The relevance of the state of growth and transformation of cells to their patterns of metabolite uptake

Evaluation of the sensitivity of three sublethal cytotoxicity assays in human HepG2 cell line using water contaminants

The in vitro toxicological index IC50 (the millimolar concentration of compound which inhibits response assay by 50% compared to the solvent control) of 11 water contaminants (acrylamide, atrazine, B[a]P, BPA, 2,4-DAT, 17-alphaEE, H(2)O(2), 4-OP, sodium bromate, sodium chlorate, sodium nitrate) was evaluated on the human hepatoma (HepG2) cells using three short-term bioassays related to their morbidity status [radiometric RNA synthesis assay (RNA), luminometric ATP assay (ATP), fluorometric Alamar blue assay (AB)]. Among all substances, we were not able to determine atrazine IC50 value whatever the test used. Furthermore, B[a]P was not cytotoxic in the ATP and AB assays. Statistical analysis revealed a correlation between the IC50 values obtained in the three assays. Except with 4-OP, RNA assay was always inhibited at lower concentrations than those required in the other assays, suggesting that this assay is a very sensitive indicator of the presence of toxic compounds. ATP and AB assays responded to a similar pattern. Due to its higher sensitivity and its reliability, RNA synthesis assay using HepG2 cell line provides the most suitable tool for the screening of water contaminants.

Uridine uptake inhibition as a cytotoxicity test for a human hepatoma cell line (HepG2 cells): comparison with the neutral red assay

This study describes a sensitive microassay for measuring cytotoxicity based on the degree of inhibition of RNA synthesis in HepG2 cells. RNA synthesis is measured by the kinetic uptake of radiolabeled uridine. A large number of compounds were tested in a wide range of concentrations. The concentration required to induce 50% inhibition of HepG2 uridine uptake rates (IC(50)) was determined for each compound and used to rank its potency. These IC(50)s were compared with IC(50)s measured with the neutral red assay. 2-acetylaminofluorene, benzo[a]pyrene and methylnitrosourea were not cytotoxic in the neutral red assay. Uridine uptake was always inhibited at lower concentrations than those required in the neutral red assay, suggesting that the uridine uptake assay is a more sensitive indicator of toxic action than the neutral red inclusion. Uridine uptake assay provides a rapid and quantitative method for assessing toxicity in a human cell line. Application of this method to bottled spring waters are described. Due to its high sensitivity and reproducibility, this method provides a suitable tool for screening a great number of samples and will be a helpful test for evaluating food safety and controlling the recycling process of wrapping materials.

System A amino acid transport in cultured human tumor cells: implications for tumor imaging with PET

The A system of amino acid transport is concentrative and thought to be a regulator of cell growth. The [11C]methyl alpha-aminoisobutyric acid (MeAIB) is prospectively an ideal tracer for transport measurements with PET, as it is not metabolized and concentrates in cells only via System A transport. We examined the factors governing [14C]MeAIB accumulation by cultured human erythroleukemic (K562) cells. Experiments were performed in growth medium and phosphate-buffered saline (PBS) +/- cycloheximide (an inhibitor of protein synthesis) on logarithmically growing cells, as well as cells that had reached a growth plateau. Both inward transport rate and net uptake of MeAIB were positively correlated with cell growth rate and showed a strong inverse relationship to amino acid supply. The observations are consistent with a body of evidence from animal tumor cells, and they suggest that the correlation between System A transport and tumor cell proliferation may be obscured in vivo by variations in amino acid supply. Thus, while [11C]MeAIB might be useful as a PET radiotracer of System A transport per se, this compound may be limited in its ability to provide measurements of tumor growth rate.

Insulin-induced enhancement of cell morphological dynamics: non-specific biophysical mechanisms for the generalized stimulation of metabolism?

Time series analysis of fluctuations in the intensity of light scattered by cells indicates the existence of a range of oscillations in cell morphology that usually occur in (pseudo) periodic bursts. Insulin has a frequency-dependent effect on the rhythms, predominantly that of stimulating the short period components believed to reflect surface movements. It is suggested that the non-specific action of the hormone on transport is due to an effective decrease in the net thickness of the surrounding diffusion layer (resulting from increased stirring of the micro-environment) and to accompanying membrane and cytoplasmic vibrations. From published data it seems likely that other mitogens and chemotactic agents act in a similar manner.

Uridine uptake inhibition as a cytotoxicity test: correlations with the Draize test

The effects of 25 xenobiotic chemicals on the uptake of [3H]-uridine by Balb/c 3T3 cells were assessed. The test compounds, which included alcohols, ethers, esters, ketones, amides, acids and a detergent, inhibited uridine uptake at concentrations lower than those required to kill the cells; thus uridine uptake inhibition is a more sensitive indicator of toxic action than is cell lethality. The concentration of agent required to induce a 50% inhibition in uridine uptake rates after 4 h of treatment was determined for each agent, and this value (UI-50) was used to rank the potency of the test agents. This ranking correlated well with published data on the chemicals' capacity to induce ocular irritation in rabbits (the Draize test). Combinations of agents with differing functional groups produced additive uridine uptake inhibitory effects, suggesting the utility of this approach for the analysis of mixtures. Cells treated with levels of agents that reduced uridine uptake by 60-80% were able to recover most of their uridine uptake capacity after refeeding, indicating that the test shares with in vivo tests the ability to demonstrate recovery from toxic insult. This uridine uptake assay system provides a quantitative and rapid method for assessing toxicity that correlates well with Draize test results.

Multiple controls on the intracellular trapping of uridine

Uridine uptake by mouse or hamster cells grown in conditions which support good growth is very sensitive to inhibition by cyanide and azide, at concentrations which only slightly reduce overall cellular ATP levels. Iodoacetate, when present alone, reduces uridine uptake only insofar as it reduces cellular ATP levels. At concentrations which by themselves do not affect uridine uptake, iodoacetate greatly reduces the sensitivity of uridine uptake to cyanide or azide. The effect of cyanide is on intracellular trapping of uridine and not on its transport into the cell. The specific effect of cyanide is confined to uridine and not found for the uptake of adenine, thymidine or 2-deoxyglucose. The effect is of rapid onset (within 2 min) and is rapidly reversible (also within 2 min). Phosphorylation of uridine in homogenised cells or in Triton X-100-permeabilised cells is unaffected by cyanide. The data are interpreted in terms of a model in which intracellular trapping of uridine is subject to multiple controls, including one regulated by some factor requiring intact functioning of the mitochondrion. These multiple control systems interact synergistically to affect trapping of uridine by the intact cell.