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B H Qu

Publications and source records attributed to B H Qu.

9 recordsLinked to original sources
PubMed2001

Insulin and IGF-1 induce different patterns of gene expression in mouse fibroblast NIH-3T3 cells: identification by cDNA microarray analysis.

The IGF-1 receptor and the related insulin receptor are similar in structure and activate many of the same postreceptor signaling pathways, yet they mediate distinct biological functions. It is still not understood how the specificity of insulin vs. IGF-1 signaling is controlled. In this study, we have used cDNA microarrays to monitor the gene expression patterns that are regulated by insulin and IGF-1. Mouse fibroblast NIH-3T3 cells expressing either the wild-type human IGF receptor or the insulin receptor were stimulated with either IGF-1 or insulin, respectively. Thirty genes, 27 of which were not previously known to be IGF-1 responsive, were up-regulated by IGF-1 but not by insulin. Nine genes, none of which was previously known to be insulin responsive, were up-regulated by insulin but not by IGF-1. The IGF- and insulin-induced regulation of 10 of these genes was confirmed by Northern blot analysis. Interestingly, more than half of the genes up-regulated by IGF-1 are associated with mitogenesis and differentiation, whereas none of the genes specifically up-regulated by insulin are associated with these processes. Our results indicate that under the conditions used in this study, IGF-1 is a more potent activator of the mitogenic pathway than insulin in mouse fibroblast NIH-3T3 cells.

3T3 Cells↗
PubMed1999

Insulin receptor substrate-4 enhances insulin-like growth factor-I-induced cell proliferation.

The insulin receptor substrates (IRSs)-1-4 play important roles in signal transduction emanating from the insulin and insulin-like growth factor (IGF)-I receptors. IRS-4 is the most recently characterized member, which has been found primarily in human cells and tissues. It interacts with SH2-containing proteins such as phosphatidylinositol 3'-kinase (PI3K), Grb2, Crk-II, and CrkL. In this study, we transfected IRS-4 in mouse NIH-3T3 cells that overexpress IGF-I receptors. Clones expressing IRS-4 showed enhanced cellular proliferation when cells were cultured in 1% fetal bovine serum without added IGF-I. Addition of IGF-I enhanced cellular proliferation in cells overexpressing the IGF-I receptor alone but had an even greater proliferative effect in cells overexpressing both the IGF-I receptors and IRS-4. When etoposide and methylmethane sulfonate (MMS), both DNA damaging agents, were added to the cells, they uniformly induced cell cycle arrest. Fluorescence-activated cell sorter analysis demonstrated that the arrest of the cell cycle occurred at the G(1) checkpoint, and furthermore no significant degree of apoptosis was demonstrated with the use of either agent. In cells, overexpressing IGF-I receptors alone, IGF-I addition enhanced cellular proliferation, even in the presence of etoposide and MMS. In cells overexpressing IGF-I receptors and IRS-4, the effect of IGF-I in overcoming the cell cycle arrest was even more pronounced. These results suggest that IRS-4 is implicated in the IGF-I receptor mitogenic signaling pathway.

3T3 Cells↗
PubMed1997

The molecular chaperone Hsc70 assists the in vitro folding of the N-terminal nucleotide-binding domain of the cystic fibrosis transmembrane conductance regulator.

The most common disease-causing mutation in the cystic fibrosis transmembrane conductance regulator is a single amino acid deletion (DeltaF508) in the N-terminal cytosolic nucleotide-binding domain (NBD1). This mutation has previously been shown to be a temperature-sensitive folding mutation that alters the folding pathway but not the native state stability of the isolated domain (Qu, B.-H., and Thomas, P. J. (1996) J. Biol. Chem. 271, 7261-7264). Here we provide evidence that the molecular chaperone Hsc70 productively interacts with NBD1 to increase the folding yield of the domain and inhibit off-pathway associations leading to the formation of high molecular weight aggregates. Furthermore, we have sublocalized a region within NBD1 where Hsc70 binds. Notably, inhibition of NBD1 aggregation is not dependent upon the presence of Hsc70 in the early stages of folding, indicating that the chaperone may act on a folding intermediate. In the presence of K+ and Mg2+-ATP, conditions where Hsp70 binds substrate rapidly and can release it, Hsc70 is less effective at inhibiting NBD1 aggregation. Thus, the rate of release of unfolded substrate is an important factor in preventing aggregation and promoting folding of the domain. These results demonstrate that Hsc70 promotes the otherwise inefficient folding of DeltaF-NBD1 and provide insight into the mechanisms by which molecular chaperones assist proteins in folding.

Adenosine Triphosphate↗
PubMed1997

Localization and suppression of a kinetic defect in cystic fibrosis transmembrane conductance regulator folding.

A growing body of evidence indicates that the most common cystic fibrosis-causing mutation, DeltaF508, alters the ability of the cystic fibrosis transmembrane conductance regulator (CFTR) protein to fold and transit to the plasma membrane. Here we present evidence that the DeltaF508 mutation affects a step on the folding pathway prior to formation of the ATP binding site in the nucleotide binding domain (NBD). Notably, stabilization of the native state with 4 mM ATP does not alter the temperature-dependent folding yield of the mutant DeltaF508 NBD1 in vitro. In contrast, glycerol, which promotes DeltaF508-CFTR maturation in vivo, increases the folding yield of NBD1DeltaF and reduces the off pathway rate in vitro, although it does not significantly alter the free energy of stability. Likewise a second site mutation, R553M, which corrects the maturation defect in vivo, is a superfolder which counters the effects of DeltaF508 on the temperature-dependent folding yield in vitro, but does not significantly alter the free energy of stability. A disease-causing mutation, G551D, which does not alter the maturation of CFTR in vivo but rather its function as a chloride channel, and the S549R maturation mutation have no discernible effect on the folding of the domain. These results demonstrate that DeltaF508 is a kinetic folding mutation that affects a step early in the process, and that there is a significant energy barrier between the native state and the step affected by the mutation precluding the use of native state ligands to promote folding. The implications for protein folding in general are that the primary sequence may not necessarily simply define the most stable native structure, but rather a stable structure that is kinetically accessible.

Adenosine Triphosphate↗
PubMed1997

Cystic fibrosis: a disease of altered protein folding.

Cystic fibrosis (CF) is caused by mutations in the gene that encodes the cystic fibrosis transmembrane conductance regulator, CFTR. Previously we demonstrated that the common delta F508 mutation in the first nucleotide binding domain (NBD1) alters the ability of the domain to fold into a functional three-dimensional structure, providing a molecular explanation for the observation that the mutant CFTR is retained in the endoplasmic reticulum and does not traffic to the apical membrane of affected epithelial cells. Notably, when conditions are altered to promote folding of the mutant protein, it can assume a functional conformation. Correcting the folding defect may have therapeutic benefit for the treatment of cystic fibrosis. Here we summarize these results and discuss the implications in vitro folding studies have for understanding the pathobiology of CF.

ATP-Binding Cassette Transporters↗
PubMed1996

Alteration of the cystic fibrosis transmembrane conductance regulator folding pathway.

The cellular phenotype of the most common cystic fibrosis-causing mutation, deletion of phenylalanine 508 (deltaF508) in the amino-terminal nucleotide binding domain (NBD1) of the cystic fibrosis transmembrane conductance regulator (CFTR), is the inability of the mutant protein to fold and transit to the apical membrane of affected epithelial cells. Expressed NBD1s were purified and folded in vitro into soluble monomers capable of binding nucleotide. Here we report that the deltaF508 mutation has little effect on the thermodynamic stability of the folded NBD1. The deltaG(0)(D,GdnHCl) is 15.5 kJ/mol for the wild type NBD1 and 14.4 kJ/mol for NBD1deltaF. In contrast, the mutation significantly reduces the folding yield at a variety of temperatures, indicating that Phe-508 makes crucial contacts during the folding process, but plays little role in stabilization of the native state. Under conditions that approximate the efficiency of maturation in vivo, the rate off-pathway is significantly increased by the disease causing mutation. These results establish a molecular mechanism for most cases of cystic fibrosis and provide insight into the complex processes by which primary sequence encodes the three-dimensional structure.

Adenosine Triphosphate↗
PubMed1995

Defective protein folding as a basis of human disease.

The ability of a polypeptide to fold into a unique, functional, three-dimensional structure in vivo is dependent upon its amino acid sequence and the function of molecular chaperone proteins and enzymes that catalyse folding. Intense study of the physical chemistry and cell biology of folding have greatly aided our understanding of the mechanisms normally employed. Evidence is accumulating that many disease-causing mutations and modifications exert their effects by altering protein folding. Here we discuss the pathobiology of these processes.

Amino Acid Isomerases↗
PubMed1993

MSR--a program for kinetic analysis of enzyme modification data on PC.

A computer program has been written for the kinetic method of substrate reaction (MSR) in enzyme modification studies. The program analyses a series of progress curves that continuously record signals (absorbance, fluorescence emission, etc.) reflecting the concentration changes of substrate or product during enzyme modification in the presence of substrate. After the apparent kinetic parameters are calculated by graphdrawing or curve-fitting method, a series of relevant plots is shown, from which information about the enzyme-inhibitor interaction can be deduced and microscopic kinetic constants calculated.

Animals↗