Polarized secretion of IGF-I and IGF-I binding protein activity by cultured aortic endothelial cells

IGF binding protein-6 expression in vascular endothelial cells is induced by hypoxia and plays a negative role in tumor angiogenesis

Hypoxia stimulates tumor angiogenesis by inducing the expression of angiogenic molecules. The negative regulators of this process, however, are not well understood. Here, we report that hypoxia induced the expression of insulin-like growth factor binding protein-6 (IGFBP-6), a tumor repressor, in human and rodent vascular endothelial cells (VECs) via a hypoxia-inducible factor (HIF)-mediated mechanism. Addition of human IGFBP-6 to cultured human VECs inhibited angiogenesis in vitro. An IGFBP-6 mutant with at least 10,000-fold lower binding affinity for IGFs was an equally potent inhibitor of angiogenesis, suggesting that this action of IGFBP-6 is IGF-independent. The functional relationship between IGFBP-6 and vascular endothelial growth factor (VEGF), a major hypoxia-inducible angiogenic molecule, was examined. While VEGF alone increased angiogenesis in vitro, co-incubation with IGFBP-6 abolished VEGF-stimulated angiogenesis. The in vivo role of IGFBP-6 in angiogenesis was tested in flk1:GFP zebrafish embryos, which exhibit green fluorescence protein in developing vascular endothelium, permitting visualization of developing blood vessels. Injection of human IGFBP-6 mRNA reduced the number of embryonic inter-segmental blood vessels by ∼40%. This anti-angiogenic activity is conserved in zebrafish because expression of zebrafish IGFBP-6b had similar effects. To determine the anti-angiogenic effect of IGFBP-6 in a tumor model, human Rh30 rhabdomyosarcoma cells stably transfected with IGFBP-6 were inoculated into athymic BALB/c nude mice. Vessel density was 52% lower in IGFBP-6-transfected xenografts than in vector control xenografts. These results suggest that the expression of IGFBP-6 in VECs is up-regulated by hypoxia and IGFBP-6 inhibits angiogenesis in vitro and in vivo.

Diabetic microangiopathy: IGFBP control endothelial cell growth by a common mechanism in spite of their species specificity and tissue peculiarity

Endothelial cells (EC) play a role in many diseases including diabetes mellitus. EC share common functions, such as angiogenesis and vascular remodeling both regulated by proliferation and apoptosis, anti-thrombotic properties, regulation of vascular tone, control in the passage of nutrients and secretion of peptides and growth factors. However, EC are characterized by site-specificity so their characteristics depend on the organs and tissues where they are. The IGF system induces important growth factors that control cell growth in different microvascular EC (mEC). This family includes IGF-I and IGF-II peptides, their receptors and regulatory proteins IGF-binding proteins (IGFBP-1 to IGFBP-6). The IGFBP modulate their interaction with the IGF membrane receptors and might be regulated at a transcriptional and post-transcriptional level, thus determining the biological IGF-dependent effects on target cells. The IGF system is also a mediator of vascular diseases, and its altered balance might contribute to endothelial dysfunction with the development and evolution of diabetic microangiopathy. We reported here the reviewed literature of IGFBP production from various sources of mEC, showing that they predominantly express IGFBP-2 through IGFBP-5 mRNA. The different pattern of IGFBP secretion depends on the anatomical district and on the species of the tissues. Nevertheless, based on our and other experimental observations, we suggested that a common mechanism of IGFBP regulation in mEC could be hypothized. In retinal and glomerular EC the IGFBP4/IGFBP5 ratio controls the response of these cells to IGF-I and high levels of glucose, in terms of cellular growth.

Elevated levels of the IGF-binding protein protease MMP-1 in asthmatic airway smooth muscle

We have previously demonstrated that the asthma-associated proinflammatory eicosanoid leukotriene D4 (LTD4) is co-mitogenic with insulin-like growth factors (IGFs) in airway smooth-muscle (ASM) cells in vitro. This synergistic effect of LTD4 and IGF on ASM cell growth involves proteolysis of ASM-produced IGF binding proteins (IGFBPs), which are cell growth-inhibitory proteins. We also identified this IGFBP protease to be the matrix metalloproteinase-1 (MMP-1), and showed that this enzyme had a significant role in modulating IGF action in ASM cells. In the present study, we tested the hypothesis that ASM hyperplasia in vivo involves induction of MMP-1 leading to IGFBP proteolysis. We detected the presence of MMP-1 and measured its levels in human airway tissue sections prepared from nonasthmatic and asthmatic subjects. Six nonasthmatic and six asthmatic airway tissue samples were analyzed for immunoreactive MMP-1 through an immunohistochemical detection method. Both the bronchial and tracheal smooth-muscle cells from different regions of the same sample were examined and documented. The immunostaining for MMP-1 was significantly elevated in both the bronchial and tracheal smooth-muscle cells of the airway sections from asthmatic samples relative to that of the nonasthmatic samples. The differences in levels of MMP-1, IGFBP-2, IGFBP-3, and IGFBP proteolytic activity were quantified using densitometric analyses of the ASM tissue extracts that were separated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The MMP-1 levels in the asthmatic airway tissue extracts were 12-fold higher than those found in control samples. In addition, IGFBP-2 and IGFBP-3, which we have previously demonstrated to be proteolytic substrates of MMP-1, were found to be cleaved in asthmatic airway tissue extracts. Furthermore, the asthmatic airway extracts contained IGFBP proteolytic activity that was shown by immunodepletion studies to be due to MMP-1. These observations demonstrate that MMP-1 may play a significant role in inducing ASM hyperplasia and airway obstruction in asthma by modulating the IGF axis.

Insulin-like growth factor: receptor and binding proteins in human retinal endothelial cell cultures of diabetic and non-diabetic origin

Human retinal endothelial cell (HREC) cultures of diabetic and non-diabetic origin were examined for the production of insulin-like growth factor I (IGF-I), IGF-I receptor and IGF-binding proteins (IGFBPs) using colloidal gold quantitative immunocytochemistry and quantitative reverse transcriptase-polymerase chain reaction (QRT-PCR). The levels of immunoreactivity for IGF-I receptor and for four IGFBPs (IGFBP-1, -2, -3 and -5) were significantly increased in diabetic HREC cultures. Moreover, diabetic HREC cultures showed significantly less immunoreactivity for IGF-I and for IGFBP-4 as compared to non-diabetic HREC cultures. Message levels for IGF-I decreased two-fold in diabetic HREC and correlated with protein levels. Message levels for IGFBP-1, -2 and -5 increased 1.5-, 1.7- and 1.6-fold, respectively, in diabetic HREC and correlated with protein levels. However, the protein levels for IGF-I receptor and IGFBP-3 and -4 did not correlate with mRNA levels. There were no differences in mRNA levels for IGF-I receptor and IGFBP-3 and -4 between diabetic and non-diabetic HREC cultures, suggesting a post-transcriptional regulation of IGF-I receptor and the two IGFBPs. The net effect, however, supports enhanced IGF-I action in HREC cultures of diabetic origin which is an important cellular event in diabetic retinopathy.

Anabolic effects of 1,25-dihydroxyvitamin D3 on osteoblasts are enhanced by vascular endothelial growth factor produced by osteoblasts and by growth factors produced by endothelial cells

Human osteoblast-like cells (HOB) produce vascular endothelial growth factor (VEGF), the steady state level of which is stimulated by 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. As osteoblasts and endothelial cells are proximally located in skeletal tissue, we investigated the anabolic effects of 1,25-(OH)2D3 and VEGF on HOB cocultured with endothelial cells. When HOB with high alkaline phosphatase (Al-P) activity and human umbilical vein endothelial cells (HUVEC) with little activity were cultured together, Al-P activity increased, accompanied by an increase in cell number. When HOB and HUVEC were cultured separately, 1,25-(OH)2D3 did not directly stimulate [3H]thymidine incorporation into HUVEC, but stimulated it in the presence of HOB. VEGF did not directly stimulate the Al-P activity of HOB but stimulated it in the presence of HUVEC. The conditioned medium of HOB stimulated the proliferation of HUVEC, and this was partially blocked by anti-VEGF antibody. Conversely, the conditioned medium of HUVEC increased Al-P activity and [3H]thymidine incorporation into HOB, and this was partially blocked by antiinsulin-like growth factor I antibody and BQ-123, a specific antagonist of the endothelin-1 (ET-1) receptor. 1,25-(OH)2D3 stimulated the release of VEGF and ET-1 from HOB and HUVEC, respectively. Furthermore, the 1,25-(OH)2D3-induced release of VEGF was enhanced in HOB cocultured with HUVEC. A quantitative reverse transcription-PCR study revealed that genes for VEGF receptors (Flt-1 and KDR) were expressed in HUVEC, but not in HOB, and that 1,25-(OH)2D3 increased the levels of expression of VEGF receptor genes in endothelial cells only when cocultured with HOB. In summary, we demonstrated that 1,25-(OH)2D3 exerts an anabolic effect on osteoblasts by enhancing their production of VEGF, which stimulates its receptors on endothelial cells, followed by increased production of osteotropic growth factors, such as insulin-like growth factor I and ET-1. These in vitro findings suggest that the VEGF/VEGF receptor system may be involved in both bone formation and bone remodeling in vivo.

Paracellular transport of insulin-like growth factor-I (IGF-I) across human umbilical vein endothelial cell monolayers

Insulin-like growth factors (IGFs) are well defined mitogens and growth promoters, which are found in blood associated with high affinity IGF binding proteins (IGFBPs). In vivo, the endothelium is potentially the primary site of uptake of IGFs or IGF-IGFBP complexes from blood for transport to the extravascular space. However, the pathway and mechanisms by which IGFs cross the endothelial cell barrier are not known. The presence of high affinity receptors for IGF-I and IGF-II on human umbilical vein endothelial (HUVE) cells was demonstrated by (i) radio-receptor assays using both IGF-I and IGF-II and (ii) affinity label cross-linking studies. In addition, Western ligand blotting and immunoblotting revealed that IGFBP-2, -3, and -4 are secreted into serum-free media conditioned by confluent HUVE cell monolayers. To study transendothelial migration of IGF-I, HUVE cells were grown on microporous membranes in a bichamber system. When compared with membranes without cells, HUVE monolayers restricted the passage of 125I-IGF-I and [3H]inulin, whereas the control Madin Darby canine kidney (MDCK) cell line virtually excluded all passage of these molecules. Transport of 125I-IGF-I across HUVE cell monolayers was not significantly different to that of [3H]inulin, a paracellular probe. Moreover, 125I-IGF-I transport was not inhibited by either excess unlabelled IGF-I or a monoclonal antibody to the type I IGF receptor at a concentration shown to inhibit 125I-IGF-I binding to HUVE cell monolayers. Our findings show that the movement of free IGF-I across HUVE cell monolayers occurs via a paracellular route and not by a receptor-mediated, transcellular pathway.

Enhanced expression of dihydrofolate reductase by bovine kidney epithelial cells results in altered cell morphology, IGF-I responsiveness, and IGF binding protein-3 expression

The kidney epithelial cell line (MDBK) secretes primarily insulin-like growth factor binding protein (IGFBP)-2 under basal conditions, but exposure to forskolin decreases the synthesis of and induces IGFBP-3. Since IGFBP-3 has been shown to both potentiate and inhibit insulin-like growth factor (IGF) bioactivity, MDBK cells were transfected with an expression vector containing bovine IGFBP-3 cDNA and the dihydrofolate reductase (DHFR) gene as a selectable marker, with the goal of obtaining an epithelial cell line which constitutively secreted IGFBP-3. Stable clones which secreted greater than 100 ng/ml of IGFBP-3 were obtained and designated MDBKpMONBP-3. Northern blotting indicated that endogenous IGFBP-3 mRNA, which was undetectable in wild-type (WT) MDBK cells, was expressed in MDBKpMONBP-3 cells while the IGFBP-3 transgene did not appear to be expressed. DHFR mRNA transcripts were also expressed by MDBKp-MONBP-3 cells, whereas these transcripts were not detected in WT MDBK cells, suggesting that gene amplification of DHFR may have allowed cells to survive in methotrexate (MTX) without taking up the expression vector. In addition to the altered pattern of IGFBP-3 secretion, a marked alteration in cell morphology was observed. MDBKpMONBP-3 cells grew in distinct islands and exhibited dome formation (a characteristic of differentiated epithelial cells) whereas the WT cells did not. The alterations in morphology and IGFBP-3 expression were irreversible, since MDBKpMONBP-3 cells failed to revert to the WT phenotype upon removal of MTX and dialyzed serum. Since vectorial secretion of proteins is often associated with epithelial cell differentiation, cells were plated on tissue culture inserts which allowed conditioned media (CM) to be collected from both the apical and basal surfaces of confluent monolayers. Release of IGFBP-2 was approximately equal from apical and basal surfaces in WT MDBK cells. In contrast, release of both IGFBP-2 and IGFBP-3 was greater (3.1-fold and 3.5-fold, respectively) from basal as compared to apical surfaces of the MDBKpMONBP-3 cells. To determine if cells which were secreting IGFBP-3 had altered growth responses to IGF-I, cells were grown in serum-free media in the presence of IGF-I (0 to 100 ng/ml). Treatment of MDBKpMONBP-3 cells with 100 ng/ml of IGF-I increased cell number 138 +/- 37% above serum-free controls compared to 73 +/- 10% in WT MDBK cells.(ABSTRACT TRUNCATED AT 400 WORDS)