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PubMed1992

Seminal plasma transferrin concentration: relationship with seminal parameters and plasma hormone levels.

Seminal plasma transferrin concentrations were determined in 155 infertile male patients and in 15 pregnancy-proven fertile males (control group); then the relationship between these concentrations and seminal parameters and plasma hormone levels was investigated. The concentrations of seminal plasma transferrin in patients with a sperm concentration below 20 x 10(6)/ml were significantly lower than those in the control group (p < 0.01). There was no significant difference in seminal plasma transferrin concentration between patients with a sperm concentration of 40 x 10(6)/ml or more and the control patients. A positive correlation was observed between sperm concentration and seminal transferrin content (r = 0.56; p < 0.05). However, correlations between seminal transferrin concentration and sperm motility and between seminal plasma transferrin content and sperm morphology did not show any significance, nor did the seminal transferrin content correlate with plasma LH, FSH, prolactin or testosterone levels. It, therefore, seems that while transferrin is indicative of certain physiopathological conditions in the germ cells, this protein is not a distinctive marker of the fertility potential of an individual.

Adult↗
PubMed1995

Inhibitory effect of transferrin on progesterone production in the granulosa cell of humans in vivo and porcine granulosa cell in vitro.

We evaluated the effect of transferrin on the regulation of granulosa cell function in humans by evaluating the production of progesterone (P) in the preovulatory phase in vivo, and in cultured porcine granulosa cells in vitro. Twenty-five women treated for in vitro fertilization and embryo transfer had their serum levels of 17 beta-estradiol (E2) and P determined on the day of administration of human chorionic gonadotropin. Transferrin concentrations were also determined in ovarian follicular fluid. In an in vitro study, porcine granulosa cells were cultured in the presence of follicle-stimulating hormone (FSH) and transferrin. Serum levels of P showed a significant negative correlation with those of transferrin (r = -0.53, p < 0.01), whereas serum levels of E2 did not (r = 0.14). When the subjects were divided into two groups by serum P concentration (low P < 1 ng/ml, high P > or = 1 ng/ml) serum concentrations of transferrin were significantly increased in the group with the low versus the high level of p (p < 0.01). Production of P by porcine granulosa cells was suppressed by transferrin in the presence of various concentrations of FSH. Increasing the dose of transferrin significantly suppressed the production of P by those cells in a dose-dependent fashion. The production of P during the preovulatory phase may be suppressed by transferrin in the granulosa cells.

Adult↗
PubMed1983

Regulation of transferrin receptor expression on human leukemic cells during proliferation and induction of differentiation. Effects of gallium and dimethylsulfoxide.

The association of transferrin receptor expression with cellular proliferation has been studied extensively, but a number of events have not been defined. We therefore assayed receptor on promyelocytic leukemia (HL-60) cells at early times after exposure to a stimulus for proliferation (subculture), as well as agents that either induce differentiation (dimethylsulfoxide [DMSO] ) or inhibit iron uptake (transferrin-gallium). Within 4 h after subculture, we found that a significant increase in total cellular immunoreactive receptor occurred that preceded by 8 h the increase in cell-surface transferrin binding. Automated fluorocytometric analysis of cells in an immunofluorescent assay indicated that increased surface receptor density appeared on cells in the S, G2, and M phases of the cell cycle. DMSO-treated cells proliferated at the same rate as untreated (control) cells for the first 72 h, but as early as 12 h after treatment transferrin receptor was significantly decreased (65% of control cells). Further decreases occurred at later time points until transferrin receptor was undetectable after 7 d, when proliferation had ceased, cells were arrested in G1 phase of the cell cycle, and myeloid differentiation occurred. After exposure to transferrin-gallium, proliferation ceased, but cells exhibited increased surface receptor and were arrested at S phase of the cell cycle without associated myeloid differentiation. We conclude that events preceding cell division provide the regulatory stimulus for the synthesis and subsequent appearance of the transferrin receptor on the cell surface. Additionally, decreased receptor expression may be important in causing cessation of proliferation and/or differentiation. Finally, the way in which gallium salts are currently being investigated as chemotherapeutic agents should be reevaluated in light of our findings concerning transferrin-gallium effects on cellular proliferation.

Antibodies, Monoclonal↗
PubMed1991

Effect of transferrin on the growth of Porphyromonas gingivalis.

This study describes the effect of transferrin as an iron source on the growth of Porphyromonas (formally Bacteroides) gingivalis. Bacterial growth was monitored spectrophotometrically. All strains of P. gingivalis tested grew well in medium containing transferrin. The growth of P. gingivalis depended not only on the concentration of transferrin, but also on the iron saturation level of the protein. However, growth was not stimulated with either the ferrous or ferric iron salts tested. The addition of dipyridyl to the medium containing transferrin suppressed the growth of P. gingivalis, which also did not show species-specificity for human transferrin. Transferrin-binding activity was found in P. gingivalis by solid-phase assay with peroxidase-conjugated human transferrin. These results suggest that P. gingivalis may be capable of utilizing transferrin as an iron source for growth in vivo.

2,2'-Dipyridyl↗
PubMed2002

Total iron binding capacity and transferrin concentration in the assessment of iron status.

Transferrin concentration and total iron binding capacity (TIBC) are currently used to assess iron status. Although correlation between TIBC and transferrin is generally considered as good, conversion factors between the two analytes found in literature show large differences. Although the price per test is lower for TIBC, there are a number of analytical advantages of serum transferrin. Due to binding of iron to other plasma proteins (mainly albumin), TIBC methods generally overestimate the iron binding capacity of transferrin. Moreover, no generic reference values are available for TIBC. In contrast to TIBC, internationally accepted interim reference ranges are available for serum transferrin. The introduction of the international CRM 470 protein standard material has lead to a significant reduction in interlaboratory variation for transferring measurements. In view of these observations, determination of transferrin concentration, rather than TIBC, is recommended. However, in non-European populations characterized by a marked genetic variation in transferrin (TF BC and TF CD variants), in certain cases, immunochemical determination of transferrin may lead to errors. In these populations, TIBC measurements may be preferred.

Blood Proteins↗
PubMed1991

Transferrin inhibits aromatase activity of rat granulosa cells in vitro.

The effect of transferrin on basal and FSH-stimulated aromatase activity of granulosa cells from immature female rats treated with diethylstilboestrol (DES) was examined in vitro by a radiometric method. The basal activity of the enzyme was very low after 3 days of incubation. Treatment with FSH (20 ng/ml) resulted in a 9.6-fold increase in activity, whereas coincubation with increasing doses of transferrin (3-300 micrograms/ml) produced a dose-dependent inhibition of FSH-stimulated aromatase activity with a projected minimal effective dose of less than 2 micrograms/ml. A time-course study showed that the inhibitory effect of transferrin on aromatase activity has become significant at 48 h of incubation. The inhibitory action of transferrin on the enzyme complex was further confirmed by showing that the FSH dose-response curve was significantly suppressed by concomitant treatment with 100 micrograms transferrin/ml with a maximum suppression of 54.1% at a dose of 30 ng FSH/ml. The possibility that transferrin may act through a non-specific inhibitory effect seems unlikely, as no changes in cell number and DNA content per well were observed. In fact, protein synthesis was enhanced after treatment with transferrin. Aromatase activity, stimulated by several promoters of cyclic AMP (cAMP), such as prostaglandin E2 (PGE2), forskolin and 8-bromo-cAMP, was significantly suppressed by 100 micrograms transferrin/ml (36.6, 47.4 and 23.4% inhibition respectively), suggesting that the effect of transferrin on FSH action may involve a site(s) distal to cAMP generation. These findings indicated that transferrin, present in follicular fluid, may play an important role in the regulation of granulosa cell differentiation.

Animals↗
PubMed1990

Transferrin and epidermal growth.

Growth of keratinocytes in explant culture of mouse ear epidermis was studied. The addition of transferrin to the culture media improved growth. Transferrin fractionated from human and fetal calf serum increased outgrowths of the cultures when compared with commercially available transferrin. An acidic transferrin fraction was present in greater amount in human serum and in fetal calf serum than that found in commercial transferrin. This fraction was more abundant in serum from psoriatic patients than in serum of healthy subjects as shown by isotachophoresis. For the culture studies, preparation of this material was done by chromatography on DEAE-Sepharose 6B-CL columns. Further on, diferric transferrin was preferentially used in order to abolish variation due to iron saturation. Iron concentration higher than 5 microM was deleterious to cell growth. The basal culture medium contained transferrin depleted fetal calf serum in RPMI 1640 with 2 microM glutamine and antibiotics. Serum-free medium was used in some experiments. The additions were 1.7 microM insulin, 1.4 microM hydrocortisone, 10 microM ethanolamine and 10 microM phosphoethanolamine. A partially purified fraction of the acidic forms of transferrin (10-20 micrograms/ml medium) improved outgrowth when compared with a neutral fraction under these circumstances.

Animals↗
PubMed1995

Further studies on targeted DNA transfer to cells using a highly efficient delivery system of biotinylated transferrin and biotinylated polylysine complexed to streptavidin.

Conjugates consisting of biotinylated transferrin and biotinylated poly-L-lysine attached to streptavidin have been prepared and found to transfer luciferase plasmid DNA very efficiently to HeLa cells in the presence of chloroquine. Transfection was dependent on (i) use of biotinylated short chain polylysine containing 70 lysine residues, (ii) biotinylated transferrin containing 1-2 biotin moieties, (iii) reaction of biotinylated transferrin with streptavidin followed by isolation of the resulting conjugate on Sephadex G-200 and (iv) interaction of streptavidin-biotinylated transferrin with biotinylated polylysine giving a complex suitable for DNA transfection. It was found that if the above sequence of steps resulting in the formation of streptavidin-biotinylated transferrin/biotinylated polylysine was followed without isolation of intermediate conjugates by Sephadex G-200 chromatography, pRSVL DNA transfer was still very efficient. Transfer of luciferase DNA by the streptavidin conjugates and subsequent expression of luciferase activity was almost completely inhibited by excess free transferrin, showing that gene transfer was through the transferrin receptor pathway via receptor-mediated endocytosis. The streptavidin (bio2-transferrin) bio10-pLys70 conjugate used in the present experiments was approximately one hundred times more efficient in pRSVL DNA transfection with the HeLa cells than the previously described avidin-pLys460 (bio-transferrin) complex.

Bacterial Proteins↗
PubMed1985

Molecular advantage of diferric transferrin in delivering iron to reticulocytes: a comparative study.

The delivery of transferrin iron from four animal species and man to homologous reticulocytes was measured at different transferrin saturations. Total iron uptake in the in vitro reticulocyte incubation model employed followed a hyperbolic curve, increasing as the transferrin saturation increased but at a progressively slower rate. In all species, there was a much greater iron delivery from diferric as compared to monoferric transferrin, the molecular advantage varying from 8:1 to 14:1. The majority of iron was delivered from diferric transferrin when transferrin saturations exceeded 13-19% depending on the species. Thus a general similarity exists in the transferrin-iron interactions in these mammalian species. Formuli have been provided whereby the iron utilization curve may be calculated when uptake has been determined at any one transferrin saturation.

Animals↗
PubMed2001

Effect of wortmannin and phorbol ester on Paramecium fluid-phase uptake in the presence of transferrin.

The kinetics of the uptake of the fluid phase marker Lucifer Yellow (LY), and its alteration by wortmannin, an inhibitor of phosphatidylinositol-3 kinase (PI-3K), and the PKC modulators: GF 109203 X, an inhibitor, and phorbol ester, an activator was studied in eukaryotic model Paramecium aurelia. Spectrophotometric quantification of LY accumulation was performed in the presence or absence of transferrin, a marker of receptor-mediated endocytosis. Internalization of LY showed a curvilinear kinetics: the high initial rate of LY uptake (575 ng LY/mg protein/hr) decreased almost 5-fold within 15 min, reaching plateau at 126 ng/mg protein/hr. Transferrin induced a small increase (7.5%) in the fluid phase uptake rate (after 5 min) followed by a small decrease at longer incubation times. Lucifer Yellow and transferrin (visualized by streptavidin-FITC) were localized in Paramecium by 3-D reconstruction by confocal microscopy. LY showed a scattered, diffuse fluorescence typical of fluid phase uptake whereas transferrin accumulated in membrane-surrounded endosomes. Wortmannin did not affect LY accumulation but decreased it when transferrin was present in the incubation medium. This suggests an effect on the transferrin uptake pathway, presumably on the stage of internalization in "mixing" endosomes to which transferrin and LY were targeted. Phorbol ester diminished LY accumulation by 22% and this effect persisted up to 25 min of incubation. PKC inhibitor did not affect LY uptake. However, in the presence of transferrin, the LY uptake increased within the first 15 minutes followed by a rapid 20% decrease in comparison to the control. Such an effect of PKC modulators suggests that PMA action on fluid phase uptake is not directly mediated by PKC.

Androstadienes↗
PubMed1989

[Study of transferrin concentration in the human seminal plasma].

Recently, the seminal plasma transferrin concentration has been reported to be a possible clinical marker of the Sertoli cell function. In the present study we measured the seminal plasma transferrin concentration in patients of male infertility by the method of radial immunodiffusion. The results obtained were as follows. 1. The seminal plasma transferrin concentration is significantly correlated to sperm density (p less than 0.001). However, no significant correlations were obtained to sperm motility, serum LH, FSH and testosterone levels. 2. The seminal plasma transferrin concentration in patients of ductal obstruction was 16.2 +/- 4.1 micrograms/ml, which was approximately 20% of pregnancy-proven men (77.5 +/- 49.0 micrograms/ml). This indicates that seminal plasma transferrin originates mainly from the testis. 3. The seminal plasma transferrin concentrations in oligozoospermic men associated with varicocele and in patients of spermatid arrest were low and their values were 22.1 +/- 7.5 micrograms/ml and 13.0 micrograms/ml, respectively. In these patients the involvement of Sertoli cell dysfunction was suggested. 4. Patients of idiopathic oligozoospermia were divided into two subgroups: patients with high seminal plasma transferrin concentrations (128.3 +/- 23.8 micrograms/ml) and those with low seminal plasma transferrin concentrations (29.8 +/- 0.9 micrograms/ml). The serum gonadotropins levels in the former group was at the same level as those of pregnancy-proven men and the levels of the latter group were significantly elevated. These observations suggest that the two groups thus divided belong to different categories of the disease.

Adult↗
PubMed1990

[Therapeutic evaluation of male infertility--the increase of seminal transferrin level and the improvement in sperm concentration following administration of clomiphene citrate].

This study was designed to evaluate the functional changes of Sertoli cells following the administration of clomiphene citrate for male infertility. Sperm count and seminal transferrin level were measured before and after the treatment in 22 cases of oligozoospermia (sperm count: less than 20 X 10(6)/ml) and 14 cases of subnormal sperm count group (sperm count: 20-30 X 10(6)/ml). Clomiphene citrate was administered per os for more than 3 months consecutively in a dose of 25 mg/day. Seminal transferrin concentration increased more than 1.5 times compared with pre-treatment level in 6 cases (16.7%). Among these patients, sperm count markedly increased (20 X 10(6)/ml or more than the pre-treatment level) in 3 cases (50%) and slightly increased (10 X 10(6)/ml or more than pre-treatment) in 1 case (16.7%). In 30 cases, in which seminal transferrin level did not increase, sperm count markedly increased in 6 cases (20%) and slightly increased in 6 cases (20%). Thus, sperm count was improved more frequently in the cases in which seminal transferrin level remained elevated than the cases with no elevation of serum transferrin level. Serum FSH level of patients whose seminal transferrin level remained elevated after the treatment was significantly higher than that of patients with no elevation of serum transferrin level (mean +/- SD = 32.8 +/- 18.0 mIU/ml v.s. 14.4 +/- 11.7 mIU/ml, respectively). These data suggested that the activation of Sertoli cells may contribute to the increase of sperm count following the administration of clomiphene citrate and the elevated seminal transferrin secretion may be related to the increase of serum FSH level following this treatment.

Clomiphene↗
PubMed1999

Relative versus absolute carbohydrate-deficient transferrin as a marker of alcohol consumption in patients with acute alcoholic hepatitis.

BACKGROUND: Carbohydrate-deficient transferrin has been described as a sensitive and specific marker for alcohol consumption. This study investigated the usefulness of carbohydrate-deficient transferrin as a marker of alcohol consumption in acute alcoholic hepatitis. METHODS: Absolute concentrations (U/I) and relative values (%) of carbohydrate-deficient transferrin determined in serum with commercial assays, as well as conventional markers for alcohol consumption, were compared with the alcohol consumption (as estimated by a questionnaire) in patients with acute alcoholic hepatitis (n = 19), alcoholic liver cirrhosis (n = 37), and nonalcoholic liver diseases (n = 16). RESULTS: The concentration of carbohydrate-deficient transferrin was increased (p < 0.001) in nonabstaining patients (median intake 80 g alcohol/day) with alcoholic liver cirrhosis (45.7 +/- 30 U/l), but not in patients with acute alcoholic hepatitis (20.0 +/- 7.8 U/l) despite higher alcohol consumption (median 130 g/d), nor in abstainers with alcoholic liver cirrhosis (19.4 +/- 6.0 U/l) or nonalcoholic liver disease (18.5 +/- 6.7 U/l). However, the relative values of carbohydrate-deficient transferrin were increased both in acute alcoholic hepatitis (7.9 +/- 2.1%) and nonabstainers with alcoholic liver cirrhosis (7.4 +/- 2.8%), but not in abstainers with alcoholic liver cirrhosis (4.6 +/- 3.5%) or nonalcoholic liver disease (3.8 +/- 0.9%) (p < 0.001). In acute alcoholic hepatitis, the sensitivity and specificity were only 32% and 87% for absolute concentrations, respectively, but 79% and 97% for relative values of carbohydrate-deficient transferrin. The concentrations of carbohydrate-deficient and total transferrin in serum were strongly correlated (r = 0.60; p = 0.008). CONCLUSIONS: The relative value (% of total), but not the absolute concentration, of carbohydrate-deficient transferrin in serum is a useful marker of alcohol consumption in acute alcoholic hepatitis.

Adult↗
PubMed2001

Direct toxic effects of clinical doses of chloroquine on transferrin secretion in immature rat sertoli cells in vitro.

We have examined the effects of increasing doses of chloroquine (CQ), on transferrin secretion in primary cultures of immature rat Sertoli cells (SC) grown on a reconstituted basement membrane (Matrigel) in bicameral chambers. SC cells were seeded in serum-free defined medium at a density of 3 x 10(6) cells/0.64cm2/well on Matrigel covered Millicell-HA filters. CQ at concentrations ranging from 0.04-1.0 microM was added to the basal compartment of the bicameral system from day 7 of the culture. The formation of the tight junction was monitored by the measurement of the transepithelial resistance (TER) at 24 hr intervals using an impedance meter TER in untreated controls was 50 Ohms/cm2 on day 1, and increased progressively to 80 Ohms/cm2 by day 7 and plateaued until day 12. On the seventh day of culture, CQ was introduced into the basal chamber During the 4 days of the experiment, the secretion of transferrin decreased with time. Maximal transferrin secretion by SC was detected during the initial 2 day collection period. During the subsequent collection period, CQ (1 microM) decreased significantly transferrin secretion by SC, while 0.04 microM CQ did not affect transferrin secretion. The polarized secretion of transferrin in response to CQ was also studied. During both collection periods there was no significant difference between controls and 0.04 microM CQ cultures in the ratio of apical to basal transferrin secretion. In the 1 microM culture medium, CQ diminished significantly the ratio of apical to basal transferrin secretion. These observations demonstrate the heterogenous effects of lower doses of CQ on immature rat SC in cultures.

Animals↗
PubMed1989

Characterization of a novel form of transferrin receptor preferentially expressed on normal erythroid progenitors and precursors.

A panel of monoclonal antibodies (MoAbs) against cell surface proteins of early BFUe progeny was characterized. Five of these antibodies (Abs) reacted with normal erythroid, but not myeloid, bone marrow cells. Each of the five antibodies, typified by Ab 69.20, immunoprecipitated a dimeric complex of 185,000, which is composed of two identical disulfide-bonded subunits. This antigen had affinity for transferrin, and was essentially identical in biochemical characteristics to transferrin receptors precipitated with the well-characterized MoAbs OKT9 and 5E9. However, this form of transferrin receptor lacked both the OKT9 and 5E9 antigenic determinants and, moreover, the 69.20 epitope was absent from the conventional transferrin receptor, as defined by Abs OKT9 and 5E9. Modulation experiments demonstrated that both 69.20 and OKT9 modulated large, virtually independent populations of transferrin receptors. Both forms of transferrin receptor appeared to be derived from the product of a single gene, but the form defined by MoAb 69.20 apparently predominates in cells of the erythroid lineage and some transformed cell types that manifest a special requirement for iron. These data suggest that cells with a high iron requirement synthesize two forms of transferrin receptor, possibly by means of differential mRNA splicing or by posttranslational modification of the transferrin receptor.

Animals↗
PubMed1989

Transferrin degradation by gastrointestinal fluids of suckling and weanling rats.

Dietary transferrins are postulated to play a number of biological roles in the developing gastrointestinal tract. A prerequisite for such roles is survival in the gastrointestinal lumen. To evaluate luminal transferrin digestion during development, 125I-transferrin was incubated in vitro with luminal fluid from the stomach and small intestine of 12-day old suckling and 31-day old weanling rats, followed by analysis of degradation products. At both ages, the rate of degradation to trichloroacetic acid soluble material was maximum in the mid-jejunum and lowest in the stomach. Transferrin hydrolysis by weanling fluid was 2-10 times greater than suckling depending upon the particular segment. Chromatography of small intestinal reaction mixtures on Sephacryl S-200 revealed label eluting between intact transferrin and free iodine: two such peaks were generated with suckling fluid and one with weanling. Electrophoresis on SDS-polyacrylamide gels showed two major bands of Mr 69K and 20K; the former was the predominant reaction product with suckling intestinal fluid and the latter with weanling. Both methods showed small amounts of apparently intact transferrin. Results indicate substantial yet incomplete luminal degradation of transferrin which is more pronounced in the weanling than in the suckling. This survival is compatible with potential biological functioning of dietary transferrin or one of its breakdown products within the gastrointestinal tract.

Animals↗
PubMed1985

Prostaglandin E2 acts at two distinct pathways of T lymphocyte activation: inhibition of interleukin 2 production and down-regulation of transferrin receptor expression.

The mechanism by which prostaglandin E2 (PGE2) inhibits human T lymphocyte activation and proliferation was studied. We analyzed the effect of physiologic concentrations of PGE2 on interleukin 2 (IL 2) production, expression of IL 2 receptor (Tac antigen), and expression of the transferrin receptor after in vitro activation with phytohemagglutinin. PGE2 inhibited T lymphocyte proliferation by 80 to 90% of control values. This was associated with a similar degree of inhibition of IL 2 production while the expression of IL 2 receptor was not affected. This was in marked contrast to the expression of the transferrin receptor, which was inhibited 65% after 72 hr of in vitro activation. The addition of exogenous, purified IL 2 reconstituted lymphocyte proliferation to 50% of control values, but had no effect on transferrin receptor expression. Because PGE2 is known to increase the intracellular concentration of 3',5' cyclic adenosine monophosphate (cAMP), we investigated the effect of another adenylate cyclase activator, i.e., isoproterenol, as well as the effect of extracellular administration of the cAMP derivative dibutyryl cAMP (dBcAMP) on IL 2 production, Tac antigen expression, and transferrin receptor expression. It was demonstrated that isoproterenol, as well as dBcAMP, inhibited transferrin receptor expression on PHA-activated T lymphocytes to the same extent as PGE2, and exogenous IL 2 could not counteract the down-regulation of the receptor expression. In contrast, neither isoproterenol nor dBcAMP had any significant effect on IL 2 receptor expression. Prostaglandin F2 alpha (PGF2 alpha), which has been reported to elevate intracellular cyclic GMP levels, had no effect on lymphocyte activation and proliferation, and did not counteract the PGE2-induced depression in IL 2 production. In contrast to its effect on peripheral blood lymphocytes, PGE2 had no effect on transferrin receptor expression or cell proliferation by IL 2-dependent T cell clones and IL 2-independent T cell lines. These studies demonstrate that PGE2 exerts its inhibitory effects on T cell activation and proliferation via two distinct pathways: inhibition of IL 2 production and inhibition of transferrin receptor expression. The transferrin receptor inhibition is mediated via the cAMP pathway and is IL 2-independent.

Antigens, Surface↗
PubMed1986

Targeted inhibition of transferrin-mediated iron uptake in Hep G2 hepatoma cells.

We have used a model system consisting of two human hepatoma cell lines, Hep G2, representing well differentiated normal hepatocytes, and PLC/PRF/5, representing poorly differentiated malignant hepatocytes, to demonstrate that the differential presence of asialoglycoprotein receptor activity in these cell lines can be used to influence transferrin-mediated iron uptake. We based our experiments on the following facts: Hep G2 cells possess receptors that bind, internalize, and degrade galactose-terminal (asialo-)glycoproteins; PLC/PRF/5 cells have barely detectable asialoglycoprotein receptor activity; both cell lines possess active transferrin-mediated iron uptake; transferrin releases iron during acidification of intracellular vesicular compartments; primary amines, e.g. primaquine, inhibit acidification and iron release from transferrin. When added to culture medium, [55Fe]transferrin delivered 55Fe well to both cell lines. As expected, in the presence of [55Fe]transferrin, free primaquine caused a concentration-dependent decrease in 55Fe uptake in both cell lines. To create a targetable conjugate, primaquine was covalently coupled to asialofetuin to form asialofetuin-primaquine. When PLC/PRF/5 (asialoglycoprotein receptor (-)) cells were preincubated with this conjugate, transferrin-mediated 55Fe uptake was unaffected. However, transferrin-mediated 55Fe uptake by Hep G2 (asialoglycoprotein receptor (+)) cells under identical conditions was specifically decreased by 55% compared to control cells incubated without the conjugate.

Biological Transport↗