Interactions of cubilin with megalin and the product of the amnionless gene (AMN): effect on its stability.

Cubilin, a 456 kDa multipurpose receptor lacking in both transmembrane and cytoplasmic domains is expressed in the apical BBMs (brush border membranes) of polarized epithelia. Cubilin interacts with two transmembrane proteins, AMN, a 45-50 kDa protein product of the amnionless gene, and megalin, a 600 kDa giant endocytic receptor. In vitro, three fragments of cubilin, the 113-residue N-terminus and CUB domains 12-17 and 22-27, demonstrated Ca2+-dependent binding to megalin. Immunoprecipitation and immunoblotting studies using detergent extracts of rat kidney BBMs revealed that cubilin interacts with both megalin and AMN. Ligand (intrinsic factor-cobalamin)-affinity chromatography showed that in renal BBMs, functional cubilin exists as a complex with both AMN and megalin. Cubilin and AMN levels were reduced by 80% and 55-60% respectively in total membranes and BBMs obtained from kidney of megalin antibody-producing rabbits. Immunohistochemical analysis and turnover studies for cubilin in megalin or AMN gene-silenced opossum kidney cells showed a significant reduction (85-90%) in cubilin staining and a 2-fold decrease in its half-life. Taken together, these results indicate that three distinct regions of cubilin bind to megalin and its interactions with both megalin and AMN are essential for its intracellular stability.

Cobalamin-mediated regulation of transcobalamin receptor levels in rat organs.

Total gastrectomy (TG) causes cobalamin (Cbl) deficiency followed by increases in tumor necrosis factor (TNF)-alpha levels in the spinal cord (SC) of the rat. In order to understand how Cbl deficiency may influence cell Cbl transport, we have measured by immunoblotting protein levels of the receptor for the Cbl-transcobalamin (TC) complex (TC-R) in both animal and cell models. TC-R protein levels were elevated in the total membranes of duodenal mucosa, kidneys, liver, and SC of rats made Cbl-deficient (Cbl-D) by means of TG or feeding with a Cbl-D diet. Postoperative Cbl-replacement treatment normalized the TC-R protein levels in each of the tested organs, regardless of whether this treatment was given during the first two post-TG or during the third and fourth post-TG mo. In Caco-2 cells, progressively increasing TNF-alpha concentrations supplemented to culture medium induced an up-regulation of TC-R protein levels. We provide the first evidence of the regulation of a Cbl-specific receptor by the vitamin itself in some rat organs.

Plasma membrane delivery, endocytosis and turnover of transcobalamin receptor in polarized human intestinal epithelial cells.

Cells that are metabolically active and in a high degree of differentiation and proliferation require cobalamin (Cbl: vitamin B(12)) and they obtain it from the circulation bound to transcobalamin (TC) via the transcobalamin receptor (TC-R). This study has investigated the plasma membrane dynamics of TC-R expression in polarized human intestinal epithelial Caco-2 cells using techniques of pulse-chase labelling, domain-specific biotinylation and cell fractionation. Endogenously synthesized TC-R turned over with a half-life (T(1/2)) of 8 h following its delivery to the basolateral plasma membrane (BLM). The T(1/2) of BLM delivery was 15 min and TC-R delivered to the BLM was endocytosed and subsequently degraded by leupeptin-sensitive proteases. However, about 15% of TC-R endocytosed from the BLM was transcytosed (T(1/2), 45 min) to the apical membranes (BBM) where it underwent endocytosis and was degraded. TC-R delivery to both BLM and BBM was inhibited by Brefeldin A and tunicamycin, but not by wortmannin or leupeptin. Colchicine inhibited TC-R delivery to BBM, but not BLM. At steady state, apical TC-R was associated with megalin and both these proteins were enriched in an intracellular compartment which also contained Rab5 and transferrin receptor. These results indicate that following rapid delivery to both plasma membrane domains of Caco-2 cells, TC-R undergoes constitutive endocytosis and degradation by leupeptin-sensitive proteases. TC-R expressed in apical BBM complexes with megalin during its transcytosis from the BLM.

Cobalamin (vitamin B12) binding, phylogeny, and synteny of human transcobalamin.

Selected residues in a highly conserved 15-residue region, 174SVDTAAMAGLAFTC L188 of human transcobalamin (TC), a cobalamin (Cbl: vitamin B12) binding protein, were subjected to site-directed mutagenesis. The mutant constructs were expressed in TC-deficient fibroblasts or in vitro to assess the effect of these mutations on Cbl binding. Phylogenetic analyses and protein parsimony indicated that TC evolved earlier than other mammalian Cbl-binding proteins, intrinsic factor and haptocorrins, and divergence occurred between mouse/rat and human dispersing TC gene to different chromosomes. These studies show that (a) two of the three polar residues, S174, T177, or D176 and two of the three conserved alanine residues, A179 and A184 present in the 15-residue evolutionary conserved region are essential for Cbl-binding by human TC, and (b) TC gene is transferred in a syntenic manner to different chromosomes, at least before the divergence of mouse/rat and human.

Rat transcobalamin: cloning and regulation of mRNA expression.

Transcobalamin (TC) has been cloned and used for studying its gene expression in the rat. TC mRNA is distributed widely in adult rat tissues, but at different levels (kidney > liver > lung > yolk sac > intestine > heart > brain > spleen > muscle). TC mRNA levels were 4-fold higher in the jejunum and ileum compared to its levels in the duodenum. During postnatal development, TC mRNA levels in the ileum declined 4-fold from day 4 to day 12, but increased by 5-fold between days 12 and 24. In contrast, TC mRNA levels increased by 2.5-fold in the kidney from day 4 to day 12 and then declined by 2-fold by day 24. Adrenalectomy of adult rats resulted in a 4-fold decline in ileal levels of TC mRNA and a 50% decline in the ileal mucosal formation of the TC-[(57)Co] cobalamin (Cbl) complex following oral administration of [(57)Co]Cbl complexed to gastric intrinsic factor (IF). Cortisone treatment reversed these changes noted in the ileum. In contrast to ileum, kidney TC mRNA levels were not altered significantly in adrenalectomized rats before and after cortisone treatment. Taken together, this study has provided evidence for the regulation of TC gene expression in the rat kidney and intestine during their postnatal development, and cortisone selectively regulates ileal but not kidney TC mRNA levels.

Cobalamin transport proteins and their cell-surface receptors.

The primary function of cobalamin (Cbl; vitamin B12) is the formation of red blood cells and the maintenance of a healthy nervous system. Before cells can utilise dietary Cbl, the vitamin must undergo cellular transport using two distinct receptor-mediated events. First, dietary Cbl bound to gastric intrinsic factor (IF) is taken up from the apical pole of ileal epithelial cells via a 460 kDa receptor, cubilin, and is transported across the cell bound to another Cbl-binding protein, transcobalamin II (TC II). Second, plasma TC II-Cbl is taken up by cells that need Cbl via the TC II receptor (TC II-R), a 62 kDa protein that is expressed as a functional dimer in cellular plasma membranes. Human Cbl deficiency can develop as a result of acquired or inherited dysfunction in either of these two transmembrane transport events. This review focuses on the biochemical, cellular and molecular aspects of IF and TC II and their cell-surface receptors.

Human transcobalamin II receptor binds to Staphylococcus aureus protein A: implications as to its structure and function.

Purified human placental transcobalamin II receptor (TC II-R) dimer of molecular mass 124 kDa bound to Sepharose-linked bacterial immunoglobulin (IgG) binding proteins protein A, protein G, and protein A/G. TC II-R dimer was detected directly, by blotting human placental and rabbit and rat kidney membrane proteins with 125I-protein A, or indirectly, using antiserum to TC II-R or IgG-Fc region and 125I-protein. TC II-R antiserum, but not protein A, protein G, protein A/G, or antiserum to the IgG-Fc region, when added to culture medium of human intestinal epithelial Caco-2 cells or umbilical vein endothelial cells, inhibited ligand binding. However, protein A, protein G, protein A/G, or antiserum to the Fc region inhibited the internalization of the ligand TC II-[57Co]cyanocobalamin. Taken together, these studies strongly suggest TC II-R is an IgG-like molecule that contains an Fc-like region which is important in ligand internalization but not binding.

Function and stability of human transcobalamin II: role of intramolecular disulfide bonds C98-C291 and C147-C187.

The current studies have investigated the role of three disulfide bonds of human transcobalamin II (TC II), a plasma transporter of cobalamin (Cbl; vitamin B12), in its function and stability. When translated in vitro in the presence or absence of microsomal vesicles, TC II constructs with a single substitution, C3S or C249S, demonstrated synthesis of a stable functional protein. However, TC II synthesized in the presence of microsomal vesicles using constructs with a single (C98S, C147S, C187S, C291S), double (C3/147/S, C98/147/S) or triple (C3/98/147/S) substitution was unstable. In the absence of microsomal vesicles, the percentage of binding to Cbl-Sepharose matrix by TC II expressed by constructs C3S, C3/147/S, C98/147/S, or C3/98/147/S was 100, 49, 52, and 35%, respectively. Upon their reductive alkylation, the binding of TC II expressed by these constructs was reduced to approximately 25-30%. TC II constructs C3S or C249S, when expressed in TC II-deficient fibroblasts, produced a stable functional protein, but those expressed by constructs C147S, C187S, C291S, C3/147/S, C98/147/S, or C3/98/147/S were rapidly degraded. The intracellular degradation of TC II expressed by these constructs was inhibited by lactacystin or MG-132 but not by the lysosomal degradation inhibitors ammonium chloride or chloroquine. These studies suggest that optimal binding of Cbl by human TC II is supported by disulfide bonds C98-C291 and C147-C187 and that their disruption results in loss of Cbl binding and their rapid degradation by the proteasomal machinery.

Effects of interferon beta on transcobalamin II-receptor expression and antitumor activity of nitrosylcobalamin.

BACKGROUND: The ubiquitous plasma membrane transcobalamin II receptor (TC II-R) mediates uptake of cobalamin (Cbl; vitamin B12), an essential micronutrient. Tumors often require more Cbl than normal tissue, and increased Cbl uptake may result from increased TC II-R expression. To examine whether Cbl could therefore be used as a carrier molecule to target a chemotherapy drug, we tested an analogue of Cbl with nitric oxide as a ligand, nitrosylcobalamin (NO-Cbl). Because interferon beta (IFN-beta) has antitumor effects and increases expression of some membrane receptors, we examined whether it may enhance the effects of NO-Cbl. METHODS: Antiproliferative effects of NO-Cbl were assessed in 24 normal and cancer cell lines. Xenograft tumors of human ovarian cancer NIH-OVCAR-3 cells were established in athymic nude mice, and tumor growth was monitored after treatment with NO-Cbl and IFN-beta, both individually and concomitantly. TC II-R expression and apoptosis was monitored in vitro and in vivo. RNA protection assays and mitochondrial membrane potential assays were used to distinguish the extrinsic and intrinsic apoptotic pathways, respectively. RESULTS: Cancer cell lines were more sensitive to NO-Cbl (with ID(50)s [the dose that inhibits growth by 50%] as low as 2 microM) than normal cell lines (with ID(50)s of 85-135 microM). Single-agent NO-Cbl and IFN-beta treatment of NIH-OVCAR-3 xenografts induced tumor regression, whereas combination treatment induced tumor eradication. IFN-beta treatment increased TC II-R expression in vitro and uptake of [(57)Co]cobalamin in vivo. Compared with NIH-OVCAR-3 cells treated with NO-Cbl, cells treated with NO-Cbl and IFN-beta were more apoptotic and expressed higher mRNA levels of various apoptosis-associated genes. No changes in mitochondrial membrane potential were observed in cells treated with NO-Cbl. CONCLUSION: NO-Cbl inhibited tumor growth in vivo by activating the extrinsic apoptotic pathway. The increased expression of TC II-R induced by IFN-beta resulted in enhanced antitumor effects with NO-Cbl both in vitro and in vivo.

Loss of albumin and megalin binding to renal cubilin in rats results in albuminuria after total body irradiation.

The role of the renal apical brush-border membrane (BBM) endocytic receptors cubilin and megalin in the onset of albuminuria in rats exposed to a single dose of total body irradiation (TBI) has been investigated. Albuminuria was evident as immunoblot (IB) analysis of the urine samples from TBI rats revealed excretion of large amounts of albumin. IB analysis of the BBM proteins did not reveal any significant changes in cubilin or megalin levels, but (125)I-albumin binding to BBM from TBI rats declined by 80% with a fivefold decrease (from 0.5 to 2.5 microM) in the affinity for albumin. IB analysis of cubilin from the BBM demonstrated a 75% loss when purified using albumin, but not intrinsic factor (IF)-cobalamin (Cbl) ligand affinity chromatography. Immunoprecipitation (IP) of Triton X-100 extract of the BBM with antiserum to cubilin followed by IB of the immune complex with an antiserum to megalin revealed a 75% loss of association between megalin and cubilin. IP studies with antiserum to cubilin or megalin and IB with antiserum to the cation-independent mannose 6-phosphate/insulin-like growth factor II-receptor (CIMPR) revealed that CIMPR interacted with both cubilin and megalin. In addition, TBI did not disrupt the association of CIMPR with either cubilin or megalin in BBM. These results suggest that albuminuria noted in TBI rats is due to selective loss of albumin and megalin, but not CIMPR or IF-Cbl binding by cubilin. Furthermore, these results also suggest that albumin and IF-Cbl binding to cubilin occur at distinct sites and that in the rat renal BBM, CIMPR interacts with both cubilin and megalin.

Basolateral sorting signal of the 300-kDa mannose 6-phosphate receptor.

In polarized cells, the delivery of numerous membrane proteins from the trans-Golgi network to the basolateral surface depends on specific sequences located in their cytoplasmic domain. We have previously shown that the insulin-like growth factor-II/mannose 6-phosphate receptor (IGF-II/MPR) exhibits a polarized cell surface distribution in the human colon adenocarcinoma (Caco-2) cell line in which there is a threefold enrichment on the basolateral surface. To investigate the role of residues in the cytoplasmic region of the receptor that facilitates its entry into the basolateral sorting pathway, we generated stably transfected Caco-2 cell lines expressing various mutant bovine IGF-II/MPRs. The steady-state surface distribution of mutant receptors was analyzed by subjecting filter-grown cell monolayers to incubation with iodinated IGF-II/MPR-specific antibody or to indirect immunofluorescence and visualization by confocal microscopy. Together, these results demonstrate that the sorting of the IGF-II/MPR to the basolateral cell surface depends on recognition of sequences located in its cytoplasmic region that are distinct from the Tyr-based internalization and dileucine-dependent endosomal trafficking motifs.