anti-PTN Antibody Research Articles

Conditioned Medium from Human Tonsil-Derived Mesenchymal Stem Cells Enhances Bone Marrow Engraftment via Endothelial Cell Restoration by Pleiotrophin.

Cotransplantation of mesenchymal stem cells (MSCs) with hematopoietic stem cells (HSCs) has been widely reported to promote HSC engraftment and enhance marrow stromal regeneration. The present study aimed to define whether MSC conditioned medium could recapitulate the effects of MSC cotransplantation. Mouse bone marrow (BM) was partially ablated by the administration of a busulfan and cyclophosphamide (Bu–Cy)-conditioning regimen in BALB/c recipient mice. BM cells (BMCs) isolated from C57BL/6 mice were transplanted via tail vein with or without tonsil-derived MSC conditioned medium (T-MSC CM). Histological analysis of femurs showed increased BM cellularity when T-MSC CM or recombinant human pleiotrophin (rhPTN), a cytokine readily secreted from T-MSCs with a function in hematopoiesis, was injected with BMCs. Microstructural impairment in mesenteric and BM arteriole endothelial cells (ECs) were observed after treatment with Bu–Cy-conditioning regimen; however, T-MSC CM or rhPTN treatment restored the defects. These effects by T-MSC CM were disrupted in the presence of an anti-PTN antibody, indicating that PTN is a key mediator of EC restoration and enhanced BM engraftment. In conclusion, T-MSC CM administration enhances BM engraftment, in part by restoring vasculature via PTN production. These findings highlight the potential therapeutic relevance of T-MSC CM for increasing HSC transplantation efficacy.

Chronic myeloid leukemia stem cells require cell-autonomous pleiotrophin signaling.

Tyrosine kinase inhibitors (TKIs) induce molecular remission in the majority of patients with chronic myelogenous leukemia (CML), but the persistence of CML stem cells hinders cure and necessitates indefinite TKI therapy. We report that CML stem cells upregulate the expression of pleiotrophin (PTN) and require cell-autonomous PTN signaling for CML pathogenesis in BCR/ABL+ mice. Constitutive PTN deletion substantially reduced the numbers of CML stem cells capable of initiating CML in vivo. Hematopoietic cell-specific deletion of PTN suppressed CML development in BCR/ABL+ mice, suggesting that cell-autonomous PTN signaling was necessary for CML disease evolution. Mechanistically, PTN promoted CML stem cell survival and TKI resistance via induction of Jun and the unfolded protein response. Human CML cells were also dependent on cell-autonomous PTN signaling, and anti-PTN antibody suppressed human CML colony formation and CML repopulation in vivo. Our results suggest that targeted inhibition of PTN has therapeutic potential to eradicate CML stem cells.

Deletion of Pleiotrophin (PTN), a Vascular Niche Paracrine Factor, Abrogates Chronic Myeloid Leukemia Pathogenesis In Vivo

Tyrosine kinase inhibitor (TKI) therapy revolutionized the treatment of patients with chronic myeloid leukemia (CML), but TKI therapy alone is not curative. Leukemic stem cells (LSCs) are considered to be resistant to TKI therapy in CML, although the mechanisms which confer LSC resistance to TKI-mediated eradication remain poorly understood. Our laboratory has previously described the role of PTN, a paracrine factor which is secreted by bone marrow endothelial cells (BM ECs) and is necessary for the self-renewal and regeneration of normal hematopoietic stem cells (HSCs) in vivo. We hypothesized that PTN signaling in the BM vascular niche might also be important in CML pathogenesis and perpetuation of LSCs. In order to test our hypothesis, we crossed SCL-tTA/BCR-ABL mice, which develop CML over time under the control of doxycycline-treatment, with either PTN-/- or PTN+/+ mice, and compared CML pathogenesis in vivo. SCLtTA/BCR-ABL;PTN+/+ mice displayed evolution of chronic phase CML (splenomegaly, increased WBCs, left-shifted myelopoiesis, decreased survival) within 90-120 days. In contrast, SCLtTA/BCR-ABL;PTN-/- mice displayed normal range WBCs, decreased splenomegaly and significantly increased survival over the same time period. At 300 days post-induction of BCR-ABL, 33% percent of SCLtTA/BCR-ABL;PTN-/- mice remain alive compared to only 7% of SCLtTA/BCR-ABL;PTN+/+ mice (p<0.0001). Importantly, treatment of human CML cells with a specific anti-PTN antibody (BD Biosciences) significantly decreased CML colony formation in methylcellulose (P=0.02), whereas normal human CD34+ cells were unaffected. This result suggested the novel possibility that human CML cells may produce PTN in an autocrine manner as a means to propagate disease. These studies suggest that genetic or pharmacologic antagonism of PTN signaling is a potent strategy to inhibit CML pathogenesis. Ongoing studies will determine if PTN-targeted therapy, with or without TKIs, can mediate the eradication of LSCs which are responsible for perpetuation of this disease. DisclosuresHimburg:Duke University: Patents & Royalties: Patent Application for use of Pleiotrophin as a hematopoietic stem cell growth factor. Chute:C2 Regenerate: Equity Ownership; Duke University: Patents & Royalties: Application to use PTN as growth factor as hematopoietic stem cell growth factor.

Elevated expression of pleiotrophin in lymphocytic leukemia CD19+ B cells

Pleiotrophin (PTN) has been demonstrated to be strongly expressed in many fetal tissues, but seldom in healthy adult tissues. While PTN has been reported to be expressed in many types of tumors as well as at high serum concentrations in patients with many types of cancer, to date, there has been no report that PTN is expressed in leukemia, especially in lymphocytic leukemia. We isolated the CD19(+) subset of B cells from peripheral blood from healthy adults, B-cell acute lymphocytic leukemia (B-ALL) patients, and B-cell chronic lymphocytic leukemia (B-CLL) patients and examined these cells for PTN mRNA and protein expression. We used immunocytochemistry, western blotting, and enzyme-linked immunosorbent assay to show that PTN protein is highly expressed in CD19(+) B cells from B-ALL and B-CLL patients, but barely expressed in B cells from healthy adults. We also examined PTN expression at the nucleic acid level using reverse transcription polymerase chain reaction (RT-PCR) and northern blotting and detected a high levels of PTN transcripts in the CD19(+) B cells from both groups of leukemia patients, but very few in the CD19(+) B cells from the healthy controls. Interestingly, the quantity of the PTN transcripts correlated with the severity of disease. Moreover, suppression of PTN activity with an anti-PTN antibody promoted apoptosis of cells from leukemia patients and cell lines SMS-SB and JVM-2. This effect of the anti-PTN antibody suggests that PTN may be a new target for the treatment of lymphocytic leukemia.

Pleiotrophin Regulates the Retention and Self-Renewal of Hematopoietic Stem Cells in the Bone Marrow Vascular Niche

The mechanisms through which the bone marrow (BM) microenvironment regulates hematopoietic stem cell (HSC) fate remain incompletely understood. We examined the role of the heparin-binding growth factor pleiotrophin (PTN) in regulating HSC function in the niche. PTN(-/-) mice displayed significantly decreased BM HSC content and impaired hematopoietic regeneration following myelosuppression. Conversely, mice lacking protein tyrosine phosphatase receptor zeta, which is inactivated by PTN, displayed significantly increased BM HSC content. Transplant studies revealed that PTN action was not HSC autonomous, but rather was mediated by the BM microenvironment. Interestingly, PTN was differentially expressed and secreted by BM sinusoidal endothelial cells within the vascular niche. Furthermore, systemic administration of anti-PTN antibody in mice substantially impaired both the homing of hematopoietic progenitor cells to the niche and the retention of BM HSCs in the niche. PTN is a secreted component of the BM vascular niche that regulates HSC self-renewal and retention invivo.

Pleiotrophin produced by multiple myeloma induces transdifferentiation of monocytes into vascular endothelial cells: a novel mechanism of tumor-induced vasculogenesis

Enhanced angiogenesis is a hallmark of cancer. Pleiotrophin (PTN) is an angiogenic factor that is produced by many different human cancers and stimulates tumor blood vessel formation when it is expressed in malignant cancer cells. Recent studies show that monocytes may give rise to vascular endothelium. In these studies, we show that PTN combined with macrophage colony-stimulating factor (M-CSF) induces expression of vascular endothelial cell (VEC) genes and proteins in human monocyte cell lines and monocytes from human peripheral blood (PB). Monocytes induce VEC gene expression and develop tube-like structures when they are exposed to serum or cultured with bone marrow (BM) from patients with multiple myeloma (MM) that express PTN, effects specifically blocked with antiPTN antibodies. When coinjected with human MM cells into severe combined immunodeficient (SCID) mice, green fluorescent protein (GFP)-marked human monocytes were found incorporated into tumor blood vessels and expressed human VEC protein markers and genes that were blocked by anti-PTN antibody. Our results suggest that vasculogenesis in human MM may develop from tumoral production of PTN, which orchestrates the transdifferentiation of monocytes into VECs.

Pleiotrophin Is a Growth Factor for Hematopoietic Stem Cells and Induces Stem Cell Self-Renewal

The ability to undergo self-renewal is a defining feature of hematopoietic stem cells (HSCs) but the extrinsic signals which regulate HSC self-renewal remain unclear. We performed a genome-wide expression analysis on primary human brain ECs (HUBECs, n=10) which support the ex vivo expansion of HSCs in non-contact culture (Blood 100:4433–4439; Blood 105:576–583) and non-brain ECs which do not support HSC expansion (n=8) in order to identify soluble proteins overexpressed by the HSC-supportive HUBECs. We identified pleiotrophin (PTN), an 18 kD heparin binding growth factor, to be 32-fold overexpressed in HUBECs as compared to non-supportive EC lines. PTN has established activity in angiogenesis, embryogenesis, neuronal cell growth and tumorigenesis, but has no known function in hematopoiesis. We first tested whether secreted PTN was responsible for the amplification of HSCs that we have observed in co-cultures of HSCs with HUBECs via “loss of function” studies in which a blocking anti-PTN antibody was added to HUBEC cultures and HSC content was measured. Competitive repopulating unit (CRU) assays were performed in which limiting doses of donor CD45.1+ bone marrow (BM) 34−c-kit+sca-1+lin− (34-KSL) HSCs (10, 30 or 100 cells) or their progeny following 7 day non-contact culture with HUBECs + IgG or HUBECs + a blocking anti-PTN were transplanted into lethally irradiated CD45.2+ C57Bl6 mice. Mice transplanted with the progeny of 34-KSL cells cultured with HUBECs demonstrated 4–6 fold increased levels of donor-derived CD45.1+ multilineage repopulation at 8-, 12- and 24-weeks post-transplantation as compared to mice transplanted with input 34-KSL cells. In contrast, mice transplanted with the progeny of 34-KSL cells following culture with HUBECs + anti-PTN demonstrated significant reduction in donor CD45.1+ cell repopulation compared to mice transplanted with the progeny of HUBEC cultures and no difference in donor CD45.1+ cell engraftment compared to mice transplanted with input 34-KSL cells. CRU frequency within day 0 34-KSL cells was estimated to be 1 in 40 cells (95% Confidence Interval [CI]: 1/22-1/72), whereas the CRU estimate within the progeny of 34-KSL cells following HUBEC culture was 1 in 4 cells (CI: 1/2-1/9). The addition of anti-PTN to the HUBEC co-culture decreased the CRU estimate to 1 in 29 cells (CI: 1/16-1/52), suggesting that PTN signaling was responsible for the expansion of HSCs observed in HUBEC co-cultures. In order to confirm whether PTN is indeed a novel growth and self-renewal factor for HSCs, we next performed “gain of function” studies in which 34-KSL cells were placed in liquid suspension cultures with cytokines (thrombopoietin 50 ng/mL, SCF 120 ng/mL, flt-3 ligand 20 ng/mL) with and without the addition of increasing doses of recombinant murine PTN (10, 50 and 100 ng/mL) and total cell expansion and HSC content were compared. The addition of 100 ng/mL PTN to cytokine cultures caused a 20-fold increase in KSL cell content at day 7 compared to input (P<0.001), whereas a decline in KSL cells was observed with cytokine cultures alone (P<0.001), suggesting that PTN caused an expansion of stem/progenitor cells in vitro. Competitive repopulating assays were performed in which CD45.2+ recipient mice were lethally irradiated and transplanted with limiting doses (10, 30 and 100 cells) of CD45.1+ donor BM 34-KSL cells or their progeny following culture with cytokines alone or cytokines + 100 ng/mL PTN. CRU analysis at 4 weeks post-transplantation revealed that the CRU frequency within input 34-KSL cells was was 1 in 32 cells (CI: 1/18-1/57) and the CRU estimate within the progeny of 34-KSL cells cultured with cytokines alone was 1 in 69 (CI: 1/36-1/130). Conversely, the CRU estimate within the progeny of 34-KSL cells cultured with cytokines + PTN was 1 in 4 cells (CI: 1/2-1/10), indicating a 8-fold increase in short term repopulating cell content in response to PTN treatment. Longer term analysis will be performed in these mice to confirm whether PTN treatment induces the self-renewal and amplification of long-term repopulating HSCs in culture. Taken together, these data demonstrate that secreted PTN is primarily responsible for amplification of HSCs that we have observed in cultures of HSCs with ECs and the addition of PTN alone induces the expansion of phenotypic and functional HSCs in culture. PTN is therefore a novel soluble growth factor for HSCs and appears to play an important role in the extrinsic regulation of HSC self-renewal.

Pleiotrophin is highly expressed by myeloma cells and promotes myeloma tumor growth

Pleiotrophin (PTN) is an important developmental cytokine that is highly expressed during embryogenesis but shows very limited expression in adult tissues, where it is largely restricted to the brain. High PTN serum levels are associated with a variety of solid tumors. We recently showed that patients with multiple myeloma (MM) also have elevated serum levels of this protein and the amount of PTN correlated with the patients' disease status and response to treatment. In this study, we demonstrate that MM cell lines and the malignant cells from MM patients' bone marrow produced PTN and secreted PTN protein into the supernatants during short-term culture. Moreover, Ptn gene expression correlated with the patients' disease status. Inhibition of PTN with a polyclonal anti-PTN antibody reduced growth and enhanced apoptosis of MM cell lines and freshly isolated bone marrow tumor cells from MM patients in vitro. Importantly, this antibody also markedly suppressed the growth of MM in vivo using a severe combined immunodeficiency (SCID)-hu murine model. This represents the first study showing the importance of PTN in the growth of any hematological disorder. Because the expression of this protein is very limited in normal adult tissues, PTN may represent a new target for the treatment of MM.

Pleiotrophin Promotes Myeloma Proliferation through the Activation of Known and Novel Receptors.

Myeloma survival and proliferation in the bone marrow (BM) depends on the expression of a variety of autocrine and paracrine growth factors, including IL-6, insulin-like growth factor I (IGF-I), vascular endothelial growth factor (VEGF), and hepatocyte growth factor (HGF). We recently discovered an additional new autocrine myeloma growth factor, pleiotrophin (PTN). PTN is an 18kD heparin-binding protein normally expressed during early development and downregulated in adults, but aberrant PTN re-expression has been associated with a variety of aggressive solid tumors including neuroblastoma, glioma, melanoma and lung, breast and prostate cancers. We found that interference with PTN using a polyclonal anti-PTN antibody inhibited the proliferation of myeloma cells in vitro and in vivo by inducing cell cycle arrest but not apoptosis. To determine the mechanism by which PTN stimulates myeloma proliferation we analyzed the expression of the known PTN receptors, syndecans 1 (CD138) and 3, the anaplastic lymphoma kinase (ALK) and the receptor tyrosine phosphatase beta/zeta (RPTPβ/ζ) on myeloma cells. The expression of ALK and RPTPβ/ζ has not previously been investigated in any hematologic malignancy. In addition to syndecan 1 we found that a subset of myeloma cell lines and BM mononuclear cells (BMMCs) from myeloma patients are RPTPβ/ζ+ by RT-PCR, Western blot and flow cytometry. Myeloma cells, however, do not express ALK. RPTPβ/ζ inhibition by PTN binding leads to the accumulation of β-catenin, and downstream activation of Wnt, NF-κB, MAPK and Akt-mediated cell signals in cells from solid tumors, signaling pathways known to contribute to myeloma cell survival and proliferation. The myeloma cell line MM-1S and the SCID-hu myeloma model LAGλ-1 are both CD138+ but RPTPβ/ζ−. Nonetheless, the growth of these cells is inhibited by anti-PTN antibody. CD138 lacks cytoplasmic signaling motifs suggesting that additional novel receptors are required for PTN-stimulated cell growth in these cells. To continue to define PTN-mediated signaling in myeloma cells we compared the induction of PTN-regulated signaling pathways in RPTPβ/ζ+ RPMI 8226 cells compared to RPTPβ/ζ − MM-1s cells by phospho-protein Western blot. We found differences in PTN-stimulated tyrosine phosphorylation between RPTPβ/ζ+ and RPTPβ/ζ −cells. Specifically, we also discovered that RPTPβ/ζ+ RPMI 8226 cells activate the MAPK Erk1/2. In contrast, we found Akt, but not Erk1/2, to be activated by PTN in RPTPβ/ζ − MM-1S cells. We are now continuing our analysis of PTN signaling in these cells. We are using also identifying new PTN receptors on the surface of myeloma cells by affinity chromatography using biotinylated-PTN. These studies will provide potential new targets that should lead to the development of novel targeted anti-myeloma therapies.

Blocking Pleiotrophin Activity Inhibits Multiple Myeloma (MM) Cell Growth In Vitro and in a Severe Combined Immunodeficient (SCID)-hu Murine Model of Human MM.

Pleiotrophin (PTN) is a heparin-binding growth factor that binds CD138 and stimulates angiogenesis, tumor growth and metastasis in some solid tumors. Recently, we have shown that this factor is highly produced by multiple myeloma (MM) cell lines including RPMI8226 and U266 and fresh malignant plasma cells, and is secreted into the culture medium following short-term culture of bone marrow from MM patients. We investigated the effects of PTN on MM growth in vitro and in vivo using a SCID-hu murine MM model. We determined the anti-proliferative effects of suppressing PTN by cloning a whole PTN sense or anti-sense cDNA construct containing the green fluorescent protein (GFP) gene into the MM cell lines RPMI8226 and U266. Cells transduced with sense PTN showed markedly increased proliferation compared to cells transduced with vector alone whereas the anti-sense-containing MM cells showed reduced cell numbers. In addition, we treated RPMI8226 and U266 cells with a polyclonal anti-PTN antibody and evaluated its effect on MM growth. These cells were cultured for 48 hours in the presence of the anti-PTN antibody at a concentration of 100 micrograms/ml or a control antibody, and effects on cell growth assessed with an MTT assay. Marked anti-MM effects were observed with the anti-PTN antibody compared to the control antibody in both cell lines [RPMI8226 (p < 0.01) and U266 (p < 0.001)]. In order to further define the importance of PTN in the growth of MM in a more clinically relevant in vivo setting, we determined whether this polyclonal anti-PTN antibody could suppress tumor growth and human paraprotein secretion using our SCID-hu murine model of human myeloma LAGλ-1. LAGλ-1 has been previously shown by our group to produce large amounts of PTN as measured in mouse serum by ELISA and by RT- PCR analysis on freshly isolated LAGλ-1 tumor cells. Thirty SCID mice (n = 5 mice/group) were implanted with a 0.4 – 0.6 cm3 LAGλ-1 tumor fragment into the left hind limb muscle. Fourteen days following implantation, mice were randomized into treatment groups, and received treatment intraperitoneally (IP) with anti-PTN antibody at doses of 0.1, 0.3, 1.0, 3.0 or 10 mg/kg or vehicle alone twice weekly. Mice receiving anti-PTN antibody at the highest doses (3.0 and 10 mg/kg) showed marked inhibition of tumor growth [3.0 mg/kg (p < 0.03), 10 mg/kg (p < 0.008)] as well as decreases in levels of human paraprotein [3.0 mg/kg (p < 0.004), 10 mg/kg (p < 0.003)]. Notably, immunohistochemical staining with an anti-CD138 antibody showed a marked reduction in cells with CD138 positivity in the LAGλ-1 tumors from animals treated with anti-PTN antibody compared to mice treated with vehicle alone. These in vitro and in vivo results demonstrate that PTN may be a potential new target for the treatment of MM. The effects of this therapy on angiogenesis and cell signaling are currently under investigation.

Identification of Novel Receptors on Myeloma Cells and Monocytes That Contribute to Myeloma Tumor Proliferation and Angiogenesis.

Hyper-activation of multiple cell signaling pathways, including Akt, IRS-1, JAK/ STAT3, NF-kB, Wnt and MAPK, contribute to myeloma cell survival and/or proliferation, and myeloma tumor vascularization. We discovered the novel expression of two cell surface receptors that are able to activate these cell signals but were not previously known to be expressed by hematopoietic cells or to be involved in hematological malignancies. We analyzed bone marrow (BM) and peripheral blood (PB) mononuclear cells (MCs) from multiple myeloma (MM) patients and normal donors for the expression of new surface receptors by flow cytometry. We found that the malignant cells from myeloma patients (N = 10) and the MM cell lines RPMI 8226 and U266 express the receptor protein tyrosine phosphatase b/z (RPTPb/z). RPTPb/z contributes to the growth of solid tumors, including glioblastoma, neuroblastoma and melanoma by aberrant accumulation of b-catenin, leading to activation of Wnt, NF-kB and Akt-mediated cell signals. We interfered with RPTPb/z stimulation by its ligand, pleiotrophin (PTN), using specific anti-PTN antibody and showed significant inhibition of MM proliferation in vitro by MTT assay. Importantly, when this antibody was administered to MM tumor-bearing mice in vivo a significant decrease in tumor volume was detected. We have also discovered that peripheral blood monocytes from MM patients (N = 9) and normal donors (N = 7), and a monocytic cell line (THP-1), display untranslocated anaplastic lymphoma kinase (ALK) on the cell surface. Dysregulated ALK expression on a variety of solid tumors is associated with tumor growth, vascularization and metastasis. ALK+ monocytes and THP-1 cells, but not ALK− peripheral B or T lymphocytes or the ALK− monocytic cell line U937, respond to ALK stimulation by modulating hematopoietic genes and inducing the expression of endothelial genes as well as exhibiting changes in morphology typical of early vascular endothelial cells (Chen et al, 2004, Blood 104: 1472). ALK cross-linking leads to activation of the signaling intermediates IRS-1, PI3′K/Akt, STAT3 and MAPK, that are known to play key roles in angiogenesis. Thus RPTPb/z and ALK, or their downstream signal mediators, could be important new molecular targets for anti-myeloma (RPTPb/z) or anti-angiogenic (ALK) therapy. We are defining the signaling pathway(s) leading from RPTPb/z stimulation to MM cellular proliferation and survival, and from ALK to tumor vascularization via monocytic transdifferentiation. These studies provide potential new targets that should lead to new therapeutic strategies for treating MM.

Circulating Tie2-Expressing Cells Are Increased in Multiple Myeloma Patients, Correlate with Serum Pleiotrophin Levels and May Develop from This Myeloma Angiogenic and Growth Factor.

Tie2, an endothelial cell-specific receptor kinase, plays an important role in tumor angiogenesis. This protein is essential to the development of embryonic vasculature as well as vascular growth and maintenance in adult tissues. Because of the increasing importance that angiogenesis has been shown to play in multiple myeloma (MM), we determined the number of Tie2-expressing cells in the peripheral blood (PB) of MM patients and its relationship to the serum levels and gene expression of a recently identified angiogenic factor, pleiotrophin (PTN). We have recently demonstrated that PTN is expressed and secreted by MM tumor cells, and serum levels of this protein are highly elevated in MM patients. We quantified the number of Tie2-positive cells in MM patients (n=15) and age-matched control subjects (n=10) using an immunohistochemical technique. Tie2-expressing cells were significantly elevated in the PB mononuclear cells (MCs) from MM patients compared to the normal controls (p<0.05). We also analyzed gene expression for Tie2 in these same samples using RT-PCR. The results showed that Tie2 mRNA was strongly expressed in the PBMCs from MM patients whereas control samples showed no or low expression of this gene. Serum levels of PTN were tested with ELISA, and PTN mRNA concentrations were quantified by RT-PCR in PBMCs from these same patients and control subjects. The results showed that serum levels of PTN correlated with the number of Tie2-expressing PBMCs in MM patients (R2=0.5778). PTN mRNA levels also correlated with Tie2 gene expression in PBMC samples. We further examined whether monocyte colony stimulating factor (mCSF), PTN and vascular endothelial growth factor (VEGF) may be capable of inducing Tie2 expression in highly purified human monocytes that lack Tie2 expression. Normal PB monocytes were purified using density centrifugation followed by anti-CD14 micro-bead affinity column selection. Although none of these three proteins alone or the combinations of either VEGF and mCSF or VEGF and PTN induced Tie2 gene expression in the monocytes following one week of incubation, the combination of PTN (100 nM) and mCSF (20 nM) led to expression of Tie2 in these cells. We quantified the proportion of cells expressing Tie2 in these samples with RT-PCR using serial dilutional analysis with B or T cells that lack Tie2 expression, and showed that approximately 0.1–1.0% of the monocytes expressed this gene following incubation with PTN and mCSF. Moreover, the addition of VEGF (20 ng/ml) to PTN and mCSF increased the proportion of cells expressing Tie2 (to >10%). Anti-PTN antibody blocked the induction of Tie2 gene expression in these monocytes by this cytokine combination. These results show that Tie2-expressing cells are elevated in the peripheral blood of MM patients, and correlate with PTN serum and PTN mRNA expression. PTN in combination with VEGF and mCSF induces Tie2 gene expression in a large proportion of circulating human monocytes. These results suggest that MM patients show increased numbers of vasculogenic progenitors in their circulation that may result from the presence of elevated levels of circulating angiogenic factors including PTN and VEGF.

Transdifferentiation of Monocytes into Endothelial Cells by Pleiotrophin.

Pleiotrophin (PTN) is a secreted angiogenic protein. We have recently shown that malignant plasma cells express high amounts of PTN and that it is elevated in MM patient serum. In this study, we show that this protein leads to angiogenesis through a novel mechanism - transdifferentiation of monocytes into endothelial cells. First, we isolated human monocytes (CD14+ cells) from peripheral blood using immunomagnetic bead isolation. We excluded the presence of endothelial cells in these CD14-expressing cells using RT-PCR on 106 monocytes with primers to genes expressed by endothelial cells including Flk-1, Tie2, CD144, and von Willebrand factor (vWF). We cultured these purified monocytes on collagen I for one week in the presence of PTN, mCSF and VEGF. Cells cultured with the combination of mCSF and PTN developed Flk-1-expressing tube-like structures, and the addition of VEGF increased tube formation. Next, we performed RT-PCR analysis with primers to these endothelial genes on monocytes cultured with PTN, mCSF and VEGF following serial dilution in cells (T or B lymphocytes) that lack monocyte or endothelial cell gene expression. Many of the cells expressed Tie2 RNA (>10%), and a smaller proportion (0.1–1%) also expressed Flk-1, CD144 and vWF RNA. In contrast, purified monocytes incubated with mCSF, VEGF, or PTN alone or the combinations of mCSF and VEGF or PTN and VEGF lacked Flk-1 staining, did not form tubes and failed to express endothelial cell RNA. We also assessed these monocytes in three dimensional matrices using Matrigel. Cells treated with mCSF and PTN invaded the matrix and began to form tube-like structures in the three dimensional gels as early as 7 days following culture whereas monocytes treated with mCSF, VEGF, or PTN alone did not form these structures. Next, we transduced human monocytic THP-1 cells that lack PTN expression with PTN-sense or -anti-sense constructs. Using RT-PCR, THP-1 cells transduced with PTN expressed endothelial cell genes and lost expression of the monocyte genes c-fms and CD68. In contrast, endothelial cell RNA was not detected in either THP-1 cells infected with anti-sense or the GFP control vectors. We used Transwell plates to co-culture THP-1 monocytes with human MM RPMI8226 or U266 cells or cell lines lacking PTN expression. We also added serum from MM patients with high levels of PTN or normal controls lacking PTN to THP-1 cells. THP-1 cells cultured with the MM cell lines or MM serum expressed endothelial genes and lost expression of monocyte RNA. Endothelial gene expression was blocked by the addition of an anti-PTN antibody but not by a control antibody. Control serum and cell lines lacking PTN did not induce endothelial gene expression or changes in monocyte RNA expression in the THP-1 cells. These experiments define a previously unrecognized novel mechanism leading to angiogenesis in cancer patients - the transdifferentiation of monocytes into endothelial cells by a factor highly produced by the malignant cells in MM. These findings also suggest a potential new specific target, PTN, to inhibit angiogenesis in cancer patients and should have profound clinical implications.

Heparin-binding Growth Factor, Pleiotrophin, Mediates Neuritogenic Activity of Embryonic Pig Brain-derived Chondroitin Sulfate/Dermatan Sulfate Hybrid Chains

Chondroitin sulfate (CS) and dermatan sulfate (DS) chains play roles in the central nervous system. Most notably, CS/DS hybrid chains (E-CS/DS) purified from embryonic pig brains bind growth factors and promote neurite outgrowth toward embryonic mouse hippocampal neurons in culture. However, the neuritogenic mechanism is not well understood. Here we showed that pleiotrophin (PTN), a heparin-binding growth factor, produced mainly by glia cells, was the predominant binding partner for E-CS/DS in the membrane-associated protein fraction of neonatal rat brain. The CS/DS chains were separated on a PTN column into unbound, low affinity, and high affinity fractions. The latter two fractions promoted outgrowth of dendrite- and axon-like neurites, respectively, whereas the unbound fraction showed no such activity. The activity of the low affinity fraction was abolished by an anti-PTN antibody or when glia cells were removed from the culture. In contrast, the high affinity fraction displayed activity under both these conditions. Hence, PTN mainly from glia cells mediated the activity of the low affinity but not the high affinity fraction. The anti-CS antibody 473HD neutralized the neuritogenic activities of both fractions. Interaction analysis indicated that the 473HD epitope and PTN-binding domains in the E-CS/DS chains largely overlap. The three affinity subfractions differed in disaccharide composition and the distribution of l-iduronic acid-containing disaccharides along the chains. Oversulfated disaccharides and nonconsecutive iduronic acid-containing units were the requirements for the E-CS/DS chains to bind PTN and to exhibit the neuritogenic activities. Thus, CS subpopulations with distinct structures in the mammalian brain play different roles in neuritogenesis through distinct molecular mechanisms, at least in part by regulating the functions of growth factors.

Pleitrophin Alone Induces Transdifferentiation of Human Monocytes and Bone Marrow Stem Cells into Endothelial Cells.

Bone marrow angiogenesis is a hallmark of multiple myeloma (MM). We have recently shown that MM patients express pleitrophin (PTN), a secreted protein that binds syndecan, and it is found at high levels in MM serum. This protein has been shown to stimulate angiogenesis. We have discovered a novel mechanism leading to blood vessel formation by tumor cells. First, we cloned human monocytic THP-1 cells with PTN sense or antisense whole sequencing DNA. We examined expression by RT-PCR of endothelial cell markers (Flk-1), Tie-2, von Willebrand factor (vWf)) and monocyte markers (c-fms and CD68). THP-1 cells infected with PTN sense strand expressed high amounts of Flk-1, Tie-2 and vWf similar to that found in human coronary artery endothelial cells and lost expression of c-fms and CD68. Endothelial cell marker RNA was not detected in either THP-1 cells infected with PTN anti-sense strand or the GFP control vector but these cells showed the continued presence of monocyte markers. Immunohistochemical studies showed THP-1 cells infected with PTN sense strand expressed endothelial markers but not cells treated with antisense or control cells. We have recently found high levels of PTN in MM serum and expression of PTN by MM cell lines including RPMI8226. Next, we cultured THP1 monocytes with RPMI8226 in transwell cultures, serum derived from MM patients with high serum levels of PTN, cell lines lacking PTN expression, and normal controls lacking serum PTN. The THP-1 cells exposed to the MM cell lines or MM serum showed expression of endothelial markers and lost expression of monocyte markers. The expression of endothelial markers was blocked by adding anti-PTN antibody. In contrast, control serum and cell lines lacking PTN expression did not change monocyte marker expression on the THP1 cells nor induce expression of endothelial markers. We examined whether PTN induced monocytes from human peripheral blood to transdifferentiate. Normal human blood monocytes were highly purified using density centrifugation followed by anti-CD14 micro-bead affinity column selection. These purified monocytes also showed transdifferentiation into endothelial cells in the presence of PTN with m-CSF unlike cells treated with m-CSF-alone or cells without these factors present. We determined whether PTN could also stimulate differentiation of bone marrow stem cells into endothelial cells. The stem cells were derived from bone marrow selected for CD34 using magnetic bead selection, and were stimulated with either m-CSF or PTN alone or a combination of m-CSF and PTN or no treatment for 7 days. Real time PCR analysis showed that the m-CSF and PTN combination markedly increased endothelial cell marker expression and decreased monocyte marker (CD68 and c-fms) expression in this stem cell population. When induced with PTN alone, the stem cells exhibited slightly increasing expression of endothelial markers with no change in monocyte marker expression whereas m-CSF alone and no treatment had no effect on either endothelial or monocyte marker expression. These experiments define a previously unrecognized novel mechanism leading to angiogenesis in cancer patients- the transdifferentiation of monocytes into endothelial cells by a factor highly produced by tumor cells. They also suggest a potential new specific target to inhibit angiogenesis-pleitrophin which may have profound clinical implications.