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Biomedical subjects

A F Stewart

Publications and source records attributed to A F Stewart.

At least 73 records · Page 4Linked to original sources
PubMed1997

Opposing mitogenic and anti-mitogenic actions of parathyroid hormone-related protein in vascular smooth muscle cells: a critical role for nuclear targeting.

Parathyroid hormone-related protein (PTHrP) is a prohormone that is posttranslationally processed to a family of mature secretory forms, each of which has its own cognate receptor(s) on the cell surface that mediate the actions of PTHrP. In addition to being secreted via the classical secretory pathway and interacting with cell surface receptors in a paracrine/autocrine fashion, PTHrP appears to be able to enter the nucleus directly following translation and influence cellular events in an "intracrine" fashion. In this report, we demonstrate that PTHrP can be targeted to the nucleus in vascular smooth muscle cells, that this nuclear targeting is associated with a striking increase in mitogenesis, that this nuclear effect on proliferation is the diametric opposite of the effects of PTHrP resulting from interaction with cell surface receptors on vascular smooth muscle cells, and that the regions of the PTHrP sequence responsible for this nuclear targeting represent a classical bipartite nuclear localization signal. This report describes the activation of the cell cycle in association with nuclear localization of PTHrP in any cell type. These findings have important implications for the normal physiology of PTHrP in the many tissues which produce it, and suggest that gene delivery of PTHrP or modified variants may be useful in the management of atherosclerotic vascular disease.

Adaptor Proteins, Signal Transducing↗
PubMed1997

The Kw recombinase, an integrase from Kluyveromyces waltii.

Site-specific recombinases of the integrase family share limited amino-acid-sequence similarity, but use a common reaction mechanism to recombine distinct DNA target sites. Here we report the characterisation of the Kw site-specific recombinase, encoded on the 2 mu-like plasmid pKWS1 from the yeast Kluyveromyces waltii. Using in vitro-translated Kw recombinase, we show that the protein is able to bind and to recombine its putative DNA target site. Recombination is conservative and the Kw target site has a spacer of seven base pairs. We show that Kw recombinase is able to mediate recombination in a mammalian cell line, thus, it has potential for use as a tool for genomic manipulation in heterologous systems.

Cell Line↗
PubMed1997

Transfection and overexpression of the calcium binding protein calbindin-D28k results in a stimulatory effect on insulin synthesis in a rat beta cell line (RIN 1046-38).

Calbindin-D28k, a calcium binding protein that is thought to act as a facilitator of calcium diffusion in intestine and kidney, is known to be regulated by vitamin D in these tissues. Calbindin-D28k is also present in pancreatic beta cells, but its function in these cells is not known. To determine a role for calbindin-D28k in the beta cell, rat calbindin-D28k was overexpressed in the pancreatic beta cell line RIN 1046-38 by transfection of calbindin in expression vector, and changes in insulin mRNA were examined. Five transfected RIN cell clones were found to overexpress calbindin 6- to 35-fold as determined by radioimmunoassay. Northern blot analysis revealed increases in abundance in calbindin mRNA (>20-fold for most clones). Overexpressed calbindin was functional because it was capable of buffering calcium in response to a rapid calcium influx induced by 1 and 5 microM calcium ionophore. In cells transfected with calbindin, there was a marked increase in the expression of insulin mRNA (>20-fold for most clones compared with vector transfected cells). Besides an increase in insulin mRNA, calbindin overexpression was also associated with an increase in insulin content and release (a 5.8-fold increase in insulin release was noted for clone C10, and a 54-fold increase was noted for clone C2). To begin to address the mechanism whereby overexpression of calbindin results in increased insulin gene expression, calbindin-overexpressing clones were transiently transfected with plasmids incorporating various regions of the rat insulin I (rInsI) promoter linked to the chloramphenicol acetyltransferase coding sequence. Transient transfection with reporter plasmids bearing the regulatory sequences of the rInsI promoter (-345/+1) or five copies of the Far-FLAT minienhancer (-247/-198) from the rInsI promoter suggests that increased insulin mRNA in calbindin transfected cells is due, at least in part, to enhanced insulin gene transcription. These studies provide the first direct evidence (to our knowledge) for a role for calbindin in beta cell function.

Animals↗
PubMed1997

Chromatin structure analysis of the human c-fos promoter reveals a centrally positioned nucleosome.

Chromatin architecture plays an important regulatory role in nuclear transcription in eukaryotes. In particular, a preset chromatin organization has been proposed to prime rapidly inducible genes for response to extracellular signals. We analyzed the chromatin architecture of the rapidly inducible human c-fos gene using a combination of nuclease digestion studies. Several regions of nuclease sensitivity in the c-fos gene are observed: (i) a hypersensitive site at position -1900; (ii) a region centered at -350 that corresponds to the locations of the serum response element (SRE) and the sis-inducible element (SIE); (iii) a region around the transcriptional start site that includes the TATA box, the cAMP-responsive element (CRE) and a directly repeated sequence (DRE); and (iv) two sites at positions +250 and +550 that appear to delineate the linker regions of two positioned nucleosomes. In contrast, the region from -280 to -90 is strongly resistant to nuclease digestion. We identify a highly positioned nucleosome in this region of the c-fos promoter that may contribute to a defined higher-order nucleoprotein structure containing the nucleosome and proteins bound to the SIE, SRE, DRE, and CRE elements and TATA box. This structure may lead to a functional juxtaposition of the regulatory elements of the c-fos promoter and suggests that such a chromatin architecture is particularly suited for presetting promoters for rapid responsiveness.

Binding Sites↗
PubMed1997

Parathyroid hormone-related protein-(1-36) is biologically active when administered subcutaneously to humans.

PTH-related protein (PTHrP) is responsible for most cases of humoral hypercalcemia of malignancy (HHM). It mimics the actions of PTH as a result of its structural homology with PTH and its ability to bind to and signal via the PTH/PTHrP receptor in bone and kidney. PTHrP-(1-36) appears to be one of several secretory forms of PTHrP. This peptide has been administered iv to normal volunteers previously and has been shown to produce effects that are qualitatively and quantitatively the same as those produced by PTH-(1-34). To determine whether PTHrP-(1-36) could be used sc in humans as a diagnostic reagent for elucidating the differences between HHM and hyperparathyroidism, we performed a 12-h dose-finding study examining whether sc PTHrP-(1-36) could elicit effects on mineral homeostasis. PTHrP-(1-36) administered sc in three doses (0.82, 1.64, and 3.28 micrograms/kg) to 21 normal women produced increases in circulating PTHrP-(1-36), reductions in serum phosphorus and the renal phosphorus threshold, increments in fractional calcium excretion and nephrogenous cAMP excretion, and increases in plasma 1,25-dihydroxyvitamin D. These changes were highly significant in statistical terms and were observed at doses that had no effect on serum calcium or endogenous PTH. These studies demonstrate the feasibility of using PTHrP-(1-36) as a diagnostic probe for future studies aimed at elucidating the differing pathophysiologies of HHM and hyperparathyroidism.

Adult↗
PubMed1997

FLP recombinase/estrogen receptor fusion proteins require the receptor D domain for responsiveness to antagonists, but not agonists.

The ligand-binding domains of steroid receptors convey ligand-dependent regulation to certain proteins to which they are fused. Here we characterize fusion proteins between a site-specific recombinase, FLP, and steroid receptor ligand-binding domains. These proteins convert ligand binding into DNA recombination. Thus, ligand binding is directly coupled to an enzyme activity that is easily measured by DNA rearrangements or heritable genetic changes in marker gene expression, as opposed to the multiple events leading to transcription. Recombination by a FLP-estrogen receptor (FLP-EBD) fusion is activated by all tested estrogens, whether agonists or antagonists, indicating that all induce EBD release from the 90-kDa heat shock protein complex. Altering the distance between FLP and the EBD domain in the fusion proteins, by reducing the included length of the estrogen receptor D domain, affects ligand efficacy. A FLP-EBD with no D domain shows reduced inducibility by agonists and, unexpectedly, complete insensitivity to induction by all antagonists tested. A FLP-EBD including some D domain shows a ligand-inducible phenotype intermediate to those displayed by FLP-EBDs containing all or none of the D domain. Thus, we observed a tethered interference between FLP and the EBD domains that differs depending on the distance between the two domains, the conformations induced by agonists or antagonists, and which presents a previously undetectable distinction between estrogen agonists and antagonists in yeast.

Amino Acid Sequence↗
PubMed1996

Different thermostabilities of FLP and Cre recombinases: implications for applied site-specific recombination.

Genomic manipulations using site-specific recombinases rely on their applied characteristics in living systems. To understand their applied properties so that they can be optimally deployed, we compared the recombinases FLP and Cre in two assays. In both Escherichia coli and in vitro, FLP shows a different temperature optimum than Cre. FLP is more thermolabile, having an optimum near 30 degrees C and little detectable activity above 39 degrees C. Cre is optimally efficient at 37 degrees C and above. Consistent with FLP thermolability, recombination in a mammalian cell line mediated by a ligand- regulated FLP-androgen receptor fusion protein is more efficient at 35 degrees C than at higher temperatures. We also document a mutation in a commercially available FLP plasmid (FLP-F70L) which renders this recombinase even more thermolabile. The different temperature optima of FLP, FLP-F70L and Cre influence their strategies of usage. Our results recommend the use of Cre for applications in mice that require efficient recombination. The thermolabilities of FLP and FLP-F70L can be usefully exploited for gain of function and cell culture applications.

Amino Acid Sequence↗
PubMed1996

Structural and physiologic characterization of the mid-region secretory species of parathyroid hormone-related protein.

Parathyroid hormone-related protein (PTHrP) is initially translated as a preprohormone which is posttranslationally processed to yield a family of mature secretory forms. Most attention has focused on the amino-terminal portion of the molecule which is homologous to parathyroid hormone. It is clear, however, that a mid-region species of PTHrP is posttranslationally cleaved from the highly conserved mid-region of PTHrP, and that the amino terminus of this peptide is Ala38. The purposes of the current study were three: 1) to confirm that Arg37 immediately preceding Ala38 serves as a posttranslational processing site in the PTHrP precursor, 2) to determine the carboxyl terminus of the mid-region secretory species of PTHrP, and 3) to synthesize this authentic mid-region secretory form of PTHrP and determine whether it is biologically active. The results indicate that: 1) Arg37 is indeed a processing site in the PTHrP precursor; 2) three distinct mid-region PTHrP species are generated by posttranslational processing, PTHrP(38-94)amide, PTHrP(38-95), and most likely, PTHrP(38-101); and 3) synthetic mid-region PTHrP(38-94)amide is active in four different biological systems. These studies confirm the finding that PTHrP is a prohormone. More importantly, they define a novel, biologically active highly conserved mid-region secretory form of PTHrP.

Amino Acid Sequence↗
PubMed1996

Cloning of human RTEF-1, a transcriptional enhancer factor-1-related gene preferentially expressed in skeletal muscle: evidence for an ancient multigene family.

Transcriptional Enhancer Factor-1 (TEF-1) is a transcription factor required for cardiac muscle gene activation. Since ablation of TEF-1 does not abolish cardiac gene expression, we sought to identify a human gene related to TEF-1 (RTEF-1) that might also participate in cardiac gene regulation. A human heart cDNA library was screened to obtain a full-length RTEF-1 cDNA. Fluorescence in situ hybridization assigned the RTEF-1 gene to chromosome 12p13.2-p13.3. In contrast, PCR screening of human/rodent cell hybrid panels identified TEF-1 on chromosome 11p15.2, between D11S1315 and D11S1334, extending a region of known synteny between human chromosomes 11 and 12 and arguing for an ancient divergence between these two closely related genes. Northern blot analysis revealed a striking similarity in the tissue distribution of RTEF-1 and TEF-1 mRNAs; skeletal muscle showed the highest abundance of both mRNAs, with lower levels detected in pancreas, placenta, and heart. Phylogenetic analysis of all known TEF-1-related proteins identified human RTEF-1 as one of four vertebrate members of this multigene family and further suggests that these genes diverged in the earliest metazoan ancestors.

Amino Acid Sequence↗
PubMed1996

A simple assay to determine the functionality of Cre or FLP recombination targets in genomic manipulation constructs.

We report the construction of two Escherichia coli strains (294-Cre and 294-FLP) which express either Cre- or FLP-recombinase. Plasmids containing authentic recognition targets for either recombinase (loxPs or FRTs) are recombined when propagated in the appropriate strain. 294-Cre and 294-FLP thus provide a simple test for the recombination competence of constructs that are designed for use in Cre- or FLP-mediated genomic manipulations.

DNA Nucleotidyltransferases↗
PubMed1996

Excision of Ets by an inducible site-specific recombinase causes differentiation of Myb-Ets-transformed hematopoietic progenitors.

BACKGROUND: The Myb-Ets protein encoded by the E26 acute avian leukemia virus is a paradigm for the function of fused transcriptional activator oncoproteins. Myb-Ets transforms hematopoietic progenitor cells (myb-Ets progenitors, MEPs) that can be induced to differentiate into eosinophilic and myeloid cells by the activation of pathways involving Ras and/or protein kinase C. The Ets portion of the fusion protein seems to be required to maintain the multipotency of MEPs: MEPs transformed with a temperature-sensitive E26 mutant with a lesion in Ets (ts 1.1) and shifted to the non-permissive temperature predominantly form erythroid cells, but also form eosinophilic and myeloid cells. This interpretation is complicated, however, by the observation that ts 1.1-transformed MEPs differ from MEPs transformed with wild-type E26 in that they expressed erythroid and eosinophil markers even at the permissive temperature. RESULTS: In order to alleviate the problems associated with the use of temperature-sensitive mutants we have designed a vector that allows the inducible deletion of the Ets domain. To this end, we introduced FLP recombinase target sites into the E26 virus on the 5' and 3' sides of Ets and included within the same retroviral vector sequences encoding and estrogen-dependent FLP recombinase. This construct, termed FRV-3, is capable of transforming cells to produce a phenotype indistinguishable from that of MEPs obtained with wild-type virus. Hormone treatment of MEPs transformed with FRV-3 induced erythroid differentiation in a subpopulation of the cells; this subpopulation was found to have completely excised ets. However, in contrast to previous results obtained with ts 1.1-transformed MEPs, no differentiation along the eosinophilic and myeloid lineages was seen in hormone-treated FRV-3-transformed MEPs. CONCLUSIONS: Our results demonstrate the feasibility of using a site-specific recombinase to excise a fused oncoprotein domain encoded by a retrovirus. More specifically, they show that the Ets portion of the Myb-Ets protein selectively inhibits differentiation of MEPs along the erythroid lineage, and suggests that Ets is also required for their differentiation along the eosinophil and, possibly, myeloid lineages.

Animals↗
PubMed1996

Alpha1-adrenergic receptor subtype mRNAs are differentially regulated by alpha1-adrenergic and other hypertrophic stimuli in cardiac myocytes in culture and in vivo. Repression of alpha1B and alpha1D but induction of alpha1C.

The three cloned alpha1-adrenergic receptor (AR) subtypes, alpha1B, alpha1C, and alpha1D, can all couple to the same effector, phospholipase C, and the reason(s) for conservation of multiple subtypes remain uncertain. All three alpha1-ARs are expressed natively in cultured neonatal rat cardiac myocytes, where chronic exposure to the agonist catecholamine norepinephrine (NE) induces hypertrophic growth and gene transcription. We show here, using RNase protection, that the alpha1-AR subtype mRNAs respond in distinctly different ways during prolonged NE exposure (12 72 h). Alpha1B and alpha1D mRNA levels were repressed by NE, whereas alpha1C mRNA was induced. Changes in mRNA levels were mediated by an alpha1-AR, were not explained by altered mRNA stability, and were reflected in receptor proteins by [3H]prazosin binding. alpha1-AR-stimulated phosphoinositide hydrolysis and myocyte growth were not desensitized. Three other hypertrophic agonists in culture, endothelin-1, PGF2alpha, and phorbol 12-myristate 13-acetate, also induced alpha1C mRNA and repressed alpha1B mRNA. In myocytes from hearts with pressure overload hypertrophy, alpha1 mRNA changes were identical to those produced by NE in culture. These results provide the first example of a difference in regulation among alpha1-AR subtypes expressed natively in the same cell. Transcriptional induction of the alpha1C-AR could be a mechanism for sustained growth signaling through this receptor and is a common feature of a hypertrophic phenotype in cardiac myocytes.

Adrenergic alpha-Agonists↗
PubMed1996

Overexpression of parathyroid hormone-related protein in the pancreatic islets of transgenic mice causes islet hyperplasia, hyperinsulinemia, and hypoglycemia.

Parathyroid hormone-related protein (PTHrP) is produced by the pancreatic islet. It also has receptors on islet cells, suggesting that it may serve a paracrine or autocrine role within the islet. We have developed transgenic mice, which overexpress PTHrP in the islet through the use of the rat insulin II promoter (RIP). Glucose homeostasis in these mice is markedly abnormal; RIP-PTHrP mice are hypoglycemic in the postprandial and fasting states and display inappropriate hyperinsulinemia. At the end of a 24-hour fast, blood glucose values are 49 mg/dl in RIP-PTHrP mice, as compared to 77 mg/dl in normal littermates; insulin concentrations at this time are 6.3 and 3.9 ng/ml, respectively. Islet perifusion studies failed to demonstrate abnormalities in insulin secretion. In contrast, quantitative islet histomorphometry demonstrates that the total islet number and total islet mass are 2-fold higher in RIP-PTHrP mice than in their normal littermates. PTHrP very likely plays a normal physiologic role within the pancreatic islet. This role is most likely paracrine or autocrine. PTHrP appears to regulate insulin secretion either directly or indirectly, through developmental or growth effects on islet mass. PTHrP may have a role as an agent that enhances islet mass and/or enhances insulin secretion.

Animals↗
PubMed1996

PTHrP(1-36) as a skeletal anabolic agent for the treatment of osteoporosis.

Parathyroid hormone-related protein (1-36) [(PTHrP(1-36)] has been shown to be one of the authentic secretory forms of PTHrP. Human studies employing this peptide have demonstrated that it is equipotent to parathyroid hormone (1-34). This observation, together with the observations that: (1) both peptides employ the same receptors; (2) both bind to this PTH/PTHrP receptor with equal affinity and activate the PTH/PTHrP receptor intracellular signaling pathways with the same efficacy; (3) both peptides are effective anabolic skeletal agents in laboratory animals; and (4) PTHrP is likely to be the endogenous skeletal ligand for the PTH/PTHrP receptor, collectively suggest that PTHrP (1-36) should be as equally effective as PTH(1-34) as an anabolic skeletal agent. In this mini-review, the theoretical basis for the exploration and development of PTHrP(1-36) as a potential therapeutic agent for osteoporosis is considered.

Animals↗
PubMed1996

Parathyroid hormone-related protein detection and interaction with NO and cyclic AMP in the renovascular system.

The presence of parathyroid hormone-related protein (PTHrP) in human kidney vasculature and the signal transduction pathways stimulated during PTHrP-induced vasodilation of the rabbit kidney were investigated. Immunostaining of human kidney revealed the abundant presence of PTHrP in media and intima of all microvessels as well as in macula densa. In isolated perfused rabbit kidney preconstricted with noradrenaline, 10(-5) M Rp-cAMPS, a direct inhibitor of protein kinase A, produced comparable inhibition of 2.5 x 10(-7) M forskolin- and 10(-7) M PTHrP-induced vasorelaxations. Renal vasorelaxation and renal microvessel adenylyl cyclase stimulation underwent comparable desensitization following exposure to PTHrP. Nitric oxide (NO)-synthase inhibition by L-NAME (10(-4) M), NO scavenging by an imidazolineoxyl N-oxide (10(-4) M) and guanylyl cyclase inhibition by methylene blue (10(-4) M) decreased PTHrP-induced vasorelaxation by 27 to 53%, abolished bradykinin-induced vasorelaxation and did not affect forskolin-induced vasorelaxation. The effects of Rp-cAMPS and L-NAME were not additive on PTHrP-induced vasorelaxation. Damaging endothelium by treating the kidney with either anti-factor VIII-related antibody and complement, gossypol or detergent, did not affect PTHrP- or forskolin-induced vasorelaxations but reduced bradykinin-induced vasorelaxation by 53 to 92%. Conversely, endothelial damage did not alter the inhibitory action of L-NAME on PTHrP-induced vasorelaxation. In conclusion, PTHrP is present throughout the human renovascular tree and juxtaglomerular apparatus. Activation of both adenylyl cyclase/protein kinase A and NO-synthase/guanylyl cyclase pathways are directly linked to the renodilatory action of PTHrP in a way that does not require an intact endothelium in the isolated rabbit kidney.

Adenylyl Cyclases↗
PubMed1996

Defining the roles of parathyroid hormone-related protein in normal physiology.

Parathyroid hormone-related protein (PTHrP) was discovered as a result of a search for the circulating factor secreted by cancers which causes the common paraneoplastic syndrome humoral hypercalcemia of malignancy. Since the identification of the peptide in 1982 and the cloning of the cDNA in 1987, it has become clear that PTHrP is a prohormone that is posttranslationally cleaved by prohormone convertases to yield a complex family of peptides, each of which is believed to have its own receptor. It is also clear that the PTHrP gene is expressed not only in cancers but also in the vast majority of normal tissues during adult and/or fetal life. In contrast to the situation in humoral hypercalcemia of malignancy in which PTHrP plays the role of a classical "endocrine" hormone, under normal circumstances PTHrP plays predominantly paracrine and/or autocrine roles. These apparent physiological functions are also complex and appear to include 1) regulation of smooth muscle (vascular, intestinal, uterine, bladder) tone, 2) regulation of transepithelial (renal, placental, oviduct, mammary gland) calcium transport, and 3) regulation of tissue and organ development, differentiation, and proliferation. In this review, the discovery of PTHrP, the structure of its gene and its cDNAs, and the posttranslational processing of the initial translation products are briefly reviewed. Attention is then focused on a detailed organ system-oriented review of the normal physiological functions of PTHrP.

Amino Acid Sequence↗
PubMed1996

Parathyroid hormone (PTH)-related protein(1-36) is equipotent to PTH(1-34) in humans.

Humoral hypercalcemia of malignancy (HHM) results from the production of PTH-related protein (PTHrP) by human tumors. One previous study has reported the results of human (h) PTHrP(1-34) infusion into humans. In that report, hPTHrP(1-34) was found to be qualitatively similar to but 3- to 10-fold less potent than hPTH(1-34). Because hPTHrP(1-36) and not hPTH(1-34) is likely to be the actual amino-terminal secretory form of PTHrP, and because this previously reported lack of potency was unexpected, we repeated these studies using hPTHrP(1-36) and compared the results with those obtained with hPTH(1-34). Healthy subjects (n = 30) were infused over 6 h with either vehicle alone, hPTH(1-34) at a dose of 8 pmol/kg.h, or hPTHrP(1-36) at doses of 8 or 80 pmol/kg.h. Both hPTH(1-34) and hPTHrP(1-36) caused an increase in serum ionized calcium, a decrease in serum phosphorus, an increase in the fractional excretion of phosphorus, a decrease in the tubular maximum for phosphorus, an increase in nephrogenous cAMP excretion, and suppression of endogenous PTH(1-84). Unlike events observed in HHM, hPTHrP(1-36) induced an increase in plasma 1,25-dihydroxyvitamin D2. In addition, fractional excretion of calcium was reduced by both hPTH(1-34) and hPTHrP(1-36). In their actions on serum calcium, renal calcium and phosphorus handling, and nephrogenous cAMP excretion, hPTHrP(1-36) and hPTH(1-34) appeared equivalent in potency. These studies indicate that short-term infusion of hPTHrP(1-36) into humans reproduces most but not all of the features of HHM. In contrast to the reported findings with hPTHrP(1-34), we found the potency of hPTHrP(1-36) to be comparable with that of hPTH(1-34) in vivo in humans. In addition, unlike the situation in HHM, hPTHrP(1-36) produces an increment in plasma 1,25-dihydroxyvitamin D2. Finally, hPTHrP(1-36) has been shown for the first time to have anticalciuric effects in humans. This would suggest that, in addition to osteoclastic bone resorption, tubular reabsorbtion of calcium by hPTHrP may contribute to the hypercalcemia in patients with HHM.

Adult↗