Characterization of the inhibition of DNA synthesis in proliferating mink lung epithelial cells by insulin-like growth factor binding protein-3

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

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

Preoperative insulin-like growth factor-binding protein-3 (IGFBP-3) blood level predicts gleason sum upgrading

BACKGROUND

About 43% of men with low Gleason grade prostate cancer (PCa) at biopsy will be finally diagnosed with high-grade PCa at radical prostatectomy (RP). Gleason sum at RP is a good indicator of biochemical recurrence and poor clinical outcome. Therefore, there is a need to improve clinical evaluation of PCa aggressiveness in order to choice appropriate treatment. To this aim an easy-available tool is represented by circulating biomarkers. Among these, the best candidates are some molecules involved in PCa pathogenesis such as IGFBP-2 and IGFBP-3, IL-6, and its soluble receptor (SIL-6R).

METHODS

In this study, we evaluated the ability of preoperative IGFBP-2, IGFBP-3, IL-6, and SIL-6R serum levels to predict Gleason score upgrade in 52 PCa patients.

RESULTS

We found that IGFBP-3 median levels were significantly lower in patients who showed Gleason upgrading from biopsy to RP (P = 0.024). We also found an association between biopsy T-stage and Gleason Upgrade (P = 0.011). Using multivariate logistic regression model, we demonstrated that the association of IGFBP-3 serum levels together with biopsy T-stage and biopsy Gleason score was useful to calculate a prognostic risk score. ROC curve analysis of risk score showed a good ability to predict GSU (AUC = 0.81; 95% CI 0.69-0.93).

CONCLUSIONS

Our results suggest that preoperative IGFBP-3 circulating levels determination may be useful to predict Gleason score upgrading alone and/or in combination with biopsy T-stage and biopsy Gleason score.

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

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

Adenovirus E1A and E1B-19K proteins protect human hepatoma cells from transforming growth factor beta1-induced apoptosis

Primary and some transformed hepatocytes undergo apoptosis in response to transforming growth factor beta1 (TGFbeta). We report that infection with species C human adenovirus conferred resistance to TGFbeta-induced apoptosis in human hepatocellular carcinoma cells (Huh-7). Protection against TGFbeta-mediated cell death in adenovirus-infected cells correlated with the maintenance of normal nuclear morphology, lack of pro-caspases 8 and 3 processing, maintenance of the mitochondrial membrane potential, and lack of cellular DNA degradation. The TGFbeta pro-apoptotic signaling pathway was blocked upstream of mitochondria in adenovirus-infected cells. Both the N-terminal sequences of the E1A proteins and the E1B-19K protein were necessary to protect infected cells against TGFbeta-induced apoptosis.

An N-terminal fragment of insulin-like growth factor binding protein-3 (IGFBP-3) induces apoptosis in human prostate cancer cells in an IGF-independent manner

OBJECTIVE

IGF-binding protein-3 (IGFBP-3) can induce apoptosis in human prostate cancer cells by direct, IGF-independent mechanisms that are poorly understood. IGFBP-3 undergoes limited proteolysis by plasmin and other proteases to generate small N-terminal fragments (e.g., amino acids 1-97) that have lost their affinity for IGF-I and IGF-II yet still can inhibit mitogenesis. The present study examines whether the N-terminal 1-97-IGFBP-3 fragment can induce apoptosis in human prostate cancer cells in an IGF-independent manner.

DESIGN

N-terminal 1-97-IGFBP-3 with or without a signal prepeptide was fused to yellow fluorescent protein (YFP) and expressed in PC-3 human prostate cancer cells. In some cases, the N-terminal IGF-binding site was mutated. Subcellular localization was determined by confocal microscopy. Loss of cell viability was determined by Annexin V-APC staining in the presence and absence of a general caspase inhibitor, z-VAD-fmk.

RESULTS

All of the fusion proteins, including those synthesized with a signal peptide, were predominantly intracellular, suggesting that they had been internalized following secretion. YFP-1-97-IGFBP-3 is present at comparable concentrations in the nucleus and cytoplasm, indicating that it does not contain a nuclear localization signal. Cells transfected with YFP-1-97-IGFBP-3 lost viability. Cell death was blocked by incubation with a caspase inhibitor suggesting that it resulted from apoptosis. Similar results were obtained with YFP-1-97-IGFBP-3 mutants that do not bind IGFs.

CONCLUSIONS

The N-terminal 1-97-IGFBP-3 fragment induces apoptosis in human prostate cancer cells in an IGF-independent manner. Generation of the fragment might contribute to the proapoptotic activity of IGFBP-3 in vivo.

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.

Effects of insulin-like growth factor (IGF) binding protein-3 (IGFBP-3) on endometrial cancer (HHUA) cell apoptosis and EGF stimulated cell proliferation in vitro

OBJECTIVE

IGFBP-3 has been demonstrated to stimulate or inhibit cell proliferation independently of its ability to bind IGF and a specific IGFBP-3 receptor has been proposed. EGF has been implicated in the cancer development and carcinogenesis. Only limited data are available on the crosstalk between IGFBP-3 signaling and EGF induced cell survival and signal transduction. The current studies were undertaken to characterize IGFBP-3 binding to endometrial cancer cells (HHUA) and determine its biological effects, as well as whether IGFBP-3 exposure alters the cell proliferation stimulated by EGF.

METHODS

Cell proliferation and apoptosis were analyzed by ELISA using specific antibodies. The interaction between HHUA cell and IGFBP-3 was analyzed using a biosensor. The phosphorylation abundance of specific proteins and their phosphorylation in response to EGF and IGFBP-3 was analyzed by immunoprecipitation followed by immunoblotting.

RESULTS

Biosensor analysis showed that IGFBP-3 could bind to HHUA cell surface. IGFBP-3 inhibited BrdU uptake, potentiated ssDNA production and induced p53 in HHUA cells. Although EGF stimulated HHUA cell proliferation and Akt phosphorylation, IGFBP-3 inhibited cell proliferation and Akt phosphorylation that had been stimulated by EGF. However, EGF receptor phosphorylation and expression were not reduced by IGFBP-3. Since HHUA cells lack IGF receptors and do not show biological response to IGF these results suggest that IGFBP-3 can bind to HHUA cells, inhibit cell proliferation and induce apoptosis independently of its ability to bind to IGFs possibly by binding to an IGFBP-3 receptor.

CONCLUSIONS

Taken together these findings demonstrate that IGFBP-3 binds to HHUA cell surface, and inhibits cell division induced by EGF, possibly by modulating the EGF-mediated signal transduction system.

TGF-beta control of cell proliferation

This article focuses on recent findings that the type V TGF-beta receptor (TbetaR-V), which co-expresses with other TGF-beta receptors (TbetaR-I, TbetaR-II, and TbetaR-III) in all normal cell types studied, is involved in growth inhibition by IGFBP-3 and TGF-beta and that TGF-beta activity is regulated by two distinct endocytic pathways (clathrin- and caveolar/lipid-raft-mediated). TGF-beta is a potent growth inhibitor for most cell types, including epithelial and endothelial cells. The signaling by which TGF-beta controls cell proliferation is not well understood. Many lines of evidence indicate that other signaling pathways, in addition to the prominent TbetaR-I/TbetaR-II/Smad2/3/4 signaling cascade, are required for mediating TGF-beta-induced growth inhibition. Recent studies revealed that TbetaR-V, which is identical to LRP-1, mediates IGF-independent growth inhibition by IGFBP-3 and mediates TGF-beta-induced growth inhibition in concert with TbetaR-I and TbetaR-II. In addition, IRS proteins and a Ser/Thr-specific protein phosphatase(s) are involved in the TbetaR-V-mediated growth inhibitory signaling cascade. The TbetaR-V signaling cascade appears to cross-talk with the TbetaR-I/TbetaR-II, insulin receptor (IR), IGF-I receptor (IGF-IR), integrin and c-Met signaling cascades. Attenuation or loss of the TbetaR-V signaling cascade may enable carcinoma cells to escape from TGF-beta growth control and may contribute to the aggressiveness and invasiveness of these cells via promoting epithelial-to-mesenchymal transdifferentiation (EMT). Finally, the ratio of TGF-beta binding to TbetaR-II and TbetaR-I is a signal controlling TGF-beta partitioning between two distinct endocytosis pathways and resultant TGF-beta responsiveness. These recent studies have provided new insights into the molecular mechanisms underlying TGF-beta-induced cellular growth inhibition, cross-talk between the TbetaR-V and other signaling cascades, the signal that controls TGF-beta responsiveness and the role of TbetaR-V in tumorigenesis.

IGFBP-3 activates TGF-beta receptors and directly inhibits growth in human intestinal smooth muscle cells

We have shown that human intestinal smooth muscle cells produce IGF-I and IGF binding protein-3 (IGFBP-3). Endogenous IGF-I acts in autocrine fashion to stimulate growth of these cells. IGFBP-3 inhibits the binding of IGF-I to its receptor and thereby inhibits IGF-I-stimulated growth. In several carcinoma cell lines and some normal cells, IGFBP-3 regulates growth independently of IGF-I. Two mechanisms for this effect have been identified: IGFBP-3 can directly activate transforming growth factor-beta (TGF-beta) receptors or it can undergo direct nuclear translocation. The aim of the present study was to determine whether IGFBP-3 acts independently of IGF-I and to characterize the mechanisms mediating this effect in human intestinal smooth muscle cells. The direct effects of IGFBP-3 were determined in the presence of an IGF-I receptor antagonist to eliminate its IGF-I-dependent effects. Affinity labeling of TGF-beta receptors (TGF-betaRI, TGF-betaRII, and TGF-betaRV) with 125I-labeled TGF-beta1 showed that IGFBP-3 displaced binding to TGF-betaRII and TGF-betaRV in a concentration-dependent fashion. IGFBP-3 stimulated TGF-betaRII-dependent serine phosphorylation (activation) of both TGF-betaRI and of its primary substrate, Smad2(Ser465/467). IGFBP-3 also caused IGF-I-independent inhibition of basal [3H]thymidine incorporation. The effects of IGFBP-3 on Smad2 phosphorylation and on smooth muscle cell proliferation were independent of TGF-beta1 and were abolished by transfection of Smad2 siRNA. Immunoneutralization of IGFBP-3 increased basal [3H]thymidine incorporation, implying that endogenous IGFBP-3 inhibits proliferation. We conclude that endogenous IGFBP-3 directly inhibits proliferation of human intestinal smooth muscle cells by activation of TGF-betaRI and Smad2, an effect that is independent of its effect on IGF-I-stimulated growth.

Identification of insulin receptor substrate proteins as key molecules for the TβR‐V/LRP‐1‐mediated growth inhibitory signaling cascade in epithelial and myeloid cells

The type V TGF-beta receptor (TbetaR-V) mediates IGF-independent growth inhibition by IGFBP-3 and mediates growth inhibition by TGF-beta1 in concert with the other TGF-beta receptor types. TbetaR-V was recently found to be identical to LRP-1. Here we find that insulin and (Q3A4Y15L16) IGF-I (an IGF-I analog that has a low affinity for IGFBP-3) antagonize growth inhibition by IGFBP-3 in mink lung epithelial cells (Mv1Lu cells) stimulated by serum. In these cells, IGFBP-3 induces serine-specific dephosphorylation of IRS-1 and IRS-2. The IGFBP-3-induced dephosphorylation of IRS-2 is prevented by cotreatment of cells with insulin, (Q3A4Y15L16) IGF-I, or TbetaR-V/LRP-1 antagonists. The magnitude of the IRS-2 dephosphorylation induced by IGFBP-3 positively correlates with the degree of growth inhibition by IGFBP-3 in Mv1Lu cells and mutant cells derived from Mv1Lu cells. Stable transfection of murine 32D myeloid cells (which lack endogenous IRS proteins and are insensitive to growth inhibition by IGFBP-3) with IRS-1 or IRS-2 cDNA confers sensitivity to growth inhibition by IGFBP-3; this IRS-mediated growth inhibition can be completely reversed by insulin in 32D cells stably expressing IRS-2 and the insulin receptor. These results suggest that IRS-1 and IRS-2 are key molecules for the TbetaR-V/LRP-1-mediated growth inhibitory signaling cascade.

Identification and Characterization of the Acidic pH Binding Sites for Growth Regulatory Ligands of Low Density Lipoprotein Receptor-related Protein-1

The type V TGF-beta receptor (TbetaR-V) plays an important role in growth inhibition by IGFBP-3 and TGF-beta in responsive cells. Unexpectedly, TbetaR-V was recently found to be identical to the LRP-1/alpha(2)M receptor; this has disclosed previously unreported growth regulatory functions of LRP-1. Here we demonstrate that, in addition to expressing LRP-1, all cells examined exhibit low affinity but high density acidic pH binding sites for LRP-1 growth regulatory ligands (TGF-beta(1), IGFBP-3, and alpha(2)M(*)). These sites, like LRP-1, are sensitive to receptor-associated protein and calcium depletion but, unlike LRP-1, are also sensitive to chondroitin sulfate and heparin and capable of directly binding ligands, which do not bind to LRP-1. Annexin VI has been identified as a major membrane-associated protein capable of directly binding alpha(2)M(*) at acidic pH. This is evidenced by: 1) structural and Western blot analyses of the protein purified from bovine liver plasma membranes by alpha(2)M(*) affinity column chromatography at acidic pH, and 2) dot blot analysis of the interaction of annexin VI and (125)I-alpha(2)M(*). Cell surface annexin VI is involved in (125)I-TGF-beta(1) and (125)I-alpha(2)M(*) binding to the acidic pH binding sites and (125)I-alpha(2)M(*) binding to LRP-1 at neutral pH as demonstrated by the sensitivity of cells to pretreatment with anti-annexin VI IgG. Cell surface annexin VI is also capable of mediating internalization and degradation of cell surface-bound (125)I-TGF-beta(1) and (125)I-alpha(2)M(*) at pH 6 and of forming ternary complexes with (125)I-alpha(2)M(*) and LRP-1 at neutral pH as demonstrated by co-immunoprecipitation. Trifluoperazine and fluphenazine, which inhibit ligand binding to the acidic pH binding sites, block degradation after internalization of cell surface-bound (125)I-TGF-beta(1) or (125)I-alpha(2)M(*). These results suggest that cell surface annexin VI may function as an acidic pH binding site or receptor and may also function as a co-receptor with LRP-1 at neutral pH.

Cellular growth inhibition by TGF-β1involves IRS proteins

In Mv1Lu cells, insulin partially reverses transforming growth factor-beta1 (TGF-beta1) growth inhibition in the presence of alpha5beta1 integrin antagonists. TGF-beta1 appears to induce phosphorylation of IRS-2 in these cells; this is inhibited by a TGF-beta antagonist known to reverse TGF-beta growth inhibition. Stable transfection of 32D myeloid cells (which lack endogenous IRS proteins and are insensitive to growth inhibition by TGF-beta1) with IRS-1 or IRS-2 cDNA confers sensitivity to growth inhibition by TGF-beta1; this IRS-mediated growth inhibition can be partially reversed by insulin in 32D cells stably expressing IRS-2 and the insulin receptor (IR). These results suggest that growth inhibition by TGF-beta1 involves IRS proteins.

Cellular growth inhibition by IGFBP‐3 and TGF‐β1requires LRP‐1

The type V TGF-beta receptor (TbetaR-V)/IGFBP-3 receptor mediates the IGF-independent growth inhibition induced by IGFBP-3. It also mediates the growth inhibitory response to TGF-beta1 in concert with other TGF-beta receptor types, and its loss may contribute to the malignant phenotype of human carcinoma cells. Here we demonstrate that TbetaR-V is identical to LRP-1/alpha2M receptor as shown by MALDI-TOF analysis of tryptic peptides of TbetaR-V purified from bovine liver. In addition, 125I-IGFBP-3 affinity-labeled TbetaR-V in Mv1Lu cells is immunoprecipitated by antibodies to LRP-1 and TbetaR-V. RAP, an LRP-1 antagonist, inhibits binding of 125I-TGF-beta1 and 125I-IGFBP-3 to TbetaR-V and diminishes IGFBP-3-induced growth inhibition in Mv1Lu cells. Absent or low levels of LRP-1, as with TbetaR-V, have been linked to the malignant phenotype of carcinoma cells. Mutagenized Mv1Lu cells selected for reduced expression of LRP-1 have an attenuated growth inhibitory response to TGF-beta1 and IGFBP-3. LRP-1-deficient mouse embryonic fibroblasts lack a growth inhibitory response to TGF-beta1 and IGFBP-3. On the other hand, stable transfection of H1299 human lung carcinoma cells with LRP-1 cDNA restores the growth inhibitory response. These results suggest that the LRP-1/TbetaR-V/IGFBP-3 receptor is required for the growth inhibitory response to IGFBP-3 and TGF-beta1.

Insulin-like growth factor-independent effects mediated by a C-terminal metal-binding domain of insulin-like growth factor binding protein-3

Insulin-like growth factors (IGFs) play a central role in the integration of proliferative and survival responses of most mammalian cell types. IGF-binding protein-3 (IGFBP-3) influences IGF action directly as a carrier of IGFs but also modulates these actions indirectly via independent mechanisms involving interactions with plasma, extracellular matrix and cell surface molecules, conditional proteolysis, cellular uptake, and nuclear transport. Here we demonstrate that a short C-terminal metal-binding domain (MBD) of IGFBP-3 mediates binding to metals. MBD epitopes, sequestered in the intact molecule, are unmasked by incubation in the presence of ferrous (but not ferric or zinc) ions. An isolated 14-mer MBD peptide triggered apoptotic effects in stressed HEK293 cells as effectively as IGFBP-3. The MBD, which encompasses a nuclear localization sequence and an adjacent putative caveolin-binding sequence, mobilizes rapid cellular uptake and nuclear localization of unrelated proteins such as green fluorescent protein and streptavidin-horseradish peroxidase conjugate. Metal ions stimulate MBD-mediated cellular/nuclear uptake in vivo. Cross-linking studies showed a direct physical interaction of MBD with integrins alphav and beta1, caveolin-1, and transferrin receptor. MBD-mediated protein mobilization and pro-apoptotic effects are inhibited by nystatin but not chlorpromazine, suggesting an involvement of caveolar-mediated endocytosis. However, MBD effects are inhibited by antibodies to transferrin receptor or integrins. These results are discussed with particular reference to the cell target specificity of IGFBP-3 in disease processes such as cancer and atherosclerosis.

Interaction of insulin-like growth factor binding protein-3 with latent transforming growth factor-beta binding protein-1

Insulin-like growth factor binding protein-3 (IGFBP-3) inhibits the replication and promotes apoptosis in various cell lines in an IGF-independent manner. We utilized a yeast two-hybrid system to identify binding partners for IGFBP-3 in a mouse embryo cDNA library. A partial cDNA encoding mouse latent transforming growth factor beta (TGF-beta) binding protein-1 (LTBP-1) was identified. This cDNA encoded a mouse LTBP-1 mRNA fragment corresponding to amino acid residues 1160-1712. Analysis of C-terminal deleted mutants indicated that the IGFBP-3 interacting domain resides in the 552 residue C-terminal fragment encoding amino acids 831-1383. The interaction of IGFBP-3 with recombinant human LTBP-1 immobilized on nitrocellulose was also demonstrated. Neither binding of IGF-1 to IGFBP-3 nor binding of latency associated protein (LAP) with LTBP-1 inhibited the interaction of IGFBP-3 with LTBP-1. Furthermore the large latent complex, 125I-TGF-beta/LAP/LTBP-1 was able to bind to immobilized IGFBP-3. These data demonstrate that IGFBP-3 can bind to LTBP-1 and provide a potential mechanism whereby IGFBP-3 can interact with the TGF-beta system.

Effects of antimicrobials and weaning on porcine serum insulin-like growth factor binding protein levels

The effects of subtherapeutic antimicrobial supplementation and weaning on serum levels of IGF-I and insulin-like growth factor binding proteins (IGFBP)-2, -3 and -4 were determined in crossbred weanling pigs. At weaning, pigs were allotted to a diet containing 21.8% crude protein and 1.15% lysine with or without Aureozol (110 mg/kg of Aureomycin chlortetracycline, 110 mg/kg of sulfathiazole, and 55 mg/kg of penicillin) for 4 wk. Insulin-like growth factor-binding proteins and IGF-I analyses were performed on blood samples that were drawn weekly. Weaning decreased serum IGFBP-3 levels in both control and Aureozol-treated groups on d 6 and d 14 (P < 0.05) relative to preweaning levels. The IGFBP-3 values returned to preweaning levels by d 21. Although the circulating levels of both the 43-kDa and the 39-kDa glycosylation variants of IGFBP-3 were affected by weaning, the level of the 39-kDa IGFBP-3 was affected relatively more than that of the 43-kDa IGFBP-3 (P < 0.05). Compared with circulating IGFBP-3 levels in control pigs, Aureozol-treated pigs had higher circulating IGFBP-3 levels on d 21 (43%, P < 0.05) and d 27 (46%, P < 0.05). In direct contrast to the effect of weaning on serum IGFBP-3 level, serum IGFBP-2 levels increased on d 6 and d 14 after weaning (P < 0.05) and decreased to preweaning levels by d 21. The IGFBP-2 levels continued to decline and were less than preweaning levels by d 27 (P < 0.05). Aureozol treatment had no effect on serum IGFBP-2 levels at any time. Serum levels of nonglycosylated IGFBP-4 were not affected by either weaning or Aureozol supplementation. Weaning decreased circulating IGF-I concentration on d 6 in both control and Aureozol-treated pigs (76 and 73%, respectively, P < 0.05) and on d 14 (62%, P < 0.05) and d 21 (32%, P < 0.05) in control pigs. Aureozol-supplemented pigs had higher serum IGF-I concentrations than control pigs on d 14 (82%, P < 0.05), d 21 (55%, P < 0.05), and d 27 (36%, P < 0.05). The Aureozol-fed pigs had a 14.2% increase in BW gain (P < 0.05) and a 59.6% increase in ADG (P < 0.05) compared with pigs fed the control diet. Both Aureozol-supplementation and weaning cause changes in serum IGFBP levels and IGF-I concentrations that might be involved in regulating rate and efficiency of growth.

Insulin-like growth factor I (IGF-I) and IGF binding protein-1 modulate prostate cancer cell growth and apoptosis: possible mediators for the effects of diet and exercise on cancer cell survival

Previous studies showed that serum from men consuming a low fat diet and undergoing exercise intervention (DE) reduced LNCaP cell growth and induced apoptosis in vitro. DE also decreased serum IGF-I and increased serum IGF binding protein-1 (IGFBP-1). The present study evaluates the effects of IGF-I and IGFBP-1 on growth and apoptosis of prostate cancer cells in vitro. When IGF-I was added to the post-DE serum, the reduction in LNCaP cell growth and the induction of apoptosis in medium containing post-DE serum alone were reversed. When IGFBP-1 was added to the pre-DE serum samples, LNCaP cell growth was reduced, and apoptosis was induced. IGF-I, long-R(3)-IGF-I (only binds IGF-I receptor), AL(31)Leu(60)-IGF-I (only binds IGFBPs), antihuman IGF-I receptor antibodies, and IGFBP-1 were then added to LNCaP cultures to determine the independent effects of IGF-I and IGFBP-1 on cell growth. Collectively, the results using these agents show that IGF-I and IGFBP-1 exert opposing effects on LNCaP cell growth and apoptosis, and IGFBP-1 acts mainly through an IGF-dependent mechanism. DE results in a decrease in serum IGF-I with increased IGFBP-1 in vivo that is associated with apoptosis and reduced LNCaP and LAPC-4 prostate cancer cell growth in vitro.

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.

Identification of STAT-1 as a molecular target of IGFBP-3 in the process of chondrogenesis

The chondrogenesis process requires the ordered proliferation and differentiation of chondrocytes. Insulin-like growth factor-binding protein (IGFBP)-3, well characterized as the carrier of insulin-like growth factor (IGF), has been reported to have intrinsic bioactivity that is independent of IGF binding. The mechanisms involved in this IGF-independent action are still unclear. Using the RCJ3.1C5.18 chondrogenic cells, which in culture progresses from undifferentiated to terminally differentiated chondrocytes, we have shown previously that IGFBP-3 has an IGF-independent, antiproliferative effect in undifferentiated and early differentiated but not in terminally differentiated chondrocytes. In the present study, cDNA microarray analysis was used to screen for genes: 1) that were regulated by IGFBP-3 in early but not in terminally differentiated chondrocytes; 2) that were regulated specifically by IGFBP-3, but not by IGF-I; and 3) whose regulation was abolished by coincubation of IGFBP-3 with IGF-I. Signal transducer and activator of transcription (STAT)-1 was the gene that, fulfilling the screening criteria, exhibited the greatest up-regulation by IGFBP-3 (>40-fold). STAT-1 gene up-regulation was confirmed by Northern analysis of cells treated with IGFBP-3 or transfected with an IGFBP-3 expression vector. Remarkably, similar results were obtained when cells were transfected with an IGFBP-3 mutant unable to bind IGFs, definitively demonstrating the IGF-independent action of IGFBP-3. Consistent with the up-regulation of STAT-1 mRNA, IGFBP-3 also increased STAT-1 protein expression. Furthermore, both IGFBP-3 and the IGFBP-3 mutant induced STAT-1 phosphorylation and its nuclear localization. An antisense STAT-1 oligonucleotide abolished the IGF-independent cell apoptosis induced by IGFBP-3. We have demonstrated that STAT-1 is a major intracellular signaling and transcriptional target of the IGF-independent apoptotic effect of IGFBP-3 in chondrogenesis.

Insulin-like growth factor (IGF)-binding protein-3 mutants that do not bind IGF-I or IGF-II stimulate apoptosis in human prostate cancer cells

Insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) can stimulate apoptosis and inhibit cell proliferation directly and independently of binding IGFs or indirectly by forming complexes with IGF-I and IGF-II that prevent them from activating the IGF-I receptor to stimulate cell survival and proliferation. To date, IGF-independent actions only have been demonstrated in a limited number of cells that do not synthesize or respond to IGFs. To assess the general importance of IGF-independent mechanisms, we have generated human IGFBP-3 mutants that cannot bind IGF-I or IGF-II by substituting alanine for six residues in the proposed IGF binding site, Ile(56)/Tyr(57)/Arg(75)/Leu(77)/Leu(80)/Leu(81), and expressing the 6m-hIGFBP-3 mutant construct in Chinese hamster ovary cells. Binding of both IGF-I and IGF-II to 6m-hIGFBP-3 was reduced >80-fold. The nonbinding 6m-hIGFBP-3 mutant still was able to inhibit DNA synthesis in a mink lung epithelial cell line in which inhibition by wild-type hIGFBP-3 previously had been shown to be exclusively IGF-independent. 6m-hIGFBP-3 only can act by IGF-independent mechanisms since it is unable to form complexes with the IGFs that inhibit their action. We next compared the ability of wild-type and 6m-hIGFBP-3 to stimulate apoptosis in serum-deprived PC-3 human prostate cancer cells. PC-3 cells are known to synthesize and respond to IGF-II, so that IGFBP-3 could potentially act by either IGF-dependent or IGF-independent mechanisms. In fact, 6m-hIGFBP-3 stimulated PC-3 cell death and stimulated apoptosis-induced DNA fragmentation to the same extent and with the same concentration dependence as wild-type hIGFBP-3. These results indicate that IGF-independent mechanisms are major contributors to IGFBP-3-induced apoptosis in PC-3 cells and may play a wider role in the antiproliferative and antitumorigenic actions of IGFBP-3.

Insulin-like growth factor binding protein-2 mediates the inhibition of DNA synthesis by transforming growth factor-beta in mink lung epithelial cells

Insulin-like growth factor binding protein-3 (IGFBP-3) has been proposed to mediate the growth inhibitory effects of transforming growth factor (TGF)-beta in breast and prostate cancer cells. Both TGF-beta and exogenous IGFBP-3 inhibit DNA synthesis in Mv1 mink lung epithelial cells (CCL64). The present study asks whether IGFBPs synthesized by CCL64 cells mediate growth inhibition by TGF-beta. CCL64 cells synthesize and secrete a single 34-kDa IGFBP that was identified as IGFBP-2 by immunoprecipitation and immunodepletion. Recombinant bovine IGFBP-2 inhibited CCL64 DNA synthesis in serum-free media in an IGF-independent manner. Coincubation with Leu(60)-IGF-I, an IGF-I analog that binds to IGFBPs with higher affinity than to IGF-I receptors, decreased the inhibition by bIGFBP-2. Leu(60)-IGF-I also decreased the inhibition of CCL64 DNA synthesis by TGF-beta by up to 70%, whereas Long-R3-IGF-I, an IGF-I analog with higher affinity for IGF-I receptors than for IGFBPs, did not decrease inhibition, suggesting that the effect of Leu(60)-IGF-I resulted from its forming complexes with endogenous IGFBPs. Leu(60)-IGF-I did not decrease TGF-beta stimulation of a Smad3-dependent reporter gene. Following incubation of intact CCL64 cells with bIGFBP-2 at 0 degrees C, bIGFBP-2 was recovered in membrane fractions; membrane association was abolished by coincubation with Leu(60)-IGF-I. If exogenous and secreted IGFBP-2 must bind to CCL64 cells to inhibit DNA synthesis, Leu(60)-IGF-I might reduce the inhibition of DNA synthesis by bIGFBP-2 or TGF-beta by inhibiting the association of IGFBP-2 in the media with CCL64 cells. Since TGF-beta does not increase IGFBP-2 abundance, we propose that TGF-beta sensitizes CCL64 cells to the latent growth inhibitory activity of endogenous IGFBP-2 by potentiating an intracellular IGFBP-2 signaling pathway or by promoting the association of secreted IGFBP-2 with the plasma membrane.