Identification of anaplastic lymphoma kinase as a receptor for the growth factor pleiotrophin

Pleiotrophin (PTN) is a secreted growth factor that induces neurite outgrow th and is mitogenic for fibroblasts, epithelial, and endothelial cells. Dur ing tumor growth PTN can serve as an angiogenic factor and drive tumor inva sion and metastasis, To identify a receptor for PTN, we penned a phage disp lay human cDNA library against immobilized PTN protein as a bait. From this we isolated a phage insert that was homologous to an amino acid sequence s tretch in the extracellular domain (ECD) of the orphan receptor tyrosine ki nase anaplastic lymphoma kinase (ALK), In parallel with PTN, ALK is highly expressed during perinatal development of the nervous system and down-modul ated in the adult. Here we show in cell-free assays as well as in radioliga nd receptor binding studies in intact cells that PTN binds to the ALK ECD w ith an apparent K-d of 32 +/- 9 pM, This receptor binding is inhibited by a n excess of PTN, by the ALK ECD, and by anti-PTN and anti-ECD antibodies. P TN added to ALK-expressing cells induces phosphorylation of both ALK and of the downstream effector molecules IRS-1, She, phospholipase C-gamma, and p hosphatidylinositol 3-kinase, Furthermore, the growth stimulatory effect of PTN on different cell lines in culture coincides with the endogenous expre ssion of ALK mRNA, and the effect of PTN is enhanced by ALK overexpression. From this we conclude that ALK is a receptor that transduces PTN-mediated signals and propose that the PTN-ALK axis can play a significant role durin g development and during disease processes.