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R C Baxter

Publications and source records attributed to R C Baxter.

At least 163 records · Page 9Linked to original sources
PubMed1991

Cytochalasin B stimulates insulin-like growth factor-binding protein-1 production by Hep G2 cells.

Hep G2 cells were used to study the early sequence of events regulating production of insulin-like growth factor-binding protein-1 (IGFBP-1). Cytochalasin B (100 microM) specifically inhibited 2-deoxyglucose uptake by Hep G2 cells and stimulated IGFBP-1 production 2-fold. Insulin (300 nM) did not stimulate hexose uptake but inhibited IGFBP-1 production more than 50%. A change in IGFBP-1 secretion was observed as early as 2 h after a 15-min or 2-h pulse exposure to either effector. In contrast to IGFBP-1, albumin production was diminished in the presence of cytochalasin B and increased by insulin. From these results we conclude that IGFBP-1 synthesis is (i) stimulated by transient inhibition of cellular glucose uptake and further stimulated by long-term glucose deprivation, and (ii) inhibited by transient exposure to insulin with further inhibition on long-term exposure. These effects are consistent with the dynamic regulation of IGFBP-1 by nutritional status.

Albumins↗
PubMed1991

Evaluation of interference by insulin-like growth factor I (IGF-I) binding proteins in a radioimmunoassay for IGF-I in serum from dairy cows.

Insulin-like growth factor I (IGF-I) circulates in serum bound to a number of different binding proteins (BPs). With antibodies currently available, BPs must be dissociated and inactivated or removed from serum prior to measurement of IGF-I by radioimmunoassay (RIA). Serum samples which spanned a 13-fold range in IGF-I concentration were obtained from lactating dairy cows and used to develop conditions for assay of IGF-I with minimal interference from BPs. Removal of BPs from serum by acid-ethanol extraction resulted in interference in the RIA. Therefore, serum was incubated with 0.1 M glycyl-glycine HCl to inactivate BPs as suggested by Underwood et al. Time, temperature and pH were optimum when serum was incubated for 48 hr at 37 C, pH 3.7. Binding protein inactivation was evaluated by ability of glycyl-glycine incubated serum to reassociate with 125I-IGF-I. In addition, BPs isolated by gel filtration of glycyl-glycine incubated serum were tested for interference in the RIA. The concentration of IGF-I in serum where inactivated BPs were removed by acid gel filtration was compared to corresponding glycyl-glycine incubated serum. There was a 1:1 relationship which intersected at zero indicating that total IGF-I could be measured. Therefore, incubation of serum with glycyl-glycine is a reliable method for measuring total IGF-I in serum from dairy cows.

Animals↗
PubMed1991

Transforming growth factor-beta stimulates production of insulin-like growth factor-binding protein-3 by human skin fibroblasts.

Human neonatal fibroblasts in monolayer culture produce insulin-like growth factor-binding protein-3 (IGFBP-3), the IGF-binding subunit of the circulating 140-kDa IGFBP complex. We now report that transforming growth factor-beta (TGF beta) is a potent stimulator of IGFBP-3 production by fibroblasts. After 72-h incubation with 1 ng/ml TGF beta, the levels of IGFBP-3 in conditioned medium were increased 5.8 +/- 1.2-fold (mean +/- SE; n = 9). Half-maximal stimulation of IGFBP-3 production was seen at 0.4 +/- 0.05 ng/ml TGF beta (n = 4). Coincubation of fibroblasts with TGF beta and either IGF-I or IGF-II at 50 ng/ml enhanced IGFBP-3 production 1.5- to 2-fold compared to TGF beta alone. As previously reported, fetal calf serum (FCS) stimulated IGFBP-3 production 5- to 6-fold; 1 ng/ml TGF beta increased the stimulated production of IGFBP-3 by FCS a further 2.5- to 3.5-fold. Acidification of FCS before addition enhanced the stimulation of IGFBP-3 compared to that caused by untreated FCS, but decreased further potentiation by TGF beta. This effect of acidified FCS was reversed by a neutralizing antibody to TGF beta. Similarly, the stimulation of IGFBP-3 levels by human serum or conditioned serum-free fibroblast medium was significantly increased by acidification of serum or medium before addition and was reversed by TGF beta antibody. These observations are consistent with acid-mediated activation of latent TGF beta added in serum or secreted by fibroblasts. Since IGFBP-3 is known to regulate IGF activity in fibroblasts, these results raise the possibility that TGF beta may modulate IGF actions in these cells by stimulating the production of IGFBP-3.

Animals↗
PubMed1991

Secretion of soluble insulin-like growth factor-II/mannose 6-phosphate receptor by rat tissues in culture.

The insulin-like growth factor-II/mannose 6 phosphate receptor, which binds both IGF-II and mannose 6-phosphate containing proteins, such as lysosomal enzymes, has been detected in the serum of several species. Neither the source nor the role of this soluble, truncated form of the membrane receptor has been determined. This study has examined the ability of a variety of rat tissues in culture to synthesize and release soluble receptor. Explants (1 mm3) of 21-day fetal, neonatal and adult rat tissues were cultured in serum-free medium and the conditioned medium analyzed for the presence of receptor. Using IGF-II binding and immunochemical techniques, receptor was detected in media from heart, kidney, liver, lung and muscle explants. [35S]cysteine labeling indicated that de novo synthesis of the soluble receptor occurs in the cultured explants and can be inhibited by cycloheximide. This rat tissue explant culture system demonstrates that soluble receptor is released by a number of tissues, and should provide a useful model for further investigations into its function and regulation.

Affinity Labels↗
PubMed1991

Insulin-like growth factor-binding protein-1 modulates blood glucose levels.

We have determined the consequences of insulin-like growth factor-binding protein-1 (IGFBP-1) administration alone and in combination with insulin-like growth factor-I (IGF-I). Human recombinant IGF-I, infused as a bolus into male Wistar rats, induced a fall in plasma glucose to 72 +/- 3% of baseline 15 min after injection. Co-infusion of equimolar concentrations of human IGFBP-1 abolished the IGF-I-induced fall (P less than 0.001). Injection of IGFBP-1 alone caused a rise in plasma glucose levels (P less than 0.002). The half life of human IGFBP-1, measured using a primate-specific RIA, was 12.5 +/- 0.7 min and was not influenced by the co-infusion of IGF-I. This study demonstrates that, in the rat, human IGFBP-1 blocks the hypoglycemic response to intravenous IGF-I and increases blood glucose levels when administered alone. Since IGFBP-1 concentrations are dependent on metabolic status, we suggest that fluctuating IGFBP-1 levels might modulate the hypoglycemic activity of unbound IGFs in the circulation.

Animals↗
PubMed1991

Impaired formation of the ternary insulin-like growth factor-binding protein complex in patients with hypoglycemia due to nonislet cell tumors.

In some subjects with hypoglycemia associated with tumors of mesenchymal origin, high insulin-like growth factor-II (IGF-II) levels have been described in serum and in the tumors. Tumor IGF-II of 10-15 kDa circulates in a 60-kDa complex, in contrast to the ternary 150-kDa complex in which serum IGFs normally circulate together with the IGF-binding subunit (IGFBP-3) and the acid-labile subunit (alpha-subunit). This study examines the molecular distribution and complex-forming activity of the components of the ternary complex in the serum of subjects with mesenchymal tumor hypoglycemia. Total serum IGFBP-3 levels were 60% of normal in tumor patients and appeared at 60 kDa on gel chromatography, shifting after tumor removal to 150 kDa. Total alpha-subunit levels were 40% of normal in patients with tumors, increasing after tumor removal to 70% of normal and changing in elution profile from a peak typical of uncomplexed alpha-subunit to the normal broad peak representing both complexed and uncomplexed alpha-subunit. Although low by RIA, alpha-subunit activity in a ternary complex formation assay was normal, indicating that the ability of free alpha-subunit in the patients' circulation to combine with exogenous IGFBP-3 plus IGF-I was not impaired. In contrast, in an assay that tested the ability of IGF-IGFBP complexes in the patients' circulation to combine with pure alpha-subunit, complex formation activity was 75-85% below normal in preoperative sera, despite low normal IGFBP-3 levels. Therefore, the cause of hypoglycemia in these patients may be the inability of complexes between the abnormal tumor IGF-II and IGFBP-3 to be sequestered in the biologically inactive ternary complex.

Aged↗
PubMed1991

Abnormal regulation of insulin-like growth factor binding proteins in adolescents with insulin-dependent diabetes.

We have measured fasting 0800 h insulin-like growth factor binding proteins (IGFBP)-1 and IGFBP-3, in 52 diabetic adolescents and 74 puberty-matched control subjects with short stature and normal hormonal status. We have also measured overnight hourly profiles of IGFBP-1, glucose, free insulin, and GH in 12 of the diabetic adolescents. With advancing age and pubertal status, IGFBP-1 declined and IGFBP-3 increased significantly in the control, but not the diabetic group. Fasting IGFBP-1 levels were elevated 4-fold compared to controls. Median IGFBP-3 was significantly lower in the diabetic compared to the control group in pubertal stages III and V. Elevated IGFBP-1 was significantly correlated with metabolic control in poorly controlled subjects (mean 12-month glycosylated haemoglobin greater than 8.5%). In the overnight profiles, mean hourly IGFBP-1 was inversely related to insulin, but not glucose. As free insulin levels declined, IGFBP-1 rose, associated with rising predawn blood sugars. The integrated 3-h IGFBP-1 value (0500-0800 h) was significantly correlated with the corresponding glucose integrated value. IGFBP-1 area under the curve for the whole overnight profile was significantly correlated with glycosylated hemoglobin in 11 of the 12 subjects. IGFBP-1 from diabetic adolescents has been shown to inhibit IGF-I bioactivity. We postulate that IGFBP-1 may have a role in growth impairment of poorly controlled diabetes and may contribute to the dawn phenomenon.

Adolescent↗
PubMed1991

Insulin-like growth factor (IGF) binding protein-3 in pregnancy serum binds native IGF-I but not iodo-IGF-I.

Although serum immunoreactive insulin-like growth factor binding protein-3 (IGFBP-3) increases during pregnancy, radioligand binding methods such as ligand blotting with iodinated IGFs fail to detect the protein in pregnancy serum. Since IGFBP-3 must bind IGF-I or IGF-II to form a complex with the acid-labile subunit (alpha-subunit), we have used ternary complex formation from [125I]alpha-subunit as a measure of IGF binding to serum IGFBP-3. High-pressure liquid chromatography fractions containing IGFBP-3 from pregnancy serum did not bind [125I]IGF-I, although the equivalent fractions from nonpregnancy serum showed dose-dependent binding. In contrast, IGFBP-3 fractions from nonpregnancy and pregnancy sera both bound [125I]alpha-subunit in the presence of either exogenous IGF-I or endogenous serum IGFs, implying that non-iodinated IGFs could bind to the IGFBP-3. Substitution of nonradioactive iodo-IGF-I for native IGF-I in the complex formation assay confirmed that the pregnancy-induced alteration in IGFBP-3, probably resulting from proteolysis, prevents it from binding iodo-IGF-I while having little effect on its binding of the native peptide. This provides an explanation for the failure to detect IGFBP-3 in pregnancy by radioligand binding methods, and raises the question of the significance of proteolysis of IGFBP-3.

Carrier Proteins↗
PubMed1990

Glycosaminoglycans inhibit formation of the 140 kDa insulin-like growth factor-binding protein complex.

The 140 kDa insulin-like growth factor (IGF)-binding protein complex in human serum consists of three subunits: an acid-labile, non-IGF-binding glycoprotein (alpha-subunit), an IGF-binding glycoprotein known as BP-53 or IGFBP-3 (beta-subunit), and IGF-I or IGF-II (gamma-subunit). This study investigates the regulation, by salt and glycosaminoglycans, of ternary (alpha-beta-gamma) complex formation, measured by incubating radioiodinated alpha-subunit with a mixture of IGF-I and IGFBP-3 and precipitating bound radioactivity with an anti-IGFBP-3 antiserum. Increasing NaCl concentrations progressively decreased ternary complex formation without any effect on binary (beta-gamma) complex formation. In 0.15 M-NaCl, the association constant for the ternary complex was 0.318 +/- 0.092 nM-1, 100-fold lower than that for the binary complex. Glycosaminoglycans also inhibited ternary complex formation without affecting the binary complex. Heparin [50% inhibition at 0.27 +/- 0.08 units/ml (1.5 +/- 0.4 micrograms/ml)] was more potent than heparan sulphate (50% inhibition at 15 +/- 7 micrograms/ml), with chondroitin sulphate even less potent. The inhibition by heparin was due principally to a decrease in binding affinity, from 0.604 +/- 0.125 to 0.151 +/- 0.024 nM-1 in the presence of 0.25 units of heparin/ml, with a slight decrease in the number of apparent binding sites from 1.05 +/- 0.08 to 0.85 +/- 0.15 mol of alpha-subunit bound/mol of beta-subunit. Since the ternary IGF-binding protein complex cannot cross the capillary barrier, it is proposed that a decrease in the affinity of the complex, mediated by circulating or cell-associated glycosaminoglycans, may be important in the passage of IGFs and IGFBP-3 to the tissues.

Binding, Competitive↗
PubMed1990

Developmental regulation of insulin-like growth factor-II/mannose 6-phosphate receptor mRNA in the rat.

This study examined levels of insulin-like growth factor-II/mannose 6-phosphate receptor (IGF-II/M6PR) mRNA in tissues of rats at different stages of growth. Northern blot analysis of total RNA from tissues of rats aged 2, 9, 21 and 42 days and from 21 day fetal rats was carried out using a cDNA probe to the IGF-II/M6PR. Northern blots showed this probe hybridized to a single 9kb band in all tissues tested. Highest hybridization signals were detected in fetal and neonatal tissues with levels rapidly decreasing after birth. For all age groups tested the highest signal was obtained with heart followed by muscle, lung, and kidney, with liver and brain showing lower levels of message. These results indicate that IGF-II/M6PR mRNA is developmentally regulated, and suggest a role for the IGF-II/M6PR in fetal and neonatal growth.

Aging↗
PubMed1990

Purification and properties of a novel insulin-like growth factor-II binding protein from transformed human fibroblasts.

We have purified an insulin-like growth factor (IGF) binding protein from culture medium conditioned by the SV40-transformed human fibroblast line AG 2804. This protein (TFBP), of apparent Mr = 34,000 on gel electrophoresis, binds IGF-II with an affinity constant of 3 x 10(11) liters/mol, 100-fold higher than its affinity for IGF-I. Amino-terminal sequencing indicated no structural relationship to any previously characterized IGF binding protein or receptor. Like a protein in cerebrospinal fluid with selective affinity for IGF-II, TFBP does not bind IGF-I lacking the amino-terminal tripeptide. In contrast with binding proteins produced by normal human fibroblasts, TFBP does not cross-react in radioimmunoassays for the plasma protein IGFBP-3 or the amniotic fluid protein IGFBP-1; however, after affinity labeling with IGF-II, it is immunoprecipitable by an anti-IGFBP-3 antiserum. The cerebrospinal fluid binding protein is not precipitable by this antiserum and appears smaller than TFBP. TFBP reacts with wheat germ agglutinin, but not with concanavalin A, or with the acid-labile subunit of the high molecular weight serum IGF binding protein complex. Furthermore, since RNA from transformed fibroblasts does not hybridize with an IGFBP-3 cDNA probe, TFBP is not derived from the IGFBP-3 gene. We conclude that, although TFBP appears to share an epitope with IGFBP-3, it is distinctly different from any previously described IGF binding protein.

Amino Acid Sequence↗
PubMed1990

Radioimmunoassay for insulin-like growth factor (IGF) II: interference by pure IGF-binding proteins.

A new radioimmunoassay for insulin-like growth factor-II (IGF-II) is described. Compared to recombinant DNA-derived IGF-II standard, the cross-reactivity of natural or recombinant IGF-I was less than 1%. The ED50 for IGF-II standard was 1.0 ng/ml, and the mean IGF-II level in acid-ethanol-extracted serum from healthy adults was 525 +/- 87 ng/ml (SD, n = 30). Addition of the IGF binding protein IGFBP-1 (BP-28, PP12) caused dose-dependent inhibition of IGF-II tracer binding to antiserum, increasing to greater than 90% inhibition at 400 ng/ml IGFBP-1. In contrast, the IGF binding protein IGFBP-3 (BP-53) caused approximately 30% inhibition of tracer binding at 20 ng/ml IGFBP-3, with no further inhibition up to 400 ng/ml IGFBP-3. The influence of added IGF binding proteins on IGF-II displacement curves varied depending on both the type and concentration of binding protein added. It is concluded that interference in IGF radioimmunoassays by IGF binding proteins depends both on the types of binding proteins present, and on the IGF concentration, in the test samples.

Binding, Competitive↗
PubMed1990

Inhibitors of glucose uptake stimulate the production of insulin-like growth factor-binding protein (IGFBP-1) by human fetal liver.

Cultured human fetal hepatic explants were used to study the role of hexose uptake in regulating production of the GH-independent insulin-like growth factor-binding protein (IGFBP-1). Cytochalasin B (100 microM), but not cytochalasin E, inhibited 2-deoxyglucose uptake by fetal liver explants and increased production of IGFBP-1 2-fold. Two agents which stimulate cyclic nucleotide-dependent pathways, cholera toxin and theophylline, also inhibited 2-deoxyglucose uptake in this system and both stimulated IGFBP-1 production. When added to a maximally inhibiting concentration of cytochalasin B (250 microM), hexose uptake was not further inhibited by theophylline (5 mM) or cholera toxin (10 micrograms/ml), suggesting that the three substances interact with the same hexose transport mechanism. However, the stimulatory effect of theophylline (5 mM) or cholera toxin (10 micrograms/ml) on IGFBP-1 production was additive to the effect of 250 microM cytochalasin B, suggesting that another pathway, possibly involving cyclic nucleotide accumulation, could further stimulate IGFBP-1 above the effect of blocking glucose transport. Insulin (300 nM) had no effect on hexose uptake by human fetal liver explants but inhibited IGFBP-1 production, both basally and when stimulated by cytochalasin B (100 microM) or cholera toxin (1 microgram/ml). These results are consistent with the conclusion that a cyclic nucleotide-dependent pathway, activated by blocking hexose transport and inhibited by insulin, is involved in the regulation of IGFBP-1 synthesis.

Biological Transport↗
PubMed1990

Insulin-like growth factor-II/mannose-6-phosphate receptors are increased in hepatocytes from regenerating rat liver.

Insulin-like growth factor-II (IGF-II) receptor levels were determined in hepatocytes from sham-operated and two thirds hepatectomized rats. [125I]IGF-II binding to confluent cultures increased 2-fold in cells isolated 24 and 48 h after hepatectomy compared to that in cells from sham-operated rats. Receptor levels increased from 1.74 +/- 0.14 x 10(4)/cell (sham-operated) to 3.60 +/- 0.31 x 10(4)/cell (hepatectomized; P = 0.002), with no change in affinity of IGF-II binding (Ka = 6.4 x 10(9) M-1). As previously reported, receptor levels increased in cells plated at low density, but this effect was decreased in cells from hepatectomized rats (80% increase) compared to that in cells from control rats (300% increase). Serum IGF-I levels decreased 50% 24 h after partial hepatectomy (P less than 0.001), but returned to normal levels by 48 h. However, IGF-I synthesis was not decreased in hepatocytes isolated 24 h after partial hepatectomy, suggesting that decreased serum levels are due to decreased liver mass. Circulating IGF-II levels were not altered by partial hepatectomy, and IGF-II production was not detected in hepatocytes from sham-operated or hepatectomized rats. Transforming growth factor-beta is thought to terminate hepatocyte proliferation upon complete liver regeneration. In hepatocytes from sham-operated or hepatectomized rats transforming growth factor-beta totally blocked DNA synthesis, but had no effect on elevated IGF-II receptor levels after partial hepatectomy. Concomitant with increased IGF-II receptor levels, hepatocytes from hepatectomized rats were more sensitive to IGF-II stimulation of DNA synthesis. [3H]Thymidine incorporation in the presence of epidermal growth factor (50 ng/ml) was stimulated 66% by IGF-II (300 ng/ml) in control cells compared with 220% in cells from hepatectomized animals. These results suggest that IGF-II and the IGF-II receptor may play a role in liver regeneration.

Animals↗
PubMed1990

Production of an insulin-like growth factor (IGF)-inducible IGF-binding protein by human skin fibroblasts.

Neonatal human skin fibroblasts produce insulin-like growth factor-binding proteins (IGFBPs) that have the potential to modulate the actions of the growth factors. We have examined the IGFBPs secreted by monolayer cultures of neonatal fibroblasts by ligand blotting with [125I]IGF-II and immunoblotting with antisera raised against three well characterized IGFBPs: IGFBP-1, IGFBP-2, and IGFBP-3. As detected by ligand blotting, medium from untreated fibroblasts contained IGFBP-3, a second IGFBP which appeared as a doublet of 29-31 K, and a smaller protein of 22 K. Within 10 h of the addition of 50 ng/ml IGF-I, a markedly increased level of production of the 29-31 K IGFBP doublet was detectable, with levels increasing 8- to 9-fold by 24 h compared to that in untreated samples. IGF-I was approximately twice as potent as IGF-II at inducing 29-31 K IGFBP, with a half-maximal response at 15.4 +/- 2.7 ng/ml IGF-I and 26.6 +/- 1.6 ng/ml IGF-II (n = 3). Insulin tested at 1 microgram/ml did not induce 29-31 K IGFBP. Neither GH nor the acid-labile subunit of the circulating high mol wt IGFBP complex induced 29-31 K IGFBP or affected its induction by IGF-I or IGF-II. Immunoblotting demonstrated that IGF-inducible IGFBP did not react with antibodies to IGFBP-1, IGFBP-2, or IGFBP-3. These results indicate that IGF-I and IGF-II induce an IGFBP that is different from previously characterized human IGFBPs.

Cell Transformation, Neoplastic↗