Mouse transcobalamin II: biosynthesis and uptake by L-929 cells

Cyanocobalamin promotes muscle development through the TGF-β signaling pathway

Cyanocobalamin (CNCbl, the compound name of Vitamin B12) is the only mineral vitamin that is essential for growth and development and cannot be produced by animals. Some studies have found that CNCbl can promote the proliferation and migration of C2C12 cells, but the mechanism by which it affects muscle development is still unknown. In this study, we elucidated the effect of CNCbl on muscle development and studied its underlying mechanism. CNCbl could promote the differentiation of C2C12 cells and upregulate Acvr1, p-Smad2 and p-Smad3 in the TGF-β signaling pathway in vitro. CD320 (the receptor in cell surface for binding with CNCbl transporter transcobalamin II) inhibition could reduce the uptake of CNCbl and significantly downregulate the expression of differentiation marker proteins MyoG and MYH2. Furthermore, the levels of p-Smad2 and p-Smad3 were also reduced with the inhibition of CD320, even though CNCbl was added to the C2C12 culture medium. In addition, the injection of CNCbl could accelerate the process of mouse muscle injury repair, enlarge the diameter of newly formed myofibers and upregulate the expression of MYH2, PAX7, CD320, Acvr1, p-Smad2 and p-Smad3 in vivo. These results suggest that CNCbl can promote muscle development and may play its role by regulating the expression of Acvr1, p-Smad2 and p-Smad3 related to the TGF-β signaling pathway.

Evidence for genetic effects on variation in plasma unsaturated transcobalamin II and cobalamin (vitamin B12)

Unsaturated plasma transcobalamin II (UTC II) and cobalamin were measured in two selected age-groups of like-sexed mono- and dizygotic twins. For UTC II, a higher mean level was found in women than in men, and in the older (57 to 61 years) than in the younger (33 to 39 years) age group. Testing of genetic-environmental models revealed that variation in plasma levels of UTC II is almost exclusively genetically determined. More than 50% of the variation in cobalamin levels was accounted for by genes, the remainder being due to person-specific environmental factors (for older males no model gave a good fit). A negative correlation was noted between UTC II and cobalamin levels. The correlation coefficient was low, and the variation in UTC II accounted for only about 4% of the variation in the cobalamin level. This finding suggests that a pathologically high value of one of the variables may have clinical significance, regardless of the value of the other variable. For 22 patients studied longitudinally, a clear tendency to maintain plasma levels at constant levels over long periods of time was found, suggesting that certain degrees of deviation from these levels may have clinical relevance.

Vitamin B12 transporters

The uptake of vitamin B12 from the intestine into the circulation is perhaps the most complex uptake mechanism of all the vitamins, involving no less than five separate VB12-binding molecules, receptors and transporters. Each molecule involved in uptake has a separate affinity and specificity for VB12 as well as a separate cell receptor. Thus VB12 is initially bound by haptocorrin in the stomach, then by IF in the small intestine. An IF receptor is then involved in uptake of the IF-VB12 complex by the intestinal epithelial cell, with the subsequent proteolytic release of VB12 and subsequent binding to TcII. The TcII receptor then transports the TcII-VB12 complex across the cell, whence it is released into the circulation. It is surprising, then, that despite its complexity, it has been possible to harness the vitamin VB12 uptake mechanism to enhance the oral uptake of peptides, proteins, and nanoparticles.

The sources and biochemical characteristics of cobalamin-binders in human amniotic fluid

The sources and biochemical characteristics of cobalamin-binders in amniotic fluid were investigated. Using gel-permeation chromatography, cobalamin-binder, extracted from amniotic fluid at term, was recovered in a single peak with the molecular size of haptocorrin. Neonatal saliva also contained only haptocorrin. However, neonatal gastric juice contained two types of cobalamin-binders: haptocorrin and intrinsic factor. Amniotic fluid in midtrimester contained three types of cobalamin-binders: haptocorrin, intrinsic factor, and transcobalamin II. The cultured amnion cells secreted mainly apo-transcobalamin II. Concentrations of both apo-haptocorrin and salivary amylase in amniotic fluid increased as gestation advanced. These results suggest that cobalamin-binder in amniotic fluid in midtrimester originates from the fetal salivary gland, fetal gastric mucosa, and amnion cells, and that the contribution by the fetal salivary gland increases at term, when almost all cobalamin-binders in amniotic fluid are haptocorrin.

Isoelectric focusing of rabbit transcobalamin from serum and cerebrospinal fluid

Isoelectric focusing in agarose gel separated rabbit transcobalamin into five to eight isopeptides with isoelectric point (pI) 5.4-6.8. Three different sets of patterns were observed in serum samples from 34 rabbits and in cerebrospinal fluid samples from 10 rabbits as a given pattern in serum corresponded to a given pattern in cerebrospinal fluid. Serum contained higher substance concentrations of acidic isopeptides than cerebrospinal fluid. No correlation was found between the isopeptide patterns and the unsaturated cobalamin-binding capacity. The unsaturated cobalamin-binding capacity of cerebrospinal fluid was high in relation to the protein concentration (0.5-1.3 nmol/l) compared to that of serum (6.6-22.8 nmol/l). The data suggest synthesis of transcobalamin into the cerebrospinal fluid.

Intrinsic factor-cobalamin accumulates in the ilea of mice treated with chloroquine

Chloroquine, which interferes with the degradation of a number of transport proteins, impedes the exit of cobalamin from the small intestine of the mouse. This study was designed to determine if treatment with the drug led to the retention of cobalamin in the form of intrinsic factor-cobalamin in the ileal mucosa. Solubilized homogenates were prepared 2-4 h after an oral dose of [57Co]cobalamin and were examined by gel chromatography. There was a progressive transfer of [57Co]cobalamin from a protein identified as intrinsic factor to transcobalamin II in control mice. In chloroquine-treated mice, the major radioactive protein peak 2-4 h after an oral dose corresponded with the position of intrinsic factor. Only a small amount of the radioactivity was associated with transcobalamin. Cobalamin binding proteins were also identified by polyacrylamide gel electrophoresis and by their reaction with specific antibodies. It is concluded that chloroquine interferes with the release of cobalamin from intrinsic factor and thus slows the release of cobalamin from the intestine.

Synthesis and secretion of the human vitamin B12-binding protein, transcobalamin II, by cultured skin fibroblasts and by bone marrow cells

Human skin fibroblasts and bone marrow cells were tested for their ability to synthesize the cobalamin-binding protein transcobalamin II. Cobalamin binders secreted in the media of cultured fibroblasts and of dextran-sedimented bone marrow cells in liquid culture could be identified as transcobalamin II on the basis of immunological, electrophoretical and chromatographical identity with serum transcobalamin II. The net secretion of transcobalamin II increased linearly with time of culture, up to 30 days after confluence. The reversible inhibition of transcobalamin II secretion by cycloheximide demonstrated that human fibroblasts are capable of de novo transcobalamin II synthesis. Addition of cyanocobalamin to the fibroblast culture medium induced a reduction of transcobalamin II net secretion, most likely due to preferred uptake of transcobalamin II saturated with cobalamin, as opposed to unsaturated protein. Addition of lysozymal enzyme inhibitors, ammonium chloride and chloroquine, resulted in a markedly increased secretion of transcobalamin II. In the culture medium of fibroblasts, obtained from two transcobalamin II-deficient patients, functionally deficient transcobalamin II was demonstrated on the basis of strongly reduced secretion of immunoreactive transcobalamin II, and the absence of apotranscobalamin II. Individual phenotypes in the culture media of the fibroblasts and bone marrow cells were identical to the corresponding serum transcobalamin II types.

Synthesis of transcobalamin II by cultured human hepatocytes

Cultured HepG2 cells, derived from a human hepatoma synthesized and released unsaturated, immunoreactive transcobalamin II. Synthesis was confirmed by the blocking with inhibitors of protein synthesis and by incorporation of tritiated leucine into transcobalamin II.

Localization of the gene for the vitamin B12 binding protein, transcobalamin II, near the centromere on mouse chromosome 11, linked with the hemoglobin alpha-chain locus

Somatic cell hybrids, recombinant inbred (RI) mouse strains, and backcross breeding experiments were used to locate the gene of transcobalamin II (Tcn-2), the vitamin B12 binding protein in mouse serum. TCN-2 was found to be useful genetic marker in the somatic cell hybrids. Selected hybrid clones were derived from fusions between GR mouse cells and the Chinese hamster cell line E36. Analysis of mouse specific chromosomal enzyme markers in relationship to TCN-2 secretion, in the hybrid clones, provided provisional evidence for assignment of the Tcn-2 locus to chromosome 11. The strain distribution pattern of the TCN-2 variants S and F in the RI series CXS, constructed from the cross of BALB/cHeA (TCN-2S) with STS/A (TCN-2F), implied a close linkage with the hemoglobin alpha-chain locus (Hba) on chromosome 11. Backcross breeding using inbred strains confirmed these findings and located the Tcn-2 gene closest to the centromere, linked with waved 2 (wa-2) and Hba with recombination frequencies of 6.9 and 19.2% each. The linkage group Tcn-2/wa-2/Hba was established.

Allelic forms of mouse transcobalamin 2

Transcobalamin 2 is the only vitamin B12-binding protein found in mouse serum. Two allelic forms of mouse transcobalamin 2 are described. The two forms differ in their mobilities on polyacrylamide gel electrophoresis. The slowly migrating form has been found in serum from 25 inbred mouse strains. The more rapidly migrating form was detected in 3 inbred mouse strains (NZB, ST/bJ, and CPB-WV). Both parental variants were expressed in F1 progeny of appropriate interstrain crosses, showing codominant expression of the transcobalamin 2 alleles. In backcrosses between F1 and parental individuals, the two electrophoretic variants were inherited as single Mendelian traits. The strain distribution pattern of the two variants in recombinant inbred lines likewise suggested a single-gene mode of inheritance and indicated a lack of close linkage with a number of genetic loci on chromosomes 1, 2, 4, 5, 6, 7, 9, 12, 14, 15, and 17. We propose the symbol Tcn-2 for the polymorphic gene locus coding for transcobalamin 2 in the mouse and Tcn-2s and Tcn-2f for the two alleles.

Uptake and metabolism of free cyanocobalamin by cultured human fibroblasts from controls and a patient with transcobalamin II deficiency

We have investigated the uptake and metabolism of free cyanocobalamin (CN-Cbl; vitamin B12) by intact cultured human skin fibroblasts. Monolayers of control fibroblasts take up free CN-[57Co]Cbl via a saturable, calcium-independent process that is inhibited by sulfhydryl reagents, inhibitors of protein synthesis, and inhibitors of electron transport, but not by inhibitors of glycolysis. CN-Cbl taken up in this manner is converted to active cobalamin (Cbl) coenzymes (adenosylcobalamin and methylcobalamin) and becomes associated with intracellular Cbl-dependent apoenzymes (methylmalonyl CoA mutase and homocysteine:methyltetrahydrofolate methyltransferase). Since fibroblasts from controls were also found to synthesize transcobalamin II (TC II), a plasma protein shown previously to facilitate the cellular uptake of Cbl, it seemed possible that the observed uptake of free CN-Cbl was TC II-mediated. This thesis was rejected by demonstrating that cells from a patient with complete TC II deficiency took up free CN-Cbl as well as control cells did. Finally, we propose a mechanism by which an uptake process for free Cbl might serve a function in intracellular metabolism of Cbl.

Transplacental transport in the rabbit of vitamin B12 bound to human transcobalamin I, II and III

Transcobalamins I, II and III (TCI, TCII, TCIII) were purified from human serum, saturated with 57Co-vitamin B12 (57Co-B12), and injected into pregnant rabbits. Whole body retention of the 57Co-B12 averaged 91% and was similar for each of the three vitamin B12 binders. A maximum of 62% of the injected 57Co-B12 was found in fetal tissues when the vitamin was injected bound to TCII, but only 6--7% when bound to TCI or TCIII. Thus TCII is responsible for the delivery of vitamin B12 to the fetus.

Binding and uptake of transcobalamin II by human fibroblasts

We have used purified, (125)I-labeled human transcobalamin II (TC II), saturated with cobalamin (Cbl), to study the uptake process for the TC II-Cbl complex by intact normal cultured human skin fibroblasts. We have also investigated the possibility that a defect in one step of this process underlies that inborn error of Cbl metabolism-designated cbl C-in which mutant cells are unable to retain Cbl intracellularly or convert it to its coenzyme forms. TC II-Cbl binding at 4 degrees C reached a plateau after 3-4 hr; 95% of the bound (125)I was releasable with trypsin. Binding of TC II-Cbl at 4 degrees C could be inhibited by human and rabbit TC II-Cbl and human TC II devoid of Cbl but not by other Cbl-binding proteins, albumin, or free Cbl. Specific binding reached saturation at congruent with5 ng TC II/ml (0.13 nM) and could be inhibited by ethylene glycol-bis (beta-aminoethyl ether) N,N,N',N'- tetraacetic acid. At 37 degrees C, the TC II-Cbl complex was internalized as shown by a progressive decrease in the trypsin-releasable fraction of bound (125)I. After 2 h at 37 degrees C, increasing amounts of acid-soluble (125)I were found in the incubation medium indicating that the labeled TC II was being degraded. Chloroquine, an inhibitor of lysosomal proteolysis, prevented this degradation. The binding, internalization, and degradation of TC II-Cbl by cbl C cells was indistingusihable from that by control cells. Our studies provide additional support for the concepts: (a) that the TC II-Cbl complex binds to a specific cell surface receptor through a site on the TC II; (b) that the interaction between the receptor and TC II is calcium dependent; (c) that the TC II-Cbl is internalized via endocytosis; (d) that the degradation of TC II and release of Cbl from the complex occurs in lysosomes. We also conclude that the defect in cbl C must reside at some step beyond this receptor-mediated uptake process.