Anaplastic lymphoma kinase (ALK) is a novel neuronal orphan receptor tyrosine kinase that is essentially and transiently expressed in specific regions of the central and peripheral nervous systems, suggesting a role in its normal development and function. To determine whether ALK could play a role in neuronal differentiation, we established a model system that allowed us to mimic the normal activation of this receptor. We expressed, in PC12 cells, a chimeric protein in which the extracellular domain of the receptor was replaced by the mouse IgG 2b Fc domain. The Fc domain induced the dimerization and oligomerization of the chimeric protein leading to receptor phosphorylation and activation, thus mimicking the effect of ligand binding, whereas the wild type ALK remained as a monomeric nonphosphorylated protein. Expression of the chimera, but not that of the wild type ALK or of a kinase inactive form of the chimera, induced the differentiation of PC12 cells. Analysis of the signaling pathways involved in this process pointed to an essential role of the mitogen-activated protein kinase cascade. These results are consistent with a role for ALK in neuronal differentiation.The common structural features of a receptor tyrosine kinase (RTK) 1 include an extracellular ligand binding region, a hydrophobic membrane-spanning segment, and a cytoplasmic domain that carries the catalytic function. Following ligand binding, the RTK dimerizes and autophosphorylates (1). The activated RTK initiates signal transduction cascades through binding of SH2 domain-containing proteins to specific receptor phosphotyrosine residues (2). RTKs can regulate a wide variety of cellular processes involved in cell division, differentiation, survival, and motility. A number of RTKs play essential roles during the development of the nervous system by contributing to neuronal differentiation, survival, and function (reviewed in Ref.3). Most of these receptors have specific or shared ligands called neurotrophic factors that have been identified (reviewed in Ref. 3). However, for some of them, named orphan receptors, their ligands are still unknown (4 -6).Anaplastic lymphoma kinase (ALK), a novel orphan neuronal receptor, was originally identified as a member of the insulin receptor subfamily of receptor tyrosine kinases that acquires transforming capability when truncated and fused in the t(2;5) chromosomal rearrangement associated with the non-Hodgkin lymphoma (7). This translocation produces a fusion gene that encodes a soluble chimeric transforming protein comprised of the N-terminal portion of the phosphoprotein nucleophosmin (NPM), a highly conserved RNA-binding nucleolar protein, linked to the cytoplasmic portion of ALK (7). The NPM-ALK fusion protein was localized within both the cytoplasm and the nucleoplasm and also within the nucleoli of t(2;5)-translocation-positive lymphoma cells (8). However, whereas the NPM sequence is essential for the transforming activity (9), the nuclear localization, occurring via the shuttling activity of NPM (10...

Elevated serum pleiotrophin levels can indicate the presence of tumors expressing this factor. Monitoring serum levels of pleiotrophin may prove useful in determining the pharmacologic efficacy of cytotoxic or anti-pleiotrophin therapy.

No abstract

Pleiotrophin (PTN) is a developmentally regulated protein that has been shown to be involved in tumor growth and metastasis presumably by activating tumor angiogenesis. To clarify the potential angiogenic activity of PTN and to analyze the signaling pathways involved in this process, we used an in vitro model of Human Umbilical Vein Endothelial Cells (HUVEC). We show that PTN was mitogenic toward a variety of endothelial cells including HUVEC, stimulated HUVEC migration across a reconstituted basement membrane and induced the formation of capillary-like structures by HUVEC grown as 3D-cultures in Matrigel or collagen. The signaling pathways triggered following endothelial cell stimulation by PTN were studied by using pharmacological inhibitors of the Phosphoinositide-3 kinase (PI3K) and endothelial Nitric Oxide Synthase (eNOS), two enzymes that have been shown to be crucial in the angiogenic response to Vascular Endothelial Growth Factor (VEGF). Whereas wortmannin (a PI3K inhibitor) and L-NAME (an eNOS inhibitor) dramatically reduced HUVEC growth induced by VEGF, only the former inhibitor reduced the growth induced by PTN and to a lesser extent that stimulated by basic Fibroblast Growth Factor. Thus, our results indicate that PTN induces angiogenesis and utilizes PI3K- but not eNOS-dependent pathways for its angiogenic activity.