EXCHANGE OF IRON AND TRANSFERRIN ACROSS ENDOTHELIAL SURFACES IN THE RAT AND RABBIT

Toxic Metal Species and 'Endogenous' Metalloproteins at the Blood-Organ Interface: Analytical and Bioinorganic Aspects

Globally, human exposure to environmental pollutants causes an estimated 9 million deaths per year and it could also be implicated in the etiology of diseases that do not appear to have a genetic origin. Accordingly, there is a need to gain information about the biomolecular mechanisms that causally link exposure to inorganic environmental pollutants with distinct adverse health effects. Although the analysis of blood plasma and red blood cell (RBC) cytosol can provide important biochemical information about these mechanisms, the inherent complexity of these biological matrices can make this a difficult task. In this perspective, we will examine the use of metalloentities that are present in plasma and RBC cytosol as potential exposure biomarkers to assess human exposure to inorganic pollutants. Our primary objective is to explore the principal bioinorganic processes that contribute to increased or decreased metalloprotein concentrations in plasma and/or RBC cytosol. Furthermore, we will also identify metabolites which can form in the bloodstream and contain essential as well as toxic metals for use as exposure biomarkers. While the latter metal species represent useful biomarkers for short-term exposure, endogenous plasma metalloproteins represent indicators to assess the long-term exposure of an individual to inorganic pollutants. Based on these considerations, the quantification of metalloentities in blood plasma and/or RBC cytosol is identified as a feasible research avenue to better understand the adverse health effects that are associated with chronic exposure of various human populations to inorganic pollutants. Exposure to these pollutants will likely increase as a consequence of technological advances, including the fast-growing applications of metal-based engineering nanomaterials.

The mechanism of 67Ga localization in malignant disease

After almost two decades of research, evidence suggests that transferrin receptors are rate-limiting in 67Ga uptake, 67Ga imaging is an in vivo detector of transferring receptors and an indicator of tumor cell proliferation. Moreover, 67Ga can be incorporated into tumor cells by a transferrin receptor-independent process. This pathway could be important in patients with saturated transferrin. Data that appear to support the concept that transferrin receptor-independent transporters are rate-limiting can be resolved by considering the presence of the gallate ion, [67Ga (OH)4]-.

Transport of transferrin across the blood-thymus barrier in young rats

The interaction of transferrin-peroxidase (Tf-HRP) with the capillary endothelium of the rat thymus was analyzed by diaminobenzidine (DAB) cytochemistry and electron microscopy. The thoracic aortas of young rats were cannulated and the upper bodies perfused with the Tf-HRP conjugate. In the thymus, plasmalemmal vesicles of the endothelium mediated the endocytosis and transport of Tf-HRP. Transcytosis of Tf-HRP appeared to occur by micropinocytosis, without morphological evidence of involvement by endothelial endosomes. DAB reaction product was commonly present in the subendothelial interstitial space and, in addition, was often localized in clathrin coated pits and vesicles in epithelial reticular cells that surround the thymic capillaries. In perfusions done at 4 degrees C, no binding of Tf-HRP to the lumenal membranes of capillaries was detected. The transport of Tf-HRP across the capillary endothelium in the thymus was not inhibited by competition with excess native transferrin; however, the uptake of Tf-HRP by epithelial reticular cells was completely inhibited by excess transferrin. Transferrin receptors were localized in the thymus by indirect immunocytochemistry using OX-26, a mouse anti-rat transferrin receptor monoclonal antibody. No transferrin receptors were detected on the capillary endothelium but diffuse reaction product was localized on the subjacent epithelial reticular cells. These results indicate that transport of Tf-HRP across the thymus capillary wall is independent of transferrin receptors at the level of the endothelial cells but that subsequent uptake of Tf-HRP by epithelial reticular cells is a transferrin receptor dependent process.

Developmental changes in transferrin and iron uptake by the brain in the rat

The uptake of transferrin and iron by the brain, liver and femurs was investigated in rats using 125I-59Fe-transferrin (Tf), and 131I-albumin in order to measure the plasma content of the organs. Measurements in rats ranging in age from birth to 70 days revealed that the rate of iron uptake by the brain increased rapidly over the first 15 days of life, peaking at 15 days and thereafter declining. A similar pattern occurred in the uptake of 125I-Tf. These changes were accompanied by rapid growth of the brain up to 15 days and a decrease in the concentration of non-haem iron. The turnover of 59Fe and 125I-Tf in the brain was also determined by measuring radioactivity in the brain of 15-day rats at various times after injection from 15 min to 13 days. The amount of 59Fe in the brain increased over the first 4 h and thereafter remained constant. By contrast, the 125I-Tf values increased rapidly during the first 15 min to reach a relatively constant level which was maintained for at least 6 h after which it declined. The patterns of uptake by the brain were different from those found in the liver and femurs, indicating that the changes in the brain were specific for that organ.(ABSTRACT TRUNCATED AT 250 WORDS)

Transferrin receptor expression during exponential and plateau phase growth of human tumour cells in culture

Transferrin receptor expression by the human tumour cell lines CCRF-CEM leukaemia and PMC-22B melanoma was studied, measuring the specific binding of fluorescein isothiocyanate (FITC)-labelled transferrin using a fluorescence-activated cell sorter. By measuring the fluorescence of cells stained at subsaturating concentrations of conjugate it was possible to calculate the average numbers of receptors per cell and the binding affinity by Scatchard analysis. These values (1.9 X 10(5) binding sites/cell, KA 1.2 X 10(9) M-1 for CCRF-CEM during exponential growth and 6.9 X 10(4) binding sites/cell, KA 1.4 X 10(-9) M-1 for PMC-22B) are in close agreement with previously published data obtained using radiolabelled transferrin. The present method, however, allowed the transferrin receptor expression of individual cells within a population to be measured and thus it has been possible to test the hypothesis that transferrin receptor is a marker for cycling cells. Frequency-distribution histograms of transferrin receptor showed a wide range of values for both cell lines during exponential growth. When the extreme ranges were sorted and the cells examined for cellular DNA content it was found that those with the highest transferrin receptor expression were enriched with cells in S, G2, and M phases of the cell cycle, whereas those with low transferrin receptor expression were mainly in G1. However, two-parameter-correlated dot plots of transferrin receptor expression versus DNA content showed there was considerable overlap between the ranges of receptor expression for the different cell cycle compartments. Using a stathmokinetic method we have measured the proportion of quiescent cells in fed plateau phase cultures. Transferrin receptor expression was downgraded under these growth conditions but, contrary to expectation, the decline affected the population uniformly, without the emergence of a distinct, transferrin receptor-negative subpopulation corresponding to the increasing proportion of quiescent cells. Thus, although transferrin receptor expression bears some relation to cell cycle phase and reflects the proliferative activity of populations of cells, it is incapable of identifying individual cells which are out of cycle.

Mechanisms of distribution of gallium 67 in mouse tumour hosts

In several mouse tumour models iron given parenterally not only inhibited the transport of 67Ga by plasma transferrin but also accelerated the binding of 67Ga in tumour tissue, particularly during the first hour after 67Ga injection. By 30 min, the tumour had already attained a high concentration though the 67Ga accumulated was still potentially reversible. With the passage of time equally high 67Ga concentrations were attained in tumour in iron-treated and untreated animals alike, and were maintained high as 67GA was totally cleared from the plasma. Given IV, 125-I-human transferrin was not preferentially bound in tumour whereas in muscle tissue loaded with transferrin, 67Ga was accumulated locally during the immediate post-injection phase and to an enhanced degree in iron-treated animals. A hypothesis is presented which, by drawing attention to the role of extravascular transferrin in binding and presenting 67Ga for cellular uptake, helps resolve the apparent contradictions between in vivo and cell culture findings.

Siderosis of lymph nodes patients with Hodgkin's disease

Lymph nodes removed from 28 untreated patients with Hodgkin's disease all contained markedly increased amounts of hemosiderine, whether or not they were histologically involved in the disease. This finding was particularly striking in patients with the nodular sclerosis type of disease. Abnormal deposits of iron were also noted frequently in lymph nodes containing metastatic carcinoma, lymphoma of non-Hodgkin's type, and reactive hyperplasia, but in each case, with the exception of metastatic squamous cell carcinoma, the amount was significantly less than seen in Hodgkin's disease. The findings suggest that in patients with Hodgkin's disease and perhaps in those with other disorders in which abnormal tissue retention of iron underlies sideropenic anemia, lymph nodes are an important site of iron retention.

Virulence and the role of iron in Pseudomonas aeruginosa infection

The virulence of Pseudomonas aeruginosa can be enhanced by passage in mice or rabbits. Enhanced virulence has some specificity for the host in which the passage is done. Experimental infection in the peritoneal cavity of cannulated rabbits has shown that the injection of iron compounds can lead to a rapid and fatal growth of an otherwise nonlethal dose of bacteria. In vitro the unsaturated iron-binding proteins present in the peritoneal fluid can halve the growth rate of P. aeruginosa. The restricted rate of growth is restored to normal if the iron-binding proteins are saturated with iron. Exactly the same results are achieved with purified transferrin. Both fatal and nonfatal infections with P. aeruginosa cause a sharp fall in the percentage of saturation with Fe of the plasma and peritoneal fluid. In both normal and infected animals the peritoneal fluid is invariably less saturated than the plasma. Specific antiserum not only protects against death but also against the fall in iron saturation of the plasma and peritoneal fluid. In both fatal and nonfatal infections a high proportion of viable bacteria are unphagocytized in the peritoneal cavity.