Disease relevance of CUBN

• In most cases, the molecular basis of the selective malabsorption and proteinuria involves a mutation in one of two genes, cubilin (CUBN) on chromosome 10 or amnionless (AMN) on chromosome 14 [1].
• Furthermore, the recent characterization of the CUBN gene encoding the intrinsic factor-vitamin B12 receptor (cubilin) provides a basis to identify the causative mutations in patients suffering from a hereditary syndrome of hyperhomocysteinemia that presents with megaloblastic anemia and proteinuria [2].
• We also found that megalin and its endocytic partner, cubilin, are coexpressed in normal murine mammary tissue, in nontransformed human mammary epithelial cell lines, and in some established human breast cancer cell lines [3].
• We examined the interaction between cubilin (gp280) and four species of light chains isolated from the urine of patients with multiple myeloma [4].
• Human colon adenocarcinoma (Caco-2) cells express both intrinsic factor-cobalamin receptor and transcobalamin II (TC II) [5].

High impact information on CUBN

• We previously mapped the MGA1 locus by linkage analysis in Finnish and Norwegian families to a 6-cM region on chromosome 10p12.1 (ref. 8). A functional candidate gene encoding the intrinsic factor (IF)-B12 receptor, cubilin, was recently cloned; the human homologue, CUBN, was mapped to the same region [6].
• Cubilin is the intestinal receptor for the endocytosis of intrinsic factor-vitamin B12 [7].
• Megalin and cubilin: multifunctional endocytic receptors [8].
• Cubilin is an albumin binding protein important for renal tubular albumin reabsorption [9].
• Using affinity chromatography and surface plasmon resonance analysis, we have identified cubilin, a 460-kDa receptor heavily expressed in kidney proximal tubule epithelial cells, as an albumin binding protein [9].

Biological context of CUBN

• Haplotype evidence excluded both CUBN and AMN conclusively in five families and tentatively in three families, suggesting the presence of at least one more gene locus that can cause MGA [10].
• Megalin-dependent cubilin-mediated endocytosis is a major pathway for the apical uptake of transferrin in polarized epithelia [11].
• We show that 1alpha-hydroxylase is expressed in human osteoblasts, as well as the vitamin D binding protein receptors megalin and cubilin [12].
• Cubilin is a high molecular weight multiligand receptor that mediates intestinal absorption of intrinsic factor-cobalamin and selective protein reabsorption in renal tubules [13].
• TGF-beta1 led to a time- and dose-dependent downregulation of megalin-cubilin-mediated endocytosis without affecting two other transport systems tested [14].

Anatomical context of CUBN

• Here we show that cubilin and AMN colocalize in the endocytic apparatus of polarized epithelial cells and copurify as a tight complex during IF-cobalamin affinity and nondenaturing gel filtration chromatography [15].
• However, in cells cotransfected with AMN and the cubilin construct, cubilin trafficked to the cell surface and endosomes, and the cells exhibited IF-cobalamin endocytosis and lysosomal degradation of IF [15].
• Mouse amnionless, which is required for primitive streak assembly, mediates cell-surface localization and endocytic function of cubilin on visceral endoderm and kidney proximal tubules [16].
• Binding to cubilin results in uptake and lysosomal degradation of CCSP in cultured cells [17].
• Surface plasmon resonance analysis demonstrated a high-affinity binding of apoA-I and HDL to cubilin, and cubilin-expressing yolk sac cells showed efficient 125I-HDL endocytosis that could be inhibited by IgG antibodies against apoA-I and cubilin [7].

Associations of CUBN with chemical compounds

• Cubilin dysfunction causes abnormal metabolism of the steroid hormone 25(OH) vitamin D(3) [18].
• In this study, we analyzed the expression of cubilin in the pre- and early post-implantation rodent embryo and tested its potential function in protein and cholesterol uptake [19].
• These studies indicate that thyroxine deficiency in rats results in decreased uptake and tissue accumulation of cbl caused mainly by destabilization and deficit of cubilin in the intestinal brush border [20].
• Endocytosis of the intrinsic factor-cobalamin-receptor complex was inhibited by two ways: at 4 degrees C and at 37 degrees C by incubating the cells with vinblastine, monensin and chloroquine [21].
• Incubation of cells with tunicamycin decreased the half-life of IFCR from 48 to 24 h, thus causing faster degradation and depletion of the surface receptor [22].

Physical interactions of CUBN

• In transfected cells expressing either AMN or a truncated IF-cobalamin-binding cubilin construct, neither protein alone conferred ligand endocytosis [15].
• Using surface plasmon resonance analysis, we demonstrate that CCSP binds to cubilin, a peripheral membrane protein on the surface of proximal tubular cells [17].
• A receptor has been found in proximal tubular cells (cubilin/megalin complex), which mediates the absorption of LCs and is involved in the pathogenesis of tubulopathies that occurs in these patients [23].
• IF-mediated import of Cbl is limited to the apical membranes of epithelial cells; it occurs via a multipurpose receptor recently termed "cubilin," and the imported Cbl is usually exported out of these cells bound to endogenous TC II [24].

Regulatory relationships of CUBN

• Cubilin is a 460-kDa endocytic receptor coexpressed with megalin, a multiligand receptor of the low-density lipoprotein receptor gene family, at the apical pole of epithelial cells in the renal proximal convoluted tubule, visceral yolk sac, ileum, and placenta [25].

Other interactions of CUBN

• This observation indicates that megalin deficiency causes failure in internalization of the cubilin-ligand complex [11].
• In a series of families clinically diagnosed with likely IGS, at least six displayed no evidence of mutations in CUBN or AMN [26].
• More specific receptors like the folate receptor, IGF-II/Man-6-P receptor, and gp280/IFR, identical to the intrinsic factor receptor, are also functioning in the apical endocytic pathway of renal proximal tubules [27].
• In this study we used a well established cell culture model (proximal-tubule-derived opossum kidney (OK) cells) in order to test the hypothesis that TGF-beta1 reduces megalin-cubilin-mediated endocytosis [14].
• It could occur either by receptor-mediated endocytosis via a somatostatin receptor or, alternatively, the multiligand megalin/cubilin tandem receptor, or by fluid-phase endocytosis [28].

Analytical, diagnostic and therapeutic context of CUBN

• We isolated apolipoprotein A-I (apoA-I), the main protein of high-density lipoprotein (HDL), using cubilin affinity chromatography [7].
• Using fluorescence in situ hybridization, radiation hybrid mapping, and screening of YAC clones, the human cubilin gene was mapped between the markers D10S1661 and WI-5445 on the short arm of chromosome 10 [29].
• By site-directed mutagenesis, mammalian expression, and functional comparison of the purified wild-type and FM1 mutant forms of the IF-Cbl-binding cubilin region (CUB domains 5-8, amino acid 928-1386), we have investigated the functional implications of the P1297L mutation [30].
• Also, by immunoblotting and immunocytochemistry we show that previously identified low-molecular-weight renal albumin binding proteins are fragments of cubilin [9].
• Using morphological and Western blot analysis, we showed that cubilin first appeared at the eight-cell stage [19].

References