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

NGF, EPO, and IGF-1 in the Male Reproductive System

Several studies have demonstrated interesting results considering the implication of three growth factors (GFs), namely nerve growth factor (NGF), erythropoietin (EPO), and the insulin-like growth factor-I (IGF-1) in the physiology of male reproductive functions. This review provides insights into the effects of NGF, EPO, and IGF-1 on the male reproductive system, emphasizing mainly their effects on sperm motility and vitality. In the male reproductive system, the expression pattern of the NGF system varies according to the species and testicular development, playing a crucial role in morphogenesis and spermatogenesis. In humans, it seems that NGF positively affects sperm motility parameters and NGF supplementation in cryopreservation media improves post-thaw sperm motility. In animals, EPO is found in various male reproductive tissues, and in humans, the protein is present in seminal plasma and testicular germ cells. EPO receptors have been discovered in the plasma membrane of human spermatozoa, suggesting potential roles in sperm motility and vitality. In humans, IGF-1 is expressed mainly in Sertoli cells and is present in seminal plasma, contributing to cell development and the maturation of spermatozoa. IGF-1 seems to modulate sperm motility, and treatment with IGF-1 has a positive effect on sperm motility and vitality. Furthermore, lower levels of NGF or IGF-1 in seminal plasma are associated with infertility. Understanding the mechanisms of actions of these GFs in the male reproductive system may improve the outcome of sperm processing techniques.

Insulin-like growth factors and their binding proteins in prostate cancer: Cause or consequence?☆

Insulin-like growth factors (IGFs) promote growth and survival of many types of tumor cells. Epidemiologic studies have implicated carcinogenesis with high levels of IGFs in circulation or in tissues. The levels of IGF binding proteins (IGFBPs) have been associated with reduced risk for prostate and other cancers. Experimental studies have implicated high levels of IGF-I directly and IGFBP-3 inversely in prostate cancer growth, survival, and progression. However, recent evidence suggests a much weaker association of IGF-I with prostate cancer development and a stronger antagonistic association of IGFBP-3 with prostate cancer progression. Considering the clonal heterogeneity and unpredictable progression pattern of prostate cancer, the role of any single growth factor or its regulator (IGFBP) as a single determining factor is limited. This review is a critical appraisal of the role of IGFs, IGFBP, and IGF-I receptor (the IGF axis) in both experimental and clinical prostate cancer genesis and progression.

Modeling of neutral solute transport in a dynamically loaded porous permeable gel: implications for articular cartilage biosynthesis and tissue engineering

A primary mechanism of solute transport in articular cartilage is believed to occur through passive diffusion across the articular surface, but cyclical loading has been shown experimentally to enhance the transport of large solutes. The objective of this study is to examine the effect of dynamic loading within a theoretical context, and to investigate the circumstances under which convective transport induced by dynamic loading might supplement diffusive transport. The theory of incompressible mixtures was used to model the tissue (gel) as a mixture of a gel solid matrix (extracellular matrix/scaffold), and two fluid phases (interstitial fluid solvent and neutral solute), to solve the problem of solute transport through the lateral surface of a cylindrical sample loaded dynamically in unconfined compression with frictionless impermeable platens in a bathing solution containing an excess of solute. The resulting equations are governed by nondimensional parameters, the most significant of which are the ratio of the diffusive velocity of the interstitial fluid in the gel to the solute diffusivity in the gel (Rg), the ratio of actual to ideal solute diffusive velocities inside the gel (Rd), the ratio of loading frequency to the characteristic frequency of the gel (f), and the compressive strain amplitude (epsilon 0). Results show that when Rg > 1, Rd < 1, and f > 1, dynamic loading can significantly enhance solute transport into the gel, and that this effect is enhanced as epsilon 0 increases. Based on representative material properties of cartilage and agarose gels, and diffusivities of various solutes in these gels, it is found that the ranges Rg > 1, Rd < 1, correspond to large solutes, whereas f > 1 is in the range of physiological loading frequencies. These theoretical predictions are thus in agreement with the limited experimental data available in the literature. The results of this study apply to any porous hydrated tissue or material, and it is therefore plausible to hypothesize that dynamic loading may serve to enhance solute transport in a variety of physiological processes.

Distribution of insulin-like growth factor-I mRNA in the mandibular condyle and rib cartilage of the rat during growth

This study makes a molecular biological comparison of primary and secondary cartilage at an early phase of postnatal development. The distribution of insulin-like growth factor-I (IGF-I) mRNA expression in the mandibular condyle and rib cartilage of 1-28-day-old rats was examined after in situ hybridisation using an oligo probe cocktail for IGF-I mRNA. In the condyle, expression was localised to a narrow strip under the articular layer where the cells are undifferentiated. Essentially, no differences were found in IGF-I synthesis within three samples from the same age group or between different age groups. In rib cartilage, IGF-I mRNA was localised within the germinative, proliferative and early hypertrophic cell layers in 1-28-day-old rats. Again, there were no differences in expression among animals of the same age or as a function of age. This pattern of IGF-I mRNA expression indicates that IGF-I synthesis during growth of the mandibular condylar cartilage is different from that of costal cartilage. The findings shed light on the problem of overgrowth often associated with the use of costochondral grafts to replace defective mandibular condyles.

Identification of insulin-like growth factor (IGF)-1 receptor in human sperm cell

The present study was conducted to examine the presence of insulin-like growth factor- receptor (IGF-1R) in human sperm cell. The IGF-1R monoclonal antibody (MAb) specifically immunoprecipitated two protein bands of approximately 135 and approximately 220 kD, respectively, from the reduced human sperm extract. These bands correspond to the alpha-subunit and partially reduced IGF-1R molecule, respectively. The immunocytochemistry experiments indicated the presence of IGF-1R primarily in the equatorial regions, along with a weak binding in the acrosomal regions of human spermatozoa. These data indicate that the IGF-1 receptor is present in the human sperm cell. Since the IGF-1 receptor has tyrosine kinase activity and its ligand IGF-1 is present in the seminal plasma, the IGF-1 system (IGF-1R/IGF-1/IGF-binding protein) may be involved in the signal transduction pathway leading to sperm capacitation and acrosome exocytosis.

Effect of chronic infusion of placental lactogen on ovine fetal growth in late gestation

To test the hypothesis that placental lactogen (PL) is a humoral regulator of fetal growth, six singleton sheep fetuses received a continuous intravenous fusion of 1.2 mg/d of purified ovine PL (oPL) for 14 d, beginning on Day 122 of gestation. The plasma concentration of oPL was approximately four-fold higher in infused fetuses than in six control fetuses that received a continuous infusion of saline. The circulating insulin-like growth factor 1 (IGF-I) concentration was also significantly elevated in PL-infused fetuses (43.1 +/- 1.7 vs. 31.9 +/- 4.1 ng/ml; P < 0.05). Animals were slaughtered on Day 136, and the placenta and all major fetal tissues were dissected, weighed, and subsampled for chemical analysis. Fetal weight and crown-rump length were not significantly affected by treatment; however, the aggregate weight of the brain, liver, lungs, and heart tended to be larger (85.3 +/- 2.1 vs. 79.9 +/- 1.5 g/kg fetus; mean +/- SE, P = 0.07) and the thyroid gland was smaller (0.18 +/- 0.1 vs. 0.26 +/- 0.02 g/kg fetus; P < 0.05) in the PL-infused fetuses. The livers of the PL-infused fetuses had also accumulated additional glycogen (13.1 +/- 1.7 vs. 8.4 +/- 0.7 g; P < 0.05). In late gestation, PL within the fetal compartment increases fetal plasma IGF-I concentration and hepatic glycogen deposition and may affect the growth of several vital organs.

Effects of free and bound insulin-like growth factors on proteoglycan metabolism in articular cartilage explants

This article describes the effects of bound forms of insulin-like growth factors (IGFs) on proteoglycan metabolism by bovine articular cartilage in explant culture. When these growth factors were added to articular cartilage explants complexed with their native serum binding proteins (BPs), both IGF-I-BP complex and IGF-II-BP complex stimulated proteoglycan synthesis to different degrees over a 3-day period. When added to the medium of cultures of articular cartilage over 5 days, IGF-II-BP complex induced high rates of synthesis and low rates of catabolism of proteoglycans, giving rise to tissue levels of proteoglycan similar to those observed in fresh tissue. When articular cartilage was maintained in culture with the same concentration of IGF-I-BP complex, tissue levels of proteoglycans fell over the culture period because of lower rates of proteoglycan synthesis. Analysis of the proteoglycans synthesized by articular cartilage in the presence of free or bound IGF-I or IGF-II showed that these growth factors stimulated the rate of synthesis of the large proteoglycan species present in cartilage but did not affect the synthesis of the small proteoglycans.

In vitro studies on the regulation of endochondral ossification by vitamin D

The research described in this article has focused on the complex autocrine, paracrine, and endocrine regulation of endochondral ossification using vitamin D metabolites and TGF-beta as models. By comparing results from a number of laboratories utilizing a diverse array of in vivo and in vitro systems, a coherent picture is beginning to emerge. Vitamin D metabolites influence cell differentiation and maturation and have direct effects on cell function. Differentiation of the mesenchymal cells into chondroblasts is regulated by both 1,25-(OH)2D3 and 24,25-(OH)2D3, as well as by TGF-beta. The resting zone chondrocytes respond primarily to 24,25-(OH)2D3 in terms of matrix synthesis and matrix vesicle biochemistry. They synthesize both metabolites and other factors that stabilize matrix vesicle enzymes like AHSG. In addition to the paracrine role these factors may play in regulating the matrix, it is possible that they may influence the cells in the growth plate itself. Growth zone chondrocytes also synthesize both metabolites, but respond primarily to 1,25-(OH)2D3 for the parameters measured in the studies described. These cells also synthesize TGF-beta which further increases alkaline phosphatase activity, perhaps via an autocrine stimulation of the cell. While cells from the calcified zone have not yet been studied directly in culture, it is likely that they respond to paracrine signals from the avascular cartilage as well as to serum-derived factors. How the signals are transferred among the cells is unknown. Certainly one can postulate information flow in both upward and downward directions. The signal transduction mechanisms for the factors at the cellular level are complex. While it is known that 1,25-(OH)2D3 stimulates gene transcription and stabilization of mRNA for proteins like alkaline phosphatase, its nongenomic effects are only beginning to emerge. Membrane effects of this metabolite have been shown in intestine and kidney in conjunction with studies on Ca flux. It is becoming increasingly evident that other steroid hormones may operate in similar ways. Studies with the rat costochondral chondrocytes are the first to show that there are specific membrane effects for at least two vitamin D metabolites and that membrane enzymes, including those involved in phospholipid metabolism, can be differentially regulated by them. Furthermore, these experiments have provided for the first time a clear hypothesis for how cells can regulate events in the extracellular matrix after the matrix vesicles are produced and incorporated into the matrix.

Skeletal muscle regeneration in young rats is dependent on growth hormone

Skeletal muscle fibres have a well known ability to regenerate after different kinds of injury. This study was undertaken to establish if regenerating skeletal muscle is dependent on growth hormone (GH) in the same manner as normal, growing skeletal muscle in young rats. Muscle regeneration was achieved by injection of notexin into the soleus muscle. Initial necrosis, which included all muscle fibres, was followed by a rapid and uniform regeneration throughout the muscle. Cell proliferation was estimated by scintillation counting and autoradiography of incorporated [3H]thymidine, injected intravenously 1 h before killing, 7 or 27 days after the initiation of regeneration. GH deficiency was accomplished by hypophysectomy 4 days before the [3H]thymidine injection. Cell proliferation was diminished in both regenerating and normal muscle of the hypophysectomized rats compared to control and GH-substituted rats. After 7 days of regeneration the reduction of cell proliferation seen in hypophysectomized rats was less pronounced in the regenerating than in the normal muscle. These findings demonstrate that GH plays an important role for muscle regeneration in young rats, although other substances appear to be of greater importance during the early stages of regeneration.

Biological activity of insulin-like growth factor-I purified from chicken serum

This study describes a rapid purification of insulin-like growth factor-I from chicken serum and the immunological, biological and receptor binding activity of the peptide. It was purified after initial extraction, by cation exchange chromatography, hydrophobic interaction chromatography and reverse phase chromatography up to 1.4 x 10(6)-fold with an overall yield ranging from 10-30%. The N-terminal amino acid sequence was the same as predicted from the nucleotide sequence of a chicken IGF-I cDNA and the partial sequence obtained from a previously reported purification. The material was both immunologically and biologically active. It had a 50% potency compared to human IGF-I in a radioimmunoassay using an antiserum raised against human IGF-I, stimulated the incorporation of [3H]-thymidine into DNA in cultured chick embryo myoblasts with a half-maximum effective dose of 5 ng/ml and displaced [125I]-labelled human IGF-I and IGF-II from binding sites in microsomal membranes prepared from both the chicken liver and the lactating rabbit mammary gland in a dose dependent manner.

Stimulation of insulin-like growth factor I production in primary cultures of chicken hepatocytes by chicken growth hormone

Extracts of conditioned media from primary cultures of chicken hepatocytes stimulated [3H]thymidine incorporation into chick embryo fibroblasts demonstrating that the cells released mitogen(s) into the medium. There was also a simultaneous release of insulin-like growth factor I (IGF-I) immunoreactivity into the medium. Acid chromatography of freeze-dried extracts of conditioned media by high performance gel permeation chromatography demonstrated that IGF-I immunoreactivity eluted in a major peak with a molecular weight of 7500 Da and a minor peak with a molecular weight of 55,000 Da. The release of IGF-I immunoreactivity was increased by pituitary-derived chicken growth hormone (cGH) in a dose-dependent manner with half-maximum stimulation occurring at a cGH concentration of 40 ng/ml and maximum stimulation at cGH concentrations greater than 800 ng/ml. These results demonstrate that cultured chicken hepatocytes produce IGF-I and that this can be stimulated by cGH in vitro.

The use of biological assays for detection of polypeptide growth factors

Polypeptide growth factors can be identified and quantified with high accuracy by the use of specific biological assays. In general these bioassays are highly sensitive for detection of growth factor activity, and enhanced specificity can be obtained by a proper choice of selective culture conditions for the target cells involved. In this paper sensitive and selective bioassays are described for growth factors acting on substrate-attached cells, in particular members of the epidermal growth factor, transforming growth factor beta, platelet-derived growth factor, insulin-like growth factor, and heparin-binding growth factor families. A cross-reactivity scheme has been worked out to identify possible contaminations in growth factor preparations.

Effects of growth hormone on skeletal muscle. II. Studies on regeneration and denervation in adult rats

The effects of human recombinant growth hormone (rhGH) on regenerating skeletal muscle after ischaemic necrosis and on denervated skeletal muscle were studied in normal adult rats. One group of rats was treated with 4 IE rhGH daily by subcutaneous injections, while control rats were injected with saline. The treatment with rhGH resulted in increased levels of insulin-like growth factor-I (IGF-I) in serum. Ischaemic necrosis was achieved in the extensor digitorum longus (EDL) muscle by cutting the supplying vessels and nerve fascicles at the entrance into the muscle. The wet weight and DNA: protein ration in the regenerating muscle were determined 2 and 4 weeks after the operation. The weight of the regenerating muscles in the rats treated with rhGH during the period of study was larger than in the control rats, while the DNA:protein ratio did not differ significantly between the groups. Denervation of the EDL and soleus muscles followed by subsequent reinnervation was obtained by freezing the sciatic nerve with a forceps chilled in liquid nitrogen. Rats treated with rhGH during the period of denervation and reinnervation, i.e. during the 4 weeks after the freezing of the sciatic nerve, revealed increased weight of both the reinnervated and normal muscles compared to corresponding muscles of control rats. Denervation of the EDL and soleus muscles without subsequent reinnervation was achieved by cutting the sciatic nerve at the level of the thigh. Four weeks after denervation the muscles showed atrophy, mainly affecting type 2 fibres in the EDL muscle and both type 1 and type 2 fibres in the soleus muscle.

Muscle regeneration. The effect of hypophysectomy on cell proliferation and expression of insulin-like growth factor-I

Against the background of the importance of growth hormone (GH) for normal muscle growth, a study was performed to investigate whether lack of GH after hypophysectomy affects the cell proliferation and the local production of insulin-like growth factor-I (IGF-I) in the early stages of muscle regeneration in adult rats. The level of IGF-I in the serum of hypophysectomised rats was reduced to about 30% of that of controls. The incorporation of [methyl-3H]thymidine into the regenerating muscle showed a peak 6 days after the operation and then gradually declined to the end of the period of study 30 days after initiation of regeneration by ischemic necrosis. The DNA content rose to a maximum level after 6-8 days, and remained high after 30 days. There was no major difference in the incorporation of [3H]thymidine in regenerating muscle of hypophysectomised and control rats, but the DNA concentration in the regenerating muscles of hypophysectomised rats was significantly reduced after 30 days. There was a corresponding reduction in the number of nuclei per muscle fibre, indicating that hypophysectomy has a small effect on the cell proliferation during the early stages of muscle regeneration. Immunohistochemical demonstration of IGF-I in the regenerating muscle revealed the transient presence of immunoreactive material in satellite cells and myotubes after 6 to 8 days of regeneration but no immunoreactivity after 30 days. No obvious difference was observed between hypophysectomised and control rats, indicating that the endogenous production of IGF-I in regenerating skeletal muscle can occur independently of GH.

Growth hormone receptors

The receptor for rabbit liver growth hormone (GH) has recently been purified and cloned. Sequencing data demonstrates that this site represents a new class of transmembrane receptor proteins containing covalently linked ubiquitin and N-linked oligosaccharides. Homology with the human GH liver receptor is estimated to be 84%. GH receptors are also present on non-liver cells, however, their function is poorly understood, and it is not yet known if there is homology with the liver site. Direct stimulation of differentiation of preadipocytes to adipocytes by GH has been demonstrated. In bone, binding of GH to the resting cell zone, but not the proliferative layers has been reported. Taken together, these data support the hypothesis of Green and colleagues that GH has a dual mechanism of action which includes direct stimulation of differentiation of precursor cells and a secondary effect of enhancing the clonal proliferative response.

Type I IGF receptors of Leydig cells are upregulated by human chorionic gonadotropin

The effects of human chorionic gonadotropin (hCG) on type I insulin-like growth factor (IGF) receptors of purified Leydig cells were investigated. Sprague-Dawley rats (50 day-old) were treated with a single injection of hCG 10 units intraperitoneally, type I IGF receptors were then determined daily for 4 days. HCG caused a rapid increase in type I IGF receptors within 24 h, which returned to basal by 72 h. There was no significant change in binding affinity. Our present study indicates that type I IGF receptors of Leydig cells are up regulated by hCG, and this may be one mechanism by which hCG and IGF-I interact to enhance Leydig cell steroidogenesis.

Insulin-like growth factor-I augments gonadotropin-stimulated androgen biosynthesis by cultured rat testicular cells

In addition to the well-known growth stimulating effects of insulin-like growth factors (IGFs), recent studies suggest that these peptides may also modulate the differentiated functions of endocrine cells. Thus, in the present studies, we have investigated the actions of IGFs on androgen biosynthesis by cultured testicular cells. Treatment of cells obtained from adult hypophysectomized rats with LH (1 ng/ml) stimulated testosterone production 60-fold over basal levels. In contrast, treatment with either synthetic human IGF-I or IGF-II failed to stimulate androgen production. However, concomitant treatment of the LH-containing cultures with increasing doses of IGF-I (10-500 ng/ml) augmented testosterone production up to 70% over that seen with LH alone (ED50 = 67 ng/ml). Similar effects were obtained with IGF-II but this peptide was about 10-fold less potent than IGF-I. In addition, these peptides also stimulated the accumulation of pregnenolone and progesterone in the culture medium. Additional studies demonstrated specific binding of [125I]iodo-IGF-I to testicular cells. This binding was competed by IGF-related peptides with the potency order IGF-II = IGF-I greater than insulin whereas unrelated peptides did not compete. The cellular localization of these receptors was examined in testicular cells separated on a metrizamide density gradient. IGF-I receptors were evenly distributed between two cell peaks containing subpopulations of Leydig cells whereas much less binding was found in other testicular cell types. Coupled with recent findings indicating testicular production of IGF-I, the present results suggest that this peptide may act as a positive intratesticular modulator of Leydig cell differentiation.

Effects of unilateral arterial infusion of GH and IGF-I on tibial longitudinal bone growth in hypophysectomized rats

We have studied the effect of local arterial infusion of bacterially produced human growth hormone (hGH), insulinlike growth factor I (IGF-I), or pituitary-derived ovine prolactin (oPRL) on longitudinal bone growth of hypophysectomized rats. The substances were infused during a 14-day period by osmotic mini-pumps through a catheter which was implanted into the femoral artery of one hindlimb. Longitudinal bone growth was measured by the intravital marker tetracycline. Infusion of 1 microgram hGH per day stimulated bone growth only of the treated limb and not of the uninfused contralateral limb. Infusion of 10 micrograms hGH per day also stimulated unilateral longitudinal bone growth, but the uninfused contralateral limb also showed a significant growth response, probably because local administration of GH at this dose caused a significant elevation of GH in the systemic circulation. As a result, the differential growth response between the GH-treated and untreated limbs decreased compared to rats that were infused with 1 microgram hGH per day. Unilateral arterial infusion of 5 micrograms human IGF-I or 10 micrograms oPRL per day did not produce a significant growth response. The results of the present study confirm the observation by Schlechter and co-workers, who demonstrated that unilateral arterial infusion of GH maintained tibial cartilage width following hypophysectomy in the rat. The results of Schlechter and coworkers and the results of the present study show that GH in vivo stimulates epiphyseal cartilage growth directly. However, an increased local production of insulinlike growth factors is probably of importance for the expression of the direct effect of GH on longitudinal bone growth.(ABSTRACT TRUNCATED AT 250 WORDS)

Preferential association of the insulin-like growth factors I and II with metabolically inactive and active carrier-bound complexes in serum

Ion-exchange chromatography of serum on DEAE-Sephadex A-50 using a stepwise NaCl gradient showed that complexes enriched with insulin-like growth factors I and II (IGF-I and IGF-II) could be preferentially eluted. A fraction eluted with 0.075 M-NaCl preferentially contained immunoreactive IGF-I with peak levels appearing in fractions of Mr approx. 110,000. The IGF-I-binding protein complex itself had low bioactivity as measured in a non-suppressible insulin-like (NSILA) bioassay. On conversion to free IGF-I by gel-permeation chromatography on Sephadex G-75 in 1% formic acid, however, the IGF-I did express its intrinsic NSILA bioactivity. In contrast, an IGF-II-enriched complex was eluted from the DEAE-Sephadex with 0.15 M-NaCl. Practically all of the recovered NSILA of the original serum was present in this fraction, in the Mr range 70,000-300,000 with a peak of 150,000. Chromatography on Sephadex G-75 in 1% formic acid separated this high-Mr NSILA into low-Mr (less than 15000) IGF-II and high-Mr acid-stable NSILA-P. The high-Mr IGF-II complex bound to concanavalin A-Sepharose, suggesting that it was a glycoprotein. The results confirm previous reports that a large portion of the NSILA of whole serum can be accounted for by a biologically active acid-dissociable complex. These data show for the first time that this active complex consists of an IGF-II-preferring binding protein. In direct contrast, the IGF-I-preferring complex does not express NSILA bioactivity until the IGF-I is liberated through acidification. The presence of a metabolically active IGF-II complex in serum raises questions as to its possible biological role in the adult.

Stimulation of proteoglycan biosynthesis by serum and insulin-like growth factor-I in cultured bovine articular cartilage

The addition of foetal calf serum to explant cultures of adult bovine articular cartilage is known to stimulate proteoglycan synthesis in a dose-dependent manner. We have now shown the activity in serum responsible for this effect to be heat- and acid-stable, to be associated with a high-Mr complex in normal serum but converted to a low-Mr form under acid conditions. The activity has an apparent Mr approximately 10,000 and isoelectric points similar to those reported for insulin-like growth factors (IGFs). Addition of a monoclonal antibody against insulin-like growth factor-I (IGF-I) prevented foetal calf serum from stimulating proteoglycan synthesis. Physiological concentrations of recombinant IGF-I or pharmacological levels of insulin when added to cartilage cultures mimicked the proteoglycan-stimulatory activity of serum. IGF-I appeared to act by increasing the rate of proteoglycan synthesis and did not change the nature of the proteoglycan synthesized nor the rate of proteoglycan catabolism by the tissue, suggesting that IGF-I may be important in the regulation of proteoglycan metabolism in adult articular cartilage. Furthermore, IGF-I can replace foetal calf serum in the culture medium, thereby allowing the use of a fully-defined medium which will maintain the synthesis and tissue levels of proteoglycan in adult articular cartilage explants for up to 5 days.

Chemical and biological characterization of low-molecular-weight human skeletal growth factor

Skeletal growth factor (SGF) activity was extracted from human bone matrix by demineralization and purified under dissociative conditions using hydroxyapatite, HPLC gel-filtration and HPLC reverse-phase chromatography. Human SGF thus purified was characterized chemically and biologically. Purified human SGF stimulated chick embryo bone cell proliferation at picomolar concentrations (half maximum at 2-3 ng/ml) and had little or no activity on other cell types tested (mouse 3T3 and normal rat kidney fibroblasts, embryonic chick intestinal and human placental cells). Human SGF did not displace 125I-labeled epidermal growth factor binding to normal rat kidney cells and did not stimulate normal rat kidney cell colony formation in soft agar. Human SGF activity was sensitive to trypsin, chymotrypsin, papain, dithiothreitol and performic acid but was resistant to heat (upto 70 degrees C), pH (3-10), cyanogen bromide, alkaline phosphatase and neuraminidase and did not bind jack bean concanavalin A or kidney bean lectin. From our chemical and biological studies it appears that human SGF is different from other known polypeptide growth factors: epidermal growth factor, fibroblast growth factor, insulin, insulin-like growth factor-I, platelet-derived growth factor and transforming growth factor.

Evidence suggesting that the direct growth-promoting effect of growth hormone on cartilage in vivo is mediated by local production of somatomedin

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.

Regulation by growth hormone of number of chondrocytes containing IGF-I in rat growth plate

Whether growth hormone stimulates longitudinal bone growth by a direct effect at the site of the growth plate or indirectly by increasing the concentration of circulating somatomedins (insulin-like growth factors) has been the subject of controversy. Immunohistochemical methods were used to explore the localization and distribution of insulin-like growth factor I (IGF-I) immunoreactivity in the epiphyseal growth plate of the proximal tibia of male rats. Cells in the proliferative zone of the growth plate of normal rats exhibited a bright immunofluorescence, whereas cells in the germinal and hypertrophic zones stained only weakly. In rats subjected to hypophysectomy, the number of fluorescent cells was markedly reduced. When the hypophysectomized rats were treated with growth hormone, either systemically or at the site of the growth plate, the number of IGF-I-immunoreactive cells in the proliferative zone was increased. The results show that IGF-I is produced in proliferative chondrocytes in the growth plate and that the number of IGF-I-containing cells is directly regulated by growth hormone. These findings suggest that IGF-I has a specific role in the clonal expansion of differentiated chondrocytes and exerts its function locally through autocrine or paracrine mechanisms.

Direct stimulatory effects of insulin-like growth factor-I on Leydig cell steroidogenesis in primary culture

Insulin-like growth factor-I (IGF-I) in concentration as low as 10 ng/ml significantly increased basal testosterone formation and 100 ng/ml of IGF-I increased testosterone production more than two fold in primary cultures of purified mature Leydig cells. IGF-I also markedly potentiated hCG-induced testosterone formation in a dose-dependent manner. Furthermore, IGF-I enhanced 8-bromo cyclic AMP induced steroidogenesis and hCG-stimulated cyclic AMP formation. The binding of 125I-IGF-I to purified Leydig cells was linear with a binding affinity of 0.56 +/- 0.07 X 10(9) M-1 and a capacity of 167 +/- 10.2 fmol/mg protein. Insulin and multiplication-stimulating activity were less potent than IGF-I in competing the binding of 125I-IGF-I to purified Leydig cells. This suggests that Leydig cells contain specific type I IGF receptor and IGF-I could modulate Leydig cell steroidogenesis.

Identification and characterization of specific binding proteins for growth hormone in normal human sera

The well-recognized "big" forms (45,000-100,000 mol wt) of immunoreactive human growth hormone (hGH) in human serum have been reported to be random aggregates or formal polymers. However, we have now investigated the possibility that they are protein-bound forms. After incubation of monomeric 125I-hGH with normal serum, gel chromatography indicated a peak of bound 125I-hGH (at approximately 120,000 mol wt), which was completely displaced by excess unlabeled hGH. When serum alone was chromatographed two peaks of specific binding were subsequently detected, the major peak, eluting between 74,000 and 85,000 mol wt corresponded to the 125I-hGH-binding protein complex observed at approximately 120,000 mol wt. Using a mini-gel filtration system for separating bound from free hormone, binding of 125I-hGH by normal human serum was dependent on time (equilibrium was reached in 2 h at 21 degrees C), temperature (21 degrees C greater than 37 degrees C), Ca2+ and serum concentrations. Binding was reversible and highly specific for hGH, not being displayed by GH or prolactins from several species. Scatchard analysis revealed linear plots with an affinity (KA) of 0.32 +/- 0.06 X 10(9) M-1 (n = 7). Human serum with low endogenous hGH levels, when added to rabbit liver membranes, decreased the binding of 125I-hGH in this tissue in a dose-dependent manner. These data indicate that human sera contain a specific, high affinity binding protein for hGH and that this may account, at least in part, for the known size heterogeneity of GH in serum. Its effect on GH binding to target tissues may indicate a role for the binding protein in the regulation of GH action.

Effects of local administration of GH and IGF-1 on longitudinal bone growth in rats

The effect of local administration of growth hormone (GH) and insulinlike growth factor 1 (IGF-1) on longitudinal bone growth was studied in the proximal tibia of hypophysectomized rats, by using the tetracycline method. Human GH (hGH) stimulated local bone growth when administered into the epiphysial growth plate, into the epiphysis through an implanted cannula, or into the knee joint intraarticularly. In contrast, hGH administration into the metaphysis did not cause such a stimulation. The effect of hGH was dose dependent, and the lowest daily dose of hGH that caused a stimulation was 50 ng. hGH produced by cloned bacteria was as effective as pituitary-derived hGH, excluding the possibility of a pituitary growth factor being the active compound. GH from other mammalian species (rat GH, ovine GH, and bacterially produced bovine GH) also stimulated local bone growth. Ovine prolactin (oPRL) stimulated local bone growth but the threshold dose of oPRL was approximately 100 times higher than that of hGH, suggesting that contamination of this preparation by GH may account for the stimulation. Reduced carboxymethylated human GH, that has a greatly reduced anabolic activity, did not stimulate local bone growth. Local administration of 5 micrograms of bacterially produced human IGF-1 per day produced a small but significant effect on unilateral bone growth. Simultaneous administration of hGH had no additive effect with, nor did it potentiate, the stimulatory effect of IGF-1. The present study confirms and extends earlier investigations, showing that local injection of GH at the site of the epiphysial growth plate stimulates unilateral bone growth. The study also shows that local administration of IGF-1 stimulates longitudinal bone growth.

Specific non-saturable binding of insulin by human platelets

Studies on the nature and characterization of the specific binding of 125I-insulin to intact human platelets have been undertaken. Although under conditions of physiological buffer osmolality, a binding equilibrium was attained in 4-6 h at 4 degrees C, at higher temperatures (17 degrees C or 24 degrees C) equilibrium was reached only in the presence of very high buffer osmolality or 25 mM NaF. Under conditions of normal osmolality and in the absence of NaF, binding at 17 degrees C was not saturable. This phenomenon was specific for insulin or insulin-like hormones (the insulin-like growth factors) and did not occur with 125I-labelled growth hormone, ACTH or beta-endorphin. The non-saturable uptake of insulin appeared due to an energy-dependent specific sequestration or internalization of insulin by mechanisms probably involving the microtubule system. This study emphasizes the need to restrict this non-saturable uptake of insulin if one wishes to adequately study the platelet plasma membrane receptors for insulin. These data also indicate that there is a major interaction of insulin and insulin-like hormones with normal human platelets and support previous demonstrations of major insulin effects on platelet physiology in both normal and diabetic states.

Somatomedin C augments FSH-induced differentiation of cultured rat granulosa cells

Growth hormone (GH) deficiency in rats is associated with decreased ovarian steroidal responsiveness to gonadotropins, possibly through a reduction in the production of the GH-dependent Somatomedin C/insulinlike growth factor I (SM C/IGF I). We have investigated the direct effects of synthetic SM C/IGF I on gonadotropin-stimulated ovarian steroidogenesis in vitro. Granulosa cells were cultured in a serum-free medium for 48 h in the presence of follicle-stimulating hormone (FSH), with or without SM C/IGF I. FSH dose-dependently increased both estrogen and progestin production. Concomitant treatment with SM C/IGF I led to a dose-dependent augmentation of progestin secretion over the full range of FSH doses tested, by a maximum of 2.3- to 2.6-fold. FSH-stimulated estrogen was enhanced by up to 2.4-fold but only at low doses of FSH. SM C/IGF I-enhanced progestin production was associated with increased pregnenolone production and 3 beta-hydroxysteroid dehydrogenase activity, whereas augmented estrogen production appeared to be due to enhanced aromatase activity. The actions of SM C/IGF I, at physiologically relevant concentrations were correlated with increased extracellular cAMP accumulation and cellular protein content but were independent of any change in cell number or viability. In contrast to SM C/IGF I, the closely related peptide multiplication-stimulating activity decreased estrogen production while increasing progestin metabolite accumulation. The present results indicate that the GH-dependent peptide SM C/IGF I may play a role in ovarian development by enhancing gonadotropin-stimulated granulosa cell steroidogenesis.

Evidence for the specific binding of growth hormone to a receptor-like protein in rabbit serum

The binding in vitro of 125I-human or bovine growth hormone (GH) to normal female rabbit serum has been studied using gel filtration to separate bound and free hormone. On Ultrogel AcA34 columns, a substantial peak of specific 125I-GH binding was observed at a MW approximately 120 000. This peak was not precipitable by 12.5% polyethylene glycol, a method used widely for solubilized hormone receptors. Assuming a 1:1 binding stoichiometry between GH (MW 21 000) and the binding protein, the MW of the binding protein would be approximately 100 000. Gel filtration of serum alone, followed by assessment of 125I-hGH binding in column fractions, indicated the binding protein had a similar MW (83 000-107 000). Specific binding of 125I-hGH was dependent on incubation time (equilibrium being reached in 2 h at 21 degrees C), Ca2+ concentration (0.5-2.0 mM) and serum concentration (a 1:5 dilution of serum giving 45.2 +/- 1.7% specific binding; mean +/- SE, n = 10). Binding was completely reversible (t 1/2 approximately 1.5 h) and specific for somatotrophic but not lactogenic hormones. Scatchard analysis revealed linear plots with a Ka 1.59 +/- 0.11 X 10(9) M-1 and capacity 3700 fmol/ml serum. The presence in rabbit serum of a high affinity, GH-specific, binding protein raises important questions regarding its identity and possible physiological role in modulating the delivery and/or activity of GH in vivo.

Chemical synthesis of a gene for somatomedin C

A synthetic gene for somatomedin C, a human growth factor, has been assembled by a single ligation of 23 oligodeoxyribonucleotides, which were chemically synthesized by an improved solid phase phosphotriester method.

Developmental and tissue-specific expression of a family of transcripts related to rat insulin-like growth factor II mRNA

We have constructed a cDNA library from the mRNA of a rat liver cell line (BRL-3A) and characterized cDNA clones encoding the protein precursor of the rat insulin-like growth factor II (pre-pro-rIGF-II). This precursor, inferred from the nucleotide sequence, consists of a signal peptide, the rIGF-II sequence, and a trailer polypeptide of unknown significance. The characterized cDNA sequence (1016 nt) is part of a 3.4 kb mRNA species. Northern analysis reveals that a probe containing the extreme 5' noncoding region hybridizes to a second RNA (1.6 kb), while a probe corresponding to the 5' noncoding region proximal to the coding region hybridizes to two other RNA species (1.75 and 1.1 kb). All four RNAs are differentially expressed in all of the neonatal tissues that were examined, while the 3.4 kb pre-pro-rIGF-II mRNA and the 1.1 kb transcript are absent from adult tissues.

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.

Partial purification of a specific inhibitor of the insulin-like growth factors by reversed-phase high-performance liquid chromatography

We report here preliminary data using reversed-phase high-performance liquid chromatography for the purification of a specific inhibitor (a molecular weight 16,000-18,000 protein) of the insulin-like growth factor (IGF) or somatomedin family. Crude inhibitor prepared from Cohn fraction IV-1 of human serum was first partially purified using an IGF/CH-Sepharose 4B affinity column. Following elution of the bound inhibitor and resuspension in 0.1% aqueous trifluoroacetic acid (mobile phase A), it was injected (100 microliter; 2.0 mg protein) onto a Brownlee Aquapore RP-300 column. Application of a linear gradient from 0% to 100% mobile phase B (45% isopropanol-0.1% trifluoroacetic acid) resulted in elution of two peaks of inhibitor activity between 31% and 34% isopropanol associated with a major homogeneous protein peak and a minor heterogeneous protein peak. No inhibitor was recovered when an acetonitrile gradient was used instead of isopropanol, indicating that the inhibitor is very hydrophobic. These data suggest that high-performance liquid chromatography offers a simple procedure for the potential purification of IGF inhibitor(s) from normal human serum.

Changes in endochondral bone elongation rates during pregnancy and lactation in rats

The rates of endochondral bone elongation during pregnancy and lactation in rats have been studied. The rate of growth at the distal femoral epiphyseal growth plate was measured using fluorescent bone markers. Endochondral growth rates were substantially increased in pregnant animals when compared with age-matched, nonmated controls. There were also increases in growth plate thickness, hypertrophic cell lacunar height, and the calculated rate of cell production during pregnancy. At parturition, this growth trend was reversed and during lactation there were significant decreases in endochondral growth rates. There were also corresponding decreases in growth plate thickness, hypertrophic cell lacunar height, and the calculated rate of cartilage cell production. These results indicate that significant changes occur in maternal endochondral growth rates during the reproductive cycle in rats.