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M T Nuñez

Publications and source records attributed to M T Nuñez.

8 recordsLinked to original sources
PubMed2000

The cellular mechanisms of body iron homeostasis.

Cells tightly regulate iron levels through the activity of iron regulatory proteins (IRPs) that bind to RNA motifs called iron responsive elements (IREs). When cells become iron-depleted, IRPs bind to IREs present in the mRNAs of ferritin and the transferrin receptor, resulting in diminished translation of the ferritin mRNA and increased translation of the transferrin receptor mRNA. Similarly, body iron homeostasis is maintained through the control of intestinal iron absorption. Intestinal epithelia cells sense body iron through the basolateral endocytosis of plasma transferrin. Transferrin endocytosis results in enterocytes whose iron content will depend on the iron saturation of plasma transferrin. Cell iron levels, in turn, inversely correlate with intestinal iron absorption. In this study, we examined the relationship between the regulation of intestinal iron absorption and the regulation of intracellular iron levels by Caco-2 cells. We asserted that IRP activity closely correlates with apical iron uptake and transepithelial iron transport. Moreover, overexpression of IRE resulted in a very low labile or reactive iron pool and increased apical to basolateral iron flux. These results show that iron absorption is primarily regulated by the size of the labile iron pool, which in turn is regulated by the IRE/IRP system.

Biological Transport↗
PubMed1999

Transferrin stimulates iron absorption, exocytosis, and secretion in cultured intestinal cells.

The cellular mechanism by which basolateral transferrin (Tf) produces an increase in apical-to-basolateral Fe flux in Caco-2 cells was analyzed. After a pulse of 59Fe from the apical medium, three types of basolateral 59Fe efflux were found: a 59Fe efflux that was independent of the presence of Tf in the basolateral medium, a 59Fe efflux in which 59Fe left the cell bound to Tf, and a Tf-dependent 59Fe efflux in which 59Fe came off the cell not bound to Tf. Furthermore, addition of Tf to the basolateral medium doubled the exocytosis rate of Tf and increased the secretion of apolipoprotein A, a basolateral secretion marker. Both apotransferrin and Fe-containing Tf produced similar increases in 59Fe efflux, Tf exocytosis, and apolipoprotein A secretion. The Ca2+ channel inhibitor SKF-96365 inhibited both the Tf-mediated increase in transepithelial Fe transport and the secretion of apolipoprotein A. Thus the activation of transepithelial Fe transport by Tf seems to be mediated by Ca2+ entry into the cells.

Apolipoproteins A↗
PubMed1990

Inhibitory effect of a toxic peptide isolated from a waterbloom of Microcystis sp. (Cyanobacteria) on iron uptake by rabbit reticulocytes.

The effect of a soluble toxin purified from the algae bloom of a eutrophic lake dominated by Microcystis on the receptor-mediated endocytosis of ferro-transferrin in rabbit reticulocytes was studied. The toxin was a very effective inhibitor of cell iron uptake. Kinetic studies using 125I, 59Fe-labeled transferrin indicated that the step of ferrotransferrin internalization was selectively inhibited by the toxin while the surface receptor-binding capacity, the externalization of previously internalized transferrin, and the cellular ATP levels were not affected. These findings indicate that the reduction of iron uptake caused by the toxin is due to inhibition of the internalization of surface-located transferrin-transferrin receptor complexes, perhaps due to a disruption of cytoskeleton integrity.

Animals↗
PubMed1985

Iron uptake in reticulocytes. Inhibition mediated by the ionophores monensin and nigerisin.

The lipophilic carboxylic ionophores monensin and nigerisin reversibly blocked iron uptake by erythroid cells. At low concentrations of ionophores (0.25-0.5 microM), the disruption of the compartment in which iron is released affected minimally the release of iron from transferrin but effectively inhibited iron uptake. Iron released from transferrin was extruded from the cell synchronously with but not bound to transferrin. The compartment disrupted by the ionophores, and in which iron is released from transferrin, is apparently contiguous to the extracellular medium. Contiguity was assessed by determining the effect of extracellular Na+ and K+ on the activity of the ionophores. The above data fit a model of iron uptake in which iron is released from transferrin in an acidic compartment in immediate contiguity with the cell plasma membrane. Iron is then bound by its membrane acceptor and is translocated to the cytosolic side of the plasma membrane. At submicromolar concentrations, the ionophores monensin and nigerisin produce a small increase in the pH of the acidic compartment. The pH change, which is not sufficient to block the release of iron from transferrin, is enough to block the binding of released iron to its acceptor in the plasma membrane, thus producing inhibition of iron uptake.

Animals↗
PubMed1983

The transferrin cycle and iron uptake in rabbit reticulocytes. Pulse studies using 59Fe, 125I-labeled transferrin.

Rabbit reticulocytes were pulsed for 10 s with diferric 59Fe, 125I-labeled transferrin followed without further manipulation by a chase with unlabeled diferric transferrin. The 125I-transferrin and 59Fe were measured in different cell compartments as a function of time thereafter. Immediately after binding, transferrin entered a phase that lasted 60 s, characterized by a slow dissociation of transferrin into the extracellular medium. During this period, most of the iron originally present in transferrin is donated to the cell. The half-time of 59Fe release from transferrin was 43 s. After the initial 60 s, transferrin, now devoid of iron, is released into the medium. The whole transferrin cycle lasted about 4 min. The iron released from transferrin could be transiently found in the cell plasma membrane, the cytosol, and the mitochondria. All these compartments behaved like intermediates in the iron uptake process as their 59Fe content rose, reached a plateau, and gradually fell. Finally, the iron was incorporated into heme with a half-time of incorporation of 173 s. We conclude that the release of iron from transferrin is one of the fastest events occurring after the initial binding of transferrin. The limiting step in the entire process of iron delivery is the dissociation of apotransferrin from its receptor, a step which will enable the latter to undergo another cycle of transferrin binding.

Animals↗
PubMed1977

The cross-linking of 125I-labelled transferrin to rabbit reticulocytes.

125I-labelled transferrin has been covalently linked to proteins of rabbit reticulocyte membranes using three different cross-linking reagents: (a) bis(methyl) suberimidate (b) 4-methyl mercaptobutirimidate and (c) Cu-o-phenanthroline. Analysis of the products of the cross-linking reactions by SDS polyacrylamide gel electrophoresis revealed three radioactively labelled components. The major component corresponds to non-cross-linked transferrin while a second smaller component represents a complex (125 000 daltons) of membrane protein and transferrin. A third component (210 000 daltons) was observed when 4-methyl mercaptobutirimidate was used as a cross-linking reagent. The possibility that the two higher molecular weight complexes contain the receptor protein for transferrin is discussed.

Animals↗
PubMedDate not supplied

Effect of copper, cadmium, mercury, manganese and lead on Fe2+ and Fe3+ absorption in perfused mouse intestine.

Body iron homeostasis is maintained by the regulation of iron absorption in the first portion of the small intestine. Previous studies have shown that some trace metal cations may have an inhibitory effect on inorganic iron absorption. In view of the possible nutritional and toxicological implications of this inhibition, we decided to study the effect of Cu2+, Cd2+, Hg2+, Pb2+, and Mn2+ on Fe2+ and Fe3+ intestinal uptake. To that end, the duodenal portion of the mouse intestine was perfused with 1 microM 55Fe either in the 2+ or 3+ redox state, in the absence or presence of a 10-fold molar excess of Cd2+, Cu2+, Hg2+, Mn2+, or Pb2+. It was found that Cd2+ and Cu2+, but not Hg2+, Mn2+, or Pb2+, significantly inhibited the uptake of Fe2+ (p < 0.001). Surprisingly, the observed inhibition by Cu2+ and Cd2+ was dependent on the iron redox state, since neither cation inhibited Fe3+ uptake by mouse duodenum. Similarly, no inhibition of Fe3+ uptake was observed with a 10-fold molar excess of Hg2+, Mn2+, or Pb2+. These results indicate that inhibition of iron uptake by Cu2+ and Cd2+ takes place only in a reducing environment. Therefore, under the common oxidative conditions found in nutrients no inhibition of non-heme Fe uptake by Cu2+ and Cd2+, as well as Mn2+, Hg2+ and Pb2+ is to be expected. We propose either that Fe2+ and Fe3+ are transported through two different mechanisms, or that the enterocyte-mediated reduction of Fe3+, and the ensuing transport of Fe2+ are processes coupled in such a way that they are refractory to inhibition by Cu2+ and Cd2+.

Animals↗