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Tubular cells have been isolated, characterized and cultured from more than 70 adult cadaver kidneys (postmortem time less than or equal to 12 hr.). Confluent monolayers were observed at 7 days after seeding (10(6) cells/ml.) and cells demonstrating normal human karyotypes have been passaged up to 6 times. Primary isolates and monolayer cultures were negative for Factor VIII activity, and strongly positive for gamma-glutamyltransferase activity and keratin. Ultrastructurally primary isolates consisted of cells with numerous mitochondria, microvilli, cytoplasmic filaments and well-developed endocytotic apparati. Monolayer cultures examined at 7, 14, 21 and 72 days demonstrated less prominent microvilli and the additional structures of desmosomes and cell junctions. Membrane-associated and cytosolic enzyme activities were measured up to 28 days in culture. The membrane-associated enzymes gamma-glutamyltransferase and alkaline phosphatase both exhibited approximately 10-fold decreases in activity during the 1st 7 days in culture. There was an approximately 5-fold increase in pyruvate kinase activity during the same time period, while fructose-1,6-bisphosphatase activity exhibited a 5-fold decrease. Glucose-6-phosphatase activity did not change during the 28 day culture period examined. From 7 to 28 days no further changes were noted in any of the enzyme activities measured. Decreased membrane-associated enzyme activity corresponded to the ultrastructural observation of less prominent microvilli. Increases in glycolytic enzyme activity and decreases in gluconeogenic enzyme activity may reflect the presence of glucose in the culture medium. The morphologic and biochemical evidence suggests that primary isolates and cultures are proximal tubule cells which should provide a well-defined in vitro human system for future studies.

This study was designed to identify the cellular component of the intestinal villus where transcobalamin II (TCII) is synthesized, because this protein provides an essential function in the intestinal absorption of vitamin B12 (cobalamin, Cbl). When a segment of proximal or distal small intestine of the guinea pig is cultured in medium containing [57Co]Cbl, TCII-[57Co]Cbl appears within 15 min. Northern blot analysis of RNA from both proximal and distal small intestine identified the TCII transcript. In situ hybridization of the distal ileum with35S-labeled TCII antisense transcript localized grains predominantly in crypts and in the lower third and central core of the villi. Grains were also evident at the base of the enterocytes in close apposition with the vascular network, whereas few grains appeared in the apical region of the columnar cells. This study provides evidence that TCII is constitutively expressed in the intestinal villi where vascular endothelium is abundant. In the distal ileum, where the intrinsic factor (IF) receptor is expressed, after uptake of IF-Cbl and the subsequent binding of free Cbl to TCII synthesized in the villi, the TCII-Cbl complex enters the microcirculation and passes into the portal blood.

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Transcobalamin II (TCII) is a plasma protein that binds vitamin B12 (cobalamin; Cbl) and facilitates the cellular uptake of the vitamin by receptor-mediated endocytosis. In genetic disorders that are characterized by congenital deficiency of TCII, intracellular Cbl deficiency occurs, resulting in an early onset of megaloblastic anemia that is sometimes accompanied by a neurologic disorder. To define the genetic basis for TCII deficiency, we have cloned and characterized the human gene that encodes this protein. The gene spans a minimum of 18 kbp and contains nine exons and eight introns, with a polyadenylation signal sequence located 509 bp downstream from the termination codon and a transcription initiation site beginning 158 bp upstream from the ATG translation start site. The 5′ flanking DNA does not have a TATA or CCAAT regulatory element, but a 34-nucleotide stretch beginning just upstream of the CAP site contains four tandemly organized 5′-CCCC-3′ tetramers. This sequence is a motif for a trans-active transcription factor (ETF) that regulates expression of the epidermal growth factor receptor gene (EGFR), which also lacks TATA and CCAAT regulatory elements. A GC-rich sequence that binds the SP1 protein is located 356 nucleotides upstream from the first of the series of CCCC tetramers. Although this GC sequence is at an unusual location with respect to the CAP site, a 507-bp fragment containing this GC box drives the chloramphenicol acetyltransferase (CAT) reporter gene after transient transfection into NIH 3T3 cells. No CAT activity was observed when a 420-bp fragment lacking this GC box but containing the ETF-binding domains was similarly transfected into this cell line. One consensus and two atypical motifs for the c-myc ligand are located downstream and upstream, respectively, of the GC box, and this could explain the elevated plasma TCII observed in some patients with multiple myeloma, as the c-myc product is overexpressed in some myeloma cells. Restriction endonuclease digestion of genomic DNA from eight normal subjects with Taq I, Hinfl, Msp I, and Bgl I identified three patterns of restriction fragment length polymorphism (RFLP). A number of the exon/intron splice junctions of human TCII, TCI, and IF genes are located in homologous regions of these proteins, providing evidence that these genes have evolved by duplication of an ancestral gene. This characterization of the TCII gene and the RFLP should facilitate the identification of the mutation(s) responsible for the genetic abnormalities of TCII expression.

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