Signaling through the Smad pathway by insulin-like growth factor-binding protein-3 in breast cancer cells. Relationship to transforming growth factor-beta 1 signaling

The role of IGFBP-3 in tumor development and progression: enlightenment for diagnosis and treatment

IGFBP-3 is aberrantly expressed in many tumor types, and its serum and tumor tissue levels provide auxiliary information for assessing the degree of tumor malignancy and patient prognosis, making it a potential therapeutic target for human malignancies and conferring it remarkable clinical value for determining patient prognosis. In this review, we provide a comprehensive overview of the aberrant expression, diverse biological effects, and clinical implications of IGFBP-3 in tumors and its role as a potential prognostic marker and therapeutic target for tumors. In addition, we summarize the signaling pathways through which IGFBP-3 exerts its effects. IGFBP-3 comprises an N-terminal, an intermediate region, and a C-terminal structural domain, each exerting different biological effects in several tumor cell types in an IGF-dependent/non-independent manner. IGFBP-3 shares an intricate relationship with the tumor microenvironment, thereby affecting tumor growth. Overall, IGFBP-3 is an essential regulatory factor that mediates tumor occurrence and progression. Gaining deeper insights into the fundamental characteristics of IGFBP-3 and its role in various tumor types will provide new perspectives and allow for the development of novel strategies for cancer diagnosis, treatment, and prognostic evaluation.

Signaling Pathways of the Insulin-like Growth Factor Binding Proteins

The 6 high-affinity insulin-like growth factor binding proteins (IGFBPs) are multifunctional proteins that modulate cell signaling through multiple pathways. Their canonical function at the cellular level is to impede access of insulin-like growth factor (IGF)-1 and IGF-2 to their principal receptor IGF1R, but IGFBPs can also inhibit, or sometimes enhance, IGF1R signaling either through their own post-translational modifications, such as phosphorylation or limited proteolysis, or by their interactions with other regulatory proteins. Beyond the regulation of IGF1R activity, IGFBPs have been shown to modulate cell survival, migration, metabolism, and other functions through mechanisms that do not appear to involve the IGF-IGF1R system. This is achieved by interacting directly or functionally with integrins, transforming growth factor β family receptors, and other cell-surface proteins as well as intracellular ligands that are intermediates in a wide range of pathways. Within the nucleus, IGFBPs can regulate the diverse range of functions of class II nuclear hormone receptors and have roles in both cell senescence and DNA damage repair by the nonhomologous end-joining pathway, thus potentially modifying the efficacy of certain cancer therapeutics. They also modulate some immune functions and may have a role in autoimmune conditions such as rheumatoid arthritis. IGFBPs have been proposed as attractive therapeutic targets, but their ubiquity in the circulation and at the cellular level raises many challenges. By understanding the diversity of regulatory pathways with which IGFBPs interact, there may still be therapeutic opportunities based on modulation of IGFBP-dependent signaling.

Insulin-like Growth Factor Binding Protein 3 Increases Mouse Preimplantation Embryo Cleavage Rate by Activation of IGF1R and EGFR Independent of IGF1 Signalling

The viability of embryos cultured in vitro is poor compared to those that develop in vivo. The lack of maternally derived growth factors in vitro may contribute to this problem. Insulin-like growth factor binding protein 3 (IGFBP3) is one such growth factor that has been identified in the maternal reproductive system. This study examined the role of autocrine and exogenous IGFBP3 in mouse preimplantation embryos. Embryos expressed IGFBP3 across all stages of preimplantation development, and addition of exogenous IGFBP3 to embryo culture media increased the rate of development to the 2-, 4-, 5-, and 8-cell stages. Addition of inhibitors of the IGF1 and EGF receptors prevented this IGFBP3-mediated improvement in developmental rate, but the effect was not cumulative, indicating that both receptors are transactivated downstream of IGFBP3 as part of the same signalling pathway. Acute exposure to IGFBP3 increased phosphorylation of Akt and rps6 in 4-8 cell embryos, suggesting activation of the PI3-kinase/Akt pathway downstream of the IGF1 and EGFR receptors to promote cell proliferation and survival. In conclusion, addition of IGFBP3 to embryo culture media increases early cleavage rates independent of IGF1 signalling and therefore, IGFBP3 addition to IVF culture media should be considered.

Roles of IGFBP-3 in cell migration and growth in an endophytic tongue squamous cell carcinoma cell line

Insulin-like growth factor binding protein-3 (IGFBP-3) is a member of the IGFBP family that has high affinity for IGFs and functions as either an oncogene or tumor suppressor in various types of cancer. We previously found that IGFBP3 mRNA levels are higher in endophytic-type human tongue squamous cell carcinoma (TSCC) that is more invasive and more prone to metastasis than exophytic and superficial types. This finding prompted us to investigate the roles of IGFBP-3 in TSCC using SAS cells, which were originally derived from endophytic-type TSCC. Specifically, we used SAS cells that express a fluorescent ubiquitination-based cell-cycle indicator (Fucci). RNA-sequencing analysis indicated that IGFBP-3 is associated with cell migration and cell growth. In fact, IGFBP-3 knockdown downregulates cell migration and causes cells to arrest in G₁. This migratory potential appears to be cell cycle–independent. IGFBP-3 knockdown also reduced levels of secreted IGFBP-3; however, decreased migratory potential was not rescued by exogenous recombinant human IGFBP-3. Furthermore, ERK activity was downregulated by IGFBP-3 depletion, which suggests that MEK/ERK signaling may be involved in IGFBP-3-mediated cell migration. We therefore conclude that intracellular IGFBP-3 enhances cell migration independently of the cell cycle in TSCC with a higher metastatic potential.

Identification of Differential Expression Cytokines in Hemolysis, Elevated Liver Enzymes, and Low Platelet Syndrome by Proteome Microarray Analysis and Further Verification

To screen the differential expression cytokines (DECs) in hemolysis, elevated liver enzymes, and low platelet (HELLP) syndrome, establish its differential cytokines spectra, and provide the clues for its diagnosis and pathogenic mechanism researches. Sera from four HELLP syndrome patients and four healthy controls were detected by proteome microarray. Then the analysis of Gene Ontology (GO) enrichment, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, and protein–protein interaction (PPI) network were performed and possible hub proteins were selected out, further verified by Enzyme Linked Immunosorbent Assay (ELISA) in sera from 21 HELLP syndrome patients and 21 healthy controls. Thirty DECs were defined according to P-value and fold change between HELLP group and control group. GO enrichment analysis showed that DECs were mainly involved in the regulation of inflammatory response and have relationship to growth factor binding, transmembrane receptor protein kinase, and cytokine receptor activity. Seven possible hub proteins were defined by PPI analysis, including IGFBP-3/Follistatin-like 1/FLRG/Fetuin A and MMP-13/Thrombospondin-5/Aggrecan. ELISA showed higher serum levels of Fetuin A/IGFBP-3/FLGR/MMP-13/Thrombospondin-5 in HELLP group than those in controls, while the levels of Follistatin-like 1 and Aggrecan were lower in HELLP patients (all P < 0.05 or <0.01).The serological DECs spectra of HELLP syndrome was established and seven possible hub proteins that may be more closely related to the disease have been verified, providing new clues for its pathogenesis, diagnosis, and clinical treatment.

SMAD2 promotes myogenin expression and terminal myogenic differentiation

SMAD2 is a transcription factor, the activity of which is regulated by members of the transforming growth factor β (TGFβ) superfamily. Although activation of SMAD2 and SMAD3 downstream of TGFβ or myostatin signaling is known to inhibit myogenesis, we found that SMAD2 in the absence of TGFβ signaling promotes terminal myogenic differentiation. We found that, during myogenic differentiation, SMAD2 expression is induced. Knockout of SMAD2 expression in primary myoblasts did not affect the efficiency of myogenic differentiation but produced smaller myotubes with reduced expression of the terminal differentiation marker myogenin. Conversely, overexpression of SMAD2 stimulated myogenin expression, and enhanced both differentiation and fusion, and these effects were independent of classical activation by the TGFβ receptor complex. Loss of Smad2 in muscle satellite cells in vivo resulted in decreased muscle fiber caliber and impaired regeneration after acute injury. Taken together, we demonstrate that SMAD2 is an important positive regulator of myogenic differentiation, in part through the regulation of Myog.

Insulin Growth Factor Binding Protein 7 (IGFBP7)-Related Cancer and IGFBP3 and IGFBP7 Crosstalk

The insulin/insulin-like growth factors (IGFs) have crucial tasks in the growth, differentiation, and proliferation of healthy and pernicious cells. They are involved in coordinated complexes, including receptors, ligands, binding proteins, and proteases. However, the systems can become dysregulated in tumorigenesis. Insulin-like growth factor-binding protein 7 (IGFBP7) is a protein belonging to the IGFBP superfamily (also termed GFBP-related proteins). Numerous studies have provided evidence that IGFBP3 and IGFBP7 are involved in a variety of cancers, including hepatocellular carcinoma (HCC), breast cancer, gastroesophageal cancer, colon cancer, prostate cancer, among many others. Still, very few suggest an interaction between these two molecules. In studying several cancer types in our laboratories, we found that both proteins share some crucial signaling pathways. The objective of this review is to present a comprehensive overview of the relationship between IGFBP7 and cancer, as well as highlighting IGFBP3 crosstalk with IGFBP7 reported in recent studies.

Insulin-Like Growth Factor Binding Protein-3 (IGFBP-3): Unraveling the Role in Mediating IGF-Independent Effects Within the Cell

Insulin-like growth factor (IGF) binding protein-3 (IGFBP-3), one of the six members of the IGFBP family, is a key protein in the IGF pathway. IGFBP-3 can function in an IGF-dependent as well as in an IGF-independent manner. The IGF-dependent roles of IGFBP-3 include its endocrine role in the delivery of IGFs from the site of synthesis to the target cells that possess IGF receptors and the activation of associated downstream signaling. IGF-independent role of IGFBP-3 include its interactions with the proteins of the extracellular matrix and the proteins of the plasma membrane, its translocation through the plasma membrane into the cytoplasm and into the nucleus. The C-terminal domain of IGFBP-3 has the ability to undergo cell penetration therefore, generating a short 8-22-mer C-terminal domain peptides that can be conjugated to drugs or genes for effective intracellular delivery. This has opened doors for biotechnological applications of the molecule in molecular medicine. The aim of this this review is to summarize the complex roles of IGFBP-3 within the cell, including its mechanisms of cellular uptake and its translocation into the nucleus, various molecules with which it is capable of interacting, and its ability to regulate IGF-independent cell growth, survival and apoptosis. This would pave way into understanding the modus operandi of IGFBP-3 in regulating IGF-independent processes and its pleiotropic ability to bind with potential partners thus regulating several cellular functions implicated in metabolic diseases, including cancer.

Insulin-like growth factor binding Protein-3 suppresses osteoblast differentiation via bone morphogenetic protein-2

Bone augmentation therapy is used in dental implantation. While techniques to induce bone formation are generally successful, the maintenance of bone mass is more difficult. Therefore, it is important to understand the mechanisms that regulate this process. Insulin-like growth factor-1 (IGF-1) is one of the most abundant growth factors that regulate bone mass, promote osteoblast differentiation, and accelerate bone formation. The activity of IGF-1 is regulated by IGF-binding proteins (IGFBPs). IGFBP-3 forms a ternary complex with IGF-1, extending its half-life in the circulating system. Therefore, IGFBP-3 acts as a stabilizer and transporter of IGF-1. Recent studies reported new IGF-1-independent functions of IGFBP-3 related with bone metabolism. In this study, we investigated the function of IGFBP-3 in osteoblast differentiation. Our results showed that IGFBP-3 decreases the expression of osteoblast differentiation markers, whose expression is enhanced by bone morphogenetic protein-2 (BMP-2). IGFBP-3 also reduced BMP-2 effect on ALP activity and mineral nodule formation. In addition, IGFBP-3 suppresses the activity of the Smad Binding Element (SBE) reporter, induced by BMP-2 signaling. These results suggest that IGFBP-3 inhibits osteoblast differentiation through the BMP-2 signal pathway, and that IGFBP-3 might play a role in bone mass maintenance in an IGF-1-dependent and -independent manner.

Contrasting effects of IGF binding protein-3 expression in mammary tumor cells and the tumor microenvironment

IGFBP-3 has both stimulatory and inhibitory effects on cancer progression. The growth of EO771 mammary carcinoma cells as syngeneic tumors in C57BL/6 mice is reduced in Igfbp3-null (BP3KO) mice, suggesting that systemic IGFBP-3 enhances tumor progression. In this study we assessed the growth of EO771 cells expressing human IGFBP-3 in BP3KO mice. Cells expressing hIGFBP-3 showed decreased proliferation in vitro and increased levels of IGF-1 receptor (IGF1R) protein but not mRNA, consistent with sequestration of endogenous IGF by IGFBP-3. The growth rate of these cells was restored by exposure to IGF-1 or analogues with reduced affinity for IGFBP-3 (long Arg³-IGF-1) or IGF1R (Leu²⁴-IGF-1). In EO771 cells implanted orthotopically into mice, hIGFBP-3 expression by the cells inhibited tumor establishment in BP3KO but not wild-type mice. For tumors that successfully established, final weight was not affected significantly by hIGFBP-3 expression. However, final tumor weight was inversely related to intratumoral T cell counts, and sera from BP3KO mice with tumors showed low-titer immunoreactivity against IGFBP-3. The contrasting effects on tumor establishment and progression of IGFBP-3 expressed by mammary carcinoma cells, compared to systemic stromal and circulating IGFBP-3, highlights the complexity of growth regulation by IGFBP-3 in mammary tumors.

Role of IGF-binding proteins in regulating IGF responses to changes in metabolism

The IGF-binding protein family contains six members that share significant structural homology. Their principal function is to regulate the actions of IGF1 and IGF2. These proteins are present in plasma and extracellular fluids and regulate access of both IGF1 and II to the type I IGF receptor. Additionally, they have functions that are independent of their ability to bind IGFs. Each protein is regulated independently of IGF1 and IGF2, and this provides an important mechanism by which other hormones and physiologic variables can regulate IGF actions indirectly. Several members of the family are sensitive to changes in intermediary metabolism. Specifically the presence of obesity/insulin resistance can significantly alter the expression of these proteins. Similarly changes in nutrition or catabolism can alter their synthesis and degradation. Multiple hormones such as glucocorticoids, androgens, estrogen and insulin regulate IGFBP synthesis and bioavailability. In addition to their ability to regulate IGF access to receptors these proteins can bind to distinct cell surface proteins or proteins in extracellular matrix and several cellular functions are influenced by these interactions. IGFBPs can be transported intracellularly and interact with nuclear proteins to alter cellular physiology. In pathophysiologic states, there is significant dysregulation between the changes in IGFBP synthesis and bioavailability and changes in IGF1 and IGF2. These discordant changes can lead to marked alterations in IGF action. Although binding protein physiology and pathophysiology are complex, experimental results have provided an important avenue for understanding how IGF actions are regulated in a variety of physiologic and pathophysiologic conditions.

Enhancement of mammary tumour growth by IGFBP-3 involves impaired T cell accumulation

Epidemiological studies show an association between obesity and poor breast cancer prognosis. We previously demonstrated that global IGFBP-3 deficiency, in IGFBP-3-null mice, resulted in a 50% reduction in mammary tumour growth over 3 weeks relative to tumours in wild-type (WT) C57BL/6 mice. This growth reduction was ameliorated by high fat feeding-induced obesity. This study aimed to examine how IGFBP-3 promotes tumour growth by influencing the immune tumour microenvironment in healthy and obese mice. Syngeneic EO771 cells, which lack detectable IGFBP-3 expression, were grown as orthotopic tumours in WT and IGFBP-3-null C57BL/6 mice placed on either a control chow or a high-fat diet (HFD), and examined by quantitative PCR and immunohistochemistry. In WT mice, increased stromal expression of IGFBP-3 was positively associated with tumour growth, supporting the hypothesis that IGFBP-3 in the microenvironment promotes tumour progression. Examining markers of immune cell subsets, gene expression of Ifng, Cd8a, Cd8b1 and Tnf and CD8 measured by immunohistochemistry were elevated in tumours of IGFBP-3-null mice compared to WT, indicating an accumulation of CD8+ T cells, but this increase was absent if the IGFBP-3-null mice had been exposed to HFD. Expression of these genes was negatively associated with tumour growth. Although similar among groups overall, Nkg2d and Tnfsf10 tumoural expression was associated with decreased tumour growth. Overall, the results of this study provide an immune-based mechanism by which host IGFBP-3 may promote breast tumour growth in the EO771 murine breast cancer model, and suggest that targeting IGFBP-3 might make a novel contribution to immune therapy for breast cancer.

What Happened to the IGF Binding Proteins?

Insulinlike growth factor (IGF) binding proteins (IGFBPs) 1 to 6 are high-affinity regulators of IGF activity. They generally inhibit IGF actions by preventing binding to the IGF-I receptor but can also enhance their actions under some conditions. Posttranslational modifications such as glycosylation and phosphorylation modulate IGFBP properties, and IGFBP proteolysis results in IGF release. IGFBPs have more recently been shown to have IGF-independent actions. A number of mechanisms are involved, including modulation of other growth factor pathways, nuclear localization and transcriptional regulation, interaction with the sphingolipid pathway, and binding to non-IGF biomolecules in the extracellular space and matrix, on the cell surface and intracellularly. IGFBPs modulate important biological processes, including cell proliferation, survival, migration, senescence, autophagy, and angiogenesis. Their actions have been implicated in growth, metabolism, cancer, stem cell maintenance and differentiation, and immune regulation. Recent studies have shown that epigenetic mechanisms are involved in the regulation of IGFBP abundance. A more complete understanding of IGFBP biology is necessary to further define their cellular roles and determine their therapeutic potential.

Acquired Resistance Mechanisms to Combination Met-TKI/EGFR-TKI Exposure in Met-Amplified EGFR-TKI-Resistant Lung Adenocarcinoma Harboring an Activating EGFR Mutation

Met-amplified EGFR-tyrosine kinase inhibitor (TKI)-resistant non-small cell lung cancer (NSCLC) harboring an activating EGFR mutation is responsive to concurrent EGFR-TKI and Met-TKI treatment in a preclinical model. Here, we determined that Met-amplified gefitinib-resistant cells acquire dual resistance to inhibition of EGFR and Met tyrosine kinase activities. PC-9 lung adenocarcinoma cells harboring 15-bp deletions (Del E746_A750) in EGFR exon 19 were treated with increasing concentrations of the Met-TKI PHA665752 and 1 μmol/L gefitinib for 1 year; three resistant clones were established via Met amplification. The three dual-resistance cell lines (PC-9DR2, PC-9DR4, and PC-9DR6, designated as DR2, DR4, and DR6, respectively) exhibited different mechanisms for evading both EGFR and Met inhibition. None of the clones harbored a secondary mutation of EGFR T790M or a Met mutation. Insulin-like growth factor (IGF)/IGF1 receptor activation in DR2 and DR4 cells acted as a bypass signaling pathway. Met expression was attenuated to a greater extent in DR2 than in PC-9 cells, but was maintained in DR4 cells by overexpression of IGF-binding protein 3. In DR6 cells, Met was further amplified by association with HSP90, which protected Met from degradation and induced SET and MYND domain-containing 3 (SMYD3)-mediated Met transcription. This is the first report describing the acquisition of dual resistance mechanisms in NSCLC harboring an activating EGFR mutation to Met-TKI and EGFR-TKI following previous EGFR-TKI treatment. These results might inform the development of more effective therapeutic strategies for NSCLC treatment. Mol Cancer Ther; 15(12); 3040-54. ©2016 AACR.

Role of IGF Binding Proteins in Regulating Metabolism

Insulin-like growth factors (IGFs) circulate in extracellular fluids bound to a family of binding proteins. Although they function in a classical manner to limit the access of the IGFs to their receptors they also have a multiplicity of actions that are independent of this property; they bind to their own receptors or are transported to intracellular and intranuclear sites to influence cellular functions that may directly or indirectly modify IGF actions. The availability of genetically modified animals has helped to determine their functions in a physiological context. These results show that many of their actions are cell type- and context-specific, and have led to a broader understanding of how these proteins function coordinately with IGF-I and -II to regulate growth and metabolism.

Understanding the Key to Targeting the IGF Axis in Cancer: A Biomarker Assessment

Type 1 insulin like growth factor receptor (IGF-1R) targeted therapies showed compelling pre-clinical evidence; however, to date, this has failed to translate into patient benefit in Phase 2/3 trials in unselected patients. This was further complicated by the toxicity, including hyperglycemia, which largely results from the overlap between IGF and insulin signaling systems and associated feedback mechanisms. This has halted the clinical development of inhibitors targeting IGF signaling, which has limited the availability of biopsy samples for correlative studies to understand biomarkers of response. Indeed, a major factor contributing to lack of clinical benefit of IGF targeting agents has been difficulty in identifying patients with tumors driven by IGF signaling due to the lack of predictive biomarkers. In this review, we will describe the IGF system, rationale for targeting IGF signaling, the potential liabilities of targeting strategies, and potential biomarkers that may improve success.

Targeting Insulin-Like Growth Factor Binding Protein-3 Signaling in Triple-Negative Breast Cancer

Insulin-like growth factor binding protein-3 (IGFBP-3) is a key regulatory molecule of the IGF axis and can function in a tissue-specific way as both a tumor suppressor and promoter. Triple-negative breast cancer (TNBC) has high tumor expression of IGFBP-3 associated with markers of poor prognosis and, although accounting for 15-20% of all breast cancers, is responsible for disproportionate rates of morbidity and mortality. Because they lack estrogen and progesterone receptors and overexpression of HER2, TNBC are resistant to treatments that target these molecules, making the development of new therapies an important goal. In addition to frequent high expression of IGFBP-3, these tumors also express EGFR highly, but targeting EGFR signaling alone in TNBC has been of little success. Identification of a functional growth-stimulatory interaction between EGFR and IGFBP-3 signaling prompted investigation into cotargeting these pathways as a novel therapy for TNBC. This involves inhibition of both EGFR kinase activity and a mediator of IGFBP-3's stimulatory bioactivity, sphingosine kinase-1 (SphK1), and has shown promise in a preclinical setting. Functional interaction between EGFR and IGFBP-3 may also promote chemoresistance in TNBC, and delineating the mechanisms involved may identify additional targets for development of therapies in cancers that express both IGFBP-3 and EGFR.

Anti-androgen flutamide suppresses hepatocellular carcinoma cell proliferation via the aryl hydrocarbon receptor mediated induction of transforming growth factor-β1

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and a member of the basic helix-loop-helix PER/ARNT/SIM family of chemosensors and developmental regulators. The AhR is widely known as a mediator of dioxin toxicity; however, it also suppresses cancer cell proliferation and recent findings have implicated its role as a tumor suppressor. We conducted a chemical library screen to identify nontoxic AhR ligands with anti-cancer effects and discovered flutamide (Eulexin) as a putative AhR ligand. Flutamide is an androgen receptor (AR) antagonist approved by the United States Food and Drug Administration for the treatment of prostate cancer. We found that flutamide inhibited the growth of several cancer cell lines independent of AR status, and that suppression of AhR expression reversed the anti-proliferative effects of flutamide. We investigated the AhR-dependent mechanism of action of flutamide in human hepatocellular carcinoma cells and identified that transforming growth factor-β1 (TGF-β1) is induced by flutamide in an AhR-dependent manner. In contrast, the potent AhR agonist 2,3,7,8-Tetrachlorodibenzo-p-dioxin had no effect on TGF-β1 expression, indicating the ligand specificity of AhR activation. We also determined that TGF-β1 induction is required for the AhR-dependent growth inhibitory effects of flutamide. Therefore, flutamide may be effective in AhR-positive cancers that are sensitive to TGF-β1 signaling, such as hepatocellular carcinoma.

Glucose intolerance in aging male IGFBP-3 transgenic mice: differential effects of human IGFBP-3 and its mutant IGFBP-3 devoid of IGF binding ability

We have reported a reduction of insulin secretion and glucose intolerance in young mice overexpressing human IGFBP-3 (phosphoglycerate kinase [PGK]BP3) or its mutant Gly56/Gly80/Gly81-IGFBP-3 (PGKmutBP3) under the PGK promoter. Here, we investigated changes in glucose and lipid homeostasis with age in PGKBP3 and PGKmutBP3 mice compared with wild-type mice. Body weight, glucose tolerance, insulin tolerance, visceral fat, interscapular brown adipose tissue (BAT), serum lipids, and pancreas histology were examined at age 3, 6, and 12 months. Murine IGFBP-3 was similar in all mouse genotypes and decreased with age in parallel with total IGF-1. Visceral fat and BAT masses increased in PGKmutBP3 mice, but not in PGKBP3 mice. Glucose tolerance was impaired in both PGKBP3 and PGKmutBP3 mice. However, PGKBP3 mice had increased expression of uncoupling protein-1 in BAT and reduced adiposity, and continued to have smaller pancreatic β-cell mass and reduced insulin secretion through age 12 months. In contrast, PGKmutBP3 mice developed insulin resistance with age in association with pancreatic β-cell hyperplasia, impaired expression of uncoupling protein-1 in BAT, and increased adiposity. In addition, both PGKBP3 and PGKmutBP3 mice had elevated glycerol in the circulation, but only PGKBP3 mice had elevated free fatty acids and only PGKmutBP3 mice had elevated triglycerides. Estimated free IGF-1 did not increase with age in transgenic mice, as it did in wild-type mice. Thus, overexpression of human IGFBP-3 or its mutant devoid of IGF binding ability leads to glucose intolerance with, however, different effects on insulin secretion, insulin sensitivity, and lipid homeostasis in aging mice.

IGFBP-3: a cell fate pivot in cancer and disease

One of the hallmarks in the advancement of cancer cells is an ability to overcome and acquire resistance to adverse conditions. There has been a large amount of cancer research on IGFBP-3 as a pro-apoptotic molecule in vitro. These pro-apoptotic properties, however, do not correlate with several studies linking high IGFBP-3 levels in breast cancer tissue to rapid growth and poor prognosis. Evidence is emerging that IGFBP-3 also exhibits pro-survival and growth-promoting properties in vitro. How IGFBP-3 pivots cell fate to either death or survival, it seems, comes down to a complex interplay between cells' microenvironments and the presence of cellular IGFBP-3 binding partners and growth factor receptors. The cytoprotective actions of IGFBP-3 are not restricted to cancer but are also observed in other disease states, such as retinopathy and brain ischaemia. Here we review the literature on this paradoxical nature of IGFBP-3, its pro-apoptotic and growth-inhibitory actions versus its cytoprotective and growth-potentiating properties, and discuss the implications of targeting IGFBP-3 for treatment of disease.

IGF binding proteins in cancer: mechanistic and clinical insights

The six members of the family of insulin-like growth factor (IGF) binding proteins (IGFBPs) were originally characterized as passive reservoirs of circulating IGFs, but they are now understood to have many actions beyond their endocrine role in IGF transport. IGFBPs also function in the pericellular and intracellular compartments to regulate cell growth and survival - they interact with many proteins, in addition to their canonical ligands IGF-I and IGF-II. Intranuclear roles of IGFBPs in transcriptional regulation, induction of apoptosis and DNA damage repair point to their intimate involvement in tumour development, progression and resistance to treatment. Tissue or circulating IGFBPs might also be useful as prognostic biomarkers.

Activation of various downstream signaling molecules by IGFBP-3

Insulin-like growth factor binding protein-3 (IGFBP-3), a secretory protein, is the most abundant IGF binding protein present in human serum among all IGF binding proteins. IGFBP-3 shows decreased level of expression in cancerous cells but has been known to be present in significant amounts in normal or non-cancerous cells. IGFBP-3 can induce apoptosis in prostate cancer cells either in an IGF-dependent manner or independently of IGF binding. Although putative cell death specific Insulin-like growth factor binding protein-3 (IGFBP-3R) receptor(s) has recently been identified by which IGFBP-3 may induce its anti-tumor effects, IGFBP-3 has also been known to activate various downstream intracellular signaling molecules via a different mechanistic pathway. Stat-1 has been known to be one of the candidate molecules activated by IGFBP-3. IGFBP-3 can also inhibit Akt/IGF-1 survival pathway in MCF- 7 breast cancer cells which ultimately leads to the induction of apoptosis in these cells. All these studies clearly demonstrate that IGFBP-3 regulates cell proliferation and promotes its pro-apoptotic effects in cancer cells in two different pathways,1) sequester IGF-I to bind to IGF-I receptor to inhibit cell proliferation and induce apoptosis, 2) independent of IGF-I pathway, IGFBP-3 binds to some putative receptor and activate various downstream pro-apoptotic molecules involved in cell death.

Gene expression profiling of early hepatic stellate cell activation reveals a role for Igfbp3 in cell migration

Background

Scarring of the liver is the result of prolonged exposure to exogenous or endogenous stimuli. At the onset of fibrosis, quiescent hepatic stellate cells (HSCs) activate and transdifferentiate into matrix producing, myofibroblast-like cells.

Aim and methods

To identify key players during early HSC activation, gene expression profiling was performed on primary mouse HSCs cultured for 4, 16 and 64 hours. Since valproic acid (VPA) can partly inhibit HSC activation, we included VPA-treated cells in the profiling experiments to facilitate this search.

Results

Gene expression profiling confirmed early changes for known genes related to HSC activation such as alphasmoothmuscleactin (Acta2), lysyloxidase (Lox) and collagen, type I, alpha 1 (Col1a1). In addition we noticed that, although genes which are related to fibrosis change between 4 and 16 hours in culture, most gene expression changes occur between 16 and 64 hours. Insulin-likegrowthfactorbinding protein 3 (Igfbp3) was identified as a gene strongly affected by VPA treatment. During normal HSC activation Igfbp3 is up regulated and this can thus be prevented by VPA treatment invitro and invivo. siRNA-mediated silencing of Igfbp3 in primary mouse HSCs induced matrix metalloproteinase (Mmp) 9 mRNA expression and strongly reduced cell migration. The reduced cell migration after Igfbp3 knock-down could be overcome by tissue inhibitor of metalloproteinase (TIMP) 1 treatment.

Conclusion

Igfbp3 is a marker for culture-activated HSCs and plays a role in HSC migration. VPA treatment prevents Igfbp3 transcription during activation of HSCs invitro and invivo.

TGF-β-induced expression of IGFBP-3 regulates IGF1R signaling in human osteosarcoma cells

Signaling pathways initiated by transforming growth factor-β (TGF-β) and insulin-like growth factors (IGFs) are important in osteosarcoma cell growth. We have investigated a role for endogenous IGF binding protein-3 (IGFBP-3) in mediating cross-talk between TGF-β receptor and type I IGF receptor (IGF1R) signaling pathways in MG-63 osteosarcoma cells. TGF-β1 indirectly activated the Ras/Raf/MAPK pathway and induced the expression of IGFBP-3, an important regulator of IGF1R activity. IGFBP-3 attenuated TGF-β1 activation of ERK1/2 and Akt in MG-63 cells, and inhibited TGF-β1-induced cell cycle progression and proliferation. This effect of IGFBP-3 was blocked by inhibiting IGF1R signaling. TGF-β1 phosphorylated Smad2 on the non-receptor substrate sites (Ser245/250/255). Blocking the TGF-β1-induced expression of IGFBP-3 enhanced pSmad2(Ser245/250/255) and increased its nuclear accumulation. These results suggest an important role for TGF-β1 in osteosarcoma cell growth, with the induction of IGFBP-3 by TGF-β1 serving in a negative-feedback loop to control cell growth by preventing activation of the IGF1R.

Stromal insulin-like growth factor binding protein 3 (IGFBP3) is elevated in the diseased human prostate and promotes ex vivo fibroblast-to-myofibroblast differentiation

Dysregulation of the IGF axis is implicated in the development of benign prostatic hyperplasia (BPH) and prostate cancer (PCa), 2 of the most common diseases affecting elderly males. PCa is the second leading cause of male-related cancer death in Western societies. Although distinct pathologies, BPH and PCa are both characterized by extensive stromal remodeling, in particular fibroblast-to-myofibroblast differentiation, thought to be induced by elevated local production of TGFβ1. We previously showed that TGFβ1-mediated fibroblast-to-myofibroblast differentiation of primary human prostatic stromal cells resulted in the dsyregulation of several components of the IGF axis, including the induction of IGF binding protein 3 (IGFBP3). Using isoform-specific lentiviral-mediated knockdown, we demonstrate herein that IGFBP3 is essential for TGFβ1-mediated differentiation. Although recombinant human IGFBP3 alone was not sufficient to induce differentiation, IGFBP3 synergistically potentiated TGFβ1-mediated stromal remodeling predominantly via an IGF-independent mechanism. Consistent with these in vitro findings, IGFBP3 immunohistochemistry revealed elevated levels of IGFBP3 in the hyperplastic fibromuscular stroma of BPH specimens and in the tumor-adjacent stroma of high-grade PCa. Collectively these data indicate that the dysregulation of the stromal IGF axis, in particular elevated IGFBP3, plays a crucial role in fibroblast-to-myofibroblast differentiation in the diseased prostatic stroma and indicate the therapeutic potential of inhibiting stromal remodeling and the resulting dysregulation of the stromal IGF axis as a novel strategy for the treatment of advanced PCa and BPH.

Insulin-like growth factor binding protein-3 (IGFBP-3): Novel ligands mediate unexpected functions

In addition to its important role in the regulation of somatic growth by acting as the major circulating transport protein for the insulin-like growth factors (IGFs), IGF binding protein-3 (IGFBP-3) has a variety of intracellular ligands that point to its function within major signaling pathways. The discovery of its interaction with the retinoid X receptor has led to the elucidation of roles in regulating the function of several nuclear hormone receptors including retinoic acid receptor-α, Nur77 and vitamin D receptor. Its interaction with the nuclear hormone receptor peroxisome proliferator-activated receptor-γ is believed to be involved in regulating adipocyte differentiation, which is also modulated by IGFBP-3 through an interaction with TGFβ/Smad signaling. IGFBP-3 can induce apoptosis alone or in conjunction with other agents, and in different systems can activate caspases -8 and -9. At least two unrelated proteins (LRP1 and TMEM219) have been designated as receptors for IGFBP-3, the latter with a demonstrated role in inducing caspase-8-dependent apoptosis. In contrast, IGFBP-3 also has demonstrated roles in survival-related functions, including the repair of DNA double-strand breaks through interaction with the epidermal growth factor receptor and DNA-dependent protein kinase, and the induction of autophagy through interaction with GRP78. The ability of IGFBP-3 to modulate the balance between pro-apoptotic and pro-survival sphingolipids by regulating sphingosine kinase 1 and sphingomyelinases may be integral to its role at the crossroads between cell death and survival in response to a variety of stimuli. The pleiotropic nature of IGFBP-3 activity supports the idea that IGFBP-3 itself, or pathways with which it interacts, should be investigated as targets of therapy for a variety of diseases.

Signal Transduction Pathways Mediated by Secreted and Non-secreted Forms of intact Insulin-like Growth Factor Binding Protein-3 (IGFBP-3) and its 1-97 N-terminal Fragment in PC-3 Human Prostate Cancer Cells

Our previous results indicated that both the secreted and the intracellular form of full length and 1-97 N-terminal fragment of IGFBP-3 induces apoptosis in PC-3 human prostate cancer cells in an IGF-dependent and independent manner. This study was undertaken to delineate possible down-stream signaling pathways that are involved in this process. Intact IGFBP-3 and its N-terminal 1-97 fragments with or without a signal pro-peptide was fused to YFP and expressed in PC-3 human prostate cancer cells. In some cases, the putative IGF-binding site present in full length IGFBP-3 and its N-terminal fragment was also mutated. Extent of apoptosis was quantified using FACS. Up-regulation of total Stat-1 and activation of phospho-Stat-1 was shown by western blot. TGF-β signal was measured by luciferase reporter assay. Results from inhibitor studies indicated that both the Caspase 8 and caspase 9 pathways are involved in IGFBP-3 (non-secreted form) induced apoptosis in PC-3 cells. Exogenous addition of IGFBP-3 to PC-3 cells increased Stat-1 protein expression/tyrosine phosphorylation. Interestingly, results also showed that knockdown of Stat-1 by siRNA potentiated the IGFBP-3 induced apoptosis in PC-3 cells. In addition, both full-length IGFBP-3 and its 1-97 N-terminal fragments inhibited TGFβ signaling in these cells. This is the first report that compares the signal transduction pathways involved in apoptotic pathways mediated by IGFBP-3 in PC-3 human prostate cancer cells. Non-secreted form of full length IGFBP-3 and its N-terminal fragments induced apoptosis in PC-3 cells via activation of caspase 8 and caspase 9. We noted that both secreted and non-secreted forms of IGFBP-3 are involved in modulating Stat-1 and TGF-β pathways to induce apoptotic actions in PC-3 cells. Surprisingly, only non-secreted form of IGFBP-3 and its N-terminal fragments are involved in the induction of apoptosis in PC-3 cells via caspase 8 and caspase 9 activation. These studies clearly demonstrate that secreted and non-secreted FL and its 1-97 N-terminal fragments induce apoptosis in PC-3 cells by regulating different mechanistic pathways.

Interaction between IGF binding protein-3 and TGFβ in the regulation of adipocyte differentiation

The development of white adipose tissue involves both the hypertrophy of existing adipocytes and the proliferation and differentiation of preadipocytes. Adipogenic differentiation is inhibited by TGFβ signaling through Smad2/3, and IGF binding protein-3 (IGFBP-3) is also known to activate Smad2/3 signaling in some cell types. We previously reported that exogenous or overexpressed IGFBP-3 inhibits adipogenesis in 3T3-L1 cells, but the role of endogenous IGFBP-3 in this process, and its possible interaction with TGFβ, is not known. During 10-d adipogenic differentiation initiated by insulin, dexamethasone, and 3-isobutyl-1-methylxanthine, 3T3-L1 cells expressed increasing levels of IGFBP-3 and TGFβ1, secreting over 1000 pg/ml of both proteins. Exogenous recombinant human IGFBP-3 paralleled TGFβ1 in stimulating Smad2 phosphorylation in 3T3-L1 preadipocytes, but no additive effect was observed for the two agents. In contrast, knockdown of endogenous IGFBP-3 by small interfering RNA (siRNA) significantly impaired Smad2 activation by 0.25 ng/ml TGFβ1. Transient expression of human IGFBP-3 significantly inhibited the induction of adipogenic markers adiponectin and resistin, and the appearance of lipid droplets, but down-regulation of endogenous IGFBP-3 by siRNA had little effect on the expression of either marker during the 10-d differentiation, compared with nonsilencing control siRNA. However, down-regulation of endogenous IGFBP-3 using two different siRNA significantly reversed the inhibitory effect of TGFβ1 on both adiponectin and resistin induction. We conclude that IGFBP-3 activates inhibitory Smad signaling in 3T3-L1 cells and that endogenous IGFBP-3 modulates their adipogenic differentiation by regulating cell sensitivity towards the inhibitory effect of TGFβ.

Signalling pathways of insulin-like growth factors (IGFs) and IGF binding protein-3

Although the insulin-like growth factor (IGF) system is essential for normal growth and development, its dysregulation has been implicated in a range of pathological states. The peptide growth factors IGF-I and IGF-II exert their effects by binding to cell-surface heterotetrameric tyrosine kinase receptors and activating multiple intracellular signalling cascades, leading to changes in the expression of proteins essential for cell proliferation, survival and differentiation. The IGF system comprises multiple ligands, receptors and high-affinity IGF binding proteins (IGFBPs), with added complexity arising from crosstalk between its receptors and other key growth-regulatory pathways such as those activated by steroid hormones, integrins and other receptor tyrosine kinases. The IGFBPs are also increasingly recognised for their intrinsic growth-regulatory activity, and the ability of IGFBP-3 to modulate signalling pathways of nuclear hormone and growth factor receptors, as well as novel receptors, is believed to play a role both in normal physiology and in disease.

Influence of the hepatic eukaryotic initiation factor 2alpha (eIF2alpha) endoplasmic reticulum (ER) stress response pathway on insulin-mediated ER stress and hepatic and peripheral glucose metabolism

Recent studies have implicated endoplasmic reticulum (ER) stress in insulin resistance associated with caloric excess. In mice placed on a 3-day high fat diet, we find augmented eIF2α signaling, together with hepatic lipid accumulation and insulin resistance. To clarify the role of the liver ER stress-dependent phospho-eIF2α (eIF2α-P) pathway in response to acute caloric excess on liver and muscle glucose and lipid metabolism, we studied transgenic mice in which the hepatic ER stress-dependent eIF2α-P pathway was inhibited by overexpressing a constitutively active C-terminal fragment of GADD34/PPP1R15a, a regulatory subunit of phosphatase that terminates ER stress signaling by phospho-eIF2α. Inhibition of the eIF2α-P signaling in liver led to a decrease in hepatic glucose production in the basal and clamped state, which could be attributed to reduced gluconeogenic gene expression, resulting in reduced basal plasma glucose concentrations. Surprisingly, hepatic eIF2α inhibition also impaired insulin-stimulated muscle and adipose tissue insulin sensitivity. This latter effect could be attributed at least in part by an increase in circulating IGFBP-3 levels in the transgenic animals. In addition, infusion of insulin during a hyperinsulinemic-euglycemic clamp induced conspicuous ER stress in the 3-day high fat diet-fed mice, which was aggravated through continuous dephosphorylation of eIF2α. Together, these data imply that the hepatic ER stress eIF2α signaling pathway affects hepatic glucose production without altering hepatic insulin sensitivity. Moreover, hepatic ER stress-dependent eIF2α-P signaling is implicated in an unanticipated cross-talk between the liver and peripheral organs to influence insulin sensitivity, probably via IGFBP-3. Finally, eIF2α is crucial for proper resolution of insulin-induced ER stress.

Bone morphogenetic protein-1 processes insulin-like growth factor-binding protein 3

The bone morphogenetic protein-1 (BMP1)-like metalloproteinases play key roles in extracellular matrix formation, by converting precursors into mature functional proteins involved in forming the extracellular matrix. The BMP1-like proteinases also play roles in activating growth factors, such as BMP2/4, myostatin, growth differentiation factor 11, and transforming growth factor β1, by cleaving extracellular antagonists. The extracellular insulin-like growth factor-binding proteins (IGFBPs) are involved in regulating the effects of insulin-like growth factors (IGFs) on growth, development, and metabolism. Of the six IGFBPs, IGFBP3 has the greatest interaction with the large pool of circulating IGFs. It is also produced locally in tissues and is itself regulated by proteolytic processing. Here, we show that BMP1 cleaves human and mouse IGFBP3 at a single conserved site, resulting in markedly reduced ability of cleaved IGFBP3 to bind IGF-I or to block IGF-I-induced cell signaling. In contrast, such cleavage is shown to result in enhanced IGF-I-independent ability of cleaved IGFBP3 to block FGF-induced proliferation and to induce Smad phosphorylation. Consistent with in vivo roles for such cleavage, it is shown that, whereas wild type mouse embryo fibroblasts (MEFs) produce cleaved IGFBP3, MEFs doubly null for the Bmp1 gene and for the Tll1 gene, which encodes the related metalloproteinase mammalian Tolloid-like 1 (mTLL1), produce only unprocessed IGFBP3, thus demonstrating endogenous BMP1-related proteinases to be responsible for IGFBP3-processing activity in MEFs. Similarly, in zebrafish embryos, overexpression of Bmp1a is shown to reverse an Igfbp3-induced phenotype, consistent with the ability of BMP1-like proteinases to cleave IGFBP3 in vivo.

IGF binding protein 3 exerts its ligand-independent action by antagonizing BMP in zebrafish embryos

IGFBP3 is a multi-functional protein that has IGF-dependent and IGF-independent actions in cultured cells. Here we show that the IGF binding domain (IBD), nuclear localization signal (NLS) and transactivation domain (TA) are conserved and functional in zebrafish Igfbp3. The in vivo roles of these domains were investigated by expression of Igfbp3 and its mutants in zebrafish embryos. Igfbp3, and its NLS and TA mutants had equally strong dorsalizing effects. Human IGFBP3 had similar dorsalizing effects in zebrafish embryos. The activities of IBD and IBD+NLS mutants were lower, but they still caused dorsalization. Thus, the IGF-independent action of Igfbp3 is not related to NLS or TA in this in vivo model. We next tested the hypothesis that Igfbp3 exerts its IGF-independent action by affecting Bmp signaling. Co-expression of Igfbp3 with Bmp2b abolished Bmp2b-induced gene expression and inhibited its ventralizing activity. Biochemical assays and in vitro experiments revealed that IGFBP3 bound BMP2 and inhibited BMP2-induced Smad signaling in cultured human cells. In vivo expression of Igfbp3 increased chordin expression in zebrafish embryos by alleviating the negative regulation of Bmp2. The elevated level of Chordin acted together with Igfbp3 to inhibit the actions of Bmp2. Knockdown of Igfbp3 enhanced the ventralized phenotype caused by chordin knockdown. These results suggest that Igfbp3 exerts its IGF-independent actions by antagonizing Bmp signaling and that this mechanism is conserved.

Low-density lipoprotein-related receptor protein 1 (LRP-1) is not required for insulin-like growth factor binding protein 3 (IGFBP-3) to suppress L6 myogenic cell proliferation

Insulin-like growth factor binding protein-3 (IGFBP-3) suppresses proliferation of numerous cell types, including myogenic cells, via both insulin-like growth factor (IGF)-dependent and IGF-independent mechanisms; however, the mechanism of IGF-independent suppression of proliferation is not clearly defined. In nonmuscle cells, binding of IGFBP-3 to the low-density lipoprotein receptor-related protein-1 (LRP-1)/activated α(2)M receptor is reportedly required for IGFBP-3 to inhibit proliferation. These findings suggest that binding to this receptor also may be required for IGFBP-3 to suppress proliferation of cultured myogenic cells. To investigate the role of the LRP-1 receptor in suppression of myogenic cell proliferation by IGFBP-3, we have examined the effect of receptor-associated protein, an LRP-1 receptor antagonist, on recombinant porcine (rp)IGFBP-3 inhibition of L6 myogenic cell proliferation. Treatment with receptor-associated protein results in a 37% decrease (P < 0.05) in the ability of rpIGFBP-3 to inhibit L6-cell proliferation. In L6 cells subjected to LRP-1 small interfering RNA treatment for 48 h (LRP-1 silenced), LRP-1 mRNA levels were reduced by greater than 80% compared with control cultures treated with nonsense small interfering RNA (mock silenced). In addition, the 85-kDa transmembrane subunit of LRP-1 was undetectable in Western immunoblots of total protein lysates from LRP-1-silenced cells. Even though LRP-1 mRNA and protein levels were dramatically reduced in LRP-1-silenced L6 cells compared with mock-silenced controls, rpIGFPB-3 suppressed proliferation rate to the same extent in both LRP-1-silenced and mock-silenced cultures. Our results strongly suggest that, in contrast to data obtained for nonmuscle cell lines, the LRP-1 receptor is not required for IGFBP-3 to suppress proliferation of L6 myogenic cells.

Potential applications for cell regulatory factors in liver progenitor cell therapy

Orthotopic liver transplant represent the state of the art treatment for terminal liver pathologies such as cirrhosis in adults and hemochromatosis in neonates. A limited supply of transplantable organs in relationship to the demand means that many patients will succumb to disease before an organ becomes available. One promising alternative to liver transplant is therapy based on the transplant of liver progenitor cells. These cells may be derived from the patient, expanded in vitro, and transplanted back to the diseased liver. Inborn metabolic disorders represent the most attractive target for liver progenitor cell therapy, as many of these disorders may be corrected by repopulation of only a portion of the liver by healthy cells. Another potential application for liver progenitor cell therapy is the seeding of bio-artificial liver matrix. These ex vivo bioreactors may someday be used to bridge critically ill patients to other treatments. Conferring a selective growth advantage to the progenitor cell population remains an obstacle to therapy development. Understanding the molecular signaling mechanisms and micro-environmental cues that govern liver progenitor cell phenotype may someday lead to strategies for providing this selective growth advantage. The discovery of a population of cells within the bone marrow possessing the ability to differentiate into hepatocytes may provide an easily accessible source of cells for liver therapies.

Insulin-like growth factor-binding protein-3 promotes transforming growth factor-{beta}1-mediated epithelial-to-mesenchymal transition and motility in transformed human esophageal cells

Insulin-like growth factor-binding protein (IGFBP)-3 is overexpressed frequently in esophageal squamous cell carcinoma. Yet, the role of IGFBP3 in esophageal tumor biology remains to be elucidated. We find that IGFBP3 facilitates transforming growth factor (TGF)-beta1-mediated epithelial-to-mesenchymal transition (EMT) in transformed human esophageal epithelial cells, EPC2-hTERT-EGFR-p53(R175H). In organotypic 3D culture, a form of human tissue engineering, laser-capture microdissection revealed concurrent upregulation of TGF-beta target genes, IGFBP3 and EMT-related genes in the cells invading into the stromal compartment. IGFBP3 enhanced TGF-beta1-mediated EMT as well as transcription factors essential in EMT by allowing persistent SMAD2 and SMAD3 phosphorylation. TGF-beta1-mediated EMT and cell invasion were enhanced by ectopically expressed IGFBP3 and suppressed by RNA interference directed against IGFBP3. The IGFBP3 knockdown effect was rescued by IGFBP3(I56G/L80G/L81G), a mutant IGFBP3 lacking an insulin-like growth factor (IGF)-binding capacity. Thus, IGFBP3 can regulate TGF-beta1-mediated EMT and cell invasion in an IGF or insulin-like growth factor 1 receptor-independent manner. IGFBP3(I56G/L80G/L81G) also promoted EMT in vivo in a Ras-transformed human esophageal cell line T-TeRas upon xenograft transplantation in nude mice. In aggregate, IGFBP3 may have a novel IGF-binding independent biological function in regulation of TGF-beta1-mediated EMT and cell invasion.

Proinflammatory and profibrotic mediators: principal effectors of leiomyoma development as a fibrotic disorder

Leiomyomas are believed to derive from the transformation of myometrial smooth muscle cells/connective tissue fibroblasts. Although the identity of the molecule(s) that initiate such cellular transformation and orchestrate subsequent growth is still unknown, conventional evidence indicates that ovarian steroids are essential for leiomyoma growth. Ovarian steroid action in their target cell/tissue is mediated in part through local expression of various growth factors, cytokines, and chemokines. These autocrine/paracrine molecules with proinflammatory and profibrotic activities serve as major contributing factors in regulating cellular transformation, cell growth and apoptosis, angiogenesis, cellular hypertrophy, and excess tissue turnover, events central to leiomyoma growth. This review addresses the key regulatory functions of proinflammatory and profibrotic mediators and their molecular mechanisms, downstream signaling that regulates cellular events that result in transformation, and commitments of specific cells into forming a cellular environment with a possible role in development and subsequent growth of leiomyomas.

Transforming growth factor-{beta} (TGF{beta}) receptors I/II differentially regulate TGF{beta}1 and IGF-binding protein-3 mitogenic effects in the human placenta

Maternal IGFs regulate cytotrophoblast proliferation and, thereby, placental growth and function. IGF bioavailability is controlled by IGF-binding proteins (IGFBPs); in placenta, IGFBP-3 is particularly abundant. In other systems, IGFBP-3 can regulate cellular events independently of IGFs; these effects are thought to be mediated by TGFbeta receptors (TbetaR). We have examined IGFBP-3 regulation of IGF-dependent and -independent cytotrophoblast proliferation in first-trimester placental explants and the role of TbetaRII in mediating these effects. In the presence of IGFBP-3 (50 nm), IGF-induced (10 nm) proliferation (monitored by immunohistochemical analysis of Ki67 expression and bromodeoxyuridine incorporation) was significantly reduced (P < 0.05). IGFBP-3 also reduced basal proliferation independently of IGF receptor signaling. Immunohistochemical analysis demonstrated that TGFbeta signaling molecules [TGFbeta receptor I (TbetaRI), TbetaRII, TbetaRV, Smad-2, and ERK] are expressed in syncytium and/or cytotrophoblast. TGFbeta1 (10 ng/ml) enhanced cytotrophoblast proliferation and activated both Smad-2 and ERK-1/2, whereas IGFBP-3 activated only Smad-2. The function of both TGFbeta1 and IGFBP-3 was attenuated by a TbetaRII function-blocking antibody and by small interfering RNA-mediated knockdown of TbetaRII (P < 0.05); this was accompanied by a reduction in Smad-2 activation. This study demonstrates that both TGFbeta1 and IGFBP-3 signal through TbetaRI/II to influence human cytotrophoblast proliferation. However, downstream pathways are distinct, because IGFBP-3 acts only through Smad-2, whereas TGFbeta1 also phosphorylates ERK, resulting in opposite effects on cytotrophoblast proliferation. The effects of maternal growth signals on placental growth and function therefore depend on the balance of ligands, receptors, and signaling molecules at the syncytiotrophoblast surface. Therapeutic manipulation of this balance might offer a strategy to optimize placental development and pregnancy outcome.

Actions of IGF binding proteins and related proteins in adipose tissue

The insulin-like growth factors (IGFs), their binding proteins (IGFBPs) and structurally related proteins have been identified in adipose tissue but their roles in adipose tissue are poorly understood. IGF-I and IGFBP-3 expression increase during human preadipocyte differentiation. However, whereas IGF-I stimulates this process, IGFBP-3 is inhibitory both to preadipocyte differentiation and to differentiated adipocyte function. The direct interaction of IGFBP-3 with peroxisome proliferator-activated receptor-gamma is believed to contribute to its inhibitory effect on differentiation. Connective tissue growth factor (CTGF/CCN2) shares weak structural homology and functional similarities with IGFBP-3, including inhibition of preadipocyte differentiation. This review examines the current knowledge of IGFBP regulation and actions in adipocytes and proposes a common regulatory pathway involving IGFBP-3 and CTGF/CCN2.

Perspectives in mammalian IGFBP-3 biology: local vs. systemic action

Insulin-like growth factor (IGF) binding protein (IGFBP)-3 has traditionally been defined by its role as a binding protein and its association with IGF delivery and availability. Development of non-IGF binding IGFBP-3 analogs and the use of cell lines devoid of type 1 IGF receptors (IGF-R) have led to critical advances in the field of IGFBP-3 biology. These studies show that IGFBP-3 has IGF-independent roles in inhibiting cell proliferation in cancer cell lines. Nuclear transcription factor, retinoid X receptor (RXR)-alpha, and IGFBP-3 functionally interact to reduce prostate tumor growth and prostate-specific antigen in vivo. Moreover, IGFBP-3 inhibits insulin-stimulated glucose uptake into adipocytes independent of IGF. The purpose of this review is to highlight IGFBP-3 as a novel effector molecule and not just another "binding protein" by discussing its IGF-independent actions on metabolism and cell growth. Although this review presents studies that assume the role of IGFBP-3 as either an endocrine or autocrine/paracrine molecule, these systems may not exist as distinct entities, justifying the examination of IGFBP-3 in an integrated model. Also, we provide an overview of factors that regulate IGFBP-3 availability, including its production, methylation, and ubiquitination. We conclude with the role of IGFBP-3 in whole body systems and possible future applications of IGFBP-3 in physiology.

Senescence-messaging secretome: SMS-ing cellular stress

Oncogene-induced cellular senescence constitutes a strong anti-proliferative response, which can be set in motion following either oncogene activation or loss of tumour suppressor signalling. It serves to limit the expansion of early neoplastic cells and as such is a potent cancer-protective response to oncogenic events. Recently emerging evidence points to a crucial role in oncogene-induced cellular senescence for the 'senescence-messaging secretome' or SMS, setting the stage for cross-talk between senescent cells and their environment. How are such signals integrated into a coordinated response and what are the implications of this unexpected finding?

IGFBP-3 sensitizes prostate cancer cells to interferon-gamma-induced apoptosis

OBJECTIVE

Insulin-like growth factor binding protein-3 (IGFBP-3) has been shown to exhibit diverse biological actions, including IGF-independent effects on cell growth and cell death. Here we report that IGFBP-3 sensitizes prostate cancer cells to interferon-gamma (IFN-gamma)-induced apoptosis and inhibition of cell proliferation.

DESIGN

The cell growth or cell death of prostate cells in response to the treatments of IGFBPs and/or IFN-gamma was measured, and the signaling pathways mediating these actions assessed.

RESULTS

Cell proliferation was minimally affected when M12 prostate cancer cells were treated with exogenous IGFBP-3 (1-5 microg/ml), IGFBP-1 (1-5 microg/ml) or IFN-gamma (20 U/ml). However, strong inhibition of cell growth and significant apoptosis were observed when M12 cells were co-treated with IGFBP-3 and IFN-gamma, but not with IGFBP-1 and IFN-gamma. These effects were IGF-independent and appear not to require intracellular localization of IGFBP-3, as similar results were obtained with mutants of IGFBP-3 that either could not bind IGF or has impaired ability to be internalized. Further analyses revealed that IGFBP-3, but not IGFBP-1, could significantly enhance the weak tyrosine phosphorylation of STAT1 induced by IFN-gamma (20 U/ml) alone. The IGFBP-3-promoted apoptosis in the presence of IFN-gamma could also be abrogated by blockade of the mTOR pathway with its pharmacological inhibitors, LY294002 or rapamycin.

CONCLUSIONS

These results demonstrated that in a cancer cell line not responsive to exogenous IGFBP-3 alone, IGFBP-3 sensitized the cells to the anti-proliferative, proapoptotic actions of IFN-gamma through an IGF-independent, STAT1- and mTOR-dependent mechanism.

Localization of insulin-like growth factor (IGFBP)-3 in cultured porcine embryonic myogenic cells before and after TGF-beta1 treatment

Insulin-like growth factor binding protein (IGFBP)-3 binds IGFs with high affinity and affects their biological activity. IGFBP-3 that is not bound to IGF also affects cells via mechanisms involving binding to specific cell surface receptors and/or transport into the cell. IGFBP-3 is produced by porcine embryonic myogenic cell (PEMC) cultures. Additionally, IGFBP-3 facilitates the proliferation-suppressing actions of TGF-beta(1) and myostatin in PEMC cultures via mechanisms that do not involve IGF binding. Moreover, these mechanisms do not involve preventing myostatin or TGF-beta(1)-induced increases in phosphosmad2 or phosphosmad3 level. Consequently, the mechanism(s) by which IGFBP-3 facilitates the proliferation-suppressing actions of TGF-beta(1) and myostatin in PEMC is unclear. Since IGFBP-3 reportedly interacts with nuclear proteins that regulate transcription, TGF-beta(1) or myostatin-induced translocation of IGFBP-3 into the nucleus may facilitate the proliferation-suppressing actions of these cytokines. Here, we show that IGFBP-3 is localized in cells containing the muscle specific protein desmin, thus establishing the presence of this IGFBP in myogenic cells. IGFBP-3 is present in the cytoplasm of all myogenic cells and approximately 50% of the nuclei of proliferating PEMC. IGFBP-3 is also detectable in fused myotubes. IGFBP-3 suppresses IGF-I-stimulated differentiation of PEMC but has no affect on Long-R3-IGF-I-stimulated differentiation of PEMC. Treatment of PEMC for 24h with TGF-beta(1) (20 ng/ml) results in a 78% (p<0.01) increase in the number of nuclei that contain detectable IGFBP-3. These results suggest that translocation of IGFBP-3 into the nucleus of PEMC could play a role in mediating the proliferation-suppressing action of TGF-beta(1).

Mechanisms of action of TGF-beta in cancer: evidence for Smad3 as a repressor of the hTERT gene

Transforming growth factor-beta (TGF-beta) induces cell differentiation and suppresses cell proliferation, but the mechanisms underlying the actions of TGF-beta remain to be fully elucidated. Recent studies suggest that TGF-beta suppresses neoplastic cell development by employing Smad3 protein to repress the gene of human telomerase reverse transcriptase (hTERT). In human breast cancer cells, TGF-beta induces rapid phosphorylation and subsequent entry of Smad3 into the nucleus. In the nucleus, Smad3 binds to the hTERT gene promoter directly and inhibits hTERT gene transcription activity. By interacting with c-myc, Smad3 also represses the c-myc gene. Thus, TGF-beta prevents continuous cell proliferation by switching off telomerase activity through Smad3 repression of the hTERT gene and the action of c-myc. Modulating the interface between Smad3 and the hTERT gene, and the potential feedback loop from telomeres to Smad3 via Smurf2, may represent a novel approach to regulate cell lifespan of proliferation.

TGF-beta-like transcriptional effects of thyroglobulin (Tg) in mouse mesangial cells

TGF-beta-like activities of proteins unrelated to the cytokine could mimic its actions in fibrosis and cell proliferation. Thyroglobulin (Tg) has been identified as having a TGF-beta receptor (TGFbetaR)-binding activity and is deposited in the glomerulus in certain immune-complex diseases. The aim of the present study is to determine whether Tg can reproduce the transcriptional activity of TGF-beta1 in the mouse glomerular mesangial cell (MC), and to examine whether such activity is manifested through TGFbetaR. Real-time RT-PCR was employed to examine the effects of TGF-beta1 and bovine Tg on the expression of three genes (TGF-beta1, plasminogen activator inhibitor 1 [PAI-1], and Pax-8) regulated by TGF-beta1 in other cell types. In addition, a pentacosapeptide TGF-beta1 antagonist, beta(1)(25) (41-65) was employed to determine whether the transcriptional activity of Tg was mediated through the TGF-beta binding site on the TGFbetaR. A 6h exposure to TGF-beta1 resulted in increased TGF-beta1 and PAI-1 transcript, and a decrease in Pax-8. Similarly, a 6h exposure to Tg resulted in increases of about 5-fold in TGF-beta1 and PAI-1 mRNA and a decrease of 53% in Pax-8. In comparison with other proteins, Tg had the greatest positive effect on TGF-beta1 transcript levels. beta(1)(25) (41-65) significantly reduced the TGF-beta1-, but not the Tg-induced changes in TGF-beta1, PAI-1 and Pax-8 transcript levels. We conclude from these studies that Tg possesses a TGF-beta-mimetic transcriptional activity in the MC that is not mediated by its binding to TGFbetaR. These results suggest that Tg and other proteins could initiate glomerular injury by reproducing the actions of TGF-beta1 in the mesangial cell.

Contribution of the orphan nuclear receptor Nur77 to the apoptotic action of IGFBP-3

Tumor suppression by insulin-like growth factor-binding protein-3 (IGFBP-3) has been demonstrated to occur via insulin-like growth factor-dependent and -independent mechanisms in vitro and in vivo. We have recently described IGFBP-3-induced mitochondrial translocation of the nuclear receptors RXRalpha/Nur77 in the induction of prostate cancer (CaP) cell apoptosis. Herein, we demonstrate that IGFBP-3 and Nur77 associate in the cytoplasmic compartment in 22RV1 CaP cells. Nur77 is a major component of IGFBP-3-induced apoptosis as shown by utilizing mouse embryonic fibroblasts (MEFs) derived from Nur77 wild-type and knockout (KO) mice. However, dose-response experiments revealed that a small component of IGFBP-3-induced apoptosis is Nur77 independent. Reintroduction of Nur77 into Nur77 KO MEFs restores full responsiveness to IGFBP-3. IGFBP-3 induces phosphorylation of Jun N-terminal kinase and inhibition of Akt phosphorylation and activity, which have been associated with Nur77 translocation. Finally, IGFBP-3 administration to CaP xenografts on SCID mice induced apoptosis and translocated Nur77 out of the nucleus. Taken together, our results verify an important role for the orphan nuclear receptor Nur77 in the apoptotic actions of IGFBP-3.

TGF-beta and cancer: is Smad3 a repressor of hTERT gene?

Transforming growth factor beta (TGF-beta) carries out tumor suppressor activity in epithelial and lymphoid cells, whereas telomerase is required for most cancers. Although the molecular mechanisms by which TGF-beta acts as a tumor suppressor are yet to be fully established, a link between TGFb and its tumor suppressor activity by telomerase has been suggested. Recently, we have noted a novel mode of action for TGF-beta through which human telomerase reverse transcriptase (hTERT) gene is repressed in immortal and neoplastic cells, confirming that one of the mechanisms underlying TGF-beta suppression of tumor growth may be through inhibiting hTERT gene transcription. Moreover, the inhibition of hTERT gene by TGF-beta suggests a cis action of the TGF-beta signaling molecule Smad3 on hTERT promoter directly. This article examines our current understanding and investigation of TGF-beta regulation of telomerase activity, and presents a model in which Smad3 participates in regulating hTERT gene transcription by acting as a repressor directly. Engineering the interface between Smad3 and hTERT gene may lead to a new strategy to inhibit telomerase activity in cancer.

IGF-I and IGFBP-3 augment transforming growth factor-beta actions in human renal carcinoma cells

Renal cell carcinoma (RCC) is the most prevalent cancer of the kidney. In human RCC cells, we recently showed that insulin-like growth factor I (IGF-I) has growth-promoting effects regulated by IGF-binding protein 3 (IGFBP-3). In this study, the analysis was expanded to include the interaction between the IGF and transforming growth factor-beta (TGF-beta) systems in the human RCC cells Caki-2 (from a primary tumor) and SK-RC-52 (from a metastasis). Functional effects such as cell proliferation, TGF-beta receptor (TbetaR) signaling, and IGFBP-3 levels were monitored after stimulation with various concentrations of IGF-I, TGF-beta, and IGFBP-3. In addition, human RCC tissues as well as experimental human RCC tumors were analyzed for cellular expression of phosphorylated Smad2 by immunohistochemistry. TGF-beta regulated the endogenous IGFBP-3 levels in these RCC cells as neutralizing anti-TGF-beta(1-3) antibodies strongly reduced the basal IGFBP-3 level. In addition, IGF-I increased the IGFBP-3 levels five- to eightfold with TGF-beta acting in synergy to enhance the IGFBP-3 levels 12- to 17-fold. Neutralizing TGF-beta(1-3) activity circumvented the growth inhibitory effects of IGFBP-3 seen in SK-RC-52, whereas it inhibited the growth-promoting effects of IGFBP-3 in Caki-2. Moreover, IGF-I interacted directly with TGF-beta activation of the TbetaR complex by enhancing phosphorylation and nuclear translocation of Smad2. This study demonstrates a direct interaction of the IGF and TGF-beta systems in human renal carcinoma cells. The observations that IGF-I enhances the TGF-beta signaling and that TGF-beta promotes IGFBP-3 production and thus influence the biological activity of IGF may be of importance for future therapeutic options.

Effect of constitutive expression of porcine IGFBP-3 on proliferation and differentiation of L6 myogenic cells

We have previously shown that exogenous recombinant porcine IGFBP-3 (rpIGFBP-3) suppresses proliferation and differentiation of L6 myogenic cells in an IGF-I-dependent manner and suppresses proliferation of L6 myogenic cells via an IGF-I-independent mechanism. In order to assess the effects of endogenously produced IGFBP-3, we have transfected L6 myogenic cells with a pEF6/V5 vector containing pIGFBP-3 cDNA under the control of the human elongation factor 1alpha (hEF-1alpha) promoter and with the empty vector. We have isolated a cell population that constitutively produces porcine IGFBP-3 (tL6 cells) and a stable mock transfected cell population containing the empty vector (mtL6 cells). Constitutive expression of IGFBP-3 slightly reduced the expression of IGFBP-5 but had no effect on IGFBP-4 production by L6 myogenic cells. Immunoneutralization of IGFBP-3 increased both IGF-I- and Long-R3-IGF-I-stimulated proliferation of tL6 cells (58 and 33%, respectively) (P<0.01). These data indicate endogenous pIGFBP-3, like exogenous rpIGFBP-3, suppresses the proliferation of L6 myogenic cells via both IGF-I-dependent and -independent pathways. Immunoneutralization of IGFBP-3 also increased IGF-I-stimulated differentiation (21%, P<0.05) but had no effect on Long-R3-IGF-I stimulated differentiation of tL6 myogenic cells. Results indicate that exogenous and endogenous IGFBP-3 affect proliferation and differentiation of L6 myogenic cells in a similar way. Immunohistochemical localization data reveal that pre-incubation with anti-pIGFBP-3 dramatically reduces the level of intracellular IGFBP-3 in tL6 myogenic cells indicating that endogenously produced IGFBP-3 must first be secreted before it is internalized and that anti-pIGFBP-3 prevents internalization of IGFBP-3. TL6 and mtL6 cells provide a good system to further investigate the mechanisms by which IGFBP-3 affects proliferation and differentiation of myogenic cells.