HIF-2alpha regulates glyceraldehyde-3-phosphate dehydrogenase expression in endothelial cells

Endothelial cells (EC) express both hypoxia inducible factor-1alpha (HIF-1alpha) and -2alpha (HIF-2alpha), yet their roles in the EC hypoxic response are unclear. Hypoxia upregulates the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in EC through a 5' hypoxic regulatory element (HRE). We compared the upregulation of GAPDH in human lung microvascular EC to that in hep3B cells, another cell type known to express both HIF-1alpha and HIF-2alpha. GAPDH mRNA increased to a lesser extent in hypoxic hep3B cells than in EC, yet upregulation occurred through the same HRE that was active in EC. HIF-1alpha protein induction in response to hypoxia was similar in both cell types. In contrast, HIF-2alpha protein levels were upregulated to a greater extent and for a longer period of time by hypoxia in EC than in hep3B cells. Correspondingly, electrophoretic mobility supershift assays showed that, in EC, there was preferential binding of HIF-2alpha to the GAPDH HRE while, in hep3B cells, there was binding of both HIF-1alpha and HIF-2alpha. The preferential binding of HIF-2alpha to the GAPDH HRE in EC may account for their higher level of induction of GAPDH. These findings suggest that cell-specific patterns of HIF-1alpha and HIF-2alpha expression lead to cell-specific gene upregulation during hypoxia.     

Organization of the constant-region gene family of the mouse immunoglobulin heavy chain

We cloned overlapping DNA segments that encompass the region from the immunoglobulin JH segments to the C gamma 3 gene of BALB/c mouse. We have now cloned the entire region (about 200 kilobases) of the constant-region gene family of the immunoglobulin heavy chain, the organization of which is 5'-JH-6.5 kb-C mu-4.5 kb-C delta-55 kb-C gamma 3-34 kb-C gamma 1-21 kb-C gamma 2b-15 kb-C gamma 2a-14 kb-C epsilon-12 kb-C alpha-3'. Using these cloned DNAs, we have characterized several structural features of the constant-region gene loci. There are no other J region segments except for those at the 5' side of the C mu gene. The S region is 5' to each CH gene except for the C delta gene, and the nucleotide sequences of the S region share some homology. There is no reasonably conserved pseudogene. There are at least two species of reiterated sequences scattered in these loci. Cloning and Southern blot hybridization analyses indicate that the general organizations of the heavy-chain gene loci of BALB/c and C57BL/6 mice, which have many different serological markers, are fundamentally similar but different in the lengths of S regions. Restriction enzyme cleavage maps of the whole constant-region gene loci were constructed with respect to eight restriction endonucleases.     

Insulin-mediated suppression of apolipoprotein B mRNA translation requires the 5' UTR and is characterized by decreased binding of an insulin-sensitive 110-kDa 5' UTR RNA-binding protein

Insulin has been shown to acutely regulate hepatic apolipoprotein B (apoB) secretion at both translational and post-translational levels; however, mechanisms of apoB mRNA translational control are largely unknown. Recent studies of apoB untranslated regions (UTRs) revealed a potentially important role for cis-trans interactions at the 5' and 3' UTRs. In the present paper, deletion constructs of the UTR regions of apoB revealed that the 5' UTR was necessary and sufficient for insulin to inhibit synthesis of apoB15. Metabolic radiolabeling and in vitro translation experiments in the presence of protease inhibitors confirmed that the effect of insulin on the apoB 5' UTR was translational in nature. Using the nondenaturing electrophoretic mobility shift assay (EMSA), protein-RNA complexes were detected binding to the apoB 5' and 3' UTRs. Denaturing EMSA identified a 110-kDa protein interacting at the 5' UTR. Nondenaturing EMSA determined that insulin altered binding of large protein complexes to the 5' UTR. Binding specificity was determined by competition with both specific and nonspecific competitors. Insulin treatment decreased binding of the 110-kDa protein to the 5' UTR as visualized by EMSA. Absence of insulin increased binding of this trans-acting factor to the 5' UTR by 2-fold. Analysis of the 3' UTR showed no significant insulin-mediated changes in binding of trans-acting factors. We thus propose the existence of a novel RNA-binding insulin-sensitive factor that binds to the 5' UTR and may regulate apoB mRNA translation. Perturbations in hepatic insulin signaling as observed in insulin-resistant states may alter cis-trans interactions at the 5' UTR, leading to alterations in the rate of apoB mRNA translation, thus contributing to apoB-lipoprotein overproduction.     

Mode of DNA binding by SopA protein of Escherichia coli F plasmid

The binding of SopA to the promoter region of its own gene, in which four copies of SopA's recognition sequence, 5'-CTTTGC-3', are arrayed asymmetrically, was examined in vitro. Titration using electrophoretic mobility shift assay showed that the stoichiometry of SopA protomers to the promoter-region DNA is 4 and that the binding is highly co-operative. The co-operativity was corroborated by EMSA and DNase I footprinting for a number of mutant DNA fragments in which 5'-CTTTGC-3' was changed to 5'-CTTACG-3'. EMSA in the style of circular permutation showed that SopA bends DNA. Mutation at either outermost binding site had a different effect on DNA bending by SopA, reflecting the asymmetry in the arrangement of the binding sites, for which the results of DNase I footprinting were in agreement. Gel filtration chromatography and analytical ultracentrifugation of free SopA showed that the protein can exist as a monomer and oligomers in the absence of ATP. Hence, the results indicate that the co-operativity in SopA's DNA binding is based on its intrinsic protein-protein interaction modified by DNA interaction.     

Dynamic conformations compared for IgE and IgG1 in solution and bound to receptors.

Dynamic conformations of two distinct immunoglobulin (Ig) isotypes, murine IgE and human IgG1, were examined with fluorescence resonance energy transfer measurements. The IgE mutant epsilon/C gamma 3* and the IgG1 mutant gamma/C gamma 3* each bind [5-(dimethylamino)naphthalen-1-yl]sulfonyl (DNS) in two identical antigen binding sites at the amino (N)-terminal ends of the Ig in the Fab segments. Eosin-DNS bound in these Fab sites served as the acceptor probe in these studies. Both Ig have a carboxy (C)-terminal domain (C gamma 3*) which contains genetically introduced cysteine residues. Modification of these cysteine sulfhydryls with fluorescein maleimide provided donor probes near the C-terminal ends of the Ig in the Fc segment. Energy transfer between the C-terminal and N-terminal ends was compared for these two Ig in solution and when they were found to their respective high-affinity receptors on plasma membranes: IgE-Fc epsilon RI on RBL cell membranes and IgG1-Fc gamma RI on U937 cell membranes. Previous energy-transfer measurements with these probes yielded an average end-to-end distance of 71 A for IgE in solution and 69 A for IgE bound to Fc epsilon RI, indicating that in both situations IgE is bent such that the axes of the Fab segments and the axis of the Fc segment do not form a planar Y-shape [Zheng, Shopes, Holowka, & Baird (1991) Biochemistry 30, 9125]. In the current study we found the average end-to-end distance for IgG1 in solution is 75 A and greater than or equal to 85 A for IgG1 bound to Fc gamma RI, suggesting an average bend conformation for IgG1 as well. The contributions of segmental flexibility to the average distances were assessed directly by measuring the efficiency of energy transfer as a function of variations in donor quantum yield caused by a collisional quencher and using these data to extract a Gaussian distribution of end-to-end distances. The distribution average (rho) and half-width (hw) were determined to be as follows: rho = 75 A, hw = 24 A for IgE in solution; rho = 71 A, hw = 12 A for IgE bound to Fc epsilon RI; and rho = 100 A, hw = 88 A for IgG in solution.(ABSTRACT TRUNCATED AT 400 WORDS)     

A B-cell-specific nuclear protein that binds to DNA sites 5'' to immunoglobulin S alpha tandem repeats is regulated during differentiation.

Immunoglobulin heavy-chain switching is effected by recombination events between sites associated with tandemly repeated switch sequences located 5' to immunoglobulin heavy-chain genes. Using the band mobility shift assay, we have identified two distinct sites 5' to the alpha heavy-chain switch sequence with affinity for a single B-cell-specific DNA-binding protein, S alpha-BP. S alpha-BP was present in nuclear extracts from pre-B and B cells but was not detected in extracts from plasmacytomas, B-cell hybridomas, T-cell lymphomas, or a macrophage cell line. It was also not detectable in other nonlymphoid cells tested. Evidence suggests there are S alpha-BP-binding sites near other immunoglobulin switch sequences. As with the S alpha sites, these sites appear to be distinct from the consensus tandem repeats characteristic of immunoglobulin switch sequences. The possible functions of S alpha-BP on contacting its binding sites are discussed in the context of immunoglobulin heavy-chain switch recombination.     

Human mb-1 gene: complete cDNA sequence and its expression in B cells bearing membrane Ig of various isotypes.

The transmembrane protein, IgM-alpha, a product of mb-1 gene, has been shown to be specifically associated with membrane-bound IgM on the plasma membrane of B lymphocytes. Recent studies have suggested that IgM-alpha may play a role in transducing signals from the Ag receptors during the activation of B cells. A large amount of information has been obtained in the mouse system regarding IgM-alpha and other components of the newly conceived B cell Ag receptor complex. Here we report the cloning and the nucleotide sequencing of cDNA clones of human mb-1, covering the entire length of the mRNA. At the amino acid sequence level, human and murine mb-1 share a high homology in their transmembrane and intracytoplasmic segments, suggesting an important biologic function for these regions of mb-1. A major difference, mainly in the 3' untranslated part, exists between our cDNA sequence and the published partial human mb-1 cDNA sequence. It has also been observed that human mb-1 is expressed not only by B cell lines expressing membrane-bound Ig of mu and delta isotypes but also those expressing membrane-bound Ig of alpha and gamma isotypes.