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C T Roberts

Publications and source records attributed to C T Roberts.

At least 91 records · Page 5Linked to original sources
PubMed1994

Role of the carboxyl-terminal domains of the insulin and insulin-like growth factor I receptors in receptor function.

The insulin and insulin-like growth factor I receptors (IR and IGF-IR, respectively) are heterotetrameric tyrosine kinases consisting of two extracellular ligand-binding alpha subunits and two transmembrane catalytic beta subunits. A number of lines of evidence have suggested that the IR and IGF-IR differ with respect to their ability to elicit mitogenic versus metabolic events upon activation by cognate ligands. To ascertain the contribution of the poorly conserved carboxyl-terminal domains to the differential functioning of the IR and IGF-IR, we have constructed receptor chimeras in which the carboxyl-terminal domain of one receptor was fused to the remainder of the heterologous receptor. The responses of a number of parameters after ligand stimulation were examined in stably transfected NIH-3T3 cells expressing the chimeric receptors or the analogous wild-type receptor sequence. Replacement of the IR carboxyl terminus with that of the IGF-IR severely affected insulin-stimulated responses, whereas substitution of the carboxyl terminus of the IGF-IR with that of the IR had a minimal effect. These data suggest that the carboxyl-terminal domains of the IR and IGF-IR are not interchangeable and that the mitogenic activity of the IR can be influenced by sequences present in the carboxyl-terminal domain. The analogous functions of the IGF-IR, on the other hand, do not appear to be greatly affected by the presence of the IR carboxyl-terminal domain.

3T3 Cells↗
PubMed1994

Transcriptional repression of the insulin-like growth factor I receptor (IGF-I-R) gene by the tumor suppressor WT1 involves binding to sequences both upstream and downstream of the IGF-I-R gene transcription start site.

The insulin-like growth factor-I receptor (IGF-I-R) has been implicated in the etiology and/or progression of Wilms' tumor, a pediatric malignancy of the kidney that is often associated with deletion or mutation of the WT1 tumor suppressor gene. The expression of the IGF-I-R gene is increased in Wilms' tumor as compared with normal kidney tissue. Furthermore, the levels of IGF-I-R mRNA in individual tumors have been shown to be inversely correlated to the levels of WT1 mRNA, suggesting that the expression of the IGF-I-R gene is under the negative control of WT1. The activity of an IGF-I-R promoter/luciferase construct in Chinese hamster ovary cells was reduced by cotransfection of a WT1 expression vector. An analysis of various reporter constructs containing different portions of the IGF-I-R 5'-flanking and 5'-untranslated regions suggested that the effect of WT1 depends on the number of WT1 binding sites present, with sites located both upstream and downstream of the IGF-I-R transcription start site involved in mediating this effect. Using the purified zinc finger domain of WT1 in gel retardation and DNase I footprinting assays, we mapped five sites in the 5'-flanking and six sites in the 5'-untranslated regions that were involved in WT1 binding. In addition, the initiator element of the IGF-I-R gene contains a sequence that binds WT1. Thus, the repression of IGF-I-R promoter activity by the WT1 tumor suppressor gene product involves multiple interactions of its zinc finger domain with WT1 binding sites located both 5' and 3' of the transcription initiation site.

Alternative Splicing↗
PubMed1994

Elevated protein tyrosine phosphatase activity and increased membrane viscosity are associated with impaired activation of the insulin receptor kinase in old rats.

Insulin resistance is very common in the elderly, and may be associated with glucose intolerance or frank diabetes. In previous studies we demonstrated that insulin resistance in old Wistar rats is associated with decreased autophosphorylation and activation of the hepatic insulin receptor kinase (IRK) in vivo. We now show that this defect can be reproduced in vitro, where the extent of insulin-induced activation of IRK in liver membranes of old rats was decreased by approximately 50% compared with young controls. The defect could be largely abolished after solubilization of the membranes with Triton X-100. We also show that: (a) the viscosity of membranes from the old rats was significantly (P < 0.001, n = 4) higher (by 15%) compared with young controls; (b) incubation of plasma membranes from old animals with lecithin liposomes, which lowered their cholesterol levels, partially abolished the defect in IRK activation; and (c) Triton extracts of liver membranes prepared from old rats did not interfere with the activation of IRK derived from young controls. Additionally, non-membrane components did contribute to the development of this defect. We observed a significant (approximately 30%) (P < 0.001, n = 18) elevation of cytosolic protein tyrosine phosphatase (PTP) activity directed against the beta subunit of the insulin receptor in livers of old rats. No such elevation of PTP activity could be demonstrated with synthetic substrates. Our findings are consistent with a model in which increased membrane viscosity as well as enhancement of a cytosolic PTP activity both markedly inhibit the activation in vivo of the hepatic IRK in old animals.

Aging↗
PubMed1994

Embryonic-maternal cell interactions at implantation in the fat-tailed dunnart, a dasyurid marsupial.

BACKGROUND: In marsupials implantation occurs about two-thirds the way through the short gestation before which time the embryo is surrounded by the permeable shell membrane which prevents physical contact between the trophoblast and uterine epithelium. Although the trophoblast has been shown to be invasive to varying degrees in several species of marsupials, the ultrastructure of the embryonic-uterine cell interactions at the time of implantation has not been described in this group. METHODS: Thick plastic sections and transmission electron microscopy were employed to investigate the cellular interactions at implantation in the fat-tailed dunnart (Sminthopsis crassicaudata), a dasyurid Australian marsupial. RESULTS: Our results show that epithelial penetration begins when the embryo is at the late presomite/early somite stage. In the trilaminar region of the yolk sac (TYS), trophoblast cells adjacent to the embryo form desmosomes with uterine epithelial cells and also appear to fuse with them to form hybrid cells, the cytoplasm of which resembles that of trophoblast. Later in the TYS, as the placenta develops, trophoblast microvilli and larger cell processes invaginate, and interdigitate with, the highly folded maternal epithelium but do not invade it. At this time in the bilaminar, or avascular, yolk sac (BYS), multinucleate trophoblast giant cells (TGCs) from an annular region adjacent to the sinus terminalis intrude between, and possibly fuse with, the maternal epithelium. The invading TGCs spread laterally above the residual basal lamina before migrating into the stroma. CONCLUSIONS: In this species of marsupial at least, the cell interactions at the time of implantation are similar to those seen in some eutherian species despite the fact that the fetal chorion is of yolk sac rather than allantoic origin.

Animals↗
PubMed1994

Insulin-like growth factor-binding protein enhancement of insulin-like growth factor-I (IGF-I)-mediated DNA synthesis and IGF-I binding in a human breast carcinoma cell line.

The insulin-like growth factors (IGFs) are potent mitogens for malignant cell proliferation. The majority of secreted IGFs are bound to specific IGF-binding proteins (IGFBPs) that are secreted by a large number of cells. These proteins may either inhibit or enhance IGF actions. Breast carcinoma cells secrete a variety of IGFBPs. We have previously demonstrated that retinoic acid (RA) inhibition of IGF-I-stimulated MCF-7 cell proliferation is associated with increased IGFBP-3 levels in the conditioned media. We therefore investigated the effect of recombinant IGFBP-3 as well as IGFBP-2, -4 and -5 on IGF-I stimulation of DNA synthesis and IGF-I binding in the MCF-7 human breast carcinoma cell line. IGFBP-2 and -3 enhanced IGF-I stimulation of DNA synthesis in MCF-7 cells while IGFBP-4 and -5 had no effect. Transfection of MCF-7 cells with an IGFBP-3 expression vector resulted in the enhanced secretion of IGFBP-3 with an accompanying increase in IGF-I binding as well as increased cell proliferation upon treatment of the cells with IGF-I. IGF-I preincubation of MCF-7 cells transfected with control pSVneo plasmids results in cells refractory to further IGF-I stimulation of thymidine incorporation while IGF-I continues to stimulate [3H]-thymidine incorporation in IGFBP-3-transfected MCF-7 cells, suggesting that IGFBP-3 protects the cells from IGF-I-mediated down regulation of its receptor. Therefore, IGFBP-3 secreted by MCF-7 cells can enhance IGF-I stimulation of DNA synthesis, increase IGF-I binding to these cells, and prevent IGF-I-induced desensitization of its own receptor, suggesting that IGFBP-3 plays a significant role in IGF-I-mediated breast carcinoma proliferation.

Breast Neoplasms↗
PubMed1994

Platelet-derived growth factor increases the activity of the promoter of the insulin-like growth factor-1 (IGF-1) receptor gene.

Stimulation by platelet-derived growth factor (PDGF) is known to increase the number of IGF-I binding sites in cells in culture. We show here that PDGF also increases the levels of IGF-1 receptor mRNA. Using cell lines stably transfected with an expression plasmid in which the reporter luciferase gene is under the control of the rat IGF-1 receptor gene promoter, we find that PDGF increases the activity of this promoter. A short IGF-1 receptor gene promoter, comprising about 100 base pairs of the sequence immediately upstream of the initiation of transcription site, is sufficient for a response to the stimulatory action of PDGF. These results suggest that an increase in RNA levels and in promoter activity may play an important role in the increase in IGF-1 receptor levels that occurs after stimulation by PDGF.

3T3 Cells↗
PubMed1994

Growth hormone (GH) and insulin-like growth factor-I (IGF-I) treatment of the GH-deficient dwarf rat: differential effects on IGF-I transcription start site expression in hepatic and extrahepatic tissues and lack of effect on type I IGF receptor mRNA expression.

The rat IGF-I gene consists of six exons, with exons 3 and 4 forming a 'core' mature IGF-I coding region to which alternate 5' and 3' regions are spliced. Transcription occurs from four dispersed start sites (ss) approximately 382 (ss 1), approximately 343 (ss 2), approximately 245 (ss 3) and approximately 30-40 (ss 4) basepairs (bp) from the 3' end of exon 1, and from a region 50-70 bp from the 3' end of exon 2. The expression of ss mRNAs displays tissue-specific and ontogenic regulation. Alternate splicing of exon 5 produces E-peptide coding domain variants (Ea and Eb mRNAs), with the Eb form found predominantly in the liver. The regulation of IGF-I mRNA expression by GH and IGF-I in the GH-deficient dwarf (dw/dw) rat was investigated using antisense RNA probes in a solution hybridization RNase protection assay to detect leader exon and E domain variant mRNAs. GH treatment of dw/dw and normal Lewis rats increased the expression of all liver leader exon ss and E domain variants coordinately (1.6-1.9-fold increase, p < 0.01), although the increase observed in Eb transcripts was significantly higher in the dw/dw compared to the normal rat (p < 0.05). In kidney, GH treatment significantly increased exon 1 ss 3 and ss 4 transcripts by approximately 40% (p < 0.05). The expression of the other start sites was not affected by GH, suggesting that transcription factors may regulate start site usage independently. GH treatment was associated with a significant increase in IGF-I mRNA expression in skeletal muscle (p < 0.05) but not cardiac muscle or spleen. IGF-I treatment was associated with minor (approximately 20%) but significant (p < 0.05) reductions in IGF-I mRNA expression in the liver and kidney of dw/dw rats, suggesting that IGF-I can suppress IGF-I mRNA expression. IGF-I treatment did not affect IGF-I mRNA expression in cardiac and skeletal muscle of dw/dw rats. IGF-I receptor mRNA was detected in extrahepatic tissues only, and was not affected by either GH or IGF-I treatment. In summary, start site-specific regulation by GH was observed in kidney. GH increased IGF-I mRNA expression in muscle, kidney and liver, but had no effect in heart or spleen in the dw/dw rat. Our data suggest that systemic IGF-I can feedback on hepatic and renal IGF-I mRNA expression in the GH-deficient state.

Animals↗
PubMed1994

Growth hormone (GH) stimulates insulin-like growth factor-I (IGF-I) and IGF-binding protein (IGFBP)-2 gene expression in spleens of juvenile rats.

Growth and development of the spleen involves the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis. To evaluate the molecular mechanism of these effects we studied the effect of hypophysectomy (Hx) and GH replacement therapy on the expression of IGF-I, the IGF-I receptor and IGF-binding protein-2 (IGFBP-2) in juvenile rats. Hx resulted in a 30% reduction in body weight. GH replacement therapy for seven days partially prevented these effects. IGF-I mRNA levels were reduced 30% by Hx, IGFBP-2 mRNA levels fell 50% whereas IGF-I receptor mRNA levels were unaffected. GH therapy prevented the reduction in IGF-I and IGFBP-2 mRNA levels. These results suggest that the GH effect on splenic growth and development is via local (paracrine) IGF-I expression, in addition to any effect by circulating (endocrine) IGF-I.

Animals↗
PubMed1994

Characterization of a salmon insulin-like growth factor I promoter.

We have identified four transcription initiation sites in the salmon insulin-like growth factor I (IGF-I) gene. Use of the most upstream transcription start site generates a minor mRNA species with a 5' untranslated region (UTR) of approximately 450-550 nucleotides, whereas transcription starting at the downstream initiation sites results in more abundant IGF-I mRNAs with 5'-UTRs approximately 250, 245, and 165 nucleotides in length. No consensus TATA box-like elements are present immediately upstream of the most upstream start site identified, nor is this region particularly GC-rich. Transient expression assays, however, demonstrated orientation-dependent promoter activity in a 386-nucleotide-long fragment containing the major downstream transcription start sites. Additionally, reverse transcriptase polymerase chain reaction (RT-PCR) analyses demonstrated tissue and developmental stage-specific use of the various transcription start sites identified.

Animals↗
PubMed1994

Isolation of a second nonallelic insulin-like growth factor I gene from the salmon genome.

We have characterized a second nonallelic insulin-like growth factor-I (IGF-I) gene in the chum salmon (Oncorhynchus keta) genome. This gene, IGF-I.2, differs from the previously described chum salmon IGF-I gene, IGF-I.1, in the E peptide-coding portion of exon 3; specifically, the IGF-I.2 gene lacks one codon present in the IGF-I gene and contains two potential splice donor sites at the 3' end of exon 3 rather than the single, more distal site present in the IGF-I.1 gene. The expression of these two IGF-I genes could give rise to as many as six IGF-I mRNA species, each of which would encode a unique E-peptide moiety of the IGF-I prohormone. Thus, the presence of multiple, distinct IGF genes adds an additional level of complexity to IGF-I gene expression and IGF-I biosynthesis in salmon.

Alleles↗
PubMed1994

Expression of the genes for insulin-like growth factors and their receptors in bone during skeletal growth.

Insulin-like growth factors (IGF) are important regulators of skeletal growth. To determine whether the capacity to produce and respond to these growth factors changes during skeletal development, we measured the protein and mRNA levels for IGF-I, IGF-II, and their receptors (IGF-IR and IGF-IIR, respectively) in the tibia and femur of rats before and up to 28 mo after birth. The mRNA levels remained high during fetal development but fell after birth, reaching a nadir by 3-6 wk. This fall was most pronounced for IGF-II and IGF-IIR mRNA and least pronounced for IGF-I mRNA. However, after 6 wk, both IGF-I and IGF-IR mRNA levels recovered toward the levels observed at birth. In the prenatal bones, the signals for the mRNAs of IGF-II and IGF-IIR were stronger than the signals for the mRNAs of IGF-I and IGF-IR, although the content of IGF-I was three- to fivefold greater than that of IGF-II. IGF-II levels fell postnatally, whereas the IGF-I content rose after birth such that the ratio IGF-I/IGF-II continued to increase with age. We conclude that, during development, rat bone changes its capacity to produce and respond to IGFs with a progressive trend toward the dominance of IGF-I.

Aging↗
PubMed1994

Insulin-like growth factor I produces renal hyperfiltration by a kinin-mediated mechanism.

Insulin-like growth factor-I (IGF-I) infusion into rats and humans reduces renal vascular resistance and raises glomerular filtration rate (GFR) and renal plasma flow (RPF). To investigate whether kinins mediate the renal vasodilatory effects of IGF-I, we infused rats with IGF-I alone or in the presence of a B2 kinin receptor antagonist. Left kidney GFR, RPF, and kinin excretion were measured during infusion of vehicle and subsequently during 60-min infusion of IGF-I or IGF-I plus kinin antagonist. IGF-I was given as a bolus (150 micrograms/kg body wt), followed by infusion at a rate of 8.3 micrograms.kg-1 x min-1 for 60 min. The kinin antagonist was infused at a dose of 1 microgram.kg-1 x min-1 for 60 min before the start of IGF-I infusion. GFR and RPF increased significantly after IGF-I infusion was begun, from baseline levels of 1.70 +/- 0.12 and 6.21 +/- 0.34 to 2.12 +/- 0.11 and 7.91 +/- 0.29 ml/min, respectively, at 20 min (P < 0.001). This effect was maintained throughout 60 min of infusion. The increase in GFR and RPF was associated with a marked rise in urinary kinin excretion, from a baseline of 8.51 +/- 6.7 to 24.7 +/- 6.7 pg/min at 20 min and 40.3 +/- 10.4 pg/min at 40 min (P < 0.001). Pretreatment with the kinin receptor antagonist blocked the rise in GFR and RPF in response to IGF-I. These data suggest that the renal vasodilatory effect of IGF-I is mediated by kinins.

Animals↗
PubMed1994

Effect of training and growth hormone suppression on insulin-like growth factor I mRNA in young rats.

The growth hormone (GH)-insulin-like growth factor I (IGF-I) axis plays a role in the adaptation to exercise training, but IGF-I gene expression in response to exercise training and GH suppression has not been studied. Twenty female rates underwent a 4-wk treadmill training program begun in the prepubertal period (day 14 of life). In 10 of the training rats, GH production was suppressed by anti-GH-releasing hormone antibodies (GH suppressed). IGF-I mRNA and protein levels were measured in liver and hindlimb skeletal muscle. GH suppression reduced IGF-I mRNA expression in the liver to a much greater extent than in the muscle. In the GH control rats, training induced significant increases in hepatic exon 1-derived IGF-I mRNA (mean increase 30%; P < 0.05) and muscle exon 2-derived mRNA (mean increase 35%; P < 0.05). In the GH-suppressed rats, only muscle exon 1-derived transcripts were significantly increased by training (55%; P < 0.05) and this was associated with a significant increase in muscle IGF-I protein levels (P < 0.05). We speculate that the anabolic response to training may involve both GH-dependent increases in IGF-I mRNA in the liver and GH-independent increases in the muscle.

Actins↗
PubMed1994

How does the mitogenic insulin-like growth factor I receptor differ from the metabolic insulin receptor?

Under normal physiological conditions, insulin regulates intermediary metabolism by interacting with insulin receptors in liver, fat and muscle cells. The insulin-like growth factors (IGF-I and IGF-II), on the other hand, are primarily involved in the regulation of growth and development of the whole organism and interact with IGF receptors expressed by most, if not all, tissues of the body. As the insulin and IGF-I receptors are both structurally and functionally similar, one of the fundamental questions in this area of research has been the basis for the distinct pathways of hormone action elicited by insulin and the IGFs. Several features are involved in the divergence of the signalling pathways of insulin and the IGFs. The insulin receptor binds insulin with high affinity, and this specificity is determined by domains lying to the N-terminal and C-terminal sides of the cysteine-rich region in the alpha-subunit. The high-affinity IGF binding by the IGF-I receptor is determined by its cysteine-rich domain. Secondly, the IGF-binding proteins (IGFBPs), of which six have been characterized, bind the IGFs with an affinity higher than that of the IGF receptors. They bind all the circulating IGFs, protect them from degradation and deliver them to the IGF receptors in the target tissues, where they modulate IGF action. As they have no affinity for insulin, insulin is free to interact with its own receptor. Finally, structural differences in the beta-subunits of the insulin and IGF-I receptors may result in divergence of the signal pathways.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗
PubMed1994

Dentists variation in the determination of orthodontic treatment need.

The criteria that dentists use to judge the need for orthodontic treatment are not clear. This study investigates variation in dentists' perception of orthodontic treatment need. Seventy-four dentists were asked to assess 320 dental casts in relation to aesthetic and dental health need. The results of this investigation revealed that the panel was divided as what constituted a need for orthodontic treatment on dental health grounds. It is suggested that one method of achieving a more uniform evaluation of orthodontic treatment need is the use of an occlusal index. Until an occlusal index is accepted and used by the profession, the distortion of need and demand for orthodontic treatment by dentists' unequal perceptions will continue.

Attitude of Health Personnel↗
PubMed1994

Use of the Index of Orthodontic Treatment Need (IOTN) in assessing the need for orthodontic treatment pre- and post-appliance therapy.

The Index of Orthodontic Treatment Need (IOTN) was used to assess the need for orthodontic treatment before and after treatment, on a systematic sample of 1225 cases. On the whole, full upper and lower fixed appliances brought about a greater improvement in Aesthetics and Dental Health Components compared to other appliance techniques, and were less likely to make the occlusion worse. It was found that certain occlusal traits were more likely to be successfully treated than others. A common problem resulting from treatment was the development of a cross-bite.

Dental Health Surveys↗
PubMed1994

Insulin and insulin-like growth factor-I receptors similarly stimulate deoxyribonucleic acid synthesis despite differences in cellular protein tyrosine phosphorylation.

Signal transduction pathways stimulated by insulin or insulin-like growth factor-I (IGF-I) were compared in transfected NIH3T3 fibroblast cell lines expressing the human insulin receptor, IGF-I receptor, or a chimeric IGF-I receptor with its carboxy-terminal tail replaced with that of the insulin receptor (approximately 1 x 10(6) receptors/cell). Although receptor autophosphorylation was very similar in the three cell lines overexpressing receptors (EC50 = 1-3 nM), there were differences detected in the protein tyrosine phosphorylation stimulated by insulin and IGF-I in these cells. Although no substrates specific for the insulin receptor were detected, phosphorylation of a 170-kilodalton (kDa; IRS-1) and a 70-kDa protein was 10 times more sensitive to insulin than to IGF-I (EC50 = 1.5-2.5 vs. 14-23 nM). The chimeric receptor stimulated significantly lower levels of phosphorylation of several proteins relative to the wild-type IGF-I receptor. Activation of phosphatidylinositol 3'-kinase paralleled phosphorylation of the 170- and 70-kDa proteins. Despite these differences in protein tyrosine phosphorylation, stimulation of mitogen-activated protein (MAP) kinase and DNA synthesis were very similar in the three cell lines overexpressing receptors. Little difference was detected in Shc phosphorylation or MAP kinase activation through the three receptors, although activation of MAP kinase was more efficiently coupled to the platelet-derived growth factor receptor than to any of the overexpressed receptors. All three receptors stimulated DNA synthesis to levels comparable to 10% serum, with similar sensitivities (EC50 = 1.5-3.5 nM).

1-Phosphatidylinositol 4-Kinase↗