Short-term growth hormone or IGF-I administration improves the IGF-IGFBP system in arthritic rats

OBJECTIVE

Adjuvant-induced arthritis is an experimental model of rheumatoid arthritis that inhibits the GH-IGF-I axis and decreases body weight gain and muscle mass. Although chronic GH or IGF-I treatment increases body weight gain in arthritic rats, muscle resistance to GH and IGF-I is a very common complication in inflammatory diseases. In this study we examine the effect of short-term administration of rhGH and rhIGF-I on liver and muscle IGF-I, IGFBP-3 and -5 as well as on the ubiquitin-ligases MuRF1 and atrogin-1 in the muscle of arthritic rats.

DESIGN

Arthritis was induced in adult male Wistar rats by an intradermal injection of 4 mg of Freund's adjuvant. Fifteen days after adjuvant injection, 300 μg/kg of rhGH or 200 μg/kg of rhIGF or saline was administrated 18 and 3h before decapitation. A pair-fed group injected with saline was included in order to discard a possible effect of decreased food intake. Gene expression of IGF-I, GHR, IGFBP-3, IGFBP-5, atrogin-1 and MuRF1 were quantified using RT-PCR. In serum, IGF-I was measured by radioimmunoassay (RIA) and IGFBP-3 by ligand blot.

RESULTS

Arthritis decreased serum IGF-I and IGF mRNA in liver (P<0.05), but not in skeletal muscle. In arthritic rats, rhGH increased serum IGF-I and liver IGF-I mRNA similar to the levels of pair-fed rats. Arthritis increased atrogin-1, MuRF1, IGFBP-3 and IGFBP-5 mRNA in muscle (P<0.01). IGFBP-3 mRNA was downregulated by rhIGF-I, but not by rhGH, administration in control and arthritic rats (P<0.05). Administration of rhGH and rhIGF-I increased IGFBP-5 in the gastrocnemius of arthritic rats.

CONCLUSIONS

Short-term rhGH and rhIGF-I administration was found to increase muscle IGFBP-5 mRNA, whereas only rhIGF-I administration decreased muscle IGFBP-3 mRNA in control and arthritic rats. These data suggest that arthritis does not induce GH or IGF-I resistance in skeletal muscle.

Does reduced IGF-1R signaling in Igf1r+/- mice alter aging?

Mutations in insulin/IGF-1 signaling pathway have been shown to lead to increased longevity in various invertebrate models. Therefore, the effect of the haplo- insufficiency of the IGF-1 receptor (Igf1r^+/−) on longevity/aging was evaluated in C57Bl/6 mice using rigorous criteria where lifespan and end-of-life pathology were measured under optimal husbandry conditions using large sample sizes. Igf1r^+/− mice exhibited reductions in IGF-1 receptor levels and the activation of Akt by IGF-1, with no compensatory increases in serum IGF-1 or tissue IGF-1 mRNA levels, indicating that the Igf1r^+/− mice show reduced IGF-1 signaling. Aged male, but not female Igf1r^+/− mice were glucose intolerant, and both genders developed insulin resistance as they aged. Female, but not male Igf1r^+/− mice survived longer than wild type mice after lethal paraquat and diquat exposure, and female Igf1r^+/− mice also exhibited less diquat-induced liver damage. However, no significant difference between the lifespans of the male Igf1r^+/− and wild type mice was observed; and the mean lifespan of the Igf1r^+/− females was increased only slightly (less than 5%) compared to wild type mice. A comprehensive pathological analysis showed no significant difference in end-of-life pathological lesions between the Igf1r^+/− and wild type mice. These data show that the Igf1r^+/− mouse is not a model of increased longevity and delayed aging as predicted by invertebrate models with mutations in the insulin/IGF-1 signaling pathway.

Characterization of three IGFBP mRNAs in Atlantic croaker and their regulation during hypoxic stress: potential mechanisms of their upregulation by hypoxia

Insulin-like growth factor-binding proteins (IGFBPs) play important roles in downregulating IGF activity and growth and development in vertebrates under hypoxic stress. However, the mechanisms of hypoxia regulation of IGFBPs in teleost fishes are unknown. The involvement of reactive oxygen species (ROS) and hypoxia-inducible factors (HIFs) in hypoxia upregulation of IGFBPs in Atlantic croaker were investigated. Three croaker IGFBPs, IGFBP-1, IGFBP-2, and IGFBP-5, were cloned and characterized. Chronic hypoxia exposure [dissolved oxygen (DO): 1.7 mg/l for 2-4 wk] caused significant increases in hepatic and neural IGFBP-1 mRNA expression compared with tissue mRNA levels in fish held under normoxic conditions (6.5 mg DO/l). Moreover, longer-term chronic hypoxia exposure (2-2.7 mg DO/l for 15-20 wk) caused significant increases in mRNA levels of all three IGFBPs in both liver and brain tissues. Hypoxia exposure also markedly increased superoxide radical (O(2)(·-), an index of ROS) production and HIF-1α mRNA and HIF-2α protein expression in croaker livers. Pharmacological treatment with an antioxidant attenuated the hypoxia-induced increases in O(2)(·-) production and HIFα mRNA and protein expression as well as the elevation of IGFBP-1 mRNA levels. These results suggest that the upregulation of IGFBP expression under hypoxia stress is due, in part, to alterations in the antioxidant status, which may involve ROS and HIFs.

Pregnancy-associated plasma protein-A2 (PAPP-A2): tissue expression and biological consequences of gene knockout in mice

Pregnancy-associated plasma protein-A2 (PAPP-A2) is a novel homolog of PAPP-A in the metzincin superfamily. However, compared with the accumulating data on PAPP-A, very little is known about PAPP-A2. In this study, we determined the tissue expression pattern of PAPP-A2 mRNA in wild-type (WT) mice and characterized the phenotype of mice with global PAPP-A2 deficiency. Tissues expressing PAPP-A2 in WT mice were more limited than those expressing PAPP-A. The highest PAPP-A2 mRNA expression was found in the placenta, with abundant expression in fetal, skeletal, and reproductive tissues. Heterozygous breeding produced the expected Mendelian distribution for the pappa2 gene and viable homozygous PAPP-A2 knockout (KO) mice that were normal size at birth. The most striking phenotype of the PAPP-A2 KO mouse was postnatal growth retardation. Male and female PAPP-A2 KO mice had 10 and 25-30% lower body weight, respectively, than WT littermates. Adult femur and body length were also reduced in PAPP-A2 KO mice, but without significant effects on bone mineral density. PAPP-A2 KO mice were fertile, but with compromised fecundity. PAPP-A expression was not altered to compensate for the loss of PAPP-A2 expression, and proteolysis of PAPP-A2's primary substrate, IGF-binding protein-5, was not altered in fibroblasts from PAPP-A2 KO embryos. In conclusion, tissue expression patterns and biological consequences of gene KO indicate distinct physiological roles for PAPP-A2 and PAPP-A in mice.

Differential gene expression profiles of invasive and non-invasive non-functioning pituitary adenomas based on microarray analysis

Non-functioning pituitary adenomas (NFPAs) may be locally invasive. Markers of invasiveness are needed to guide patient management and particularly the use of adjuvant radiotherapy. To examine whether invasive NFPAs display a specific gene expression profile relative to non-invasive tumors, we selected 40 NFPAs (38 of the gonadotroph type) and classified them as invasive (n=22) or non-invasive (n=18) on the basis of magnetic resonance imaging and surgical findings. We then performed pangenomic analysis with the 44k Agilent human whole genome expression oligonucleotide microarray in order to identify genes with differential expression between invasive and non-invasive NFPAs. Candidate genes were then tested in qRT-PCR. Prediction class analysis showed that the expression of 346 genes differed between invasive and non-invasive NFPAs (P<0.001), of which 233 genes were up-regulated and 113 genes were down-regulated in invasive tumors. On the basis of Ingenuity networks and the degree of up- or down-regulation in invasive versus non-invasive tumors, 35 genes were selected for expression quantification by qRT-PCR. Overexpression of only four genes was confirmed, namely IGFBP5 (P=0.02), MYO5A (P=0.04), FLT3 (P=0.01), and NFE2L1 (P=0.02). At the protein level, only myosin 5A (MYO5A) immunostaining was stronger in invasive than in non-invasive NFPAs. Molecular signature allows to differentiate 'grossly' invasive from non-invasive NFPAs. The product of one of these genes, MYO5A, may be a useful marker of tumor invasiveness.

C2C12 myoblasts release micro-vesicles containing mtDNA and proteins involved in signal transduction

Micro-vesicles can be released by different cell types and operate as 'safe containers' mediating inter-cellular communication. In this work we investigated whether cultured myoblasts could release exosomes. The reported data demonstrate, for the first time, that C2C12 myoblasts release micro-vesicles as shown by the presence of two exosome markers (Tsg101 and Alix proteins). Using real-time PCR analysis it was shown that these micro-vesicles, like other cell types, carry mtDNA. Proteomic characterization of the released micro-vesicle contents showed the presence of many proteins involved in signal transduction. The bioinformatics assessment of the Disorder Index and Aggregation Index of these proteins suggested that C2C12 micro-vesicles mainly deliver the machinery for signal transduction to target cells rather than key proteins involved in hub functions in molecular networks. The presence of IGFBP-5 in the purified micro-vesicles represents an exception, since this binding protein can play a key role in the modulation of the IGF-1 signalling pathway. In conclusion, the present findings demonstrate that skeletal muscle cells release micro-vesicles, which probably have an important role in the communication processes within skeletal muscles and between skeletal muscles and other organs. In particular, the present findings suggest possible new diagnostic approaches to skeletal muscle diseases.

Osteogenic properties of human dental pulp stem cells

Stem cells are a promising tool for bone tissue regeneration. Dental pulp stem cells (DPSCs) can be easily obtained even in human young adults. In this study we investigated the capability of DPSCs, to express the osteoblastic phenotype when cultured with osteogenic medium. DPSCs isolated from the dental pulp of impacted third molar teeth were cultured with appropriate medium to induce osteoblast differentiation. Using Western-Blot, RT-PCR and microarray analysis, we studied the expression of osteoblastic parameter, and by Von Kossa staining we evaluated the production of mineralized matrix nodules. The results were compared with controls represented by undifferentiated DPSCs. DPSCs, differentiated into osteoblast-like cells, express large amount of alkaline phosphatase (ALP), collagen I (Coll I), osteopontin (OPN) and osteocalcin (OCN), all these parameters characterizing the osteoblastic phenotype. Differentiated DPSCs express Runx2 and JunB, a member of the AP-1 complex; both the transcription factors are associated with osteoblast differentiation and skeletal morphogenesis. Moreover, DPSCs express insulin growth factor-binding protein 5 (IGFBP-5), one of the regulating proteins of IGFs function. Finally, DPSCs can form mineralized matrix nodules that are a feature exclusive to osteoblasts. DPSCs could represent a potential source of osteoblasts to be used for bone regeneration.

Genetic screen identifies insulin-like growth factor binding protein 5 as a modulator of tamoxifen resistance in breast cancer

Tamoxifen resistance is one of the overarching challenges in the treatment of patients with estrogen receptor (ER)-positive breast cancer. Through a genome-wide RNA interference screen to discover genes responsible for tamoxifen resistance in vitro, we identified insulin-like growth factor binding protein 5 (IGFBP5) as a determinant of drug sensitivity. Specific knockdown of IGFBP5 by retroviral infection with short hairpin RNA-expressing cassette in MCF7 human breast cancer cells (pRS-shIGFBP5) conferred tamoxifen resistance in vitro due to concomitant loss of ERalpha expression and signaling. IGFBP5 expression was also reduced in MCF7 cells selected for tamoxifen resistance in culture (TAMR). Both tamoxifen-resistant MCF7-TAMR and MCF7-pRS-shIGFBP5 cells could be resensitized to drug by treatment with exogenous recombinant IGFBP5 (rIGFBP5) protein. Treatment with rIGFBP5 protein in mouse tumor xenografts reversed the in vivo tamoxifen resistance of MCF7-pRS-shIGFBP5 cell-derived tumors by reducing tumor cell proliferation. IGFBP5 immunohistochemical staining in a cohort of 153 breast cancer patients showed that low IGFBP5 expression was associated with shorter overall survival after tamoxifen therapy. Thus, IGFBP5 warrants investigation as an agent to reverse tamoxifen resistance.

Involvement of pregnancy-associated plasma protein-A2 in insulin-like growth factor (IGF) binding protein-5 proteolysis during pregnancy: a potential mechanism for increasing IGF bioavailability

CONTEXT

During pregnancy, circulating IGF binding protein-5 (IGFBP-5) undergoes substantial molecular redistribution from ternary complexes to either binary complexes or the uncomplexed protein.

OBJECTIVE

This study aimed to characterize the proteolysis of circulating IGFBP-5 during pregnancy and to determine whether it can increase IGF bioavailability.

DESIGN

Biochemical methods were used to purify and characterize IGFBP-5 fragments and IGFBP-5-specific proteolytic activity from pregnancy plasma.

RESULTS

Circulating IGFBP-5 was fully proteolyzed at all stages of pregnancy. Cleavage after either Ser143 or Lys144 resulted in two complementary fragments. Of two pools of proteolytic activity (>150 kDa and approximately 40 kDa) identified in pregnancy plasma, only the greater than 150-kDa proteolytic activity was specific to pregnancy. The approximately 40-kDa proteolytic activity, also present in nonpregnancy plasma, appeared largely inactive against IGF-I-complexed IGFBP-5. The greater than 150-kDa proteolytic activity was inhibited by alpha-PAPP-A2 but not alpha-PAPP-A1 antibody, cleaved recombinant IGFBP-5 at Ser143-Lys144 similar to PAPP-A2, and was inactive against IGFBP-5 (Ala128), a PAPP-A2-resistant analog. Compared to nonpregnancy plasma, incubation with pregnancy plasma resulted in release of more bioactive IGF-I from IGF-I-IGFBP-5 complexes as measured by stimulation of IGF-I receptor phosphorylation.

CONCLUSIONS

Circulating IGFBP-5 is proteolyzed by PAPP-A2 during pregnancy, resulting in increased IGF bioavailability, which may have important consequences for the development of the fetus and/or the well-being of the mother.

IGF-I system, atrogenes and myogenic regulatory factors in arthritis induced muscle wasting

The aim of this work was to analyse the evolution of the ubiquitin-proteasome, the myogenic regulatory factors, and the IGF-I system during the development of experimental arthritis. Arthritis was induced by adjuvant injection and rats were killed 10, 15 and 22 days later. Gastrocnemius was progressively atrophied in arthritic rats. Arthritis induced a rapid increase in muscular IGFBP-3 and IGFBP-5 and, to a lesser extent, in IGF-I mRNA. An increased expression of the muscle-specific ubiquitin ligases atrogin-1/MAFbx and MuRF-1 was observed in the gastrocnemius from day 10, reaching its maximum value on day 15. Concomitantly, the proliferation marker PCNA and the early myogenic regulatory factor MyoD were also maximally increased on day 15. Myogenin, a late-acting myogenic regulatory factor, was maximally increased on days 15 and 22. These results suggest that muscle wasting in arthritis is secondary to an increase in muscle proteolysis, rather to a decrease in muscle regeneration.

IGFBP-5 induces epithelial and fibroblast responses consistent with the fibrotic response

Fibrosis involves activation of fibroblasts, increased production of collagen and fibronectin and transdifferentiation into contractile myofibroblasts. The process resembles aspects of wound-healing but remains unresolved and can be life-threatening when manifest in the kidneys, lungs and liver, in particular. The causes are largely unknown, but recent suggestions that repetitive micro-injury results in the eventual failure of epithelial cell repair due to replicative senescence are gaining favour. This is consistent with the onset of fibrotic diseases in middle age. Because epithelial injury often involves blood loss, inflammatory responses associated with the fibrotic response have been considered as therapeutic targets. However, this has proved largely unsuccessful and focus is now switching to earlier events in the process. These include EMT (epithelial-mesenchymal transition) and fibroblast activation in the absence of inflammation. TGFbeta1 (transforming growth factor-beta1) induces both EMT and fibroblast activation and is considered to be a major pro-fibrotic factor. Recently, IGFBP-5 [IGF (insulin-like growth factor)-binding protein-5] has also been shown to induce similar effects on TGFbeta1, and is strongly implicated in the process of senescence. It also stimulates migration of peripheral blood mononuclear cells, implicating it in the inflammatory response. In this paper, we examine the evidence for a role of IGFBP-5 in fibrosis and highlight its structural relationship with other matrix proteins and growth factors also implicated in tissue remodelling.

A mouse mammary gland involution mRNA signature identifies biological pathways potentially associated with breast cancer metastasis

Mouse mammary gland involution resembles a wound healing response with suppressed inflammation. Wound healing and inflammation are also associated with tumour development, and a 'wound-healing' gene expression signature can predict metastasis formation and survival. Recent studies have shown that an involuting mammary gland stroma can promote metastasis. It could therefore be hypothesised that gene expression signatures from an involuting mouse mammary gland may provide new insights into the physiological pathways that promote breast cancer progression. Indeed, using the HOPACH clustering method, the human orthologues of genes that were differentially regulated at day 3 of mammary gland involution and showed prolonged expression throughout the first 4 days of involution distinguished breast cancers in the NKI 295 breast cancer dataset with low and high metastatic activity. Most strikingly, genes associated with copper ion homeostasis and with HIF-1 promoter binding sites were the most over-represented, linking this signature to hypoxia. Further, six out of the ten mRNAs with strongest up-regulation in cancers with poor survival code for secreted factors, identifying potential candidates that may be involved in stromal/matrix-enhanced metastasis formation/breast cancer development. This method therefore identified biological processes that occur during mammary gland involution, which may be critical in promoting breast cancer metastasis that could form a basis for future investigation, and supports a role for copper in breast cancer development.

Molecular characterization, tissue distribution patterns and nutritional regulation of IGFBP-1, -2, -3 and -5 in yellowtail, Seriola quinqueradiata

Insulin-like growth factor-binding proteins (IGFBPs) play a vital role in regulating the biological activities of IGFs. In this study, we cloned and determined full-length cDNA sequences of yellowtail IGFBP-1, -2, -3 and -5. Their tissue distribution was determined by real-time quantitative RT-PCR, which revealed that IGFBP-1, -2, -3 and -5 are widely distributed in yellowtail tissues. In yellowtail, both IGFBP-1 and -2 are highly expressed in the liver, IGFBP-3 is predominantly expressed in the heart and skin, with the lowest expression in the liver, and IGFBP-5 is highly expressed in the liver and kidneys. The widespread tissue expression of the yellowtail IGFBPs suggests that they may act in an autocrine and/or paracrine manner in the regulation of IGF activity. The effects of nutritional deprivation on yellowtail IGFBPs and IGF-I were also examined. During a 15-day starvation period, significant elevation was observed in hepatic yellowtail IGFBP-1. Refeeding restored its level to that of the control. No significant change was observed in the hepatic yellowtail IGFBP-2 mRNA levels in starved fish compared with control fish during the starvation period. Interestingly, during the early period of food deprivation, a significant increase was observed in hepatic yellowtail IGFBP-3 and -5 mRNA levels, concomitant to the significant elevation in hepatic IGF-I mRNA from day 3 to day 9. The unexpected increase in growth stimulatory IGFBPs and IGF-I during nutritional deprivation may represent a species-specific response to changes in nutritional condition.

Differential regulation of RGS-2 by constant and oscillating PTH concentrations

PTH has diverse effects on bone metabolism: anabolic when given intermittently, catabolic when given continuously. The cellular mechanisms underlying the varying target cell response are not clear yet. PTH induces RGS-2, a member of the Regulator of G-protein Signaling protein family, via cAMP/PKA, and inactivates PKC-mediated signaling. To investigate intracellular signaling pathways with different PTH concentration-time patterns, we treated UMR 106-01 osteoblast-like cells in a perfusion system. PTH was administered intermittently (4 min/h, 10(-7) M) or continuously at an equivalent cumulative dose (6.6 x 10(-9) M). cAMP was measured using radioimmunoassay, mRNA levels using real-time rtPCR and ribonuclease protection assay, and protein levels using Western immunoblotting. A single PTH pulse transiently increased cAMP levels by 2000% +/- 1200%. In contrast to continuous PTH exposure, cAMP induction remained unchanged with intermittent PTH, ruling out desensitization of the PTH receptor. In continuously perfused cells, RGS-2 abundance was three to five times higher than in cells intermittently exposed to PTH for up to 12 h. MKP-1 and -3 were significantly less induced with pulsatile PTH; exposure-mode-dependent differences in MMP-13 and IGFBP-5 were small. Pulsatile but not continuous PTH administration prevents PTHrP receptor desensitization and accumulation of RGS-2 in osteoblasts, which should preserve PKC-dependent signaling.

Prostate-specific antigen enhances bioavailability of insulin-like growth factor by degrading insulin-like growth factor binding protein 5

In the bone matrix, insulin-like growth factors (IGFs) are the most abundant growth factors and IGF binding protein 5 (IGFBP-5) is the major IGFBP. Our previous study suggested that IGFs stored in the bone matrix and prostate-specific antigen (PSA) play an important role in prostate cancer (PC) bone metastasis. However, it is not clear how IGF signaling is activated in the bone microenvironment of PC metastasis. Therefore, we investigated whether PSA degrades IGFBP-5 and enhances biological activity of IGF. Enzymatically active PSA degraded the recombinant IGFBP-5 protein in a dose- and time-dependent manner and a serine protease inhibitor suppressed this degradation. Furthermore, PSA induced IGF-mediated type I IGF receptor phosphorylation that was inhibited by coincubation with IGFBP-5. The present study indicates PSA derived from PC cells can enhance IGF bioavailability in the bone microenvironment of PC metastasis, thereby permitting PC survival and malignant progression in the bone microenvironment.

Epithelial injury induces an innate repair mechanism linked to cellular senescence and fibrosis involving IGF-binding protein-5

Fibrosis is associated with epithelial repair. It involves the activation of fibroblasts, increased production of extracellular matrix proteins and transdifferentiation to contractile, myofibroblasts that aid in wound contraction. This provisional matrix plugs the injured epithelium and provides a scaffold for epithelial cell migration, involving an epithelial-mesenchymal transition (EMT). When epithelial injury involves blood loss, this leads to platelet activation, the production of several growth factors and an acute inflammatory response. Under normal circumstances, the epithelial barrier is repaired and the inflammatory response resolves. However, in fibrotic disease, the fibroblast response continues, resulting in unresolved wound healing. The fibrotic diseases range from scleroderma, where the problem may be restricted to the skin and where it is not life-threatening, through to systemic forms that can manifest as, for example, idiopathic pulmonary fibrosis, in which death is inevitable within 3-5 years. Anti-inflammatory treatments have failed to ameliorate the disease condition and focus has instead turned to transforming growth factor-beta1 (TGFB1), since it induces many of the processes involved, including fibroblast activation and EMT. Most recently, however, a new player in this process has been described, IGF-binding protein-5 (IGFBP5). IGFBP5 has also been shown to induce similar effects to TGFB1, but, in addition, it is strongly implicated in the process of senescence which is now believed to be a significant factor in these diseases. We examine the evidence for this role of IGFBP5 and identify some of the therapeutic targets which might be used to ameliorate these diseases of unknown cause.

Beneficial synergistic interactions of TNF-alpha and IL-6 in C2 skeletal myoblasts--potential cross-talk with IGF system

The interaction effects of tumour necrosis factor-alpha (TNF-alpha) and interlukin-6 (IL-6) on skeletal muscle proliferation and differentiation remains controversial. We therefore investigated the potential interactive effects of TNF-alpha and IL-6 on murine C2 skeletal myoblast survival, differentiation and proliferation. A novel and unexpected positive temporal interaction between TNF-alpha and IL-6 on cell growth was identified (90%), with maximal beneficial effects obtained in myoblasts treated with TNF-alpha (10 ng/ml) for 24 h prior to being dosed with IL-6 (2.5 ng/ml) for a further 24 h. This combined treatment significantly (p < 0.05) increased the level of total cellular protein (330%), extracellular signal-regulated kinase (ERK) phosphorylation (55%), and S-phase of cell cycle (2.5-fold), confirming cell growth. The expression of mRNAs of key regulators of muscle mass: insulin-like growth factor binding protein-5, insulin-like growth factor-II (IGF-II), IGF-I receptor (IGF-IR) and IGF-II receptor (IGF-IIR) were also significantly (p < 0.05) increased by 1600-, 1.6-, 27- and 6-fold, respectively, giving an indication of the regulatory mechanisms of this interaction. Moreover, in response to this treatment, the expression level of signal-transducing glycoprotein 130 (gp130) was induced up to 3.5-fold but not after either treatments alone. This may not only explain the beneficial effects of this treatment on skeletal myoblast numbers but also define a functional role of gp130 in skeletal muscle cells. Our data suggest that in the presence of TNF-alpha/IL-6 functions positively and potentially also cooperatively with the IGF system to achieve the maximal beneficial effect on skeletal myoblast numbers.

Cell proliferation and migration in glioblastoma multiforme cell lines are influenced by insulin-like growth factor I in vitro

BACKGROUND

Malignant gliomas continue to be a therapeutic challenge. One of the major problems is the early and rapidly infiltrating tumor growth. The role of the insulinlike growth factor (IGF) system in the progression of malignant glioma growth is poorly understood. In this study we investigated the expression of different members of the IGF system in malignant glioma cells and the influence of IGF-I and -II on the proliferation and migration of human glioma cell lines in vitro.

MATERIALS AND METHODS

Expression of IGF-I and -II, IGF-receptor I and II and IGF binding proteins (IGFBP) 1 to 6 was analysed by PCR in cell lines T98G, A172, 86HG39 (glioblastoma multiforme) and U87MG (anaplastic astrocytoma). To investigate effects on cell-proliferation, the cells were treated with IGF-I or -II (0.001-100 ng/ml). The cell viability was assessed by MTT-assay. For testing migratory effects, the Boyden-chamber-assay with different combinations of IGF-I or -II or fetal calf serum (FCS) as chemotactic agents was used.

RESULTS

All cell lines expressed IGF-I- and IGF-II-receptor, whereas none of the cells expressed IGF-I or -II. IGFBP 2-6 were found in all cell lines. IGF-I treated cell lines T98G and 86HG39 showed a weak dose-independent enhanced proliferation compared to controls, whereas A172 did not respond. None of the investigated cell lines changed proliferation when treated with IGF-II. All IGF-I (100 ng/ml) treated cells showed increased migration compared to controls. This effect was markedly enhanced by supplementation with 0.5% FCS. Again, IGF-II had no effect.

CONCLUSION

These data demonstrate that IGF-I modulates proliferation and strongly stimulates migration of glioma cell lines in vitro.

A functional nuclear localization signal in insulin-like growth factor binding protein-6 mediates its nuclear import

IGF binding protein (IGFBP)-6 is a member of the IGFBP family that regulates the actions of IGFs. Although IGFBPs exert their functions extracellularly in an autocrine/paracrine manner, several members of the family, such as IGFBP-3 and -5, possess nuclear localization signals (NLS). To date, no NLS has been described for IGFBP-6, an IGFBP that binds preferentially to IGF-II. We report here that both exogenous and endogenous IGFBP-6 could be imported into the nuclei of rhabdomyosarcoma and HEK-293 cells. Nuclear import of IGFBP-6 was mediated by a NLS sequence that bears limited homology to those found in IGFBP-3 and -5. IGFBP-6 nuclear translocation was an active process that required importins. A peptide corresponding to the IGFBP-6 NLS bound preferentially to importin-alpha. A comprehensive peptide array study revealed that, in addition to positively charged residues such as Arg and Lys, amino acids, notably Gly and Pro, within the NLS, played an important part in binding to importins. Overexpression of wild-type IGFBP-6 increased apoptosis, and the addition of IGF-II did not negate this effect. Only the deletion of the NLS segment abolished the apoptosis effect. Taken together, these results suggest that IGFBP-6 is translocated to the nucleus with functional consequences and that different members of the IGFBP family have specific nuclear import mechanisms.

Insulin-like growth factor-binding protein-5 enters vesicular structures but not the nucleus

In addition to its extracellular function as a secreted protein, IGF-binding protein (IGFBP)-5 has been postulated to act as a signaling molecule in the nucleus. This study aims to assess the significance of this postulated nuclear localization. By confocal immunofluorescence microscopy, we detected IGFBP-5 in the vesicular compartment of mammary epithelial cells in culture, while no nuclear staining was observed. Immunohistochemistry performed on paraffin sections of the involuting mammary gland revealed IGFBP-5 positive staining of epithelial cells only outside the nucleus. To evaluate the contribution of reuptake of extracellular IGFBP-5, T47D cells were incubated with Alexa Fluor 647-labeled IGFBP-5. The protein was taken up into intracellular vesicles and again was neither detectable in the cytoplasm outside of vesicular structures nor in the nucleus. Quantification of the time and concentration dependence of uptake by immunoblotting revealed that the process was saturable at IGFBP-5 concentrations between 1 and 2 mum and partially reversible with 30% remaining in the cell after a 1-h chase. The observation of nuclear uptake of IGFBP-5 was restricted to artificial conditions such as expression of non-secreted forms of IGFBP-5 or selective permeabilization of the plasma membrane by digitonin.

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.

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.

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.

The mitogen-activated protein kinase pathway tonically inhibits both basal and IGF-I-stimulated IGF-binding protein-5 production in mammary epithelial cells

The IGF system plays a key role in mammary gland growth and development. Our lab previously reported that IGF-I primarily regulates IGF-binding protein (BP)-3 in bovine mammary epithelial cells (MEC) and IGFBP-5 in mammary fibroblasts (MF). Presently, we examined the signaling pathways used by IGF-I to elicit this distinct, cell-type specific regulation. The phosphatidylinositol-3 kinase pathway was required for IGF-I to increase IGFBP-3 and -5 in MF and IGFBP-3 in MEC. Surprisingly, inhibiting the mitogen-activated protein kinase (MAPK) pathway in MEC increased IGFBP-5 mRNA levels 2- to 4-fold under basal conditions and 8- to 12-fold in cells treated with IGF-I within 4 h. Similar patterns of IGFBP-3 and -5 regulation were observed in murine MEC. Cells treated with IGF-I in the presence of MAPK inhibitors secreted more IGFBP-5 protein into conditioned media relative to cells treated with IGF-I alone; however, IGFBP-5 protein was not detected in conditioned media of cells treated with only a MAPK inhibitor. The IGFBP-5 mRNA response to MAPK inhibitors was specific for MEC, as blocking MAPK activity decreased the ability of IGF-I to induce IGFBP-5 in MF. In addition, no other IGFBP was increased in either cell type when MAPK activity was inhibited. These increases in IGFBP-5 expression in response to inhibition of the MAPK pathway corresponded with the induction of apoptosis. In conclusion, we report the novel observation that the MAPK/extracellular signal regulated kinase (ERK) pathway specifically represses IGFBP-5 expression in MEC. The corresponding changes in apoptosis and IGFBP-5 expression support a role for this specific IGFBP in mammary gland involution.