Molecular and pharmacologic targeting of angiogenesis factors--the example of pleiotrophin

Pleiotrophin (PTN) expression and function and in the mouse mammary gland and mammary epithelial cells

Expression of the heparin-binding growth factor, pleiotrophin (PTN) in the mammary gland has been reported but its function during mammary gland development is not known. We examined the expression of PTN and its receptor ALK (Anaplastic Lymphoma Kinase) at various stages of mouse mammary gland development and found that their expression in epithelial cells is regulated in parallel during pregnancy. A 30-fold downregulation of PTN mRNA expression was observed during mid-pregnancy when the mammary gland undergoes lobular-alveolar differentiation. After weaning of pups, PTN expression was restored although baseline expression of PTN was reduced significantly in mammary glands of mice that had undergone multiple pregnancies. We found PTN expressed in epithelial cells of the mammary gland and thus used a monoclonal anti-PTN blocking antibody to elucidate its function in cultured mammary epithelial cells (MECs) as well as during gland development. Real-time impedance monitoring of MECs growth, migration and invasion during anti-PTN blocking antibody treatment showed that MECs motility and invasion but not proliferation depend on the activity of endogenous PTN. Increased number of mammospheres with laminin deposition after anti-PTN blocking antibody treatment of MECs in 3D culture and expression of progenitor markers suggest that the endogenously expressed PTN inhibits the expansion and differentiation of epithelial progenitor cells by disrupting cell-matrix adhesion. In vivo, PTN activity was found to inhibit ductal outgrowth and branching via the inhibition of phospho ERK1/2 signaling in the mammary epithelial cells. We conclude that PTN delays the maturation of the mammary gland by maintaining mammary epithelial cells in a progenitor phenotype and by inhibiting their differentiation during mammary gland development.

Pleiotrophin, a multifunctional angiogenic factor: mechanisms and pathways in normal and pathological angiogenesis

PURPOSE OF REVIEW

This study seeks to integrate recent studies that identify new critical mechanisms through which the 136 amino acid secreted heparin-binding cytokine pleiotrophin (PTN, Ptn) stimulates both normal and pathological angiogenesis.

RECENT FINDINGS

Pleiotrophin is directly angiogenic; it initiates an angiogenic switch in different cancer models in vivo. It acts as an angiogenic factor through multiple mechanisms that include a unique signaling pathway that activates newly identified downstream tyrosine kinases through a unique mechanism, an interaction with endothelial cells to initiate proliferation, migration, and tube formation, the regulation of basic fibroblast growth factor and vascular endothelial growth factor signaling, the remodeling of the stromal microenvironment, and induction of transdifferentiation of monocytes into endothelial cells. Recently also, domains of PTN that stimulate angiogenesis and peptides that function to inhibit PTN signaling have been identified.

SUMMARY

Recent studies have identified new mechanisms dependent on activation of the PTN signaling pathway that regulate angiogenesis and new targets to use PTN to both stimulate angiogenesis and block its activity to control pathological angiogenesis.

RNA interference-mediated gene silencing of pleiotrophin through polyethylenimine-complexed small interfering RNAs in vivo exerts antitumoral effects in glioblastoma xenografts

RNA interference (RNAi) is a powerful strategy to inhibit gene expression through specific mRNA degradation mediated by small interfering RNAs (siRNAs). In vivo, however, the application of siRNAs is severely limited by their instability and poor delivery into target cells and target tissues. Glioblastomas are the most frequent and malignant brain tumors with, so far, limited treatment options. To develop novel and more efficacious therapies, advanced targeting strategies against glioblastoma multiforme (GBM)-relevant target genes must be established in vivo. Here we use RNAi-based targeting of the secreted growth factor pleiotrophin (PTN), employing a polyethylenimine (PEI)/siRNA complex strategy. We show that the complexation of chemically unmodified siRNAs with PEI leads to the formation of complexes that condense and completely cover siRNAs as determined by atomic force microscopy (AFM). On the efficient cellular delivery of these PEI/siRNA complexes, the PTN downregulation in U87 glioblastoma cells in vitro results in decreased proliferation and soft agar colony formation. More importantly, in vivo treatment of nude mice through systemic application (subcutaneous or intraperitoneal) of PEI-complexed PTN siRNAs leads to the delivery of intact siRNAs into subcutaneous tumor xenografts and a significant inhibition of tumor growth without a measurable induction of siRNA-mediated immunostimulation. Likewise, in a clinically more relevant orthotopic mouse glioblastoma model with U87 cells growing intracranially, the injection of PEI-complexed PTN siRNAs into the CNS exerts antitumoral effects. In conclusion, we present the PEI complexation of siRNAs as a universally applicable platform for RNAi in vitro and in vivo and establish, also in a complex and relevant orthotopic tumor model, the potential of PEI/siRNA-mediated PTN gene targeting as a novel therapeutic option in GBM.

PTEN deletion leads to up-regulation of a secreted growth factor pleiotrophin

Tumor suppressor gene PTEN is highly mutated in a wide variety of human tumors. To identify unknown targets or signal transduction pathways that are regulated by PTEN, microarray analysis was performed to compare the gene expression profiles of Pten null mouse embryonic fibroblasts (MEFs) cell lines and their isogenic counterparts. Expression of a heparin binding growth factor, pleiotrophin (Ptn), was found to be up-regulated in Pten-/- MEFs as well as Pten null mammary tumors. Further experiments revealed that Ptn expression is regulated by the PTEN-PI3K-AKT pathway. Knocking down the expression of Ptn by small interfering RNA resulted in the reduction of Akt and GSK-3beta phosphorylation and suppression of the growth and the tumorigenicity of Pten null MEFs. Our results suggest that PTN participates in tumorigenesis caused by PTEN loss and PTN may be a potential target for anticancer therapy, especially for those tumors with PTEN deficiencies.

Pleiotrophin inhibits HIV infection by binding the cell surface-expressed nucleolin

The growth factor pleiotrophin (PTN) has been reported to bind heparan sulfate and nucleolin, two components of the cell surface implicated in the attachment of HIV-1 particles to cells. Here we show that PTN inhibits HIV-1 infection by its capacity to inhibit HIV-1 particle attachment to the surface of permissive cells. The beta-sheet domains of PTN appear to be implicated in this inhibitory effect on the HIV infection, in particular the domain containing amino acids 60-110. PTN binding to the cell surface is mediated by high and low affinity binding sites. Other inhibitors of HIV attachment known to bind specifically surface expressed nucleolin, such as the pseudopeptide HB-19 and the cytokine midkine prevent the binding of PTN to its low affinity-binding site. Confocal immunofluorescence laser microscopy revealed that the cross-linking of surface-bound PTN with a specific antibody results in the clustering of cell surface-expressed nucleolin and the colocalization of both PTN and nucleolin signals. Following its binding to surface-nucleolin, PTN is internalized by a temperature sensitive mechanism, a process which is inhibited by HB-19 and is independent of heparan and chondroitin sulfate proteoglycans. Nevertheless, proteoglycans might play a role in the concentration of PTN on the cell surface for a more efficient interaction with nucleolin. Our results demonstrate for the first time that PTN inhibits HIV infection and suggest that the cell surface-expressed nucleolin is a low affinity receptor for PTN binding to cells and it is also implicated in PTN entry into cells by an active process.

Ribozyme-targeting reveals the rate-limiting role of pleiotrophin in glioblastoma

Glioblastomas (GBMs) are the most frequent malignant brain tumors with very limited treatment options and nearly all GBM patients dying within 1 year. Pleiotrophin (PTN, HB-GAM, HBNF, OSF-1) is a secreted growth factor that shows mitogenic, chemotactic and transforming activity. While PTN expression is tightly regulated during embryogenesis and very limited in normal adult tissues, a marked PTN upregulation is seen in tumors including glioblastomas. Targeting of the PTN receptors, ALK and RPTP-zeta, indicates a contribution of PTN-activated signaling pathways in glioblastomas. However, the relevance of PTN expression itself is unknown especially since, besides PTN, at least one more growth factor, midkine (MK), signals through ALK and is expressed in glioblastoma. Here we demonstrate the biologic relevance of PTN in 2 glioblastoma cell lines in vitro and in vivo. We show that stable ribozyme-targeting leads to a robust reduction of PTN mRNA and protein levels. This results in decreased cell proliferation, cell migration and soft agar colony formation in vitro. Comparing clonal ribozyme-transfected cells with different residual PTN levels, we establish a PTN gene-dose effect of glioblastoma cell proliferation. In a subcutaneous tumor xenograft mouse model, in vivo growth is markedly reduced upon PTN depletion, which is paralleled by decreased PTN serum levels. Furthermore, the immunohistochemical analysis of the tumors shows reduced angiogenesis in PTN-depleted tumors. We conclude that PTN is a rate-limiting growth factor in glioblastoma. Since PTN is overexpressed in glioblastomas but rarely found in normal tissue, PTN may represent an attractive therapeutic target.

Ribozyme targeting of the growth factor pleiotrophin in established tumors: a gene therapy approach

The growth and metastasis of solid tumors relies on the activities of polypeptide growth factors to recruit stromal tissue and expand the tumor mass. Pleiotrophin (PTN) is a secreted growth factor with angiogenic activity that has been found to contribute to the growth and metastasis of tumors including melanoma. Here, we present a gene therapy approach of targeting PTN in established tumors using ribozymes. Tetracycline-regulated ribozyme expression vectors were used to deplete conditionally PTN mRNA from melanoma xenograft tumors in vivo. We found that tetracycline-mediated initiation of ribozyme expression in established tumors reduced further tumor growth. Next, we generated synthetic anti-PTN ribozymes that inhibit PTN-dependent colony formation of cells in soft agar. Intraperitoneal administration of these synthetic ribozymes into nude mice inhibited growth of PTN-positive, subcutaneous melanoma. Furthermore, PTN released from the tumors into the circulation of mice was reduced after ribozyme treatment. These data show that ribozyme targeting of rate-limiting tumor growth factors could provide an efficient tool for cancer therapy and that the efficacy may be reflected in the reduction of the serum levels of the targeted protein, PTN.

Expression and Function of the Receptor Protein Tyrosine Phosphatase ζ and Its Ligand Pleiotrophin in Human Astrocytomas

Using subtractive cloning combined with cDNA array analysis, we previously identified the genes encoding for the protein tyrosine phosphatase zeta/receptor-type protein tyrosine phosphatase beta (PTPzeta/RPTPbeta) and its ligand pleiotrophin (PTN) as overexpressed in human glioblastomas compared to normal brain. Both molecules have been implicated in neuronal migration during central nervous system development, and PTN is known to be involved in tumor growth and angiogenesis. We confirm overexpression of both molecules at the protein level in astrocytic gliomas of different malignancy grades. PTPzeta/RPTPbeta immunoreactivity was associated with increasing malignancy grade and localized predominantly to the tumor cells. PTN immunoreactivity as determined by ELISA and immunohistochemistry analysis was increased in low-grade astrocytomas compared to normal brain. Further increase in malignant gliomas was marginal, and thus no correlation with malignancy grade or microvessel density was present. However, PTN levels were significantly associated with those of fibroblast growth factor-2, suggesting co-regulation of both factors. Functionally, PTN induced weak chemotactic and strong haptotactic migration of glioblastoma and cerebral microvascular endothelial cells. Haptotaxis of glioblastoma cells towards PTN was specifically inhibited by an anti-PTPzeta/RPTPbeta antibody. Our findings suggest that upregulated expression of PTN and PTPzeta/RPTPbeta in human astrocytic tumor cells can create an autocrine loop that is important for glioma cell migration. Although PTN is a secreted growth factor, it appears to exert its mitogenic effects mostly in a matrix-immobilized form, serving as a substrate for migrating tumor cells.

Cellular distribution of the angiogenic factor heparin affin regulatory peptide (HARP) mRNA and protein in the human mammary gland

The heparin affin regulatory peptide (HARP) growth factor, also known as pleiotrophin, is a developmentally regulated protein that displays biological functions during cell growth and differentiation. To study the physiological role of this protein, we investigated the cellular distribution of HARP mRNA and protein in the resting human mammary gland. In situ hybridization histochemistry revealed that HARP mRNA was localized in alveolar myoepithelial cells, whereas alveolar epithelial cells were negative. In the stroma, HARP mRNA was localized in endothelial cells and smooth muscle cells of blood vessels. Interestingly, HARP protein and mRNA were not always co-localized. HARP protein immunocytochemistry staining was observed in an area including both alveolar myoepithelial and epithelial cells, although epithelial cells do not express HARP transcript. In contrast, the distribution of HARP protein is parallel to that of HARP mRNA in endothelial and vascular smooth muscle cells. In the light of these results, the putative role of HARP in controlling the proliferation and/or differentiation of the different mammary cell types is proposed and discussed.