Insulin-like growth factor-binding protein 5 (Igfbp5) compromises survival, growth, muscle development, and fertility in mice

Role of insulin-like growth factor binding proteins in mammary gland development

Insulin-like growth factors (IGFs) play an important role in mammary gland development and their effects are, in turn, influenced by a family of 6 IGF-binding proteins (IGFBPs). The IGFBPs are expressed in time- and tissue-specific fashion during the periods of rapid growth and involution of the mammary gland. The precise roles of these proteins in vivo have, however, been difficult to determine. This review examines the indirect evidence (evolution, chromosomal location and roles in lower life-forms) the evidence from in vitro studies and the attempts to examine their roles in vivo, using IGFBP-deficient and over-expression models. Evidence exists for a role of the IGFBPs in inhibition of the survival effects of IGFs as well as in IGF-enhancing effects from in vitro studies. The location of the IGFBPs, often associated with the extracellular matrix, suggests roles as a reservoir of IGFs or as a potential barrier, restricting access of IGFs to distinct cellular compartments. We also discuss the relative importance of IGF-dependent versus IGF-independent effects. IGF-independent effects include nuclear localization, activation of proteases and interaction with a variety of extracellular matrix and cell surface proteins. Finally, we examine the increasing evidence for the IGFBPs to be considered as part of a larger family of extracellular matrix proteins involved in morphogenesis and tissue re-modeling.

IGFBP-5 regulates muscle cell differentiation by binding to IGF-II and switching on the IGF-II auto-regulation loop

IGF-II stimulates both mitogenesis and myogenesis through its binding and activation of the IGF-I receptor (IGF-IR). How this growth factor pathway promotes these two opposite cellular responses is not well understood. We investigate whether local IGF binding protein-5 (IGFBP-5) promotes the myogenic action of IGF-II. IGFBP-5 is induced before the elevation of IGF-II expression during myogenesis. Knockdown of IGFBP-5 impairs myogenesis and suppresses IGF-II gene expression. IGF-II up-regulates its own gene expression via the PI3K-Akt signaling pathway. Adding IGF-II or constitutively activating Akt rescues the IGFBP-5 knockdown-caused defects. However, an IGF analogue that binds to the IGF-IR but not IGFBP has only a limited effect. When added with low concentrations of IGF-II, IGFBP-5 restores IGF-II expression and myogenic differentiation, whereas an IGF binding–deficient IGFBP-5 mutant has no effect. These findings suggest that IGFBP-5 promotes muscle cell differentiation by binding to and switching on the IGF-II auto-regulation loop.

Loss of core fucosylation of low-density lipoprotein receptor-related protein-1 impairs its function, leading to the upregulation of serum levels of insulin-like growth factor-binding protein 3 in Fut8-/- mice

alpha1,6-Fucosyltransferase (Fut8) catalyzes the transfer of a fucose residue from GDP-fucose to the innermost N-acetylglucosamine residue of N-glycans. Here we report that the loss of core fucosylation impairs the function of low-density lipoprotein (LDL) receptor-related protein-1 (LRP-1), a multifunctional scavenger and signaling receptor, resulting in a reduction in the endocytosis of insulin like growth factor (IGF)-binding protein-3 (IGFBP-3) in the cells derived from Fut8-null (Fut8-/-) mice. The reduced endocytosis was restored by the re-introduction of Fut8. Serum levels of IGFBP-3 were markedly upregulated in Fut8-/- mice. These data clearly indicate that core fucosylation is crucial for the scavenging activity of LRP-1 in vivo.

Expression level of insulin-like growth factor binding protein 5 mRNA is a prognostic factor for breast cancer

The role of insulin-like growth factor binding protein 5 (IGFBP5) in tumorigenesis and development of cancer is not well-defined. IGFBP5 has been shown to either stimulate or inhibit cell proliferation via an IGF-dependent mechanism and to promote cell proliferation and migration in an IGF-independent manner. In the authors' previous study, IGFBP5 was found to be significantly up-regulated in lymph node metastases compared with their paired primary breast cancers. To further determine the role of IGFBP5 in breast cancer development and to evaluate its clinical significance in breast cancer, the mRNA expression level was detected in 30 normal breast tissues, 108 primary tumors, and 30 lymph node metastases using real time reverse transcription-polymerase chain reaction. The expression levels were correlated with several clinical parameters, including clinical stage, pathologic tumor size, axillary lymph node status, nuclear grade, estrogen receptor status, Her2 status, and local relapse or distant metastasis of the patients. As a result, the expression of IGFBP5 mRNA correlated positively with the invasion of axillary lymph nodes and the status of hormonal receptor. Furthermore, overexpression of IGFBP5 was associated with poor outcome of breast cancer patients with positive lymph nodes and negative ER. Thus, the expression level of IGFBP5 may contribute to the development of breast cancer and is a prognostic factor for breast cancer.

Insulin-like growth factor (IGF) binding protein-5 blocks skeletal muscle differentiation by inhibiting IGF actions

Signaling through the IGF-I receptor by locally produced IGF-I or -II is critical for normal skeletal muscle development and repair after injury. In most tissues, IGF action is modulated by IGF binding proteins (IGFBPs). IGFBP-5 is produced by muscle cells, and previous studies have suggested that when overexpressed it may either facilitate or inhibit IGF actions, and thus potentially enhance or diminish IGF-mediated myoblast differentiation or survival. To resolve these contradictory observations and discern the mechanisms of action of IGFBP-5, we studied its effects in cultured muscle cells. Purified wild-type (WT) mouse IGFBP-5 or a variant with diminished extracellular matrix binding (C domain mutant) each prevented differentiation at final concentrations as low as 3.5 nm, whereas analogs with reduced IGF binding (N domain mutant) were ineffective even at 100 nm. None of the IGFBP-5 variants altered cell number. An IGF-I analog (R(3)IGF-I) with diminished affinity for IGFBPs promoted full muscle differentiation in the presence of IGFBP-5(WT), showing that IGFBP-5 interferes with IGF-dependent signaling pathways in myoblasts. When IGFBP-5(WT) or variants were overexpressed by adenovirus-mediated gene transfer, concentrations in muscle culture medium reached 500 nm, and differentiation was inhibited, even by IGFBP-5(N). As 200 nm of purified IGFBP-5(N) prevented activation of the IGF-I receptor by 10 nm IGF-II as effectively as 2 nm of IGFBP-5(WT), our results not only demonstrate that IGFBP-5 variants with reduced IGF binding affinity impair muscle differentiation by blocking IGF actions, but underscore the need for caution when labeling effects of IGFBPs as IGF independent because even low-affinity analogs may potently inhibit IGF-I or -II if present at high enough concentrations in biological fluids.

Induction of cellular senescence by insulin-like growth factor binding protein-5 through a p53-dependent mechanism

The insulin-like growth factor (IGF) signaling pathway plays a crucial role in the regulation of cell growth, differentiation, apoptosis, and aging. IGF-binding proteins (IGFBPs) are important members of the IGF axis. IGFBP-5 is up-regulated during cellular senescence in human dermal fibroblasts and endothelial cells, but the function of IGFBP-5 in cellular senescence is unknown. Here we show that IGFBP-5 plays important roles in the regulation of cellular senescence. Knockdown of IGFBP-5 in old human umbilical endothelial cells (HUVECs) with IGFBP-5 micro-RNA lentivirus caused partial reduction of a variety of senescent phenotypes, such as changes in cell morphology, increases in cell proliferation, and decreases in senescence-associated beta-galactosidase (SA-beta-gal) staining. In addition, treatment with IGFBP-5 protein or up-regulation of IGFBP-5 in young cells accelerates cellular senescence, as confirmed by cell proliferation and SA-beta-gal staining. Premature senescence induced by IGFBP-5 up-regulation in young cells was rescued by knockdown of p53, but not by knockdown of p16. Furthermore, atherosclerotic arteries exhibited strong IGFBP-5-positive staining along intimal plaques. These results suggest that IGFBP-5 plays a role in the regulation of cellular senescence via a p53-dependent pathway and in aging-associated vascular diseases.

Experimental arthritis inhibits the insulin-like growth factor-I axis and induces muscle wasting through cyclooxygenase-2 activation

Chronic arthritis induces cachexia associated with an inhibition of the growth hormone (GH)-insulin-like growth factor-I (IGF-I) system and an activation of the E3 ubiquitin-ligating enzymes muscle atrophy F-box (MAFbx) and muscle Ring finger 1 (MuRF1) in the skeletal muscle. The aim of this work was to study the role of cyclooxygenase (COX)-2 in chronic arthritis-induced cachexia. Arthritis was induced in rats by Freund's adjuvant injection, and the effects of two COX inhibitors (indomethacin, a nonspecific inhibitor, and meloxicam, a selective COX-2 inhibitor on pituitary GH and on liver and serum IGF-I levels) were tested. Arthritis decreased body weight gain and GH and liver IGF-I gene expression. In the arthritic rats, both inhibitors, indomethacin and meloxicam, prevented the inhibitory effect of arthritis on body weight gain. Indomethacin and meloxicam administration to arthritic rats increased pituitary GH and liver IGF-I mRNA as well as serum levels of IGF-I. These data suggest that induction of COX-2 during chronic inflammation is involved in the inhibition of the GH-IGF-I axis and in the body weight loss. In the gastrocnemius muscle, arthritis increased the gene expression of tumor necrosis factor (TNF)-alpha, the E3 ubiquitin-ligating enzymes MAFbx and MuRF1, as well as of IGF-I and IGF-binding protein-5 (IGFBP-5). Inhibition of COX-2 by meloxicam administration increased gastrocnemius weight and decreased MAFbx, MuRF1, TNF-alpha, and IGFBP-5 gene expression. In summary, our data indicate that chronic arthritis-induced cachexia and muscle wasting are mediated by the COX-2 pathway resulting in a decreased GH-IGF-I secretion and increased expression of MAFbx and MuRF1 mRNA.

Delayed mammary gland involution in mice with mutation of the insulin-like growth factor binding protein 5 gene

IGFs (IGF-I and IGF-II) are essential for development, and their bioactivities are tightly regulated by six related IGF-binding proteins (IGFBPs). IGFBP-5 is the most highly conserved binding protein and is expressed in several key developmental lineages as well as in multiple adult tissues including the mammary gland. To explore IGFBP-5 actions in vivo, we produced IGFBP-5 knockout (KO) mice. Whole-body growth, selected organ weights, and body composition were essentially normal in IGFBP-5 KO mice, presumably because of substantial compensation by remaining IGFBP family members. The IGFBP-5 KO mice also exhibited normal mammary gland development and were capable of nursing their pups. We then directly evaluated the proposed role of IGFBP-5 in apoptosis and remodeling of mammary gland during involution. We found that the process of involution after forced weaning was delayed in IGFBP-5 KO mice, with both the appearance of apoptotic cells and the reappearance of adipocytes retarded in mutant mice, compared with controls. We also determined the effects of IGFBP-5 deletion on mammary gland development in pubertal females after ovariectomy and stimulation with estradiol/progesterone. In this paradigm, IGFBP-5 KO mammary glands exhibited enhanced alveolar bud formation consistent with enhanced IGF-I action. These results demonstrate that IGFBP-5, although not essential for normal growth, is required for normal mammary gland involution and can regulate mammary gland morphogenesis in response to hormone stimulation.

Identification of proteins from feeder conditioned medium that support human embryonic stem cells

The maintenance of undifferentiated human embryonic stem cells (hESC) requires feeder cells, either in co-culture or feeder-free with conditioned medium (CM) from the feeders. In this study, we compared the CM of a supporting primary mouse embryonic feeder (MEF) and an isogenic but non-supporting MEF line (DeltaE-MEF) in order to gain an insight to the differential expression profile of secreted factors. Using two-dimensional electrophoresis (2-DE) and matrix-assisted laser desorption/ionization-time of flight (MALDI) tandem mass spectrometry, 13 protein identities were found to be downregulated in DeltaE-MEF compared to MEF, of which 4 were found to be soluble factors and 3 proteins were membrane-associated or related to the extracellular matrix. In addition, four other proteins were identified to be differentially expressed in MEF-CM using high pressure liquid chromatography (HPLC) and cytokine arrays. In functional experiments where CM was replaced with six of the factors identified, hESC were able to proliferate for five continuous passages whilst maintaining 68-82% and 74-98% expression of pluripotent markers, Oct-4 and Tra-1-60, respectively. Using proteomic tools, important proteins from CM that supports hESC culture have been identified, which when replaced with recombinant proteins, continue to support undifferentiated hESC growth in a feeder-free culture platform.

Altered muscle development and expression of the insulin-like growth factor system in growth retarded fetal pigs

We have used a porcine model of spontaneous differential fetal growth to investigate the effects of fetal size on muscle development. We hypothesized that altered muscle development may occur in small fetuses as a consequence of modified expression of selected genes of the insulin-like growth factor system. We examined the development of the Longissimus muscle (m. Longissimus) in small fetuses and their average sized littermates. We collected small for gestational age fetuses and their average sized sibling on days 45, 65 and 100 of gestation (term is 113-116 days). Small fetuses had significantly lower body weight at all three stages of gestation (p<0.05) and significantly reduced secondary to primary muscle fibre ratio in m. Longissimus on day 100 (p<0.05) compared to their littermates. On day 65, the expression of insulin-like growth factor receptor 1 and insulin-like growth factor binding protein 3 were significantly higher (p<0.05) in m. Longissimus of the small fetuses compared with their average sized littermates. On day 100, the expression of insulin-like growth factor receptor 1 remained significantly higher (p=0.001), in addition to significantly higher levels of insulin-like growth factor receptor 2 and insulin-like growth factor binding protein 5 in the small fetuses (p<0.05). No difference in levels of myogenin was observed between the small and average sized littermates. In conclusion, we demonstrate that reduced fetal muscle development is associated with an increased expression of several genes of the insulin-like growth factor system in small fetuses in mid to late gestation.

IGF-binding protein-2 protects against the development of obesity and insulin resistance

Proliferation of adipocyte precursors and their differentiation into mature adipocytes contributes to the development of obesity in mammals. IGF-I is a potent mitogen and important stimulus for adipocyte differentiation. The biological actions of IGFs are closely regulated by a family of IGF-binding proteins (IGFBPs), which exert predominantly inhibitory effects. IGFBP-2 is the principal binding protein secreted by differentiating white preadipocytes, suggesting a potential role in the development of obesity. We have generated transgenic mice overexpressing human IGFBP-2 under the control of its native promoter, and we show that overexpression of IGFBP-2 is associated with reduced susceptibility to obesity and improved insulin sensitivity. Whereas wild-type littermates developed glucose intolerance and increased blood pressure with aging, mice overexpressing IGFBP-2 were protected. Furthermore, when fed a high-fat/high-energy diet, IGFBP-2-overexpressing mice were resistant to the development of obesity and insulin resistance. This lean phenotype was associated with decreased leptin levels, increased glucose sensitivity, and lower blood pressure compared with wild-type animals consuming similar amounts of high-fat diet. Our in vitro data suggest a direct effect of IGFBP-2 preventing adipogenesis as indicated by the ability of recombinant IGFBP-2 to impair 3T3-L1 differentiation. These findings suggest an important, novel role for IGFBP-2 in obesity prevention.

Growth factor messenger ribonucleic acid expression during differentiation of porcine embryonic myogenic cells

The growth factors, IGF-I and II, their binding proteins, IGFBP, and members of the transforming growth factor (TGF) superfamily (myostatin and TGFbeta1) are known to regulate proliferation and differentiation of myogenic cells. We hypothesized that changes in the relative expression of members of the IGF and TGFbeta systems play a significant role in regulating myogenesis in porcine embryonic myogenic cell (PEMC) cultures. Therefore, determining the expression patterns of these factors during PEMC myogenesis is important. Consequently, we used real-time PCR to explore the pattern of IGF-I; IGF-II; IGFBP-2, -3, and -5; IGF-type-I receptor; myogenin; myostatin; and TGFbeta1 mRNA expression during PEMC myogenesis. The progression of differentiation was assessed using creatine kinase activity and myogenin mRNA expression. As anticipated, creatine kinase activity was low in PEMC cultures at 48 h and increased 20-fold (P < 0.0001) between 48 h and its peak at 144 h. Similarly, myogenin mRNA was low at 48 h and increased approximately 5-fold (P < 0.0001) as differentiation progressed, peaking at 120 h and decreasing at 144 h. The patterns of IGF-I and IGFBP-2 mRNA expression were similar and were relatively lower in 48-h PEMC cultures, increasing approximately 5-fold (P < 0.0001) to their greatest levels at 120 h. In contrast, IGF-II and IGFBP-5 mRNA levels were relatively high at 48 h, peaking at 72 h, and steadily decreasing by 60 and 80%, respectively (P < 0.001), at 144 h. The level of IGF-type-I receptor mRNA was relatively high until 96 h of culture, after which it decreased 40% (P < 0.01), reaching a low at 144 h. Levels of IGFBP-3 mRNA were relatively high at 48 h, dropped approximately 40% to their lowest level at 72 h (P < 0.001), and then increased approximately 60% (P < 0.001) to their greatest levels at 144 h. Levels of TGFbeta1 mRNA decreased approximately 30% (P < 0.0001) between 48 and 96 h, then quickly rebounded to a peak at 120 h, and by 144 h had dropped to the levels seen at 72 h. Myostatin mRNA was at its greatest level at 48 h and declined rapidly between 72 and 96 h, finally decreasing by approximately 80% at 144 h (P < 0.0001). Our data demonstrate that these factors are differentially regulated during PEMC myogenesis and provide new information about their pattern of mRNA expression in cultured porcine muscle cells.

Insulin-like growth factor-binding protein-5 induces pulmonary fibrosis and triggers mononuclear cellular infiltration

We have recently shown that insulin-like growth factor-binding protein (IGFBP)-5 is overexpressed in idiopathic pulmonary fibrosis lung tissues and increases collagen and fibronectin deposition. Here, we further examined the effect of IGFBP-5 in vivo by intratracheal administration of replication-deficient adenovirus expressing human IGFBP-5 (Ad5), IGFBP-3 (Ad3), or no cDNA (cAd) to wild-type mice. Increased cellular infiltration and extracellular matrix deposition were observed in mice after Ad5 administration compared with Ad3 and cAd. Mononuclear cell infiltration consisted predominantly of T lymphocytes at day 8. By day 14, the number of infiltrating T cells decreased, whereas that of B cells and monocytes/macrophages increased. IGFBP-5 also induced migration of peripheral blood mononuclear cells in vitro, suggesting that in vivo mononuclear cell infiltration may be the direct result of IGFBP-5 expression. alpha-Smooth muscle actin and Mucin-1 co-localized in cells of mice treated with Ad5, suggesting that IGFBP-5 induced epithelial-mesenchymal transition. In addition, exogenous IGFBP-5 induced alpha-smooth muscle actin expression in primary fibroblasts and epithelial-mesenchymal transition of pulmonary epithelial cells in vitro. In conclusion, our results suggest that overexpression of IGFBP-5 in mouse lung results in fibroblast activation, increased extracellular matrix deposition, and myofibroblastic changes. Thus, the IGFBP-5-induced fibrotic phenotype in vivo may represent a novel model to better understand the pathogenesis of fibrosis.

Insulin-like growth factor binding protein 5 induces skin fibrosis: A novel murine model for dermal fibrosis

OBJECTIVE

To determine the role of insulin-like growth factor binding protein 5 (IGFBP-5) in the development of skin fibrosis in vivo, by examining the effect of overexpression of IGFBP-5 in mouse skin.

METHODS

Wild-type C57BL/6J mice were injected subcutaneously with replication-deficient serotype 5 adenovirus expressing human IGFBP-3 (Ad3), IGFBP-5 (Ad5), or no complementary DNA (cAd). Mice were killed 3, 8, or 22 days postinjection. The dermal thickness and dermal collagen bundle thickness in skin sections were measured. The deposition of collagen in the extracellular matrix (ECM) was quantified using the Sircol assay. Expression of proliferating cell nuclear antigen (PCNA) and fibronectin, as determined by immunohistochemical analysis, was used to evaluate fibroblast activation, and vimentin and alpha-smooth muscle actin (alpha-SMA) were used to evaluate the fibroblast phenotype.

RESULTS

Adenovirally expressed IGFBP was detected in dermal fibroblasts, endothelial cells, epithelial cells, and muscle bundles in Ad3- and Ad5-injected mice. Increased collagen deposition, denser dermal connective tissue, and increased collagen bundle thickness were observed in IGFBP-5-overexpressing mice. Dermal thickness and collagen bundle thickness were significantly increased in Ad5-injected mice compared with cAd- and Ad3-injected mice. Treatment with Ad5 resulted in a dose-dependent increase in dermal and collagen bundle thickness. Increased deposition of collagen and fibronectin, increased numbers of PCNA-positive fibroblasts, as well as increased numbers of vimentin- and alpha-SMA-double-positive fibroblasts were detected in the dermis of IGFBP-5-overexpressing mouse skin.

CONCLUSION

IGFBP-5 is a key mediator of fibrosis. IGFBP-5 mediates its profibrotic effects through fibroblast activation, increased ECM deposition, and myofibroblastic transformation of dermal fibroblasts. Overexpression of IGFBP-5 provides a novel model for studying the pathogenesis of skin fibrosis in systemic sclerosis.

A locus on mouse Chromosome 9 (Adip5) affects the relative weight of the gonadal but not retroperitoneal adipose depot

To identify the gene or genes on mouse Chromosome 9 that contribute to strain differences in fatness, we conducted an expanded mapping analysis to better define the region where suggestive linkage was found, using the F(2 )generation of an intercross between the C57BL/6ByJ and 129P3/J mouse strains. Six traits were studied: the summed weight of two adipose depots, the weight of each depot, analyzed individually (the gonadal and retroperitoneal depot), and the weight of each depot (summed and individual) relative to body size. We found significant linkage (LOD = 4.6) that accounted for the relative weight of the summed adipose depots, and another for the relative weight of the gonadal (LOD = 5.3) but not retroperitoneal (LOD = 0.9) adipose depot. This linkage is near marker rs30280752 (61.1 Mb, Build 34) and probably is equivalent to the quantitative trait locus (QTL) Adip5. Because the causal gene is unknown, we identified and evaluated several candidates within the confidence interval with functional significance to the body fatness phenotype (Il18, Acat1, Cyp19a1, Crabp1, Man2c1, Neil1, Mpi1, Csk, Lsm16, Adpgk, Bbs4, Hexa, Thsd4, Dpp8, Anxa2, and Lipc). We conclude that the Adip5 locus is specific to the gonadal adipose depot and that a gene or genes near the linkage peak may account for this QTL.

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.

Sepsis and inflammatory insults downregulate IGFBP-5, but not IGFBP-4, in skeletal muscle via a TNF-dependent mechanism

The purpose of the present study was to determine whether catabolic stimuli that induce muscle atrophy alter the muscle mRNA abundance of insulin-like growth factor binding protein (IGFBP)-4 and -5, and if so determine the physiological mechanism for such a change. Catabolic insults produced by endotoxin (LPS) and sepsis decreased IGFBP-5 mRNA time- and dose-dependently in gastrocnemius muscle. This reduction did not result from muscle disuse because hindlimb immobilization increased IGFBP-5. Continuous infusion of a nonlethal dose of tumor necrosis factor-α (TNF-α) decreased IGFBP-5 mRNA 70%, whereas pretreatment of septic rats with a neutralizing TNF binding protein completely prevented the reduction in muscle IGFBP-5. The addition of LPS or TNF-α to cultured C2C12myoblasts also decreased IGFBP-5 expression. Although exogenously administered growth hormone (GH) increased IGFBP-5 mRNA 2-fold in muscle from control rats, muscle from septic animals was GH resistant and no such elevation was detected. In contrast, exogenous administration of IGF-I as part of a binary complex composed of IGF-I/IGFBP-3 produced comparable increases in IGFBP-5 mRNA in both control and septic muscle. Concomitant determinations of IGF-I mRNA content revealed a positive linear relationship between IGF-I and IGFBP-5 mRNA in the same muscle in response to LPS, sepsis, TNF-α, and GH treatment. Although dexamethasone decreased muscle IGFBP-5, pretreatment of rats with the glucocorticoid receptor antagonist RU486 did not prevent the sepsis-induced decrease in IGFBP-5 mRNA. In contrast, muscle IGFBP-4 mRNA abundance was not significantly altered by LPS, sepsis, or hindlimb immobilization. In summary, these data demonstrate that various inflammatory insults decrease muscle IGFBP-5 mRNA, without altering IGFBP-4, by a TNF-dependent glucocorticoid-independent mechanism. Finally, IGF-I appears to be a dominant positive regulator of IGFBP-5 gene expression in muscle under both normal and catabolic conditions.

The Sirt1 deacetylase modulates the insulin-like growth factor signaling pathway in mammals

The lifespan of the nematode, Caenorhabditis elegans, can be extended by mutations affecting components of the insulin-like growth factor (IGF) signaling cascade or by overexpression of SIR2, an NAD+-dependent protein deacetylase. The mammalian homologue of SIR2, Sirt1, has been shown to modulate the activity of FoxO, a transcription factor that is downstream of the IGF signaling system. These results suggest that Sirt1 ought to affect the IGF pathway. We report here evidence that this is the case in mice. The loss of Sirt1 protein in mice results in increased expression of the IGF binding protein IGFBP1, a secreted modulator of IGF function. A number of the anatomical characteristics of Sirt1-null mice closely resemble those of transgenic mice overexpressing IGFBP1. Our data suggest that Sirt1 is part of a regulatory loop that limits the production of IGFBP1 thereby modulating IGF signaling.

Neprilysin participates in skeletal muscle regeneration and is accumulated in abnormal muscle fibres of inclusion body myositis

Neprilysin (NEP, EP24.11), a metallopeptidase originally shown to modulate signalling events by degrading small regulatory peptides, is also an amyloid-beta- (Abeta) degrading enzyme. We investigated a possible role of NEP in inclusion body myositis (IBM) and other acquired and hereditary muscle disorders and found that in all myopathies NEP expression was directly associated with the degree of muscle fibre regeneration. In IBM muscle, NEP protein was also strongly accumulated in Abeta-bearing abnormal fibres. In vitro, during the experimental differentiation of myoblasts, NEP protein expression was regulated at the post-transcriptional level with a rapid increase in the early stage of myoblast differentiation followed by a gradual reduction thereafter, coincident with the progression of the myogenic programme. Treatment of differentiating muscle cells with the NEP inhibitor dl-3-mercapto-2-benzylpropanoylglycine resulted in impaired differentiation that was mainly associated with an abnormal regulation of Akt activation. Therefore, NEP may play an important role during muscle cell differentiation, possibly through the regulation, either directly or indirectly, of the insulin-like growth factor I-driven myogenic programme. In IBM muscle increased NEP may be instrumental in (i) reducing the Abeta accumulation in vulnerable fibres and (ii) promoting a repair/regenerative attempt of muscle fibres possibly through the modulation of insulin-like growth factor I-dependent pathways.

Insulin-like growth factor binding proteins in development

IGFBPs regulate growth and development by regulating IGF transport to tissues and IGF bioavailability to IGF receptors at cell membrane level. IGFBP excess leads predominantly to inhibition of IGF action and growth retardation with impaired organogenesis. Absence of human and also mouse ALS leads to decreased IGF-I levels in circulation and causes mild growth retardation. Although IGFBP KO mice demonstrate relatively minor phenotypes, the possibility of compensatory mechanisms that mask the phenotypic manifestation of lack of individual binding proteins needs to be further investigated. Recent studies of hepatic regeneration in IGFBP-1 KO mice and also with mutant IGFBP-3 Tg mice provide some limited support for the existence of IGF-independent mechanism of action in vivo.

Ghrelin receptor agonist GHRP-2 prevents arthritis-induced increase in E3 ubiquitin-ligating enzymes MuRF1 and MAFbx gene expression in skeletal muscle

Chronic arthritis is a catabolic state associated with an inhibition of the IGF system and a decrease in body weight. Cachexia and muscular wasting is secondary to protein degradation by the ubiquitin-proteasome pathway. The aim of this work was to analyze the effect of adjuvant-induced arthritis on the muscle-specific ubiquitin ligases muscle ring finger 1 (MuRF1) and muscle atrophy F-box (MAFbx) as well as on IGF-I and IGF-binding protein-5 (IGFBP-5) gene expression in the skeletal muscle. We also studied whether the synthetic ghrelin receptor agonist, growth hormone releasing peptide-2 (GHRP-2), was able to prevent arthritis-induced changes in the skeletal muscle. Arthritis induced an increase in MuRF1, MAFbx (P < 0.01), and tumor necrosis factor (TNF)-alpha mRNA (P < 0.05) in the skeletal muscle. Arthritis decreased the serum IGF-I and its gene expression in the liver (P < 0.01), whereas it increased IGF-I and IGFBP-5 gene expression in the skeletal muscle (P < 0.01). Administration of GHRP-2 for 8 days prevented the arthritis-induced increase in muscular MuRF1, MAFbx, and TNF-alpha gene expression. GHRP-2 treatment increased the serum concentrations of IGF-I and the IGF-I mRNA in the liver and in the cardiac muscle and decreased muscular IGFBP-5 mRNA both in control and in arthritic rats (P < 0.05). GHRP-2 treatment increased muscular IGF-I mRNA in control rats (P < 0.01), but it did not modify the muscular IGF-I gene expression in arthritic rats. These data indicate that arthritis induces an increase in the activity of the ubiquitin-proteasome proteolytic pathway that is prevented by GHRP-2 administration. The parallel changes in muscular IGFBP-5 and TNF-alpha gene expression with the ubiquitin ligases suggest that they can participate in skeletal muscle alterations during chronic arthritis.

Segmental Igfbp5 expression is specifically associated with the bent structure of zigzag hairs

The murine hair coat consists of four different hair types that are characterised by hair length, the number of medulla columns, and the presence and number of bends. The molecular mechanisms underlying the establishment and maintenance of distinct hair follicle fates are unknown. We identify Igfbp5 as the first molecular marker that distinguishes among different hair follicle types. High-resolution expression analysis revealed that its expression in the medulla of hair shafts is associated with the bend-forming zones of zigzag hairs. To directly examine the functional importance of segmental gene expression in the hair follicle, we have generated transgenic mice expressing Igfbp5 in differentiating keratinocytes of the medulla and inner root sheath. Ectopic expression of Igfbp5 resulted in the appearance of remarkable curvatures and thinning of hair shafts, two hallmarks of hair bends. Both effects and the natural bending process are under negative control of IGF signalling. Thus, our data identify Igfbp5 as a central regulator of hair shaft differentiation and hair type determination.

Interleukin-11 inhibits expression of insulin-like growth factor binding protein-5 mRNA in decidualizing human endometrial stromal cells

Differentiation of endometrial stromal cells into decidual cells is essential for successful embryo implantation. Interleukin (IL)-11 signalling is critical for normal decidualization in the mouse. The expression of IL-11 and its receptors during the menstrual cycle, and the effect of exogenous IL-11 on the decidualization of human endometrial stromal cells in vitro, suggests a role for this cytokine in human decidualization. As the downstream target genes of IL-11 are also likely to be critical mediators of this process, this study aimed to identify genes regulated by IL-11 in decidualizing human endometrial stromal cells in vitro. Stromal cells isolated from endometrial biopsies were decidualized with 17beta estradiol (E) and medroxyprogesterone acetate (EP) in the presence or absence of exogenous IL-11, and total RNA used for cDNA microarray analysis and real-time RT-PCR. Microarray analysis revealed 16 up-regulated and 11 down-regulated cDNAs in EP + IL-11-treated compared with EP-treated cells. The most down-regulated gene was insulin-like growth factor binding protein-5 (IGFBP-5) (3.6-fold). Using real-time RT-PCR, IL-11 was confirmed to decrease IGFBP-5 transcript abundance 102-fold (P = 0.016; n = 6). No difference in IGFBP-5 immunostaining intensity was detected in stromal cells decidualized in the presence or absence of IL-11, and there was no effect of exogenous IGFBP-5 on the progression of steroid-induced in vitro decidualization. Interactions between IL-11 and its target genes, including IGFBP-5, may contribute to the regulation of decidualization and/or mediate communication between the decidua and invading trophoblast at implantation.

Pregnancy-associated plasma protein-A regulates myoblast proliferation and differentiation through an insulin-like growth factor-dependent mechanism

Pregnancy-associated plasma protein-A (PAPP-A), a member of the metalloproteinase superfamily, is an important regulator of mammalian growth and development. However, the role of PAPP-A and its mechanism of action in various cellular processes remain unknown. In this study, we have investigated the role of PAPP-A in skeletal myogenesis using C2C12 myoblasts. Recombinant PAPP-A was purified from the conditioned medium of HT1080 cells overexpressing PAPP-A. Treatment of C2C12 myoblasts with PAPP-A increased their proliferation in a dose- and time-dependent manner. Addition of exogenous PAPP-A also increased the myotube formation and the activity of creatine kinase in C2C12 cultures. Transient overexpression of the full-length PAPP-A-(1-1547), but not truncated protease-inactive N-terminal PAPP-A-(1-920) or C-terminal PAPP-A-(1100-1547), significantly enhanced the proliferation of C2C12 myoblasts. In vitro and in situ experiments demonstrated that PAPP-A cleaves insulin-like growth factor-binding protein (IGFBP)-2, but not IGFBP-3, in the conditioned medium of C2C12 myoblasts. Overexpression of PAPP-A led to degradation of the IGFBP-2 produced by C2C12 myoblasts and increased free IGF-I concentrations without affecting total IGF-I concentrations. Addition of protease-resistant IGFBP-4 completely abolished the PAPP-A-induced proliferation of C2C12 myoblasts. Our results demonstrate that 1) PAPP-A increases the proliferation and differentiation of myoblasts, 2) the stimulatory effect of PAPP-A on myogenesis is governed by its proteolytic activity, and 3) PAPP-A promotes skeletal myogenesis by increasing the amount of free IGFs via specific degradation of IGFBP-2 produced by myoblasts.

Early developmental changes in IGF-I, IGF-II, IGF binding protein-1, and IGF binding protein-3 concentration in the cerebrospinal fluid of children

IGF-I and IGF-II are ubiquitously expressed growth factors that have profound effects on the growth and differentiation of many cell types and tissues, including cells of the CNS. In biologic fluids, most IGFs are bound to one of six IGF binding proteins (IGFBPs 1-6). Increasing evidence strongly supports a role for IGF-I in CNS development, as it promotes neuronal proliferation and survival. However, little is known about IGF-I and its homolog IGF-II and their carrier proteins, IGFBPs, during the neonatal period in which brain size increases dramatically, myelination takes place, and neurons show limited capacity to proliferate. Herein, we have determined the concentrations of IGF-I, IGF-II, IGFBP-1, and IGFBP-3 in cerebral spinal fluid (CSF) samples that were collected from children who were 1 wk to 18 y of age. The concentrations of IGF-I, IGFBP-1, and IGFBP-3 in CSF from children <6 mo of age were significantly higher than in older children, whereas IGF-II was higher in the older group. This is in contrast to what is observed in the peripheral circulation, where IGF-I and IGFBP-3 are low at birth and rise rapidly during the first year, reaching peak levels during puberty. Higher concentrations of IGF-I, IGFBP-1, and IGFBP-3 in the CSF of very young children suggest that these proteins might participate in the active processes of myelination and synapse formation in the developing nervous system. We propose that IGF-I and certain IGFBPs are likely necessary for normal CNS development during critical stages of neonatal brain growth and development.

Roles of insulin-like growth factor (IGF) binding proteins in regulating IGF actions

The insulin-like growth factor (IGF) system is an evolutionarily conserved signaling pathway that is composed of two IGF ligands, two IGF receptors, and six IGF binding proteins. Studies in a variety of species suggest that the IGF signaling system plays a fundamental role in regulating embryonic growth and differentiation as well as in maintaining homeostasis in the adults. In extracellular fluids, IGFs are present in a complex with an IGF-binding protein (IGFBP). These IGFBPs are traditionally thought to function as carrier proteins and regulate circulating IGF turnover, transport, and distribution. Locally expressed IGFBPs can also inhibit and/or potentiate IGF activities. Recent studies have shown that some IGFBPs, in particular IGFBP-3 and -5, possess intrinsic biological activities and can act through IGF-independent mechanisms. In this article, we provide a brief overview of our current understanding of the IGF signaling system with particular reference to IGFBPs.

Insulin-like growth factor binding proteins 3 and 5 are overexpressed in idiopathic pulmonary fibrosis and contribute to extracellular matrix deposition

Idiopathic pulmonary fibrosis (IPF) is a fibrotic disease of unknown etiology that results in significant morbidity and mortality. The pathogenesis of IPF is not completely understood. Because recent studies have implicated insulin-like growth factor-I (IGF-I) in the pathogenesis of fibrosis, we examined the expression and function of insulin-like growth factor binding proteins (IGFBP)-3 and -5 in IPF. IGFBP-3 and -5 levels were increased in vivo in IPF lung tissues and in vitro in fibroblasts cultured from IPF lung. The IGFBPs secreted by IPF fibroblasts are functionally active and can bind IGF-I, and IGFBPs secreted by primary fibroblasts bind extracellular matrix components. Our results also suggest that IGFBPs may be involved in the initiation and/or perpetuation of fibrosis by virtue of their ability to induce the production of extracellular matrix components such as collagen type I and fibronectin in normal primary adult lung fibroblasts. Although transforming growth factor-beta increased IGFBP-3 production by primary fibroblasts in a time-dependent manner, IGFBP-5 levels were not increased by transforming growth factor-beta. Taken together, our results suggest that IGFBPs play an important role in the development of fibrosis in IPF.

Functional consequences of IGFBP excess-lessons from transgenic mice

The functions of insulin-like growth factor-binding proteins (IGFBPs) have been studied extensively in vitro, revealing IGF-dependent and also IGF-independent effects on cell growth, differentiation, and survival. In contrast, the biological relevance of IGFBPs in vivo is only partially understood. In the past decade, mouse models lacking or overexpressing specific IGFBPs have been generated by transgenic technology. Phenotypic analysis revealed features that are common for most IGFBPs (growth inhibition), but also effects that appear to be specific for some but not all IGFBPs, such as disturbed glucose homeostasis (IGFBP-1 and -3) or impaired fertility (IGFBP-1, -5, and -6). Future systematic comparison of IGFBP functions in transgenic mice will be facilitated by targeted insertion of IGFBP expression vectors and by standardized phenotype assessment. Furthermore, analysis of IGFBP expression in growth-selected mouse lines or pedigrees segregating for growth phenotypes will be important to understand the roles of IGFBPs in multigenic growth regulation.

Growth hormone/insulin-like growth factor system in children with chronic renal failure

Disturbances of the somatotropic hormone axis play an important pathogenic role in growth retardation and catabolism in children with chronic renal failure (CRF). The apparent discrepancy between normal or elevated growth hormone (GH) levels and diminished longitudinal growth in CRF has led to the concept of GH insensitivity, which is caused by multiple alterations in the distal components of the somatotropic hormone axis. Serum levels of IGF-I and IGF-II are normal in preterminal CRF, while in end-stage renal disease (ESRD) IGF-I levels are slightly decreased and IGF-II levels slightly increased. In view of the prevailing elevated GH levels in ESRD, these serum IGF-I levels appear inadequately low. Indeed, there is both clinical and experimental evidence for decreased hepatic production of IGF-I in CRF. This hepatic insensitivity to the action of GH may be partly the consequence of reduced GH receptor expression in liver tissue and partly a consequence of disturbed GH receptor signaling. The actions and metabolism of IGFs are modulated by specific high-affinity IGFBPs. CRF serum has an IGF-binding capacity that is increased by seven- to tenfold, leading to decreased IGF bioactivity of CRF serum despite normal total IGF levels. Serum levels of intact IGFBP-1, -2, -4, -6 and low molecular weight fragments of IGFBP-3 are elevated in CRF serum in relation to the degree of renal dysfunction, whereas serum levels of intact IGFBP-3 are normal. Levels of immunoreactive IGFBP-5 are not altered in CRF serum, but the majority of IGFBP-5 is fragmented. Decreased renal filtration and increased hepatic production of IGFBP-1 and -2 both contribute to high levels of serum IGFBP. Experimental and clinical evidence suggests that these excessive high-affinity IGFBPs in CRF serum inhibit IGF action in growth plate chondrocytes by competition with the type 1 IGF receptor for IGF binding. These data indicate that growth failure in CRF is mainly due to functional IGF deficiency. Combined therapy with rhGH and rhIGF-I is therefore a logical approach.

Insulin-like growth factor-binding protein-5 induces a gender-related decrease in bone mineral density in transgenic mice

IGF-binding protein-5 (IGFBP-5) is abundant in serum and bone during normal skeletal development, but levels decrease in osteoporosis. Studies have shown that IGFBP-5 stimulates markers of bone formation by potentiating IGF actions and by IGF-independent actions. To test the hypothesis that IGFBP-5 promotes the acquisition of bone mineral density (BMD), we generated transgenic (Tg) mice overexpressing Igfbp5 using a cytomegalovirus enhancer and beta-actin promoter (CMV/betaA). Tg animals showed an increase in serum IGFBP-5 concentrations by 7.7- to 3.5-fold at 3-8 wk of age, respectively. Concentrations were 6-49% higher for males compared with females in both wild-type and Tg mice. Surprisingly, BMD decreased in a gender-dependent manner, with Tg male adults affected more severely than Tg females (31.3% vs. 19.2% reduction, respectively, compared with wild-type mice, assessed by dual energy x-ray absorptiometry). Significant gender differences in BMD were confirmed by peripheral quantitative computed tomography. Histomorphometry revealed that although the bone formation rate and mineralizing surface at the periosteum decreased in Tg mice, they increased at the endosteum, suggesting opposing effects of IGFBP-5 on periosteal and endosteal osteoblasts (by altering proliferation or survival). These findings differ from previous observations in Igf1- and Igf2-null animals. In conclusion, IGFBP-5 has a significant influence on BMD acquisition and maintenance that is dependent on gender and age. The phenotype of Igfbp5 mice cannot be explained solely by IGF inhibition; thus, this study provides the first in vivo evidence, by genetic manipulation, for IGF-independent actions of IGFBP-5 in bone function. These findings have implications for the gender-biased progression of osteoporosis.

Osteoblast-specific expression of insulin-like growth factor-1 in bone of transgenic mice induces insulin-like growth factor binding protein-5

The activities of insulin-like growth factors (IGFs) in bone are modulated by a family of binding proteins (IGFBPs) whose physiological roles remain poorly understood. We have previously shown that targeted overexpression of IGF-I in osteoblasts of transgenic (OC-IGF-I) mice stimulates bone formation. In this model, bone formation is markedly but transiently increased in an age-dependent manner, raising the possibility that IGF-I may be influencing IGFBPs to in turn modulate its paracrine actions within bone. We sought to characterize the IGFBPs in normal mouse bone during development and to determine whether osteoblast-targeted overexpression of IGF-I influenced bone IGFBP abundance in vivo. Femoral bone IGFBP content was assessed in control nontransgenic and OC-IGF-I mice by I125-IGF-I ligand and immunoblotting. Bone IGFBP-5 and IGF-I mRNA abundance was determined using real-time reverse transcription (RT)-PCR. Ligand blot of bone extract showed a 30-kDa band, identified as IGFBP-5 by immunoblot, predominated. The abundance of IGFBP-5 declined with age in both control and transgenic bone. Ligand and immunoblot analysis revealed a 5-fold increase in IGFBP-5 protein levels at 3 weeks in transgenic bone (P<0.0001). The elevated IGFBP-5 protein levels were associated with a similar increase in IGF-I mRNA abundance (4-fold, P<0.01) and a significant increase in IGFBP-5 mRNA abundance (1.5-fold). Despite the age-related decline at 6 weeks, IGFBP-5 remained significantly (P<0.01) more abundant in transgenic bone compared to controls. In contrast, bone IGFBP-4 abundance was relatively unchanged by either age or IGF-I overexpression. These studies demonstrate a distinctive developmental pattern of IGFBP-5 content in mouse bone and show that osteoblast-derived IGF-I determines skeletal IGFBP-5 abundance, at least in part by inducing its synthesis. In that IGFBP-5 is thought to stimulate bone formation, directly or via IGF-I action, such changes in bone IGFBP-5 may be important to ensure robust bone acquisition in the early postnatal period.

Gene expression profiling of human lung tissue from smokers with severe emphysema

The mechanism by which inhaled smoke causes the anatomic lesions and physiologic impairment of chronic obstructive pulmonary disease remains unknown. We used high-density microarrays to measure gene expression in severely emphysematous lung tissue removed from smokers at lung volume reduction surgery (LVRS) and normal or mildly emphysematous lung tissue from smokers undergoing resection of pulmonary nodules. Class prediction algorithms identified 102 genes that accurately distinguished severe emphysema from non-/mildly emphysematous lung tissue. We also defined a number of genes whose expression levels correlated strongly with lung diffusion capacity for carbon monoxide and/or forced expiratory volume at 1 s. Genes related to oxidative stress, extracellular matrix synthesis, and inflammation were increased in severe emphysema, whereas expression of endothelium-related genes was decreased. To identify candidate genes that might be causally involved in the pathogenesis of emphysema, we linked gene expression profiles to chromosomal regions previously associated with chronic obstructive pulmonary disease in genome-wide linkage analyses. Unsupervised hierarchical clustering of the LVRS samples revealed distinct molecular subclasses of severe emphysema, with body mass index as the only clinical variable that differed between the groups. Class prediction models established a set of genes that predicted functional outcome at 6 mo after LVRS. Our findings suggest that the gene expression profiles from human emphysematous lung tissue may provide insight into pathogenesis, uncover novel molecular subclasses of disease, predict response to LVRS, and identify targets for therapeutic intervention.