Mutants of Chinese hamster cells deficient in thymidylate synthetase

Isolation and characterization of a thymidylate synthase-deficient human colon tumor cell line

Following mutagenesis of the human colorectal tumor cell line HCT C with ethyl methanesulfonate, clonal sublines were isolated that survived on medium toxic to cells expressing thymidylate synthase (TS). The subline exhibiting the lowest TS activity, designated as C18, was characterized. Extracts from C18 cells were mixed with extracts from parental C cells to determine whether the TS-deficient phenotype is trans-acting. No effect was observed on the activity of TS in parental extracts. The levels of functional TS in C18 cells were analyzed by the binding of the mechanism-based inhibitor 5-fluoro-2'-deoxyuridylate (FdUMP) under conditions that allowed for the detection of 10 fmol of TS. Only a low level of FdUMP-TS complexes was detected in C18 extracts. The level of TS expression in C18 cells was similar to that in parental C cells, as indicated by immunoblot and RNA analyses. DNA sequence analysis of TS cDNA from C18 cells revealed the existence of a point mutation (C-->T) at nucleotide 647 that predicts the replacement of Ser216 by a leucine residue. That the C18 cell line was homozygous for this mutation was indicated by restriction fragment-length polymorphism analysis and by primer extension analysis. To provide additional evidence that substitution of Ser216 by a leucine residue created a defective protein, a TS-deficient bacterial strain was transformed with an expression vector containing the mutated human TS cDNA. The transformed strain exhibited thymidine auxotrophy, indicating that the mutant TS (Leu216) is nonfunctional.

Cellular responses to antimetabolite anticancer agents: cytostasis versus cytotoxicity

Thymineless death is an important cytotoxic response to several classes of antimetabolite agents used in the treatment of patients with carcinomas and hematopoeitic malignancies. Cell death induced by lack of dThd results in the formation of DNA nucleosomal ladders, and hence would be defined as a form of apoptosis. Although drug resistance to these agents has been extensively studied, relatively little attention has been focused on events downstream of dTTP depletion that determine the ultimate fate of the cancer cell. In this article we review some of the emerging data that suggests the role of p53 in determining whether the cellular response to dThd deprivation is cytostasis or cytotoxicity (apoptosis).

A study of deoxyribonucleotide metabolism and its relation to DNA synthesis. Supercomputer simulation and model-system analysis

A model system (1) was established to analyze purine and pyrimidine metabolism. This system has been expanded to include macrosimulation of DNA synthesis and the study of its regulation by terminal deoxynucleoside triphosphates (dNTPs) via a complex set of interactions. Computer experiments reveal that our model exhibits adequate and reasonable sensitivity in terms of dNTP pool levels and rates of DNA synthesis when inputs to the system are varied. These simulation experiments reveal that in order to achieve maximum DNA synthesis (in terms of purine metabolism), a proper balance is required in guanine and adenine input into this metabolic system. Excessive inputs will become inhibitory to DNA synthesis. In addition, studies are carried out on rates of DNA synthesis when various parameters are changed quantitatively. The current system is formulated by 110 differential equations.

Acquisition of thymidylate by the obligate intracytoplasmic bacterium Rickettsia prowazekii

The pathway for the acquisition of thymidylate in the obligate bacterial parasite Rickettsia prowazekii was determined. R. prowazekii growing in host cells with or without thymidine kinase failed to incorporate into its DNA the [3H]thymidine added to the culture. In the thymidine kinase-negative host cells, the label available to the rickettsiae in the host cell cytoplasm would have been thymidine, and in the thymidine kinase-positive host cells, it would have been both thymidine and TMP. Further support for the inability to utilize thymidine was the lack of thymidine kinase activity in extracts of R. prowazekii. However, [3H]uridine incorporation into the DNA of R. prowazekii was demonstrable (973 +/- 57 dpm/3 x 10(8) rickettsiae). This labeling of rickettsial DNA suggests the transport of uracil, uridine, uridine phosphates (UXP), or 2'-deoxyuridine phosphates, the conversion of the labeled precursor to thymidylate, and subsequent incorporation into DNA. This is supported by the demonstration of thymidylate synthase activity in extracts of R. prowazekii. The enzyme was determined to have a specific activity of 310 +/- 40 pmol/min/mg of protein and was inhibited greater than or equal to 70% by 5-fluoro-dUMP. The inability of R. prowazekii to utilize uracil was suggested by undetectable uracil phosphoribosyltransferase activity and by its inability to grow (less than 10% of control) in a uridine-starved mutant cell line (Urd-A) supplemented with 50 microM to 1 mM uracil. In contrast, the rickettsiae were able to grow in Urd-A cells that were uridine starved and supplemented with 20 microM uridine (117% of control). However, no measurable uridine kinase activity could be measured in extracts of R. prowazekii. Normal rickettsial growth (92% of control) was observed when the host cell was blocked with thymidine so that the host cell's dUXP pool was depressed to a level inadequate for growth and DNA synthesis in the host cell. Taken together, these data strongly suggest that rickettsiae transport UXP from the host cell's cytoplasm and that they synthesize TTP from UXP.

The G1 interval in the mammalian cell cycle: dual control by mass accumulation and stage-specific activities

The temporal determinants of the G1 cell cycle interval were investigated using nine mammalian cell lines. In each case, cells were allowed to proliferate for many cell cycles under conditions that slowed progress through S phase without an equivalent impairment of overall mass accumulation. This disproportionate inhibition of progress through the cell cycle caused newly produced cells to be more massive than usual. Under these growth conditions, the determinants of the length of the G1 interval became evident. For two cell lines, HeLa S3 and NIH 3T3, a protracted S phase, and the resultant increase in mass, resulted in a dramatically shortened G1 interval. Thus, for these cell lines, a major portion of G1 time exists to accommodate mass accumulation needed to initiate the subsequent S phase. Nevertheless, under conditions that protracted S phase and shortened the G1 interval, cells still exhibited a measurable G1 time, reflecting the stage-specific activities within G1. One activity that may be responsible for this obligatory G1 time is the synthesis of a labile protein. For other cells studied here, protraction of S phase also caused proliferating cells to become more massive, but in these cases there was no diminution of the G1 time. For these cells, the entire G1 interval must accommodate G1-specific activities necessary to initiate a new cell cycle. A unifying view of the G1 interval recognizes the two distinct influences that determine the time spent in G1: the need to accumulate sufficient mass to initiate a new DNA-division sequence; and the stage-specific events necessary for the subsequent S phase. The length of the G1 interval is dictated by the longer of these two time-consuming activities.

Thymidylate synthetase gene as a quantitative mutation marker in Chinese hamster cells

A quantitative mutation marker for cultured mammalian cells is presented which uses a selective medium containing folinic acid, aminopterin and thymidine (the 'FAT' medium) to select for mutants deficient in thymidylate synthetase (TS) activity. Optimization of FAT medium was carried out using Chinese hamster V79 cell lines having 3 levels of TS activity. By manipulating the concentration of folinic acid in FAT medium, TS-deficient mutants can be readily selected. TS mutation is inducible in a dose-dependent manner by either ethyl methanesulfonate or ultraviolet light irradiation. Expression time for TS mutation was also determined using two concentrations of ethyl methanesulfonate and found to be very short, being 1 or 2 days. This newly characterized TS mutation marker should be useful in the study of both spontaneous and induced mutagenesis.

Supercomputer analysis of purine and pyrimidine metabolism leading to DNA synthesis

A model-system is established to analyze purine and pyrimidine metabolism leading to DNA synthesis. The principal aim is to explore the flow and regulation of terminal deoxynucleoside triophosphates (dNTPs) in various input and parametric conditions. A series of flow equations are established, which are subsequently converted to differential equations. These are programmed (Fortran) and analyzed on a Cray chi-MP/48 supercomputer. The pool concentrations are presented as a function of time in conditions in which various pertinent parameters of the system are modified. The system is formulated by 100 differential equations.

Mutants of human colon adenocarcinoma, selected for thymidylate synthase deficiency

GC3/c1 human colon adenocarcinoma cells were treated with the mutagen ethyl methanesulfonate, and three clones deficient in thymidylate synthase (5,10-methylenetetrahydrofolate:dUMP C-methyltransferase, EC 2.1.1. 45) activity were selected and characterized. Growth in medium deficient in thymidine caused cell death in two clones (TS- c1 and TS- c3), whereas one clone (TS- c2) showed limited growth. Growth correlated with thymidine synthase activity and 5-fluoro-2'-deoxyuridine 5'-monophosphate-binding capacity and with incorporation of 2'-deoxy[6-3H]uridine into DNA. In the presence of optimal thymidine, growth rates were only 5-18% that of the parental clone (GC3/c1), which grew equally well in thymidine-deficient or -replete medium. Analysis of poly(A)+ RNA showed normal levels of a 1.6-kilobase transcript in TS- c1 and TS- c2 but decreased levels (approximately 6% control) in TS- c3. Clone TS- c3 was 32-, 750-, and greater than 100,000-fold more resistant than the parental clone to 5-fluorouracil, 5-fluoro-2'-deoxyuridine, and methotrexate, respectively. When inoculated into athymic nude mice, each TS- clone produced tumors, demonstrating continued ability to proliferate in vivo.

Anchorage of the Chinese hamster dihydrofolate reductase gene to the nuclear scaffold occurs in an intragenic region

We have identified a region near the center of the dihydrofolate reductase gene dhfr in Chinese hamster ovary cells that is attached to nuclear scaffolds isolated by extraction with lithium diiodosalicylate. Detailed analysis presented here reveals the presence of only two closely linked sites in 35,000 base-pairs scanned that mediate attachment of the dhfr gene to the nuclear scaffold. Sequence analysis of one of the sites reveals a high A + T content, the presence of cleavage consensus sequences for topoisomerase II, and direct and inverted repeated sequence motifs that are localized to a small region of the attachment site. Attachment of these two regions to the nuclear scaffold is observed in wild-type, hemizygous, and amplified cell lines. Attachment is also retained in dhfr mutants isolated in our laboratory, in which chromosomal lesions have occurred directly adjacent to the scaffold-associated regions. These two regions are not bound to scaffolds prepared from isolated metaphase chromosomes, suggesting that attachment of the dhfr gene is lost during mitosis.