Nuclear localization signal in insulin-like growth factor-binding protein type 3

Colostrogenesis: Role and Mechanism of the Bovine Fc Receptor of the Neonate (FcRn)

Colostrogenesis is a separate and unique phase of mammary epithelial cell activity occurring in the weeks before parturition and rather abruptly ending after birth in the bovine. It has been the focus of research to define what controls this process and how it produces high concentrations of specific biologically active components important for the neonate. In this review we consider colostrum composition and focus upon components that appear in first milked colostrum in concentrations exceeding that in blood serum. The Fc Receptor of the Neonate (FcRn) is recognized as the major immunoglobulin G (IgG) and albumin binding protein that accounts for the proteins' long half-lives. We integrate the action of the pinocytotic (fluid phase) uptake of extracellular components and merge them with FcRn in sorting endosomes. We define and explore the means of binding, sorting, and the transcytotic delivery of IgG1 while recycling IgG2 and albumin. We consider the means of releasing the ligands from the receptor within the endosome and describe a new secretion mechanism of cargo release into colostrum without the appearance of FcRn itself in colostrum. We integrate the insulin-like growth factor family, some of which are highly concentrated bioactive components of colostrum, with the mechanisms related to FcRn endosome action. In addition to secretion, we highlight the recent findings of a role of the FcRn in phagocytosis and antigen presentation and relate its significant and abrupt change in cellular location after parturition to a role in the prevention and resistance to mastitis infections.

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.

Effects of the Insulin-like Growth Factor Pathway on the Regulation of Mammary Gland Development

The insulin-like growth factor (IGF) pathway is a key signal transduction pathway involved in cell proliferation, migration, and apoptosis. In dairy cows, IGF family proteins and binding receptors, including their intracellular binding partners, regulate mammary gland development. IGFs and IGF receptor interactions in mammary glands influence the early stages of mammogenesis, i.e., mammary ductal genesis until puberty. The IGF pathway includes three major components, IGFs (such as IGF-I, IGF-II, and insulin), their specific receptors, and their high-affinity binding partners (IGF binding proteins [IGFBPs]; i.e., IGFBP1–6), including specific proteases for each IGFBP. Additionally, IGFs and IGFBP interactions are critical for the bioactivities of various intracellular mechanisms, including cell proliferation, migration, and apoptosis. Notably, the interactions between IGFs and IGFBPs in the IGF pathway have been difficult to characterize during specific stages of bovine mammary gland development. In this review, we aim to describe the role of the interaction between IGFs and IGFBPs in overall mammary gland development in dairy cows.

Insider trading: Extracellular matrix proteins and their non-canonical intracellular roles

In metazoans, the extracellular matrix (ECM) provides a dynamic, heterogeneous microenvironment that has important supportive and instructive roles. Although the primary site of action of ECM proteins is extracellular, evidence is emerging for non-canonical intracellular roles. Examples include osteopontin, thrombospondins, IGF-binding protein 3 and biglycan, and relate to roles in transcription, cell-stress responses, autophagy and cancer. These findings pose conceptual problems on how proteins signalled for secretion can be routed to the cytosol or nucleus, or can function in environments with diverse redox, pH and ionic conditions. We review evidence for intracellular locations and functions of ECM proteins, and current knowledge of the mechanisms by which they may enter intracellular compartments. We evaluate the experimental methods that are appropriate to obtain rigorous evidence for intracellular localisation and function. Better insight into this under-researched topic is needed to decipher the complete spectrum of physiological and pathological roles of ECM proteins.

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.

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.

Core functional sequence of C-terminal GAG-binding domain directs cellular uptake of IGFBP-3-derived peptides

The current study clarifies the role of the Glycosaminoglycan (GAG)-binding domain of insulin-like growth factor binding protein-3 (IGFBP-3) in cell penetration. The cell penetration function of IGFBP-3 has been mapped to an 18-residue GAG-binding domain in the C-terminal region that mobilizes cellular uptake and nuclear localization of unrelated proteins. Uptake of KW-22, a 22-residue peptide that encompasses the 18-residue GAG-binding domain, and another IGFBP-3 peptide carrying a streptavidin protein cargo was investigated in Chinese hamster ovary (CHO) cells defective at several steps of biosynthesis of cell surface GAGs. The severity of GAG truncation was highly correlated to the impairment of uptake ranging from complete abrogation to only a partial reduction, suggesting that GAG-binding is required for uptake. The 18-residue GAG-binding domain consists of an 8-residue KK-8 basic sequence devoid of Arg and an adjacent 10-residue QR-10 sequence rich in Arg. Peptide mapping of uptake and GAG-binding activities within the KW-22 peptide showed that the 8-residue KK-8 basic peptide retained 80% of GAG-binding activity with no uptake activity while the 10-residue QR-10 peptide retained 53% of uptake activity and 18% of GAG-binding activity. This suggests that KK-8 carries out the majority of GAG-binding function while QR-10 carries out the majority of the cell entry function. To our knowledge, this is the first report of physical separation of the uptake and GAG-binding functions within a short cell penetrating peptide and may shed light on the general mechanism of uptake of Arg-rich CPPs and guide new design of Arg-rich CPP-assisted drug/gene delivery systems.

Molecular characterization and expression pattern of insulin-like growth factor binding protein-3 (IGFBP-3) in common carp, Cyprinus carpio

A full-length cDNA encoding the insulin-like growth factor binding protein-3 (IGFBP-3) was cloned from the liver of common carp (Cyprinus carpio) by RT-PCR. The IGFBP-3 cDNA sequence is 1,680 bp long and has an open reading frame of 882 bp encoding a predicted polypeptide of 293 amino acid residues. The deduced amino acid sequence contains a putative signal peptide of 25 amino acid residues resulting in a mature protein of 268 amino acids. A single band of approximate 1.9 kb was found in liver by Northern blot analysis. IGFBP-3 mRNA was observed in all regions of brain with high levels. In peripheral tissues, high levels of IGFBP-3 mRNA were found in retina, red muscle, liver, heart, posterior intestine, spleen, and testis. Relatively lower levels were found in white muscle, kidney, thymus gland, and ovary, while in head kidney, blood, skin, gill, middle intestine, and anterior intestine, the IGFBP-3 mRNA levels were much lower. IGFBP-3 mRNA was first detected in the blastula stage with significantly high level. The level sharply decreased in gastrula stage, and it became to increase in the following stages. During the reproductive cycle, the abundance of IGFBP-3 mRNA significantly decreased between the recrudescing stage and the matured stage in ovary, although in testis, IGFBP-3 mRNA expression level did not exhibit a significant change. The mRNA expression profiles in the present study imply that the IGFBP-3 may play important physiological functions in common carp development and reproduction.

The insulin-like growth factor system in chronic kidney disease: Pathophysiology and therapeutic opportunities

The growth hormone-insulin-like growth factor-insulin-like growth factor binding protein (GH-IGF-IGFBP) axis plays a critical role in the maintenance of normal renal function and the pathogenesis and progression of chronic kidney disease (CKD). Serum IGF-I and IGFBPs are altered with different stages of CKD, the speed of onset, the amount of proteinuria, and the potential of remission. Recent studies demonstrate that growth failure in children with CKD is due to a relative GH insensitivity and functional IGF deficiency. The functional IGF deficiency in CKD results from either IGF resistance due to increased circulating levels of IGFBPs or IGF deficiency due to increased urinary excretion of serum IGF-IGFBP complexes. In addition, not only GH and IGFs in circulation, but locally produced IGFs, the high-affinity IGFBPs, and low-affinity insulin-like growth factor binding protein-related proteins (IGFBP-rPs) may also affect the kidney. With respect to diabetic kidney disease, there is growing evidence suggesting that GH, IGF-I, and IGFBPs are involved in the pathogenesis of diabetic nephropathy (DN). Thus, prevention of GH action by blockade either at the receptor level or along its signal transduction pathway offers the potential for effective therapeutic opportunities. Similarly, interrupting IGF-I and IGFBP actions also may offer a way to inhibit the development or progression of DN. Furthermore, it is well accepted that the systemic inflammatory response is a key player for progression of CKD, and how to prevent and treat this response is currently of great interest. Recent studies demonstrate existence of IGF-independent actions of high-affinity and low-affinity-IGFBPs, in particular, antiinflammatory action of IGFBP-3 and profibrotic action of IGFBP-rP2/CTGF. These findings reinforce the concept in support of the clinical significance of the IGF-independent action of IGFBPs in the assessment of pathophysiology of kidney disease and its therapeutic potential for CKD. Further understanding of GH-IGF-IGFBP etiopathophysiology in CKD may lead to the development of therapeutic strategies for this devastating disease. It would hold promise to use of GH, somatostatin analogs, IGFs, IGF agonists, GHR and insulin-like growth factor-I receptor (IGF-IR) antagonists, IGFBP displacer, and IGFBP antagonists as well as a combination treatment as therapeutic agents for CKD.

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.

Oncoprotein metastasis and its suppression revisited

The past two decades have witnessed an increasing appreciation of the role of the tumor microenvironment, of genetic and epigenetic alterations in normal cells adjacent to tumors and of the migration of normal cells with aberrant intrinsic properties in cancer pathophysiology. Aside from these insights, a novel concept termed "oncoprotein metastasis" (OPM) has recently been advanced and proposed to reflect protein-based neoplastic phenomena that might occur even before any modifications relating to the morphology, location or (epi)genetic outfit of cells during the malignant process. Here, evidence is presented that supports the OPM perception and thus should contribute not only to further rethink the definition of a normal cell, but also the treatment of cancer disease in the years to come.

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.

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.

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.

Multifunctional roles of insulin-like growth factor binding protein 5 in breast cancer

The insulin-like growth factor axis, which has been shown to protect cells from apoptosis, plays an essential role in normal cell physiology and in cancer development. The family of insulin-like growth factor binding proteins (IGFBPs) has been shown to have a diverse spectrum of functions in cell growth, death, motility, and tissue remodeling. Among the six IGFBP family members, IGFBP-5 has recently been shown to play an important role in the biology of breast cancer, especially in breast cancer metastasis; however, the exact mechanisms of action remain obscure and sometimes paradoxical. An in-depth understanding of IGFBP-5 would shed light on its potential role as a target for breast cancer therapeutics.

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 (IGF)-binding protein-3 (IGFBP-3) is closely associated with the chondrocyte nucleus in human articular cartilage

OBJECTIVE

Insulin-like growth factor-I (IGF-I) is critically involved in the control of cartilage matrix metabolism. It is well known that IGF-binding protein-3 (IGFBP-3) is increased during osteoarthritis (OA), but its function(s) is not known. In other cells, IGFBP-3 can regulate IGF-I action in the extracellular environment and can also act independently inside the cell; this includes transcriptional gene control in the nucleus. These studies were undertaken to localize IGFBP-3 in human articular cartilage, particularly within cells.

DESIGN

Cartilage was dissected from human femoral heads derived from arthroplasty for OA, and OA grade assessed by histology. Tissue slices were further characterized by extraction and assay of IGFBPs by IGF ligand blot (LB) and by enzyme-linked immunosorbent assay (ELISA). Immunohistochemistry (IHC) for IGF-I and IGFBP-3 was performed on cartilage from donors with mild, moderate and severe OA. Indirect fluorescence and immunogold-labeling IHC studies were included.

RESULTS

LBs of chondrocyte lysates showed a strong signal for IGFBP-3. IHC of femoral cartilage sections at all OA stages showed IGF-I and IGFBP-3 matrix stain particularly in the top zones, and closely associated with most cells. A prominent perinuclear/nuclear IGFBP-3 signal was seen. Controls using non-immune sera or antigen-blocked antibody showed negative or strongly reduced stain. In frozen sections of human ankle cartilage, immunofluorescent IGFBP-3 stain co-localized with the nuclear 4',6-diamidino-2-phenyl indole (DAPI) stain in greater than 90% of the cells. Immunogold IHC of thin sections and transmission electron immunogold microscopy of ultra-thin sections showed distinct intra-nuclear staining.

CONCLUSIONS

IGFBP-3 in human cartilage is located in the matrix and within chondrocytes in the cytoplasm and nuclei. This new finding indicates that the range of IGFBP-3 actions in articular cartilage is likely to include IGF-independent roles and opens the door to studies of its nuclear actions, including the possible regulation of hormone receptors or transcriptional complexes to control gene action.

Calpain proteolysis of insulin-like growth factor binding protein (IGFBP) -2 and -3, but not of IGFBP-1

Calpains are cytoplasmic Ca(2+)-regulated cysteine proteases that may regulate insulin-like growth factor (IGF)-independent actions of insulin-like growth factor binding proteins (IGFBPs) through IGFBP proteolysis. In this study, [(125)I]-labeled IGFBP-2 and -3, but not IGFBP-1, were proteolyzed by Ca(2+)-activated m-calpain in vitro. Degradation of higher concentrations of the recombinant proteins IGFBP-2 and -3 by m-calpain was dose-dependent, but was terminated within 20 min by autolysis. By subjecting proteolytic fragments to N-terminal amino acid sequence analysis, the primary cleavage sites in IGFBP-2 and -3 were localized to the non-conserved central linker regions. Using the biosensor technique, in vitro binding of m-calpain to IGFBP-3 was demonstrated to be a Ca(2+)-dependent reaction with a rapid on/off rate.

One for all and all for one: RB defends the cell while IDE, PTEN and IGFBP-7 antagonize insulin and IGFs to protect RB

Tumor suppressor proteins are the main cellular barrier opposing neoplastic transformation. Among these host molecules, retinoblastoma protein (RB) plays a central role. A novel insight is now advanced to suggest that various inhibitors of insulin and insulin-like growth factor (IGF) signalling such as the putative tumor suppressor insulin-degrading enzyme (IDE) as well as the anti-oncogenic proteins PTEN and insulin-like growth factor-binding protein 7 (IGFBP-7) serve the common goal of ensuring that RB remains active. Since, moreover, IDE and IGFBP-7 each potentially achieves RB protection through preventing both binding and inactivation of RB by insulin, IGF-1 or IGF-2, the present perception also vindicates the importance of previous findings on the physical interaction of any of these growth factors with RB for cell fate. Notably, the therapeutic counterpart of this natural principle for maintaining or restoring RB function through insulin/IGF neutralization is the innovative class of anti-cancer agents termed MCR peptides and developed over the past decade.

Nonsecreted insulin-like growth factor binding protein-3 (IGFBP-3) can induce apoptosis in human prostate cancer cells by IGF-independent mechanisms without being concentrated in the nucleus

Insulin-like growth factor binding protein-3 (IGFBP-3), a secreted protein, has the intrinsic ability to induce apoptosis directly without binding insulin-like growth factors. Previous studies suggested that IGFBP-3 must be secreted to exert its biological functions. IGFBP-3 contains a nuclear localization signal (NLS), and exogenous IGFBP-3 is translocated into the nucleus, suggesting that both secretion and nuclear localization may play important roles in IGFBP-3 action. To address these questions, we fused yellow fluorescent protein (YFP) to mature IGFBP-3 lacking its signal peptide so that it would remain intracellular and mutated the C-terminal NLS of IGFBP-3, (228)KGRKR(232), to MDGEA. Following transfection of PC-3 human prostate cancer cells with these constructs, Western blots indicated that YFP-IGFBP-3 lacking a signal peptide was cell-associated and not present in the extracellular media. Moreover, the fusion protein was not N-glycosylated, indicating that it had not entered the secretory pathway. Confocal imaging showed that intracellular YFP-MDGEA-IGFBP-3 was predominantly cytoplasmic. Transient transfection of nonsecreted YFP-wild-type IGFBP-3 decreased cell viability, as assessed by staining with annexin V followed by flow cytometry. Induction of cell death was caspase-dependent, indicative of apoptosis. Apoptosis also was induced by the nonsecreted NLS mutant (YFP-MDGEA-IGFBP-3) alone and when the IGF-binding site also had been mutated. These results indicate that IGFBP-3 can induce apoptosis in an IGF-independent manner without being secreted or concentrated in the nucleus.

Several acidic amino acids in the N-domain of insulin-like growth factor-binding protein-5 are important for its transactivation activity

Insulin-like growth factor-binding protein (IGFBP)-5 is a secreted protein that binds to IGFs and modulates IGF actions. IGFBP-5 is also found in the nuclei of cultured cells and has transactivation activity. Here we report the nuclear localization of endogenous IGFBP-5 in mouse embryonic skeletal cells. Chromatin immunoprecipitation experiments indicated that IGFBP-5 interacts with the nuclear histone-DNA complex. Using a series of deletion mutants, the transactivation domain of IGFBP-5 was mapped to its N-terminal region. Intriguingly, the transactivation activity of IGFBP-5 is masked by negative regulatory elements located in the L- and C-domains. Among the other IGFBPs, the N-domains of IGFBP-2 and -3 also had strong transactivation activity, whereas those of IGFBP-1 and -6 had no activity. The IGFBP-4 N-domain had modest activity. Sequence analysis revealed several amino acids in the IGFBP-5 N-domain that are not present in IGFBP-1. The activities of mutants in which these residues were changed to the corresponding IGFBP-1 sequence were determined. Mutations that changed acidic residues to neutral residues (e.g. E8A, D11S, E12A, E30S/P31A, E43L, and E52A) or a polar to a basic residue (e.g. Q56R) significantly reduced transactivation activity. The E8A/D11S/E12A triple mutant and E52A/Q56R double mutants showed further reduced activity. The combinatory mutants had essentially no transactivation activity. Taken together, our results indicate that there are several conserved residues in the IGFBP-5 N-terminal region that are critical for transactivation and that IGFBP-2 and -3 also have strong transactivation activity in their N-domains.

Combination therapy of insulin-like growth factor binding protein-3 and retinoid X receptor ligands synergize on prostate cancer cell apoptosis in vitro and in vivo

We have previously identified the retinoid X receptor-alpha (RXRalpha) as an insulin-like growth factor binding protein-3 (IGFBP-3) nuclear binding partner, which is required for IGFBP-3-induced apoptosis. In the current study, we investigated the biological interactions of the RXR ligand, VTP194204 and rhIGFBP-3, in vitro and in vivo. In vitro, IGFBP-3 and VTP194204 individually induced apoptosis, and suppressed cell growth in prostate cancer cell lines in an additive manner. In vivo, LAPC-4 xenograft-bearing severe combined immunodeficiency mice treated daily with saline, IGFBP-3, and/or VTP194204 for 3 weeks showed no effect of individual treatments with IGFBP-3 or VTP194204 on tumor growth. However, the combination of IGFBP-3 and VTP194204 treatments inhibited tumor growth by 50% and induced a significant reduction in serum prostate-specific antigen levels. In terminal nucleotidyl transferase-mediated nick end labeling immunohistochemistry of LAPC-4 xenografts, there was modest induction of apoptosis with either IGFBP-3 or VTP194204 individual treatment, but combination therapy resulted in massive cell death, indicating that IGFBP-3 and VTP194204 have a synergistic effect in preventing tumor growth by apoptosis induction. In summary, this is an initial description of the successful therapeutic use of IGFBP-3 as a cancer therapy in vivo, and shows that combination treatment of IGFBP-3 and RXR ligand has a synergistic effect on apoptosis induction leading to substantial inhibition of prostate cancer xenograft growth. Taken together, these observations suggest that combination therapy with IGFBP-3 and RXR ligands may have therapeutic potential for prostate cancer treatment.

The insulin-like growth factor system and its pleiotropic functions in brain

In recent years, much interest has been devoted to defining the role of the IGF system in the nervous system. The ubiquitous IGFs, their cell membrane receptors, and their carrier binding proteins, the IGFBPs, are expressed early in the development of the nervous system and are therefore considered to play a key role in these processes. In vitro studies have demonstrated that the IGF system promotes differentiation and proliferation and sustains survival, preventing apoptosis of neuronal and brain derived cells. Furthermore, studies of transgenic mice overexpressing components of the IGF system or mice with disruptions of the same genes have clearly shown that the IGF system plays a key role in vivo.

The insulin-like growth factor system: IGFs, IGF-binding proteins and IGFBP-proteases

Insulin-like growth factors (IGF-I/-II) are not only the endocrine mediators of growth hormone-induced metabolic and anabolic actions but also polypeptides that act in a paracrine and autocrine manner to regulate cell growth, differentiation, apoptosis and transformation. The IGF system is a complex network comprised of two growth factors (IGF-I and -II), cell surface receptors (IGF-IR and -IIR), six specific high affinity binding proteins (IGFBP-I to IGFBP-6), IGFBP proteases as well as several other IGFBP-interacting molecules, which regulate and propagate IGF actions in several tissues. Besides their broad-spectrum physiological and pathophysiological functions, recent evidence suggests even a link between IGFs and different malignancies.

Peri/nuclear localization of intact insulin-like growth factor binding protein-2 and a distinct carboxyl-terminal IGFBP-2 fragment in vivo

Insulin-like growth factor binding protein-2 (IGFBP-2) as one of the most important IGFBPs has never been assessed in the intracellular compartment in vivo. Since there is evidence for novel intracellular functions of distinct IGFBPs, we investigated the presence of IGFBP-2 inside the cell. In peri/nuclear fractions of various tissues isolated from IGFBP-2 transgenic and non-transgenic mice we were able to show the presence of intact IGFBP-2. In addition, we demonstrate the presence of a highly conserved carboxyl-terminal IGFBP-2 fragment in the peri/nuclear fraction by using different peptide-induced antibodies. In pancreatic sections, confocal microscopy revealed the presence of IGFBP-2 on the nuclear surface but not within the nucleus. Our findings suggest novel functions of intact IGFBP-2 and IGFBP-2 fragments within the cell.

The role of IGFBP-5 in mammary gland development and involution

Insulin-like growth factor-I (IGF-I) plays an important role as a survival factor during mammary gland development and remodelling during involution of the mature/lactating mammary gland, and elevated concentrations have been associated with increased risk of breast cancer. The actions of IGF-I are modulated by a family of binding proteins (IGFBPs) and we have shown that IGFBP-5 is associated with cell death in the mammary gland and more recently provided the first evidence that it is causally related to apoptosis of the mammary gland. A transgenic mouse expressing IGFBP-5 on a mammary-specific promoter led to impaired mammary development involving inhibition of IGF-signalling and involving members of the Bcl-2 family. Subsequent studies in vitro and in vivo using exogenous IGFBP-5 treatment have added support to this concept. Although the effects of IGFBP-5 did appear to involve inhibition of IGF action, a role for IGF-independent effects cannot be ruled out. Such IGF-independent effects involve potential interactions with components of the extracellular matrix involved in tissue remodelling including plasminogen activator inhibitor-1 (PAI-1). In addition, intracellular events involving nuclear localisation of IGFBP-5 have been shown to have the ability to inhibit cell proliferation. Thus, IGFBP-5 seems important for regulating both apoptosis and cell proliferation in the mammary gland during development and post-lactation involution.

IGFBP-3, a marker of cellular senescence, is overexpressed in human papillomavirus-immortalized cervical cells and enhances IGF-1-induced mitogenesis

Human ectocervical cells, following retroviral transduction with the human papillomavirus type 16 E6/E7 oncogenes, are altered in their array of transcribed cellular genes, including increased mRNA for the insulin-like growth factor binding protein 3 (IGFBP-3). IGFBP-3 expression is associated with cellular senescence, and its addition to many cell types inhibits growth or induces apoptosis. By immunoblotting and enzyme-linked immunosorbent assay methods, we demonstrate that late-passage, immortalized E6/E7-transduced cells secrete high levels of IGFBP-3 (25 ng/ml), which represent a 500-fold increase compared to levels in early-passage, nonimmortalized transduced cells (<0.05 ng/ml). Concomitantly, these late-passage cervical cells exhibit an increase in sensitivity to IGF-1, including enhanced phosphorylation of the IGF receptor (IGF-R) and insulin receptor substrate as well as increased DNA synthesis (5-fold) and cell proliferation (3.7-fold). However, there was no change in the level of IGF-R in these cells (surface or total), and the cells did not synthesize IGF-1, indicating that these arms of the IGF pathway were independently regulated and not responsible for the augmented signaling. Consistent with a causal relationship between IGFBP-3 expression and enhanced IGF-1 responses, we found that early-passage cells could be converted to the late-passage, IGF-1-responsive phenotype by preincubation with IGFBP-3. Thus, in contrast to findings with some cell types, IGFBP-3 expression in cervical cells is associated with augmented IGF-1 signaling and cell proliferation and correlates with the timing of cellular immortalization.

Insulin-like growth factor binding protein-3 is a novel mediator of apoptosis in insulin-secreting cells

Insulin-like growth factor binding protein-3 (IGFBP-3) is emerging as a critical regulator of cell survival. There has been no study which directly examined the potential role for this major growth factor in the programmed cell death (apoptosis) of insulin-secreting cells. To determine whether IGFBP-3 mediates apoptosis in insulin-secreting cells, we performed a rigorous series of experiments with the rat insulinoma (RIN) cell line m5F and the hamster insulin-secreting tumor (HIT) T-15. Within 24 h exogenous IGFBP-3 induced significant DNA fragmentation in RIN and HIT cells, at doses ranging from 4.4 to 2000 ng/ml (P<0.05) without a classic dose-response relationship. DNA fragmentation induced by rhIGFBP-3 occurred in the presence of immunoglobulin to block the type 1 IGF receptor. As detected by flow cytometry for Annexin V exposure to the cell surface, rhIGFBP-3 treatment doubled the proportion of apoptotic HIT cells from 1.7 +/- 0.4% (serum-free control) to 3.4 +/- 0.2% (P<0.02), an effect completely reversed by co-treatment with 1000 ng/ml rhIGF-I. Immunofluorescent microscopy disclosed that pro-inflammatory Th1 cytokines increased intranuclear aggregation of endogenous IGFBP-3. Cytokine-induced DNA fragmentation was completely blocked by relatively brief pre-treatment with antisense IGFBP-3 phosphorothioate oligodeoxynucleotides. In conclusion, we have presented the first evidence that IGFBP-3 contributes to cytokine-mediated apoptosis in insulin-secreting cells.

Epidermal homeostasis: the role of the growth hormone and insulin-like growth factor systems

GH and IGF-I and -II were first identified by their endocrine activity. Specifically, IGF-I was found to mediate the linear growth-promoting actions of GH. It is now evident that these two growth factor systems also exert widespread activity throughout the body and that their actions are not always interconnected. The literature highlights the importance of the GH and IGF systems in normal skin homeostasis, including dermal/epidermal cross-talk. GH activity, sometimes mediated via IGF-I, is primarily evident in the dermis, particularly affecting collagen synthesis. In contrast, IGF action is an important feature of the dermal and epidermal compartments, predominantly enhancing cell proliferation, survival, and migration. The locally expressed IGF binding proteins play significant and complex roles, primarily via modulation of IGF actions. Disturbances in GH and IGF signaling pathways are implicated in the pathophysiology of several skin perturbations, particularly those exhibiting epidermal hyperplasia (e.g., psoriasis, carcinomas). Additionally, many studies emphasize the potential use of both growth factors in the treatment of skin wounds; for example, burn patients. This overview concerns the role and mechanisms of action of the GH and IGF systems in skin and maintenance of epidermal integrity in both health and disease.

Cellular internalization of insulin-like growth factor binding protein-3: distinct endocytic pathways facilitate re-uptake and nuclear localization

Insulin-like growth factor binding protein-3 (IGFBP-3) is well established as a growth-inhibitory, apoptosis-inducing secreted molecule that acts via insulin-like growth factor (IGF)-independent as well as IGF-dependent pathways. Nuclear localization of IGFBP-3 has been observed and nuclear binding partners for IGFBP-3 demonstrated. However, little is known about the mechanism of IGFBP-3 internalization. We hypothesized that IGFBP-3 is first secreted then taken up again into cells and that its internalization could occur via binding to transferrin or caveolin. Incubation of cells with an IGFBP-3-neutralizing antibody demonstrated that nuclear translocation of endogenous IGFBP-3 requires IGFBP-3 secretion and re-uptake. Nuclear localization of exogenously added IGFBP-3 was rapid, occurring within 15 min, inhibited by co-incubation and extracellular sequestration with IGF-I, and dependent on the transferrin-binding C-terminal peptide region of IGFBP-3. Co-immunoprecipitation assays confirmed that IGFBP-3 binds transferrin but not directly to the transferrin receptor (TfR1); however, transferrin binds TfR1 and a ternary complex is formed. Specific binding to caveolin scaffolding docking sequence was confirmed utilizing radiolabeled IGFBP-3. Blocking TfR1-mediated endocytosis prevents both endogenous and exogenous IGFBP-3 re-uptake and inhibitors of caveolae formation also retard IGFBP-3 nuclear entry. Co-treatment with anti-transferrin receptor antibody and cholesterol depletion agents completely abolished endogenous and exogenous IGFBP-3 uptake. Suppression of IGFBP-3 internalization by TfR1 blockade inhibited IGFBP-3-induced apoptosis. Together, these data indicate that the actions of IGFBP-3 are mediated by internalization via distinct endocytic pathways.

A reassessment of insulin-like growth factor binding protein gene expression in the human retinal pigment epithelium

The role of insulin-like growth factors (IGF) in regulating cell differentiation and proliferation is in part modulated by the IGF binding protein (IGFBP) family of genes. Previous studies of the human retinal pigment epithelium (RPE) have detected expression of IGFBP-2, -3, and -6. However, recent experiments in our lab have suggested a broader pattern of IGFBP gene family expression in the RPE cell than has previously been recognized. We have examined the gene expression profile of IGFBP-1 to -6 and the related protein, IGFBP-rP1, in RPE cell lines derived from ten donors eyes using RT-PCR, ELISA, and Western methods. Transcripts of IGFBP-1 to -6 and -rP1 were consistently detected in human RPE cells. IGFBP-3, -5, -6, and -rP-1, appear to be constitutively expressed in the RPE, whereas IGFBP-1, -2, and -4, were expressed at variable levels in the cell lines examined. IGFBP secretion by the RPE in vitro was confirmed by ELISA (IGFBP-1, -2, -3, -4, and -6) and Western blot analysis (IGFBP-5 and -rP1). There was, in general, a strong correlation between gene-specific transcription levels and protein secretion by the RPE. Our studies demonstrate that the major IGFBP family genes are ubiquitously expressed in explanted human RPE cells in vitro. This broad expression profile and the recent evidence that IGFBPs have IGF-independent biological activity suggest that the IGFBP family genes may constitute a previously unrecognized and complex regulatory system in the human retina and RPE.

Insulin-like growth factor binding protein-3 is upregulated in LPS-treated THP-1 cells

BACKGROUND

Lipopolysaccharide (LPS) is a potent activator of human monocytic cells. We have determined that LPS stimulation of the human monocytic cell line, THP-1, results in an increased apoptotic rate. We hypothesized that cDNA expression array analysis could be used to identify target genes involved in the regulation of this process.

METHODS

THP-1 cells (1 x 10(6)/mL) were stimulated with LPS (1 microg/mL) or vehicle control. Apoptosis was measured at 0, 24, 48, 72 and 96 h using propidium iodide staining and flow cytometry to determine the percentage of cells with hypodiploid DNA. At 16 h, the Atlas Human cDNA expression array system, containing probes for 205 genes related to apoptosis, was used to survey and quantify transcript expression. The experiment was performed in duplicate and the membranes were normalized to cytoplasmic beta-actin. Standard Western blotting was performed on the conditioned medium to correlate secreted protein expression with RNA expression. Pretreatment with insulin-like growth factor I (IGF-I) was performed to determine whether the effects of insulin-like growth factor binding protein-3 (IGFBP-3) on apoptosis were IGF-dependent.

RESULTS

LPS stimulation of THP-1 cells resulted in a greater than 2-fold increase in the rate of apoptosis when compared to vehicle control. When the cDNA expression arrays were compared, there was a 500-fold increase in the expression of the IGFBP-3 transcript in the LPS-stimulated cells. Western blotting of culture medium verified an approximately 2-fold increase in secreted IGFBP-3. Pretreatment with IGF-I did not prevent the increase in apoptosis seen with LPS stimulation.

CONCLUSIONS

THP-1 cell apoptosis is increased in response to LPS stimulation and is associated with a significant induction of IGFBP-3 mRNA and protein. IGFBP-3, which reportedly promotes apoptosis and modulates the bioavailability of the pro-survival insulin-like growth factor 1, may serve to regulate apoptosis in monocytic cells in an IGF-independent manner. These data further support the investigation of the role of the IGF axis in programmed cell death of immune cells.

Cellular actions of the insulin-like growth factor binding proteins

In addition to their roles in IGF transport, the six IGF-binding proteins (IGFBPs) regulate cell activity in various ways. By sequestering IGFs away from the type I IGF receptor, they may inhibit mitogenesis, differentiation, survival, and other IGF-stimulated events. IGFBP proteolysis can reverse this inhibition or generate IGFBP fragments with novel bioactivity. Alternatively, IGFBP interaction with cell or matrix components may concentrate IGFs near their receptor, enhancing IGF activity. IGF receptor-independent IGFBP actions are also increasingly recognized. IGFBP-1 interacts with alpha(5)beta(1) integrin, influencing cell adhesion and migration. IGFBP-2, -3, -5, and -6 have heparin-binding domains and can bind glycosaminoglycans. IGFBP-3 and -5 have carboxyl-terminal basic motifs incorporating heparin-binding and additional basic residues that interact with the cell surface and matrix, the nuclear transporter importin-beta, and other proteins. Serine/threonine kinase receptors are proposed for IGFBP-3 and -5, but their signaling functions are poorly understood. Other cell surface IGFBP-interacting proteins are uncharacterized as functional receptors. However, IGFBP-3 binds and modulates the retinoid X receptor-alpha, interacts with TGFbeta signaling through Smad proteins, and influences other signaling pathways. These interactions can modulate cell cycle and apoptosis. Because IGFBPs regulate cell functions by diverse mechanisms, manipulation of IGFBP-regulated pathways is speculated to offer therapeutic opportunities in cancer and other diseases.

The Twisted gastrulation family of proteins, together with the IGFBP and CCN families, comprise the TIC superfamily of cysteine rich secreted factors

AIMS

To analyse the similarities between the Twisted gastrulation (TSG) proteins known to date; in addition, to determine phylogenetic relations among the TSG proteins, and between the TSGs and other protein families--the CCN (for example, CCN2 (CTGF), CCN1 (CYR61), and CCN3 (NOV)) and IGFBP (insulin-like growth factor binding protein) families.

METHODS

TBLASTN and FASTA3 were used to identify new tsg genes and relatives of the TSG family. The sequences were aligned with ClustalW. The predictions of sites for signal peptide cleavage, post-translational modifications, and putative protein domains were carried out with software available at various databases. Unrooted phylogenetic trees were calculated using the UPGMA method.

RESULTS

Several tsg genes from vertebrates and invertebrates were compared. Alignment of protein sequences revealed a highly conserved family of TSG proteins present in both vertebrates and invertebrates, whereas the slightly less well conserved IGFBP and CCN proteins are apparently present only in vertebrates. The TSG proteins display strong homology among themselves and they are composed of a putative signal peptide at the N-terminus followed by a cysteine rich (CR) region, a conserved domain devoid of cysteines, a variable midregion, and a C-terminal CR region. The most striking similarity between the TSGs and the IGFBP and CCN proteins occurs in the N-terminal conserved cysteine rich domain and the characteristic 5' cysteine rich domain(s), spacer region, and 3' cysteine rich domain structure.

CONCLUSION

The family of highly conserved TSG proteins, together with the IGFBP and CCN families, constitute an emerging multigene superfamily of secreted cysteine rich factors. The TSG branch of the superfamily appears to pre-date the others because it is present in all species examined, whereas the CCN and IGFBP genes are found only in vertebrates.

Insulin-like growth factor binding protein-5 (IGFBP-5) potentially regulates programmed cell death and plasminogen activation in the mammary gland

This study aims to investigate the mechanism by which prolactin and GH interact to maintain mammary epithelial cell function in the rat. IGF-I is an important survival factor for the mammary gland and we have demonstrated that the effects of GH and prolactin involve IGF-I. GH acts by increasing IGF-I whilst prolactin acts by inhibiting the expression of IGFBP-5 from the mammary epithelium. During mammary involution, when serum prolactin levels decline, IGFBP-5 expression is dramatically upregulated and it binds with high affinity to IGF-I preventing IGF-I interaction with the IGF-receptor and thus leading to epithelial cell apoptosis. We have identified a specific interaction of IGFBP-5 with alpha s2-casein. This milk protein has also been shown to bind plasminogen and its activator tissue-type plasminogen activator (tPA) leading to enhanced conversion of plasminogen to plasmin. Plasmin is an important initiator of re-modelling of the extracellular matrix during mammary involution. A potential interaction between the cell death and extracellular matrix remodelling is evident from the observation that IGFBP-5 binds to plasminogen activator inhibitor-I (PAI-1). We thus hypothesized that IGFBP-5 could activate cell death by sequestration of IGF-I and activate plasminogen cleavage by sequestering PAI-1. In support of this hypothesis we have shown that both prolactin and GH inhibit tPA activity and plasminogen activation in the involuting mammary gland. Our results suggest that GH and prolactin inhibit cell death and ECM remodelling via the IGF-axis and also indicate a novel role for the milk protein alpha s2-casein in this process. We have now established lines of transgenic mice expressing IGFBP-5 on the beta-lactoglobulin promoter to explore its function in greater detail.

The complex of recombinant human insulin-like growth factor-I (rhIGF-I) and its binding protein-5 (IGFBP-5) induces local bone formation in murine calvariae and in rat cortical bone after local or systemic administration

The influence of recombinant human insulin-like growth factor-I (rhIGF-I), its binding protein-5 (IGFBP-5) or their equimolar complexes on calvarial osteogenesis was investigated by quantitative radiography and histomorphometry after local administration to adult mice or mature rats. The systemic effects of these proteins were investigated in aged Sprague-Dawley rats with regard to their ability to prevent or restore bone mass in ovariectomy induced osteopenia as assessed by radiography, dual-energy X-ray absorptiometry (DEXA) analyses, peripheral computerized tomography (pQCT) and mineral analyses after daily s.c. administration for 3 or 8 weeks following a bone depletion period of 8 weeks. Bone mass of murine calvariae was significantly increased in a dose-dependent manner by the complex 7 days after discontinuation of local administration for 19 days in mice, whereas IGF-I alone expressed only weak effects. IGFBP-5 alone was ineffective in this respect. In the same model, only the complex had a weak osteogenetic potential in 7 week or 5 month old rats. Systemic long-term treatment with the complex of rhIGF-I/IGFBP-5 (2.0/7.6 mg/kg/day, s.c.) for 8 weeks resulted in significantly increased cortical thickness, area and mineral density in femoral midshaft or tibial metaphysis suggesting periosteal bone formation. This was obviously related to increased muscle strength since these effects were parallelled by increased body weight. No effect on trabecular bone occurred as demonstrated by site-specific analyses (vertebrae, proximal tibia) using DEXA, pQCT and radiography. This selective action of rhIGF-I/IGFBP-5 on periosteal bone formation is unique for an IGFBP. Femoral ash and calcium content, both corrected for tissue volume, increased slightly. However, when the increase in cortical thickness and bone mass was corrected for bone size, the effects are nearly abolished, suggesting an additional effect of bone growth. This potential deserves further evaluation in order to differentiate between effects on cortical bone via muscle strength and lack of efficacy on trabecular bone balance.

Down-regulation of insulin-like growth factor-I receptor and insulin receptor substrate-1 expression in advanced human breast cancer

The ligands, receptors and related signaling proteins of the insulin-like growth factor family are involved in the regulation of breast-cancer cell growth. We investigated the expression pattern of insulin-like growth factor-I receptor (IGF-IR), insulin receptor (IR) and insulin receptor substrate-1 (IRS-1), a core downstream signaling protein, in 69 primary breast-cancer specimens of different grades and in 21 control tissues by immunohistochemistry. In addition, cell proliferation (percentage of Ki67(+) nuclei) and estrogen receptor (ER) expression were determined. IGF-IR, IRS-1 and IR were expressed mainly in epithelial cells. IRS-1 and IGF-IR were expressed at high levels in control tissues and in well and moderately differentiated carcinomas but at low levels in poorly differentiated breast cancers. IR expression did not show a significant correlation with the differentiation grade of the tissues investigated. Statistical analysis (ROC analysis for tumor grade) demonstrated that down-regulation of IGF-IR and IRS-1 correlated better with tumor progression than reduction of ER expression or increase in cell proliferation, IGF-IR showing the best correlation, followed by IRS-1 and, less significant, ER and Ki67. Our findings clearly show that progression of breast cancer is accompanied by a reduction of IGF-IR/IRS-1 expression and that IGF-IR/IRS-1 expression inversely correlates with high proliferation rate in dedifferentiated breast cancers. The strong correlation of IGF-IR and IRS-1 down-regulation with tumor progression suggests the use of IGF-IR and IRS-1 as a novel set of marker proteins for tumor grading.

Nuclear import of insulin-like growth factor-binding protein-3 and -5 is mediated by the importin beta subunit

Although insulin-like growth factor-binding protein (IGFBP)-3 and IGFBP-5 are known to modulate cell growth by reversibly sequestering extracellular insulin-like growth factors, several reports have suggested that IGFBP-3, and possibly also IGFBP-5, have important insulin-like growth factor-independent effects on cell growth. These effects may be related to the putative nuclear actions of IGFBP-3 and IGFBP-5, which we have recently shown are transported to the nuclei of T47D breast cancer cells. We now describe the mechanism for nuclear import of IGFBP-3 and IGFBP-5. In digitonin-permeabilized cells, where the nuclear envelope remained intact, nuclear translocation of wild-type IGFBP-3 appears to occur by a nuclear localization sequence (NLS)-dependent pathway mediated principally by the importin beta nuclear transport factor and requiring both ATP and GTP hydrolysis. Under identical conditions, an NLS mutant form of IGFBP-3, IGFBP-3[(228)KGRKR --> MDGEA], was unable to translocate to the nucleus. In cells where both the plasma membrane and nuclear envelope were permeabilized, wild-type IGFBP-3, but not the mutant form, accumulated in the nucleus, implying that the NLS was also involved in mediating binding to nuclear components. By fusing wild-type and mutant forms of NLS sequences (IGFBP-3 [215-232] and IGFBP-5 [201-218]) to the green fluorescent protein, we identified the critical residues of the NLS necessary and sufficient for nuclear accumulation. Using a Western ligand binding assay, wild-type IGFBP-3 and IGFBP-5, but not an NLS mutant form of IGFBP-3, were shown to be recognized by importin beta and the alpha/beta heterodimer but only poorly by importin alpha. Together these results suggest that the NLSs within the C-terminal domain of IGFBP-3 and IGFBP-5 are required for importin-beta-dependent nuclear uptake and probably also accumulation through mediating binding to nuclear components.

Insulin-like growth factor binding protein-3 mediates IGF-I action in a bovine mammary epithelial cell line independent of an IGF interaction

IGF-I is mitogenic for the bovine mammary epithelial cell line MAC-T. In addition, IGF-I specifically upregulates IGFBP-3 synthesis in these cells. To investigate this effect on cell growth and IGF-I responsiveness, cell lines were developed that constitutively express IGFBP-3. MAC-T cells transfected with IGFBP-3 (+BP3) or vector alone (Mock) grew similarly over 7 days in 10 or 1% fetal calf serum. Basal DNA synthesis was lower (70%) in +BP3 cells compared to Mock cells. However, DNA synthesis was increased by IGF-I (1-50 ng/ml) relative to untreated controls to a greater extent in +BP3 cells compared to Mock cells. IGF-I (20 ng/ml) increased DNA synthesis 11- and threefold in +BP3 and Mock cells, respectively. Additionally, +BP3 cells were more sensitive to the lower concentrations of IGF-I (1-5 ng/ml). In contrast, preincubation of Mock cells with exogenous IGFBP-3 did not enhance responsiveness or sensitivity to IGF-I. Basal DNA synthesis was unaffected by either an IGF neutralizing antibody or exogenous IGFBP3, indicating the differences observed between +BP3 and Mock cells were not attributable to sequestration of endogenous IGF-I by IGFBP-3. There were no differences between +BP3 and Mock cells in IGF-I receptor number or affinity. DNA synthesis was also increased in +BP3 cells, compared to controls, in response to 5 microg/ml insulin and 2.5 ng/ml Long R(3)IGF-I, indicating that the potentiated response did not require an interaction with IGFBP-3. These results suggest that IGF-I regulation of IGFBP-3 represents a regulatory loop, the function of which is to increase IGF-I bioactivity, using a mechanism that does require an IGF-I-IGFBP-3 interaction.

Insulin-like growth factor binding proteins: IGF-dependent and -independent effects in the mammary gland

The insulin-like growth factor binding proteins (IGFBPs) are a family of proteins which bind to the IGFs with high affinity. Their expression within the mammary gland is species specific; it has thus been difficult to determine the biological roles of these binding proteins during lactation. In this article we propose a role for IGFBP-5 in the mammary gland involving the initiation of apoptosis induced by sequestration of IGF-1, an important survival factor for the mammary gland. We have shown that this binding protein retains its high affinity for IGF-1 and that it is present in extremely high concentrations compared with the growth factor. These observations make it likely that IGFBP-5 is capable of preventing interaction of IGF-1 with its receptor on the epithelial cells synthesizing milk. We have also demonstrated that IGFBP-5 interacts with alpha(s2)-casein and that this interaction implicates it in the regulation of plasminogen activation in the mammary gland. The generation of plasmin is a key initiating event in the remodeling of the extracellular matrix during mammary involution. As such, IGFBP-5 may play a key role in coordinating cell death and tissue remodeling processes. Many of the molecules involved in embryological development are also expressed in the developing and involuting mammary gland. We believe that our studies may offer mechanistic explanations for apoptotic events in a wide variety of tissues. We have recently shown that IGFBP-5 is apoptotic in the chick embryonic limb bud, adding further support to our belief that IGFBP-5 serves this function in the mammary gland. We hope to be able to explore the role of this binding protein in the mammary gland with a transgenic mouse model expressing IGFBP-5 on the beta-lactoglobulin promoter.

Insulin-like growth factor (IGF) system in the bovine mammary gland and milk

Primary bovine mammary cells express the two IGF receptors (IGF-IR, IGF-IIR), insulin receptor, and four IGFBPs (IGFBP-2, -3, -4, and -5). Examination of the IGF-IR during the mammary gland lactation cycle shows that IGF-IR number declines at parturition, a change that coincides with decreases in the blood level of its ligand, IGF-I. IGF-II and IGF-IIR are largely unchanged. IGFBP-3 is the predominant mammary IGFBP and its concentration also declines in blood and milk during lactation compared to prepartum and involution periods. Time of lactation and pregnancy were the main determinants of milk but not blood IGFBP-3 levels. IGFBP-3 binds to membrane proteins of bovine mammary tissue; an IGFBP-3 binding protein has been identified as bovine lactoferrin. Lactoferrin has the capacity to compete with IGF binding to IGFBP-3. Appearance of both IGFBP-3 and lactoferrin in conditioned media of primary cultures of bovine mammary cells was stimulated by all trans retinoic acid (atRA). Furthermore, atRA was necessary for the entry of exogenously added lactoferrin into the mammary cell nucleus, while IGFBP-3 entry into the nuclei of atRA treated cells required the presence of lactoferrin. These findings reveal a novel role for lactoferrin, suggesting that lactoferrin is critically involved in the regulation of the IGF system during the involution period.

The insulin-like growth factor-binding protein (IGFBP) superfamily

Over the last decade, the concept of an IGFBP family has been well accepted, based on structural similarities and on functional abilities to bind IGFs with high affinities. The existence of other potential IGFBPs was left open. The discovery of proteins with N-terminal domains bearing striking structural similarities to the N terminus of the IGFBPs, and with reduced, but demonstrable, affinity for IGFs, raised the question of whether these proteins were "new" IGFBPs (22, 23, 217). The N-terminal domain had been uniquely associated with the IGFBPs and has long been considered to be critical for IGF binding. No other function has been confirmed for this domain to date. Thus, the presence of this important IGFBP domain in the N terminus of other proteins must be considered significant. Although these other proteins appear capable of binding IGF, their relatively low affinity and the fact that their major biological actions are likely to not directly involve the IGF peptides suggest that they probably should not be classified within the IGFBP family as provisionally proposed (22, 23). The conservation of this single domain, so critical to high-affinity binding of IGF by the six IGFBPs, in all of the IGFBP-rPs, as well, speaks to its biological importance. Historically, and perhaps, functionally, this has led to the designation of an "IGFBP superfamily". The classification and nomenclature for the IGFBP superfamily, are, of course, arbitrary; what is ultimately relevant is the underlying biology, much of which still remains to be deciphered. The nomenclature for the IGFBP related proteins was derived from a consensus of researchers working in the IGFBP field (52). Obviously, a more general consensus on nomenclature, involving all groups working on each IGFBP-rP, has yet to be reached. Further understanding of the biological functions of each protein should help resolve the nomenclature dilemma. For the present, redesignating these proteins IGFBP-rPs simplifies the multiple names already associated with each IGFBP related protein, and reinforces the concept of a relationship with the IGFBPs. Beyond the N-terminal domain, there is a lack of structural similarity between the IGFBP-rPs and IGFBPs. The C-terminal domains do share similarities to other internal domains found in numerous other proteins. For example, the similarity of the IGFBP C terminus to the thyroglobulin type-I domain shows that the IGFBPs are also structurally related to numerous other proteins carrying the same domain (87). Interestingly, the functions of the different C-terminal domains in members of the IGFBP superfamily include interactions with the cell surface or ECM, suggesting that, even if they share little sequence similarities, the C-terminal domains may be functionally related. The evolutionary conservation of the N-terminal domain and functional studies support the notion that IGFBPs and IGFBP-rPs together form an IGFBP superfamily. A superfamily delineates between closely related (classified as a family) and distantly related proteins. The IGFBP superfamily is therefore composed of distantly related families. The modular nature of the constituents of the IGFBP superfamily, particularly their preservation of an highly conserved N-terminal domain, seems best explained by the process of exon shuffling of an ancestral gene encoding this domain. Over the course of evolution, some members evolved into high-affinity IGF binders and others into low-affinity IGF binders, thereby conferring on the IGFBP superfamily the ability to influence cell growth by both IGF-dependent and IGF-independent means (Fig. 10). A final word, from Stephen Jay Gould (218): "But classifications are not passive ordering devices in a world objectively divided into obvious categories. Taxonomies are human decisions imposed upon nature--theories about the causes of nature's order. The chronicle of historical changes in classification provides our finest insight into conceptual revolutions

Cytoplasmic and nuclear cytokine receptor complexes

Much of our understanding on how hormones and cytokines transmit their message into the cell is based on the receptor activation at the plasma membrane. Many experimental in vitro models have established the paradigm for cytokine action based upon such activation of their cell surface receptor. The signaling from the plasma membrane activated cytokine receptor is driven to the nucleus by a rapid ricochet of protein phosphorylation, ultimately integrated as a differentiative, proliferative, or transcriptional message. The Janus kinase (JAK)--signal transducers and activators of transcription (STAT) pathway that was first thought to be cytokine receptor specific now appears to be activated by other noncytokine receptors. Also, evidence is accumulating showing that cytokines modulate the signal transduction machinery of the tyrosine kinase receptors and that of the heterotrimeric guanosine triphosphate (GTP)-binding protein-coupled receptors. Thus cytokine receptor signaling has become much more complex than originally hypothesized, challenging the established model of specificity of the action of a given cytokine. This review is focused on another level of complexity emerging within cytokine receptor superfamily signaling. Over the past 10 years, data from different laboratories have shown that cytokines and their receptors localize to intracellular compartments including the nucleus, and, in some cases, biological responses have been correlated with this unexpected location, raising the possibility that cytokines act as their own messenger through inter-actions with nuclear proteins. Thus, the interplay between cytokine receptor engagement and cellular signaling turns out to be more dynamic than originally suspected. The mechanisms and regulations of intracellular translocation of the cytokines, their receptors, and their signaling proteins are discussed in the context that such compartmentalization provides some of the specificity of the responses mediated by each cytokine.

Intranuclear localization of insulin-like growth factor binding protein-3 (IGFBP-3) during cell division in human keratinocytes

Insulin-like growth factor-I (IGF-I) stimulation of basal keratinocytes is an essential component of normal epidermal homeostasis. In addition to the IGF receptor, basal keratinocytes synthesize insulin-like growth factor binding protein-3 (IGFBP-3). The HaCaT keratinocyte cell line, which has many characteristics of basal keratinocytes, synthesizes IGFBP-3 that in vitro reduces its IGF-I responsiveness. IGFBP-3 has attracted interest as a potential growth arrest protein, both via its ability to modulate IGF-I responsiveness, and more controversially via IGF-I-independent mechanisms. Intracellular modes of action have been proposed, and a nuclear localization consensus sequence has previously been identified within IGFBP-3. Using immunocytochemistry with a biotinylated antibody specific for IGFBP-3, we investigated the intracellular localization of IGFBP-3 in subconfluent monolayer cultures of HaCaT cells. Diffuse cellular staining was visible, potentially corresponding to cell surface and nascent cytoplasmic IGFBP-3. Of particular interest however, was the localization of staining over the nuclei of a large proportion of cells that were undergoing cell division. Antibody staining was specific for IGFBP-3 because addition of recombinant human IGFBP-3 to the antibody prior to incubation with the cells inhibited these staining patterns. Optical sections obtained using a confocal laser scanning microscope showed that in keratinocytes undergoing cell division, IGFBP-3 was localized inside the nucleus. These results show that intracellular IGFBP-3 localization is altered during the cell cycle and suggest a possible nuclear role for IGFBP-3 during cell division.

Insulin-like growth factor-binding protein (IGFBP)-3 and IGFBP-5 share a common nuclear transport pathway in T47D human breast carcinoma cells

Insulin-like growth factor-binding proteins (IGFBPs) play an integral role in modifying insulin-like growth factor actions in a wide variety of cell types. Recent evidence suggests that IGFBP-3 and IGFBP-5 also have effects on cell growth that are insulin-like growth factor-independent. In investigating possible mechanisms for this effect, the intracellular trafficking of IGFBP-3 and IGFBP-5, both of which contain sequences with the potential for nuclear localization, was studied in T47D cells. Nuclear uptake of fluorescently labeled IGFBP-3 and IGFBP-5 was observed in a proportion of T47D cells that appeared to be rapidly dividing. IGFBP-1 and IGFBP-2, which do not possess the putative domain for nuclear translocation, were not transported to the nuclei of T47D cells. When T47D cells were preincubated with excess unlabeled IGFBP-3, nuclear localization of labeled IGFBP-3 or IGFBP-5 was not detected, indicating that their nuclear translocation involves a common pathway. Inhibition of receptor-mediated endocytosis did not affect nuclear uptake of IGFBP-3, suggesting that it uses an alternative non-classical import pathway for transport across the plasma membrane. In addition, a variant form of IGFBP-3 with a mutation in the putative nuclear localization sequence was unable to translocate to the nuclei of T47D cells, suggesting that nuclear translocation of IGFBP-3 was dependent on these carboxyl-terminal basic residues.

Regulatory Actions of Insulin-like Growth Factor-binding Proteins

The six insulin-like growth factor-binding proteins (IGFBPs) are important regulators of insulin-like growth factor (IGF) action. Circulating high molecular weight complexes that contain IGF and IGFBP-3 restrict IGF bioavailability, and excess IGFBPs inhibit IGF action by forming biologically inactive complexes. IGFs can be released from these complexes by proteolysis. Potentiation of IGF activity might occur under specific circumstances, and involves the slow dissociation of IGFs from IGFBP complexes localized in the pericellular space, whose affinity has been reduced by dephosphorylation or association with the cell surface or extracellular matrix. Several IGFBPs or IGFBP fragments also have activities that do not involve IGFs or IGF receptors. The mechanisms by which IGFBPs regulate IGF action and exert their independent actions will be examined.