The level of TS protein, as determined by western immunoblot analysis, was increased markedly in resistant H630-R1 (23-fold), H630-R10 (33-fold), and H630-R (26-fold) cells. Northern analysis revealed elevations in TS mRNA levels in H630-R1 (18-fold), H630-R10 (39-fold), and H630-R (36-fold) cells relative to parent H630 cells. Although no major rearrangements of the TS gene were noted by Southern analysis, there was significant amplification of the TS gene in 5-FU-resistant cells, which was confirmed by DNA slot blot analysis. These studies demonstrate that continuous exposure of human colon cancer cells to 5-FU leads to TS gene amplification and overexpression of TS protein with resultant development of fluoropyrimidine resistance.

The role of thymidylate synthase expression in prognosis and outcome of adjuvant chemotherapy in patients with rectal cancer

PURPOSE

We assessed the prognostic importance of the level of thymidylate synthase (TS) expression in patients with primary rectal cancer and whether, for Dukes' B and C cancer patients, the benefit of chemotherapy was associated with TS expression.

PATIENTS AND METHODS

The level of TS expression in the primary rectal cancers of 294 of 801 patients enrolled on protocol R-01 of the National Surgical Adjuvant Breast and Bowel Project (NSABP) was immunohistochemically assessed with the monoclonal antibody TS 106.

RESULTS

Forty-nine percent of patients whose tumors had low TS levels (n = 91) were disease free at 5 years compared with 27% of patients with high levels of TS (n = 203; P < .01). Moreover, 60% of patients with low TS levels were alive after 5 years compared with 40% of patients with high TS levels (P < .01). The level of TS protein was significantly associated with Dukes' stage (P < .01); patients with a more advanced Dukes' stage had a significantly higher level of TS. The level of TS expression remained prognostic for both disease-free survival (P < .01) and survival (P < .05) independent of Dukes' stage and other pathologic characteristics evaluated. Thirty-eight percent and 54% of patients with high TS levels (n = 71) were disease free and alive, respectively, after 5 years when treated with chemotherapy, compared with 17% and 31%, respectively, of similar patients when treated with surgery alone (n = 64) (P < .01). No difference was noted in disease-free survival (P = .46) or survival (P = .43) in patients with low TS levels.

CONCLUSION

The expression of TS is an important independent prognosticator of disease-free survival and survival in patients with rectal cancer. Adjuvant fluorouracil (5-FU)-based chemotherapy demonstrated significant improvement in disease-free and overall survival for patients with high TS levels. Prospective studies measuring TS levels will be needed to understand further the role of TS as a prognosticator of survival and chemotherapeutic benefit.

Measurement of aspartate carbamoyltransferase activity by high performance liquid chromatography

We developed an assay which permits measurement of aspartate carbamoyltransferase (ACTase) activity. Cytosol from human peripheral blood mononuclear cells was used as the enzyme source. Using [14C]carbamoyl phosphate as the radiolabeled substrate, the formation of [14C]carbamoyl aspartate was quantitated by high performance liquid chromatography (HPLC) using an anion-exchange column with UV detection at 200-280 nm and an on-line liquid scintillation detector. A gradient method from an initially low concentration of ammonium phosphate, 1 mM (pH 3.0), to a higher concentration, 38 mM (pH 4.5), was used. The apparent Km values of carbamoyl phosphate and aspartate were 58 microM and 1.9 mM, respectively. ACTase inhibition by N-(phosphonacetyl)-l-aspartate (PALA) was consistent with a competitive model with respect to carbamoyl phosphate. The assay conditions were optimized to permit measurement of ACTase activity prior to and following therapy with PALA; ACTase was inhibited in a dose-dependent manner. This HPLC method permits direct quantitation of both the product of the reaction and the initial integrity of the substrate, [14C]carbamoyl phosphate, which is unstable in aqueous solutions.

Immunological quantitation of thymidylate synthase-FdUMP-5,10-methylenetetrahydrofolate ternary complex with the monoclonal antibody TS 106

Thymidylate synthase (TS) is responsible for the conversion of deoxyuridine monophosphate to deoxythymidine monophosphate. One of the principal mechanisms of action of 5-fluorouracil (5-FU) is the inhibition of TS by formation of a ternary covalent complex consisting of TS-5-fluorodeoxyuridylate-5,10-methylenetetrahydrofolate. We have developed a Western immunoblot assay using the monoclonal antibody TS 106 to measure ternary complex and free TS in intact human carcinoma cells following exposure to either 5-FU alone or 5-FU plus leucovorin. Lysates from cells treated with either 5-FU or 5-FU/leucovorin were resolved in 15% polyacrylamide gel, transferred onto nitrocellulose and immunoblotted using TS 106 antibody. Detection of positive bands was by a chromogenic substrate strain. Immunoblotting detected free TS at 36 kDa and TS in ternary complex at 38.5 kDa which were quantitated by densitometric scanning. This assay was able to detect a ternary complex from intact cells treated with 5-FU or 5-FU/leucovorin up to 96 h after drug removal. The ratio of complex to free TS was up to 2-fold greater in 5-FU/leucovorin-treated cells compared to those treated with 5-FU alone. This assay may be applied to measuring the formation and stability of ternary complex and free TS in patient tissue samples.

Quantitation of thymidylate synthase in human tumors using an ultrasensitive enzyme-linked immunoassay

Thymidylate synthase (TS; EC 2.1.1.45) is an important therapeutic target for fluoropyrimidine cytotoxic drugs that are widely used for the treatment of solid tumors. Using the monoclonal antibody TS 106, we have developed an ultrasensitive enzyme-linked immunoassay (ELISA) for the detection and quantitation of TS. Using a chemiluminescent ELISA technique, TS was detectable in serially diluted lysates from NCI H630 and HCT 116 human colon carcinoma cell lines. The ELISA assay was reliably able to detect activity down to a level of 30 attamol of TS protein above background (P2 = 0.016). The usable range of detection was from 0.03 to 500 fmol of enzyme. There was a close correlation between the optical density signal and the total TS enzyme between both cell lines (r2 = 0.96). The ELISA was used to measure TS in cytosolic extracts from human tumor samples, and it was able to quantitate TS levels using as little as 1-mg tumor biopsy samples. The mean total TS measured by ELISA in seven tumor samples from patients with breast cancer and sarcomas was 131 fmol/mg cytosolic protein (range 60-240) compared with a mean TS of 85 fmol/mg cytosolic protein (range 35-163) using the fluorodeoxyuridine monophosphate binding assay. While the TS levels were uniformly higher when measured by ELISA, there was close proportional agreement between both assays (r2 = 0.84). Thus, the chemiluminescent TS ELISA would appear to be an extremely sensitive and specific assay that may be used to quantitate TS in tumor tissue specimens.

Identification of an RNA binding site for human thymidylate synthase

Previous studies from this laboratory have shown that human TS mRNA translation is regulated by its protein product in a negative autoregulatory manner. In this paper, we identify an RNA binding site for TS protein located within the first 188 nt of TS RNA. A 36-nt RNA sequence contained within this 188-nt fragment, corresponding to nt 75-110 and including the translational initiation site, binds TS protein with an affinity similar to that of both the full-length and the 188-nt TS RNA sequences. Variant RNAs with either a deletion or a mutation at the translational initiation region are unable to compete for TS protein binding. UV crosslinking studies reveal that an RNA fragment of approximately 36 nt is protected from RNase T1 digestion by TS protein binding. A second TS protein-binding site is localized within the protein-coding region corresponding to nt 434-634. These findings demonstrate a specific interaction between human TS protein and its TS RNA and identify an RNA binding site that includes the translational initiation site.

Dose-dependent inhibition of aspartate carbamoyltransferase in peripheral blood mononuclear cells in patients receiving N-(phosphonacetyl)-L-aspartate

Forty-eight patients with adenocarcinoma of the gastrointestinal tract were treated on this trial. The MTD of 5-FU given as a 72 hour infusion with high-dose leucovorin was initially determined to be 2000 mg/m2/d. Patients were treated at PALA dose levels ranging from 250 to 2848 mg/m2. Biochemical assessment of target enzyme activity was performed at each PALA dose level. We conclude that compared to each patient's own baseline, PALA at 250 mg/m2 failed to appreciably inhibit ACTase activity at 24 hours in most patients. More consistent inhibition of ACTase activity was seen with PALA at or above 1266 mg/m2, but toxicity was prohibitive with 2848 mg/m2 PALA. Even with the highest PALA doses, ACTase activity was back to baseline by 96 hours in most patients. PALA at 1266 mg/m2 given 24 hours prior to the start of 72 hour infusional 5-FU plus high-dose leucovorin was associated with acceptable toxicity and did not appear to compromise 5-FU dose-intensity. Finally, because of interpatient variability in the degree of ACTase inhibition following PALA, biochemical monitoring of target enzyme activity may permit more rational adjustment of the PALA dose in individual patients.

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.

Autoregulation of human thymidylate synthase messenger RNA translation by thymidylate synthase

Thymidylate synthase (TS; 5,10-methylenetetrahydrofolate:dUMP C-methyltransferase, EC 2.1.1.45) is essential for the de novo synthesis of thymidylate, a precursor of DNA. Previous studies have shown that the cellular level of this protein is regulated at both the transcriptional and posttranscriptional levels. The regulation of human TS mRNA translation was studied in vitro with a rabbit reticulocyte lysate system. The addition of purified human recombinant TS protein to in vitro translation reactions inhibited translation of TS mRNA. This inhibition was specific in that recombinant TS protein had no effect on the in vitro translation of mRNA for human chromogranin A, human folate receptor, preplacental lactogen, or total yeast RNA. The inclusion of dUMP, 5-fluoro-dUMP, or 5,10-methylene-tetrahydrofolate in in vitro translation reactions completely relieved the inhibition of TS mRNA translation by TS protein. Gel retardation assays confirmed a specific interaction between TS protein and its corresponding mRNA but not with unrelated mRNAs, including human placenta, human beta-actin, and yeast tRNA. These studies suggest that translation of TS mRNA is controlled by its own protein end product, TS, in an autoregulatory manner.

Induction of thymidylate synthase associated with multidrug resistance in human breast and colon cancer cell lines

A series of Adriamycin-resistant human breast MCF-7 and human colon DLD-1 cancer cell lines were established by stepwise selection. The concentration of Adriamycin required to inhibit cell proliferation by 50% (IC50) in the parent breast line (MCF-7), Adriamycin-resistant lines (MCF-Ad5 and MCF-Ad10), and a 5-fluorouracil (5-FU)-revertant line (MCF-R) was 0.005, 3.3, 6, and 4.9 microM, respectively. The Adriamycin IC50 value for the resistant colon line (DLD-Ad) was 8.2 microM, 68-fold higher than that for its parent line (DLD-1) (IC50 = 0.12 microM). The MCF-Ad5 and MCF-Ad10 cells were cross-resistant to 5-FU, with respective 5-FU IC50 values of 11.7 and 22.5 microM, or 7.3- and 14-fold less sensitive than their parent MCF-7 (IC50 = 1.6 microM) line. The MCF-R line completely reverted in sensitivity to 5-FU, with an IC50 of 1.7 microM. The resistant DLD-Ad line was 3.5-fold more resistant to 5-FU than was the parent DLD-1 line. Using both the 5-fluoro-2'-deoxyuridine-5'-monophosphate binding and catalytic assays for measurement of thymidylate synthase (TS) activity, there was significantly increased TS activity in the resistant MCF-Ad5 (2.4- and 2.5-fold), MCF-Ad10 (11.5- and 6.8-fold), and DLD-Ad (4.8- and 10.7-fold) lines, for binding and catalytic assays, respectively, compared with their parent MCF-7 and DLD-1 lines. The level of TS in cytosolic extracts, as determined by Western immunoblot analysis, was markedly increased for the resistant MCF-Ad5 (31-fold), MCF-Ad10 (46-fold), and DLD-Ad (52-fold) cells. Measurement of TS mRNA levels by Northern analysis revealed elevation of TS mRNA in the resistant MCF-AD5 (16.7-fold), MCF-Ad10 (31-fold), and DLD-Ad (55-fold) cells. Southern analysis showed that this increase in TS mRNA was not accompanied by any major rearrangements or amplification of the TS gene. Incorporation of 5-FU into the RNA and DNA of the resistant MCF-Ad10 cells was not significantly different, compared with that for parent MCF-7 cells. These studies suggest that exposure of human breast and human colon cancer cells to Adriamycin leads to overexpression of TS, with concomitant development of resistance to 5-FU.

Mechanism of thymidylate synthase inhibition by methotrexate in human neoplastic cell lines and normal human myeloid progenitor cells

We have studied the roles of 5,10-methylenetetrahydrofolate (5,10-methylene-H4PteGlu) depletion and dihydrofolate (H2PteGlu) accumulation in the inhibition of de novo thymidylate synthesis by methotrexate (MTX) in human MCF-7 breast cancer cells. Using both a high pressure liquid chromatography system and a modification of the 5-fluoro-2'-deoxyuridine-5'-monophosphate radioenzymatic binding assay, we determined that the 5,10-methylene-H4PteGlu pool is 50-60% depleted in human MCF-7 breast cancer cells following exposure to 1 micron MTX for up to 21 h. Similar alterations in the 5,10-methylene-H4PteGlu pools were obtained when human promyelocytic HL-60 leukemia cells and normal human myeloid precursor cells were incubated with 1 micron MTX. The H2PteGlu pools within the MCF-7 cells increased significantly after 15 min of 1 micron MTX exposure, reaching maximal levels by 60 min. Thymidylate synthesis, as measured by labeled deoxyuridine incorporation into DNA, decreased to less than 20% of control activity within 30 min of 1 micron MTX exposure. The inhibition of thymidylate synthesis coincided temporally with the rapid intracellular accumulation of H2PteGlu, a known inhibitor of thymidylate synthase. Furthermore, inhibition of this pathway was associated in a log-linear fashion with the intracellular level of dihydrofolate. These studies provide further evidence that depletion of the thymidylate synthase substrate 5,10-methylene-H4PteGlu is inadequate to account completely for diminished thymidylate synthesis resulting from MTX treatment. Our findings suggest that acute inhibition of de novo thymidylate synthesis is a multifactorial process consisting of partial substrate depletion and direct enzymatic inhibition by H2PteGlu polyglutamates.