A radioimmunoassay has been developed for Somatomedin B, a growth hormone-dependent factor that stimulates DNA synthesis in human glia-like cells. The sensitivity permits detection of this factor in human plasma diluted 1: 20,000 and in monkey plasma diluted 1: 5,000. It is not measurable in nonprimate plasma diluted 1: 20. The concentration in growth hormone-deficient adult patients is equivalent to 6.6plus or minus0.5 ug/ml of a highly purified somatomedin preparation. In acromegaly the concentration is 19.3plus or minus2.3 ug/ml and falls after definitive therapy that results in a decrease in plasma growth hormone. In unextracted human plasma the immunoreactive Somatomedin B is associated with a plasma protein at least as large as gamma-globulin and with an electrophoretic mobility on paper resembling the alpha-globulins. The level of Somatomedin B in the bound form in human plasma under steady-state conditions may depend on the rate of production of the peptide and/or the concentration of the plasma-binding protein. At present there is no information concerning which of these is modulated by growth hormone. Immunoreactive Somatomedin B is found predominantly in Cohn plasma fractions III and IV, largely dissociated from the plasma-binding protein. The disappearance curves of labeled purified Somatomedin B and of immunoreactive Somatomedin B from acromegalic plasma administered intravenously to a dog were superposable; the terminal portion of the disappearance curve having a half time of almost an hour.
Insulin and insulin-like growth factors (IGIs) stimulate the growth of human breast cancer cells in vitro. The type I somatomedin receptor (SR) expressed in these cells may mediate the growth effects of these peptides. We have examined the role of this receptor on human breast cancer growth with a monoclonal antibody (alpha-IR-3) that blocks the receptor binding domain and inhibits IGF-I-induced growth. alpha-IR-3 inhibited clonal growth in vitro and blocked the mitogenic effect of exogenous IGF-I in both MCF-7 and MDA-231 breast cancer cell lines. Antibody-induced blockade of the type I SR also inhibited the estrogen-independent MDA-231 cells growing in vivo in nude mice, but growth of the estrogen-dependent MCF-7 cells was unaffected. IGIs are important growth regulators of MDA-231 breast cancer cells. Blockade of this growth stimulatory pathway may provide a new treatment strategy.
A radioreceptor assay utilizing human fetal brain plasma membrane as matrix and somatomedin A as ligand (fetal brain RRA-SMA) was developed. Increased levels of fetal brain RRA-SMA were found in the fetal circulation. The concentration was approximately 4-fold higher in the fetal as compared to the adult human. At birth, values fell within the adult range. In contrast, adult somatomedins determined by somatomedin radioimmunoassay were undetectable in the fetus and below the adult range at birth. Levels of fetal brain RRA-SMA were decreased in fetuses with different clinical disorders. In healthy newborns at cesarean section a significant correlation between serum fetal brain RRA-SMA values and birth weight and length was found. These results indicate the presence of an embryonic somatomedin in humans. The fetal brain RRA-SMA may provide a reliable index of fetal growth.
Recent studies have shown that confluent cultured fibroblasts must be rendered competent by a factor contained in platelets before they can respond to plasma components and progress from G0 to S phase of the cell cycle. Somatomedin-C is one of the components of platelet-poor plasma necessary for cell cycle progression, but other factors present in somatomedin-C-deficient plasma are also required. Because both competence and progression factors contained in somatomedin-C-deficient serum modulate somatomedin-C action in fibroblasts, this study was undertaken to examine their possible influence on the binding of 125I-labeled somatomedin-C. Exposure of BALB/c 3T3 fibroblasts to high concentrations of platelet-derived growth factor alone prevented the time-dependent decrease in somatomedin-C binding which occurs in serum-free medium. Lower concentrations of the growth factor or 5% (vol/vol) platelet-poor plasma reduced but did not abolish this decrease. In contrast, sequential addition of platelet-derived growth factor followed by platelet-poor plasma resulted in a 75% increase in 125I-labeled somatomedin-C binding over basal levels during the first 2 hr. Binding increased by 125% when somatomedin-C-deficient platelet-poor plasma was substituted for normal platelet-poor plasma. The hormonal induction of somatomedin-C receptors appears to be a mechanism whereby peptide growth factors such as platelet-derived growth factor may condition the cell to respond optimally to somatomedin-C.
Human growth hormone (hGH) is known to be a potent stimulator of somatomedin secretion in vivo. The induction of somatomedin by growth hormone has been difficult to study in vitro, however, because no organ containing a high concentration of somatomedin has been identified. Because fetal mouse explants have been shown to produce somatomedin in vitro, we have undertaken studies to determine whether postnatal human fibroblast monolayers also produce somatomedin, and if so, whether its production is regulated by other hormones. Quiescent human fibroblasts were exposed to serum-free minimum essential medium, and the medium was assayed for somatomedin concentration using a specific radioimmunoassay for somatomedin-C. A progressive rise in immunoreactive somatomedin to 0.08 U/ml per 105 cells per 24 h was observed over 72 h of incubation. This was an underestimation of the actual concentration of immunoreactive somatomedin since the amount measured following acid treatment was at least fourfold higher than in the untreated medium. Growth hormone stimulated immunoreactive somatomedin production in a dose-dependent manner: 5 ng hGH/ml = 0.1 U/ml per 105 cells; 50 ng hGH/ml = 0.25 U/ml per 105 cells. Platelet-derived growth factor and fibroblast growth factor were also stimulatory...
We have used a catheterization system that permits chronic infusion into the arterial supply of one hindlimb of rats to study the direct effects of rat growth hormone and human somatomedin C on growth of the tibial epiphyseal cartilage plate in hypophysectomized rats. Rat growth hormone (0.4 microgram per day) or human somatomedin C (0.25, 1, or 4 micrograms per day) stimulated growth of the epiphyseal plate of the infused limb but not of that of the contralateral noninfused limb. The somatomedin C had a dose-related effect. Rabbit antiserum to human somatomedin C, but not normal rabbit serum, completely abolished the direct growth effect of the rat growth hormone when it was co-infused with the hormone. These results support the concept that growth hormone stimulates long bone growth by inducing local production of somatomedin, which in turn stimulates cell proliferation in an autocrine or paracrine fashion. However, they do not exclude the possibility that serum somatomedin may also play a role.
Cultured human lymphoid (IM-9) cells were labeled with [35S]methionine in the presence and absence of monensin, a carboxylic ionophore that inhibits post-translational protein maturation. Labeled receptors for insulin and somatomedin C were immunoprecipitated with antibodies specific for each receptor. Monensin inhibits the biosynthesis of mature α and β subunits of both receptors and leads to the accumulation of immunoreactive polypeptides with molecular weights of 180,000. These 180,000 molecular weight polypeptides exist as disulfide-linked dimers and may be biosynthetic precursors of both α and β subunits. In the presence of monensin, small amounts of immunoreactive polypeptides with molecular weights 115,000 and 89,000 also are produced. These may be abnormally processed forms of the α and β subunits lacking residues normally added during terminal glycosylation. In cells treated with monensin, the polypeptides of molecular weights 180,000 and 115,000 can be affinity-labeled with 125I-labeled insulin. These labeled polypeptides are immunoprecipitated by antibodies specific for insulin receptors but not by antibodies specific for somatomedin-C receptors. This indicates that the putative precursors for insulin and somatomedin-C receptors are distinct polypeptides...
The effect of phorbol esters on the extent of phosphorylation of receptors for insulin and somatomedin C (insulin-like growth factor I) was studied in intact IM-9 cells that were labeled by incubation with H332PO4. The tumor-promoting phorbol esters phorbol tetradecanoate acetate (TPA) and phorbol dibutyrate, but not the inactive 4 alpha-phorbol, enhanced phosphorylation of the beta subunit of both receptors approximately 4-fold; 70 nM TPA maximally stimulated phosphorylation of both receptors, whereas concentrations less than or equal to 0.7 nM had no observable effect. Insulin also enhanced the phosphorylation of the beta subunit of the insulin receptor, and its effects appeared to be additive to those of TPA. Peptide maps indicated that at least some of the residues phosphorylated by these two agents are distinct. These results suggest a possible role of protein kinase C in regulating insulin and somatomedin C receptors.
The action of growth hormone on skeletal tissue is mediated through somatomedin, a low molecular weight peptide found in serum. This peptide, formerly known as “sulfation factor” or “thymidine factor,” produces marked insulin-like effects in various target tissues. Since at least some of the metabolic effects of insulin on target cells are initiated by a highly specific interaction with receptors on cell membranes, this study was undertaken to determine whether somatomedin might interact with the same binding sites. It was found that somatomedin, at physiological concentrations, competes with 125I-labeled insulin for receptor sites on isolated fat cells, liver membranes, and isolated chondrocytes, and that the relative binding affinities of insulin and somatomedin reflect the in vitro biological potencies of the two hormones in these tissues. This is the first demonstration of a peptide other than insulin, proinsulin, or derivatives of insulin competing for insulin-binding sites, and implies a structural, as well as a functional, homology between somatomedin and insulin. Somatomedin may be only one of a larger group of homologous pleiotypic peptides with different target organ specificities.
Dietary components responsible for the regulation of somatomedin-C in humans were assessed in five adult volunteers of normal weight who were fasted for 5 d on three occasions, then refed three diets of differing composition. The serum somatomedin-C decreased from a mean prefasting value of 1.85 +/- 0.39 U/ml (+/- 1 SD) to 0.67 +/- 0.16 U/ml at the end of fasting (P less than 0.005). After refeeding for 5 d with a normal diet, the mean serum somatomedin-C increased to 1.26 +/- 0.20 U/ml. A protein-deficient (32% of control), isocaloric diet resulted in a significantly smaller increase, to a mean value of 0.90 +/- 0.24 U/ml (P less than 0.05). A diet deficient in both protein and energy led to a further fall 0.31 +/- 0.06 U/ml. The changes in somatomedin-C during fasting and refeeding correlated significantly with mean daily nitrogen balance (r = 0.90). We conclude that both protein and energy intake are regulators of serum somatomedin-C concentrations in adult humans, and energy intake may be of greater importance. The correlation between changes in somatomedin-C and nitrogen balance suggests that the former are directly related to changes in protein synthesis and may be helpful in assessing the response to nutritional therapy.
This study examines the ontogenesis of somatomedin and insulin receptors in man. Particulate plasma membranes were prepared by ultracentrifugation from various tissues removed from fetuses after abortion and classified as less than 17, 17-25, and greater than 25 cm in length. The binding of iodinated insulinlike growth factors 1 (IGF-1) and 2 (IGF-2), somatomedin A (SMA), multiplication-stimulating activity (MSA), and insulin was examined at the different ages. In the liver, cross-reaction studies revealed separate insulin and IGF-2 receptors. The Scatchard plots of insulin binding to liver membranes were curvilinear and showed an increase in the concentration of insulin receptors with advancing age. A single IGF-2 receptor was found on liver and no alteration was observed during development. The brain contained a lower concentration of insulin receptors. A change in the brain receptors for somatomedins occurred during development. Early in gestation, a high concentration of a low-affinity IGF-1 receptor was found. After approximately the 17th wk of gestation a higher affinity IGF-1 receptor appeared, which then increased in concentration. Cross-reaction studies also revealed changes in the specificity of these receptors during development. In the youngest fetal group IGF-2 was preferentially bound. Around midgestation a separate IGF-1 receptor...
Abnormalities of synovial fluid, as a lubricant and nutrient, may have relevance to the causation of certain articular diseases. The somatomedin activity in normal synovial fluid obtained from the knee joint of the ox has been studied and compared with the activity in serum from the same animal. The porcine costal cartilage bioassay of Van den Brande and Du Caju (1974) has been used with the isotopes 35S-sulphate and 3H-thymidine. The mean potency ratio of ox synovial fluid in terms of ox serum for 35S-sulphate incorporation was 0-28 (range 0-19-0-47) and for 3H-thymidine incorporation 0-35 (range 0-21-0-63). A significant correlation was found between the somatomedin activity (as measured by 35S-sulphate incorporation) and the total protein and albumin concentrations in the ox synovial fluids and the ox sera, but there was no significant relationship between the somatomedin potency ratios and the globulin concentrations. The possible relevance of these findings to injury and disease in synovial joint is discussed.
1. A simple method is described for the determination of small amounts of [35S]sulphated polysaccharide with 95–100% recovery in the range from 0.3 to 150μg of polysaccharide. 2. The method is based on precipitation with cetylpyridinium chloride of polysaccharide samples applied to filter paper. It is not significantly disturbed by the presence in the sample of a large excess of inorganic 35SO42−. 3. Sulphated glucosamino- and galactosaminoglycans may be determined separately by treatment of the sample with chondroitinase ABC. 4. The method is applicable to the assay of [35S]sulphated polysaccharide biosynthesis in cell cultures. A stimulation of sulphate incorporation obeying a linear dose–response curve, was demonstrated in somatomedin-incubated fibroblast and glia cell cultures. 5. The described system provides a new assay method for somatomedin. 6. The stimulatory effect of somatomedin on the synthesis of [35S]sulphated polysaccharide appeared to be general, rather than specific, for a particular type of polysaccharide.
Somatomedin activity was measured using an embryonic chick cartilage assay in 33 normal and short normal children, 23 children with pituitary growth hormone (GH) deficiency, 14 children with sexual precocity, and 13 children with chronic renal insufficiency. In normal children somatomedin activity correlated well with chronological age: low valles in early childhood rose to higher than adult levels at puberty. Children with GH deficiency had significantly lower activities and those with sexual precocity significantly higher activities than normal children. In all three groups somatomedin activity correlated well with bone age. In children with chornic renal insufficiency there was a significant correlation between decreasing somatomedin activity and both a reduced growth velocity and a falling glomerular filtration rate. Somatomedin activity and growth velocity were within normal limits in children with glomerular filtration rates above 30 ml/min/1-73 m2.
A child with Beckwith-Wiedemann syndrome is described. Growth hormone levels were normal, but circulating somatomedin activity was increased. The role of somatomedins in this condition, and the possibility of a feedback mechanism controlling somatomedin production are discussed.
A child presenting with the clinical features of hyposomatotropism but with high immunoreactive plasma growth hormone is described. During short-term administration of human growth hormone (HGH) his response with regard to fasting blood-glucose and free fatty acids, plasma-somatomedin, urinary excretion of calcium, nitrogen, and hydroxyproline was minimal or absent. 6 months of treatment with HGH did not reduce the endogenous HGH secretion. Insulin secretion had not increased and plasma somatomedin levels remained extremely low. Over a period of 2 years of treatment, growth response and loss of subcutaneous fat were minimal. On serial dilution in radioimmunoassay, his growth hormone (GH) molecule yielded a parallel line with the HGH standard. In electrofocusing experiments the GH molecule was in the same pH range as growth hormone in acromegalic plasma and the major peak of clinical grade HGH (5·03 against 5·01 and 4·98).
Vitronectin and plasminogen activator inhibitor-1 (PAI-1) are important
physiological binding partners that work in concert to regulate cellular
adhesion, migration, and fibrinolysis. The high affinity binding site for
PAI-1 is located within the N-terminal somatomedin B domain of vitronectin;
however, several studies have suggested a second PAI-1-binding site within
vitronectin. To investigate this secondary site, a vitronectin mutant lacking
the somatomedin B domain (rΔsBVN) was engineered. The short deletion had
no effect on heparin-binding, integrin-binding, or cellular adhesion. Binding
to the urokinase receptor was completely abolished while PAI-1 binding was
still observed, albeit with a lower affinity. Analytical ultracentrifugation
on the PAI-1-vitronectin complex demonstrated that increasing NaCl
concentration favors 1:1 versus 2:1 PAI-1-vitronectin complexes and
hampers formation of higher order complexes, pointing to the contribution of
charge-charge interactions for PAI-1 binding to the second site. Furthermore,
fluorescence resonance energy transfer between differentially labeled PAI-1
molecules confirmed that two independent molecules of PAI-1 are capable of
binding to vitronectin. These results support a model for the assembly of
higher order PAI-1-vitronectin complexes via two distinct binding sites in
Vitronectin and plasminogen activator inhibitor-1 (PAI-1) are proteins that
interact in the circulatory system and pericellular region to regulate
fibrinolysis, cell adhesion, and migration. The interactions between the two
proteins have been attributed primarily to binding of the somatomedin B (SMB)
domain, which comprises the N-terminal 44 residues of vitronectin, to the
flexible joint region of PAI-1, including residues Arg-103, Met-112, and
Gln-125 of PAI-1. A strategy for deletion mutagenesis that removes the SMB
domain demonstrates that this mutant form of vitronectin retains PAI-1 binding
(Schar, C. R., Blouse, G. E., Minor, K. M., and Peterson, C. B. (2008) J.
Biol. Chem. 283, 10297–10309). In the current study, the
complementary binding site on PAI-1 was mapped by testing for the ability of a
battery of PAI-1 mutants to bind to the engineered vitronectin lacking the SMB
domain. This approach identified a second, separate site for interaction
between vitronectin and PAI-1. The binding of PAI-1 to this site was defined
by a set of mutations in PAI-1 distinct from the mutations that disrupt
binding to the SMB domain. Using the mutations in PAI-1 to map the second site
suggested interactions between α-helices D and E in PAI-1 and a site in
vitronectin outside of the SMB domain. The affinity of this second interaction
exhibited a KD value ∼100-fold higher than that of the
PAI-1-somatomedin B interaction. In contrast to the PAI-1-somatomedin B
The development of a radioimmunoassay for somatomedin-C has for the first time made it possible to discriminate between serum concentrations of a single peptide or closely related group of peptides and the net somatomedin activity measured by less specific bioassay and radioreceptor techniques. Antibodies to human somatomedin-C were raised in rabbits using a somatomedin-C ovalbumin complex as the antigen. A variety of peptide hormones at concentrations up to 1 μM are not recognized by the antibody. Insulin at concentrations >0.1 μM cross reacts in a non-parallel fashion; purified somatomedin-A is only 3% as active as somatomedin-C; and radiolabeled cloned rat liver multiplication stimulating activity does not bind to the antibody. Immunoreactive somatomedin-C can also be quantitated in the sera of a variety of subhuman species.
The somatomedin activity in synovial fluids from 50 patients with a variety of joint diseases has been studied and compared with the activity in each of the patient's own serum and a standard reference serum (SRS). The porcine costal cartilage bioassay of Van den Brande and Du Caju (1974a) has been used with the isotopes 3H-thymidine and 35S-sulphate. Synovial fluids from most patients with post-traumatic and post-operative effusions, osteoarthritis and arthritis associated with psoriasis, Reiter's disease, and ankylosing spondylitis stimulated the synthesis of DNA and proteoglycans in cartilage. Synovial fluids from patients with rheumatoid arthritis either had impaired capacity to stimulate DNA synthesis, or they inhibited it; a similar, but less evident pattern was observed for proteoglycan synthesis. Some synovial fluids from patients with miscellaneous synovitides stimulated, while others inhibited cartilage metabolism. It is concluded that the synovial fluid from patients with rheumatoid arthritis and from some patients with miscellaneous synovitides contained an inhibitor(s) to DNA and possibly proteoglycan synthesis. The sera from nearly all the patients stimulated both DNA and proteoglycan synthesis, but the somatomedin potency ratios for serum in terms of SRS were generally less than 1.0. There was a significant inverse correlation between the serum somatomedin potency ratio and the age of the patient.