Insulin-like growth factor binding protein-3 prevents retinoid receptor heterodimerization: implications for retinoic acid-sensitivity in human breast cancer cells

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.

IGFBP5 is released by senescent cells and is internalized by healthy cells, promoting their senescence through interaction with retinoic receptors

Cells that are exposed to harmful genetic damage, either from internal or external sources, may undergo senescence if they are unable to repair their DNA. Senescence, characterized by a state of irreversible growth arrest, can spread to neighboring cells through a process known as the senescence-associated secretory phenotype (SASP). This phenomenon contributes to both aging and the development of cancer. The SASP comprises a variety of factors that regulate numerous functions, including the induction of secondary senescence, modulation of immune system activity, remodeling of the extracellular matrix, alteration of tissue structure, and promotion of cancer progression. Identifying key factors within the SASP is crucial for understanding the underlying mechanisms of senescence and developing effective strategies to counteract cellular senescence. Our research has specifically focused on investigating the role of IGFBP5, a component of the SASP observed in various experimental models and conditions.Through our studies, we have demonstrated that IGFBP5 actively contributes to promoting senescence and can induce senescence in neighboring cells. We have gained valuable insights into the mechanisms through which IGFBP5 exerts its pro-senescence effects. These mechanisms include its release following genotoxic stress, involvement in signaling pathways mediated by reactive oxygen species and prostaglandins, internalization via specialized structures called caveolae, and interaction with a specific protein known as RARα. By uncovering these mechanisms, we have advanced our understanding of the intricate role of IGFBP5 in the senescence process. The significance of IGFBP5 as a pro-aging factor stems from an in vivo study we conducted on patients undergoing Computer Tomography analysis. In these patients, we observed an elevation in circulating IGFBP5 levels in response to radiation-induced organismal stress.Globally, our findings highlight the potential of IGFBP5 as a promising therapeutic target for age-related diseases and cancer.

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.

Insulin-like Growth Factor-1 and IGF Binding Proteins Predict All-Cause Mortality and Morbidity in Older Adults

While the growth hormone/insulin-like growth factor-1 (GH/IGF-1) pathway plays essential roles in growth and development, diminished signaling via this pathway in model organisms extends lifespan and health-span. In humans, circulating IGF-1 and IGF-binding proteins 3 and 1 (IGFBP-3 and 1), surrogate measures of GH/IGF-1 system activity, have not been consistently associated with morbidity and mortality. In a prospective cohort of independently-living older adults (n = 840, mean age 76.1 ± 6.8 years, 54.5% female, median follow-up 6.9 years), we evaluated the age- and sex-adjusted hazards for all-cause mortality and incident age-related diseases, including cardiovascular disease, diabetes, cancer, and multiple-domain cognitive impairment (MDCI), as predicted by baseline total serum IGF-1, IGF-1/IGFBP-3 molar ratio, IGFBP-3, and IGFBP-1 levels. All-cause mortality was positively associated with IGF-1/IGFBP-3 molar ratio (HR 1.28, 95% CI 1.05–1.57) and negatively with IGFBP-3 (HR 0.82, 95% CI 0.680–0.998). High serum IGF-1 predicted greater risk for MDCI (HR 1.56, 95% CI 1.08–2.26) and composite incident morbidity (HR 1.242, 95% CI 1.004–1.538), whereas high IGFBP-1 predicted lower risk for diabetes (HR 0.50, 95% CI 0.29–0.88). In conclusion, higher IGF-1 levels and bioavailability predicted mortality and morbidity risk, supporting the hypothesis that diminished GH/IGF-1 signaling may contribute to human longevity and health-span.

IGFBP-3/IGFBP-3 Receptor System as an Anti-Tumor and Anti-Metastatic Signaling in Cancer

Insulin-like growth factor binding protein-3 (IGFBP-3) is a p53 tumor suppressor-regulated protein and a major carrier for IGFs in circulation. Among six high-affinity IGFBPs, which are IGFBP-1 through 6, IGFBP-3 is the most extensively investigated IGFBP species with respect to its IGF/IGF-I receptor (IGF-IR)-independent biological actions beyond its endocrine/paracrine/autocrine role in modulating IGF action in cancer. Disruption of IGFBP-3 at transcriptional and post-translational levels has been implicated in the pathophysiology of many different types of cancer including breast, prostate, and lung cancer. Over the past two decades, a wealth of evidence has revealed both tumor suppressing and tumor promoting effects of IGF/IGF-IR-independent actions of IGFBP-3 depending upon cell types, post-translational modifications, and assay methods. However, IGFBP-3′s anti-tumor function has been well accepted due to identification of functional IGFBP-3-interacting proteins, putative receptors, or crosstalk with other signaling cascades. This review mainly focuses on transmembrane protein 219 (TMEM219), which represents a novel IGFBP-3 receptor mediating antitumor effect of IGFBP-3. Furthermore, this review delineates the potential underlying mechanisms involved and the subsequent biological significance, emphasizing the clinical significance of the IGFBP-3/TMEM219 axis in assessing both the diagnosis and the prognosis of cancer as well as the therapeutic potential of TMEM219 agonists for cancer treatment.

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.

Nuclear localization and actions of the insulin-like growth factor 1 (IGF-1) system components: Transcriptional regulation and DNA damage response

Insulin-like growth factor (IGF) system stimulates growth, proliferation, and regulates differentiation of cells in a tissue-specific manner. It is composed of two insulin-like growth factors (IGF-1 and IGF-2), six insulin-like growth factor-binding proteins (IGFBPs), and two insulin-like growth factor receptors (IGF-1R and IGF-2R). IGF actions take place mostly through the activation of the plasma membrane-bound IGF-Rs by the circulating ligands (IGFs) released from the IGFBPs that stabilize their levels in the serum. This review focuses on the IGF-1 part of the system. The IGF-1 gene, which is expressed mainly in the liver as well as in other tissues, comprises six alternatively spliced exons that code for three protein isoforms (pro-IGF-1A, pro-IGF-1B, and pro-IGF-1C), which are processed to mature IGF-1 and E-peptides. The IGF-1R undergoes autophosphorylation, resulting in a signaling cascade involving numerous cytoplasmic proteins such as AKT and MAPKs, which regulate the expression of target genes. However, a more complex picture of the axis has recently emerged with all its components being translocated to the nuclear compartment. IGF-1R takes part in the regulation of gene expression by forming transcription complexes, modifying the activity of chromatin remodeling proteins, and participating in DNA damage tolerance mechanisms. Four IGFBPs contain a nuclear localization signal (NLS), which targets them to the nucleus, where they regulate gene expression (IGFBP-2, IGFBP-3, IGFBP-5, IGFBP-6) and DNA damage repair (IGFBP-3 and IGFBP-6). Last but not least, the IGF-1B isoform has been reported to be localized in the nuclear compartment. However, no specific molecular actions have been assigned to the nuclear pro-IGF-1B or its derivative EB peptide. Therefore, further studies are needed to shed light on their nuclear activity. These recently uncovered nuclear actions of different components of the IGF-1 axis are relevant in cancer cell biology and are discussed in this review.

Retinoic Acids in the Treatment of Most Lethal Solid Cancers

Although the use of oral administration of pharmacological all-trans retinoic acid (ATRA) concentration in acute promyelocytic leukaemia (APL) patients was approved for over 20 years and used as standard therapy still to date, the same use in solid cancers is still controversial. In the present review the literature about the top five lethal solid cancers (lung, stomach, liver, breast, and colon cancer), as defined by The Global Cancer Observatory of World Health Organization, and retinoic acids (ATRA, 9-cis retinoic acid, and 13-cis retinoic acid, RA) was compared. The action of retinoic acids in inhibiting the cell proliferation was found in several cell pathways and compartments: from membrane and cytoplasmic signaling, to metabolic enzymes, to gene expression. However, in parallel in the most aggressive phenotypes several escape routes have evolved conferring retinoic acids-resistance. The comparison between different solid cancer types pointed out that for some cancer types several information are still lacking. Moreover, even though some pathways and escape routes are the same between the cancer types, sometimes they can differently respond to retinoic acid therapy, so that generalization cannot be made. Further studies on molecular pathways are needed to perform combinatorial trials that allow overcoming retinoic acids resistance.

IGF-Binding Proteins: Why Do They Exist and Why Are There So Many?

Insulin-like growth factors (IGFs) are key growth-promoting peptides that act as both endocrine hormones and autocrine/paracrine growth factors. In the bloodstream and in local tissues, most IGF molecules are bound by one of the members of the IGF-binding protein (IGFBP) family, of which six distinct types exist. These proteins bind to IGF with an equal or greater affinity than the IGF1 receptor and are thus in a key position to regulate IGF signaling globally and locally. Binding to an IGFBP increases the half-life of IGF in the circulation and blocks its potential binding to the insulin receptor. In addition to these classical roles, IGFBPs have been shown to modulate IGF signaling locally under various conditions. Although members of the IGFBP family share significant sequence homology, they each have unique structural features and play distinct roles. These IGFBP genes also have different modes of regulation and distinct expression patterns. Some IGFBPs have been found to bind to their own receptors or to translocate into the interior compartments of cells where they may execute IGF-independent actions. In spite of this functional and regulatory diversity, it has been puzzling that loss-of-function studies have yielded relatively little information about the physiological functions of IGFBPs. In this review, we suggest that evolution has tended to retain an array of IGFBPs in order to facilitate fine-tuning of IGF signaling. We explore the emerging explanation that many IGFBP functions have evolved to allow the targeted adjustment of IGF signaling under stressful or irregular conditions, which would likely not be revealed in a standard laboratory setting.

Hypoxia Suppresses Spontaneous Mineralization and Osteogenic Differentiation of Mesenchymal Stem Cells via IGFBP3 Up-Regulation

Hypoxia has diverse stimulatory effects on human adipose-derived stem cells (ASCs). In the present study, we investigated whether hypoxic culture conditions (2% O₂) suppress spontaneous mineralization and osteogenic differentiation of ASCs. We also investigated signaling pathways and molecular mechanisms involved in this process. We found that hypoxia suppressed spontaneous mineralization and osteogenic differentiation of ASCs, and up-regulated mRNA and protein expression of Insulin-like growth factor binding proteins (IGFBPs) in ASCs. Although treatment with recombinant IGFBPs did not affect osteogenic differentiation of ASCs, siRNA-mediated inhibition of IGFBP3 attenuated hypoxia-suppressed osteogenic differentiation of ASCs. In contrast, overexpression of IGFBP3 via lentiviral vectors inhibited ASC osteogenic differentiation. These results indicate that hypoxia suppresses spontaneous mineralization and osteogenic differentiation of ASCs via intracellular IGFBP3 up-regulation. We determined that reactive oxygen species (ROS) generation followed by activation of the MAPK and PI3K/Akt pathways play pivotal roles in IGFBP3 expression under hypoxia. For example, ROS scavengers and inhibitors for MAPK and PI3K/Akt pathways attenuated the hypoxia-induced IGFBP3 expression. Inhibition of Elk1 and NF-κB through siRNA transfection also led to down-regulation of IGFBP3 mRNA expression. We next addressed the proliferative potential of ASCs with overexpressed IGFBP3, but IGFBP3 overexpression reduced the proliferation of ASCs. In addition, hypoxia reduced the osteogenic differentiation of bone marrow-derived clonal mesenchymal stem cells. Collectively, our results indicate that hypoxia suppresses the osteogenic differentiation of mesenchymal stem cells via IGFBP3 up-regulation.

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.

Nuclear actions of insulin-like growth factor binding protein-3

In addition to its actions outside the cell, cellular uptake and nuclear import of insulin-like growth factor binding protein-3 (IGFBP-3) has been recognized for almost two decades, but knowledge of its nuclear actions has been slow to emerge. IGFBP-3 has a functional nuclear localization signal and interacts with the nuclear transport protein importin-β. Within the nucleus IGFBP-3 appears to have a role in transcriptional regulation. It can bind to the nuclear receptor, retinoid X receptor-α and several of its dimerization partners, including retinoic acid receptor, vitamin D receptor (VDR), and peroxisome proliferator-activated receptor-γ (PPARγ). These interactions modulate the functions of these receptors, for example inhibiting VDR-dependent transcription in osteoblasts and PPARγ-dependent transcription in adipocytes. Nuclear IGFBP-3 can be detected by immunohistochemistry in cancer and other tissues, and its presence in the nucleus has been shown in many cell culture studies to be necessary for its pro-apoptotic effect, which may also involve interaction with the nuclear receptor Nur77, and export from the nucleus. IGFBP-3 is p53-inducible and in response to DNA damage, forms a complex with the epidermal growth factor receptor (EGFR), translocating to the nucleus to interact with DNA-dependent protein kinase. Inhibition of EGFR kinase activity or downregulation of IGFBP-3 can inhibit DNA double strand-break repair by nonhomologous end joining. IGFBP-3 thus has the ability to influence many cell functions through its interactions with intranuclear pathways, but the importance of these interactions in vivo, and their potential to be targeted for therapeutic benefit, require further investigation.

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.

Involvement of insulin-like growth factor binding protein-3 in peroxisome proliferator-activated receptor gamma-mediated inhibition of breast cancer cell growth

We have previously reported that insulin-like growth factor binding protein-3 (IGFBP-3), a protein with dichotomous effects on both cell proliferation and cell survival, interacts with peroxisome proliferator-activated receptor gamma (PPARγ) and inhibits adipogenic PPARγ signaling. We now show that IGFBP-3 and PPARγ interact in breast cancer cells, through amino- and carboxyl-terminal residues of IGFBP-3. IGFBP-3 and the PPARγ ligands, rosiglitazone or 15-deoxy-Δ(12,14)-prostaglandin J2, separately inhibited the proliferation of MCF-7, MDA-MB-231 and MDA-MB-468 breast cancer cells. However, growth inhibition by IGFBP-3 and PPARγ ligand combined was greater than by either alone. Two IGFBP-3 mutants with reduced PPARγ binding caused no growth inhibition when used alone and abolished the inhibitory effect of rosiglitazone when used in combination with PPARγ ligand. Cell growth inhibition by PPARγ ligands was substantially blocked by IGFBP-3 siRNA and restored by exogenous IGFBP-3. We conclude that the interaction between IGFBP-3 and PPARγ is important for the growth-inhibitory effect of PPARγ ligands in human breast cancer cells, suggesting that IGFBP-3 expression by breast tumors may regulate their sensitivity toward PPARγ ligands.

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.

Retinoids and breast cancer: from basic studies to the clinic and back again

All-trans retinoic acid (ATRA) is the most important active metabolite of vitamin A controlling segmentation in the developing organism and the homeostasis of various tissues in the adult. ATRA as well as natural and synthetic derivatives, collectively known as retinoids, are also promising agents in the treatment and chemoprevention of different types of neoplasia including breast cancer. The major aim of the present article is to review the basic knowledge acquired on the anti-tumor activity of classic retinoids, like ATRA, in mammary tumors, focusing on the underlying cellular and molecular mechanisms and the determinants of retinoid sensitivity/resistance. In the first part, an analysis of the large number of pre-clinical studies available is provided, stressing the point that this has resulted in a limited number of clinical trials. This is followed by an overview of the knowledge acquired on the role played by the retinoid nuclear receptors in the anti-tumor responses triggered by retinoids. The body of the article emphasizes the potential of ATRA and derivatives in modulating and in being influenced by some of the most relevant cellular pathways involved in the growth and progression of breast cancer. We review the studies centering on the cross-talk between retinoids and some of the growth-factor pathways which control the homeostasis of the mammary tumor cell. In addition, we consider the cross-talk with relevant intra-cellular second messenger pathways. The information provided lays the foundation for the development of rational and retinoid-based therapeutic strategies to be used for the management of breast cancer.

Inhibition of insulin-like growth factor-binding protein-3 signaling through sphingosine kinase-1 sensitizes triple-negative breast cancer cells to EGF receptor blockade

The type I EGF receptor (EGFR or ErbB1) and insulin-like growth factor-binding protein-3 (IGFBP-3) are highly expressed in triple-negative breast cancer (TNBC), a particularly aggressive disease that cannot be treated with conventional therapies targeting the estrogen or progesterone receptors (ER and PR), or HER2. We have shown previously in normal breast epithelial cells that IGFBP-3 potentiates growth-stimulatory signaling transduced by EGFR, and this is mediated by the sphingosine kinase-1 (SphK1)/sphingosine 1-phosphate (S1P) system. In this study, we investigated whether cotargeting the EGFR and SphK1/S1P pathways in TNBC cells results in greater growth inhibition compared with blocking either alone, and might therefore have novel therapeutic potential in TNBC. In four TNBC cell lines, exogenous IGFBP-3 enhanced ligand-stimulated EGFR activation, associated with increased SphK1 localization to the plasma membrane. The effect of exogenous IGFBP-3 on EGFR activation was blocked by pharmacologic inhibition or siRNA-mediated silencing of SphK1, and silencing of endogenous IGFBP-3 also suppressed EGF-stimulated EGFR activation. Real-time analysis of cell proliferation revealed a combined effect of EGFR inhibition by gefitinib and SphK1 inhibition using SKi-II. Growth of MDA-MB-468 xenograft tumors in mice was significantly inhibited by SKi-II and gefitinib when used in combination, but not as single agents. We conclude that IGFBP-3 promotes growth of TNBC cells by increasing EGFR signaling, that this is mediated by SphK1, and that combined inhibition of EGFR and SphK1 has potential as an anticancer therapy in TNBC in which EGFR and IGFBP-3 expression is high.

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.

Role of insulin-like growth factor binding protein-3 in 1, 25-dihydroxyvitamin-d 3 -induced breast cancer cell apoptosis

Insulin-like growth factor I (IGF-I) is implicated in breast cancer development and 1, 25-dihydroxyvitamin D₃ (1, 25-D₃) has been shown to attenuate prosurvival effects of IGF-I on breast cancer cells. In this study the role of IGF binding protein-3 (IGFBP-3) in 1, 25-D₃-induced apoptosis was investigated using parental MCF-7 breast cancer cells and MCF-7/VD^R cells, which are resistant to the growth inhibitory effects of 1, 25-D₃. Treatment with 1, 25-D₃ increased IGFBP-3 mRNA expression in both cell lines but increases in intracellular IGFBP-3 protein and its secretion were observed only in MCF-7. 1, 25-D₃-induced apoptosis was not associated with activation of any caspase but PARP-1 cleavage was detected in parental cells. IGFBP-3 treatment alone produced cleavage of caspases 7, 8, and 9 and PARP-1 in MCF-7 cells. IGFBP-3 failed to activate caspases in MCF-7/VD^R cells; however PARP-1 cleavage was detected. 1, 25-D₃ treatment inhibited IGF-I/Akt survival signalling in MCF-7 but not in MCF-7/VD^R cells. In contrast, IGFBP-3 treatment was effective in inhibiting IGF-I/Akt pathways in both breast cancer lines. These results suggest a role for IGFBP-3 in 1, 25-D₃ apoptotic signalling and that impaired secretion of IGFBP-3 may be involved in acquired resistance to vitamin D in breast cancer.

IGFBP-2 nuclear translocation is mediated by a functional NLS sequence and is essential for its pro-tumorigenic actions in cancer cells

IGFBP-2 is highly expressed in both the serum and tumor tissues of most cancers, and is considered one of the most significant genes in the signature of major cancers. IGFBP-2 mainly modulates IGF actions in the pericellular space; however, there is considerable evidence to suggest that IGFBP-2 may also act independently of the IGFs. These IGF-independent actions of IGFBP-2 are exerted either via interactions at the cell surface or intracellularly, via interaction with cytoplasmic or nuclear-binding partners. The precise mechanism underlying the intracellular/intranuclear localization of IGFBP-2 remains unclear. In this study, we investigated IGFBP-2 nuclear localization in several common cancer cells with the aim of dissecting the mechanism of its nuclear trafficking. IGFBP-2 is detected in the nuclei of common cancer cells, including breast, prostate and several neuroblastoma cell lines, using cell fractionation and confocal microscopy. Via nuclear import assays, we show that nuclear entry of IGFBP-2 is mediated by the classical nuclear import mechanisms, primarily through importin-α, as demonstrated by the use of blocking, competition and co-immunoprecipitation assays. Bioinformatics analysis of the IGFBP-2 protein sequence with PSORT II identified a classical nuclear localization signal (cNLS) sequence at 179PKKLRPP185, within the IGFBP-2 linker domain, mutagenesis of which abolishes IGFBP-2 nuclear import. Accordingly, the NLSmutIGFBP-2 fails to activate the VEGF promoter, which would otherwise occur in the presence of wild-type IGFBP-2. As a consequence, no activation of angiogenic processes were observed in NLSmutIGFBP-2 expressing SHEP cells when implanted onto our in vivo quail chorio-allantoic membrane model. Taken together, these data show for the first time that IGFBP-2 possesses a functional NLS sequence and that IGFBP-2 actively translocates into the nucleus by a classical nuclear import mechanism, involving formation of IGFBP-2 complexes with importin-α. Nuclear IGFBP-2 is required for the activation of VEGF expression and consequent angiogenesis.

IGFBP-3 binds GRP78, stimulates autophagy and promotes the survival of breast cancer cells exposed to adverse microenvironments

Despite the established role of insulin-like growth factor binding protein-3 (IGFBP-3) as a growth inhibitor in vitro, a high level of IGFBP-3 in breast tumor tissue is associated with the stimulation of xenograft growth in mice and poor prognosis in patients. To understand the contribution of IGFBP-3 to breast cancer progression, tandem affinity purification was used to identify novel interacting proteins. The endoplasmic reticulum protein, glucose-regulated protein 78 (GRP78), was shown to bind to IGFBP-3, confirmed by colocalization, coimmunoprecipitations, glutathione S-transferase (GST) pulldowns and a nanomolar binding affinity. GST pulldowns also indicated that the GRP78 ATPase domain mediated the interaction with IGFBP-3. The critical roles of GRP78 in the unfolded protein response and macroautophagy led to an investigation of possible links between IGFBP-3, GRP78 and cellular stress responses. IGFBP-3 was found to stimulate the survival of breast cancer cells subjected to glucose starvation and hypoxia. Pharmacological inhibitors and small interfering RNA knockdown established that the increased survival of IGFBP-3-expressing cells was dependent on an intact autophagy response, as well as GRP78. The contribution of autophagy was confirmed by the demonstration that IGFBP-3 expression increases both the formation of autophagic puncta and flux through the system. In conclusion, we have shown that IGFBP-3 stimulates autophagy and thereby promotes the survival of breast cancer cells exposed to conditions that represent the adverse microenvironments encountered by solid tumor cells in vivo.

Insulin-like growth factor binding protein-3 inhibits migration of endometrial cancer cells

Cell migration and invasion leading to metastasis is a major cause of death from endometrial cancer (EC). We have shown that the rate of EC cell migration is inversely related to the level of insulin-like growth factor protein-3 (IGFBP-3). Down-regulation of IGFBP-3 by siRNA in EC cells accelerated migration without affecting proliferation and cells displayed a more migratory phenotype, with co-localization of migration-associated markers at the leading edge of cell membranes. Opposite effects were seen with either the addition of recombinant IGFBP-3 or overexpression of IGFBP-3. Cells with mutated PTEN had the highest IGFBP-3 expression and the slowest migration rates. This study demonstrates that endogenous IGFBP-3 modulates adhesion-migration dynamics in EC cells, implying that it may be important in regulating metastasis in this disease.

Insulin-like growth factor binding protein-6 interacts with the thyroid hormone receptor α1 and modulates the thyroid hormone-response in osteoblastic differentiation

Insulin-like growth factor binding protein-6 (IGFBP-6) is a member of the insulin-like growth factor binding protein family, which has both Insulin-like growth factor-dependent and independent effects on cell growth. In previous studies, we have shown that recombinant IGFBP-6 could be translocated into the cell nucleus. But the effect in the nucleus of IGFBP-6 is not clear. In the present study, we use multiple methodologies including Glutathione S-transferase pull-down assay, co-immunoprecipitation, fluorescence resonance energy transfer to demonstrate that IGFBP-6 can directly interact with thyroid hormone receptor alpha 1 (TRα1) in vitro and in vivo. We also demonstrate that the DNA-binding domains and Ligand-binding domains of TRα1 and N-terminal domains and C-terminal domains of IGFBP-6 are involved in the interaction. This interaction also can block the formation of TR: retinoid X receptor heterodimers. Furthermore, immunofluorescence co-localization studies show IGFBP-6 and TRα1 could co-localize in the nucleus of the cells. Reporter gene experiment shows that IGFBP-6 negatively regulates the growth hormone promoter activity induced by ligand activated TRα1. Moreover, real-time RT-PCR demonstrates that IGFBP-6 could inhibit the osteocalcin mRNA transcription induced by Triiodothyronine (3,3',5-Triiodo-L-thyronine, T3) in osteoblastic cells. Finally, alkaline phosphatase activity was significantly decreased in osteoblastic cells when the cells were transfected with IGFBP-6 in the presence of T3. In conclusion, these studies provide evidence that overexpression of IGFBP-6 suppresses osteoblastic differentiation regulated by TR in the present of T3.

The retinoid X receptors and their ligands

This chapter presents an overview of the current status of studies on the structural and molecular biology of the retinoid X receptor subtypes α, β, and γ (RXRs, NR2B1-3), their nuclear and cytoplasmic functions, post-transcriptional processing, and recently reported ligands. Points of interest are the different changes in the ligand-binding pocket induced by variously shaped agonists, the communication of the ligand-bound pocket with the coactivator binding surface and the heterodimerization interface, and recently identified ligands that are natural products, those that function as environmental toxins or drugs that had been originally designed to interact with other targets, as well as those that were deliberately designed as RXR-selective transcriptional agonists, synergists, or antagonists. Of these synthetic ligands, the general trend in design appears to be away from fully aromatic rigid structures to those containing partial elements of the flexible tetraene side chain of 9-cis-retinoic acid. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

IGFBP-2 enhances VEGF gene promoter activity and consequent promotion of angiogenesis by neuroblastoma cells

IGF binding protein (IGFBP)-2 is one of the most significant genes in the signature of major aggressive cancers. Previously, we have shown that IGFBP-2 enhances proliferation and invasion of neuroblastoma cells, suggesting that IGFBP-2 activates a protumorigenic gene expression program in these cells. Gene expression profiling in human neuroblastoma SK-N-SHEP (SHEP)-BP-2 cells indicated that IGFBP-2 overexpression activated a gene expression program consistent with enhancement of tumorigenesis. Regulation was significant for genes involved in proliferation/survival, migration/adhesion, and angiogenesis, including the up-regulation of vascular endothelial growth factor (VEGF) mRNA (>2-fold). Specific transcriptional activation of the VEGF gene by IGFBP-2 overexpression was demonstrated via cotransfection of a VEGF promoter Luciferase construct in SHEP-BP-2. Cotransfection of VEGF promoter Luciferase construct with IGFBP-2 protein in wild-type SHEP cells indicated that transactivation of VEGF promoter only occurs in the presence of intracellular IGFBP-2. Cell fractionation and immunofluorescence in SHEP-BP-2 cells demonstrated nuclear localization of IGFBP-2. These findings suggest that transcriptional activation of VEGF promoter is likely to be mediated by nuclear IGFBP-2. The levels of secreted VEGF (up to 400 pg/10(6) cells) suggested that VEGF might elicit angiogenic activity. Hence, SHEP-BP-2 cells and control clones cultured in collagen sponge were xenografted onto chick embryo chorioallantoic membrane. Neomicrovascularization was observed by 72 h, solely in the SHEP-BP-2 cell xenografts. In conclusion, our data indicate that IGFBP-2 is an activator of aggressive behavior in cancer cells, involving nuclear entry and activation of a protumorigenic gene expression program, including transcriptional regulation of the VEGF gene and consequent proangiogenic activity of NB cell xenografts in vivo.

Insulin-like growth factor binding protein-6 delays replicative senescence of human fibroblasts

Cellular senescence can be induced by a variety of mechanisms, and recent data suggest a key role for cytokine networks to maintain the senescent state. Here, we have used a proteomic LC-MS/MS approach to identify new extracellular regulators of senescence in human fibroblasts. We identified 26 extracellular proteins with significantly different abundance in conditioned media from young and senescent fibroblasts. Among these was insulin-like growth factor binding protein-6 (IGFBP-6), which was chosen for further analysis. When IGFBP-6 gene expression was downregulated, cell proliferation was inhibited and apoptotic cell death was increased. Furthermore, downregulation of IGFBP-6 led to premature entry into cellular senescence. Since IGFBP-6 overexpression increased cellular lifespan, the data suggest that IGFBP-6, in contrast to other IGF binding proteins, is a negative regulator of cellular senescence in human fibroblasts.

A novel interaction between insulin-like growth factor binding protein-6 and the vitamin D receptor inhibits the role of vitamin D3 in osteoblast differentiation

Insulin-like growth factor binding protein-6 (IGFBP-6) is a secreted glycoprotein that reduces the bioavailability of IGFs. It has both IGF-dependent and -independent effects on cell growth, however the mechanisms responsible for its IGF-independent actions of IGFBP-6 are not fully understood. In previous studies, we have shown that recombinant IGFBP-6 can be internalized and translocated to the nucleus. The present study shows that IGFBP-6 interacts with the vitamin D receptor (VDR). Physical interactions between IGFBP-6 and the VDR were confirmed by GST pulldown and co-immunoprecipitation assays. We also determined that the interaction binding sites were on the C-terminal region of the VDR. This interaction can influence retinoid X receptor (RXR):VDR heterodimerization. Furthermore, immunofluorescence colocalization studies showed that IGFBP-6 colocalized with the VDR predominantly in the cell's nucleus. Inductions of osteocalcin and growth hormone promoter activities by 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) were significantly decreased when cells were co-transfected with IGFBP-6 and the VDR compared with cells transfected with the VDR only. Moreover, we found that alkaline phosphatase activity (ALP, a general marker of osteoblast differentiation) was significantly decreased in osteoblast-like cells when they were transfected with IGFBP-6 in the presence of 1,25(OH)(2)D(3). No obvious difference in ALP activity was observed when cells were transfected with IGFBP-6 and endogenous VDR was knocked down by siRNA. These results demonstrate that IGFBP-6 inhibits osteoblastic differentiation mediated by 1,25(OH)(2)D(3) and the VDR through interacting with the VDR and inhibiting its function. This is a novel mechanism for IGFBP-6.

Small is beautiful: insulin-like growth factors and their role in growth, development, and cancer

Insulin-like growth factors were discovered more than 50 years ago as mediators of growth hormone that effect growth and differentiation of bone and skeletal muscle. Interest of the role of insulin-like growth factors in cancer reached a peak in the 1990s, and then waned until the availability in the past 5 years of monoclonal antibodies and small molecules that block the insulin-like growth factor 1 receptor. In this article, we review the history of insulin-like growth factors and their role in growth, development, organism survival, and in cancer, both epithelial cancers and sarcomas. Recent developments regarding phase I to II clinical trials of such agents are discussed, as well as potential studies to consider in the future, given the lack of efficacy of one such monoclonal antibody in combination with cytotoxic chemotherapy in a first-line study in metastatic non-small-cell lung adenocarcinoma. Greater success with these agents clinically is expected when combining the agents with inhibitors of other cell signaling pathways in which cross-resistance has been observed.

Nuclear export and mitochondrial and endoplasmic reticulum localization of IGF-binding protein 3 regulate its apoptotic properties

Tumor suppression by IGF-binding protein 3 (IGFBP3) may occur in an IGF-independent manner, in addition to its role as a regulator of IGF bioavailability. After secretion, IGFBP3 is internalized, rapidly localized to the nucleus, and is later detected in the cytoplasm. We identified a putative nuclear export sequence (NES) in IGFBP3 between amino acids 217 and 228, analogous to the leucine-rich NES sequence of p53 and HIV Rev. Mutation of the NES prevents nucleocytoplasmic shuttling of IGFBP3 and blocks its ability to induce apoptosis. Targeting of IGFBP3 to the mitochondria and endoplasmic reticulum (ER) was confirmed by co-localization with organelle markers using fluorescence confocal microscopy and subcellular fractionation. Mitochondrial targeting was also demonstrated in vivo in IGFBP3-treated prostate cancer xenografts. These results show that IGFBP3 shuttles from the nucleus to the mitochondria and ER, and that nuclear export is essential for its effects on prostate cancer apoptosis.

Identification of a novel cell death receptor mediating IGFBP-3-induced anti-tumor effects in breast and prostate cancer

Insulin-like growth factor-binding protein-3 (IGFBP-3), a major regulator of endocrine actions of IGFs, is a p53-regulated potent apoptotic factor and is significantly suppressed in a variety of cancers. Recent epidemiologic studies suggest that IGFBP-3 contributes to cancer risk protection in a variety of cancers, and a polymorphic variation of IGFBP-3 influences cancer risk, although other studies vary in their conclusions. Some antiproliferative actions of IGFBP-3 have been reported to be independent of IGFs, but the precise biochemical/molecular mechanisms of IGF-independent, antiproliferative actions of IGFBP-3 are largely unknown. Here we report a new cell death receptor, IGFBP-3R, that is a single-span membrane protein and binds specifically to IGFBP-3 but not other IGFBP species. Expression analysis of IGFBP-3 and IGFBP-3R indicates that the IGFBP-3/IGFBP-3R axis is impaired in breast and prostate cancer. We also provide evidence for anti-tumor effect of IGFBP-3R in vivo using prostate and breast cancer xenografts in athymic nude mice. Further in vitro studies demonstrate that IGFBP-3R mediates IGFBP-3-induced caspase-8-dependent apoptosis in various cancer cells. Knockdown of IGFBP-3R attenuated IGFBP-3-induced caspase activities and apoptosis, whereas overexpression of IGFBP-3R enhanced IGFBP-3 biological effects. IGFBP-3R physically interacts and activates caspase-8, and knockdown of caspase-8 expression or activity inhibited IGFBP-3/IGFBP-3R-induced apoptosis. Here, we propose that IGFBP-3R represents a novel cell death receptor and is essential for the IGFBP-3-induced apoptosis and tumor suppression. Thus, the IGFBP-3/IGFBP-3R axis may provide therapeutic and prognostic value for the treatment of cancer.

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.

GalNAc-T14 may be involved in regulating the apoptotic action of IGFBP-3

Insulin-like growth factor binding protein-3 (IGFBP-3) is known to induce apoptosis in an insulin-like growth factor (IGF)-dependent and IGF-independent manner, but the mechanism underlying the IGF-independent effects remains unclear. Polypeptide N-acetylgalactosaminyltransferase 14 (GalNAc-T14) is a novel IGFBP-3 binding partner. In this paper, small interference RNA (siRNA) targeting GalNAc-T14 was used to examine whether GalNAc-T14 affects the apoptotic action of IGFBP-3. Using semi-quantitative reverse-transcriptase polymerase chain reaction (RT-PCR) and western blot analysis, we determined that GalNAc-T14 expression was downregulated by the siRNA directed against GalNAc-T14. Apoptosis analysis of IGFBP-3-overexpressing cells treated with siRNA against GalNAc-T14 was performed to determine if GalNAc-T14 was specifically involved in IGFBP-3 signalling. The results, as determined by flow cytometric analysis and caspase-3 assay, showed that the extent of apoptosis induced by IGFBP-increased with RNA interference (RNAi) knockdown of GalNAc-T14. Our data suggest that GalNAc-T14 influences the apoptotic action of IGFBP-3 and might mediate the signalling pathway of IGFBP-3. Experiments to determine the role of GalNAc-T14 in the regulation of apoptosis induced by IGFBP-3 are under way.

Unraveling insulin-like growth factor binding protein-3 actions in human disease

The IGF system plays critical roles in somatic growth in an endocrine fashion (somatomedin hypothesis) as well as proliferation and differentiation of normal and malignant cells in a paracrine/autocrine fashion. IGFBP-3 is known to modulate the actions of IGFs in circulation as well as the immediate extracellular environment. Interestingly, apart from the ability to inhibit or enhance IGF actions, IGFBP-3 also exhibits very clear, distinct biological effects independent of the IGF/IGF-I receptor axis. Over the past decade it has become widely appreciated that IGF/IGF-IR-independent actions of IGFBP-3 (antiproliferative and proapoptotic effects) contribute to improving the pathophysiology of a variety of human diseases, such as cancer, diabetes, and malnutrition. Recent studies have implicated interaction of IGFBP-3 with a variety of proteins or signaling cascades critical to cell cycle control and apoptosis; however, the actual mechanism of IGFBP-3 action is still unclear. This review reinforces the concept in support of the IGF/IGF-IR axis-independent actions of IGFBP-3 and delineates potential underlying mechanisms involved and subsequent biological significance, focusing in particular on functional binding partners and the clinical significance of IGFBP-3 in the assessment of cancer risk.

Potentiation of growth factor signaling by insulin-like growth factor-binding protein-3 in breast epithelial cells requires sphingosine kinase activity

We have investigated the mechanism underlying potentiation of epidermal growth factor receptor (EGFR) and type 1 insulin-like growth factor receptor (IGFR1) signaling by IGF-binding protein-3 (IGFBP-3) in MCF-10A breast epithelial cells, focusing on a possible involvement of the sphingosine kinase (SphK) system. IGFBP-3 potentiated EGF-stimulated EGF receptor activation and DNA synthesis, and this was blocked by inhibitors of SphK activity or small interference RNA-mediated silencing of SphK1, but not SphK2, expression. Similarly, IGFR1 phosphorylation and DNA synthesis stimulated by LR3-IGF-I (an IGF-I analog not bound by IGFBP-3), were enhanced by IGFBP-3, and this was blocked by SphK1 silencing. SphK1 expression and activity were stimulated by IGFBP-3 approximately 2-fold over 24 h. Silencing of sphingosine 1-phosphate receptor 1 (S1P1) or S1P3, but not S1P2, abolished the effect of IGFBP-3 on EGF-stimulated EGFR activation. The effects of IGFBP-3 could be reproduced with exogenous S1P or medium conditioned by cells treated with IGFBP-3, and this was also blocked by inhibition of S1P1 and S1P3. These data indicate that potentiation of growth factor signaling by IGFBP-3 in MCF-10A cells requires SphK1 activity and S1P1/S1P3, suggesting that S1P, the product of SphK activity and ligand for S1P1 and S1P3, is the "missing link" mediating IGF and EGFR transactivation and cell growth stimulation by IGFBP-3.

Inhibition of adipocyte differentiation by insulin-like growth factor-binding protein-3

Insulin-like growth factor-binding protein-3 (IGFBP-3) interacts with the type II nuclear receptors retinoid X receptor (RXR)alpha and retinoic acid receptor-alpha and modulates their transcriptional activity. Peroxisome proliferator-activated receptor (PPAR)gamma, a related nuclear receptor that dimerizes with RXRalpha, plays an important role in adipocyte differentiation. IGFBP-3 is regulated during adipocyte differentiation, but its role in this process is unknown. We demonstrate that IGFBP-3 interferes with the PPARgamma-dependent processes of adipocyte differentiation and maintenance of the gene expression characteristic of mature adipocytes. Treatment of adipocytes with exogenous IGFBP-3, but not an IGFBP-3 mutant that does not bind RXRalpha or PPARgamma, decreased markers of adipocyte differentiation, PPARgamma, and resistin but increased the preadipocyte marker plasminogen activator inhibitor-1. Furthermore, expression of human IGFBP-3, but not the IGFBP-3 mutant, by preadipocytes inhibited preadipocyte differentiation as determined by gene markers and lipid accumulation. IGFBP-3 interacted with PPARgamma in vitro and in 3T3-L1 adipocyte lysates and inhibited PPARgamma heterodimerization with RXRalpha in vitro. Wild-type IGFBP-3, but not mutant IGFBP-3, blocked ligand-induced transactivation of PPAR response element in 3T3-L1 cells. The observation that IGFBP-3 inhibits adipocyte differentiation and impacts on the PPARgamma system suggests a role for IGFBP-3 in the pathogenesis of obesity and insulin resistance.

IGFBP-3, hypoxia and TNF-alpha inhibit adiponectin transcription

The thiazolidinedione rosiglitazone, an agonist ligand for the nuclear receptor PPAR-gamma, improves insulin sensitivity in part by stimulating transcription of the insulin-sensitizing adipokine adiponectin. It activates PPAR-gamma-RXR-alpha heterodimers bound to PPAR-gamma response elements in the adiponectin promoter. Rosiglitazone-stimulated adiponectin protein synthesis in 3T3-L1 mouse adipocytes has been shown to be inhibited by IGFBP-3, which can be induced by hypoxia and the proinflammatory cytokine, TNF-alpha, two inhibitors of adiponectin transcription. The present study demonstrates that IGFBP-3, the hypoxia-mimetic agent cobalt chloride, and TNF-alpha inhibit rosiglitazone-induced adiponectin transcription in mouse embryo fibroblasts that stably express PPAR-gamma2. Native IGFBP-3 can bind RXR-alpha and inhibited rosiglitazone stimulated promoter activity, whereas an IGFBP-3 mutant that does not bind RXR-alpha did not. These results suggest that IGFBP-3 may mediate the inhibition of adiponectin transcription by hypoxia and TNF-alpha, and that IGFBP-3 binding to RXR-alpha may be required for the observed inhibition.

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.

Subcellular distribution of the insulin-like growth factor (IGF) binding proteins (IGFBPs) 2 and 3 in articular chondrocytes

The insulin-like growth factor (IGF) is a major anabolic regulator in articular cartilage. The IGF-binding proteins (IGFBPs) are increased during osteoarthritis (OA), but the function of the later proteins remains unknown. In general, the IGFBPs are pluripotential effectors capable of IGF regulation and of acting on their own to control key cell functions, including survival and proliferation. The independent functions are often associated with their cell location, and therefore this study explores the distribution of IGFBP-2 and IGFBP-3 in articular chondrocytes. Immunohistochemistry was used to localize IGFBP-2 in normal human articular cartilage. Bovine chondrocytes were used for subcellular fractionation (hypotonic cell lysis) under nonreducing conditions and nuclear purification (centrifugation on sucrose cushions). Cell fraction markers and IGFBPs were assayed in the subcellular fractions by Western immunoblot. The IHC results showed association of IGFBP-2 with chondrocytes, but not with the nuclei. Subcellular fractionation of isolated chondrocytes yielded intact nuclei as assessed at the light microscopic level; the nuclear marker histone H1 was exclusively associated with this fraction. More than 90% of the cytoplasmic marker GAPDH and all the detectable IGFBP-2 were in the cytoplasmic fraction. Immunoreactive IGFBP-3 was found in the cytoplasmic and peri-nuclear/nuclear fractions. Chondrocytes contain intracellular IGFBP-2 and IGFBP-3 but only IGFBP-3 is associated with nuclei. This suggests the hypothesis that the actions of these IGFBPs in articular cartilage extend beyond the classic modulation of IGF receptor action.

Local IGFBP-3 mRNA expression, apoptosis and risk of colorectal adenomas

Background

IGF binding protein-3 (IGFBP-3) regulates the bioavailability of insulin-like growth factors I and II, and has both anti-proliferative and pro-apoptotic properties. Elevated plasma IGFBP-3 has been associated with reduced risk of colorectal cancer (CRC), but the role of tissue IGFBP-3 is not well defined. We evaluated the association between tissue or plasma IGFBP-3 and risk of colorectal adenomas or low apoptosis.

Methods

Subjects were consenting patients who underwent a clinically indicated colonoscopy at UNC Hospitals and provided information on diet and lifestyle. IGFBP-3 mRNA in normal colon was assessed by real time RT-PCR. Plasma IGFBP-3 was measured by ELISA and apoptosis was determined by morphology on H & E slides. Logistic regression was used to compute odds ratio (OR) and 95% confidence intervals.

Results

We observed a modest correlation between plasma IGFBP-3 and tissue IGFBP-3 expression (p = 0.007). There was no significant association between plasma IGFBP-3 and adenomas or apoptosis. Tissue IGFBP-3 mRNA expression was significantly lower in cases than controls. Subjects in the lowest three quartiles of tissue IGFBP-3 gene expression were more likely to have adenomas. Consistent with previous reports, low apoptosis was significantly associated with increased risk of adenomas (p = 0.003). Surprisingly, local IGFBP-3 mRNA expression was inversely associated with apoptosis.

Conclusion

Low expression of IGFBP-3 mRNA in normal colonic mucosa predicts increased risk of adenomas. Our findings suggest that local IGFBP-3 in the colon may directly increase adenoma risk but IGFBP-3 may act through a pathway other than apoptosis to influence adenoma risk.

Induction of apoptosis in human prostate cancer cells by insulin-like growth factor binding protein-3 does not require binding to retinoid X receptor-alpha

IGF binding protein (IGFBP)-3 can induce apoptosis in human prostate cancer cells directly without sequestering IGF-I and -II. The molecular mechanisms responsible for the IGF-independent actions of IGFBP-3 remain unclear. IGFBP-3, a secreted protein, can be internalized and translocate to the nucleus. It binds to the nuclear retinoid X receptor (RXR)-alpha. Binding to RXR-alpha has been proposed to be required for IGFBP-3 to induce apoptosis. The present study tests this hypothesis in the PC-3 human prostate cancer cell line. PC-3 cells express RXR-alpha, and apoptosis is induced by incubation with RXR-specific ligand. A COOH-terminal region in IGFBP-3 (residues 215-232) contains a nuclear localization signal, and binding domains for RXR-alpha and heparin (HBD). Different combinations of the 11 amino acids in this region that differ from IGFBP-1, a related IGFBP, which does not localize to the nucleus or bind RXR-alpha, were mutated to the IGFBP-1 sequence. By confocal imaging, mutation of residues 228-KGRKR-232 in nonsecreted IGFBP-3 diminished its nuclear localization. IGFBP-3 binding to glutathione S-transferase-RXR-alpha only was lost when all 11 sites were mutated (HBD-11m-IGFBP-3). Expressed nuclear RXR-alpha did not transport cytoplasmic IGFBP-3 nuclear localization signal mutants that can bind RXR-alpha to the nucleus even after treatment with RXR ligand. Expressed HBD-11m-IGFBP-3 still induced apoptosis in PC-3 cells in an IGF-independent manner as determined by flow cytometric analysis of Annexin V staining. We conclude that in PC-3 cells, RXR-alpha is not required for the nuclear translocation of IGFBP-3 and that IGFBP-3 can induce apoptosis in human prostate cancer cells without binding RXR-alpha.

Insulin-like growth factor binding protein-5 interacts with the vitamin D receptor and modulates the vitamin D response in osteoblasts

The 1,25 dihydroxyvitamin D3 [1,25(OH)2D3]-induced differentiation of osteoblasts comprises the sequential induction of cell cycle arrest at G0/G1 and the expression of bone matrix proteins. Reports differ on the effects of IGF binding protein (IGFBP)-5 on bone cell growth and osteoblastic function. IGFBP-5 can be growth stimulatory or inhibitory and can enhance or impair osteoblast function. In previous studies, we have shown that IGFBP-5 localizes to the nucleus and interacts with the retinoid receptors. We now show that IGFBP-5 interacts with nuclear vitamin D receptor (VDR) and blocks retinoid X receptor (RXR):VDR heterodimerization. VDR and IGFBP-5 were shown to colocalize to the nuclei of MG-63 and U2-OS cells and coimmunoprecipitate in nuclear extracts from these cells. Induction of osteocalcin promoter activity and alkaline phosphatase activity by 1,25(OH)2D3 were significantly enhanced when IGFBP-5 was down-regulated in U2-OS cells. Moreover, we found IGFBP-5 increased basal alkaline phosphatase activity and collagen alpha1 type 1 expression, and that 1,25(OH)2D3 was unable to further induce the expression of these bone differentiation markers in MG-63 cells. Expression of IGFBP-5 inhibited MG-63 cell growth and caused cell cycle arrest at G0/G1 and G2/M. Furthermore, IGFBP-5 reduced the effects of 1,25(OH)2D3 in blocking cell cycle progression at G0/G1 and decreased the expression of cyclin D1. These results demonstrate that IGFBP-5 can interact with VDR to prevent RXR:VDR heterodimerization and suggest that IGFBP-5 may attenuate the 1,25(OH)2D3-induced expression of bone differentiation markers while having a modest effect on the 1,25(OH)2D3-mediated inhibition of cell cycle progression in bone cells.

Molecular basis of the interaction between IGFBP-3 and retinoid X receptor: role in modulation of RAR-signaling

IGFBP-3 interacts with the retinoid X receptor-alpha (RXRalpha) and retinoic acid receptor-alpha (RARalpha) and thereby interferes with the formation of RXR:RAR heterodimers. Here we identify the domains in RXRalpha and IGFBP-3 that participate in this interaction. When different regions of RXRalpha were expressed independently, we found that only the DNA-binding domain (C-domain) bound IGFBP-3. Residues in the second Zn-finger loop (Gln49, Arg52), which contribute to C-domain dimerization on DR1 response elements, proved essential to IGFBP-3 binding. In complementary studies, we found that residues within the N-terminal domain of IGFBP-3 (Thr58, Arg60) and motifs in its C-terminal domain ((220)LysLysLys, (228)LysGlyArgLysArg) were required for interaction with RXRalpha and RARalpha. Unlike wild-type IGFBP-3, the non-retinoid receptor-binding mutants of IGFBP-3 were unable to attenuate all-trans-retinoic acid-induced transactivation of the RAR response element by RXR:RAR heterodimers. We conclude that residues in both the N- and C-terminal domains of IGFBP-3 are involved in binding the retinoid receptors, and that this interaction is essential to the modulation of RAR-signaling by IGFBP-3.

The in vivo phosphorylation and glycosylation of human insulin-like growth factor-binding protein-5

Mass spectrometry is often used to determine post-translational modifications by analysis of tryptic digests of proteins. Here we demonstrate that the analysis of tryptic peptides together with analysis of the full-length protein provided optimal characterization of insulin-like growth factor-binding protein-5 (IGFBP-5) phosphorylation and glycosylation. IGFBP-5 binds insulin-like growth factors with high affinity and has important roles in cell survival, differentiation, and apoptosis. Until now, the primary structure of IGFBP-5 has been incompletely defined. We analyzed human IGFBP-5 from T47D cells by mass spectrometry to determine all of the in vivo post-translational modifications. In full-length IGFBP-5, 31% of the protein was unmodified, 37% was monophosphorylated, and 4% was diphosphorylated with no other modification. The remaining 27% was glycosylated, more than half of which was also monophosphorylated. The major phosphorylation site was Ser(96) in the central domain, and a minor phosphorylation site was Ser(248) near the C terminus. Neither site was phosphorylated in vitro by casein kinase 2, ruling it out as the in vivo kinase. An in vivo phosphorylation site was also found in IGFBP-2 at an analogous position, Ser(106). IGFBP-5 was heterogeneously O-glycosylated mainly by sialylated core 1 type glycans. The most abundant structure contained N-acetylhexosamine, hexose, and two N-acetylneuraminic acid carbohydrates. A small amount of sialylated core 2 type glycan was also present. Phosphorylation and O-glycosylation both affected IGFBP-5 binding to heparin but not insulin-like growth factor binding or ternary complex formation with the acid-labile subunit. The results reveal the first description of the in vivo phosphorylation of IGFBP-5 and its glycan composition.

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.

IGFBP-3 and IGFBP-10 (CYR61) up-regulation during the development of Barrett's oesophagus and associated oesophageal adenocarcinoma: potential biomarkers of disease risk

Dys-regulation of the insulin-like growth factor (IGF) system increases the risk of a number of malignancies. The aim of this study was to investigate the role of members of the IGF binding protein (IGFBP) superfamily in the development of oesophageal adenocarcinoma (EAC) and their possible use as markers of disease risk. Expression of IGFBP-2, IGFBP-3, IGFBP-4, and IGFBP-10/CYR61 was assessed using Real-Time-polymerase chain reaction (PCR) and immunohistochemistry in oesophageal tissues from Barrett's oesophagus (BE) patients with and without associated EAC, and in control subjects. IGFBP-3, IGFBP-4, and IGFBP-10/CYR61 mRNA levels were up-regulated in Barrett's (n=17) and tumour tissue of EAC patients (n=18) compared with normal tissue of control subjects without BE or EAC (n=18) (p<0.001). Over-expression of IGFBP-3 and IGFBP-10/CYR61 proteins was observed in Barrett's, dysplastic and tumour tissue of EAC cases (n=47 for IGFBP-10; n=39 for IGFBP-3) compared with adjacent normal epithelium (p<0.050). Notably, IGFBP-3, IGFBP-4, and IGFBP-10/CYR61 expression in Barrett's tissue of EAC cases (n=17) was significantly (p<0.001) higher than in Barrett's tissue of BE patients with no sign of progression to cancer (n=15). Overall, the results suggest that members of the IGFBP superfamily are up-regulated during oesophageal carcinogenesis and merit further investigation as markers of EAC risk.

Early effects of imatinib mesylate on the expression of insulin-like growth factor binding protein-3 and positron emission tomography in patients with gastrointestinal stromal tumor

BACKGROUND

Imatinib has demonstrated marked clinical efficacy against gastrointestinal stromal tumor (GIST). Microarray technology, real-time polymerase chain reaction (PCR) validation, and fluorodeoxyglucose-positron emission tomography (FDG-PET) imaging were used to study the early molecular effects of imatinib antitumor activity in GIST.

METHODS

After exposure of sensitive and resistant sarcoma cell lines to imatinib for 24 to 48 hours, the changes in gene expression were evaluated using a 1146 unique pathway array with Western blot validation. Real-time PCR was used to confirm changes in gene expression in human GIST samples (preimatinib biopsy and postimatinib surgical specimen after 3-7 days of therapy). FDG-PET was performed to correlate radiographic findings with the effects of imatinib on gene expression in GIST.

RESULTS

In all, 55 genes demonstrated a > or = 2-fold change after imatinib treatment of the GIST882 cells. Among these genes there was up-regulation of insulin-like growth factor binding protein-3 (IGFBP-3), a protein that modulates proliferation and apoptosis. Western blot analysis confirmed the increase of IGFBP-3 only in imatinib-sensitive GIST882 cells. Up to a 7-fold induction (49% mean increase; P = .08) of IGFBP-3 mRNA was found in tumor samples from patients with low residual FDG uptake, whereas there was an up to 12-fold reduction (-102% mean decrease; P = .03) in IGFBP-3 in those patients with high residual FDG uptake after imatinib therapy.

CONCLUSIONS

In the current study, imatinib appears to regulate numerous genes and specifically induces IGFBP-3 in GIST cells and tumor samples. IGFBP-3 levels also were found to be inversely correlated with residual FDG uptake in GIST patients early in imatinib therapy. These initial observations suggest that IGFBP-3 is an important early marker of antitumor activity of imatinib in GIST.

Phosphorylation by DNA-dependent protein kinase is critical for apoptosis induction by insulin-like growth factor binding protein-3

Insulin-like growth factor (IGF) binding protein-3 (IGFBP-3) promotes apoptosis of cancer cells by both IGF-dependent and IGF-independent mechanisms. In vitro phosphorylation of IGFBP-3 by DNA-dependent protein kinase (DNA-PK) has been reported but with unknown functional relevance. Using a chemical inhibitor for DNA-PK in prostate cancer cells and a paired system of glioblastoma cell lines that either lack or express DNA-PK, we show that the apoptosis-promoting and growth-inhibitory actions of IGFBP-3 are completely abrogated in the absence of catalytically active DNA-PK. In the absence of DNA-PK activity, IGFBP-3 has reduced nuclear accumulation and is unable to bind its nuclear binding partner retinoid X receptor (RXR) alpha. We assessed the importance of the three potential DNA-PK phosphorylation sites in IGFBP-3 using PCR-based site-directed mutagenesis. When transfected into 22RV1 cells, IGFBP-3-S165A and IGFBP-3-T170A functioned in an identical manner to wild-type IGFBP-3 to induce apoptosis. In contrast, IGFBP-3-S156A was unable to promote apoptosis and exhibited reduced nuclear accumulation, suggesting a key role for DNA-PK-dependent phosphorylation in the regulation of IGFBP-3 action. These studies reveal a novel regulatory mechanism for the actions of IGFBP-3 in prostate cancer and show phosphorylation of Ser(156) to be functionally critical in its apoptosis-inducing actions.