Recombinant PAPP-A resistant insulin-like growth factor binding protein 4 (dBP4) inhibits angiogenesis and metastasis in a murine model of breast cancer

Background The Insulin-like growth factor (IGF) pathway plays a role in tumour development and progression. In vivo, IGF1 activity is regulated by the IGF binding proteins (IGFBPs). IGFBP4 inhibits the activity of IGF1 but proteolytic cleavage by pregnancy-associated plasma protein-A (PAPP-A) releases active IGF1. A modified IGFBP4, dBP4, which was resistant to PAPP-A cleavage but retained IGF1 binding capacity, was engineered, expressed in Human Embryonic Kidney (HEK) 293 cells and purified. This study examined the effects of dBP4 on IGF1-induced cell migration, invasion and angiogenesis in vitro. The effect of intra-tumour injections of dBP4 on tumour angiogenesis and metastasis was examined using the 4T1.2luc orthotopic model of breast cancer. Methods PAPP-A resistance and IGF binding capacity of dBP4 were characterized by Western blot and surface plasmon resonance, respectively. 4T1.2luc are mouse mammary adenocarcinoma cells transfected with luciferase to allow in vivo imaging. The effect of dBP4 on IGF1-induced Akt activation in 4T1.2luc cells was assessed by Western blot. Cell migration and invasion assays were performed using 4T1.2luc cells. Angiokit™ assays and Matrigel® implants were used to assess the effects of dBP4 on angiogenesis in vitro and in vivo, respectively. An orthotopic breast cancer model – 4T1.2luc cells implanted in the mammary fat pad of BALB/c mice – was used to assess the effect of intra tumour injection of purified dBP4 on tumour angiogenesis and metastasis. Tumour growth and lung metastasis were examined by in vivo imaging and tumour angiogenesis was evaluated by CD31 immunohistochemistry. Results Our engineered, PAPP-A resistant IGFBP4 (dBP4) retained IGF1 binding capacity and inhibited IGF1 activation of Akt as well as IGF1-induced migration and invasion by 4T1.2 mammary adenocarcinoma cells. dBP4 inhibited IGF1-induced angiogenesis in vitro and in Matrigel implants in vivo. Direct intra-tumour injection of soluble dBP4 reduced angiogenesis in 4T1.2 luc mammary tumours tumour and reduced lung metastasis. Conclusion A PAPP-A resistant IGFBP4, dBP4, inhibits angiogenesis and metastasis in 4T1.2 mammary fat pad tumours. This study highlights the therapeutic potential of dBP4 as an approach to block the tumour-promoting actions of IGF1.


Background
The insulin-like growth factor (IGF) pathway is involved in proliferation, differentiation, survival, metastasis and drug resistance in various cancers, including breast. The IGF pathway consists of the IGF ligands, IGF1 and 2, their receptors, IGF1R and IGF2R, the insulin receptor (IR), and the IGF binding proteins. Circulating IGF1 and IGF2 are mainly produced by the liver but also by neoplastic tissue [1]. IGF1 binding to the type I IGF receptor (IGF1R), activates downstream signalling pathways, including the phosphatidylinositol-3-kinase (PI3K)/Akt and mitogen-activated protein kinase (MAPK) pathways [2] leading to cellular transformation, proliferation and differentiation [3][4][5]. Biological activity of IGFs is regulated by the insulin-like binding proteins (IGFBPs) which function as carrier proteins, protecting IGFs from degradation, increasing their half-life and regulating bioavailability. IGFBPs have a higher affinity for IGFs than the IGF1R and the IGFBPs can either inhibit or enhance IGF activity [6][7][8]. IGFBP4 inhibits IGF1-induced cell proliferation and differentiation in vitro in a number of cell types including bone, muscle and prostate cancer cells [9][10][11][12][13][14]. IGF is released from IGFBP4 when IGFBP4 is cleaved by the IGFBP4-specific protease, pregnancyassociated plasma protein A (PAPP-A). PAPP-A cleaves IGFBP4 at a single site and its activity is IGF-dependent [15][16][17]. PAPP-A is expressed by a range of cell types including fibroblasts and osteoblasts [18] and we previously presented evidence that PAPP-A is expressed by host cells within 4T1.2 mammary tumours [19]. There is increasing evidence in the literature that PAPP-A is involved in the growth of a number of different tumour types [20][21][22]. More recently, a PAPP-A neutralising antibody has shown promise in the treatment of ovarian tumours utilising in vivo models [23].
IGFs are required for normal mammary gland development but are also implicated in the development and maintenance of tumours. We have previously shown that when transfected with a plasmid expressing PAPP-A resistant rat IGFBP4 (dBP4), growth of 4T1.2 cells in the mammary fat pad was inhibited. In addition, there was increased endothelial cell apoptosis within 4T1.2 tumours expressing dBP4 relative to tumours expressing wild type IGFBP4 or transfected with empty vector [19]. Having identified the anti-cancer potential of inhibiting the IGF pathway, we set about to accurately recapitulate the clinical setting through the development of a novel therapeutic for exogenous administration.
Here we show that purified, recombinant murine PAPP-A resistant IGFBP4 (dBP4) retains IGF1 binding capacity and inhibits IGF1 activation of Akt signalling in 4T1.2 cells as well as IGF1-induced cell migration and invasion. dBP4 also inhibited IGF1-induced angiogenesis in vitro and in Matrigel implants in vivo. Direct intra-tumour injection of soluble dBP4 reduced angiogenesis in 4T1.2 luc mammary tumours tumour and reduced lung metastasis. These results demonstrate the therapeutic potential of PAPP-A resistant IGFBP4 as a breast cancer therapeutic.

dBP4 and wtIGFBP4 protein purification
The PAPP-A cleavage site of mouse wild type IGFBP4 (Accession number NM_010517) was changed from 119-KHMAKVRDRSKMK-133 to 119-AAMAAVADASA MA-133 to generate a cleavage-resistant IGFBP4 variant, which we termed dBP4 (Genscript, UK). The signal sequence was retained and the termination codon removed such that dBP4 was cloned in frame into the XbaI and XhoI sites of pTriEX4neo (Novagen, UK) with a vectorderived 3'HIS tag. wtIGFBP4 and dBP4 were expressed from pTriEx4neo under the control of the CMV promoter with a 3' HIS-tag. HEK 293 cells which do not express endogenous IGFBP4 were transfected with wtIGFBP4 or dBP4 and selected in G418 (neomycin) to produce a stably transfected cell line designated HEK-wtIGFBP4 or HEK-dBP4. HIS-tagged recombinant protein was purified from serum free conditioned medium using Ni-MAC columns (Novagen, UK). Protein purity was evaluated by Silver staining (Thermo Scientific) following fractionation by SDS-PAGE.
Migration and invasion assay 10 5 cells in 200 μl serum free medium (MEMα, (Biosera, UK), 0.05% (w/v) BSA) were seeded into 8 μm Transwell chambers (Falcon, Becton Dickinson, Ireland) and allowed to adhere for 1 h at 37°C. Inserts containing the adhered cells were transferred to a second plate and allowed to migrate for 6 h at 37°C towards 600 μl serum free medium alone, or serum free mediumsupplemented with 5% (v/v) FBS, 100 ng/ml (13.16 nM) recombinant human IGF1, or IGF1 (13.16 nM) preincubated with 2.5 μg/ml (73.53 nM) dBP4. Cells were fixed in 10% (v/v) buffered formalin at 4°C for 16 h. Adhered cells were washed for 5 min in 0.1% (v/v) Triton X-100 followed by 5 min in water. Unmigrated cells were wiped off upper surface of membranes which were then placed on a glass slide in Prolong® gold antifade reagent with DAPI (Invitrogen) and a cover slip. Migrated cells (three fields of view/membrane) were counted under a fluorescent microscope (Olympus, BX51) at 200× magnification using Image J software (NIH, USA). The invasion assay protocol was similar to the migration assay protocol except cells were added to Transwells in Matrigel (1:1 cells in SFM:Matrigel®) (BD Biosciences). Cells were allowed to invade for 16 h at 37°C and processed as above.
In vitro angiogenesis assay TCS cellworks Angiokit™ (TCS cellworks, UK) was used to determine the effect of dBP4 on tubule formation in vitro. Human microvascular endothelial cells were treated with IGF1 (100 ng/ml), dBP4 (2.5 μg/ml) or IGF1 (100 ng/ml) pre-incubated for 1 h with dBP4 (2.5 μg/ml). VEGF (10 ng/ml) and suramin (20 μM) were included as positive and negative controls respectively. Medium was changed every 2-3 days and on day 10 tubules were stained with CD31. Number of tubules and junctions in 3 fields of view/well (n = 3) were scored using the TCS cellworks Angiosystem image analysis software.

PAPP-A cleavage assay
10 μg purified recombinant dBP4 or recombinant wtIGFBP4 (R&D Systems, UK) was incubated with 10 μg/ml IGF1 (R&D Systems, UK) with or without conditioned medium from cells expressing recombinant PAPP-A for 24 h at 37°C. Following digestion, IGFBP4 cleavage was analysed by western blot using an anti-IGFBP4 antibody which identifies both the intact protein and the cleavage products (Millipore, Ireland).

Surface Plasmon resonance
Surface plasmon resonance experiments were carried out on a Biacore T100 instrument (GE Healthcare). Recombinant mouse IGF1, 15 μg/ml in 10 mM sodium acetate pH 4.75, (R&D systems) was coupled to the EDC/NHS activated dextran matrix in flow cell 4 of a series S CM5 sensor chip (GE Healthcare), to a density of 130 response units. Flow cell 3 was left blank to serve as a reference cell. Subsequently, unreacted groups were blocked by a 7 min injection of 1 M ethanolamine, pH 8.0. To collect kinetic binding data, a twofold serial dilution of analyte (mouse wtIGFBP4/ dBP4), ranging from 50 nM to 0.4 nM, in 10 mM HEPES pH 7.4, 150 mM NaCl, 1 mM CaCl 2 and 0.05% Tween-20, was injected over flow cells 3 and 4 at a flow rate of 30 μl/min. The association phase was 120 s, followed by a 240 s dissociation phase. Binding analysis was performed at 37°C. The surfaces were regenerated by a 60 s injection of 10 mM glycine pH 2.0.
Data were collected at a rate of 10 Hz. Recorded signals were subtracted the background signal, as determined by the response obtained from the reference cell. Global fitting of a 1:1 L model was performed, using the Biacore T200 Evaluation Software, version 1.0.

In vivo models
Animals were housed in a licensed biomedical research facility (RCSI, St. Stephen's Green) and had ad libitum access to animal chow. All procedures were reviewed by RCSI Ethics Committee, carried out under animal license guidelines of the Department of Health, Ireland and in accordance with the UK Co-ordinating Committee on Cancer Research (UKCCCR) Guidelines for the Welfare of Animals in Experimental Neoplasia [25].

Angiogenesis assay
Matrigel® basement membrane matrix (BD Biosciences, Ireland) was placed on ice overnight then mixed with PBS (12.5 μl; vehicle control), VEGF (10 ng/ml; positive control), 100 ng/ml IGF1 (13.16 nM) or 100 ng/ml IGF1 (13.16 nM) pre-incubated with 2.5 μg/ml dBP4 (73.53 nM). 12 week old female BALB/C mice (n = 5/ group) were anaesthetised under 5% (v/v) isoflurane in oxygen. 500 μl of the prepared Matrigel® solution was injected slowly subcutaneously above right hind limb. 7 days later mice were anaesthetised and euthanised by cervical dislocation. Matrigel® implants were excised, fixed in 10% (v/v) buffered formalin overnight then stained with CD31. 4T1.2luc model of metastatic breast cancer 5 × 10 4 4T1.2luc cells were implanted into the mammary fat pad of 12 week old female BALB/c mice (n = 3/ group). Tumour growth was monitored by caliper measurements every 2-3 days and the mean tumour diameter (MTD) calculated (square root of the product of length by breadth). When tumours reached a MTD of 8-8.5 mm, mice received intra tumour injections of 50 μg dBP4 or PBS (vehicle control) every 2-3 days. The schedule of injections was chosen after demonstrating that dBP4 remained intact when incubated in mouse plasma for up to 96 h (data not shown). Injections were administered in clockwise rotation, with a single injection per treatment. Mice were paired and treated for 12 (mouse pair 3), 16 (mouse pair 2) or 22 days (mouse pair 3) and the experiment terminated when the tumours in the control animals approached an MTD of 17 mm. Mice were euthanized by cervical dislocation under anaesthesia. 10 min prior to euthanasia, mice received an intraperitoneal injection of 150 mg/Kg luciferin. Primary tumours were imaged prior to euthanasia, and post-mortem lungs were excised and imaged ex vivo using the IVIS (Living Image 3.02 software).

Results
Purified, recombinant dBP4 binds IGF1 and is resistant to PAPP-A cleavage Purified dBP4 was visualized by silver staining following SDS-PAGE fractionation (Fig. 1a). To confirm that mutation of the PAPP-A cleavage site from 119-KHMAK VRDRSKMK-133 to 119-AAMAAVADASAMA-133 rendered IGFBP4 resistant to cleavage by PAPP-A, purified dBP4 was treated with recombinant PAPP-A in the presence of IGF1 and digestion products were identified by Western blotting with anti-IGFBP4 antibody after SDS-PAGE fractionation (Fig. 1b). Although PAPP-A cleaved recombinant wtIGFBP4, with cleavage products visible mainly at 14 kDa and 18 kDa, dBP4 remained intact migrating at approximately 34 kDa. In another unpublished study we found that purified HIS tagged wtIGFBP4 was also cleaved by PAPP-A (data not shown). IGF1 binding affinity of purified wtIGFBP4 and dBP4 was determined by Biacore analysis (Fig. 1c). The fitted constants are: k a = 2.110 ± 0.004 × 10 5 M − 1 s − 1 , k d = 8.491 ± 0.038 × 10 − 4 s − 1 , resulting in a K D = 4.02 × 10 − 9 M (wtIGFBP4) and k a = 3.465 ± 0.005 × 10 5 M − 1 s − 1 , k d = 1.258 ± 0.004 × 10 − 3 s − 1 , resulting in a K D = 3.63 × 10 − 9 M (dBP4) showing that IGF1 binding capacity of dBP4 was comparable to wtIGFBP4. These data also demonstrate that the HIS tag and purification protocol did not interfere with IGF binding capacity or susceptibility to PAPP-A cleavage.

dBP4 inhibits IGF1 induced migration and invasion of 4T1.2luc cells
Migration and invasion are important steps in the formation of metastases in distant organs. Cell migration towards FBS or IGF1 was significantly (p < 0.001 and p < 0.01, respectively) increased compared to PBS controls.

dBP4 inhibits IGF1 induced angiogenesis in vitro
As dBP4 inhibited 4T1.2luc cell migration and invasion, key steps in metastasis, we also examined the effect of dBP4 on IGF1 induced angiogenesis which is also crucial to metastasis formation. The Angiokit™ is a 24 well plate pre-coated with human endothelial cells co-cultured with other human cells in medium. Following treatment, cells were stained with CD31 to visualise tubule formation. Tubule formation by untreated endothelial cells is shown in Fig. 3a. VEGF, the positive control (Fig. 3b), increased tubule formation relative to the untreated controls (Fig. 3a). Suramin, an angiogenesis inhibitor (Fig.  3c), decreased tubule formation compared to the untreated and VEGF treated controls (Figs. 3a & 2b, respectively). However, IGF1 pre-incubated with dBP4 or dBP4 alone markedly decreased tubule formation compared to the other treatments ( Fig. 3e & f, respectively). As dBP4 alone blocked tubule formation it suggests that there is IGF1 present in the assay already from either the growth factor cocktail or produced by the endothelial or other cells present in the assay.
CD31 positive tubules were quantified using the TCS Cellworks Angiosystem image analysis software. Fig. 3g and h show the effect of dBP4 on tubule number and junction number, respectively. Cells treated with VEGF formed significantly more tubules than untreated controls (p < 0.001) and suramin decreased tubule number compared to untreated controls (p < 0.01). IGF1 treatment did not increase tubule number compared to untreated controls. However, the medium supplied in the Angiokit™ contains a proprietary cocktail of growth factors and it is likely that IGF1 is included in this cocktail possibly accounting for the lack of difference between IGF1 treated and untreated controls. Treatment with IGF1 and dBP4 or dBP4 alone significantly decreased tubule number compared to untreated controls or IGF1treated cells (p < 0.001). These results further suggest the presence of IGF1 in the growth factor cocktail as dBP4 significantly reduced tubule formation in both the presence and absence of exogenous IGF1. When the number of tubule junctions was quantified, a similar effect to that on tubule number was seen. As expected, cells treated with the positive control, VEGF, and the negative control, suramin, had significantly increased or decreased junctions, respectively, compared to untreated controls (p < 0.001). IGF1 treated cells did not have a significantly different number of junctions compared to untreated controls. Treatment with IGF1 and dBP4 or dBP4 alone significantly decreased junction numbers compared to IGF1 treated or control cells (p < 0.001).
CD31+ cells were counted in 3 fields of view for each Matrigel® implant and (Fig. 4g). Positive control implants containing VEGF had significantly more CD31+ endothelial cells compared to the negative control implants containing PBS (p < 0.01). IGF1 implants also had significantly increased endothelial cells compared to PBS controls (p < 0.05). Implants containing IGF1 and dBP4 had significantly fewer endothelial cells than IGF1 implants (p < 0.001). Implants containing dBP4 alone had comparable numbers of endothelial cells to negative controls (p = n.s.). These results indicate that dBP4 inhibits IGF1-induced angiogenesis in vivo.

dBP4 inhibits angiogenesis and metastasis of 4T1.2luc tumours
The preceding data and our previous study using 4T1.2 cells transfected with dBP4 expression plasmid [19] suggested that administration of purified dBP4 protein would inhibit tumour angiogenesis. As a precursor to future studies to examine the effects of systemic administration, we conducted a pilot study using direct intratumour injection of purified, dBP4. 4T1.2luc cells were implanted into the mammary fat pad of female BALB/c mice (n = 3/group). When tumours reached a mean tumour diameter (MTD) of 8-8.5 mm, mice received intra tumour injections of 50 μg purified, recombinant dBP4 or PBS (vehicle control) every 2-3 days. Mice were culled once a maximum MTD of 17 mm was reached from either group. Following treatment, control and dBP4-treated tumours were stained for CD31 and the blood vessels were quantified. Representative images of CD31 stained control and dBP4-treated tumours are shown in Fig. 5a. dBP4 treated tumours had significantly fewer blood vessels than PBS treated tumours (Fig. 5b). The vessels in dBP4-treated tumours appeared broken and incomplete compared to PBS treated tumours, as shown in the enlarged area in the upper panels of Fig.  5a. These results indicate that dBP4 treatment decreased angiogenesis within 4T1.2luc2 mammary tumours.
dBP4 treatment significantly decreased metastatic burden compared to PBS treated controls (p < 0.01), based on BLI imaging of lungs ex vivo following treatment ( Fig. 6a and b). Treatment with dBP4 inhibited primary tumour growth in two of three mice compared to PBS treated mice (Fig. 6c).

Discussion
The activity of IGF is regulated by the IGF binding proteins which can either inhibit or potentiate IGF activity. When bound to IGFBP4, IGF is inactive and active IGF is released when IGFBP4 is cleaved by PAPP-A [26]. We mutated the PAPP-A cleavage site of mouse IGFBP4 and cloned the construct, dBP4, into pTriExneo expression vector with a 3' HIS tag to facilitate purification. Treatment of wtIGFBP4 with PAPP-A resulted in proteolytic cleavage, whereas dBP4 was resistant to PAPP-A cleavage. Surface Plasmon Resonance analysis demonstrated that IGF binding capacity of dBP4 was comparable to previously published values for wtIGFBP4 [27].
We previously demonstrated that IGF1 increased proliferation (by 50%) of 4T1.2 cells and to a much greater extent (approx. 400%) proliferation of microvascular endothelial cells [19]. Here we have shown that IGF1 activates Akt in a PI3K dependent manner in 4T1.2luc cells as the PI3K inhibitor, wortmannin, abolished IGF1induced pAkt. Treatment with dBP4 blocked Akt activation by IGF1.
Angiogenesis is essential for the survival and metastasis of tumours [28]. We previously demonstrated that wild type IGFBP4 inhibited IGF1-induced proliferation of microvascular endothelial cells in vitro and that IGF1 increased VEGF production by 4T1.2 mammary adenocarcinoma cells [19], suggesting that dBP4 would have anti-angiogenic effects. In a multicellular in vitro angiogenesis assay, dBP4 either alone or in the presence of exogenous IGF1 inhibited tubule formation. Exogenous IGF1 alone had no effect and dBP4 alone inhibited tubule formation, suggesting the presence of endogenous IGF1 in the assay from either the growth factor cocktail or produced by cells present in the assay. However, both wild type IGBP4 and a C-terminal fragment of IGFBP4 inhibited VEGF-induced angiogenesis in an in vitro Matrigel assay [29]. Although the mechanisms of IGF-independent activity of IGFBP4 are not fully understood, Moreno et al. implicated cathepsin inhibition by the C terminal region of IGFBP4 [29]. In contrast,wild type IGFBP4 inhibited IGF1 or FGF-2 induced, but not VEGF-induced angiogenesis in an in vivo CAM (chick chorioallantoic membrane) angiogenesis model [30]. It is therefore possible that the observed reduction in tubule formation by dBP4 in the absence of exogenous IGF1 is due to presence of endogenous IGF1, IGF1-independent effects or VEGF-dependent effects. A recent review has highlighted the importance of context (when, where, how, how much and what else) to explain apparently conflicting results in the IGF1 signalling literature [31].
We have shown here that dBP4 inhibited IGF1-mediated angiogenesis in Matrigel® subcutaneous implants. In contrast to the in vitro angiogenesis data where dBP4 reduced angiogenesis in the absence of exogenous IGF1 most likely  3). b Metastatic lung burden following treatment expressed as BLI. Data are expressed as mean ± SEM (*P < 0.05, **P < 0.01, unpaired Students t test). c Primary tumour growth curves of individual mice due to an inhibitory effect on the cocktail of growth factors present, angiogenesis in Matrigel implants with dBP4 in the absence of exogenous growth factors was similar to Matrigel controls.
We examined the effects of purified dBP4 on tumour angiogenesis and metastasis using direct intra-tumour injections in the 4T1.2luc model of metastatic breast cancer. For this preliminary study we chose intra-tumour injection of purified dBP4 every 2-3 days to ensure dBP4 reached the tumour and avoided confounding variables of systemic delivery systems. A recent review has highlighted differences between locally and systemically delivered IGF1 in other disease models [31] so our future studies will include comparisons between intra-tumour (local) and systemic delivery. We had established that dBP4 was stable in mouse serum from healthy and 4T1.2 tumour bearing mice at 37°C for over 4 days (data not shown). 4T1.2luc mammary adenocarcinoma tumours were established in wild type BALB/c mice before being treated with purified recombinant dBP4 allowing the effect of exogenously administered dBP4 to be evaluated in a clinically relevant model.
Tumours treated with dBP4 had significantly fewer blood vessels compared to PBS treated tumours. Blood vessels appeared broken and incomplete and large areas of necrosis were visible in dBP4 treated tumours. We previously observed that vessels in 4T1.2 tumours constitutively expressing rat dBP4 were of poor quality with occluded lumens [19]. It has been established that anti-angiogenic therapies result in tumour necrosis which eventually causes the centre of a tumour to become hollow (cavitation), a process which may explain why there was no significant reduction in primary tumour volume, despitedecreased vascularisation and lung metastasis [32]. Although 4T1.2 cells do not produce PAPP-A, host cells within 4T1.2 tumours do produce PAPP-A [19] and is the likely reason why wtIGFBP4 had no effect on 4T1.2 tumour growth or angiogenesis in that study as host-cell derived PAPP-A will cleave the wtIGFBP4 rendering it unable to sequester IGF. The differing reports on the effects of wtIGFBP4 and dBP4 on tumour growth and angiogenesis most likely depend on whether PAPP-A is expressed by the tumour cells or tumour microenvironment [13,19,33].
In addition to inhibiting angiogenesis within primary mammary tumours dBP4 treatment reduced lung metastatic burden with no obvious toxicity. The IGF pathway has previously been implicated in breast cancer metastasis. IGF1 increases cell migration and invasion in vitro in a range of cancer cells, including human colorectal carcinoma and human multiple myeloma cells [34,35]. IGF1 increased motility of metastatic MDA-MB-231BO breast cancer cells through activation of the IGF1R [36] and a dominant negative mutant of IGF1R decreased adhesion, invasion and metastasis of MDA-MB-435 cells [37]. In vitro treatment of 4T1.2luc cells with dBP4 inhibited IGF1 induced migration and invasion. The reduction in lung metastases in dBP4 mice may be secondary toangiogenesis inhibition in the primary tumour and/or inhibition of tumour cell migration and invasion.
Several strategies targeting IGF1 directly have proved successful in in vivo studies. The involvement of the IGF pathway in cancer has led to the investigation of other components of the pathway such as IGFBP4 and PAPP-A.
The results of those studies support the findings we present here. Infusion of a rat protease resistant IGFBP4 into the perilesion environment in a porcine model of neointimal hyperplasia inhibited IGF1 and neointimal expansion [38]. In a transgenic mouse model where both human protease resistant IGFBP4 and PAPP-A were overexpressed in osteoblasts, the protease resistant form of IGFBP4 inhibited PAPP-A mediated bone formation through sequestration of IGF [39]. Overexpression of PAPP-A in SKOV3 cells increased anchorage-independent growth and invasion in vitro and promoted tumour growth and vascularisation in vivo compared to a mutated version with decreased protease activity [21]. PAPP-A inhibition by a monoclonal antibody inhibited IGF1-induced Akt activation and intraperitoneal administration inhibited growth of A549 xenografts [40]. A recent study identified high levels of PAPP-A in the ascites of ovarian cancer patients and more cleaved than intact IGFBP4, indicating that PAPP-A increased IGF activity and therefore stimulated tumour growth via IGF1-R stimulation [41]. Our approach was to use dBP4 to sequester IGF1 rather than blocking PAPP-A itself.
The data presented here demonstrate that a purified, recombinant PAPP-A-resistant IGFBP4, dBP4, inhibits IGF1-induced angiogenesis in vitro and in vivo. Direct injection of purified dBP4 into 4T1.2 tumours inhibited both angiogenesis and metastasis of mammary 4T1.2luc tumours. Future studies will optimise large scale production of dBP4 and evaluate systemic delivery systems such as osmotic pumps to thoroughly assess the clinical potential of dBP4 as a cancer therapeutic. dBP4 is small enough to cross the endothelial cell barrier [42] and due to its resistance to PAPP-A cleavage we anticipate a long half-life in the circulation. As a way of blocking the IGF pathway, dBP4 via sequestering IGF-I may have advantages over monoclonal antibody approaches blocking IGF, IGF1-R or PAPP-A directly.

Conclusion
These studies demonstrated that a PAPP-A resistant IGFBP4, dBP4, inhibits angiogenesis and metastasis in 4T1.2 mammary fat pad tumours. The data highlight the therapeutic potential of dBP4 as an approach to blocking the tumour-promoting actions of IGF1.