Purification and characterization of multiplication-stimulating activity (MSA) carrier protein

Expression of insulin-like growth factor II (IGF-II), IGF binding protein-2 and myogenin during differentiation of myogenic satellite cells derived from the turkey

Myogenic satellite cells are essential for the development of postnatal skeletal muscle. The proliferation and differentiation of these cells are, in part, regulated by the insulin-like growth factors (IGFs), and it has been shown that the IGF binding proteins (IGFBPs) are capable of modulating the actions of IGFs. We have examined the endogenous expression of IGF-II, IGFBP-2 and the myogenic regulatory factor, myogenin, during differentiation of clonally derived turkey muscle satellite cells. Cells were harvested at approximately 80% of confluent density. Additional cultures were rinsed, fed differentiation medium and harvested when approximately 20%, 60% and 80% differentiated (fused). Northern blot analyses were performed using total cellular RNA and labeled rat cDNAs specific for IGF-II, IGFBP-2 and myogenin. A single IGF-II mRNA transcript of approximately 4.0 kb was observed. The relative mRNA abundance was highest in proliferating cultures and decreased with the onset of differentiation, to approximately 60% of initial levels where it remained throughout differentiation. Use of the IGFBP-2 cDNA probe indicated a single mRNA transcript of approximately 2.0 kb. The level of expression of IGFBP-2 mRNA was highest in proliferating cells and decreased to 25%, 16% and 11% of initial levels as differentiation progressed. A single 1.8 kb mRNA transcript was detected with the myogenin probe. Expression of myogenin was undetectable in proliferating cultures and increased significantly as differentiation progressed. Serum-free medium was conditioned for 24 h (CM) at each time point and collected from similar cultures. An IGFBP species of M(r) approximately 30,000 was detected in CM by probing western blots with [125I] IGF-I (ligand blot analysis). The intensity of this band decreased with differentiation to 35%, 24% and 18% of the level for proliferating cultures. Western blots were also probed with an antibody raised against the M(r)-34,000 bovine IGFBP-2. This antibody specifically bound to the M(r)-30,000 IGFBP, and the level of antibody binding decreased as differentiation progressed. It therefore appears that IGF-II, IGFBP-2 and myogenin are expressed in a differentiation-dependent manner by turkey myogenic satellite cells and may thus be involved in the process of differentiation of avian muscle cells.

Heterogeneity of insulin-like growth factor binding proteins in swine

Proteins in porcine amniotic fluid and sera (both fetal and adult) were separated electrophoretically in sodium dodecyl sulfate-containing polyacrylamide gels and transferred to nitrocellulose sheets. Western blots were analysed for proteins that would bind (a) radioiodinated insulin-like growth factor-I (IGF-I) and (b) antibodies to a rat insulin-like growth factor binding protein. Multiple insulin-like growth factor binding proteins were identified in sera. The binding proteins ranged in size from Mr 192,000 to 26,000. One immunologically cross-reactive protein (Mr 36,000) was detected. No binding proteins were detected routinely in amniotic fluids. Sera from adult swine were fractionated by preparative isoelectric focusing. Two binding proteins (Mr 192,000, 46,000) were located in acidic fractions which also contained IGF-I and IGF-II. Two binding proteins (Mr 36,000, 26,000) were located in neutral to basic fractions which contained primarily IGF-II. Immunoglobulin-sized material from adult sera fractionated over Sephadex G-200 contained two binding proteins (Mr 46,000, 42,000) whereas albumin-sized material contained one (Mr 36,000). Porcine insulin-like growth factor binding proteins are as heterogeneous as those from humans.

Characterization of insulin-like growth factor binding proteins of cultured rat astroglial and neuronal cells

Insulin-like growth factor binding proteins produced by cultured rat neurons, astrocytes, and rat cell lines BRL-3A and B104 were compared to binding proteins found in rat serum, using affinity labeling, deglycosylation, and Western ligand blotting studies. Each source elaborated an unique pattern of heterogeneous binding proteins. Some of the binding proteins from different sources behaved similarly in each experimental system suggesting that subsets of these binding proteins may be structurally related. In particular, our data suggest that cultured astrocytes and neurons make the major binding protein produced by BRL-3A cells.

Binding proteins for the insulin-like growth factors: structure, regulation and function

Binding proteins for the insulin-like growth factors (IGF-I and IGF-II) are increasingly being recognized as modulators of IGF actions in both inhibitory and stimulatory ways. At least three distinct classes of binding protein are thought to exist, differing in their primary structures and binding characteristics, although all are able to bind both IGF-I and IGF-II. This review outlines the purification and characterization of the binding proteins that have been identified to date, and describes the regulation of their production and of their levels in the circulation. Current views on their potential biological roles are also discussed.

Binding proteins for insulin-like growth factors in adult rat serum. Comparison with other human and rat binding proteins

Insulin-like growth factor (IGF) binding protein has been purified from adult rat serum by affinity chromatography on agarose-IGF-II and high performance reverse-phase chromatography. The final preparation contains two components, of apparent molecular mass 50 and 56 kDa nonreduced, or 44 and 48 kDa reduced, both of which specifically bind IGF-I and IGF-II. Competitive binding data indicate association constants of 5-10 X 10(10) l/mol for both IGFs, with a slightly higher affinity for IGF-II than IGF-I. Amino-terminal sequence analysis yields a unique sequence, identical in 11 of the first 15 amino acids with that of a human plasma IGF binding protein (Martin, J. L., and Baxter, R. C. (1986) J. Biol Chem. 261, 8754-8760), and with slight homology to other human and rat IGF binding proteins characterized to date. By analogy with the binding protein from human plasma, it is likely that the rat protein is part of the growth-hormone dependent complex which appears to carry most or all of the circulating IGFs.

Radioimmunoassay for the measurement of insulin-like growth factor I in patients with pituitary disease in comparison with commercially available somatomedin-C radioimmunoassays

A highly sensitive and specific radioimmunoassay for the measurement of insulin-like growth factor I (IGF I) has been developed. The IGF I concentrations were measured in sera of normal subjects, patients with acromegaly, hypophysectomized patients and patients with hyperprolactinaemia. The results were compared with the results obtained after measurement of IGF I in the sera of the same patients with two commercially available radioimmunoassays for somatomedin-C. IGF I was separated from its carrier protein using Sep-Pak C18 cartridges. The total recovery of IGF I by this method was about 100%. The in-house assay shows a high specificity for IGF I and a high sensitivity. As little as 0.1 microgram/l of IGF I can be detected. Only a small amount of serum (25 microliter) is necessary for the IGF I determination and a great number of serum samples (more than 100 per day) can be processed. The mean immunoreactive IGF I concentration was 379 +/- 159 micrograms/l in normal adult subjects, 3340 +/- 1094 micrograms/l in acromegalic patients, and 52 +/- 11 micrograms/l in growth-hormone deficient patients. Patients with hyperprolactinaemia had an IGF I concentration of 433 +/- 112 micrograms/l.

Characterization of Multiplication-Stimulating Activity (MSA) carrier protein

A cloned rat liver cell line (BRL-3A) synthesizes and secretes the somatomedin, Multiplication-Stimulating Activity (MSA), in association with its specific carrier protein (MCP). Affinity-purified MCP is a single-chain polypeptide with a molecular weight of 31,500 under non-reducing conditions and 34,000 when fully reduced. The formation of a Mr 42,000 complex following chemical crosslinking of purified MSA (Mr 8700) and MCP (Mr 34,000) suggests that these components bind in a 1:1 molar ratio on the basis of the sum of their combined molecular weights. The amino acid composition and the N-terminal amino acid sequence of MCP were also determined. Polyclonal MCP-antibody preparations were used to determine if MCP could be detected in normal rat sera. MCP could not be detected in adult rat serum, but was present at high concentrations in fetal rat serum. These results suggest that MCP is a fetal somatomedin carrier protein and that MSA-MCP complexes may be important during fetal development. The availability of antibodies directed against a purified somatomedin carrier protein will provide the opportunity to investigate further the role of carrier proteins in the biological activity of the somatomedins.

Affinity-labeled plasma somatomedin-C/insulinlike growth factor I binding proteins. Evidence of growth hormone dependence and subunit structure

By using disuccinimidyl suberate, we have covalently cross-linked 125I-labeled somatomedin-C (Sm-C)/insulinlike growth factor I to specific binding proteins in human plasma. In unfractionated plasma samples from normal and acromegalic donors, 125I-Sm-C binding-protein complexes with relative molecular weights (Mr) of 160,000, 135,000, 110,000, 80,000, 50,000, 43,000-35,000, and 28,000-24,000 were consistently observed. In contrast, the 43,000-35,000-mol wt species were frequently the only specific complexes observed in hypopituitary plasma and were consistently more intensely labeled in such samples. Reduction of samples with beta-mercaptoethanol did not alter the electrophoretic pattern of these 125I-Sm-C binding-protein complexes. All Sm-C binding proteins, with the exception of the 43,000-35,000-mol wt complex, were adsorbed by concanavalin A-Sepharose. When acromegalic or normal plasma was fractionated on a Sephadex G-200 column and affinity labeled, the same complexes that were adsorbed by concanavalin A were found in fractions that eluted near the gamma-globulin peak. On the other hand, the 43,000-35,000-mol wt complex consistently eluted in size-appropriate fractions near the albumin peak. These data suggest that the growth hormone (GH)-dependent Sm-C binding protein, represented by the 160,000-mol wt complex, is in some way composed of smaller species, i.e., the 135,000-, 110,000-, 80,000-, 50,000-, and 28,000-24,000-mol wt complexes. Acid incubation of plasma prior to Sephadex G-200 chromatography results in the elimination of specific 125I-Sm-C binding-protein complexes which elute near gamma-globulin and a concurrent increase in the labeling intensity of the 28,000-24,000-mol wt complexes. We speculate, therefore, that each of the GH-dependent Sm-C binding-protein complexes represents an oligomer composed of 28,000-24,000-mol wt protomers. The 43,000-35,000-mol wt species is not dependent upon GH and appears to represent a different type of Sm-C binding protein.

Differentiation between carrier-bound forms of non-suppressible insulin-like activity (NSILA) in serum

Gel-permeation chromatography of serum on Sephacryl S-300 at pH 7.4 has shown that NSILA was detected over a range of MW 50,000-400,000 with a peak at about MW 200,000. When fractions from the above chromatography were rechromatographed on Sephadex G-75 at pH 2.4 major amounts of acid-stable NSILA were found in a fraction of MW 200,000-600,000 (77% of the fraction NSILA or 28% of total serum NSILA). Further evidence was obtained for the presence of an active acid-dissociable complex in serum. This was present in both the MW 100,000-200,000 and 35,000-100,000 fractions and corresponded to 37% of total serum NSILA. Con-A Sepharose affinity chromatography of the serum fractions from Sephacryl S-300 chromatography, followed by Sephadex G-75 chromatography under acid conditions, showed that the acid-stable complex was consistently found in weakly bound materials. The active acid-dissociable complex was found in the bound fractions, especially in the Sephacryl S-300 pool of MW 35,000-100,000. Low MW NSILA (less than 15,000) was also released on acid treatment from an otherwise inactive high MW complex(es) of MW 35,000-600,000. This complex was not bound by Con-A Sepharose.

Plasma forms of somatomedin and the binding protein phenomenon

Somatomedin and peptides are bound tightly to a plasma complex in vivo. There are at least two macromolecular forms of somatomedin complexes: one with a molecular weight of 150 000 the ('150 K' complex) and one of 45 000 (the '45 K' complex). These macromolecular complexes clearly increase the half-life of somatomedins in plasma. The 150 K complex appears to have a three-subunit structure consisting of the somatomedin peptide, a specific Sm/IGF binding protein, and an acid-labile component. In addition to the binding proteins contained in the 150 K and 45 K complexes, plasma also contains unsaturated Sm/IGF binding proteins of approximately 40 000 molecular weight. The somatomedin complex forms not only have longer half-lives, but in this form their biological actions (both insulin-like and growth-promoting) are clearly modified. In addition, it is possible that the complex forms play a role in tissue selectivity and/or delivery of active somatomedin peptide to the intracellular space. An understanding of the somatomedin complexes, plasma complexes and binding proteins is necessary before the physiological roles of the somatomedin peptides can be defined more clearly.

Effect of ultraviolet radiation on production of epidermal cell thymocyte-activating factor/interleukin 1 in vivo and in vitro

UV radiation was found to enhance the release by keratinocytes of epidermal cell thymocyte-activating factor (ETAF), a hormone-like molecule that is physiochemically identical to interleukin 1 (IL-1). This conclusion was supported by the following observations: (i) the keratinocyte cell line PAM 212 retained ETAF/IL-1-producing potential after exposure to UV radiation despite significant loss in cell viability; (ii) epidermal cells from normal and UV radiation-exposed mice were found to produce equivalent amounts of ETAF/IL-1 on a per cell basis with the density of epidermal cells in UV radiation-exposed skin being at least 5-fold above normal values; (iii) under the conditions used, ETAF/IL-1 could be detected in the serum of UV radiation-exposed, but not normal, animals; and (iv) many of the biologic consequences known to be mediated by elevations in ETAF/IL-1--i.e., neutrophilia, elevated levels of complement component 3, serum amyloid P, and plasma fibrinogen--were all observed in animals following a single UV radiation exposure. Animals subjected to chronic UV radiation showed an initial elevation in their levels of acute-phase reactants that returned to normal concentrations within 7 days. This correlates with observations made by others of a "desensitization" to ETAF/IL-1-mediated effects after chronic administration of known exogenous stimulators of inflammatory responses. Further, the UV radiation-induced desensitization took place in spite of demonstrable serum levels of ETAF/IL-1. These results suggest that the mechanism(s) responsible for desensitization is not an inhibition of ETAF/IL-1 synthesis but rather may result from inability of the target cells to perceive this endogenous mediator or to unavailability of serum-associated ETAF/IL-1 for the appropriate targets.

Somatomedin carrier proteins

The somatomedins, such as the insulin-like growth factors I and II, somatomedins A and C, and multiplication-stimulating activity, are small, growth hormone dependent polypeptides which are the assumed mediators of growth hormone action on extraskeletal tissues. The somatomedins are unique among polypeptide hormones in that they exist in serum as two high molecular weight complexes of approximately 150 000 and 60 000 molecular weight. Little, if any, free hormone is present in plasma or serum. These circulating complexes appear to represent the reservoir for these hormones since none is stored in any other tissue and the circulating concentration (approx. 1 microgram/ml) exceeds the biologically active concentration of the free hormone by many fold. Many recent investigations regarding the subunit composition of the two serum carrier protein complexes and their effect on the biological activity of the somatomedins have revealed an elaborate system of intense physiological control ultimately aimed at delivering somatomedin to its specific receptor on the surface of the target cell.

Recent advances in the biochemistry and physiology of the insulin-like growth factor/somatomedin family

The insulin-like growth factors (IGF) or somatomedins (Sm) are a family of low molecular weight circulating growth factors which have a major, but not absolute, dependence on GH, and have been shown to stimulate body growth and skeletal metabolism in vivo. They are thus considered to mediate the effects of GH on skeletal growth. In humans, the family consists of two well-characterized forms--IGF-I or SmC (a basic peptide) and IGF-II (a "neutral" peptide)--as well as perhaps two less well characterized forms--SmA (a neutral peptide) and an acidic insulin-like activity (ILA pI 4.8). Similar IGF/Sm species have been found and well-characterized in rat serum. Some higher mol wt forms also exist in tissues and body fluids and may represent possible precursor forms. On the basis of in vitro, clinical and in vivo evidence it has been postulated that IGF-I is the primary growth factor in the adult, whilst IGF-II is probably a major foetal growth factor. In vitro the IGF/Sms have a variety of effects including (1) acute insulin-like metabolic actions, which are observed primarily in insulin target tissues and are initiated largely at insulin receptors, and (2) longer term effects, associated with cell growth and skeletal tissue metabolism, and which occur in traditionally non-insulin target tissues, primarily via IGF/Sm receptors. These observations, together with the circumstantial clinical evidence favouring a close association between IGF levels and growth status, clearly indicate a role for IGF/Sms in growth regulation. This concept is now fully supported by the demonstration that IGF-I infused into hypophysectomized (GH-deficient) rats results in increased growth and skeletal metabolism. The physiological regulation of the expression of net IGF activity in vivo is complex and is controlled by the following three determinants: the levels of IGFs, the levels of the specific carrier-proteins and the levels of IGF inhibitors. Both IGFs and their carrier-proteins are influenced by the GH status of the animal as well as by other hormones, nutritional status and chronic illness. Little is known yet about the control of the various IGF inhibitors that have been described. Of importance, however, is the general concept that normal growth is dependent on an adequate balance between all three determinants and that some regard must be had for the contribution of each in determining the overall potential for growth under given circumstances.

A method for culturing chick melanocytes: the effect of BRL-3A cell conditioning and related additives

A method for growing chick embryo melanocytes is described that utilizes medium conditioned by Buffalo Rat liver (BRL-3A) cells. The dissected trunk region of each 72 h (Stages 14 to 19) embryo produces approximately 200,000 melanocytes (purity, 80%) when processed and cultured for 8 d. Thus, a typical experiment involving 20 embryos would produce a total of 4 x 10(6) melanocytes. Choice of serum, serum concentration, and cell density were determined experimentally. Partially purified multiplication stimulating activity (MSA) from BRL-3A cells and insulin were also tested as medium additives. MSA was not stimulatory, whereas insulin gave a positive response in 2% but not 10 or 0% serum. The final protocol used a modified F12 medium with 10% bovine calf serum conditioned by BRL-3A cells. Cultures were fed every other day. Small colonies of cells became evident by culture Day 3 and increased rapidly to Day 5 when pigmentation became obvious. Colony size continued to increase but more slowly from Days 5 to 8, whereas pigmentation increased rapidly and maximized on Day 8. There is a factor, or factors, present in BRL-3A conditioned medium that stimulates embryonic chick melanocytes to divide preferentially over contaminating cell types. This results in cultures that can provide adequate numbers and purity for biochemical studies.

Developmental pattern of a serum binding protein for multiplication stimulating activity in the rat

The concentration of multiplication stimulating activity (MSA), an insulinlike growth factor (IGF), is high in fetal rat serum. We now report that MSA is exclusively associated wth an albumin-size binding protein in fetal rat serum; the growth hormone-dependent, gamma globulin-size binding protein, which predominates in the older animal, is absent from fetal rat serum. When (125)I-MSA was incubated with fetal rat serum and then gel filtered on Sephadex G-200, specific radioactivity eluted in the void volume (peak I) and the albumin region (peak III); by contrast, specific radioactivity eluted mainly in the gamma globulin region (peak II) in adult rat serum. Pools of the Sephadex G-200 fractions were chromatographed on Sephadex G-50, in 1 M acetic acid, to separate the binding protein from IGF activity. Analysis of IGF activity by chick embryo fibroblast bioassay, competitive protein binding assay, and MSA by radioimmunoassay revealed that all the IGF activity and MSA in fetal rat serum resided in peak III. Measurement of MSA binding capacity of the stripped binding protein by Scatchard analysis demonstrated that the majority of binding capacity also was found in peak III in fetal rat serum; most of MSA binding capacity was in peak II in adult rat serum. In fetal rat sera, in addition to the peak III binding protein, which is the major carrier of endogenous MSA, there is a component in peak I capable of specifically binding (125)I-MSA. This component elutes as a single species from a Sepharose-6B column. As MSA associated with peak III gradually declined in early neonatal life, peak II-associated IGF activity measured by chick embryo fibroblast bioassay showed a rise of activity with a peak at 5 d of neonatal life, a nadir at 20 d, with an increase again to attain adult levels. These studies demonstrate that the MSA binding protein in the fetus is different from the growth hormone-dependent binding protein in adult life.