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

IGFBPs mediate IGF-1's functions in retinal lamination and photoreceptor development during pluripotent stem cell differentiation to retinal organoids

Development of the retina is regulated by growth factors, such as insulin-like growth factors 1 and 2 (IGF-1/2), which coordinate proliferation, differentiation, and maturation of the neuroepithelial precursors cells. In the circulation, IGF-1/2 are transported by the insulin growth factor binding proteins (IGFBPs) family members. IGFBPs can impact positively and negatively on IGF-1, by making it available or sequestering IGF-1 to or from its receptor. In this study, we investigated the expression of IGFBPs and their role in the generation of human retinal organoids from human pluripotent stem cells, showing a dynamic expression pattern suggestive of different IGFBPs being used in a stage-specific manner to mediate IGF-1 functions. Our data show that IGF-1 addition to culture media facilitated the generation of retinal organoids displaying the typical laminated structure and photoreceptor maturation. The organoids cultured in the absence of IGF-1, lacked the typical laminated structure at the early stages of differentiation and contained significantly less photoreceptors and more retinal ganglion cells at the later stages of differentiation, confirming the positive effects of IGF-1 on retinal lamination and photoreceptor development. The organoids cultured with the IGFBP inhibitor (NBI-31772) and IGF-1 showed lack of retinal lamination at the early stages of differentiation, an increased propensity to generate horizontal cells at mid-stages of differentiation and reduced photoreceptor development at the later stages of differentiation. Together these data suggest that IGFBPs enable IGF-1's role in retinal lamination and photoreceptor development in a stage-specific manner.

Teratogenic effect of isotretinoin in both fertile females and males (Review)

Isotretinoin is an oral derivate of vitamin A that has been used since 1982 for the treatment of multiple dermatologic conditions such as severe acne, rosacea, scarring alopecia, ichthyosis or non-melanoma skin cancer prophylaxis. The recommended dose is 0.5-1 mg/kg/day for a period of 4-6 months in sebaceous gland pathologies. There are many adverse effects caused by isotretinoin but by far the most important is the teratogenicity induced by this drug which is estimated to have a 20-35% risk to infants that are exposed to isotretinoin in utero and includes numerous congenital defects such as craniofacial defects, cardiovascular and neurological malformations or thymic disorders. Isotretinoin induces apoptosis and cell cycle arrest in human sebocytes, emphasizing these as processes associated with its teratogenic effect. The aim of this review is to analyze the latest literature data regarding the teratogenic effect of isotretinoin for both fertile females and males and its biological effects underlying the occurrence of congenital malformations under the influence of isotretinoin.

Growth Hormone/Insulin Growth Factor Axis in Sex Steroid Associated Disorders and Related Cancers

To date, almost all solid malignancies have implicated insulin-like growth factor (IGF) signalling as a driver of tumour growth. However, the remarkable level of crosstalk between sex hormones, the IGF-1 receptor (IGF-1R) and its ligands IGF-1 and 2 in endocrine driven cancers is incompletely understood. Similar to the sex steroids, IGF signalling is essential in normal development as well as growth and tissue homoeostasis, and undergoes a steady decline with advancing age and increasing visceral adiposity. Interestingly, IGF-1 has been found to play a compensatory role for both estrogen receptor (ER) and androgen receptor (AR) by augmenting hormonal responses in the absence of, or where low levels of ligand are present. Furthermore, experimental, and epidemiological evidence supports a role for dysregulated IGF signalling in breast and prostate cancers. Insulin-like growth factor binding protein (IGFBP) molecules can regulate the bioavailability of IGF-1 and are frequently expressed in these hormonally regulated tissues. The link between age-related disease and the role of IGF-1 in the process of ageing and longevity has gained much attention over the last few decades, spurring the development of numerous IGF targeted therapies that have, to date, failed to deliver on their therapeutic potential. This review will provide an overview of the sexually dimorphic nature of IGF signalling in humans and how this is impacted by the reduction in sex steroids in mid-life. It will also explore the latest links with metabolic syndromes, hormonal imbalances associated with ageing and targeting of IGF signalling in endocrine-related tumour growth with an emphasis on post-menopausal breast cancer and the impact of the steroidal milieu.

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.

IGFBP-3 Interacts with the Vitamin D Receptor in Insulin Signaling Associated with Obesity in Visceral Adipose Tissue

Adipose tissue has traditionally only been considered as an energy storage organ. Nevertheless, the importance of this tissue in systemic physiology and, especially, in systemic inflammation has been highlighted in recent years. Adipose tissue expresses proteins related to vitamin D (VD) metabolism, and it has been proposed that it can act as a VD storage tissue. The active form of VD, 1,25-dihydroxyvitamin D3 (1,25(OH)₂D₃), is able to modify adipocyte and adipose tissue physiology via the VD receptor (VDR), decreasing the expression of pro-inflammatory cytokines in adipose tissue. Moreover, VD deficiency and VDR has been reported to be associated with obesity and diabetes. However, the results of the different studies are not conclusive. Insulin growth binding proteins (IGFBPs) have been identified in adipose tissue, but their roles are poorly understood. Therefore, the objective of this study was to analyze the plasma levels of VD and the gene expression of VDR in the adipose tissue of subjects with morbid obesity (MO) and with different degrees of insulin resistance (IR), as well as the functionality of direct interaction between IGFBP-3 and VDR, which could explain its inhibitory role in adipogenesis. Our results show a novel role of the VD system in the regulation and activation of IGFBP-3 in visceral adipose tissue (VAT) of patients with MO, as a new and alternative mechanism proposed in the insulin signaling associated with obesity.

p53: key conductor of all anti-acne therapies

This review based on translational research predicts that the transcription factor p53 is the key effector of all anti-acne therapies. All-trans retinoic acid (ATRA) and isotretinoin (13-cis retinoic acid) enhance p53 expression. Tetracyclines and macrolides via inhibiting p450 enzymes attenuate ATRA degradation, thereby increase p53. Benzoyl peroxide and hydrogen peroxide elicit oxidative stress, which upregulates p53. Azelaic acid leads to mitochondrial damage associated with increased release of reactive oxygen species inducing p53. p53 inhibits the expression of androgen receptor and IGF-1 receptor, and induces the expression of IGF binding protein 3. p53 induces FoxO1, FoxO3, p21 and sestrin 1, sestrin 2, and tumour necrosis factor-related apoptosis-inducing ligand (TRAIL), the key inducer of isotretinoin-mediated sebocyte apoptosis explaining isotretinoin's sebum-suppressive effect. Anti-androgens attenuate the expression of miRNA-125b, a key negative regulator of p53. It can thus be concluded that all anti-acne therapies have a common mode of action, i.e., upregulation of the guardian of the genome p53. Immortalized p53-inactivated sebocyte cultures are unfortunate models for studying acne pathogenesis and treatment.

Non-viral strategies for ocular gene delivery

The success of gene therapy relies on efficient gene transfer and stable transgene expression. The in vivo efficiency is determined by the delivery vector, route of administration, therapeutic gene, and target cells. While some requirements are common to several strategies, others depend on the target disease and transgene product. Consequently, it is unlikely that a single system is suitable for all applications. This review examines current gene therapy strategies, focusing on non-viral approaches and the use of natural polymers with the eye, and particularly the retina, as their gene delivery target.

Suppression of IGF binding protein-3 by palmitate promotes hepatic inflammatory responses

IGF-binding protein-3 (IGFBP-3) is a liver-derived, anti-inflammatory molecule that is decreased in obesity, a key risk factor for nonalcoholic fatty liver disease (NAFLD). It was not known whether IGFBP-3 levels were altered in NAFLD, whether such alterations could be the result of lipotoxicity, and whether altered IGFBP-3 could affect pathways that are involved in hepatic and systemic inflammation. Serum IGFBP-3 was decreased in patients with NAFLD, whereas liver and circulating IL-8 levels were increased. Palmitate inhibited IGFBP-3 secretion by THP-1 macrophages and enhanced IL-8 expression. Exposure of palmitate-treated THP-1 macrophages to IGFBP-3-deficient conditioned medium led to a 20-fold increase in palmitate-induced IL-8 expression by hepatocytes. Conversely, overexpression of IGFBP-3 suppressed JNK and NF-κB activation and blocked palmitate-induced IL-8 expression in hepatocytes. Silencing IGFBP-3 in Huh7 cells enhanced JNK and NF-κB activity and increased palmitate-induced IL-8 secretion. These data indicate that IGFBP-3 serves as an anti-inflammatory brake in hepatocytes against JNK and NF-κB and limits their activation and downstream production of proinflammatory cytokines. Under lipotoxic conditions, palmitate inhibits hepatic macrophage secretion of IGFBP-3, thereby releasing the brake and enhancing palmitate-induced IL-8 synthesis and secretion.-Min, H.-K., Maruyama, H., Jang, B. K., Shimada, M., Mirshahi, F., Ren, S., Oh, Y., Puri, P., Sanyal, A. J. Suppression of IGF binding protein-3 by palmitate promotes hepatic inflammatory responses.

Endogenous IGFBP-3 Mediates Intrinsic Apoptosis Through Modulation of Nur77 Phosphorylation and Nuclear Export

In nontransformed bovine mammary epithelial cells, the intrinsic apoptosis inducer anisomycin (ANS) induces IGFBP-3 expression and nuclear localization and knockdown of IGFBP-3 attenuates ANS-induced apoptosis. Others have shown in prostate cancer cells that exogenous IGFBP-3 induces apoptosis by facilitating nuclear export of the orphan nuclear receptor Nur77 and its binding partner, retinoid X receptor-α (RXRα). The goal of the present work was to determine whether endogenous IGFBP-3 plays a role in ANS-induced apoptosis by facilitating nuclear transport of Nur77 and/or RXRα in nontransformed cells. Knockdown of Nur77 with siRNA decreased ANS-induced cleavage of caspase-3 and -7 and their downstream target, PARP, indicating a role for Nur77 in ANS-induced apoptosis. In cells transfected with IGFBP-3, IGFBP-3 associated with RXRα but not Nur77 under basal conditions, however, IGFBP-3 co-precipitated with phosphorylated forms of both proteins in ANS-treated cells. Indirect immunofluorescence and cell fractionation techniques showed that ANS induced phosphorylation and transport of Nur77 from the nucleus to the cytoplasm and these effects were attenuated by knockdown of IGFBP-3. These data suggest that endogenous IGFBP-3 plays a role in intrinsic apoptosis by facilitating phosphorylation and nuclear export of Nur77 to the cytoplasm where it exerts its apoptotic effect. Whether this mechanism involves a physical association between endogenous IGFBP-3 and Nur77 or RXRα remains to be determined.

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.

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.

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.

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.

Regulation of retinal endothelial cell apoptosis through activation of the IGFBP-3 receptor

The goal of this study was to investigate whether insulin-like growth factor binding protein-3 receptor (IGFBP-3 receptor) is required for IGFBP-3 to inhibit retinal endothelial cell (REC) apoptosis. REC were grown in normal glucose (5 mM) or high glucose medium (25 mM) for 3 days. Once cells reached confluence, they were transfected with an endothelial- specific IGFBP-3 plasmid DNA (non-IGF binding; IGFBP-3 NB) at 1 μg/ml for 24 h. Cell proteins were extracted and analyzed for IGFBP-3 receptor expression by Western blotting or use in coimmunoprecipitation or co-localization experiments for detection of IGFBP-3 and IGFBP-3 receptor binding. REC were also transfected with or without IGFBP-3 receptor siRNA before IGFBP-3NB plasmid DNA transfection. Cell lysates were processed for a cell death ELISA, a cleaved caspase 3 ELISA, and Western blotting to measure key pro- and anti-apoptotic markers: Bcl-xL, Bax, Cytochrome C and Akt. The IGFBP-3 receptor is present on REC. Overexpression of IGFBP-3 in REC significantly increased protein levels of IGFBP-3 receptor (p < 0.05). Significant increases in cell death were found in cells transfected with IGFBP-3 receptor siRNA versus not treated samples (p < 0.05). Data suggest that IGFBP-3 inhibits retinal endothelial cell death through activation of an IGFBP-3 receptor in a hyperglycemic environment. This is the first demonstration of the involvement of IGFBP-3 receptor in inhibition of REC cell death. Future studies will investigate the mechanism by which IGFBP-3 receptor may inhibit retinal endothelial cell death.

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.

IGF binding protein-3 mediates stress-induced apoptosis in non-transformed mammary epithelial cells

Mammary epithelial cell (MEC) number is an important determinant of milk production in lactating dairy cows. IGF-I increases IGF binding protein-3 (IGFBP-3) production in these cells, which plays a role in its ability to enhance proliferation. In the present study, we show that the apoptotic factor anisomycin (ANS) also increases IGFBP-3 mRNA and protein in a dose- and concentration-dependent manner that mirrors activation of caspase-3 and -7, with significant increases in both IGFBP-3 protein and caspase activation observed by 3 h. Knock-down of IGFBP-3 with small interfering (si) RNA attenuated the ability of ANS to induce apoptosis, while knock-down of IGFBP-2, the other major IGFBP made by bovine MEC, had no effect. Reducing IGFBP-3 also decreased the ability of ANS to induce mitochondrial cytochrome c release, indicating its involvement in the intrinsic apoptotic pathway. In contrast, transfection with IGFBP-3 in the absence of ANS failed to induce apoptosis. Since both the mitogen IGF-I and the apoptotic inducer ANS increase IGFBP-3 production in MEC, we proposed that cellular localization might determine IGFBP-3 action. While both IGF-I and ANS stimulated the release of IGFBP-3 into conditioned media, only ANS induced nuclear localization of IGFBP-3. A pan-caspase inhibitor had no effect on ANS-induced nuclear localization of IGFBP-3, indicating that nuclear entry of IGFBP-3 precedes caspase activation. Treatment with IGF-I had no effect on ANS-induced nuclear localization, but did block ANS-induced apoptosis. In summary, our data indicate that IGFBP-3 plays a role in stress-induced apoptosis that may require nuclear localization in non-transformed MEC.

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.

Protection of blood retinal barrier and systemic vasculature by insulin-like growth factor binding protein-3

Previously, we showed that insulin growth factor (IGF)-1 binding protein-3 (IGFBP-3), independent of IGF-1, reduces pathological angiogenesis in a mouse model of the oxygen-induced retinopathy (OIR). The current study evaluates novel endothelium-dependent functions of IGFBP-3 including blood retinal barrier (BRB) integrity and vasorelaxation. To evaluate vascular barrier function, either plasmid expressing IGFBP-3 under the regulation of an endothelial-specific promoter or a control plasmid was injected into the vitreous humor of mouse pups (P1) and compared to the non-injected eyes of the same pups undergoing standard OIR protocol. Prior to sacrifice, the mice were given an injection of horseradish peroxidase (HRP). IGFBP-3 plasmid-injected eyes displayed near-normal vessel morphology and enhanced vascular barrier function. Further, in vitro IGFBP-3 protects retinal endothelial cells from VEGF-induced loss of junctional integrity by antagonizing the dissociation of the junctional complexes. To assess the vasodilatory effects of IGFBP-3, rat posterior cerebral arteries were examined in vitro. Intraluminal IGFBP-3 decreased both pressure- and serotonin-induced constrictions by stimulating nitric oxide (NO) release that were blocked by L-NAME or scavenger receptor-B1 neutralizing antibody (SRB1-Ab). Both wild-type and IGF-1-nonbinding mutant IGFBP-3 (IGFBP-3NB) stimulated eNOS activity/NO release to a similar extent in human microvascular endothelial cells (HMVECs). NO release was neither associated with an increase in intracellular calcium nor decreased by Ca²⁺/calmodulin-dependent protein kinase II (CamKII) blockade; however, dephosphorylation of eNOS-Thr⁴⁹⁵ was observed. Phosphatidylinositol 3-kinase (PI3K) activity and Akt-Ser⁴⁷³ phosphorylation were both increased by IGFBP-3 and selectively blocked by the SRB1-Ab or PI3K blocker LY294002. In conclusion, IGFBP-3 mediates protective effects on BRB integrity and mediates robust NO release to stimulate vasorelaxation via activation of SRB1. This response is IGF-1- and calcium-independent, but requires PI3K/Akt activation, suggesting that IGFBP-3 has novel protective effects on retinal and systemic vasculature and may be a therapeutic candidate for ocular complications such as diabetic retinopathy.

Insulin-like growth factors in the gastrointestinal tract and liver

The liver is a major source of insulin-like growth factors (IGFs) and IGF binding proteins (IGFBPs) that are present in the circulation and have important endocrine activities relating to energy metabolism, body size, carcinogenesis, and various organ-specific functions. Although IGFs have only minor effects on the normal liver itself, production of IGFs and IGFBPs in a tissue-specific manner in the gastrointestinal tract exert important regulatory effects on cellular proliferation, survival, and apoptosis. IGFs and IGFBPs play important regulatory roles in the response of both the liver and the gastrointestinal tract to inflammation and in the development of neoplasia.

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.

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.

Free insulin-like growth factor binding protein-3 (IGFBP-3) reduces retinal vascular permeability in association with a reduction of acid sphingomyelinase (ASMase)

PURPOSE

To examine the effect of free insulin-like growth factor (IGF) binding protein-3 (IGFBP-3), independent of the effect of insulin-like growth factors, in modulating retinal vascular permeability.

METHODS

We assessed the ability of a form of IGFBP-3 that does not bind IGF-1 (IGFBP-3NB), to regulate the blood retinal barrier (BRB) using two distinct experimental mouse models, laser-induced retinal vessel injury and vascular endothelial growth factor (VEGF)-induced retinal vascular permeability. Additionally, in vitro studies were conducted. In the animal models, BRB permeability was quantified by intravenous injection of fluorescein labeled serum albumin followed by digital confocal image analysis of retinal flat-mounts. Claudin-5 and vascular endothelial-cadherin (VE-cadherin) localization at interendothelial junctions was studied by immunofluorescence. In vitro changes in transendothelial electrical resistance (TEER) and flux of fluorescent dextran in bovine retinal endothelial monolayers (BREC) were measured after IGFBP-3NB treatment. Acid (ASMase) and neutral (NSMase) sphingomyelinase mRNA levels and activity were measured in mouse retinas.

RESULTS

Four days postinjury, laser-injured mouse retinas injected with IGFBP-3NB plasmid demonstrated reduced vascular permeability compared with retinas of laser-injured mouse retinas injected with control plasmid. IGFBP-3NB administration resulted in a significant decrease in laser injury-associated increases in ASMase and NSMase mRNA and activity when compared with laser alone treated mice. In vivo, intravitreal injection of IGFBP-3NB reduced vascular leakage associated with intravitreal VEGF injection. IGFBP-3NB partially restored VEGF-induced in vivo permeability and dissociation of claudin-5 and VE-cadherin at junctional complexes. When IGFBP-3NB was applied basally to bovine retinal endothelial cells (BREC) in vitro, TEER increased and macromolecular flux decreased.

CONCLUSIONS

Intravitreal administration of IGFBP-3NB preserves junctional integrity in the presence of VEGF or laser injury by reducing BRB permeability in part by modulating sphingomyelinase levels.

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.

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.

Insulin-like growth factor-binding protein-5-induced laminin gamma1 transcription requires filamin A

Insulin-like growth factor-binding protein-5 (IGFBP-5) has IGF-1-independent intranuclear effects that are poorly defined. Treatment of cells with IGFBP-5 induces migration, prevents apoptosis, and leads to increased laminin subunit transcription. Similarly, filamin A (FLNa), an actin-binding protein that participates in cell attachment, plays important additional roles in signal transduction and modulation of transcriptional responses. In this report, we show that IGFBP-5 leads to dephosphorylation of FLNa with subsequent FLNa cleavage. Following cleavage, there is enhanced recruitment of Smad3/4 to a C-terminal FLNa fragment with nuclear translocation and subsequent binding to the promoter region of the laminin gamma1 (lamc1) gene. FLNa knockdown prevents IGFBP-5-mediated increases in lamc1 transcription. These data indicate that IGFBP-5 induces formation of a FLNa-based nuclear shuttle that recruits transcription factors and regulates transcription of IGFBP-5 target genes. These studies provide new insights into the mechanisms whereby IGFBP-5 and FLNa exert intranuclear effects.

Discovery and characterization of IGFBP-mediated endocytosis in the human retinal pigment epithelial cell line ARPE-19

Insulin-like growth factor binding proteins (IGFBPs) are prime regulators of IGF-action in numerous cell types including the retinal pigment epithelium (RPE). The RPE performs several functions essential for vision, including growth factor secretion and waste removal via a phagocytic process mediated in part by vitronectin (Vn). In the course of studying the effects of IGFBPs on IGF-mediated VEGF secretion and Vn-mediated phagocytosis in the RPE cell line ARPE-19, we have discovered that these cells avidly ingest synthetic microspheres (2.0 mum diameter) coated with IGFBPs. Given the novelty of this finding and the established role for endocytosis in mediating IGFBP actions in other cell types, we have explored the potential role of candidate cell surface receptors. Moreover, we have examined the role of key IGFBP structural motifs, by comparing responses to three members of the IGFBP family (IGFBP-3, IGFBP-4 and IGFBP-5) which display overlapping variations in primary structure and glycosylation status. Coating of microspheres (FluoSpheres((R)), sulfate modified polystyrene filled with a fluorophore) was conducted at 37 degrees C for 1 h using 20 microg/mL of test protein, followed by extensive washing. Binding of proteins was confirmed using a microBCA assay. The negative control consisted of microspheres treated with 0.1% bovine serum albumin (BSA), and all test samples were post-treated with BSA in an effort to coat any remaining free protein binding sites, which might otherwise encourage non-specific interactions with the cell surface. Serum-starved cultures of ARPE-19 cells were incubated with microspheres for 24 h, using a ratio of approximately 100 microspheres per cell. Uptake of microspheres was quantified using a fluorometer and was confirmed visually by confocal fluorescence microscopy. The ARPE-19 cells displayed little affinity for BSA-treated microspheres, but avidly ingested large quantities of those pre-treated with Vn (ANOVA; p < 0.001). Strong responses were also observed towards recombinant formulations of non-glycosylated IGFBP-3, glycosylated IGFBP-3 and glycosylated IGFBP-5 (all p < 0.001), while glycosylated IGFBP-4 induced a relatively minor response (p < 0.05). The response to IGFBP-3 was unaffected in the presence of excess soluble IGFBP-3, IGF-I or Vn. Likewise, soluble IGFBP-3 did not induce uptake of BSA-treated microspheres. Antibodies to either the transferrin receptor or type 1 IGF-receptor displayed slight inhibitory effects on responses to IGFBPs and Vn. Heparin abolished responses to Vn, IGFBP-5 and non-glycosylated IGFBP-3, but only partially inhibited the response to glycosylated IGFBP-3. Our results demonstrate for the first time IGFBP-mediated endocytosis in ARPE-19 cells and suggest roles for the IGFBP-heparin-binding domain and glycosylation status. These findings have important implications for understanding the mechanisms of IGFBP actions on the RPE, and in particular suggest a role for IGFBP-endocytosis.

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.

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.

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.