Zenia Nimec

Characteristics of methotrexate polyglutamate formation in cultured hepatic cells

Abstract Upon exposure of primary monolayer cultures of hepatocytes and H35 hepatoma cells, methptrexate (MTX) is taken up by carrier-mediated mechanisms and converted to γ-glutamyl derivatives with one to four residues being added. Under conditions that result in 90% or greater conversion, the primary metabolite in both cell types is MTX with three additional glutamates (4-NH 2 -10-CH 3 PteGlu 4 ). When the time-dependent synthesis of MTX polyglutamates (4-NH 2 -10-CH 3 PteGlu 2 and higher) at extracellular concentrations of 10 and 100 μ m methotrexate is measured, both cell types exhibit linear synthesis for 4 to 6 hr, at which time an apparent steady state intracellular concentration of approximately 40 μ m is reached. The concentration of MTX polyglutamate synthesized is not due a restriction in MTX since the hepatocytes and H35 cells accumulated 400 and 138 μ m intracellular methotrexate, respectively, after 24 h in the presence of 100 μ m extracellular MTX. Examination of MTX polyglutamate formation following a 24-h incubation showed concentration dependence with respect to intra- and extracellular MTX. Saturation was reached at a medium concentration of approximately 2 μ m with both cell types which corresponded to 10 to 12 μ m intracellular MTX. Placement of cells at steady state in medium lacking MTX results in the rapid equilibration of all free intracellular MTX with the medium. The MTX polyglutamates leave the cell by a slow loss of intact polyglutamates and also by intracellular cleavage to MTX followed by efflux. The longer-chain-length γ-glutamyl derivatives (Glu 4–5 ) are more avidly retained by the cells than the shorter ones (Glu 2–3 ).

Regulatory aspects of the glutamylation of methotrexate in cultured hepatoma cells

The glutamylation of methotrexate has been evaluated in H35 hepatoma cells in vitro as a function of the conditions of culture. Glutamylation yields methotrexate polyglutamate with two to five additional glutamate residues and is a saturable process. The rate of glutamylation increases little above 10 microM extracellular methotrexate which corresponds to an intracellular concentration of approximately 4 microM. The rate of glutamylation measured over a 6-h period was stimulated by a reduction in cellular folates and prior incubation of the cells with insulin. Glutamylation was also more rapid in dividing cultures than in confluent cells. The combination of insulin inclusion and folate reduction, which was additive, caused approximately a fourfold increase in the rate of glutamylation over control cells under the conditions tested. The maximal rate of methotrexate glutamylation, which was 100 nmol/g/h, occurred in folate-depleted, insulin-supplemented cells. Supplementing folate-depleted cells with reduced folate coenzymes caused the glutamylation to be reduced by more than 90%. The turnover of methotrexate polyglutamates in cells saturated with these derivatives occurred at approximately one-half the rate of net synthesis and was stimulated to nearly the same extent by folate depletion and insulin. In addition to showing that folates can modify the rates of methotrexate polyglutamate formation, data are presented suggesting that methotrexate polyglutamates can regulate their own synthesis. The consequences of the formation of these retained forms of methotrexate in H35 hepatoma cells (M. Balinska, J. Galivan, and J.K. Coward (1981) Cancer Res. 41,2751-2756) and the effects of potential regulators of this process are discussed in terms of the glutamylation of folates in the cells and the chemotherapeutic effects of antifolates.

IDENTIFICATION, CLONING, AND SEQUENCING OF A CDNA CODING FOR RAT GAMMA -GLUTAMYL HYDROLASE

Purified -glutamyl hydrolase secreted from rat H35 hepatoma cells has been characterized as a diffuse band of 55 kDa on SDS-polyacrylamide gel electrophoresis that is converted to bands of 35 and 33 kDa after enzymatic removal of N-linked carbohydrate. Polyclonal antibodies against 55-kDa -glutamyl hydrolase captured the enzyme activity and recognized the glycosylated and both deglycosylated forms of -glutamyl hydrolase. A complete cDNA sequence of -glutamyl hydrolase was obtained using degenerate oligonucleotides derived from peptide sequences, screening of a rat hepatoma cDNA library, and reverse transcription polymerase chain reaction. Based upon the deduced amino acid sequence the peptide component of -glutamyl hydrolase had a molecular weight of 33,400. The results of amino acid analysis of the purified protein agreed with the deduced amino acid sequence in which there are seven potential asparagine-containing glycosylation sites.

The properties of the secreted γ-glutamyl hydrolases from H35 hepatoma cells

γ-Glutamyl hydrolase has been partially purified and characterized from conditioned culture medium of H35 hepatoma cells. Evidence for heterogeneity of the enzyme is derived from its elution as three distinct peaks of enzymatic activity when the enzyme is purified by TSK-butyl-Sepharose column chromatography. These three enzyme fractions appear to have identical catalytic properties but, as yet, the basis for their resolution is not understood. A rapid, sensitive and simple assay based on reverse-phase HPLC fluorescent detection with pre-column derivatization using o-phthalaldehyde (OPA) was developed to separate OPA-derivatives of poly-γ-glutamates and glutamic acid. Using this assay and the standard HPLC assay for pteroylpolyglutamates, the enzyme appears to be an endopeptidase with respect to pteroylpenta-γ-glutamate (PteGlu5), methotrexate penta-γ-glutamate (4-NH2-10-CH3PteGlu5) and p-aminobenzoyl-penta-γ-glutamate (pABAGlu5). The initial products are PteGlu1 (or 4-NH2-10-CH3PteGlu1 or pABAGlu1) and intact tetra-γ-glutamate, which is subsequently degraded to glutamic acid. When penta-γ-glutamate is the substrate, the cleavage of the γ-bonds by the enzyme is less ordered, with the early appearance of mono-, di-, tri- and tetraglutamate. Poly-α-glutamate is not a substrate nor are pABA-γ-Glu5 or penta-γ-glutamate covalently linked to albumin. 4-NH2-10-CH3PteGlu2 or Glu5 bound to dihydrofolate reductase is not a substrate for the enzyme, offering further evidence that protein-associated poly-γ-glutamates are poor substrates for γ-glutamyl hydrolase from H35 hepatoma cells.

The Chromophore and Polypeptide Composition of Aplysia Ink

The composition of the ink of the sea hare, Aplysia, was studied in regard to its tetrapyrrole and polypeptide content. The ink was separated into three pigment components by both thin-layer and gel filtration chromatography. These three pigments have distinctive visible absorption spectra, and--by comparison with known tetrapyrroles--we have demonstrated that they are derived from the three tetrapyrrole chromophores (bilins) found on the biliproteins of certain red algae, which constitute a portion of the Aplysia diet. The red component is phycourobilin; the purple is phycoerythrobilin; and the blue is phycocyanobilin. Sodium dodecyl sulfate gel electrophoresis experiments were also performed. The results of these experiments showed several polypeptides, and major bands at 78,000 and 61,000 molecular weight were noted. Biliproteins, at most, would be minor components of the ink.

The properties and function of γ-glutamyl hydrolase and poly-γ-glutamate

gamma-Glutamyl hydrolase is a ubiquitous enzyme that has the capacity to cleave gamma-glutamyl bonds of cellular folyl- and antifolylpoly-gamma-glutamates. This study has revealed that the enzyme is secreted by primary cultures of rat hepatocytes and by H35 hepatoma cells. It was found that more than 99% of the total enzyme from H35 cells accumulated in the medium after 48 hr incubation with the serum-free medium. The cells were shown to remain intact during the secretion period since lactate dehydrogenase, dihydrofolate reductase and lysosomal hydrolases other than gamma-glutamyl hydrolase were retained within the cell. When PteGlu5 (folylGlu4) is used as a substrate the initial product is PteGlu (folate), and there is no appearance of intermediate chain length pteroyl polyglutamates. Therefore, the secreted and cellular gamma-glutamyl hydrolase from hepatoma cells appears to be an endopeptidase. Polyclonal antibodies to the poly-gamma-glutamate substrates of the enzyme were prepared and characterized. The antibodies recognize the structural differences between alpha- and gamma-glutamyl linkages but appear equally active with PteGlu5 and its analogs such as 4-NH2-10-CH3PteGlu5 and pABAGlu5. The affinity of the antibodies is related to the gamma-glutamyl structure since L-glutamic acid, folate or p-aminobenzoic acid are inactive with the antibodies. Furthermore, poly-gamma-glutamate has lower affinity for the antibodies than the poly-gamma-glutamate derivatives of PteGlu, 4-NH2-10-CH3PteGlu or pABA.

Glutamylation of methotrexate in hepatoma cells in vitro: Regulation and the development of specific inhibitors

Methotrexate is glutamylated in cultured hepatoma cells to derivatives that contain a total of 2 to 5 gamma-glutamyl residues. The rate of polyglutamate formation and extent of accumulation are saturable with respect to both medium concentration of methotrexate and time. Maximal rates of glutamylation and accumulation of methotrexate polyglutamates at steady state occur at approximately 10 microM extracellular methotrexate. Inclusion of physiologic concentrations of insulin or removal of folate from the medium each cause a doubling of the rate of glutamylation, and these effects are additive. Insulin and folate restriction also enhance the accumulation of methotrexate polyglutamates. In combination they result in a doubling in the intracellular methotrexate polyglutamate pool at steady state and a shift in the polyglutamate distribution to longer-chain-length species. The importance of the longer-chain-length polyglutamates is apparent from the 6-hr retention of the polyglutamate species: Glu2, 15%; Glu3, 21%; Glu4, 50%; and Glu5, 83%. In probing the glutamylation reaction, a new series of inhibitors have been initiated. These are based upon replacing the incoming glutamate with 4-fluoroglutamate or synthesizing methotrexate with the glutamate replaced by 4-fluoroglutamate. The 4-fluoroglutamyl analogs of methotrexate are effective inhibitors of dihydrofolate reductase but cannot be glutamylated. They will be utilized to probe the role of glutamylation in antifolate activity and folate metabolism.

Factors controlling the concentrations of methotrexate in cultured hepatic cells

The polyglutamate metabolites of methotrexate are as inhibitory to the target enzyme dihydrofolate reductase as is methotrexate. Because of their greater retention they have a longer half-life within the cells and thus a greater potential for cytotoxicity. These metabolites have been found in numerous cells and tissues and are extensively synthesized in cultured hepatic cells. Uptake of methotrexate by primary cultures of rat hepatocytes occurs by a pathway which is independent of the folate coenzymes but appears to be related in some way to cholic acid and organic anion uptake. The evidence for the commonality of these pathways is (a) an instability of both uptake systems in the absence of hormones in the culture medium, (b) nearly equal inhibition of uptake by PCMS and NEM, and (c) cross competition of cholic acid and methotrexate for entry into the cells. Cholic acid and BSP can also selectively inhibit methotrexate polyglutamate formation in hepatocytes. Methotrexate entry into H35 hepatoma cells is mediated by the transport system which is shared by folate coenzymes and is not inhibited by cholic acid, BSP or sulfhydryl reagents. At concentrations of cholic acid or BSP which inhibit methotrexate polyglutamate formation in hepatocytes there is little or no loss of polyglutamate formation in H35 cells, possibly because BSP and cholic acid are taken up less by H35 cells than by hepatocytes.

Two Novel HPLC Methods which Rapidly Detect the Substrates and Cleavage Products of γ-Glutamyl Hydrolase

γ-Glutamyl hydrolase (EC 3.4.22.12) is widely distributed throughout the phylogenetic spectrum and is thought to play an important role in the cellular homeostasis and nutritional absorption of folates and antifolates1,2. The enzyme catalyzes the endo-or exopeptidyl hydrolysis of the γ-glutamyl side chains of naturally occurring polyglutamylated folates and synthetic antifolates2.

The characteristics and consequences of folate depletion in hepatoma cells in vitro by inhibition of dihydrofolate reductase

Growth of rat hepatoma cells in subtoxic concentrations of the DHFR inhibitor metoprine caused a marked time and concentration dependent reduction in cellular folates. As much as 75% total cellular folates can be lost without impairing growth. Increasing the concentration of metoprine into a range that causes inhibition of growth results in no further reduction in cellular folates. This effect is presumably mediated through inhibition of DHFR and several mechanisms are discussed which may account for these results. Cells grown in medium in which the concentration of folate is changed from 4 microM to 20 nM had intracellular folate levels that were reduced 85%. This is nearly the same reduction caused by treating cells grown in normal medium (4 microM folate) with continuous, subtoxic levels of metoprine. The reduction in cellular folates caused by growth in nM folic acid caused enhanced growth inhibitory activity of several antifolates. On a concentration basis metoprine was 12-fold more active under these conditions, PDDF was 37-fold more active and DDATHF was 44-fold more active. The reason for the enhanced sensitivity to PDDF and DDATHF may also be analogous to the reason for their synergism with the low concentration of metoprine and trimetrexate (12).