Receptor-mediated endocytosis of asialoglycoproteins and diferric transferrin is independent of second messengers.

The intracellular concentrations of cyclic AMP, polyphosphoinosides and free Ca2+ were unaffected during receptor-mediated endocytosis of the neoglycoprotein beta-D-galactosyl-bovine serum albumin (D-Gal-BSA) by isolated hepatocytes. Elevation of either intracellular cyclic AMP by glucagon or inositol phosphates and Ca2+ by vasopressin were without effect on the binding and internalization of D-Gal-BSA. The normal response of this cell to glucagon- and vasopressin-mediated mobilization of these second messengers was not modified in the presence of saturating concentrations of D-Gal-BSA. Receptor-mediated endocytosis of diferric transferrin (Fe3+-TRF) by both hepatocytes and HL60 cells was also shown to be independent of second messengers, although the unequivocal expression of the transferrin receptor by hepatocytes could not be satisfactorily demonstrated. The results of the present study are at variance with a suggested regulatory role for second messengers in receptor-mediated endocytosis of serum-derived ligands such as asialoglycoproteins and Fe3+-TRF. Receptor phosphorylation by protein kinase C in particular has been proposed to regulate the distribution and recycling of these receptors in actively endocytosing cells. We would suggest that if receptor phosphorylation has a regulatory role during endocytosis, it is likely to be mediated by a second-messenger-independent protein kinase analogous to casein kinase II. An alternative interpretation is that phosphorylation has no physiological significance and receptor-mediated endocytosis is a constitutive event coupled to membrane turnover.     

Retinoic acid inhibition of transplasmalemma diferric transferrin reductase

All trans retinoic acid inhibited diferric transferrin reduction by HeLa cells. The NADH diferric transferrin reductase activity of isolated liver plasma membranes was also inhibited by retinoic acid. Retinol and retinyl acetate had very little effect. Transplasma membrane ferricyanide reduction by HeLa cells and NADH ferricyanide reductase of liver plasma membrane was also inhibited by retinoic acid, therefore the inhibition was in the electron transport system and not at the transferrin receptor. Since the transmembrane electron transport has been shown to stimulate cell growth, the growth inhibition by retinoic acid thus may be based on inhibition of the NADH diferric transferrin reductase.     

Rapid endocytosis of the transferrin receptor in the absence of bound transferrin

The rate of endocytosis of transferrin receptors, occupied or unoccupied with transferrin, was measured on the cell line K562. At 37 degrees C, receptors, radioiodinated on the cell surface at 4 degrees C, were internalized equally rapidly in the presence or absence of transferrin. In both cases, 50% of the labeled receptors became resistant to externally added trypsin in 5 min. An antitransferrin antibody was used to show directly that the receptors had entered the cells without bound transferrin. The distribution of the receptors on the cell surface was revealed by antibody and protein A-gold staining after prolonged incubation in the presence or absence of transferrin. The receptors were concentrated in coated pits under both conditions. The data suggest that endocytosis of transferrin receptors is not "triggered" by ligand binding and raise the possibility that ligand-induced down-regulation of surface receptors may not occur by this mechanism. Instead receptors may be recognized as being ligand-occupied, not at the cell surface, but at some other site in the recycling pathway such as the endosome.     

Casein kinase II activity is required for transferrin receptor endocytosis.

The effect of protein kinase inhibitors on transferrin receptor (TR) internalization was examined in HeLa, A431, 3T3-L1 cells, and primary chicken embryo fibroblasts. We show that TR endocytosis is not affected by tyrosine kinase or protein kinase C inhibitors, but is inhibited by one serine/threonine kinase inhibitor, H-89. Inhibition occurred within 15 min, was completely reversible after H-89 withdrawal, and was specific for endocytosis rather than pinocytosis since a TR mutant lacking an internalization signal was not affected. Interestingly, H-89 also inhibited the internalization of a TR chimera containing the major histocompatibility complex class II invariant chain cytoplasmic tail, indicating that the effect was not specific for the TR. Since H-89 inhibits a number of kinases, we employed a permeabilized cell endocytosis assay to further characterize the kinase. In permeabilized 3T3-L1 cells, addition of pseudosubstrate inhibitor peptides of casein kinase II (CKII) blocked TR internalization by more than 50%, whereas pseudosubstrates of cyclic AMP-dependent kinase A, protein kinase C, and casein kinase I had no effect. Furthermore, addition of purified CKII to the cell-free reactions containing CKII pseudosubstrates reversed the endocytosis block, suggesting that CKII or a CKII-like activity is required for constitutive endocytosis.     

Prelysosomal divergence of transferrin and epidermal growth factor during receptor-mediated endocytosis

The routes followed by epidermal growth factor and transferrin during their endocytosis by human epithelial cells were compared in double-label studies by using density gradient centrifugation of cell homogenates and fluorescence microscopy with intact cells. Gradient centrifugation studies of cells incubated with radioactively labeled epidermal growth factor and transferrin indicated that both ligands initially were associated with a class of vesicles having a density of 1.037 g/mL and then were rapidly transferred to a membrane compartment having a slightly higher density (1.039 g/mL). Subsequently, the two ligands diverged. Epidermal growth factor ultimately was transferred to a membranous compartment containing lysosomal enzymes (density (1.08 g/mL) where it was degraded. Transferrin was released intact from the cells; very little was transferred to lysosomes. Using fluorescently labeled ligands, it was observed that after cells were warmed to 37 degrees C for 5 min, transferrin and epidermal growth factor gave coincident, punctate fluorescent patterns, strongly suggesting they were localized within the same endocytic vesicles. Subsequently, the epidermal growth factor signal was observed in lysosomes whereas the transferrin signal became weaker and diffuse and did not coincide with the punctate epidermal growth factor fluorescence. The time course of the divergence of the radioactive and fluorescent ligands coupled with the previous morphologic studies on the pathway of epidermal growth factor internalization [Willingham, M. C., & Pastan, I. (1982) J. Cell Biol. 94, 207-212] suggests that the sorting process is prelysosomal and possibly Golgi associated.     

Receptor-mediated endocytosis of transferrin and recycling of the transferrin receptor in rat reticulocytes

At 4 degrees C transferrin bound to receptors on the reticulocyte plasma membrane, and at 37 degrees C receptor-mediated endocytosis of transferrin occurred. Uptake at 37 degrees C exceeded binding at 4 degrees C by 2.5-fold and saturated after 20-30 min. During uptake at 37 degrees C, bound transferrin was internalized into a trypsin- resistant space. Trypsinization at 4 degrees C destroyed surface receptors, but with subsequent incubation at 37 degrees C, surface receptors rapidly appeared (albeit in reduced numbers), and uptake occurred at a decreased level. After endocytosis, transferrin was released, apparently intact, into the extracellular space. At 37 degrees C colloidal gold-transferrin (AuTf) clustered in coated pits and then appeared inside various intracellular membrane-bounded compartments. Small vesicles and tubules were labeled after short (5-10 min) incubations at 37 degrees C. Larger multivesicular endosomes became heavily labeled after longer (20-35 min) incubations. Multivesicular endosomes apparently fused with the plasma membrane and released their contents by exocytosis. None of these organelles appeared to be lysosomal in nature, and 98% of intracellular AuTf was localized in acid phosphatase-negative compartments. AuTf, like transferrin, was released with subsequent incubation at 37 degrees C. Freeze-dried and freeze-fractured reticulocytes confirmed the distribution of AuTf in reticulocytes and revealed the presence of clathrin-coated patches amidst the spectrin coating the inner surface of the plasma membrane. These data suggest that transferrin is internalized via coated pits and vesicles and demonstrate that transferrin and its receptor are recycled back to the plasma membrane after endocytosis.     

Comparison of the kinetics of cycling of the transferrin receptor in the presence or absence of bound diferric transferrin.

The kinetics of cycling of the transferrin receptor in A431 human epidermoid-carcinoma cells was examined in the presence or absence of bound diferric transferrin. In order to investigate the properties of the receptor in the absence of transferrin, the cells were maintained in defined medium without transferrin. It was demonstrated that Fab fragments of a monoclonal anti-(transferrin receptor) antibody (OKT9) did not alter the binding of diferric 125I-transferrin to the receptor or change the accumulation of [59Fe]diferric transferrin by cells. OKT9 125I-Fab fragments were prepared and used as a probe for the function of the receptor. The first-order rate constants for endocytosis (0.16 +/- 0.02 min-1) and exocytosis (0.056 +/- 0.003 min-1) were found to be significantly lower for control cells than the corresponding rate constants for endocytosis (0.22 +/- 0.02 min-1) and exocytosis (0.065 +/- 0.004 min-1) measured for cells incubated with 1 microM-diferric transferrin (mean +/- S.D., n = 3). The cycling of the transferrin receptor is therefore regulated by diferric transferrin via an increase in both the rate of endocytosis and exocytosis. Examination of the accumulation of OKT9 125I-Fab fragments indicated that diferric transferrin caused a marked decrease in the amount of internalized 125I-Fab fragments associated with the cells after 60 min of incubation at 37 degrees C. Diferric transferrin therefore increases the efficiency of the release of internalized 125I-Fab fragments compared with cells incubated without diferric transferrin. These data indicate that transferrin regulates the sorting of the transferrin receptor at the cell surface and within endosomal membrane compartments.     

Transplasmalemma electron transport from cells is part of a diferric transferrin reductase system

Intact cells are known to reduce external, impermeable electron acceptors. We now show that cells can reduce the iron in diferric transferrin at the cell surface and that this reduction reaction depends on the transferrin receptor as well as the transmembrane electron transport system. Reduction of external diferric transferrin is accompanied by oxidation of internal NADH which indicates that the transmembrane enzyme is an NADH diferric transferrin reductase. Highly purified liver plasma membranes have NADH diferric transferrin reductase activity which shows properties similar to the diferric transferrin reductases activity of intact cells. Cell growth stimulation by diferric transferrin and other impermeable oxidants which can react with the diferric transferrin reductase can be based on electron transport through he plasma membrane.     

Inhibition of receptor-mediated but not fluid-phase endocytosis in polymorphonuclear leukocytes

We have found that hypertonic medium inhibited the receptor-mediated uptake of the chemotactic peptide N-formylnorleucylleucylphenylalanine without affecting fluid-phase endocytosis by polymorphonuclear leukocytes (PMNs). Morphological and biochemical evidence demonstrated that cells in hypertonic medium did not accumulate peptide in a receptor-mediated manner. However, the cells continued to form endosomes containing fluid-phase markers. Furthermore, the content of these endosomes was processed normally, i.e., both digested and intact material were released into the medium. The inhibition of receptor- mediated uptake was a function of the tonicity. Partial inhibition occurred in 0.45 and 0.6 osmolar medium and maximal inhibition occurred in 0.75 osmolar medium. The inhibition was independent of the solute used to increase the tonicity: sodium chloride, sucrose, and lactose all inhibited uptake to similar extents. Hypertonic medium had little effect on saturable peptide binding. However, it did prevent the clustering of surface molecules as indicated by the inhibition of capping of fluorescent concanavalin A. In addition, hypertonic medium prevented the peptide-stimulated increase in cytosolic calcium levels as measured by quin 2 fluorescence. The tonicity dependence of the inhibition of quin 2 fluorescence paralleled the inhibition of receptor- mediated uptake.     

Binding to the transferrin receptor is required for endocytosis of HFE and regulation of iron homeostasis

HFE, the protein that is mutated in hereditary haemochromatosis, binds to the transferrin receptor (TfR). Here we show that wild-type HFE and TfR localize in endosomes and at the basolateral membrane of a polarized duodenal epithelial cell line, whereas the primary haemochromatosis HFE mutant, and another mutant with impaired TfR-binding ability accumulate in the ER/Golgi and at the basolateral membrane, respectively. Levels of the iron-storage protein ferritin are greatly reduced and those of TfR are slightly increased in cells expressing wild-type HFE, but not in cells expressing either mutant. Addition of an endosomal-targeting sequence derived from the human low-density lipoprotein receptor (LDLR) to the TfR-binding-impaired mutant restores its endosomal localization but not ferritin reduction or TfR elevation. Thus, binding to TfR is required for transport of HFE to endosomes and regulation of intracellular iron homeostasis, but not for basolateral surface expression of HFE.     

Receptor-mediated endocytosis: the intracellular journey of transferrin and its receptor

A variety of ligands and macromolecules enter cells by receptor-mediated endocytosis. Ligands bind to their receptors on the cell surface and ligand-receptor complexes are localized in specialized regions of the plasma membrane called coated pits. Coated pits invaginate and give rise to intracellular coated vesicles containing ligand-receptor complexes which are thus internalized. Transferrin, a major serum glycoprotein which transports iron into cells, enters cells by this pathway. It binds to its receptor on the cell surface, transferrin-receptor complexes cluster in coated pits and are internalized in coated vesicles. Coated vesicles then lose their clathrin coat and fuse with endosomes, an organelle with an internal pH of about 5-5.5. Most ligands dissociate from their receptors in endosomes and they finally end up in lysosomes where they are degraded, while their receptors remain bound to membrane structures and recycle to the cell surface. Transferrin has a different fate: in endosomes iron dissociates from transferrin but apotransferrin remains bound to its receptor because of its high affinity for the receptor at acid pH. Apotransferrin thus recycles back to the plasma membrane still bound to its receptor. When the ligand-receptor complex reaches the plasma membrane or a compartment at neutral pH, apotransferrin dissociates from its receptor with a half-life of 18 s because of its low affinity for its receptor at neutral pH. The receptor is then ready for a new cycle of internalization, while apotransferrin enters the circulation, reloads iron in the appropriate organs and is ready for a new cycle of iron transport.     

Inhibition of receptor-mediated endocytosis demonstrates generation of amyloid beta-protein at the cell surface

Sequential cleavages of the amyloid beta-protein precursor (APP) by the beta- and gamma-secretases generate the amyloid beta-protein (A beta), which plays a central role in Alzheimer's disease. Previous work provided evidence for involvement of both the secretory and endocytic pathways in A beta generation. Here, we used HeLa cells stably expressing a tetracycline-regulated dominant-negative dynamin I (dyn K44A), which selectively inhibits receptor-mediated endocytosis, and analyzed the effects on the processing of endogenous APP. Upon induction of dyn K44A, levels of mature APP rose at the cell surface, consistent with retention of APP on the plasma membrane. The alpha-secretase cleavage products of APP were increased by dyn K44A, in that alpha-APPs in medium and the C83 C-terminal stub in the membrane both rose. The beta-secretase cleavage of APP, C99, also increased modestly. The use of specific gamma-secretase inhibitors to study the accumulation of alpha- and beta-cleavage products independent of their processing by gamma-secretase confirmed that retention of APP on the plasma membrane results in increased processing by both alpha- and beta-secretases. Unexpectedly, endogenous A beta secretion was significantly increased by dyn K44A, as detected by three distinct methods: metabolic labeling, immunoprecipitation/Western blotting, and enzyme-linked immunosorbent assay. Levels of p3 (generated by sequential alpha- and gamma-cleavage) also rose. We conclude that endogenous A beta can be produced directly at the plasma membrane and that alterations in the degree of APP endocytosis may help regulate its production. Our findings are consistent with a role for the gamma-secretase complex in the processing of numerous single-transmembrane receptors at the cell surface.     

Release of iron from diferric transferrin in the presence of rat liver plasma membranes: no evidence of a plasma membrane diferric transferrin reductase

The transfer of iron from diferric transferrin to bathophenanthroline disulfonate was measured under varying conditions by spectrophotometry and EPR spectroscopy. Intact rat hepatocytes efficiently mediated the transfer of iron from human diferric transferrin to bathophenanthroline disulfonate. Isolated rat liver plasma membranes, in contrast, failed to facilitate the reaction at pH 7.4 in the presence of NADH, although the membranes were able to reduce ferricyanide and to oxidize NADH. Oxidation of NADH was stimulated by diferric transferrin. However, ferricyanide reductase and transferrin-stimulated NADH oxidase activities were apparently not linked to release of iron from transferrin. Our results, together with theoretical considerations, show that the ability (or inability) of intact cells or isolated plasma membranes to facilitate the transfer of iron from transferrin to strong diferric iron chelators does not allow interferences about the existence of an iron reduction step as part of the process of cellular uptake of iron from transferrin.     

Unconventional endocytosis and trafficking of transferrin receptor induced by iron

The posttranslational regulation of transferrin receptor (TfR1) is largely unknown. We investigated whether iron availability affects TfR1 endocytic cycle and protein stability in HepG2 hepatoma cells exposed to ferric ammonium citrate (FAC). NH₄Cl and bafilomycin A1, but not the proteasomal inhibitor MG132, prevented the FAC-mediated decrease in TfR1 protein levels, thus indicating lysosomal involvement. Knockdown experiments showed that TfR1 lysosomal degradation is independent of 1) endocytosis mediated by the clathrin adaptor AP2; 2) Tf, which was suggested to facilitate TfR1 internalization; 3) H-ferritin; and 4) MARCH8, previously implicated in TfR1 degradation. Notably, FAC decreased the number of TfR1 molecules at the cell surface and increased the Tf endocytic rate. Colocalization experiments confirmed that, upon FAC treatment, TfR1 was endocytosed in an AP2- and Tf-independent pathway and trafficked to the lysosome for degradation. This unconventional endocytic regulatory mechanism aimed at reducing surface TfR1 may represent an additional posttranslational control to prevent iron overload. Our results show that iron is a key regulator of the trafficking of TfR1, which has been widely used to study endocytosis, often not considering its function in iron homeostasis.     

Chemical modification of recombinant hirudin with palmitic acid in mixed aqueous-organic solutions

In this study, chemical modification of recombinant hirudin variant-2 (HV2) with palmitic acid (PAL) was proposed as an alternative approach to circumvent the limitations of PEGylation. To facilitate a sufficient contact of the hydrophilic HV2 to the hydrophobic PAL, thereby improving the reaction specificity to achieve the desired mono-PAL-HV2 with high retained bioactivity, the reaction was developed in mixed aqueous-organic solutions. Compared with HV2 conjugation with PAL-benzotriazole (PAL-BTA) in mixed aqueous-NMP solution, the conjugation of HV2 with PAL-N-hydroxysuccinimide (PAL-NHS) in mixed aqueous-DMSO solution could improve the site-specific conjugation of one PAL molecule to a particular lysine residue. Furthermore, the reaction mixture of the latter was further purified by preparative liquid chromatography. Three mono-PAL-HV2 isomers were obtained and retained 36%, 4% and 89% of the in vitro anticoagulant activity of unmodified HV2, respectively. One of the mono-PAL-HV2 isomers, namely, mono-PAL-HV2-3, was isolated with the highest selectivity and exhibited the highest in vitro anticoagulant bioactivity. Modification site analysis of mono-PAL-HV2-3 revealed that a single PAL molecule was conjugated at Lys27 of HV2. This study presented a successful PAL modification in which site-specific reaction was improved to achieve the desired mono-PAL-HV2 with highly retained bioactivity in mixed aqueous-organic solutions.     

Structural requirements for high efficiency endocytosis of the human transferrin receptor.

Wild-type and mutant human transferrin receptors (TR) have been expressed in chicken embryo fibroblasts using a helper-independent retroviral vector. By functional studies of the mutant TRs, we have identified the tetrapeptide sequence, YXRF, in the cytoplasmic tail of the receptor as the internalization signal required for high efficiency endocytosis and shown that transplanted internalization signals from the low density lipoprotein receptor (LDLR) and the cation-independent mannose-6-phosphate receptor (Man-6-PR) are able to promote rapid internalization of the human TR. A six-residue LDLR signal, FDNPVY, is required for activity in TR, whereas a four-residue Man-6-PR signal, YSKV, is sufficient. These data indicate that internalization signals are interchangeable self-determined structural motifs and that signals from type I membrane proteins are active in a type II receptor. Putative internalization signals in the cytoplasmic tails of other receptors and membrane proteins can be identified based on the sequence patterns of the LDLR, Man-6-PR, and TR signals. Two such putative four-residue internalization signals, one from the poly-Ig receptor and one from the asialoglycoprotein receptor, were tested for activity by transplantation into TR and were found to promote high efficiency internalization. These results suggest that an exposed tight turn is the conformational motif for high efficiency endocytosis.     

Inhibition of growth of Brochothrix thermosphacta by palmitic acid

Palmitic acid was inhibitory to the growth of Brochothrix thermosphacta in liquid culture at 0.5 mmol/l. Uptake of [1-¹⁴C]-palmitic acid by the organism has been demonstrated, and was reduced at acid pH. These findings are discussed in relation to the known effects of fatty acids on bacteria.     

Stochastic simulations of nanoparticle internalization through transferrin receptor dependent clathrin-mediated endocytosis

Background

Receptor dependent clathrin-mediated endocytosis (CME) is one of the most important endocytic pathways for the internalization of bioparticles into cells. During CME, the ligand-receptor interactions, development of clathrin-coated pit (CCP) and membrane evolution all act together to drive the internalization of bioparticles. In this work, we develop a stochastic computational model to investigate the CME based on the Metropolis Monte Carlo simulations.

The effects of an antibody to the rat transferrin receptor and of rat serum albumin on the uptake of diferric transferrin by rat hepatocytes

The role of high-affinity specific transferrin receptors and low-affinity, non-saturable processes in the uptake of transferrin and iron by hepatocytes was investigated using fetal and adult rat hepatocytes in primary monolayer culture, rat transferrin, rat serum albumin and a rabbit anti-rat transferrin receptor antibody. The intracellular uptake of transferrin and iron occurred by saturable and non-saturable mechanisms. Treatment of the cells with the antibody almost completely eliminated the saturable uptake of iron but had little effect on the non-saturable process. Addition of albumin to the incubation medium reduced the endocytosis of transferrin by the cells but had no significant effect on the intracellular accumulation of iron. The maximum effect of rat serum albumin was observed at concentrations of 3 mg/ml and above. At a low incubation concentration of transferrin (0.5 microM), the presence of both rat albumin and the antibody decreased the rate of iron uptake by the cells to about 15% of the value found in their absence, but to only 40% when the diferric transferrin concentration was 5 microM. These results confirm that the uptake of transferrin-bound iron by both fetal and adult rat hepatocytes in culture occurs by a specific, receptor-mediated process and a low-affinity, non-saturable process. The low-affinity process increases in relative importance as the iron-transferrin concentration is raised.