PubMed Health⌕ Search

Biomedical subjects

L S Mathews

Publications and source records attributed to L S Mathews.

33 records · Page 2Linked to original sources
PubMed1992

Cloning of a second type of activin receptor and functional characterization in Xenopus embryos.

A complementary DNA coding for a second type of activin receptor (ActRIIB) has been cloned from Xenopus laevis that fulfills the structural criteria of a transmembrane protein serine kinase. Ectodermal explants from embryos injected with activin receptor RNA show increased sensitivity to activin, as measured by the induction of muscle actin RNA. In addition, injected embryos display developmental defects characterized by inappropriate formation of dorsal mesodermal tissue. These results demonstrate that this receptor is involved in signal transduction and are consistent with the proposed role of activin in the induction and patterning of mesoderm in Xenopus embryos.

Activin Receptors↗
PubMed1992

Effect of activin A on globin gene expression in purified human erythroid progenitors.

The regulatory control of human erythropoiesis through a purified protein, activin A, was examined. Previous studies using mixed populations of bone marrow cells suggested that activin A has an indirect effect on cellular proliferation and DNA synthesis of erythroid progenitors through the mediation of accessory cells. In present studies, the cultures of purified erythroid progenitors were used to examine the effect of activin A on globin gene expression. Human erythroid burst-forming units (BFU-E) were partially purified from peripheral blood, and after 8 days of culture the cells generated consisted mainly of erythroid colony-forming units (CFU-E). It was found that the subsequent 7-day cultures of these purified progenitors yielded similar numbers and size distributions of erythroid colonies, regardless of the presence of activin A in the cultures. In addition, these erythroid progenitor cells were responsive, in terms of stimulation of DNA synthesis, to the addition of erythropoietin, but not to treatment by activin A. Therefore, once the erythroid progenitors are depleted of accessory cells, activin A has little effect on both the proliferation and the DNA synthesis of these progenitors. However, when these purified erythroid progenitors were cultured in the presence of activin A, the levels of all alpha, beta, and epsilon globin transcripts and hemoglobins were significantly increased. In addition, disuccinimidyl suberate was found to chemically cross-link 125I-activin A to cell surface binding proteins (45 to 54 Kd) in both purified erythroid progenitors and K562 cells. The labeling of these binding proteins was specifically inhibited by the presence of unlabeled activin A, but not transforming growth factor-beta. These results suggest that, in addition to its indirect effect on DNA synthesis and cellular proliferation of erythroid progenitors, activin A directly affects the levels of globin mRNAs and hemoglobins in developing human erythroid cells through its specific surface binding receptor(s).

Activins↗
PubMed1991

Expression cloning of an activin receptor, a predicted transmembrane serine kinase.

Activins are involved in the regulation of multiple biological events, ranging from early development to pituitary function. To characterize the cellular mechanisms involved in these processes, cDNAs coding for an activin receptor were cloned from AtT20 mouse corticotropic cells by screening COS cell transfectants for binding of 125I-activin A. The cDNAs code for a protein of 494 amino acids comprising a ligand-binding extracellular domain, a single membrane-spanning domain, and an intracellular kinase domain with predicted serine/threonine specificity. 125I-activin A binds to transfected COS cells with an affinity of 180 pM and can be competed by activin A, activin B, and inhibin A, but not by transforming growth factor beta 1. The kinase domain, but not the extracellular sequence, of the activin receptor is most closely related to the C. elegans daf-1 gene product, a putative transmembrane serine/threonine-specific protein kinase for which the ligand is not known.

Activin Receptors↗
PubMed1990

Introduction of exogenous growth hormone receptors augments growth hormone-responsive insulin biosynthesis in rat insulinoma cells.

The stimulation of insulin biosynthesis in the pancreatic insulinoma cell line RIN5-AH by growth hormone (GH) is initiated by GH binding to specific receptors. To determine whether the recently cloned rat hepatic GH receptor is able to mediate the insulinotropic effect of GH, we have transfected a GH receptor cDNA under the transcriptional control of the human metallothionein promoter into RIN5-AH cells. The transfected cells were found to exhibit an increased expression of GH receptors and to contain a specific GH receptor mRNA that was not expressed in the parent cell line. The expression of GH receptors in one clone (1.24) selected for detailed analysis was increased 2.6-fold compared to untransfected cells. The increased GH receptor expression was accompanied by an increased responsiveness to GH. Thus, the maximal GH-stimulated increase of insulin biosynthesis was 4.1-fold in 1.24 cells compared to 1.9-fold in the nontransfected RIN5-AH cells. The expression of the transfected receptor was stimulated 1.6- and 2.3-fold when cells were cultured in the presence of 25 or 50 microM Zn2+, respectively. The increased expression of the GH receptor by Zn2+ was associated with an increased magnitude of GH-stimulated insulin biosynthesis. A close stoichiometric relationship between the level of receptor expression and the level of GH-stimulated insulin biosynthesis was observed. We conclude from these results that the hepatic GH receptor is able to mediate the effect of GH on insulin biosynthesis in RIN5-AH cells.

Animals↗
PubMed1990

Growth hormone (GH) stimulates protein synthesis in cells transfected with GH receptor complementary DNA.

An expression vector containing a rat GH receptor cDNA was transfected into Chinese hamster ovary (CHO) cells, and stable cell lines expressing GH receptors were established. In contrast to nontransfected CHO cells, expression of GH receptors in transfected cells resulted in the appearance of high affinity (Kd = 1.53 nM) specific binding of GH. Cross-linking of [125I]hGH to the receptors and subsequent sodium dodecyl sulfate (SDS)-electrophoresis gave an estimated receptor mol wt of 84,000. GH treatment stimulated protein synthesis 60% over basal levels in GH receptor-expressing CHO cells, but not in the receptor-negative parental cells. The effect was observed only under serum-free conditions and was time and dose dependent. These results show that heterologous expression of the rat GH receptor results in the appearance of specific binding of GH and the acquisition of a functional GH response.

Animals↗
PubMed1989

Regulation of rat growth hormone receptor gene expression.

A cDNA encoding the growth hormone (GH) receptor was cloned from rat liver. Both the nucleotide and translated amino acid sequence share greater than 70% similarity with the GH receptors from rabbit and human. An RNA probe was generated from this sequence for use in a solution hybridization assay to quantitate GH receptor mRNA expression in rat tissues. Expression was detected in 9/12 tissues examined, with the highest levels observed in the liver. Expression in liver, kidney, heart and muscle was developmentally regulated, being low at birth and rising to adult levels in 5 weeks. No difference was observed between hepatic expression in males and females, although livers from pregnant rats had elevated levels. Hypophysectomy and GH treatment did not affect hepatic GH receptor mRNA levels.

Aging↗
PubMed1989

Histopathology associated with elevated levels of growth hormone and insulin-like growth factor I in transgenic mice.

Serum levels of GH and insulin-like growth factor I (IGF-I) were genetically increased to investigate the physiological activities of these proteins. Lines of mice expressing chimeric genes composed of bovine GH, human GRF, or human IGF-I coding sequences fused to the mouse metallothionein I promoter were examined for consequences of chronic exposure to high levels of these peptides. Animals with elevated serum levels of GH (either bovine GH or mouse GH) have selective splanchnomegaly coupled with glomerular sclerosis and hepatocellularmegaly. Serum levels of insulin and cholesterol are increased. In contrast (with the exception of selective enlargement of organs), the chronic expression of IGF-I results in a different pattern of abnormalities. These findings suggest that the pathogenesis of GH-related disorders is not mediated solely by IGF-I.

Animals↗
PubMed1989

Regulation of insulin-like growth factor-I and growth hormone receptor gene expression by diabetes and nutritional state in rat tissues.

Insulin-like growth factor-I (IGF-I) mRNA and GH receptor mRNA levels were analysed in different tissues from rats made diabetic with streptozotocin, fasted rats and rats fed with a protein-reduced diet. Diabetes decreased IGF-I mRNA levels in liver, heart, diaphragm, kidney and aorta, but not in brain. GH receptor mRNA levels were decreased in heart and diaphragm, but not in liver and kidney. Fasting decreased IGF-I mRNA in all tissues studied except brain, and decreased GH receptor mRNA in liver, heart and diaphragm, but not in kidney. A protein-reduced diet decreased hepatic IGF-I mRNA levels but did not significantly affect other tissues, while GH receptor mRNA levels were reduced in liver and diaphragm. In conclusion, both diabetes and limited nutrition affected IGF-I and GH receptor mRNA in different tissues, but the two mRNAs were not co-ordinately regulated in all tissues studied. While reduced GH receptor gene expression may thus be responsible for decreased IGF-I gene expression in some states and tissues, additional regulatory mechanisms may be of importance.

Animals↗
PubMed1988

Dwarf mice produced by genetic ablation of growth hormone-expressing cells.

Fusion of the 310 bp located 5' of the rat growth hormone (GH) gene to the human GH structural gene resulted in somatotrope-specific expression in transgenic mice. Human GH transcripts were detected only in pituitaries of these mice, and immunocytochemical analyses revealed that this expression was limited to GH-expressing cell types. The rat GH 5' sequences were then used to direct the expression of diphtheria toxin to the GH-expressing cells of transgenic mice. A line of mice was established which lacks detectable levels of circulating GH. This deficiency resulted in dwarfism; transgenic animals grew only to half the size of nontransgenic littermates. Nearly all somatotropes were absent, as shown by GH immunostaining in the transgenic pituitaries. Prolactin (PRL)-producing lactotropes, thought to share a common cellular origin with somatotropes, were also reduced in numbers. A model for the lineal relationships between GH- and PRL-synthesizing cells is proposed.

Animals↗
PubMed1988

Regulation of insulin-like growth factor messenger ribonucleic acid in rat growth plate by growth hormone.

The mechanism of action for the stimulatory effect of GH on longitudinal bone growth is not yet clarified. Several recent reports indicate that GH has a direct effect at the site of the epiphyseal growth plate, as opposed to the somatomedin hypothesis which holds that the effect of GH is mediated by circulating insulin-like growth factors (IGFs). Using a RNA probe in a solution hybridization assay we investigated the presence of IGF-I mRNA in rat rib growth plate. Hypophysectomy resulted in a decrease in the number of IGF-I mRNA copies compared to that in normal rats. Replacement treatment with GH restored the number of transcripts in a specific and dose-dependent manner. The results show that GH regulates the level of IGF-I mRNA in rat rib growth plate and give further support to the hypothesis that locally produced IGF-I might contribute to the stimulatory effect of GH on longitudinal bone growth.

Actins↗
PubMed1988

Expression of insulin-like growth factor I in transgenic mice with elevated levels of growth hormone is correlated with growth.

To study the role of insulin-like growth factor I (IGF-I) in mediating the growth-promoting function of GH, IGF-I expression was assayed in transgenic mice carrying either GH or GRF fusion genes. These mice exhibit enhanced growth as a result of high level ectopic expression of GH and have 2-fold elevation of both hepatic IGF-I mRNA levels and circulating IGF-I levels. Hepatic IGF-I mRNA levels are low at birth regardless of GH levels; they increase approximately 10-fold during postnatal development and become GH inducible 2 weeks after birth. The ontogeny of circulating IGF-I is similar. Accelerated growth in transgenic mice with GH fusion genes commences 3 weeks after birth despite high circulating GH levels at least as early as birth. In addition, endogenous mouse GH-secreting somatotroph cells in the pituitary are present in normal numbers at 3 weeks, but are undetectable 4 weeks after birth. Because the time at which IGF-I expression becomes GH responsive slightly precedes both the initiation of accelerated growth and the time when endogenous GH disappears, we conclude that IGF-I is directly involved in mediating the GH signal and that delayed induction of IGF-I gene expression is responsible, at least in part, for the delayed onset of other GH-responsive events.

Aging↗
PubMed1988

Growth enhancement of transgenic mice expressing human insulin-like growth factor I.

A line of transgenic mice carrying a chimeric gene composed of human insulin-like growth factor I (IGF-I) coding sequences fused to the mouse metallothionein I promoter was generated to study the effects of chronically elevated exposure to IGF-I. Mice in this line overexpress IGF-I in most tissues studied and have circulating IGF-I levels 1.5 times the normal value. This results in a growth response manifested by a 1.3-fold increase in weight as a result of selective organomegaly without an apparent increase in skeletal growth. In addition, expression of the endogenous GH and IGF-I genes is inhibited. These results are consistent with IGF-I playing an important role in the control of somatic growth.

Animals↗
PubMed1986

Regulation of insulin-like growth factor I gene expression by growth hormone.

Recombinant clones containing exon 3 of the insulin-like growth factor I (IGF-I) gene were isolated from a mouse genomic library. These sequences were used to generate an RNA probe, which was used in a solution hybridization assay to quantitate IGF-I mRNA in various murine tissues as a function of growth hormone status. The liver is the major site of IGF-I synthesis and the level of IGF-I mRNA is regulated about 10-fold by growth hormone in the growth hormone-deficient lit/lit mouse. Nuclear run-on assays were used to show that growth hormone regulation is manifested in part at the transcriptional level. Growth hormone also affects the size distribution of hepatic IGF-I mRNAs. Pancreas showed the highest non-hepatic expression, but every tissue analyzed contained some IGF-I mRNA. Expression was not growth hormone-dependent in most non-hepatic tissues.

Animals↗
PubMedDate not supplied

Role of calcium and arachidonic acid metabolites in creatine kinase release from isolated rat skeletal muscles damaged by organic cosolvents.

The objective of the present work is to provide additional experimental evidence which can help to differentiate between a direct solubilization mechanism versus an intracellular calcium mechanism of organic cosolvent-induced creatine kinase release from isolated rat skeletal muscles. An increase in cytosolic free calcium levels following exposure to propylene glycol was evaluated indirectly through the measurement of skeletal muscle glycogen levels. Skeletal muscle glycogen levels decreased significantly in propylene glycol-treated muscles compared to normal saline-treated controls; providing further evidence for the possible role of increased cytosolic calcium in organic cosolvent-induced muscle damage. The possible involvement of prostaglandins and leukotrienes which disrupt membrane integrity leading to propylene glycol-induced creatine kinase release was investigated via the use of cyclo-oxygenase and lipoxygenase inhibitors. Organic cosolvent-induced creatine kinase release was not significantly reduced when the muscles were incubated in the presence of cyclo-oxygenase inhibitors, lipoxygenase inhibitors, or a combination of a cyclo-oxygenase and lipoxygenase inhibitor. These findings suggest that in this experimental system prostaglandins and leukotrienes do not seem to be involved in organic cosolvent-induced alterations in sarcolemma integrity leading to the release of creatine kinase.

Animals↗