Characterization of insulin-like growth factor-binding protein 5-degrading proteases produced throughout murine osteoblast differentiation

ADAM 12-S cleaves IGFBP-3 and IGFBP-5 and is inhibited by TIMP-3

ADAMs are a family of multidomain proteins having proteolytic and cell adhesion activities. We have previously shown that ADAM 12-S, the secreted soluble form of human ADAM 12, is a catalytically active protease. We now describe the purification of full-length recombinant ADAM 12-S and demonstrate that it cleaves insulin-like growth factor binding protein-3 (IGFBP-3). This result supports a role for ADAM 12-S in the degradation of IGFBP-3 in the blood of pregnant women. Furthermore, we tested for proteolysis of other members of the IGF binding protein family and found that ADAM 12-S cleaves IGFBP-5 in addition to IGFBP-3, but does not cleave IGFBP-1, -2, -4, or -6. ADAM 12-S may therefore be the IGFBP-5 protease that is secreted by osteoblasts and other cells. Cleavage of both IGFBP-3 and -5 by ADAM 12-S was inhibited by TIMP-3, raising the possibility that TIMP-3 is a physiological inhibitor of ADAM 12-S.

The expression of metalloproteinase-2, -9, and -14 and of tissue inhibitors-1 and -2 is developmentally modulated during osteogenesis in vitro, the mature osteoblastic phenotype expressing metalloproteinase-14

During osteogenesis, in vitro, of tibial-derived rat osteoblasts (ROB) and derived clones, changes occur in the interactions of mature osteoblasts with the endogenous extracellular matrix (ECM) and these culminate in the formation of tridimensional nodules, which become sites of mineral deposition. We investigated if these changes might be mediated by remodeling of ECM, and we focused our study on the neutral metalloproteinases (MMPs), known agents of matrix remodeling, and on their tissue inhibitors (TIMPs). We report that during in vitro differentiation, osteoblasts express the secreted MMP-2 and -9 and the membrane gelatinase MMP-14. These, along with the tissue inhibitors TIMP-1 and -2, are developmentally regulated according to the maturation stage of osteoblasts. Their levels change in a similar association with osteoblast phenotypic maturation in different populations of ROB, which take different times to complete osteogenesis in vitro. MMP-14 expression coincides in both cell populations with the mature osteoblastic phenotype and is localized in the cells forming nodules. MMP-2 and -9 are expressed diffusely in the osteoblast population. Developmentally associated changes in the activation of MMP-2 are detected, associated in their timing with the expression of MMP-14 in both populations of ROB, and MMP-14 activates pro-MMP-2 in vitro. Expression of messenger RNAs (mRNAs) for the three MMPs increases up to the time of nodule formation. At this stage, TIMP-1 mRNA levels are lowest. TIMP-2 mRNA decreases throughout osteogenesis. In situ hybridization in 7-day-old rat tibias shows the strongest expression of MMP-14 among osteogenic cells, in lining osteoblasts on the newly formed trabeculae under the growth plate, and on the endosteal surface of cortical bone. Our data support the concept that the developmentally regulated expression of MMP-14 triggers localized proteolysis within the osteogenic population, concomitant in vitro to nodule formation.

Ionic products of bioactive glass dissolution increase proliferation of human osteoblasts and induce insulin-like growth factor II mRNA expression and protein synthesis

Bioglass 45S5 is an osteoproductive material, which resorbs by releasing its constitutive ions into solution. Treatment with the ionic products of Bioglass 45S5 dissolution in DMEM for 4 days increased human osteoblast proliferation to 155% of control. Two days after treatment, differential gene expression was analyzed by cDNA microarrays. Expression of a potent osteoblast mitogenic growth factor, insulin-like growth factor II (IGF-II), was increased to 290%. Additionally, there was a 168% increase in the concentration of unbound IGF-II protein in the conditioned media of treated osteoblasts. Expression levels of IGFBP-3, an IGF-II carrier protein, metalloproteinase-2 and cathepsin-D were also increased to 200, 340, and 310% of control levels, respectively. Metalloproteinase-2 and cathepsin-D are proteases that cleave IGF-II from its carrier proteins, resulting in the release of the unbound biologically active IGF-II. We suggest that the stimulatory effect of the ionic products of Bioglass 45S5 dissolution on osteoblast proliferation may be mediated by IGF-II.

Disruption of the tissue inhibitor of metalloproteinase-1 gene results in altered reproductive cyclicity and uterine morphology in reproductive-age female mice

Tissue inhibitor of metalloproteinase-1 (TIMP-1) is a multifunctional protein expressed in the uterus of essentially all species, yet the function of this protein is uncertain. To assess the role of TIMP-1 in the uterine events that occur during the murine estrous cycle, mature female TIMP-1 wild-type and null mice were monitored for reproductive cyclicity. Mice were sacrificed in each stage of the estrous cycle, and peripheral blood was collected and assayed for serum estradiol and progesterone content by RIA. Uterine morphology and TIMP-1, TIMP-2, TIMP-3, and TIMP-4 mRNA expression were also examined between genotypes in each stage of the estrous cycle. Disruption of the TIMP-1 gene product was associated with an altered reproductive cycle characterized by a significant decrease in the length of the estrus period in the null mice. Also during the period of estrus, null mice expressed significantly lower levels of uterine TIMP-3 mRNA expression, altered uterine morphology, significantly higher serum estradiol levels, and significantly lower serum progesterone levels compared to their wild-type counterparts. It is concluded from this study that TIMP-1 has a multifaceted role in regulating the murine reproductive cycle, and this control appears to be at the level of both the uterus and the ovary.

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.

Regulation and localization of insulin-like growth factor binding protein-5 gene expression in the uterus and placenta of the cyclic and early pregnant ewe

The insulin-like growth factor (IGF) system plays an important role in the regulation of uterine function and placental growth. However, there is little information regarding the localization and regulation of IGF binding protein-5 (IGFBP-5) in the reproductive tract. The distribution of this IGFBP was therefore investigated using in situ hybridization in sections of utero-placental tissue obtained throughout the estrous cycle, up to Day 55 of gestation, and on Days 16-17 from both horns of ewes with unilateral pregnancies that followed uterine transection. In nonpregnant ewes, IGFBP-5 mRNA was present at high concentrations in the maternal caruncles and luminal epithelium, and at moderate levels in myometrium. In these regions IGFBP-5 mRNA showed cyclic variations, with concentrations peaking around ovulation, whereas low expression in the endometrial stroma remained constant. During pregnancy, there was additional localization to the endometrial glands; and in all regions, with the exception of the caruncles, concentrations increased significantly with gestational age. In transected uteri, concentrations in the luminal epithelium of the pregnant horn were significantly higher than those in the nonpregnant horn. In the caruncles, IGFBP-5 mRNA formed an intense band just below the tips of the invading fetal villi. Below this band, IGFBP-5 mRNA localized to form a series of rings, which could create a route to allow the fetal villi access into the caruncular stroma for nutrient exchange. In conclusion, IGFBP-5 is abundantly expressed in the ovine reproductive tract, with both the concentration and localization differentially regulated during the cycle and pregnancy.

Different bone growth rates are associated with changes in the expression pattern of types II and X collagens and collagenase 3 in proximal growth plates of the rat tibia

Skeletal growth depends on endochondral ossification in growth plate cartilage, where proliferation of chondrocytes, matrix synthesis, and increases in chondrocyte size all contribute to the final length of a bone. To learn more about the potential role of matrix synthesis/degradation dynamics in the determination of bone growth rate, we investigated the expression of matrix collagens and collagenase 3 in tibial growth plates in three age groups of rats (21, 35, and 80 days after birth), each characterized by specific growth rates. By combining stereological and in situ hybridization techniques, it was found that the expression of matrix collagens and collagenase 3 was specifically turned on or off at specific stages of the chondrocyte-differentiation cycle, and these changes occurred as a temporal sequence that varied depending of animal growth rate. Furthermore, the expression of these matrix proteins by a growth plate chondrocyte was found to be sped up or slowed down depending of the growth rate. In addition to expression of types II and X collagen, collagenase-3 expression was found to constitute a constant event in the series of changes in gene expression that takes place during the chondrocyte-differentiation process. Collagenase-3 expression was found to show a biphasic pattern: it was intermittently expressed at the proliferative phase and uniformly expressed at the hypertrophic stage. An intimate relationship between morphological and kinetic changes associated with chondrocyte hypertrophy and changes in the expression pattern of matrix collagens and collagenase 3 was observed. Present data prove that the matrix synthesis/degradation dynamics of the growth plate cartilage varied depending on growth rate; these results support the hypothesis that changes in matrix degradation and synthesis are a critical link in the sequence of tightly regulated events that lead to chondrocytic differentiation.

Activation of the insulin-like growth factor-binding protein-5 promoter in osteoblasts by cooperative E box, CCAAT enhancer-binding protein, and nuclear factor-1 deoxyribonucleic acid-binding sequences

Insulin-like growth factor (IGF)-binding protein-5 (IGFBP-5) has IGF-dependent and -independent actions. PGE2 rapidly increases IGFBP-5 expression by osteoblasts through cAMP-dependent processes. A minimal DNA sequence required for basal and PGE2-stimulated IGFBP-5 promoter activity spans -69 to -35 bp. This region adjoins a functional TATA box and contains E box, CCAAT enhancer-binding protein (C/EBP), nuclear factor-1 (NF-1), and activator protein-2 (AP-2) transcription factor related binding motifs. In this study we compared minimal promoter sequences of -74 to +120 bp, without or with mutations in each potential regulatory element, by reporter gene expression and electrophoretic mobility shift assays. Mutation of the E box-related element reduced basal promoter activity by 50% and eliminated the 2-fold stimulatory effect of PGE2. In contrast, mutations in the C/EBP- or NF-1-related elements also reduced basal promoter activity without fully eliminating the PGE2 effect. Overexpression of C/EBPdelta stimulated basal IGFBP-5 promoter activity, and this effect was eliminated by mutating the C/EBP-binding site. However, mutation of the AP-2-binding site or overexpression of AP-2 did not correlate with basal or PGE2-induced promoter activation. By electrophoretic mobility shift assay, prominent gel shift complexes occurred with osteoblast nuclear extracts and 32P-labeled probes spanning the E box-, C/EBP-, and NF-1-related motifs. These gel shift complexes were depleted by specific binding site mutations and were enhanced by PGE2. Increased binding by extracts from PGE2-treated cultures was blocked by cycloheximide treatment. These results identify several elements as integral binding sequences for both basal and PGE2-stimulated IGFBP-5 promoter activity. They further reveal that multiple sequences within this cluster form a basic transcription unit where nuclear factors can accumulate in a protein synthesis-dependent way and enhance IGFBP-5 expression by osteoblasts in response to PGE2.

Insulin-like growth factor II signaling in neoplastic proliferation is blocked by transgenic expression of the metalloproteinase inhibitor TIMP-1

Insulin-like growth factor (IGF) II is overexpressed in many human cancers and is reactivated by, and crucial for viral oncogene (SV40 T antigen, [TAg])-induced tumorigenesis in several tumor models. Using a double transgenic murine hepatic tumor model, we demonstrate that tissue inhibitor of metalloproteinase 1 (TIMP-1) blocks liver hyperplasia during tumor development, despite TAg-mediated reactivation of IGF-II. Because the activity of IGFs is controlled by IGF-binding proteins (IGFBPs), we investigated whether TIMP-1 overexpression altered the IGFBP status in the transgenic liver. Ligand blotting showed that IGFBP-3 protein levels were increased in TIMP-1-overexpressing double transgenic littermates, whereas IGFBP-3 mRNA levels were not different, suggesting that TIMP-1 affects IGFBP-3 at a posttranscriptional level. IGFBP-3 proteolysis assays demonstrated that IGFBP-3 degradation was lower in TIMP-1-overexpressing livers, and zymography showed that matrix metalloproteinases (MMPs) were present in the liver homogenates and were capable of degrading IGFBP-3. As a consequence of reduced IGFBP-3 proteolysis and elevated IGFBP-3 protein levels, dissociable IGF-II levels were significantly lower in TIMP-1-overexpressing animals. This decrease in bioavailable IGF-II ultimately resulted in diminished IGF-I receptor signaling in vivo as evidenced by diminished receptor kinase activity and decreased tyrosine phosphorylation of the IGF-I receptor downstream effectors, insulin receptor substrate 1 (IRS-1), extracellular signal regulatory kinase (Erk)-1, and Erk-2. Together, these results provide evidence that TIMP-1 inhibits liver hyperplasia, an early event in TAg-mediated tumorigenesis, by reducing the activity of the tumor-inducing mitogen, IGF-II. These data implicate the control of MMP-mediated degradation of IGFBPs as a novel therapy for controlling IGF bioavailability in cancer.

Timp-1 is important for epithelial proliferation and branching morphogenesis during mouse mammary development

The dynamic process of mammary ductal morphogenesis depends on regulated epithelial proliferation and extracellular matrix (ECM) turnover. Epithelial cell-matrix contact closely dictates epithelial proliferation, differentiation, and survival. Despite the fact that tissue inhibitors of metalloproteinases (Timps) regulate ECM turnover, their function in mammary morphogenesis is unknown. We have delineated the spatiotemporal expression of all Timps (Timp-1 to Timp-4) during discrete phases of murine mammary development. Timp mRNAs were abundant in mammary tissue, each displaying differential expression patterns with predominant localization in luminal epithelial cells. Timp-1 mRNA was unique in that its expression was limited to the stage at which epithelial proliferation was high. To assess whether Timp-1 promotes or inhibits epithelial cell proliferation we manipulated mammary Timp-1 levels, genetically and biochemically. Down-regulation of epithelial-derived Timp-1 in transgenic mice, by mouse mammary tumor virus promoter-directed Timp-1 antisense RNA expression, led to augmented ductal expansion and increased number of ducts (P < 0.004). In these transgenics the integrity of basement membrane surrounding epithelial ducts, as visualized by laminin-specific immunostaining, was breached. In contrast to these mice, ductal expansion was markedly attenuated in the proximity of implanted recombinant Timp-1-releasing pellets (rTIMP-1), without an increase in basement membrane deposition around migrating terminal end buds. Epithelial proliferation and apoptosis were measured to determine the basis of altered ductal expansion. Luminal epithelial proliferation was increased by 55% (P < 0.02) in Timp-1-reduced transgenic mammary tissue and, conversely, decreased by 38% (P < 0.02) in terminal end buds by implanted rTIMP-1. Epithelial apoptosis was minimal and remained unaffected by Timp-1 manipulations. We conclude that Timps have an integral function in mammary morphogenesis and that Timp-1 regulates mammary epithelial proliferation in vivo, at least in part by maintaining basement membrane integrity.

Collagenase 3 is a target of Cbfa1, a transcription factor of the runt gene family involved in bone formation

Collagenase 3 (MMP-13) is a recently identified member of the matrix metalloproteinase (MMP) gene family that is expressed at high levels in diverse human carcinomas and in articular cartilage from arthritic patients. In addition to its expression in pathological conditions, collagenase 3 has been detected in osteoblasts and hypertrophic chondrocytes during fetal ossification. In this work, we have evaluated the possibility that Cbfa1 (core binding factor 1), a transcription factor playing a major role in the expression of osteoblastic specific genes, is involved in the expression of collagenase 3 during bone formation. We have functionally characterized a Cbfa motif present in the promoter region of collagenase 3 gene and demonstrated, by cotransfection experiments and gel mobility shift assays, that this element is involved in the inducibility of the collagenase 3 promoter by Cbfa1 in osteoblastic and chondrocytic cells. Furthermore, overexpression of Cbfa1 in osteoblastic cells unable to produce collagenase 3 leads to the expression of this gene after stimulation with transforming growth factor beta. Finally, we show that mutant mice deficient in Cbfa1, lacking mature osteoblasts but containing hypertrophic chondrocytes which are also a major source of collagenase 3, do not express this protease during fetal development. These results provide in vivo evidence that collagenase 3 is a target of the transcriptional activator Cbfa1 in these cells. On the basis of these transcriptional regulation studies, together with the potent proteolytic activity of collagenase 3 on diverse collagenous and noncollagenous bone and cartilage components, we proposed that this enzyme may play a key role in the process of bone formation and remodeling.

Inhibition of Matrix Metalloproteinases Prevents Allergen-Induced Airway Inflammation in a Murine Model of Asthma

Although matrix metalloproteinases (MMPs) have been reported to play crucial roles in the migration of inflammatory cells through basement membrane components in vitro, the role of MMPs in the in vivo accumulation of the cells to the site of inflammation in bronchial asthma is still obscure. In this study, we investigated the role of MMPs in the pathogenesis of bronchial asthma, using a murine model of allergic asthma. In this model, we observed the increase of the release of MMP-2 and MMP-9 in bronchoalveolar lavage fluids after Ag inhalation in the mice sensitized with OVA, which was accompanied by the infiltration of lymphocytes and eosinophils. Administration of tissue inhibitor of metalloproteinase-2 to airways inhibited the Ag-induced infiltration of lymphocytes and eosinophils to airway wall and lumen, reduced Ag-induced airway hyperresponsiveness, and increased the numbers of eosinophils and lymphocytes in peripheral blood. The inhibition of cellular infiltration to airway lumen was observed also with tissue inhibitor of metalloproteinase-1 and a synthetic matrix metalloproteinase inhibitor. These data suggest that MMPs, especially MMP-2 and MMP-9, are crucial for the infiltration of inflammatory cells and the induction of airway hyperresponsiveness, which are pathophysiologic features of bronchial asthma, and further raise the possibility of the inhibition of MMPs as a therapeutic strategy of bronchial asthma.

Differential expression and biological effects of insulin-like growth factor-binding protein-4 and -5 in vascular smooth muscle cells

Insulin-like growth factor-I (IGF-I) plays an important role in regulating vascular smooth muscle cell (VSMC) proliferation, migration, and apoptosis. The bioactivity of IGF-I is modulated by a group of high affinity, specific binding proteins (IGF-binding proteins; IGFBPs) that are present in the interstitial fluid. Previously, we have reported that porcine VSMCs synthesize and secrete IGF-I and several forms of IGFBPs, including IGFBP-2, IGFBP-4, and IGFBP-5. In this study, we examined the role of autocrine/paracrine secreted IGF-I in controlling the expression of IGFBP-4 and IGFBP-5 as well as the effects of these IGFBPs in modulating the cellular replication response to IGF-I. The concentrations of IGFBP-4 in the conditioned medium increased significantly from <50 ng/ml to 742 +/- 105 ng/ml. This increase was associated with a decrease in the activity of an IGF-I-regulated IGFBP-4 protease. In contrast, the synthesis of IGFBP-5 was inversely correlated with culture density, and its concentration decreased from 792 +/- 91 to 44 +/- 14 ng/ml. IGFBP-5 mRNA in sparse cultures was 3-fold higher compared with those in confluent cultures. This culture density-dependent change in IGFBP-5 mRNA correlated closely with endogenous IGF-I levels. Since treatment of VSMC with exogenous IGF-I increased IGFBP-5 mRNA levels, we neutralized the effect of endogenously secreted IGF-I with an anti-IGF-I antibody to determine if it would alter IGFBP-5 mRNA abundance. This resulted in a 4.4-fold decrease in IGFBP-5 mRNA levels. When added together with IGF-I, exogenous IGFBP-4 inhibited IGF-I-induced DNA synthesis in a concentration-dependent manner. IGFBP-5, on the other hand, potentiated the effect of IGF-I. Therefore, IGFBP-4 and IGFBP-5 appear to be differentially regulated by autocrine/paracrine IGF-I through distinct mechanisms. These two proteins, in turn, play opposing roles in modulating IGF-I action in stimulating VSMC proliferation.

Dual regulation of stromelysin-3 by fibroblast growth factor-2 in murine osteoblasts

Osteoblasts express stromelysin-3, a matrix metalloproteinase associated with normal remodeling processes and with stromal fibroblasts surrounding many invasive carcinomas. Fibroblast growth factors (FGFs) play an important role in skeletal development, fracture repair, and osteoblast function. The osteoblastic cell line MC3T3 was used to study the regulation of stromelysin-3 by FGF-2. Acutely, FGF-2 decreased stromelysin-3 mRNA levels, whereas prolonged treatment caused an induction of stromelysin-3 mRNA. RNA stability studies and nuclear run-off assays indicated that acute treatment with FGF-2 decreased stromelysin-3 mRNA stability but did not alter gene transcription. However, the induction of stromelysin-3 after prolonged treatment with FGF-2 resulted from increased gene transcription, with no effect on RNA stability. The stimulatory effect was protein synthesis-dependent, whereas the inhibitory effect was not. This study demonstrates dual regulation of stromelysin-3 by FGF-2: acute destabilization of stromelysin-3 mRNA, followed by induction of gene transcription. This complex regulation may be important in the function of stromelysin-3 in bone and in remodeling processes, such as wound and fracture repair.

Insulin-like growth factor-binding protein-5 is cleaved by physiological concentrations of thrombin

Insulin-like growth factor (IGF)-binding protein-5 (IGFBP-5) is cleaved by a serine protease that is secreted by fibroblasts and porcine smooth muscle cells (pSMC) in culture. To investigate whether other serine proteases could cleave this substrate at physiologically relevant concentrations, we determined the proteolytic effects of thrombin on IGFBP-5. Human alpha-thrombin (0.0008 NIH U/ml) cleaved IGFBP-5 into 24-, 23-, and 20-kDa non-IGF-I-binding fragments. Cleavage occurred at a physiologically relevant thrombin concentration. The effect was specific for IGFBP-5, as other forms of IGFBPs, e.g. IGFBP-1, IGFBP-2, and IGFBP-4 were not cleaved by thrombin. Although IGFBP-3 was cleaved by thrombin, this effect required a 50-fold greater thrombin concentration. [35S]Methionine labeling followed by immunoprecipitation confirmed that IGFBP-5 that was constitutively synthesized by pSMC cultures was also degraded by thrombin into 24-, 23-, and 20-kDa fragments. The binding of IGF-I to IGFBP-5 partially inhibited IGFBP-5 degradation by thrombin, and an IGF analog that does not bind to IGFBP-5 had no effect. Thrombin did not account for the serine protease activity that had been shown previously to be present in pSMC-conditioned medium. This was proven by showing that 1) no immunoreactive thrombin could be detected in the pSMC-conditioned medium; 2) the IGFBP-5 fragments that were generated by thrombin showed three cleavage sites (Arg192-Ala193, Arg156-Ile157, and Lys120-His121), whereas the serine protease in conditioned medium cleaves IGFBP-5 at a different site; and 3) hirudin had no effect on IGFBP-5 cleavage by the protease in pSMC medium; however, it inhibited IGFBP-5 degradation by thrombin. To determine the physiological significance of IGFBP-5 cleavage, the effect of an IGFBP-5 mutant that is resistant to cleavage by the pSMC protease and has been shown to inhibit IGF-I actions in pSMC was determined. This mutant inhibited IGF-I-stimulated DNA synthesis, but if thrombin was added simultaneously, IGF-I was fully active. In summary, physiological concentrations of thrombin degrade IGFBP-5. Degradation can be blocked by hirudin and is partially inhibited by IGF-I binding. Generation of active thrombin in vessel walls may be a physiologically relevant mechanism for controlling IGF-I bioactivity.

Insulin-like growth factor-binding protein-5 (IGFBP-5) stimulates phosphorylation of the IGFBP-5 receptor

The finding that insulin-like growth factor (IGF)-binding protein-5 (IGFBP-5) binding to mouse osteoblasts was capable of being downregulated by IGFBP-5 suggested that the 420-kDa membrane protein, which interacted with IGFBP-5, may be a signaling receptor (Andress, D. L. J. Biol. Chem. 270: 28289-28296, 1995). In the current study, a carboxy-terminal IGFBP-5 peptide, IGFBP-5-(201-218), which was found to competitively inhibit 125I-IGFBP-5 binding and to specifically bind to osteoblast monolayers, was used to affinity-purify the 420-kDa membrane protein. Co-incubation of the affinity-purified membrane protein with [32P]ATP resulted in autophosphorylation at serine residues. Serine phosphorylation of the 420-kDa protein was enhanced by intact IGFBP-5, IGFBP-5-(1-169), and IGFBP-5-(201-218). When the IGFBP-5 receptor was incubated with dephosphorylated casein in the presence of [32P]ATP, casein became phosphorylated on serine residues. These data indicate that IGFBP-5 stimulates the phosphorylation of the IGFBP-5 receptor and suggest that serine/threonine kinase activation may be important in mediating some of the IGF-independent effects of IGFBP-5.

Identification of insulin-like growth factor-binding protein-1 as a potential physiological substrate for human stromelysin-3

To elucidate the physiological role of human stromelysin-3 (hST-3) in tumor progression and/or wound healing, insulin-like growth factor-binding protein-1 (IGFBP-1) was analyzed as a potential physiological substrate. hST-3 proteolysis generates two fragments of 16 and 9 kDa that react with IGFBP-1 monoclonal antibody, although they do not bind insulin-like growth factor-I (IGF-I) in ligand blot. N-terminal sequencing shows that hST-3 cleaves IGFBP-1 at the His140-Val141 bond located in the IGFBP-1 midregion. We show that IGFBP-1 inhibits IGF-I-induced survival and proliferation of BAF/3 cells, as well as IGF-I-mediated activation of phosphatidylinositol 3-kinase (PI 3-K). Co-incubation of the IGF-I. IGFBP-1 complex with hST-3 restores IGF-I-induced proliferation and PI 3-K kinase activity in these cells. BAF/3 proliferation is significantly increased with the hST-3-treated IGF-I.IGFBP-1 complex compared with that obtained using IGF-I alone. To produce this enhanced proliferation, IGF-I must bind to IGFBP-1 before hST-3 proteolysis, demonstrated using an IGF-I variant that does not bind IGFBP. IGFBP-1 also inhibits IGF-I-induced proliferation of the MCF-7 breast adenocarcinoma, and this inhibition was not seen in hST-3-transfected MCF-7 cells. Such proteolysis may thus play a role in in vivo tumor progression. These results indicate that hST-3 may regulate IGF-I bioavailability by proteolyzing IGFBP, thus favoring cell survival and proliferation.

Interleukin-6 and its soluble receptor regulate the expression of insulin-like growth factor binding protein-5 in osteoblast cultures

Interleukin-6 (IL-6), a cytokine produced by bone cells, is known to influence bone resorption by stimulating the development of osteoclasts from precursor cells and to have mitogenic actions on osteoblastic cells. Insulin-like growth factors (IGFs) are important local regulators of bone formation, and IGF binding protein (IGFBP)-5 stimulates bone cell growth and enhances the effects of IGF-I. We tested the effects of IL-6 in the presence and absence of its soluble receptor (sIL-6R) on IGFBP-5 expression in cultures of osteoblast-enriched cells from 22-day-old fetal rat calvariae (Ob cells). When tested individually, IL-6 and sIL-6R had a modest stimulatory effect on IGFBP-5 messenger RNA (mRNA) levels. In contrast, when IL-6 and sIL-6R were tested in combination, they caused a considerable increase in IGFBP-5 mRNA levels, and IL-6 at 100 ng/ml and sIL-6R at 125 ng/ml increased IGFBP-5 transcripts by 5- to 7-fold after 24 h. The effect of IL-6 and sIL-6R on IGFBP-5 transcripts was not blocked by indomethacin, but cycloheximide markedly inhibited IGFBP-5 mRNA levels in control and treated cultures. IL-6 and sIL-6R did not modify the decay of IGFBP-5 mRNA in transcriptionally arrested Ob cells, and stimulated the rate of IGFBP-5 transcription as demonstrated by a nuclear run-on assay. IL-6 and sIL-6R did not increase intact IGFBP-5 levels in the extracellular matrix and increased IGFBP-5 fragments in the culture medium. Conditioned medium from Ob cells induced the proteolytic fragmentation of an IGFBP-5 standard, an effect that was accelerated and enhanced by conditioned medium from IL-6/sIL-6R-treated cultures and prevented by metalloprotease inhibitors. In conclusion, IL-6, in the presence of sIL-6R, stimulates IGFBP-5 mRNA expression in Ob cells by transcriptional mechanisms, and accelerates the fragmentation of the protein.

Insulin-like growth factor binding protein (IGFBP) substrate zymography. A new tool to identify and characterize IGFBP-degrading proteinases

Insulin-like growth factor binding protein (IGFBP) degrading proteinase activities have been described in biological fluids and conditioned media from numerous cell lines. To identify and characterize IGFBP-degrading proteinases, our laboratory has developed IGFBP substrate zymography. Herein, we illustrate how IGFBP substrate zymography can be used both to identify candidate IGFBP-degrading proteinases and characterize their degradative capabilities. For this purpose, human matrix metalloproteinase-3 (MMP-3), a proteinase that degrades IGFBP-3 in human fibroblast cultures, was first electrophoresed through a polyacrylamide gel containing IGFBP-3 as substrate and then analyzed for its ability to degrade the substrate into immunoreactive fragments that were absorbed onto a polyvinylidene difluoride membrane. IGFBP-3 substrate zymography was capable of detecting as little as 20 ng of human MMP-3, demonstrating a sensitivity similar to casein substrate zymography. Using the zymogram as a template, MMP-3 was identified in a standard SDS-polyacrylamide gel run in parallel with the zymogram, and the corresponding area of the gel was excised. Electroelution of the gel slice yielded active MMP-3 when examined by casein substrate zymography. Furthermore, digestion of IGFBP-3 in solution by the electroeluted MMP-3 revealed the same fragmentation pattern of the binding protein as that produced by MMP-3, which had not been electroeluted. Together, these studies demonstrate that IGFBP substrate zymography can be a useful tool for both the identification and the characterization of IGFBP-degrading proteinases.

Cloning and sequencing of human PEX from a bone cDNA library: evidence for its developmental stage-specific regulation in osteoblasts

Inactivating mutations of the neutral endopeptidase, PEX, have been identified as the cause of X-linked hypophosphatemia (XLH). Though the function of PEX is unknown, current information suggests that impaired renal phosphate conservation in XLH is due to the failure of PEX to either degrade an undefined phosphaturic factor or activate a novel phosphate-conserving hormone. The physiologically relevant target tissue for the XLH mutation has not been identified. An apparent intrinsic defect of osteoblast function in XLH implicates bone as a possible site of PEX expression. In the current investigation, we employed a polymerase chain reaction (PCR) strategy to amplify a PEX cDNA from a human bone cell cDNA library. We found that the human PEX cDNA encodes a 749 amino acid protein belonging to the type II integral membrane zinc-dependent endopeptidase family. The predicted PEX amino acid sequence shares 96.0% identify to the recently cloned mouse Pex cDNA and has 27-38% identity to other members of the metalloendopeptidase family. Using reverse transcriptase (RT)-PCR with PEX-specific primers, we detected PEX transcripts in both human osteosarcoma-derived MG-63 osteoblasts and in differentiated mouse MC3T3-E1 clonal osteoblasts but not in immature MC3T3-E1 preosteoblasts. The association of impaired mineralization of bone in XLH and the apparent developmental stage-specific expression of PEX in osteoblasts suggest that bone is a physiologically relevant site of PEX expression and that PEX may play an active role in osteoblast-mediated mineralization.

Heparin-binding, highly basic regions within the thyroglobulin type-1 repeat of insulin-like growth factor (IGF)-binding proteins (IGFBPs) -3, -5, and -6 inhibit IGFBP-4 degradation

MC3T3-E1 murine osteoblasts produce insulin-like growth factor (IGF)-binding protein-4 (IGFBP-4)-degrading proteinase activity, which is inhibited by IGFBP-3 and a highly basic, C-terminal domain of IGFBP-3. Of all the other five IGFBPs, IGFBP-5 and -6 share the highest degree of homology with this domain of IGFBP-3; therefore, we investigated whether these two IGFBPs inhibit IGFBP-4 degradation. Both IGFBP-5 and IGFBP-6 inhibit the degradation of 125I-IGFBP-4 by MC3T3-E1-conditioned media, and their inhibitory effects are variably reversed by IGFs. Synthetic peptides containing highly basic, C-terminal regions of IGFBP-5 and IGFBP-6 inhibit 125I-IGFBP-4 degradation, as does an homologous IGFBP-3 peptide, yet each peptide displays a different IC50, with the IGFBP-5 peptide being the most potent and the IGFBP-6 peptide being the least potent. In contrast, a homologous, yet neutral, IGFBP-4 peptide does not inhibit 125I-IGFBP-4 proteolysis, confirming the role of basic residues in the inhibitory process. The IGFBP-3, -5, and -6 peptides, each of which contains the heparin-binding consensus sequence XBBBXXBX, bind heparin, yet the IGFBP-3 and -5 peptides bind heparin with the highest affinities, whereas the IGFBP-6 peptide binds heparin with approximately 10-fold less affinity. Consistent with these regions being involved in proteinase inhibition, heparin completely reverses their inhibitory effects on 125I-IGFBP-4 proteolysis. Together, these data demonstrate that IGFBP-3, -5, and -6 can function as IGF-reversible inhibitors of IGFBP-4 proteolysis, likely through homologous, highly basic, heparin-binding domains contained within the conserved thyroglobulin type-1 motif present in the C-termini of these IGFBPs.

Insulin-like growth factor-I (IGF-I) and IGF binding protein-5 in Schwann cell differentiation

Schwann cells (SCs) are the myelin producing cells of the peripheral nervous system. During development, SCs cease proliferation and differentiate into either a myelin-forming or non-myelin forming mature phenotype. We are interested in the role of insulin-like growth factor-I (IGF-I) in SC development. We have shown previously SCs proliferate in response to IGF-I in vitro. In the current study, we investigated the role of IGF-I in SC differentiation. SC differentiation was determined by morphological criteria and expression of myelin proteins. Addition of 1 mM 8-bromo cyclic AMP (cAMP) or growth on Matrigel matrix decreased proliferation and induced differentiation of SCs. IGF-I enhanced both cAMP and Matrigel matrix-induced SC differentiation, as assessed by both morphological criteria and myelin gene expression. Cultured SCs also express IGF binding protein-5 (IGFBP-5), which can modulate the actions of IGF-I. We examined the expression of IGFBP-5 during SC differentiation. Both cAMP and Matrigel matrix treatment enhanced IGFBP-5 protein expression and cAMP increased IGFBP-5 gene expression five fold. These findings suggest IGF-I potentiates SC differentiation. The concomitant up-regulation of IGFBP-5 may play a role in targeting IGF-I to SCs and thus increase local IGF-I bioavailability.

Identification and characterization of a novel human matrix metalloproteinase with unique structural characteristics, chromosomal location, and tissue distribution

We have cloned a novel member of the matrix metalloproteinase (MMP) family of proteins from a human liver cDNA library. The isolated cDNA contains an open reading frame coding for a polypeptide of 508 amino acids, which has been tentatively called MMP-19. This protein exhibits the domain structure characteristic of previously described MMPs, including a signal sequence, a prodomain with the cysteine residue essential for maintaining the latency of these enzymes, an activation locus with the zinc-binding site, and a COOH-terminal fragment with sequence similarity to hemopexin. However, it lacks a series of structural features distinctive of the diverse MMP subclasses, including the Asp, Tyr, and Gly residues located close to the zinc-binding site in collagenases, the fibronectin-like domain of gelatinases, the transmembrane domain of membrane-type (MT) MMPs, and the furin-activation sequence common to stromelysin-3 and MT-MMPs. In addition, the 9-residue insertion rich in hydrophobic amino acids present at the hinge region in stromelysins is replaced in MMP-19 by a longer insertion very rich in acidic residues. On the basis of these structural characteristics, we propose that MMP-19 does not belong to any of the previously defined MMP-subclasses and may represent the first member of a new MMP subfamily. Chromosomal location of the MMP-19 gene revealed that it maps to chromosome 12q14, which is also a unique location for any MMPs mapped to date. The cDNA encoding a full-length MMP-19 was expressed in Escherichia coli, and after purification and refolding, the recombinant protein was able to degrade synthetic substrates for MMPs. MMP-19 proteolytic activity was abolished by TIMP-2 and EDTA, thus providing additional evidence that the isolated cDNA codes for an authentic MMP. Northern blot analysis of polyadenylated RNAs isolated from a variety of human tissues revealed that MMP-19 is mainly expressed in placenta, lung, pancreas, ovary, spleen, and intestine, suggesting that it may play a specialized role in these tissues.

Effects and fate of human IGF-binding protein-5 in rat osteoblast cultures

Osteoblasts prepared from calvaria of newborn rats produce insulin-like growth factors (IGF) and IGF-binding proteins (IGFBP), IGFBP-5 was discovered in bone extracts. However, we could not detect IGFBP-5 in the medium of newborn rat osteoblasts, although we found mRNA expression. To find an explanation for this discrepancy and to learn more about the physiological role of IGFBP-5 in these cells, we studied the biological activity and the fate of recombinant human (rh) IGFBP-5 in comparison to rhIGFBP-3. IGFBP-5 but not IGFBP-3 stimulated thymidine incorporation into DNA both in the absence and presence of IGF-I. However, IGFBP-5 did not enhance uridine incorporation into RNA and glucose incorporation into glycogen. 125I-rhIGFBP-5 but not 125I-rhIGFBP-3 rapidly disappeared from the culture medium consistent with the observation that endogenous (rat) IGFBP-3 but not IGFBP-5 accumulated in the medium. However, intact 125I-labeled or unlabeled rhIGFBP-5 was associated with the cell-layer matrix, whereas IGFBP-5 fragments appeared in the medium. Trapping of IGFBP-5 in the cell layer matrix may enhance local availability of IGF.

Retinoic acid stimulates the transcription of insulin-like growth factor binding protein-6 in skeletal cells

Retinoic acid has important actions on cell differentiation and osteoblastic function, and some of these actions may be mediated by changes in the insulin-like growth factor (IGF) axis. Skeletal cells synthesize IGF I and II and the six known IGF binding proteins (IGFBP). IGFBP-6 binds IGF II with high affinity and prevents IGF II-mediated effects. In fibroblasts, IGFBP-6 levels are regulated by retinoic acid, and we postulated that retinoic acid may regulate IGF II in bone by altering IGFBP-6 synthesis. We examined the effect of retinoic acid on IGFBP-6 expression in cultures of osteoblast-enriched cells from 22-day fetal rat calvariae (Ob cells). Retinoic acid caused a time- and dose-dependent increase in IGFBP-6 mRNA levels, as determined by Northern blot analysis. The effect was maximal after 48 h of treatment and observed with retinoic acid at concentrations of 10 nM to 1 microM. Retinoic acid increased IGFBP-6 polypeptide levels in the culture medium, as determined by Western immunoblot analysis. Cycloheximide at 3.6 microM slightly decreased IGFBP-6 transcripts but did not prevent the stimulatory effect of retinoic acid. The decay of IGFBP-6 mRNA in transcriptionally arrested Ob cells was similar in control and retinoic acid-treated cells, and retinoic acid increased the rates of IGFBP-6 transcription, as determined by nuclear run on assays. In conclusion, retinoic acid enhances IGFBP-6 expression in Ob cells by transcriptional mechanisms. Since IGFBP-6 prevents the effects of IGF II, increased synthesis of IGFBP-6 could mediate selected actions of retinoic acid in bone.

Degradation of interleukin 1beta by matrix metalloproteinases

Matrix metalloproteinases (MMPs) and interleukin 1 (IL-1) are implicated in inflammation and tissue destruction, where IL-1 is a potent stimulator of connective tissue cells to produce the extracellular matrix-degrading MMPs. Here, we report that IL-1beta, but not IL-1alpha, is degraded by MMP-1 (interstitial collagenase), MMP-2 (gelatinase A), MMP-3 (stromelysin 1), and MMP-9 (gelatinase B). This degradation was effectively blocked by tissue inhibitor of metalloproteinases (TIMP)-1. When IL-1beta was treated with MMPs it lost the ability to enhance the synthesis of prostaglandin E2 and pro-MMP-3 in human fibroblasts. The primary cleavage site of IL-1beta by MMP-2 was identified at the Glu25-Leu26 bond. These results suggest that IL-1beta stimulates connective tissue cells to produce MMPs, but activated MMPs in turn negatively regulate the activity of IL-1beta.

Cortisol inhibits the synthesis of insulin-like growth factor-binding protein-5 in bone cell cultures by transcriptional mechanisms

Glucocorticoids inhibit the synthesis of insulin-like growth factor-binding protein-5 (IGFBP-5) in osteoblasts, but the mechanisms involved are unknown. IGFBP-5 stimulates bone cell growth, and its inhibition by glucocorticoids may be relevant to the action of this binding protein on bone formation. We tested the effects of cortisol on IGFBP-5 expression in cultures of osteoblast-enriched cells from fetal rat calvariae (Ob cells). Cortisol decreased IGFBP-5 polypeptide levels in the extracellular matrix and caused a time- and dose-dependent decrease in IGFBP-5 mRNA. IGFBP-5 transcripts were markedly decreased by cycloheximide, and further suppressive effects of cortisol could not be determined. Cortisol did not modify the decay of IGFBP-5 mRNA in transcriptionally arrested Ob cells. Cortisol decreased IGFBP-5 hnRNA, the rate of IGFBP-5 transcription, and the activity of the murine IGFBP-5 promoter by 35% in transient transfection experiments. Deletion analysis showed that the region responsive to cortisol is from base pairs -70 to +22, and E-box-binding proteins or c-Myb-related nuclear factors may be involved in its regulation. In conclusion, cortisol inhibits IGFBP-5 transcription in Ob cells through the Myb-binding domain. This effect may be partly responsible for the effect of glucocorticoids on bone formation.

Promoter-dependent and -independent activation of insulin-like growth factor binding protein-5 gene expression by prostaglandin E2 in primary rat osteoblasts

Insulin-like growth factor (IGF) action is mediated by high affinity cell surface IGF receptors and modulated by a family of secreted IGF binding proteins (IGFBPs). IGFBP-5, the most conserved of six IGFBPs characterized to date, uniquely potentiates the anabolic actions of IGF-I for skeletal cells. In osteoblasts, IGFBP-5 production is stimulated by prostaglandin E2 (PGE2), a local factor that mediates certain effects induced by parathyroid hormone, cytokines such as interleukin-1 and transforming growth factor-beta, and mechanical strain. In this study, we show that transcriptional and post-transcriptional events initiated by PGE2 collaborate to enhance IGFBP-5 gene expression in primary fetal rat osteoblast cultures. PGE2 treatment stimulated up to a 7-fold rise in steady-state levels of IGFBP-5 mRNA throughout 32 h of incubation. Analysis of nascent IGFBP-5 mRNA suggested that PGE2 had only a modest stimulatory effect on IGFBP-5 gene transcription, and transient transfection studies with IGFBP-5 promoter-reporter genes confirmed that PGE2 enhanced promoter activity by approximately 2-fold. Similar stimulatory effects were seen with forskolin. A DNA fragment with only 51 base pairs of the 5'-flanking sequence retained hormonal responsiveness, which may be mediated by a binding site for transcription factor AP-2 located at positions -44 to -36 in the proximal IGFBP-5 promoter. Incubation of osteoblasts with the mRNA transcriptional inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole demonstrated that PGE2 enhanced IGFBP-5 mRNA stability by 2-fold, increasing the t1/2 from 9 to 18 h. The effects of PGE2 on steady-state IGFBP-5 transcripts were abrogated by preincubating cells with cycloheximide, indicating that the effects of PGE2 on both gene transcription and mRNA stability required ongoing protein synthesis. Therefore, both promoter-dependent and -independent pathways converge to enhance IGFBP-5 gene expression in response to PGE2 in osteoblasts.

Insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) functions as an IGF-reversible inhibitor of IGFBP-4 proteolysis

Previous studies have shown that insulin-like growth factor (IGF)-binding protein-4 (IGFBP-4) is degraded only in the presence of exogenous IGFs; however, we found that cation-dependent proteinase activity present in conditioned medium of MC3T3-E1 osteoblasts degrades 125I-recombinant human (rh)IGFBP-4 in the absence of IGFs. Addition of IGF-I, IGF-II, or insulin to conditioned medium had little affect on 125I-rhIGFBP-4 proteolysis, while extraction of IGFs resulted in only a approximately 10% reduction in proteinase activity. Since factors other than IGFs appeared to be involved in regulating IGFBP-4 proteolysis, we hypothesized that IGFBP-3, an IGFBP produced by many cell lines, but not MC3T3-E1 cells, might function as an inhibitor of IGFBP-4 proteolysis. Addition of rhIGFBP-3 to conditioned media inhibited 125I-rhIGFBP-4 proteolysis by 90%, while IGF-I and IGF-II reversed the inhibitory effects of rhIGFBP-3 in a dose-dependent manner. 125I-rhIGFBP-4 proteolysis was not inhibited by N-terminal rhIGFBP-3 fragments that bind IGFs, but was inhibited by two synthetic peptides corresponding to sequences contained in the mid-region or C-terminal region of IGFBP-3. Both inhibitory peptides contain highly basic, putative heparin-binding domains and heparin partially reversed the inhibitory effects of rhIGFBP-3 on 125I-rhIGFBP-4 proteolysis. These data demonstrate that rhIGFBP-3 inhibits IGFBP-4-degrading proteinase activity and binding of IGFs or glycosaminoglycans to IGFBP-3 may induce conformational changes in the binding protein, causing disinhibition of the proteinase.

Insulin-like growth factor binding protein proteolysis in bone cell models

Insulin-like growth factor binding protein (IGFBP) proteases-their identification, regulation, and biological significance-are currently an area of ardent investigation. This has developed from the very recent realization that IGFBP availability and bioactivity is determined not only by gene expression, but also by the controlled proteolytic processing of the protein in the pericellular environment. In each case identified so far, the modified IGFBP acts dramatically different from native or recombinant IGFBP in solution. This post-translational modification of IGFBP structure/function could have widespread significance since IGFBPs modulate the diverse growth-promoting activities of the IGFs. In fact, it may be argued that local IGF action is largely controlled by this mechanism. Therefore, knowledge of the form, function, and control of the various IGFBP proteases is likely to have major implications for our understanding of the physiology and pathophysiology of the IGFs.