A unique receptor-independent mechanism by which insulinlike growth factor I regulates the availability of insulinlike growth factor binding proteins in normal and transformed human fibroblasts

The Origination of Growth Hormone/Insulin-Like Growth Factor System: A Story From Ancient Basal Chordate Amphioxus

The growth hormone/insulin-like growth factor (GH/IGF) system, also called the pituitary-liver axis, has a somatotrophic role in the body. Although the GH/IGF system has always been regarded as a vertebrate-specific endocrine system, its actual origin remained unknown for a long time. The basal chordate, amphioxus, occupies an evolutionary position between vertebrates and invertebrates. Impressively, most of the members of the GH/IGF system are present in the amphioxus. The GH-like molecule in the amphioxus is mainly expressed in Hatschek's pit. It functions similarly to vertebrate GH and has a GH receptor-like binding partner. The amphioxus IGF-like peptide shows mitogenic activity and an expression pattern resembling that of vertebrate IGF-I. The receptor of IGF-like peptide and IGF binding protein (IGFBP) have also been demonstrated to exist in the amphioxus. These results reveal the origin of the gene families in the GH/IGF system, providing strong evidence that this system emerged in the amphioxus.

Insulin- like Growth Factor-Binding Protein Action in Bone Tissue: A Key Role for Pregnancy- Associated Plasma Protein-A

The insulin-like growth factor (IGF) axis is required for the differentiation, development, and maintenance of bone tissue. Accordingly, dysregulation of this axis is associated with various skeletal pathologies including growth abnormalities and compromised bone structure. It is becoming increasingly apparent that the action of the IGF axis must be viewed holistically taking into account not just the actions of the growth factors and receptors, but also the influence of soluble high affinity IGF binding proteins (IGFBPs).There is a recognition that IGFBPs exert IGF-dependent and IGF-independent effects in bone and other tissues and that an understanding of the mechanisms of action of IGFBPs and their regulation in the pericellular environment impact critically on tissue physiology. In this respect, a group of IGFBP proteinases (which may be considered as ancillary members of the IGF axis) play a crucial role in regulating IGFBP function. In this model, cleavage of IGFBPs by specific proteinases into fragments with lower affinity for growth factor(s) regulates the partition of IGFs between IGFBPs and cell surface IGF receptors. In this review, we examine the importance of IGFBP function in bone tissue with special emphasis on the role of pregnancy associated plasma protein-A (PAPP-A). We examine the function of PAPP-A primarily as an IGFBP-4 proteinase and present evidence that PAPP-A induced cleavage of IGFBP-4 is potentially a key regulatory step in bone metabolism. We also highlight some recent findings with regard to IGFBP-2 and IGFBP-5 (also PAPP-A substrates) function in bone tissue and briefly discuss the actions of the other three IGFBPs (-1, -3, and -6) in this tissue. Although our main focus will be in bone we will allude to IGFBP activity in other cells and tissues where appropriate.

Key questions and answers about pregnancy-associated plasma protein-A

Twenty-five years after it was identified as a circulating protein of unknown function derived from the placenta, pregnancy-associated plasma protein-A (PAPP-A) was discovered to be a novel zinc metalloproteinase expressed by a variety of cell types. Great progress has been made in understanding the biology of PAPP-A and its regulation during recent years, especially in regard to physiological and pathophysiological inflammatory injury responses. However, much remains to be learned about this complex protein and its potential clinical implications outside pregnancy. In this article we address some of the outstanding questions about PAPP-A, in particular about its newly emerging role in the insulin-like growth factor (IGF) system.

Insulin-like growth factor binding protein (IGFBP)-mediated hair cell survival on the mouse utricle exposed to neomycin: the roles of IGFBP-4 and IGFBP-5

CONCLUSION

This study suggests for the first time that 1) IGF-I, IGFBP-4, and -5 alone and IGF-I+IGFBP-5 mixture stimulated hair cell survival and prevented neomycin-induced hair cell loss in the sensory epithelial culture of mouse utricles, 2) When administered together, IGFBP-4 diminished the effect of IGF-I, 3) In P3-5 mice utricle, IGF-I, IGFBP-4, and IGFBP-5 are expressed in the cytoplasm of hair cells. And Insulin/IGF-I Receptor is expressed in the nucleus of hair cells.

OBJECTIVES

Several growth factors have been demonstrated to protect auditory sensory cells in vitro and in vivo from aminoglycoside toxicity. IGF-I is one of the most well-known mitogenic and protective substance working in the inner ear. However, there are no reports available regarding the function of IGFBPs in the inner ear. In the present study, the effects of IGFBP-4 and -5 on hair cell survival were investigated in mouse utriclular organ cultures.

MATERIALS AND METHODS

The amount of cellular damage and cell viability in vestibular organs were assessed by counting hair cells stained with a rhodamine-phalloidin probe. The expressions of IGFBP-4, IGFBP-5, IGF-IR, and IGF-I were localized by immunohistochemistry.

RESULTS

When treated with IGF-I, IGFBP-4, or IGFBP-5 for 24 h, explant culture showed hair cell survival rates of 136+/-18%, 140+/-15%, and 133+/-6%, respectively, compared to controls. Neomycin (1 mM) induced hair cell loss resulted in 45+/-17% of hair cell survival. However, pre-treatment of IGF-I, IGFBP-4, or -5 before neomycin insult showed survival rates of 113+/-14%, 98+/-8%, and 73+/-24%, respectively. Similar to IGF-I, IGFBP-4 and IGFBP-5 were significantly protective. IGFBP-4 and -5 immunoreactivities were observed in the cytoplasm of normal explanted vestibular hair cells as well as in the P3 mouse utricular hair cells in vivo.

Molecular interactions in the insulin-like growth factor (IGF) axis: a surface plasmon resonance (SPR) based biosensor study

This review describes a comprehensive analysis of a surface plasmon resonance (SPR)-based biosensor study of molecular interactions in the insulin-like growth factor (IGF) molecular axis. In this study, we focus on the interaction between the polypeptide growth factors IGF-I and IGF-II with six soluble IGF binding proteins (IGFBP 1-6), which occur naturally in various biological fluids. We have describe the conditions required for the accurate determination of kinetic rate constants for these interactions and highlight the experimental and theoretical pitfalls, which may be encountered in the early stages of such a study. We focus on IGFBP-5 and describe a site-directed mutagenesis study, which examines the contribution of various residues in the protein to high affinity interaction with IGF-I and -II. We analyse the interaction of IGFBP-5 (and IGFBP-3) with heparin and other biomolecules and describe experiments, which were designed to monitor multi-protein complex formation in this molecular axis.

The insulin-like growth factor system and colorectal cancer: clinical and experimental evidence

OBJECTIVE

The aim of this review is to clarify the involvement of the insulin-like growth factor (IGF) system in the development of colorectal malignancy.

MATERIALS AND METHODS

Medline searches were used to identify key articles relating the IGF system with the development of colorectal cancer.

RESULTS

The IGF system has been linked to colorectal malignancy by a convergence of data from epidemiological, clinical and laboratory-based sources.

CONCLUSION

Further work is needed to characterise the IGF system expression in the colon. Such clarification could lead to the identification of targets that can be manipulated for clinical advantage.

Insulin-like growth factor-binding protein-5 (IGFBP-5): a critical member of the IGF axis

The six members of the insulin-like growth factor-binding protein family (IGFBP-1-6) are important components of the IGF (insulin-like growth factor) axis. In this capacity, they serve to regulate the activity of both IGF-I and -II polypeptide growth factors. The IGFBPs are able to enhance or inhibit the activity of IGFs in a cell- and tissue-specific manner. One of these proteins, IGFBP-5, also has an important role in controlling cell survival, differentiation and apoptosis. In this review, we report on the structural and functional features of the protein which are important for these effects. We also examine the regulation of IGFBP-5 expression and comment on its potential role in tumour biology, with special reference to work with breast cancer cells.

Isolation and validation of human prepubertal skeletal muscle cells: maturation and metabolic effects of IGF-I, IGFBP-3 and TNFalpha

We have developed a primary skeletal muscle cell culture model derived from normal prepubertal children to investigate the effects of insulin-like growth factor-I (IGF-I), insulin-like growth factor binding protein-3 (IGFBP-3) and tumour necrosis factor alpha (TNFalpha) on growth, differentiation and metabolism. Cells of myoblast lineage were characterized morphologically by desmin staining and differentiated successfully into multinucleated myotubes. Differentiation was confirmed biochemically by an increase in creatine kinase (CK) activity and IGFBP-3 secretion over time. IGF-I promoted whilst TNFalpha inhibited myoblast proliferation, differentiation and IGFBP-3 secretion. IGF-I partially rescued the cells from the inhibiting effects of TNFalpha. Compared to adult myoblast cultures, children's skeletal muscle cells demonstrated higher basal and day 7 CK activities, increased levels of IGFBP-3 secretion, diminished IGF-I/TNFalpha action and absence of the inhibitory effect of exogenous IGFBP-3 on differentiation. Additional studies demonstrated that TNFalpha increased basal glucose transport via GLUT1, nitric oxide synthase and p38MAPK-dependent mechanisms. These studies provide baseline data to study the interactivity effects of growth factors and cytokines on differentiation and metabolism in muscle in relation to important metabolic disorders such as obesity, type II diabetes or chronic wasting diseases.

Insulin-like Growth Factor-binding Protein-3 Plays an Important Role in Regulating Pharyngeal Skeleton and Inner Ear Formation and Differentiation

Insulin-like growth factor-binding protein (IGFBP)-3 is the major insulin-like growth factor (IGF) carrier protein in the bloodstream. IGFBP-3 prolongs the half-life of circulating IGFs and prevents their potential hypoglycemic effect. IGFBP-3 is also expressed in many peripheral tissues in fetal and adult stages. In vitro, IGFBP-3 can inhibit or potentiate IGF actions and even possesses IGF-independent activities, suggesting that local IGFBP-3 may also have paracrine/autocrine function(s). The in vivo function of IGFBP-3, however, is unclear. In this study, we elucidate the developmental role of IGFBP-3 using the zebrafish model. IGFBP-3 mRNA expression is first detected in the migrating cranial neural crest cells and subsequently in pharyngeal arches in zebrafish embryos. IGFBP-3 mRNA is also persistently expressed in the developing inner ears. To determine the role of IGFBP-3 in these tissues, we ablated the IGFBP-3 gene product using morpholino-modified antisense oligonucleotides (MOs). The IGFBP-3 knocked down embryos had delayed pharyngeal skeleton morphogenesis and greatly reduced pharyngeal cartilage differentiation. Knockdown of IGFBP-3 also significantly decreased inner ear size and disrupted hair cell differentiation and semicircular canal formation. Furthermore, reintroduction of a MO-resistant form of IGFBP-3 "rescued" the MO-induced defects. These findings suggest that IGFBP-3 plays an important role in regulating pharyngeal cartilage and inner ear development and growth in zebrafish.

Cellular actions of the insulin-like growth factor binding proteins

In addition to their roles in IGF transport, the six IGF-binding proteins (IGFBPs) regulate cell activity in various ways. By sequestering IGFs away from the type I IGF receptor, they may inhibit mitogenesis, differentiation, survival, and other IGF-stimulated events. IGFBP proteolysis can reverse this inhibition or generate IGFBP fragments with novel bioactivity. Alternatively, IGFBP interaction with cell or matrix components may concentrate IGFs near their receptor, enhancing IGF activity. IGF receptor-independent IGFBP actions are also increasingly recognized. IGFBP-1 interacts with alpha(5)beta(1) integrin, influencing cell adhesion and migration. IGFBP-2, -3, -5, and -6 have heparin-binding domains and can bind glycosaminoglycans. IGFBP-3 and -5 have carboxyl-terminal basic motifs incorporating heparin-binding and additional basic residues that interact with the cell surface and matrix, the nuclear transporter importin-beta, and other proteins. Serine/threonine kinase receptors are proposed for IGFBP-3 and -5, but their signaling functions are poorly understood. Other cell surface IGFBP-interacting proteins are uncharacterized as functional receptors. However, IGFBP-3 binds and modulates the retinoid X receptor-alpha, interacts with TGFbeta signaling through Smad proteins, and influences other signaling pathways. These interactions can modulate cell cycle and apoptosis. Because IGFBPs regulate cell functions by diverse mechanisms, manipulation of IGFBP-regulated pathways is speculated to offer therapeutic opportunities in cancer and other diseases.

Pregnancy-associated plasma protein-A (PAPP-A) cleaves insulin-like growth factor binding protein (IGFBP)-5 independent of IGF: implications for the mechanism of IGFBP-4 proteolysis by PAPP-A

Pregnancy-associated plasma protein-A (PAPP-A) has recently been identified as the proteinase responsible for cleavage of insulin-like growth factor binding protein (IGFBP)-4, an inhibitor of IGF action, in several biological fluids. Cleavage of IGFBP-4 by PAPP-A is believed to occur only in the presence of IGF. We here report that in addition to IGFBP-4, PAPP-A also cleaves IGFBP-5. Cleavage occurs at one site, between Ser-143 and Lys-144 of IGFBP-5. In the presence of IGF, IGFBP-4 and -5 are cleaved with similar rates by PAPP-A. Interestingly, cleavage of IGFBP-5 by PAPP-A does not require the presence of IGF, but is slightly inhibited by IGF. These findings have implications for the mechanism of proteolysis of IGFBP-4 by PAPP-A, suggesting that IGFBP-4 binds IGF, which then becomes a PAPP-A substrate. Using highly purified, recombinant proteins, we establish that (1) PAPP-A cleavage of IGFBP-4 can occur in the absence of IGF, although the rate of hydrolysis is very slow, and (2) IGF is unable to bind to PAPP-A. We thus conclude that IGF enhances proteolysis by binding to IGFBP-4, not by interaction with PAPP-A, which could not previously be ruled out.

Evidence that the interaction between insulin-like growth factor (IGF)-II and IGF binding protein (IGFBP)-4 is essential for the action of the IGF-II-dependent IGFBP-4 protease

A variety of human cell types, including human osteoblasts (hOBs), produce an IGFBP-4 protease, which cleaves IGFBP-4 in the presence of IGF-II. Recently, the pregnancy-associated plasma protein (PAPP)-A has been determined to be the IGF-II-dependent IGFBP-4 protease produced by human fibroblasts. This study sought to define the mechanism by which IGF-II enhances IGFBP-4 proteolysis. Addition of PAPP-A antibody blocked the IGFBP-4 proteolytic activity in hOB conditioned medium (CM), suggesting that PAPP-A is the major IGFBP-4 protease in hOB CM. Pre-incubation of IGFBP-4 with IGF-II, followed by removal of unbound IGF-II, led to IGFBP-4 proteolysis without further requirement of the presence of IGF-II in the reaction. In contrast, prior incubation of the partially purified IGFBP-4 protease from either hOB CM or human pregnancy serum with IGF-II did not lead to IGFBP-4 proteolysis unless IGF-II was re-added to the assays. To further confirm that the interaction between IGF-II and IGFBP-4 is required for IGFBP-4 protease activity, we prepared IGFBP-4 mutants, which contained the intact cleavage site (Met135-Lys136) but lacked the IGF binding activity, by deleting the residues Leu72-His74 in the IGF binding domain or Cys183-Glu237 that contained an IGF binding enhancing motif. The IGFBP-4 protease was unable to cleave these IGFBP-4 mutants, regardless of whether or not IGF-II was present in the assay. Conversely, an IGFBP-4 mutant with His74 replaced by an Ala, which exhibited normal IGF binding activity, was effectively cleaved in the presence of IGF-II. Taken together, these findings provided strong evidence that the interaction between IGF-II and IGFBP-4, rather than the direct interaction between IGF-II and IGFBP-4 protease, is required for optimal IGFBP-4 proteolysis.

Studies on the role of human insulin-like growth factor-II (IGF-II)-dependent IGF binding protein (hIGFBP)-4 protease in human osteoblasts using protease-resistant IGFBP-4 analogs

To characterize the insulin-like growth factor binding protein-4 (IGFBP-4) protease produced by human osteoblasts (hOBs), we localized and determined the role of the proteolytic domains in human IGFBP-4 (hIGFBP-4) in modulating IGF-II actions. N-terminal amino acid sequence and mass spectrometric analyses of the 6xHis-tagged IGFBP-4 proteolytic fragments revealed that Met135-Lys136 was the only cleavage site recognized by the IGF-II-dependent IGFBP-4 protease produced by hOBs. This cleavage site was confirmed by the finding that deletion of His121 to Pro141 blocked proteolysis. However, unexpectedly, deletion of Pro94 to Gln119 containing no cleavage site had no effect on IGF-II binding activity but blocked proteolysis. Addition of the synthetic peptide corresponding to this region at concentrations of 250 or 1000 molar excess failed to block IGFBP-4 proteolysis. These data suggest that residues 94-119 may be involved in maintaining the IGFBP-4 conformation required to expose the cleavage site rather than being involved in direct protease-substrate binding. To determine the physiological significance of the IGF-II-dependent IGFBP-4 protease, we compared the effect of the wild-type IGFBP-4 and the protease-resistant IGFBP-4 analogs in blocking IGF-II-induced cell proliferation in normal hOBs, which produce IGFBP-4 protease, and MG63 cells, which do not produce IGFBP-4 protease. It was found that protease-resistant IGFBP-4 analogs were more potent than the wild-type protein in inhibiting IGF-II-induced cell proliferation in hOBs but not in MG63 cells. These data suggest that IGFBP-4 proteolytic fragments are not biologically active and that IGFBP-4 protease plays an important role in regulating IGFBP-4 bioavailability and consequently the mitogenic activity of IGFs in hOBs.

Response of bipotential human marrow stromal cells to insulin-like growth factors: effect on binding protein production, proliferation, and commitment to osteoblasts and adipocytes

Insulin-like growth factors (IGFs) are important regulators of the activity of mature osteoblasts, but their effects on osteoprogenitor cells in human bone marrow stroma are unclear. In this study, we assessed the effects of IGFs on a conditionally immortalized human marrow stromal cell line, hMS(3-4), which has the ability to differentiate to either mature osteoblasts or adipocytes. hMS(3-4) cells expressed functional receptors for IGFs as well as specific IGF-binding proteins (IGFBP-3, -4, -5, and -6). IGF treatment of hMS(3-4) cells did not alter IGFBP expression, but resulted in distinct posttranslational modifications of secreted IGFBP-3 and IGFBP-4 proteins. IGF-I, IGF-II, and their receptor-activating analogs significantly increased by 2-fold the proliferation rate of the hMS(3-4) cells, but had a more complex effect on hMS(3-4) cell differentiation. Treatment with IGFs did not affect gene expression of Cbfa1 or peroxisome proliferator-activated receptor gamma2 (transcription factors involved in commitment to osteoblast and adipocyte pathways, respectively), alkaline phosphatase, type I collagen, and osteocalcin (markers of the osteoblast lineage), or lipoprotein lipase and adipsin (markers of the adipocyte lineage) and did not change alkaline phosphatase activity or type I collagen and osteocalcin protein relative to total protein production. In contrast, IGFs significantly increased type I collagen expression in differentiated hMS(3-4) cells as well as mature osteoblasts and promoted lipid accumulation in differentiated adipocytes. In summary, hMS(3-4) cells express essential components of the IGF system and respond to IGF treatment with increased proliferation. There was no evidence for IGFs directly modulating the commitment of hMS(3-4) cells to either osteoblast or adipocyte pathways, and their effects on differentiation within these lineages were dependent on the stage of cell maturation.

Attenuated in vitro coronary arteriolar vasorelaxation to insulin-like growth factor I in experimental hypercholesterolemia

Insulin and insulin-like growth factor (IGF) 1 affect coronary vasoactivity. Experimental hypercholesterolemia is associated with coronary atherogenesis and altered vasomotor regulation. Because the IGF axis is altered during atherogenesis, we postulated that experimental hypercholesterolemia is associated with an altered coronary vasoactive response to IGF-1 in vitro. Coronary arteries and arterioles from pigs fed either a normal or high-cholesterol diet for 10 weeks were contracted with endothelin-1 and relaxed with cumulative concentrations of insulin or IGF-1 (10(-12) to 10(-7) mol/L). Control arterioles were also incubated with the nitric oxide synthase inhibitor 10(-4) mol/L N(G)-monomethyl-L-arginine (L-NMMA) or the potassium channel blocker 10(-2) mol/L tetraethylammonium (TEA), contracted with endothelin-1, and relaxed with insulin or IGF-1. Experimental hypercholesterolemia (1) increased serum cholesterol (9.5+/-1.0 versus 1.9+/-0.08 mmol/L; P<0.0001), (2) caused coronary arterial and arteriolar endothelial dysfunction in vitro (attenuated vasorelaxation to bradykinin), (3) did not alter the epicardial response to either insulin (P=0.80) or IGF-1 (P=0.12), and (4) significantly attenuated the arteriolar response to IGF-1 (maximal relaxation of 79+/-6% versus 42+/-8%; P=0.01) but not insulin (43+/-6% versus 53+/-7%; P=0.99). Control arteriolar vasorelaxation to IGF-1 was attenuated by both L-NMMA (P<0.001) and TEA (P=0.01), whereas only L-NMMA attenuated insulin (P<0.001). Staining for IGF-1 and IGF binding protein 2 was increased (P<0.05) in arterioles of cholesterol-fed pigs. IGF-1 and insulin are therefore coronary arteriolar vasorelaxants through different mechanisms. Experimental hypercholesterolemia is associated with resistance to the coronary arteriolar vasorelaxing effects of IGF-1 but not insulin, in conjunction with increased ligand and binding-protein expression. The IGF axis may contribute to the altered coronary vasoactivity in hypercholesterolemia.

Effect of insulin-like growth factor (IGF)-I and Des (1-3) IGF-I on the level of IGF binding protein-3 and IGF binding protein-3 mRNA in cultured porcine embryonic muscle cells

Insulin-like growth factor binding protein (IGFBP)-3 effects proliferation and differentiation of numerous cell types by binding to insulin-like growth factors (IGF) and attenuating their activity or by directly affecting cells in an IGF-independent manner. Consequently, IGFBPs produced by specific cells may affect their differentiation and proliferation. In this study we show that embryonic porcine myogenic cells, unlike murine muscle cell lines, produce significant quantities of a binding protein immunologically identified as IGFBP-3. Nonfusing cells subcultured from highly fused porcine myogenic cell cultures do not produce detectable IGFBP-3 protein or mRNA, thus suggesting the IGFBP-3 is produced by muscle cells in the porcine myogenic cell cultures. Treatment of porcine myogenic cultures with 20 ng of IGF-I or 20 ng of Des (1-3) IGF-I/ml serum-free media for 24 h results in a threefold reduction in the level of IGFBP-3 in conditioned media. This reduction is not affected by cell density over a sixfold range. Additionally, treatment for 24 h with 20 ng of IGF-I/ml media results in a sevenfold decrease in the steady-state level of IGFBP-3 mRNA. This IGF-I-induced decrease in IGFBP-3 mRNA level appears to be relatively unique to myogenic cells. IGF-I treatment also causes a fourfold increase in the steady-state level of myogenin mRNA. This increase in myogenin mRNA suggests that, as expected, IGF-I treatment accelerates differentiation of myogenic cells. The simultaneous decrease in IGFBP-3 mRNA and protein that accompanies IGF-I-induced myogenin expression suggests that differentiation of myogenic cells may be preceded or accompanied by decreased production of IGFBP-3.

Regulations of IGF binding proteins in human aorta vascular smooth muscle cells by cAMP, dexamethasone and IGF-I

Human vascular smooth muscle cells produce IGFBP-3, IGFBP-4, IGFBP-6 and proteases specific for IGFBP-3 and IGFBP-4. This study evaluated the regulation of IGFBPs in human aorta smooth muscle cells by cyclic AMP, dexamethasone and IGF-I. cAMP decreased IGFBP-3, increased IGFBP-4 and increased IGFBP-6. Dexamethasone decreased IGFBP-3, slightly increased IGFBP-4 and increased IGFBP-6. IGF-I increased IGFBP-3 and IGFBP-6 while decreasing IGFBP-4. Co-incubation with IGF-I and dexamethasone or cAMP increased media IGFBP-3, despite a decrease in IGFBP-3 mRNA, due to the dominant effect of IGF-I-induced dissociation of cell surface-bound IGFBP-3. In cells incubated with cAMP and IGF-I, media IGFBP-4 was decreased, despite increased IGFBP-4 mRNA, in this case secondary to the dominant effect of IGF-I-stimulated IGFBP-4 protease. These findings suggest that cAMP, dexamethasone and IGF-I regulate IGFBP production in human aorta smooth muscle cells via a complex interplay of changes in transcription, protease activation and dissociation of cell surface-bound IGFBPs.

Expression of insulin-like growth factor binding proteins in bronchoalveolar lavage fluid of patients with pulmonary sarcoidosis

Pulmonary sarcoidosis involves development of parenchymal granulomata that usually resolve spontaneously; however, it remains unclear what pathogenic mechanisms are responsible for the progression to local or diffuse fibrosis with irreversible lung remodeling that occurs in 20% of patients. Alveolar macrophages have a pivotal role in sarcoidosis, releasing mediators including insulin-like growth factor (IGF)-1, a potent profibrogenic molecule. IGF-1 bioavailability in the lung is dependent on at least six high-affinity IGF-binding proteins (IGFBP), which mainly inhibit IGF-1 action. We have investigated their presence in patients with established stage III sarcoidosis to determine whether IGF-1 and IGFBP contribute to the fibrogenic process in these patients and as such contribute to the (clinical) progression of the disease. The fibroblast mitogenic potential of bronchoalveolar lavage fluid (BALF) was more than 3-fold higher (P < 0.005) in sarcoid patients. Sarcoid BALF-induced activity could be inhibited (P < 0.0005) by neutralizing antibodies to IGF-1. We established the IGFBP profile of BALF with Western ligand analysis and quantified expression of IGFBP-3 by immunoblotting. IGFBP-2 and IGFBP-4 predominate in normal and sarcoid BALF, but IGFBP-3 occurs only as a modified, smaller, 29-kD form, expression of which was raised (P < 0.003) in sarcoid patients. Gene expression of IGF-1 and IGFBP-3 was demonstrated by reverse transcription-polymerase chain reaction in BAL cells. Thus, local production of pro-fibrogenic IGF-1 may be subject to substantial post-translational regulation by associated IGFBP and IGFBP proteases that may contribute to enhanced fibrogenesis in sarcoidosis patients with evidence of progression or (development) of fibrosis.

Insulin and insulin-like growth factor-I cause coronary vasorelaxation in vitro

Insulin and insulin-like growth factor-I (IGF-I) may play a role in the modulation of coronary artery tone, yet there are few data regarding their vasoactive effects on the coronary vascular bed. We evaluated the vasorelaxation effects of insulin and IGF-I on porcine coronary epicardial vessels in vitro and elucidated possible mechanisms. Porcine epicardial arteries were contracted with 10(-7) mol/L endothelin-1 and relaxed with cumulative concentrations of either insulin or IGF-I (10(-12) to 10(-7) mol/L). The above experiments were repeated in vessels without endothelium. Vessels were also incubated with the nitric oxide synthase inhibitor NG-monomethyl-L-arginine (L-NMMA; 10(-4) mol/L) with and without 10(-3.5) mol/L L-arginine, the potassium channel blocker tetraethylammonium (TEA; 10(-2) mol/L), and the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-alpha]quinoxalin-1-one (ODQ; 10(-5.5) mol/L); vessels were then contracted with endothelin-1 and relaxed with insulin or IGF-I. Insulin and IGF-I were also added after contraction with 60 mmol/L KCl. Insulin and IGF-I caused a similar decrease in coronary epicardial tension after contraction with endothelin-1 (relaxation of 28+/-4% [n=7] and 25+/-3% [n=8] with insulin and IGF-I, respectively; P<0.0001 for both peptides). Removal of the endothelium did not affect these responses. Incubation with L-NMMA, but not ODQ, attenuated the vasorelaxation response to insulin and IGF in vessels without endothelium. L-Arginine did not reverse this effect of L-NMMA. KCl and TEA attenuated the vasorelaxation effect of both insulin and IGF-I. Thus, both insulin and IGF-I caused non-endothelium-dependent coronary vasorelaxation in vitro, probably through a mechanism involving the activation of potassium channels. These findings suggest that insulin and IGF-I participate in the regulation of coronary vasomotor tone.

Molecular regulation of insulin-like growth factor-I and its principal binding protein, IGFBP-3

The insulin-like growth factors (IGFs) have diverse anabolic cellular functions, and structure similar to that of proinsulin. The distribution of IGFs and their receptors in a wide variety of organs and tissues enables the IGFs to exert endocrine, paracrine, and autocrine effects on cell proliferation and differentiation, caloric storage, and skeletal elongation. IGF-I exhibits particular metabolic responsiveness, and circulating IGF-I originates predominantly in the liver. Hepatic IGF-I production is controlled at the level of gene transcription, and transcripts are initiated largely in exon 1. Hepatic IGF-I gene transcription is reduced in conditions of protein malnutrition and diabetes mellitus, and our laboratory has used in vitro transcription to study mechanisms related to diabetes. We find that the presence of sequences downstream from the major transcription initiation sites in exon 1 is necessary for the diabetes-induced decrease in IGF-I transcription. Six nuclear factor binding sites have been identified within the exon 1 downstream region, and footprint sites III and V appear to be necessary for metabolic regulation; region V probes exhibit a decrease in nuclear factor binding with hepatic nuclear extracts from diabetic animals. IGFs in biological fluids are associated with IGF binding proteins, and IGFs circulate as a 150-kDa complex that consists of an IGF, an IGFBP-3, and an acid-labile subunit. Circulating IGFBP-3 originates mainly in hepatic nonparenchymal cells, where IGF-I increases IGFBP-3 mRNA stability, but insulin increases IGFBP-3 gene transcription. Regulation of IGFBP-3 gene transcription by insulin appears to be mediated by an insulin-responsive element, which recognizes insulin-responsive nuclear factors in both gel mobility shift assays and southwestern blots. Studies of mechanisms underlying the modulation of IGF-I and IGFBP-3 gene transcription, and identification of critical nuclear proteins involved, should lead to new understanding of the role and regulation of these important growth factors in diabetes mellitus and other metabolic disorders.

Insulin-like growth factor system abnormalities in hepatitis C-associated osteosclerosis. Potential insights into increasing bone mass in adults

Hepatitis C-associated osteosclerosis (HCAO) is a rare disorder characterized by a marked increase in bone mass during adult life. Despite the rarity of HCAO, understanding the mediator(s) of the skeletal disease is of great interest. The IGFs-I and -II have potent anabolic effects on bone, and alterations in the IGFs and/or IGF-binding proteins (IGFBPs) could be responsible for the increase in bone formation in this disorder. Thus, we assayed sera from seven cases of HCAO for IGF-I, IGF-II, IGF-IIE (an IGF-II precursor), and IGFBPs. The distribution of the serum IGFs and IGFBPs between their ternary ( approximately 150 kD) and binary (approximately 50 kD) complexes was also determined to assess IGF bioavailability. HCAO patients had normal serum levels of IGF-I and -II, but had markedly elevated levels of IGF-IIE. Of the IGFBPs, an increase in IGFBP-2 was unique to these patients and was not found in control hepatitis C or hepatitis B patients. IGF-I and -II in sera from patients with HCAO were carried, as in the case of sera from control subjects, bound to IGFBP-3 in the approximately 150-kD complex, which is retained in the circulation. However, IGF-IIE was predominantly in the approximately 50-kD complex in association with IGFBP-2; this complex can cross the capillary barrier and access target tissues. In vitro, we found that IGF-II enhanced by over threefold IGFBP-2 binding to extracellular matrix produced by human osteoblasts and that in an extracellular matrix-rich environment, the IGF-II/IGFBP-2 complex was as effective as IGF-II alone in stimulating human osteoblast proliferation. Thus, IGFBP-2 may facilitate the targeting of IGFs, and in particular IGF-IIE, to skeletal tissue in HCAO patients, with a subsequent stimulation by IGFs of osteoblast function. Our findings in HCAO suggest a possible means to increase bone mass in patients with osteoporosis.

Insulin-like growth factor (IGF)-binding protein 5 forms an alternative ternary complex with IGFs and the acid-labile subunit

Up to 90% of circulating insulin-like growth factors (IGF-I and IGF-II) are carried in heterotrimeric complexes with a binding protein (IGFBP) and a liver-derived glycoprotein known as the acid-labile subunit. IGFBP-3 is considered unique among the six well characterized IGFBPs in its ability to complex with the acid-labile subunit. However, a basic carboxyl-terminal domain of IGFBP-3, known to be involved in its interaction with the acid-labile subunit, is shared by IGFBP-5, suggesting the possibility of ternary complexes containing IGFBP-5. We now demonstrate using three independent methods that human IGFBP-5, when occupied by IGF-I or IGF-II, forms ternary complexes of approximately 130 kDa with the acid-labile subunit. IGFBP-3 competes with approximately twice the potency of IGFBP-5 for the formation of such complexes. No other IGFBP complexes with the acid-labile subunit itself or competes with IGFBP-5 for complex formation. As observed for IGFBP-3, ternary complexes containing IGFBP-5 form preferentially in the presence of IGF-I, even though IGFBP-5 has a preferential affinity for IGF-II over IGF-I. By size fractionation chromatography, serum IGFBP-5 co-elutes predominantly with ternary complexes. The demonstration of IGFBP-5-containing ternary complexes indicates an unrecognized form of IGF transport in the circulation and an additional mechanism for regulating IGF bioavailability.

Insulin-like growth factor binding protein production in bovine retinal endothelial cells

Retinopathy is the most frequent microangiopathic complication in diabetes. Many circulating hormones and locally produced mitogenic factors have been involved. Bovine retinal endothelial cells (BRECs) were cultured to investigate if insulin, insulin-like growth factors (IGFs), IGF binding proteins (IGFBPs), and a chronic high-glucose condition could control endothelial cell growth. Specific IGF-I receptors with two binding sites with high (Kd 0.03 nmol/L) and low (Kd 1.3 nmol/L) affinity were found when analyzing families of displacement curves between IGF-I versus IGF-I and IGF-I versus insulin. However, IGFs failed to be mitogenic factors in these cells. This could be explained by an inhibitory effect due to the presence of specific IGFBPs with a molecular weight between 24 and 43 kd. Using Western blot and immunoblot analysis, Northern blot study, and specific radioimmunoassay (RIA), these IGFBPs have been identified as IGFBP-3, -2, -5, and -4. Insulin, which does not bind to IGFBPs, was a potent mitogenic factor in these cells at a high concentration (10 nmol/L), suggesting a cross-reaction to IGF-I receptor. These IGFBPs, except the 24-kd form (IGFBP-4), were modulated by both IGF-I and IGF-II, with a maximum effect at 100 and 10 nmol/L, respectively. This regulation on IGFBPs was IGF-I receptor-independent. In fact, (1) IGFBP mRNA levels were not modified after stimulation with 100 nmol/L IGF-I, (2) 100 nmol/L IGF plus an equimolar concentration of alpha IR3 did not affect IGFBP production, (3) Des(1-3)IGF-I had no effect on IGFBP modulation, whereas at 10 nmol/L it enhanced BREC thymidine cell incorporation, and (4) 100 nmol/L insulin, which at this concentration can cross-react with the IGF-I receptor, did not modify the IGFBP pattern. Chronic exposure (4 weeks) of BRECs to 25 mmol/L glucose had no effect on cell growth. However, after 3 weeks, we observed a decreased IGFBP detection, and addition of 100 nmol/L IGF-I did not change IGFBP levels and did not modify cell growth. Conversely, BRECs grown in regular medium for 4 weeks showed increased IGFBP production. In conclusion, we showed that conditions mimicking hyperinsulinemia, rather than high levels of IGFs, could regulate BREC growth and that the IGF-I analog, Des(1-3), even with reduced affinity for IGFBPs but in part capable of binding to IGFBP-3, significantly stimulated BRECs growth only at 10 nmol/L. IGF actions are modulated by locally produced endothelial IGFBPs, and in turn, these endothelial IGFBPs are regulated, via in IGF-I receptor-independent mechanism, by the presence of IGFs. The autoregulatory IGF system together with the direct glucose modulation of IGFBPs could contribute in diabetic subjects to the retinal endothelial cell growth and metabolism through local changes in IGF bioavailability.

The type V transforming growth factor beta receptor is the putative insulin-like growth factor-binding protein 3 receptor

Insulin-like growth factor-binding protein 3 (IGFBP-3) has been shown to inhibit cell growth by IGF-dependent and -independent mechanisms. The putative cell-surface IGFBP-3 receptor that mediates the IGF-independent growth inhibition has not been identified. Here we show that recombinant human IGFBP-3 inhibits 125I-transforming growth factor (TGF)-beta1 binding to the type V TGF-beta receptor (Mr 400,000) in mink lung epithelial cells. We also demonstrate that the approximately 400-kDa 125I-IGFBP-3 affinity-labeled putative IGFBP-3 receptor is immunoprecipitated by specific antiserum to the type V TGF-beta receptor. The 125I-IGFBP-3 affinity labeling of the putative receptor and IGFBP-3-induced growth inhibition as measured by DNA synthesis in these cells is blocked by a TGF-beta1 peptide antagonist. The 125I-IGFBP-3 affinity-labeled putative receptor can only be detected in cells expressing the type V TGF-beta receptor, but not in cells lacking the type V TGF-beta receptor. These results indicate that the type V TGF-beta receptor is the putative IGFBP-3 receptor and that IGFBP-3 is a functional ligand for the type V TGF-beta receptor.

Standardization of nutrient media for isolated human articular chondrocytes in gelified agarose suspension culture

Human articular cartilage cells were cultured in 1.5% agarose in Dulbecco's modified Eagle's medium (DMEM) with 10% fetal calf serum or in serum-free DMEM with 0.15% bovine serum albumin. 35S-aggrecan synthesis in serum-free DMEM was between 20% and 30% of the value observed in DMEM supplemented with 10% fetal calf serum. The extent to which different growth or differentiation factors were able to restore 35S incorporation in aggrecan in serum-free DMEM was determined: human serum transferrin had no effect on aggrecan synthesis levels; bovine pancreas insulin, insulin-like growth factor (IGF)-1 and IGF-2 restored 35S-aggrecan synthesis to 35-50% of the control levels. The effects were dose-dependent, to level off at 100 ng/mL for the three factors. No cumulative or synergistic activities were observed when these factors were combined. Transforming growth factor (TGF)-beta, at concentrations ranging from 10-50 ng/mL stimulated aggrecan synthesis to approximately 50% of the control values in the chondrocytes obtained from two out of four donors, while the cells of the other two maintained within the range of the control levels. In th presence of insulin (100 ng/mL) 10 ng/mL of TGF-beta stimulated aggrecan synthesis to more than 90% of the control level in the chondrocytes of all donors.

Insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) functions as an IGF-reversible inhibitor of IGFBP-4 proteolysis

Previous studies have shown that insulin-like growth factor (IGF)-binding protein-4 (IGFBP-4) is degraded only in the presence of exogenous IGFs; however, we found that cation-dependent proteinase activity present in conditioned medium of MC3T3-E1 osteoblasts degrades 125I-recombinant human (rh)IGFBP-4 in the absence of IGFs. Addition of IGF-I, IGF-II, or insulin to conditioned medium had little affect on 125I-rhIGFBP-4 proteolysis, while extraction of IGFs resulted in only a approximately 10% reduction in proteinase activity. Since factors other than IGFs appeared to be involved in regulating IGFBP-4 proteolysis, we hypothesized that IGFBP-3, an IGFBP produced by many cell lines, but not MC3T3-E1 cells, might function as an inhibitor of IGFBP-4 proteolysis. Addition of rhIGFBP-3 to conditioned media inhibited 125I-rhIGFBP-4 proteolysis by 90%, while IGF-I and IGF-II reversed the inhibitory effects of rhIGFBP-3 in a dose-dependent manner. 125I-rhIGFBP-4 proteolysis was not inhibited by N-terminal rhIGFBP-3 fragments that bind IGFs, but was inhibited by two synthetic peptides corresponding to sequences contained in the mid-region or C-terminal region of IGFBP-3. Both inhibitory peptides contain highly basic, putative heparin-binding domains and heparin partially reversed the inhibitory effects of rhIGFBP-3 on 125I-rhIGFBP-4 proteolysis. These data demonstrate that rhIGFBP-3 inhibits IGFBP-4-degrading proteinase activity and binding of IGFs or glycosaminoglycans to IGFBP-3 may induce conformational changes in the binding protein, causing disinhibition of the proteinase.

Insulin-like growth factor-I and epidermal growth factor regulate insulin-like growth factor binding protein-3 (IGFBP-3) in the human keratinocyte cell line HaCaT

The human keratinocyte cell line HaCaT has a basal phenotype and secretes an insulin-like growth factor (IGF) binding protein, IGFBP-3, which modulates its IGF-I response. Keratinocytes are highly responsive to mitogenic stimulation by IGF-I and epidermal growth factor (EGF), but the effect of these growth factors on IGFBP secretion by keratinocytes is not known. We investigated the effects of IGF-I and EGF, as well as three other skin-growth regulators, retinoic acid, basic fibroblast growth factor, and dexamethasone, on mitogenic stimulation and IGFBP-3 production in HaCaT cells. IGF-I and EGF were strongly mitogenic, whereas retinoic acid, basic fibroblast growth factor, and dexamethasone were not significantly mitogenic. IGF-I increased the level of IGFBP-3 in cell-conditioned medium by up to twofold, whereas EGF caused a twentyfold reduction in IGFBP-3. Retinoic acid and basic fibroblast growth factor had only minor effects on IGFBP-3 and dexamethasone had no effect. IGF-I stimulation of IGFBP-3 did not involve increases in IGFBP-3 mRNA; however, EGF, consistent with its effect on IGFBP-3 protein, caused a fivefold reduction in IGFBP-3 mRNA. In summary, EGF profoundly inhibited IGFBP-3 synthesis in basal keratinocytes, whereas IGF-I increased IGFBP-3 levels by a posttranscriptional mechanism. We hypothesize that by inhibiting IGFBP-3 production in basal keratinocytes, epidermal mitogens such as EGF might stimulate epidermal growth indirectly by increasing local IGF-I availability.

Regulation of insulin-like growth factor (IGF) binding protein-2 synthesis and degradation by platelet-derived growth factor and the IGFs is enhanced by serum deprivation in vascular smooth muscle cells

Growth factors such as platelet-derived growth factor (PDGF) and insulin-like growth factor (IGF-I) stimulated proliferation and migration of vascular smooth muscle cells (SMC). IGF-I bioactivity is modulated by high-affinity binding proteins (IGFBP) which are important regulators of these processes. Porcine vascular SMC synthesize IGFBP-2 and IGFBP-4 in vitro. In the present study, levels of IGFBP-2 in conditioned media (CM) were increased approximately 1.6 to 2.2-fold when cells were exposed to PDGF (20 ng/ml) or insulin (5 micrograms/ml) for 24 hr following a 24 hr incubation in serum-free media, or following a 72 hr exposure to either growth factor. Similar increases in IGFBP-2 mRNA levels were observed. Exposure of cells to PDGF for 24 hr without prior serum deprivation resulted in smaller (47 +/- 11%) increases in IGFBP-2 protein levels but failed to alter mRNA levels. IGF-I, FGF, TGF-beta and EGF failed to increase IGFBP-2 using either experimental paradigm. In contrast, IGFBP-2 protein levels were consistently decreased (75 +/- 14%) after 72 hr of exposure to IGF-II without corresponding decreases in IGFBP-2 mRNA levels. Immunoprecipitation of [35S]methionine-labeled IGFBP-2 indicated that this decrease was not due to a decrease in synthesis of IGFBP-2. Immunoblot analysis of CM from cells treated with IGF-II indicated that the decrease in intact protein corresponded with an increase in two non-IGF binding IGFBP-2 fragments of 22 and 14 kD. Increased abundance of these fragments was also observed following IGF-I exposure, although corresponding decreases in intact IGFBP-2 were not usually observed. The relative abundance of these fragments did not appear to be affected by treatment with PDGF or insulin. In contrast to IGFBP-2, regulation of the levels of IGFBP-4 in CM did not appear to be altered by serum deprivation. Insulin consistently increased IGFBP-4 mRNA and protein levels under all situations. PDGF tended to increase IGFBP-4 protein levels, although this effect was less consistent and not as great as the increase observed with insulin. Treatment with IGF-I or -II consistently decreased IGFBP-4 levels in CM but tended to increase their mRNA levels under all situations. These data indicate that insulin, PDGF, and the IGFs regulate both IGFBP-2 and IGFBP-4. While PDGF and insulin stimulate IGFBP-2 and 4 synthesis, the IGFs appear to activate protease(s) which regulate IGFBP-2 and -4 levels post-translationally.(ABSTRACT TRUNCATED AT 400 WORDS)

Complex pattern of insulin-like growth factor binding protein expression in primary rat osteoblast enriched cultures: regulation by prostaglandin E2, growth hormone, and the insulin-like growth factors

Primary osteoblast-enriched (Ob) cultures from fetal rat bone synthesize insulin-like growth factor (IGF) I and IGF-II, which each enhance Ob function. While a number of agents modulate IGF-I production, IGF-II is constitutively expressed in this culture model. Independent of their expression, however, the activity of the IGFs can be modified by a small group of proteins termed IGF binding proteins (IGFBPs), but little is known about the regulation of individual IGFBPs that are synthesized by Ob cells. Northern blot analysis revealed that serum-deprived primary rat Ob cells express transcripts encoding IGFBP-2, IGFBP-3, IGFBP-4, IGFBP-5, and IGFBP-6, but undetectable levels of IGFBP-1 transcripts. Western ligand blots of Ob culture medium probed with 125I-IGF-I or 125I-IGF-II showed predominant IGFBPs migrating at 30/32 kDa, with minor bands at 24 and 38-47 kDa. Western antibody analysis identified IGFBP-2 and IGFBP-5 within the 30/32 kDa complex, while gel mobility shift on SDS-PAGE following deglycosylation determined that IGFBP-3 comprised the 38-47 kDa complex. By Northern analysis, 6 h treatment with prostaglandin E2 (PGE2), growth hormone (hGH), IGF-I, or IGF-II revealed a complex pattern of regulatory effects on steady-state IGFBP transcript expression. PGE2 increased the transcript levels of IGFBP-3, IGFBP-4, and IGFBP-5, (approximately 22-, approximately 2- and approximately 4-fold respectively), but had no effect on IGFBP-2 or IGFBP-6 transcripts. hGH enhanced IGFBP-3 and IGFBP-5 transcripts (each approximately twofold). IGF-I and IGF-II had no effect on IGFBP-2 steady-state transcript levels but enhanced the level of IGFBP-5 transcripts (approximately fourfold). By Western ligand blot analysis, 24 h treatment with PGE2 elevated the 24 and 38-47 kDa IGFBPs and to a lesser extent the 30/32 kDa complex, hGH elevated the 38-47 kDa IGFBPs, and IGF-I and IGF-II each increased the 30/32 kDa IGFBP complex. Therefore, a comparison of results obtained from Northern, Western ligand, and Western antibody studies indicates that multiple IGFBPs are expressed by primary rat Ob cultures. While IGFBP-2 and IGFBP-6 synthesis in Ob cultures is relatively unaffected by short-term treatment with PGE2, hGH, or the IGFs, these agents modify IGFBP-3, IGFBP-4, and IGFBP-5 expression with individual patterns of effects. In addition, some changes in IGFBP polypeptide levels that are independent of alterations in transcript expression may result from the formation of complexes between IGFs and certain IGFBPs, which could serve to store IGFs for future utilization in the formation phase of bone remodeling.

IGF-I-induced IGFBP-3 potentiates the mitogenic actions of IGF-I in mammary epithelial MD-IGF-I cells

Limited information is available concerning the molecular and cellular mechanisms that regulate expression of insulin-like growth factor-I (IGF-I) binding proteins (IGFBPs) in bovine mammary epithelial cells. Here, we report on the autocrine mechanisms of action of IGF-I and hormonal regulation of expression of IGFBPs in bovine mammary epithelial MD-IGF-I cells which express recombinant IGF-I under the control of the glucocorticoid-inducible mouse mammary tumor virus-long terminal repeat (MMTV-LTR). Levels of IGFBP-3 mRNA and secretion of IGFBP-3 by MD-IGF-I cells were stimulated by IGF-I, insulin (INS), and IGF-I analogs but not prolactin (PRL). Conversely, parental MAC-T cells expressed little IGF-I and secreted primarily IGFBP-2 (29-32 kDa) in response to stimulation with INS, dexamethasone (DEX), or IGF-I analogs. Secretion of recombinant IGF-I caused a 26.5-fold increase in secretion of IGFBP-3, as measured by densitometric analysis of ligand blots, which was associated with a 1.7-fold increase in total DNA. Conditioned media (CM) from MD-IGF-I cells induced with DEX stimulated a 2.8-fold increase in [3H]thymidine incorporation into DNA of parental MAC-T cells, compared with uninduced cells. Moreover, inclusion of exogenous IGF-I with CM from MD-IGF-I cells triggered an additional 3.0-fold increase in label incorporation, but only a 1.6-fold increase in the presence of IGFBP-2-containing media conditions by MAC-T cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Autocrine growth factors produced in the thyroid

This catalogue of autocrine growth factors is limited to proteins--metabolites of iodine and prostaglandins are omitted and they are undoubtedly of autocrine importance in the thyroid, as elsewhere. However, this summary of polypeptide growth factors secreted by the thyroid illustrates the potential cells have to condition their environment to modify their responses to external stimuli. This enables cells in different tissues to respond to agonists in different ways. The effects of TSH on IGF, IGFBP and IGF receptor production and the effects of IGFBPs on IGF action are good examples of this amplified response. Many pieces of the jigsaw, however, remain to be found and put in place before a clear picture of the regulation and roles of these factors can be made.

IGF-I stimulates granulosa cell-derived insulin-like growth factor binding protein-5: evidence for medication via type I IGF receptors

Although the precise role of insulin-like growth factor binding proteins (IGFBPs) in ovarian physiology remains a matter of study, existing data suggest a possible antigonadotropic role in the context of follicular atresia. Given the above and the need for improved understanding of the regulation of ovarian IGFBPs, we have set out to explore the ability of IGF-I to modulate IGFBP levels in cultured rat granulosa cells. Specifically, granulosa cells (5 x 10(5) viable cells/dish) from immature (23-25 days old), estrogen-primed rats were cultured under serum-free conditions for 72 h in the absence or presence of IGF-I. At the conclusion of this incubation period, media samples were collected and subjected to Western ligand blotting. Treatment with IGF-I (100 ng/ml) resulted in a substantial (P < 0.05) increase in the accumulation of IGFBP-5, the major 28-29 kDa IGFBP species. Subsequent studies revealed this effect of IGF-I to be both dose- and time-dependent. A similar effect was noted for insulin at dose levels 1-10 micrograms/ml at which cross-reaction with the type I IGF receptor (but not with IGFBPs) has been amply documented. Des (1-3) IGF-I, a type I receptor-selective ligand with markedly reduced avidity for IGFBPs, proved substantially more potent (as a promoter of IGFBP-5 accumulation) than its native counterpart. In contrast, treatment with IGF-II or [Leu27]IGF-II, type II IGF receptor-selective ligands, yielded a more limited effect on IGFBP-5 accumulation in keeping with an overall rank order of potency of des (1-3) IGF-I > IGF-I > IGF-II > or = [Leu27]IGF-II.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of insulin-like growth factor binding protein synthesis and secretion in human retinal pigment epithelial cells

Cultured human retinal pigment epithelial cells (RPE) secrete insulin-like growth factor binding proteins (IGFBPs), a family of polypeptides which modulate the actions of the insulin-like growth factors. RPE cells secrete two IGFBPs with Mr estimates of 34,000 and 46,000, respectively. Treatment of RPE cells with IGF-I markedly stimulated the secretion of the 46,000 Mr form. This stimulation occurred via an IGF-I receptor independent mechanism because both [QAYL]IGF-I (an IGF-I analogue with decreased affinity for the IGFBPs but normal affinity for the IGF-I receptor) and alpha-IR3 (a blocking monoclonal antibody against the IGF-I receptor) had no effect on IGF-I stimulated increases in IGFBPs. Additionally, [QAYL]IGF-I enhanced RPE cell proliferation to the same magnitude as IGF-I. Treatment with IGF-I, [QAYL]IGF-I, or alpha-IR3 had no effect on steady-state levels of the 2.5 kb IGFBP-3 or the 1.3 kb IGFBP-6 mRNA transcripts as measured by Northern blotting and quantitative autoradiography. Forskolin and a group of candidate growth factors, including platelet-derived growth factor, epidermal growth factor, and acidic and basic fibroblast growth factor, modestly increased IGFBP secretion when compared to untreated cells, but these effects were small when compared to IGF-I treatment. Fetal calf serum enhanced the presence of the 2.5 kb IGFBP-3 mRNA transcript in a dose-dependent fashion but had no effect on the 1.3 kb IGFBP-6 mRNA transcript. IGF-I, forskolin, and the candidate growth factors had no effect on either IGFBP-3 or IGFBP-6 mRNA. These data suggest that the production of IGFBPs in human RPE cells is regulated by distinct mechanisms which include (1) an IGF-I receptor independent interaction of IGF-I with secreted IGFBPs and (2) de novo synthesis of IGFBPs by serum-containing factors.

IGFs and muscle differentiation

The Role of IGFs in Myogenesis. Thus we are now convinced that the control of myogenesis by IGFs is a general phenomenon that occurs in all skeletal muscle cells, whether or not IGFs are added to the "differentiation" medium. We believe that several medium components contribute to the suppression of IGF-II expression in myoblasts incubated in high serum "growth" medium, and conclude that the IGF-I receptor mediates the feedback inhibition of IGF-II gene expression in muscle cells. Mechanism of Induction of Myogenesis by IGFs. The observations summarized here now permit a reasonably coherent overview of the stimulation of myogenic differentiation by the IGFs. It seems clear that all IGFs act by binding to the Type I IGF receptor, and that this process is inhibited to a significant extent by IGF binding proteins secreted by the target myoblasts. A major, but possibly not the only relevant effect of this binding is the induction of expression of the myogenin gene; this induction appears to require the presence of myf-5 protein, at least during the early part of the response. Cells capable of a mitogenic response undergo a round of division in response to IGF-I, thus delaying their entry into the final processes of postmitotic terminal differentiation. Other laboratories have shown that myogenin complexes with one or more widely occurring proteins such as E12 or E47 to form an active complex that interacts with CAnnTG elements in muscle specific genes, turning on expression of those genes and thus initiating the phenotype associated with terminally differentiated skeletal muscle.

Regulation of insulin-like growth factor binding protein 4 by a specific insulin-like growth factor binding protein 4 proteinase in normal human osteoblast-like cells: implications in bone cell physiology

Insulin-like growth factor binding protein 4 (IGFBP-4) is secreted by normal human osteoblast-like cells (hOB) and is a potent inhibitor of insulin-like growth factor (IGF) action in vitro. In previous studies, IGF treatment of hOB in culture led to markedly reduced medium levels of IGFBP-4 as detected by western ligand blotting. In the present study, incubation of hOB-conditioned medium (hOB-CM) with IGF under cell-free conditions resulted in a similar loss of IGFBP-4. Both IGF-I and IGF-II were capable of inducing a decrease in IGFBP-4; however, IGF-II was more effective. When the six characterized IGFBP were added to hOB-CM, only IGFBP-4 disappeared in response to IGF-II addition. This IGF-regulated loss of IGFBP-4 was inhibited by metalloproteinase inhibitors and appeared to be due to a proteinase that cleaved IGFBP-4 in 18 and 14 kD fragments identified by western immunoblotting. Conditioned media from eight of eight different donor hOB lines tested exhibited IGFBP-4 proteinase activity. To assess the biologic consequences of IGF-II-induced IGFBP-4 proteolysis, we treated hOB with IGF-II for 5 h, which decreased medium IGF-BP-4 by 70%, and then measured IGF-I and insulin stimulation of [3H]thymidine incorporation. IGF-II itself was not mitogenic and had no effect on insulin-stimulated [3H]thymidine incorporation. However, pretreatment of cultured hOB with IGF-II enhanced IGF-I-stimulated [3H]thymidine incorporation threefold.(ABSTRACT TRUNCATED AT 250 WORDS)

Ligand-dependent inhibition of myoblast differentiation by overexpression of the type-1 insulin-like growth factor receptor

The insulin-like growth factors (IGFs) have paradoxical effects on skeletal myoblast differentiation. While low concentrations of IGF stimulate myoblast differentiation, high concentrations of IGF induce a progressive decrease in myoblast differentiation. The mechanism of this inhibition is unknown. Using a retroviral expression vector, we developed a subline of mouse P2 mouse myoblasts (P2-LISN) which expressed 7.5 times higher levels of type-1 IGF receptors than control (P2-LNL6) myoblasts, which were infected with a virus lacking the type-1 IGF receptor sequence. Overexpression of the type-1 IGF receptor caused the IGF dose-response curves of stimulation and progressive inhibition of differentiation to shift to the left. Additionally, at high insulin and IGF-I concentrations, complete inhibition of P2-LISN myoblast differentiation occurred. These results suggest that inhibition of differentiation at high ligand concentrations was not due to the primary involvement of other species of receptors for IGF. Type-1 IGF receptor downregulation as a mechanism for inhibition of differentiation was also ruled out since P2-LISN myoblasts constitutively expressed high levels of type-1 IGF receptors. Additionally, inhibition of differentiation at high concentrations of IGF-I was not correlated with overt stimulation of proliferation or with IGF binding protein (IGF-BP) release into the culture medium. These results indicate that the type-1 IGF receptor mediates two conflicting signal pathways in myogenic cells, differentiation-inducing and differentiation-inhibitory, which predominate at different ligand concentrations.

Accumulation of insulin-like growth factor binding protein-3 in conditioned medium of human fibroblasts increases with chronologic age of donor and senescence in vitro

We have found that insulin-like growth factor binding protein-3 (IGFBP-3) accumulates to higher levels in medium conditioned by a strain of normal fibroblasts at late passage (LP) and a strain derived from subjects with Werner syndrome (WS) of premature aging, compared to medium conditioned by the same normal cells at early passage (EP) (Goldstein et al., Proc. Natl. Acad. Sci. USA, 88:9680-9684, 1991). To explore the generality of this phenomenon with respect to chronological age of donor (in vivo aging) and LP (in vitro senescence) we assayed IGFBP-3 in medium conditioned by 18 normal fibroblast strains at EP and LP and two WS strains at the midpoint of their curtailed replicative lifespans and assessed IGFBP-3 mRNA levels in cells by Northern analysis. The lowest accumulations of IGFBP-3 were found in medium conditioned by fetal cells with progressively increasing amounts postnatally; direct correlations between IGFBP-3 levels and donor age were seen in EP cells 3 days after subculture (during logarithmic growth) r = 0.80, P < 0.001, and 7 days after subculture (at confluence) r = 0.77, P < 0.001. With two exceptions, conditioned medium of cell strains accumulated more IGFBP-3 at LP; IGFBP-3 levels correlated with chronological age after 3 days, r = 0.50, P = 0.05, and after 7 days, r = 0.75, P < 0.001. IGFBP-3 content of WS culture medium fell within the range of LP normal cells. Cumulative IGFBP-3 levels were inversely proportional to the thymidine labeling index, a measure of proliferative vigor. With some exceptions IGFBP-3 mRNA levels were commensurate with the amount of IGFBP-3 accumulated in the medium, suggesting that distal translational and posttranslational mechanisms also regulate IGFBP-3 production in some strains. The trend toward augmented IGFBP-3 output of fibroblasts as a direct function of chronological age and in vitro senescence and as an inverse function of proliferative vigor is consistent with the known inhibitory effect of excess IGFBP-3 on IGF-mediated DNA synthesis and the reduced regenerative potential that is evident during biological aging in vivo.

Regulation of insulin-like growth factor-binding-protein-1, 2, 3, 4, 5, and 6: synthesis, secretion, and gene expression in estrogen receptor-negative human breast carcinoma cells

Insulin-like growth-factors I and II (IGF-I, II) are potent mitogens for breast carcinoma proliferation. In extracellular fluids, most of the IGF-I and II is associated with specific IGF-binding proteins (IGFBPs). The role of these IGFBPs in IGF action is still not clear, but it has been demonstrated that these proteins may either enhance or inhibit IGF-mediated cellular effects. Synthesis and secretion of IGFBPs have been demonstrated in breast carcinoma cells. In this study, we examined retinoic acid (RA) and IGF-I modulation of IGFBP mRNA and IGFBP levels in two ER-negative human breast carcinoma cell lines. Treatment of MDA-MB-231 and MDA-MB-468 cells with RA increased the levels in conditioned media of a M(r) 42-46-kDa IGFBP, which was immunoprecipitated by an IGFBP-3 antibody. IGF-I also increased the accumulated levels of IGFBP-3 in the conditioned media of both cell lines. Both cell lines expressed high basal levels of IGFBP-3 mRNA; the addition of RA increased IGFBP-3 mRNA levels by 1.5-fold, whereas the addition of IGF-I had no effect on IGFBP-3 mRNA levels in either cell line. The difference in the magnitude of the RA enhancement of IGFBP-3 mRNA levels (1.5-fold) and RA stimulation of IGFBP-3 levels in conditioned media (3.5-4-fold) suggests that some of the effect of RA is at a posttranscriptional level. IGF-I increased the levels of IGFBP-2 and IGFBP-5 in conditioned media by greater than tenfold but had no effect on IGFBP-2 and IGFBP-5 mRNA levels, again suggesting the involvement of posttranscriptional controls. Pretreatment of MDA-MB-468 and MDA-MB-231 cells with IGF-I receptor antibody (alpha IR3) blocked the IGF-I effect on IGFBP-3 levels in the media in both cell lines and IGFBP-2 and IGFBP-5 secreted levels in MDA-MB-468 cell conditioned media. The addition of RA also blocked IGF-I stimulation of IGFBP-2 and IGFBP-5 levels. Cycloheximide treatment completely blocked the RA and/or IGF-I-mediated modulation of these binding proteins, suggesting that these agents enhance IGFBP-3, IGFBP-2, and IGFBP-5 synthesis and consequent secretion. MDA-MB-468 cells expressed IGFBP-5 mRNA, whereas both MDA-MB-231 and MDA-MB-468 expressed IGFBP-6 mRNA. RA enhanced IGFBP-6 gene expression by threefold in MDA-MB-231 cells, whereas IGF-1 had no effect on IGFBP-6 gene expression in either cell line.(ABSTRACT TRUNCATED AT 400 WORDS)

Posttranslational regulation of insulin-like growth factor binding protein-4 in normal and transformed human fibroblasts. Insulin-like growth factor dependence and biological studies

Insulin-like growth factor binding protein-4 (IGFBP-4) is a 24-26-kD protein expressed by a variety of cell types in vivo and in vitro. Treatment of normal adult human fibroblasts with 10 nM insulin-like growth factor II (IGF-II) for 24 h resulted in an 85% decrease in endogenous IGFBP-4, as assessed by Western ligand blot analysis of the conditioned medium. Incubation of human fibroblast-conditioned medium (HFCM) with IGF-II under cell-free conditions led to a similar loss of IGFBP-4. This posttranslationally regulated decrease in IGFBP-4 appeared to be due to a protease in HFCM: (a) It could be prevented with specific protease inhibitors or incubation at 4 degrees C; (b) proteolysis of recombinant human (rh) IGFBP-4 required HFCM; (c) immunoblotting and radiolabeling confirmed cleavage of IGFBP-4 into 18- and 14-kD IGFBP-4 fragments. The protease was specific for IGFBP-4, and was strictly dependent on IGFs for activation. IGF-II was the most effective of the natural and mutant IGFs tested, inducing complete hydrolysis of rhIGFBP-4 at a molar ratio of 0.25:1 (IGF/IGFBP-4). Simian virus 40-transformed adult human fibroblasts also expressed IGFBP-4 and IGFBP-4 protease, as well as an inhibitor of IGFBP-4 proteolysis. In biological studies, intact rhIGFBP-4 potently inhibited IGF-I-stimulated [3H]aminoisobutyric acid uptake, whereas proteolyzed rhIGFBP-4 had no inhibitory effect. In conclusion, these data provide evidence for a novel IGF-dependent IGFBP-4-specific protease that modifies IGFBP-4 structure and function, and indicate a preferential role for IGF-II in its activation. Posttranslational regulation of IGFBP-4 may provide a means for cooperative control of local cell growth by IGF-I and IGF-II.

Developmental regulation of insulin-like growth factor binding protein production: studies in fetal, postnatal, and pregnant sheep

To assess the roles of developmental factors in the regulation of sheep IGFBP production at the cellular level, we characterized and compared the IGFBPs released by fetal, postnatal, and maternal sheep skin fibroblasts in culture with those in fetal, postnatal, and maternal sheep plasma. Sheep fibroblasts produced seven IGFBPs: a 36.5-41 kDa protein induced in vitro by IGF-I, likely representing oIGFBP-3; a 28.5 kDa protein that reacted with antisera to human IGFBP-2, likely representing oIGFBP-2; 25 and 27 kDa proteins induced in fetal fibroblasts by IGF-I; a 22 kDa protein that was inhibited by IGF-I, likely representing oIGFBP-4; and 21 and 23 kDa proteins labelled only by IGF-II, suggesting their similarities to IGFBP-6. The developmental pattern of IGFBP production by sheep fibroblasts in culture was similar in several respects to that observed in sheep plasma. For example, relative amounts of the 21, 22, and 28.5 kDa IGFBPs exceeded that of the 36.5-41 kDa protein in early fetal fibroblast conditioned media and in fetal plasma, while the relative concentrations of the 36.5-41 kDa protein increased markedly during the perinatal period. Sheep plasma differed, however, in two major respects from fibroblast conditioned media: First, fetal, and to a far lesser extent maternal, plasma contained a 200 kDa IGF-II-selective BP, likely to be the circulating form of the IGF-II receptor; and second, plasma, unlike conditioned media, contained a 26 kDa IGFBP, likely to be oIGFBP-1. The results of our studies suggest that the production and release of IGFBPs by isolated sheep fibroblasts is regulated by developmental factors operative under in vitro culture conditions. The differences in the relative levels of IGFBPs in conditioned media from fetal, postnatal, and maternal sheep fibroblasts resemble in several respects the differences in the relative concentrations of the various IGFBPs in fetal, postnatal, and maternal sheep plasma. Thus, sheep fibroblasts provide a useful though imperfect model system by which to examine the nutritional and hormonal regulation of sheep IGFBP production at various developmental stages.