Biomedical subjects

Y Xu

Publications and source records attributed to Y Xu.

At least 19 recordsLinked to original sources

PubMed1995

Desensitization and internalization of the m2 muscarinic acetylcholine receptor are directed by independent mechanisms.

The phenomenon of acute desensitization of G-protein-coupled receptors has been associated with several events, including receptor phosphorylation, loss of high affinity agonist binding, receptor:G-protein uncoupling, and receptor internalization. However, the biochemical events underlying these processes are not fully understood, and their contributions to the loss of signaling remain correlative. In addition, the nature of the kinases and the receptor domains which are involved in modulation of activity have only begun to be investigated. In order to directly measure the role of G-protein-coupled receptor kinases (GRKs) in the desensitization of the m2 muscarinic acetylcholine receptor (m2 mAChR), a dominant-negative allele of GRK2 was used to inhibit receptor phosphorylation by endogenous GRK activity in a human embryonic kidney cell line. The dominant-negative GRK2K220R reduced agonist-dependent phosphorylation of the m2 mAChR by approximately 50% and prevented acute desensitization of the receptor as measured by the ability of the m2 mAChR to attenuate adenylyl cyclase activity. In contrast, the agonist-induced internalization of the m2 mAChR was unaffected by the GRK2K220R construct. Further evidence linking receptor phosphorylation to acute receptor desensitization was obtained when two deletions of the third intracellular loop were made which created m2 mAChRs that did not become phosphorylated in an agonist-dependent manner and did not desensitize. However, the mutant mAChRs retained the ability to internalize. These data provide the first direct evidence that GRK-mediated receptor phosphorylation is necessary for m2 mAChR desensitization; the likely sites of in vivo phosphorylation are in the central portion of the third intracellular loop (amino acids 282-323). These results also indicate that internalization of the m2 receptor is not a key event in desensitization and is mediated by mechanisms distinct from GRK phosphorylation of the receptor.

Alleles↗

PubMed1995

Selective inhibition of thrombin receptor-mediated Ca2+ entry by protein kinase C beta.

Thrombin initiates many physiological processes in platelets and other megakaryocyte-lineage cells by interacting with surface receptors and generating rises in cytoplasmic Ca2+; these rises result from both Ca2+ release from intracellular stores and receptor-mediated Ca2+ entry. Regulators that limit Ca2+ entry after its initiation by thrombin have not been identified. In this study, prevention of expression of a single protein kinase C isoenzyme (PKC beta) by antisense cDNA overexpressed in HEL cells, a human megakaryoblastic cell line that expresses thrombin receptors, promotes thrombin receptor-mediated Ca2+ entry without altering thrombin-induced intracellular release of Ca2+. The cytoplasmic Ca2+ rise initiated by endoperoxide analogs was not affected by inhibiting PKC beta. Overexpression of a cDNA encoding wild-type PKC beta mutated to prevent recognition by the antisense cDNA abolished the enhancement of Ca2+ influx following thrombin. Thus, PKC beta appears to be a specific negative regulator of thrombin receptor-mediated Ca2+ entry.

Base Sequence↗

PubMed1995

Lysophospholipids activate ovarian and breast cancer cells.

We have investigated the effects of phospholipids on activation and proliferation of ovarian and breast cancer cells. Lysophosphatidic acid (LPA), lysophosphatidylserine (LPS) and sphingosylphosphorylcholine (SPC) all induce transient increases in cytosolic free Ca2+ ([Ca2+]i) in both ovarian and breast cancer cell lines. The ability of LPA, LPS and SPC to induce increases in [Ca2+]i in ovarian and breast cancer cells is likely to be due to an interaction with cell-surface receptors as the increases in [Ca2+]i were: (1) due to release of calcium from intracellular stores and not from transmembrane uptake due to changes in permeability; (2) blocked by lanthanum and suramin which do not enter cells; (3) blocked by phorbol esters which interrupt increases in [Ca2+]i induced through a number of different receptors; and (4) not detected in freshly isolated peripheral blood mononuclear cells, indicating cell type specificity. In addition, increases in [Ca2+]i induced by LPA, LPS and SPC in ovarian and breast cancer cells completely self-desensitized and cross-desensitized each other, but did not block increases in [Ca2+]i induced by thrombin. Lysophosphatidylglycerol (LPG), but not other lysophospholipids, inhibited LPA- but not LPS- or SPC-induced increases in [Ca2+]i, suggesting that LPA may interact with a different receptor(s) to LPS or SPC and that their downstream signalling pathways converge or interact. LPA, SPC and LPS also induced rapid increases in tyrosine phosphorylation of specific cellular proteins, including p125FAK. Strikingly, LPA, but not LPS or SPC, induced activation of mitogen-activated protein (MAP) kinases. Despite an ability to activate similar intracellular signaling events, LPA, LPS and SPC exhibited markedly different effects on cell proliferation. Whereas LPA induced a significant increase in cell proliferation, LPS did not substantially alter cell proliferation and SPC inhibited cell proliferation. Surprisingly, phosphatidic acid (PA), which did not induce increases in [Ca2+]i, p125FAK activation or activation of MAP kinases, did induce proliferation of ovarian cancer cells, albeit at higher concentrations that LPA. The discordance between sensitivity to LPG, early biochemical events stimulated, and the eventual proliferation response combine to suggest that LPA probably utilizes a different receptor from LPS, SPC and PA. Therefore ovarian and breast cancer cells are sensitive to the effects of a number of different phospholipids which may play a role in the growth of these tumour cells in the cancer patient and are thus potential targets for therapy.

Breast Neoplasms↗

PubMed1995

Genetic and redox determinants of nitric oxide cytotoxicity in a Salmonella typhimurium model.

Paradoxically, nitric oxide (NO) has been found to exhibit cytotoxic, antiproliferative, or cytoprotective activity under different conditions. We have utilized Salmonella mutants deficient in antioxidant defenses or peptide transport to gain insights into NO actions. Comparison of three NO donor compounds reveals distinct and independent cellular responses associated with specific redox forms of NO. The peroxynitrite (OONO-) generator 3-morpholinosydnonimine hydrochloride mediates oxygen-dependent Salmonella killing, whereas S-nitrosoglutathione (GSNO) causes oxygen-independent cytostasis, and the NO. donor diethylenetriamine-nitric oxide adduct has no antibacterial activity. GSNO has the greatest activity for stationary cells, a characteristic relevant to latent or intracellular pathogens. Moreover, the cytostatic activity of GSNO may best correlate with antiproliferative or antimicrobial effects of NO, which are unassociated with overt cell injury. dpp mutants defective in active dipeptide transport are resistant to GSNO, implicating heterolytic NO+ transfer rather than homolytic NO. release in the mechanism of cytostasis. This transport system may provide a specific pathway for GSNO-mediated signaling in biological systems. The redox state and associated carrier molecules are critical determinants of NO activity.

Base Sequence↗

PubMed1995

Irreversible inhibitors of nicotinic acetylcholine receptors: isolation and structural characterization of the biologically active solvolysis products of bipinnatin-A and bipinnatin-C.

The lophotoxins irreversibly inhibit nicotinic acetylcholine receptors by covalent modification of Tyr190 in the alpha-subunits of the receptor. Previous studies have shown that the naturally occurring lophotoxin analogs bipinnatin-A, -B, and -C are actually inactive protoxins and that their ability to irreversibly inhibit nicotinic receptors is enhanced by preincubation in buffer. However, the ability of lophotoxin to irreversibly inhibit nicotinic receptors does not appear to be enhanced by preincubation in buffer. These observations led to the current effort to isolate and determine the structures of biologically active bipinnatins. Disappearance of the lophotoxins from solution followed a simple first-order exponential decay function. Lophotoxin, however, was approximately 40-fold more stable then bipinnatin-A, -B, or -C. Solvolysis of the bipinnatins, but not of lophotoxin, resulted in production of an equimolar amount of acetic acid at a rate similar to the rate of solvolysis, suggesting that the initial event in solvolysis of these toxins involves hydrolysis of an acetate ester. Proton NMR and fast-atom bombardment mass spectroscopy were used to confirm the structures of the active solvolysis products of bipinnatin-A and -C. Their structures and the relative pH insensitivity of the solvolysis reaction suggest that biological activation of the bipinnatins may proceed through an SN1 type of substitution reaction involving elimination of acetate followed by reaction of a carbocation intermediate with solvent.

Cell Line↗

PubMed1995

Phytochromes: photosensory perception and signal transduction.

The phytochrome family of photoreceptors monitors the light environment and dictates patterns of gene expression that enable the plant to optimize growth and development in accordance with prevailing conditions. The enduring challenge is to define the biochemical mechanism of phytochrome action and to dissect the signaling circuitry by which the photoreceptor molecules relay sensory information to the genes they regulate. Evidence indicates that individual phytochromes have specialized photosensory functions. The amino-terminal domain of the molecule determines this photosensory specificity, whereas a short segment in the carboxyl-terminal domain is critical for signal transfer to downstream components. Heterotrimeric GTP-binding proteins, calcium-calmodulin, cyclic guanosine 5'-phosphate, and the COP-DET-FUS class of master regulators are implicated as signaling intermediates in phototransduction.

Photoreceptor Cells↗

PubMed1995

Effects of tetrandrine on left ventricle hypertrophy in deoxycorticosterone acetate-salt hypertensive rats.

The effect of regression of left ventricular hypertrophy was studied in deoxycorticosterone-acetate-salt hypertensive rats (DOCA-salt hypertensive rats) treated with tetrandrine. Treatment with tetrandrine (by gastric intubation, 50 mg/kg per day for 9 weeks) lowered systolic blood pressure, left ventricular weight, Ca2+ of mitochondria, and markedly decreased the density (Bmax) and total number of dihydropyridine binding sites in hypertrophic left ventricle (P < 0.001). There was no difference between groups in dissociation constant (Kd) values of dihydropyridine binding sites. These facts indicate that tetrandrine decreased cardiac mass in DOCA-salt hypertensive rats through mechanisms that may be associated with the density and the total number of dihydropyridine binding sites, Ca2+ and blood pressure control.

Alkaloids↗

PubMed1995

Determination of plasma non-esterified fatty acids and triglyceride fatty acids by gas chromatography of their methyl esters after isolation by column chromatography on silica gel.

Non-esterified fatty acids (NEFA) and triglycerides were isolated from human plasma by column chromatography on silica gel. Eight principal fatty acids of each of these lipid classes were determined by gas chromatography of their methyl ester derivatives and quantified relative to multipoint standard curves. Within-day relative standard deviations for plasma non-esterified fatty acid and triglyceride fatty acid determinations were 2.4 and 3.2%, respectively. Day-to-day relative standard deviations for plasma non-esterified fatty acid and triglyceride fatty acid determinations were 1.4 and 1.1%, respectively. The total plasma concentration and the relative proportions of the eight non-esterified fatty acids determined by this method were significantly different from results obtained according to two generally accepted methods for direct plasma non-esterified fatty acid determination without a specific isolation step. These comparisons suggested that considerable fatty acid ester lipid hydrolysis occurred during these direct determination procedures, and that this hydrolysis resulted in 3-fold overestimation of plasma NEFA content by those methods. Measured levels of arachidonic acid are substantially overestimated by these direct determination methods in which non-esterified fatty acids are not isolated before derivatization.

Chromatography, Gas↗

PubMed1995

Sexual isolation in Drosophila melanogaster: a possible case of incipient speciation.

It is generally believed that Drosophila melanogaster has no closely related species with which it can produce the viable and fertile hybrids that are essential for the genetic analysis of speciation. Following the recent report of molecular differentiation between a Zimbabwe, Africa, population and two United States populations, we provide evidence that strong sexual isolation exists between the D. melanogaster population in Zimbabwe and populations of other continents. In the presence of males of their own kind, females from most isofemale lines of Zimbabwe would not mate with males from elsewhere; the reciprocal mating is also significantly reduced, but to a lesser degree. The genes for sexual behaviors are apparently polymorphic in Zimbabwe and postmating reproductive isolation between this and other populations has not yet evolved. Whole chromosome substitutions indicate significant genetic contributions to male mating success by both major autosomes, whereas the X chromosome effect is too weak to measure. In addition, the relative mating success between hybrid and pure line males supports the interpretation of strong female choice. These observations suggest that we are seeing the early stages of speciation in this group and that it is driven by sexual selection. The genetic and molecular tractability of D. melanogaster offers great promise for the detailed analysis of this apparent case of incipient speciation.

Animals↗

PubMed1995

ARE- and TRE-mediated regulation of gene expression. Response to xenobiotics and antioxidants.

Antioxidant response elements (AREs) containing 12-O-tetradecanoylphorbol-13-acetate response element (TRE) (perfect AP1) and TRE-like (imperfect AP1) elements mediate high basal transcription of the NAD(P)H:quinone oxidoreductase1 (NQO1) and glutathione S-transferase Ya genes in tumor cells and its induction in response to xenobiotics and antioxidants. Mutations in the human NQO1 gene ARE (hARE) revealed the requirement for two TRE or TRE-like elements arranged in inverse orientation at the interval of three base pairs and a GC box for optimal expression and beta-naphthoflavone induction of the NQO1 gene. A single TRE element from the human collagenase gene failed to respond to beta-naphthoflavone. These results demonstrate that ARE (2 x TRE or TRE-like elements)-containing detoxifying enzyme genes and not genes that contain 1 x TRE are responsive to xenobiotics and antioxidants. Bandshift assays showed shifting of a complex of more or less similar mobility with hARE and TRE that could be competed by each other. Mutations in the 3'-TRE of the NQO1 gene hARE eliminated binding of nuclear proteins to the hARE and resulted in the loss of basal and induced expression, indicating that 3'-TRE is the most important element within the hARE. 5'-TRE-like element within the NQO1 gene hARE is required for xenobiotic response but may not bind to the nuclear proteins by itself. The GC box located immediately following the 3'-TRE is required for optimal expression and induction of the NQO1 gene. The comparison of AREs from several different genes indicated the requirement for specific arrangement and spacing of two TRE and TRE-like elements within the AREs.

Antioxidants↗

PubMed1995

Substance P and bradykinin are natural inhibitors of CD13/aminopeptidase N.

Aminopeptidase N (EC 3.4.11.2) is an important enzyme that is involved in the degradation of regulatory peptides including enkephalins. We report here that purified and native membrane-bound aminopeptidase N will sequentially and completely hydrolyze both Leu-enkephalin and Met-enkephalin from the amino terminus. Both purified pig aminopeptidase N and the enzyme on live HL60 cells displayed similar Km values for enkephalin. The naturally occurring neuropeptides substance P and bradykinin, and the morphine agonist, morphiceptin, were not hydrolyzed by aminopeptidase N and each inhibited the enzymatic activity. Each of these peptides contains a proline at the second residue. The Ki values for substance P (0.44 microM), bradykinin (9.4 microM), and morphiceptin (169 microM) were obtained with the enzyme on live HL60 cells. The values for the purified enzyme from pig were similar. The potent inhibition of aminopeptidase N by substance P and bradykinin suggests that these peptides may be natural inhibitors of the enzyme.

Amino Acid Sequence↗

PubMed1995

Cyclic ADP-ribose-induced calcium release in sea urchin egg homogenates is a cooperative process.

Cyclic ADP-ribose (cADPR), an enzymatic product formed from NAD+ in sea urchin eggs and in several mammalian cell types, has been proposed as a second messenger for the regulation of cytosolic free Ca2+ concentration. Although cADPR induces Ca2+ release from an intracellular Ca2+ pool by a mechanism that differs from that used by inositol 1,4,5-trisphosphate, the precise action is unknown. We have analyzed the kinetics of cADPR-induced Ca2+ release at 15 degrees C in sea urchin egg homogenates using the calcium indicator fluo-3. In this system, Ca2+ release induced by cADPR was a positively cooperative process with a Hill coefficient of 1.8 +/- 0.32 (mean +/- SE, n = 15). We also examined the effect of caffeine, which potentiates the action of cADPR, on the kinetics of cADPR-induced Ca2+ release. Caffeine (3 mM) did not affect the degree of cooperativity induced by cADPR, but it did cause an increase in the apparent affinity of cADPR for its cellular target; the apparent Kd, derived from the kinetic analysis, decreased from 176 +/- 50 to 68 +/- 13 nM. We conclude that cADPR-induced Ca2+ release in sea urchin eggs is a cooperative process. A straightforward interpretation of the analysis is that opening of the Ca(2+)-release channel requires the binding of two molecules of cADPR (Hill coefficient of about 2). However, it is not yet known whether cADPR binds directly to the release channel. It is possible that potentiation of the action of cADPR by newly released Ca2+ could also contribute to the observed cooperative effect.

Adenosine Diphosphate Ribose↗

PubMed1995

Location of the catalytic site for phosphoenolpyruvate formation within the primary structure of Clostridium symbiosum pyruvate phosphate dikinase. 1. Identification of an essential cysteine by chemical modification with [1-14C]bromopyruvate and site-directed mutagenesis.

Pyruvate phosphate dikinase (PPDK) catalyzes the interconversion of adenosine 5'-triphosphate (ATP), orthophosphate (Pi), and pyruvate with adenosine 5'-monophosphate (AMP), pyrophosphate (PPi), and phosphoenolpyruvate (PEP). The reaction takes place according to the following steps: (1) E+ATP+P(i)<-->E-PP.AMP.P(i), (2) E-PP.AMP.P(i)<-->E-P+AMP+PP(i), and (3) E-P+pyruvate<-->E+PEP, where E represents free enzyme; E-PP, pyrophosphorylenzyme; and E-P, phosphorylenzyme. Steps 1 and 2 comprise the nucleotide partial reaction, and step 3 comprises the pyruvate partial reaction. The present studies were carried out to locate amino acid residues within the primary structure of Clostridium symbiosum PPDK participating in the catalysis of the pyruvate partial reaction. The enzyme was treated with the affinity label [1-14C]bromopyruvate, reduced with NaBH4, proteolyzed with trypsin, and chromatographed on an HPLC column. The radiolabeled tryptic peptide isolate was sequenced to reveal Cys 831 as the site of alkylation. Using PCR techniques Cys 831 was replaced by Ala, and the C831A PPDK mutant formed was then subjected to kinetic analysis. Rapid quench studies of single turnover reactions on the enzyme showed that the mutant is as efficient as wild-type PPDK in catalyzing the nucleotide partial reaction while it is unable to catalyze the pyruvate partial reaction. These results were interpreted as evidence for a role of Cys 831 in pyruvate/PEP binding and/or catalysis.

Affinity Labels↗

PubMed1995

Location of the catalytic site for phosphoenolpyruvate formation within the primary structure of Clostridium symbiosum pyruvate phosphate dikinase. 2. Site-directed mutagenesis of an essential arginine contained within an apparent P-loop.

Pyruvate phosphate dikinase catalyzes the interconversion of adenosine 5'-triphosphate (ATP), orthophosphate (P(i)), and pyruvate with adenosine 5'-monophosphate (AMP), pyrophosphate (PP(i)), and phosphoenolpyruvate (PEP). The Arg 561 residue of Clostridium symbiosum PPDK is contained within a Gly-rich stretch of sequence spanning positions 553-563 (viz., GAEGIGLCRTE) located in the 35 kDa C-terminal domain of the enzyme. The possible role of this stretch of sequence as a phosphate binding loop participating in catalysis of the PEP/pyruvate partial reaction (viz., E+PEP<-->E-P+pyruvate, where E-P represents enzyme phosphorylated at the catalytic histidine) was deduced from the similarity of this sequence to other known phosphate binding loops and by its location in the 35 kDa PEP/pyruvate binding domain of PPDK. To test the proposed role of Arg 561, and hence, the signature sequence, in catalysis of the E+PEP<-->E-P+pyruvate partial reaction, the C. symbiosum PPDK site-directed mutants Arg 561-->Leu 561 and Arg 561-->Lys 561 were constructed and expressed in Escherichia coli JM101. Neither mutant catalyzed the full PPDK reaction, ATP+P(i)+pyruvate<-->AMP+PP(i)+PEP, but both catalyzed the E+ATP+P(i)<-->E-P+AMP+PP(i) partial reaction as efficiently as wild-type PPDK. Both mutants were shown to be unable to catalyze the PEP/pyruvate partial reaction. On the basis of these results it was proposed that Arg 561 and, possibly, the Gly-rich stretch of sequence spanning positions 553-563 are essential components of the active site of the PEP/pyruvate partial reaction.

Amino Acid Sequence↗

PubMed1995

Separate site catalysis by pyruvate phosphate dikinase as revealed by deletion mutants.

Previous studies had indicated that pyruvate phosphate dikinase (PPDK), an enzyme which catalyzes the interconversion of adenosine 5'-triphosphate (ATP), orthophosphate (P(i)), and pyruvate with adenosine 5'-monophosphate (AMP), pyrophosphate (PP(i)), and phosphoenolpyruvate (PEP), is made up of 25, 13, 18, and 35 kDa domains [Carroll, L. J., Xu, Y., Thrall, S. H., Martin, B. M. & Dunaway-Mariano, D. (1994) Biochemistry 33, 1134]. The catalytic histidine (which mediates the phosphoryl group transfers from ATP to P(i) and pyruvate) is located on the 18 kDa domain while the 25 and 13 kDa domains appear to contain the ATP binding site and the 35 kDa domain appears to contain the pyruvate binding site, respectively. The goal of this investigation was to examine functional interdependency of the putative ATP and pyruvate binding domains. Two truncated forms of PPDK were created by using recombinant DNA techniques. The 35 kDa (C-terminal) deletion mutant was found to catalyze the E+ATP+P(i)<-->E-P+AMP+PP(i) partial reaction but not the E-P+pyruvate<-->E+PEP partial reaction. The 25 kDa (N-terminal) deletion mutant was found to catalyze the E-P+pyruvate<-->E+PEP partial reaction but not the E+ATP+P(i)<-->E-P+AMP+PP(i) partial reaction. Neither mutant catalyzes the full ATP+P(i)+pyruvate<-->AMP+PP(i)+PEP reaction. These results are interpreted to mean that the ATP and pyruvate binding domains in PPDK are functionally independent, thus providing evidence for separate active sites for catalysis of the two partial reactions.

Base Sequence↗

PubMed1995

High-yield bacterial expression, purification, and functional reconstitution of the tricarboxylate transport protein from rat liver mitochondria.

The rat liver mitochondrial tricarboxylate transport protein has been overexpressed in E. coli. The expressed transporter, which contains a 21 amino acid N-terminal fusion sequence, accumulates in inclusion bodies. Subsequent extraction of the tricarboxylate transporter from isolated inclusion bodies yields approximately 90 mg of transport protein per liter of E. coli culture at a purity of greater than 90%. Upon incorporation into phospholipid vesicles the purified, overexpressed transporter catalyzes a 1,2,3-benezenetricarboxylate-sensitive citrate/citrate exchange (i.e., the defining reaction of the mitochondrial tricarboxylate transporter). Kinetic characterization of the reconstituted transporter indicates a Km of 0.37 mM and a Vmax of 101 nmol/min/mg protein. The substrate specificity of the reconstituted, expressed transporter is virtually identical to that of the native transporter. These studies represent the first overexpression of the rat liver mitochondrial tricarboxylate transporter. By providing a large amount of highly-purified, functionally competent transporter this system will now enable a variety of structural studies, including site-directed mutagenesis, which heretofore could not be performed.

Animals↗

PubMed1995

FLK-2/FLT-3 ligand regulates the growth of early myeloid progenitors isolated from human fetal liver.

The effects of the recently identified FLK-2/FLT-3 ligand (FL) on the growth of purified human fetal liver progenitors were investigated under serum-deprived culture conditions. FL alone was found to stimulate modest proliferation in short-term cultures of CD34++ CD38+ lineage (Lin)- light-density fetal liver (LDFL) cells and the more primitive CD34++ CD38- Lin- LDFL cells. However, the low levels of growth induced by FL were insufficient for colony formation in clonal cultures. Synergism between FL and either granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3) or KIT ligand (KL) was observed in promoting the growth of high-proliferative potential (HPP) colony-forming cells (CF) and/or low-proliferative potential (LPP)-CFC in cultures of CD34++ CD38+ Lin- and CD34++ CD38- Lin- LDFL-cells. FL, alone or in combination with other cytokines, was not found to affect the growth of CD34+ Lin- LDFL cells, the most mature subpopulation of fetal liver progenitors investigated. The growth of the most primitive subset of progenitors studied, CD34++ CD38- Lin- LDFL cells, required the interactions of at least two cytokines, because only very low levels of growth were observed in response to either FL, GM-CSF, IL-3 or KL alone. However, the results of delayed cytokine-addition experiments suggested that individually these cytokines did promote the survival of this early population of progenitors. Although two-factor combinations of FL, KL, and GM-CSF were observed to promote the growth of early progenitors in a synergistic manner, neither of these factors was found to make fetal liver progenitors more responsive to suboptimal concentrations of a second cytokine. Only myeloid cells were recovered from liquid cultures of CD34++ CD38- Lin- LDFL cells grown in the presence of combinations of FL, KL, and GM-CSF. These results indicate that FL is part of a network of growth factors that regulate the growth and survival of early hematopoietic progenitors.

ADP-ribosyl Cyclase↗

PubMed1995

Characterization of the proximal promoter region of the rat somatostatin receptor gene, SSTR4.

In the present study, the putative promoter region of rSSTR4 gene has been cloned and characterized. Primer extension and RNAase protection analysis reveals the existence of multiple transcriptional initiation sites, the five major ones mapping between -126 and -18 relative to the ATG initiation codon. Sequence analysis reveals that the region lacks TATA and CCAAT promoter elements, possesses a high GC content and contains a number of potential SP1 binding sites. In addition, potential AP-2, GCF, and PuF binding sites and an octimer motif were also identified. Thus, the promoter of rSSTR4 shows similar features to promoters of highly regulated growth factor receptors and oncogenes. The potential role these transcriptional binding sites play in regulating rSSTR4 expression is currently being investigated.

Animals↗