Use of lanthanum to accurately quantify insulin-like growth factor binding to proteins on cell surfaces

Characterization, physicochemical properties and biocompatibility of La-incorporated apatites

In this study, the physicochemical properties and biocompatibilities of La-containing apatites were intensively investigated together with their characterizations in terms of composition, structure, valent state and morphology using X-ray diffraction, Fourier-transform infrared spectra, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. The results indicate that the La(3+) ion can be incorporated into the crystal lattice of hydroxyapatite resulting in the production of La-incorporated apatites (La(x)Ca(10-x)(PO(4))(6)(OH)(2+x-2y)O(y square y-x) (x> or =0.5, y<1+x/2) or La(x)Ca(10-x)(PO(4))(6)O(y square y-x) (0.5<x<2, y=1+x/2)) by high-temperature solid phase synthesis. For La content <20%, the product is composed of the major phase, La(x)-OAP, as well as a small amount of tricalcium phosphate, but for a La content of 20%, the product is pure La-incorporated oxyapatite with the formula La(2)Ca(8)(PO(4))(6)O(2) (La(2)-OAP, x=2, y=2). It is also found that the La content plays important roles in both the physicochemical properties and biocompatibilities of the La-incorporated apatites. In contrast to La-free apatite, La-incorporated apatites possess a series of attractive properties, including higher thermal stability, higher flexural strength, lower dissolution rate, larger alkaline phosphatase activity, preferable osteoblast morphology and comparable cytotoxicity. In particular, the sintered La-incorporated apatite block achieves a maximal flexure strength of 66.69+/-0.98 MPa at 5% La content (confidence coefficient 0.95), increased 320% in comparison with the La-free apatite. The present study suggests that the La-incorporated apatite possesses application potential in developing a new type of bioactive coating material for metal implants and also as a promising La carrier for further exploring the beneficial functions of La in the human body.

Association of total insulin-like growth factor-I, insulin-like growth factor binding protein-1 (IGFBP-1), and IGFBP-3 levels with incident coronary events and ischemic stroke

CONTEXT

Prior observational studies have demonstrated that the GH/IGF axis is associated with cardiovascular disease. However, this association has not been extensively studied among older adults.

OBJECTIVE

The objective of this study was to assess the association between levels of total IGF-I and IGF binding proteins (IGFBP-1, IGFBP-3) and risk of incident coronary events and ischemic stroke.

DESIGN AND PARTICIPANTS

A case-cohort analysis was conducted among adults 65 yr and older in the Cardiovascular Health Study.

MAIN OUTCOME MEASURES

A total of 534 coronary events [316 nonfatal myocardial infarctions (MIs), 48 fatal MIs, and 170 fatal coronary heart disease events] and 370 ischemic strokes were identified on follow-up. Comparison subjects were 1122 randomly selected participants from the Cardiovascular Health Study.

RESULTS

Mean follow-up time was 6.7 yr for coronary events, 5.6 yr for strokes, and 9.3 yr for comparison subjects. Hazard ratios (95% confidence intervals) associated with baseline levels of total IGF-I and IGFBPs were estimated using multivariate adjusted Cox proportional hazards models. Neither IGF-I nor IGFBP-1 levels predicted risk of incident coronary events or stroke. IGFBP-3 had an inverse association with risk of coronary events [adjusted hazard ratio per sd=0.88 (0.78-1.00), P=0.05] but was not associated with stroke. Exploratory analyses suggested that low IGF-I and low IGFBP-3 levels were significantly associated with higher risk of nonfatal MI (P<0.05) but not with risk of fatal MI or fatal coronary heart disease.

CONCLUSION

Circulating levels of total IGF-I or IGFBP-1 were not associated with risk of total coronary events or ischemic stroke among older adults, whereas low IGFBP-3 level was associated with increased risk of incident coronary events.

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.

Apoptosis resistance of senescent human fibroblasts is correlated with the absence of nuclear IGFBP-3

Signaling through the insulin/IGF axis plays a major role in determining the rate of aging in many species. IGF-binding proteins (IGFBPs) modulate the IGF pathway in higher organisms. IGFBP-3 accumulates in conditioned medium of senescent human fibroblasts, suggesting that it may contribute to the senescent phenotype. IGFBP-3 can enhance apoptotic cell death in tumor cells due to its ability to target intracellular regulators of apoptosis, including nuclear transcription factors. Senescent fibroblasts are highly resistant to apoptosis, suggesting that IGFBP-3 fails to induce apoptosis in this cell type; however, mechanisms of apoptosis resistance in senescent cells are poorly understood. To address this question, we studied the production and intracellular localization of IGFBP-3 in senescent fibroblasts. Whereas IGFBP-3 is highly overexpressed by senescent fibroblasts, IGFBP-3 was not detectable in the nucleus of senescent fibroblasts. In tumor cells, IGFBP-3 can be internalized by endocytosis, which is considered as a prerequisite for the intracellular functions of IGFBP-3 and probably also for its transport to the nucleus; we show here that endocytotic uptake of IGFBP-3 does not occur in senescent human fibroblasts. This is correlated with a generally decreased endocytotic activity of these cells, as shown with the model substrate transferrin. The data are consistent with a model where IGFBP-3 accumulation in conditioned medium of senescent fibroblasts contributes to growth arrest of these cells, whereas the failure to endocytose IGFBP-3 and the absence of nuclear IGFBP-3 may contribute to the well-established apoptosis resistance of senescent human fibroblasts.

Zinc partitions insulin-like growth factors (IGFs) from soluble IGF binding protein (IGFBP)-5 to the cell surface receptors of BC3H-1 muscle cells

Zinc (Zn(2+)) is a multifunctional micronutrient. The list of functions for this micronutrient expanded with the recent discovery that Zn(2+) retains insulin-like growth factors binding proteins (IGFBPs) on the surface of cultured cells, lowers the affinity of cell-associated IGFBPs, and increases the affinity of the cell surface insulin-like growth factor (IGF)-type 1 receptor (IGF-1R). However, currently there is no information concerning the effect of Zn(2+) on soluble IGFBPs. In the current study, the soluble IGFBP-5 secreted by BC(3)H-1 cells is shown to bind approximately 50% more [(125)I]-IGF-II than [(125)I]-IGF-I at pH 7.4. Zn(2+) is shown to depress the binding of both IGF-I and IGF-II to soluble secreted IGFBP-5; [(125)I]-IGF-I binding is affected more so than [(125)I]-IGF-II binding. Zn(2+) acts by lowering the affinity (K(a)) of IGFBP-5 for the IGFs. Scatchard plots are non-linear indicating the presence of high and low affinity binding sites; Zn(2+) affects only binding to the high affinity site. In contrast, Zn(2+) increases the affinity by which either [(125)I]-IGF-I or [(125)I]-R(3)-IGF-I binds to the IGF-1R, but depresses [(125)I]-IGF-II binding to the IGF-type 2 receptor (IGF-2R) on BC(3)H-1 cells. By depressing the association of the IGFs with soluble IGFBPs, Zn(2+) is shown to repartition either [(125)I]-IGF-I or [(125)I]-IGF-II from soluble IGFBP-5 onto cell surface IGF receptors. Zn(2+) was active at physiological doses depressing IGF binding to IGFBP-5 and the IGF-2R at 15-20 microM. Hence, a novel mechanism is further characterized by which the trace micronutrient Zn(2+) could regulate IGF activity.

Mutation of the RGD sequence does not affect plasma membrane association and growth inhibitory effects of elevated IGFBP-2 in vivo

Using insulin-like growth factor-binding protein-2 (IGFBP-2) transgenic mice (D mice) as a model of elevated IGFBP-2 expression, which is often found in unphysiological conditions, we found association of IGFBP-2 to purified plasma membranes of many organs. To determine whether the RGD (Arg-Gly-Asp) motif of IGFBP-2 mediates cell surface binding in vivo, we mutated the RGD motif of IGFBP-2 into an RGE (Arg-Gly-Glu) sequence and produced transgenic mice (E mice) which express elevated amounts of mutated IGFBP-2. Our data demonstrate that in vivo IGFBP-2 cell surface association is not dependent on the RGD motif and that mutation of this sequence does not alter growth inhibitory effects of IGFBP-2.

IGFBP-3 binding to endothelial cells inhibits plasmin and thrombin proteolysis

Insulin-like growth factor-binding protein (IGFBP)-3 contains a highly basic COOH-terminal heparin-binding region, the P3 region, which is thought to be important in the binding of IGFBP-3 to endothelial cells. IGFBP-3 and IGFBP-4, and their chimeras IGFBP-3(4) and IGFBP-4(3), were treated with plasmin and with thrombin, proteases known to cleave IGFBP-3. IGFBP-3 was highly susceptible to plasmin, whereas IGFBP-4 was less so. Substitution of the P3 region for the P4 region in IGFBP-4 (IGFBP-4(3)) increased the ability of the protease to digest IGFBP-4(3); substitution of the P4 region for the P3 region in IGFBP-3 (IGFBP-3(4)) decreased the digestion of IGFBP-3(4). When 125I-labeled IGFBP-3 or 125I-IGFBP-4(3) was first bound to vascular endothelial cells, subsequent proteolysis by either plasmin or thrombin was substantially inhibited. Proteolysis of 125I-IGFBP-3(4) was not inhibited in the presence of endothelial cells. The P3 peptide was cleaved by plasmin but not by thrombin. We conclude that the P3 region is central to proteolysis of IGFBP-3 by plasmin and thrombin, processes which were inhibited by association of IGFBP-3 with endothelial cells.

Insulin-like growth factor (IGF)-binding proteins: interactions with IGFs and intrinsic bioactivities

The insulin-like growth factor (IGF)-binding proteins (IGFBPs) are a family of six homologous proteins with high binding affinity for IGF-I and IGF-II. Information from NMR and mutagenesis studies is advancing knowledge of the key residues involved in these interactions. IGF binding may be modulated by IGFBP modifications, such as phosphorylation and proteolysis, and by cell or matrix association of the IGFBPs. All six IGFBPs have been shown to inhibit IGF action, but stimulatory effects have also been established for IGFBP-1, -3, and -5. These generally involve a decrease in IGFBP affinity and may require cell association of the IGFBP, but precise mechanisms are unknown. The same three IGFBPs have well established effects that are independent of type I IGF receptor signaling. IGFBP-1 exerts these effects by signaling through alpha(5)beta(1)-integrin, whereas IGFBP-3 and -5 may have specific cell-surface receptors with serine kinase activity. The regulation of cell sensitivity to inhibitory IGFBP signaling may play a role in the growth control of malignant cells.

Absence of growth hormone-induced avian muscle growth in vivo

Growth hormone (GH) clearly has the potential to dramatically enhance skeletal muscle accretion in red meat animals such as swine. It is generally accepted that this anabolic effect is mediated by insulin-like growth factor-I (IGF-I), a potent stimulator of proliferation and differentiation of satellite cells that are important for myofiber hypertrophy and for regeneration in postnatal muscle tissue. All available evidence suggests that the capacity for IGF-I-mediated actions of GH on avian myogenic cells is intact, and recent evidence is accumulating that GH may even have direct effects on avian skeletal muscle satellite cell proliferation and differentiation. However, with little exception, exogenous GH does not improve skeletal muscle mass, carcass protein, or any measure of muscle anabolism in domestic poultry. A primary lesion would appear to be the inability of GH to induce significant increases in circulating IGF-I concentrations in sexually immature, growing poultry. This is the case despite clear evidence of GH binding to hepatic receptors, GH-induced tyrosine phosphorylation of Janus kinase 2 (JAK2), and GH-induced expression of hepatic IGF-I mRNA and protein. Factors that should be explored with respect to this apparent discrepancy are discussed, including the regulation of IGF-I release, uptake, and interaction with cell-associated IGF binding proteins or receptors. In addition to its growth-promoting effects via IGF-I, GH has direct metabolic effects that are expressed as changes in circulating regulatory hormone and metabolite concentrations. The possibility that such changes may influence IGF-I release and action is also proposed.

Multivalent cations and ligand affinity of the type 1 insulin-like growth factor receptor on P2A2-LISN muscle cells

Mouse P2A2-LISN myoblasts are transfected cells that overexpress the human type 1 insulin-like growth factor (IGF) receptor. Because the type 1 IGF receptor is the major binding site for both IGF-I and IGF-II, this cell line is an excellent model to determine the effect of multivalent cations on ligand binding specifically to this type of receptor. Competitive binding assays were performed to characterize IGF binding and Scatchard analysis to quantify affinity (Ka). 125I-IGF-I, 125I-IGF-II, and 125I-R3-IGF-I bind only to the type 1 IGF receptor on these cells. Zn2+ increased binding of the three ligands to the type 1 IGF receptor by 17 to 35%. Cd2+ significantly increased binding of 125I-IGF-I, although by only 8%. La3+ and Cr3+ did not effect binding. Au3+ decreased IGF binding by approximately 56%. Scatchard analysis produced nonlinear concave-down plots yielding binding constants for high and low affinity sites. Zn2+ increased the strength of only the high affinity sites. Au3+ decreased the affinity of both high and low affinity sites. Zn2+ increased binding with a half-maximal effect between 40 microM and 60 microM. Half-maximal dose of Au3+ was >130 microM. Zinc, gold, and cadmium bind to similar regions within proteins (a zinc-binding motif) and only these cations were found to affect receptor binding indicating similar mechanisms of action. Thus, multivalent cations may alter IGF binding to cell surface receptors ultimately controlling growth. Physiologically this may be especially important for the growth promoting effects of Zn2+.

Controlling insulin-like growth factor activity and the modulation of insulin-like growth factor binding protein and receptor binding

The insulin-like growth factors (IGF) and insulin perform seemingly unique roles by causing the same metabolic effect: cellular hypertrophy. Although overlapping, there are different consequences to cellular hypertrophy induced by IGF and that induced by insulin. The IGF enhance the cell hypertrophy that is requisite for cell survival, hyperplasia, and differentiation, and insulin enhances cell hypertrophy primarily as a means to increase nutrient stores. The effects of IGF and insulin are controlled by the segregation of their receptors between different cell types. A model is discussed that describes the need for three hormones (IGF-I, IGF-II, and insulin) to control nutrient partitioning. Insulin receptor localization, as well as an episodic mode of secretion, evolved to perform the short-term action of clearing excess nutrients from the circulation. In contrast, a complex and interactive set of factors ensure that maximal IGF activity occurs only when conditions are optimal for growth. A relatively invariant rate of secretion and the IGF binding proteins serve to maintain a large mutable pool of IGF. This pool exists to ensure a constant supply of IGF to maintain the basal metabolic rate and to ensure that, once a cell begins to proliferate or differentiate, adequate exposure is available to complete the process even after severe short-term physiological insults. The IGF concentrations only change in response to prolonged differences in protein and energy availabilities, environmental and body temperatures, and external stress. Also, evidence is now emerging that describes a discrete role for trace nutrients in the regulation of IGF activity. In this latter regard, zinc has the notable role of targeting IGF binding proteins to the cell surface. New data are presented showing that zinc also changes the affinity of the type 1 IGF receptor and cell-associated IGF binding proteins to optimize IGF activity.

Multivalent cations depress ligand binding to cell-associated insulin-like growth factor binding protein-5 on human glioblastoma cells

The current studies quantified the effect of the multivalent cations zinc, cadmium, lanthanum, chromium, and gold (Zn2+, Cd2+, La3+, Cr3+, and Au3+) on [125I]-insulin-like growth factor ([125I]-IGF) binding to T98G human glioblastoma cells. The major binding site for the IGFs on T98G cells is IGF binding protein-5 (IGFBP-5), as determined by affinity labeling. Competitive binding studies, using either [125I]-IGF-I or [125I]-IGF-II, indicated that La3+ and Cr3+ did not affect [125I]-IGF-I or [125I]-IGF-II binding to cell-associated IGFBP-5. Zn2+, Au3+, and Cd2+ depressed binding of both [125I]-IGF-I and [125I]-IGF-II. [125I]-IGF-I and [125I]-IGF-II binding resulted in nonlinear concave-down Scatchard plots, indicating the presence of high- and low-affinity equilibrium constant of association (Ka) sites. Assuming a preexisting asymmetric model with independent high (KaHi) and low (KaLo) sites; Zn2+, Cd2+, and Au3+ eliminated KaHi and Zn2+, and Au3+ lowered KaLo, compared with control values. The same results were found, independent of whether [125I]-IGF-I or [125I]-IGF-II was used. Similarly, assuming a ligand-induced model of negative cooperativity, all three cations eliminated the initial affinity for the high affinity sites (Ka), whereas Zn2+ and Au2+ reduced the final affinity for the low affinity sites (Kf). Dose-response studies indicated that Zn2+, Au3+, and Cd2+ depressed binding with half-maximal activities of approximately 20 microM, 14-60 microM, and 50-65 microM, respectively. Zn2+, Au3+, and Cd2+ bind to similar sites on proteins (a zinc-binding motif), indicating similar mechanisms of action. A zinc-binding motif is present within the IGFBPs but not the IGFs. We demonstrate, for the first time, that multivalent cations have the potential to modulate IGF activity by decreasing the amount of IGF bound to cell-associated IGFBP-5.