Uptake of thymidine into isolated rat hepatocytes. Evidence for two transport systems

The concentrative nucleoside transporter family, SLC28

The SLC28 family consists of three subtypes of sodium-dependent, concentrative nucleoside transporters, CNT1, CNT2, and CNT3 (SLC28A1, SLC28A2, and SLC28A3, respectively), that transport both naturally occurring nucleosides and synthetic nucleoside analogs used in the treatment of various diseases. These subtypes differ in their substrate specificities: CNT1 is pyrimidine-nucleoside preferring, CNT2 is purine-nucleoside preferring, and CNT3 transports both pyrimidine and purine nucleosides. Recent studies have identified key amino acid residues that are determinants of pyrimidine and purine specificity of CNT1 and CNT2. The tissue distributions of the CNTs vary: CNT1 is localized primarily in epithelia, whereas CNT2 and CNT3 have more generalized distributions. Nucleoside transporters in the SLC28 and SLC29 families play critical roles in nucleoside salvage pathways where they mediate the first step of nucleotide biosynthesis. In addition, these transporters work in concert to terminate adenosine signaling. SLC28 family members are crucial determinants of response to a variety of anticancer and antiviral nucleoside analogs, as they modulate the entry of these analogs into target tissues. Further, this family is involved in the absorption and disposition of many nucleoside analogs. Several CNT single nucleoside polymorphisms (SNPs) have been identified, but have yet to be characterized.

Adenosine and the nervous system: pharmacological data and therapeutic perspectives

1. Adenosine acts on a family of G-protein-coupled receptors called purinoreceptors. 2. Four subtypes have been cloned and pharmacologically characterized. 3. The principal pharmacological data and structure-function relations for agonist interactions with P1 receptors are presented. 4. We conclude that the potent role of adenosine in the nervous system may be interesting for the development of drugs targeted at purines and their receptors.

Saturation kinetics, specificity and NBMPR sensitivity of thymidine entry into the central nervous system

It was not until the development of a technique that could measure the brain uptake of slowly moving substrates, that the saturable transport system at the blood-brain barrier (BBB) for the pyrimidine deoxyribonucleoside, thymidine, was demonstrated. The aim of this present study was to further characterize this saturable uptake system at the blood-brain and blood-CSF barriers in terms of specificity, 6-(4-nitrobenzyl)thio-9-beta-D-ribofuranosylpurine (NBMPR) sensitivity and saturation kinetics by means of the in situ brain perfusion technique in anaesthetized guinea pigs. The results indicated that the transport system identified for [3H]thymidine can also transport other pyrimidine deoxyribonucleosides (deoxycytidine) and pyrimidine ribonucleosides (uridine) and is partially NBMPR-sensitive. In addition, guanosine, monocarboxylic acids, hexoses or amino acids were not substrates for the transport system. Further studies revealed that the transport system for [3H]thymidine at the BBB has a low affinity (Km 0.20 +/- 0.06 mM), but a relatively high capacity (Vmax 1.06 +/- 0.08 nmol min(-1) g(-1)). Overall, this study is indicative of a NBMPR-sensitive (es) facilitative transport system for [3H]thymidine and the likely presence of a NBMPR-insensitive and/or sodium-dependent transport system of the N2 (cit) type at the blood-brain and blood-CSF barriers of the guinea pig.

Nucleoside uptake in rat liver parenchymal cells

Rat liver parenchymal cells express Na(+)-dependent and Na(+)- independent nucleoside transport activity. The Na(+)-dependent component shows kinetic properties and substrate specificity similar to those reported for plasma membrane vesicles [Ruiz-Montasell, Casado, Felipe and Pastor-Anglada (1992) J. Membr. Biol. 128, 227-233]. This transport activity shows apparent K(m) values for uridine in the range 8-13 microM and a Vmax of 246 pmol of uridine per 3 min per 10(5) cells. Most nucleosides, including the analogue formycin B, cis-inhibit Na(+)-dependent uridine transport, although thymidine and cytidine are poor inhibitors. Inosine and adenosine inhibit Na(+)-dependent uridine uptake in a dose-dependent manner, reaching total inhibition. Guanosine also inhibits Na(+)-dependent uridine uptake, although there is some residual transport activity (35% of the control values) that is resistant to high concentrations of guanosine but may be inhibited by low concentrations of adenosine. The transport activity that is inhibited by high concentrations of thymidine is similar to the guanosine-resistant fraction. These observations are consistent with the presence of at least two Na(+)-dependent transport systems. Na(+)-dependent uridine uptake is sensitive to N-ethylmaleimide treatment, but Na(+)-independent transport is not. Nitrobenzylthioinosine (NBTI) stimulates Na(+)-dependent uridine uptake. The NBTI effect involves a change in Vmax, it is rapid, dose-dependent, does not need preincubation and can be abolished by depleting the Na+ transmembrane electrochemical gradient. Na(+)-independent uridine transport seems to be insensitive to NBTI. Under the same experimental conditions, NBTI effectively blocks most of the Na(+)-independent uridine uptake in hepatoma cells. Thus the stimulatory effect of NBTI on the concentrative nucleoside transporter of liver parenchymal cells cannot be explained by inhibition of nucleoside efflux.

Nucleoside transport in normal and neoplastic cells

The permeation of nucleosides across the plasma membrane of mammalian cells is complex and mediated by at least five distinct transporters that differ in their sensitivity to inhibitors and in their specificity for nucleosides. The basic properties and permeant specificity of these transporters are summarized in Table 3. It appears that there may be differences in the distribution of these transporters in tumors and normal tissues that might be exploited for chemotherapeutic purposes. The human tumor cell lines examined express predominantly the NBMPR-sensitive equilibrative transporter es which can be blocked by low concentrations of NBMPR and dipyridamole. It is reasonable to expect that tumors with transport properties similar to the CCRF-CEM and Rh28 cell lines (Table 1) that have no detectable NBMPR-insensitive transport activity will be highly susceptible to the therapeutic approach of combining a transport inhibitor such as dipyridamole or NBMPR with an inhibitor of de novo pyrimidine biosynthesis. On the other hand, this approach to therapy is unlikely to succeed against tumors with transport phenotypes similar to the WI-L2 cell line that may permit the salvage nucleosides in the presence of these inhibitors. The majority of tumor cells examined, however, fall between these extremes, and it is not yet known what level of NBMPR-insensitive transport activity can be tolerated without seriously compromising this therapeutic approach. With respect to normal tissues, the mature absorptive cells of the intestine have predominantly Na(+)-dependent nucleoside transporters that are insensitive to NBMPR and dipyridamole. The proliferating crypt cells also appear to have Na(+)-dependent nucleoside transport, although they may also have an NBMPR-sensitive component of transport (Belt, unpublished data). Bone marrow granulocyte-macrophage progenitor cells also appear to have one or more concentrative nucleoside transporters. Thus these tissues, which are most vulnerable to the toxicity of antimetabolites, may be able to salvage nucleosides in the presence of inhibitors of equilibrative transport and be protected from the toxicity of de novo synthesis inhibitors. It is likely, however, that a successful application of this therapeutic approach will require the analysis of the nucleoside transport phenotype of individual tumors in order to identify those patients that may benefit from such therapy. Since the development of antibodies and cDNA probes for the various nucleoside transporters is currently underway in several laboratories, it is likely that analysis of the nucleoside transport phenotype of tumors from biopsy material will be feasible in the future.

Investigations on the hepatic uptake systems for organic cations with a photoaffinity probe of procainamide ethobromide

Azido procainamide methoiodide (APM), a photolabile derivative of the transport model compound procainamide ethobromide (PAEB), shows a close resemblance to PAEB from a physicochemical point of view. Like PAEB it is effectively taken up by the liver and excreted into bile. Kinetics of the uptake of APM in isolated hepatocytes revealed that in addition to a non-saturable process, two saturable uptake systems are involved (Km1 = 3 microM, Vmax1) = 80 pmol/min/10(6) cells, Km2 = 100 microM, Vmax2 = 130 pmol/min x 10(6) cells). The uptake rate of APM was inhibited markedly in the presence of other organic cations. Organic anions and uncharged compounds generally had no inhibitory effect on the APM uptake. These results support the theory that there is a separate hepatic uptake system for organic cations like APM. Photoaffinity labeling of intact hepatocytes as well as plasma membrane sub-fractions enriched with sinusoidal domains disclosed two major binding polypeptides with apparent M(r) of 48,000 and 72,000. Such labeling patterns were not observed in membranes from hepatoma cells that are deficient in organic solute uptake. Differential photoaffinity labeling with other cationic compounds such as tributylmethyl ammonium and d-tubocurarine reduced the incorporation of APM in these polypeptides. The 48- and 72-kDa proteins might be involved in carrier-mediated transport of type I organic cations at the hepatic uptake level.

Evaluation of the mutagenic effects of SV40 in mouse, hamster, and mouse-human hybrid cells

We have examined the ability of SV40 to induce changes in drug or temperature resistance in mouse, hamster, and mouse-human hybrid cells. SV40 induced a substantial increase of cells resistant to 5-bromodeoxyuridine + trifluorothymidine in Balb/c 3T3 cells and induced an increase of hybrid cells resistant to 6-thioguanine. SV40 was found to be nonmutagenic or weakly mutagenic in other test systems. The 3T3 cells were T-antigen positive, exhibited a marked reduction in TK activity, were heterogeneous for [3H]BrdU incorporation by autoradiography, and exhibited instability of the drug-resistance phenotype, suggesting that SV40 may be inducing resistance by an epigenetic process. SV40-induced 6-thioguanine resistance in the hybrids appears to occur predominantly by chromosome loss.

Sodium-dependent and inhibitor-insensitive uptake of adenosine by mouse peritoneal exudate cells

[8-3H]Adenosine uptake in mouse peritoneal exudate cells, harvested following i.p. challenge with Complete Freund's Adjuvant from BALB/c mice, was found to be insensitive to common nucleoside transport inhibitors such as dilazep or 6-[(4-nitrobenzyl)mercapto]purine ribonucleoside and to require sodium ion, being inactive when sodium was replaced by lithium or potassium. These findings also applied to the adherent (macrophages) and nonadherent (polymorphonuclear cells) cell fractions prepared from the peritoneal cell mixture. Uptake was inhibited by several nucleosides including deoxyadenosine, inosine, uridine, thymidine and, to a lesser extent, by the adenosine analog tubercidin, while adenine, fructose, glucose and ribose were without effect. Uptake [8-3H]adenosine was fully matched by rapid intracellular phosphorylation to AMP, ADP and ATP. Inosine was a substrate for the transporter, but tubercidin was not. The system clearly is distinct from carrier-mediated, nonconcentrative transport and has similarities to concentrative, sodium-dependent nucleoside transporters described in other cell types.

Sodium-dependent uptake of nucleosides by dissociated brain cells from the rat

Sodium-dependent 3H-labeled nucleoside transport was studied using a mixed population of dissociated brain cells from adult rats. The accumulation of [3H]adenosine during brief (15-s) incubation periods was significantly greater in the presence of 110 mM Na+ than in its absence. This occurred at substrate concentrations that ranged from 0.25 to 100 microM. Similar findings were observed for the rapid accumulation of [3H]uridine. Kinetically, the rapid accumulation of [3H]adenosine in both the absence and the presence of Na+ was best described by a two-component system. In the presence of Na+, the KT and Vmax values for the high-affinity affinity component were 0.9 microM and 8.9 pmol/mg of protein/15 s, and those for the low-affinity component were 313 microM and 3,428 pmol/mg of protein/15 s, respectively. In the absence of Na+, the KT value for the high-affinity component was significantly higher (1.8 microM). [3H]Uridine accumulation was best described kinetically by a one-component system that in the presence of Na+ had KT and Vmax values of 1.0 mM and 2.6 nmol/mg of protein/15 s, respectively. As was found for [3H]adenosine, in the absence of Na+, the KT value was significantly higher (1.8 mM). The sodium-dependent transport of [3H]adenosine was inhibitable by ouabain and 2,4-dinitrophenol. Of the three nucleoside transport inhibitors tested, only nitrobenzylthioninosine demonstrated high affinity and selectivity in blocking the sodium component. Thus, high-affinity sodium-dependent nucleoside transport systems, in addition to facilitated diffusion systems, exist on brain cells from adult rats.

Pharmacological characterization of rapidly accumulated adenosine by dissociated brain cells from adult rat

Mechanically dissociated brain cells from adult rats were used to study biochemically and pharmacologically their capacity to accumulate rapidly [3H]adenosine. The assay, which used an inhibitor-stop method to prevent further uptake into cells, was characterized with respect to protein and optimal substrate concentrations, and incubation times that ranged from 5 to 180 s. The accumulation of [3H]adenosine using 15-s incubation periods, conditions under which less than 10% of accumulated [3H]adenosine was metabolized, was best described kinetically by a two-component system with Km and Vmax values for the high-affinity component of 0.8 microM and 6.2 pmol/mg protein/15 s and for the low-affinity component 259 microM and 2,217 pmol/mg protein/15 s, respectively. The potencies with which nucleosides, adenosine deaminase resistant adenosine receptor agonists, and nucleoside uptake inhibitors competed for these uptake components were determined. Of the nucleosides examined, adenosine was the "preferred" substrate for the uptake site. The Ki value of adenosine for the high-affinity component was 10.7 microM. Inosine and uridine competed for a single lower affinity uptake system: Ki values were 142 and 696 microM, respectively. Nucleoside uptake inhibitors--nitrobenzylthioinosine, dipyridamole, and dilazep--were the most potent inhibitors of [3H]adenosine accumulation tested: the Ki values for the high-affinity system were 0.11, 1.3, and 570 nM, respectively. The adenosine analogs S-phenylisopropyladenosine, R-phenylisopropyladenosine, and cyclohexyladenosine inhibited the high-affinity component with Ki values of 2.3, 9.3, and 14.5 microM, respectively. N-Ethylcarboxamidoadenosine competed for a single lower affinity uptake system: Ki, 292 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane transport and the antineoplastic action of nucleoside analogues

This article summarizes recent studies characterizing nucleoside transport in mammalian cells and discusses evidence for a role of membrane transport in the pharmacologic action of nucleoside analogues. Some of these studies have also addressed the controversy concerning the multiplicity in transport routes. It seems clear that erythrocytes and, perhaps, some other mammalian cells possess a single, broadly specific system for transporting nucleosides. However, substantial evidence from valid studies discriminating between transport and intracellular metabolism suggests that at least some mammalian cells, including some tumor cells, possess more than a single system. Evidence now exists for a determining role of membrane transport of nucleoside analogues in their cytotoxicity and, in the case of one pyrimidine nucleoside (AraC), in therapeutic responsiveness in leukemic patients. There are also numerous examples of transport-related resistance to nucleoside analogues. Included in this article are the results of studies from the authors' laboratory pertaining to the therapeutic activity of the purine nucleoside, FAraA, in murine tumor models. These studies provide evidence for a determining role of both membrane transport and intracellular phosphorylation in the selective antitumor action of this agent against murine leukemia. Substantially increased transport inward of FAraA occurs at pharmacologically achievable concentrations of this agent in tumor cells as compared to drug-limiting, normal proliferative epithelium of the small intestine. The basis for this differential appears to be the kinetic duality of FAraA and adenosine transport inward found in tumor cells, but not in proliferative intestinal epithelial cells. Tumor cells have highly saturable (low influx Km) and poorly saturable (high influx Km) systems for adenosine transport, both of which are shared by FAraA. In contrast, proliferative epithelial cells have only a poorly saturable system for these substrates. If a similar kinetic duality of nucleoside transport is found in other tumor cells certain implications arise concerning the significance of the duality to neoplastic transformation.

Pathobiochemical mechanisms of hepatocellular damage following lipid peroxidation

In hepatocytes, cytotoxic events induced by haloalkanes or acute iron-overload exhibit neither a quantitative nor a temporal correlation to lipid peroxidation or covalent binding. Thus, secondary pathological mechanisms have been postulated linking initial focal reactions of free radicals and end stage pathological consequences. Due to the crucial role of plasma-membrane integrity in the cytotoxic process it has to be supposed that relevant secondary pathological mechanisms finally impair the physico-chemical and functional properties of this membrane. Based on recent developments a chain of causality is proposed as a two-step activation of phospholipase A2 producing cytolytic amounts of lysophosphatides. In this cascade, the initial activating step is a decrease of membrane lipid fluidity induced by lipid peroxidation and/or by calcium binding and intramembranous formation of 4-hydroxynonenal. This enzyme activation is further amplified by the early rise of cytosolic calcium. Consequently, increasing amounts of lysophosphatides progressively impair membrane configuration thus improving the substrate accessibility for phospholipase A2 in a second activation step. Finally, the lysophosphatides reach plasma membrane-lytic concentrations by this autocatalytic enzyme activation.

Sodium-dependent, concentrative nucleoside transport in cultured intestinal epithelial cells

In a simple salts medium, monolayers of IEC-6 intestinal cells achieved concentrations of unmetabolized formycin B (an analog of inosine) about 6-fold higher than in the medium. Rates of formycin B influx were a saturable function of Na+ concentrations in the medium. Although IEC-6 cells possess sites with high affinity for nitrobenzylthioinosine, a potent inhibitor of equilibrative (facilitated diffusion) nucleoside transport systems in certain cell types, the inhibitor had only minor effects on formycin B uptake in IEC-6 cells, but reduced efflux of the analog from these cells. These findings indicate the joint presence in IEC-6 cells of nucleoside transporters of two types, one that is concentrative and Na+-dependent, and another that is sensitive to nitrobenzylthioinosine and apparently equilibrative.

Differential sensitivity of RSVts (temperature-sensitive Rous-sarcoma virus)-infected rat kidney cells to nucleoside antibiotics at permissive and non-permissive temperatures

Among a variety of anti-tumour agents tested, oxanosine and 5-azacytidine were found to be significantly more effective in inhibiting growth of rat kidney cells infected with a temperature-sensitive mutant of Rous sarcoma virus at a permissive temperature (33 degrees C) than at a non-permissive temperature (39 degrees C). These two nucleoside antibiotics were antagonistic to each other in cytotoxicity. They seem to share the same carrier-mediated membrane-transport system, because dipyridamole, a potent inhibitor of nucleoside transport, protected cells from the cytotoxicity of both drugs. Thymidine transport, which is twice as fast in cells at 33 degrees C as at 39 degrees C, was competitively inhibited by both drugs. Thus the differential toxicity of oxanosine and 5-azacytidine at the two temperatures is thought to be due to their increased transport via the thymidine-transport system, which is somehow under the influence of the active src-gene product.

Uphill transport of pyrimidine nucleosides in renal brush border vesicles

The uptake of cytidine, of thymidine and of uridine in brush border vesicles prepared from the cortex of rat kidney has been studied by the technique of rapid filtration. The nucleosides were not metabolized in the vesicles. The time-courses of uptake in the presence of inwardly directed gradients of Na+ and of K+ showed an overshoot, indicating uphill transport. The overshoot was much more pronounced with Na+ than with K+; it was not observed when Na+ was at equilibrium across the membrane. The uptake of the nucleosides was stimulated by an inside negative potential in the presence of Na+. These results provide evidence for a cotransport of pyrimidine nucleosides with Na+. The apparent Km's for the uptake of cytidine, of thymidine and of uridine were 3.76 mumol X 1(-1), 4.18 mumol X 1(-1) and 7.21 mumol X 1(-1) respectively. The uptake of the pyrimidine nucleosides was insensitive to 6-nitrobenzylthioinosine. This insensitivity as well as the high affinity for the nucleosides and the capacity for uphill transport indicate that the nucleoside carrier(s) is renal brush border is (are) different from the carriers found in most other cell types.

Specific increase in thymidine transport at a permissive temperature in the rat kidney cells infected with srcts-Rous sarcoma virus

Thymidine transport was found to be increased two-fold at a permissive temperature in the cells of a normal rat kidney (NRK) line which was infected with a temperature-sensitive Rous sarcoma virus. This increase in thymidine transport was independent of cell density and coincided with the changes in cellular morphology that result from this temperature shift. A double reciprocal plot of the data demonstrated two saturable components (Km values of 60 microM and 250 microM at 39 degrees, non-permissive temperature) of the uptake of thymidine, and the drop of temperature (at 33 degrees, the permissive temperature for transformation) decreased Km to one half, but did not change Vmax. These results indicate that a qualitative alteration of thymidine transport took place in the presence of the active src gene product.

Reutilization of preformed pyrimidines for the synthesis of DNA in rat liver and kidney

The specific activities of rat liver and kidney DNA pyrimidines increase in animals which have had [2-14C]-orotic acid administered several days before killing. This phenomenon is caused by the reutilization of the preformed labeled pyrimidine components originating mainly from degraded RNAs of the organism.

The human blood platelet: a cellular model to study the degradation of thymidine and its inhibition

Intact platelets catabolize extracellular thymidine into thymine. Studies of the concentration dependent degradation of thymidine by intact platelets indicate a Michaelis mechanism with an apparent Km of about 0.12 mM and a Vmax of 2.5 nmoles/min for 3 X 10(8) platelets. This degradation process is inhibited by various nucleosides, pyrimidine bases and C-5 or C-6 substituted uracils. Cytidine, deoxycytidine, adenosine and deoxyadenosine seem to inhibit thymidine degradation by reducing the intracellular transport of thymidine. Uridine inhibits both the thymidine transport and the activity of the phosphorolytic enzyme, thymidine phosphorylase (EC 2.4.2.4). Some substituted uracils are specific inhibitors of thymidine phosphorylase activity. 6-Amino-5-bromouracil, the most active of them, either with acellular extracts or purified thymidine phosphorylase, is also the best inhibitor of thymidine degradation in intact human platelets. Platelets constitute a new model to study the efficiency of specific inhibitors on thymidine catabolism in an 'human intact cell' which contains only one pyrimidine nucleoside phosphorylase, the thymidine phosphorylase.

Preincubation accelerates taurocholate uptake into isolated liver cells

Initial rates of taurocholate uptake into isolated hepatocytes stored at 0 degrees C increased 3-fold during a 25 min preincubation. Concomitantly, V increased while Km remained unaffected. There are several possible explanations for the preincubation effects, such as new synthesis of carrier protein, altered fluidity of the membrane or stimulation of the sodium-dependent taurocholate uptake via a change in the cation distribution. The experiments presented strongly favor the latter explanation as the sodium gradient as well as the uptake of the bile acid reach their steady state within 20-30 min and replacement of sodium by potassium in the medium abolished the effect.

Effect of local anesthetics on cellular respiration and thymidine transport in hepatocytes from young and old rats

The effect of local anesthetics (procaine, nicotinoyl-procaine, tetracaine, and dibucaine) on thymidine uptake and cellular respiration was investigated in hepatocytes from 3- and 24-month-old rats. All local anesthetics inhibited the "high-affinity" as well as the "low-affinity" thymidine transport system in a non-competitive manner immediately upon addition. Nicotinoyl-procaine, tetracaine, and dibucaine showed a similar inhibition profile with an inhibition of 10--15% at 50 mumol/l, or of more than 60% at 1 mmol/l. The less-lipophilic procaine showed a distinctly lower inhibition (10% at 1 mmol/l). The inhibitory effect was reversible and not dependent on Ca2+. All local anesthetics exerted identical effects in hepatocytes from young and old rats. Nicotinoyl-procaine and tetracaine inhibited cellular respiration in young and old rats up to a maximum of 50%. Procaine did not reduce O2 consumption below 1 mmol/l. This inhibition appeared also immediately upon addition, was not reversible, and not dependent on Ca2+. It is concluded that local anesthetics impair quite different biological processes like thymidine transport and cellular respiration in hepatocytes from young and old animals even at concentrations below 100 mumol/l. The magnitude of inhibition was correlated to the lipid solubility of the local anesthetics. The mechanism seemed to be different to the anesthetic action and it is supposed that it is a direct hydrophobic interaction with membrane proteins. Thus the local anesthetic-induced increase in membrane fluidity could not improve the age-dependent impairment of thymidine transport. The reduction of respiration is considered to be due to reduced O2 diffusion. This inhibition is in striking contrast to previously observed stimulatory effects which in part lead to the use of local anesthetics in geriatrics.

Age-dependence of molecular and functional changes in biological membrane properties

Some general aspects including results on the possible mechanisms of membrane ageing are reviewed. The liquid-crystalline fluid state of a biological membrane is an essential condition for maintenance of different membrane functions. The liquid-crystalline state of different plasma membranes changes with age of the organism. The degree of unsaturated fatty acids decreases and the content of cholesterol increases during ageing. It could be shown that superoxide radicals originate from minor side-reactions of oxidoreductase enzymes. Ageing increases the amount of superoxide radicals. A small amount of radicals escape quenching by superoxide dismutase. The formation of radicals leads to degradation of membrane lipids. The age-dependent changes in membrane lipid composition influence respiratory activity in rat heart mitochondria of old animals. Rat liver plasma membrane lipids also show a decrease in membrane fluidity which results in a change in transport parameters of cholic acid and thymidine. The change in age-dependent lipid-protein interactions was demonstrated by spin-label measurements in model membranes. The results demonstrated that peroxidative break-down of lipids is an ongoing post-transcriptional process of ageing. The possible role of protective repair mechanisms is discussed.

Thymidine transport in hepatocytes. An assay for hepatotoxicity

Thymidine transport and phosphorylation were investigated in isolated rat hepatocytes and AS 30 D hepatoma cells. In contrast to hepatoma cells, hepatocytes exhibited a minimum of thymidine phosphorylation due to a 100-fold smaller thymidine kinase activity. In hepatocytes thymidine is transported by two transport systems: a specific concentrative "high affinity" system and an unspecific non-concentrative "low affinity" system. In hepatoma cells only the "low affinity" system could be detected. A single dose of 20 or 50 mg diethylnitrosamine/kg body weight induced in hepatocytes a remakable increas of thymidine kinase activity and a decrease of the transport by the "high affinity" system. Thymidine transport and phosphorylation by hepatocytes are considered to be sensitive markers for early recognition of toxin-induced liver regeneration.

Alteration of human breast tumor cell membrane functions by chromosome-mediated gene transfer

BOT-2 cells (human breast tumor origin) have an impaired ability to utilize exogenous thymidine. Previous studies revealed this deficiency to be the permeation event rather than phosphorylation, since the cells have active thymidine kinase. Chromosome-mediated gene transfer was used to transfer genetic information in the form of metaphase chromosomes, from HeLa-65 cells to the BOT-2 cells, correcting the permease deficiency. Poly-L-ornithine or lipochromes were used for facilitation of chromosome uptake. After selection on HAT medium, transferant clones were isolated at a frequency of 4 x 10(-5) and 1 x 10(-5), respectively. Transferants MGP-1 and MGL-1 are stable after 18 months and have been characterized on the bases of purine and pyrimidine nucleoside uptake, relative thymidine kinase activities, alkaline phosphatase activities, and hydrocortisone-induced alkaline phosphatase activity. MGP-1 demonstrates positive thymidine uptake and incorporates radiolabeled thymidine into DNA. MGL-1 remains thymidine transport-deficient and surveys on HAT by increasing endogenous dihydrofolate reductase activity. Alkaline phosphatase activity in MGL-1 is similar to HeLa-65, 2% of that in BOT-2, and in addition, is inducible 25-30-fold by 3 micro M hydrocortisone. We have separated, genetically, a thymidine permease function from phosphorylation in cells of human origin and have transferred genetic information for the regulation of alkaline phosphatase.