Pleiotrophin (PTN) is a secreted growth factor that induces neurite outgrowth and is mitogenic for fibroblasts, epithelial, and endothelial cells. During tumor growth PTN can serve as an angiogenic factor and drive tumor invasion and metastasis. To identify a receptor for PTN, we panned a phage display 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 stretch in the extracellular domain (ECD) of the orphan receptor tyrosine kinase anaplastic lymphoma kinase (ALK). In parallel with PTN, ALK is highly expressed during perinatal development of the nervous system and down-modulated in the adult. Animal studies demonstrated that PTN can serve as a ratelimiting angiogenic factor during tumor growth, invasion, and metastasis (8 -12). Clinical studies showed elevated serum levels and tumor expression of PTN in samples from patients with colon, stomach, pancreatic, and breast cancer (5, 13). Furthermore, PTN has been implicated in neonatal brain development as well as in neurodegenerative disorders (reviewed in Ref. 14).Obviously, understanding of PTN-mediated signal transduction as well as identification of a receptor for PTN would enhance studies on the biology and pathology of this growth factor family. Our previous studies have shown that the activation of mitogen-activated protein kinase and PI 3-kinase pathways is required for mitogenic activity of PTN, and we had found that the adaptor molecule Shc participated in signal transduction (15). Based on the work of different laboratories in various cell types, it was hypothesized that proteins of 170 -220 kDa that are tyrosine-phosphorylated in response to PTN could be part of the receptor complex (15-17). More recently, several cell membrane-located proteins were shown to bind PTN at low affinity and serve as potential coreceptors or modulators of signal transduction (18 -21), but none of these molecules carried the hallmarks of a signal transducing receptor predicted from the earlier work.To identify a receptor for PTN, we rationalized that panning of a phage display cDNA library against immobilized PTN as a bait would allow us to isolate phage containing a ligand binding fragment of the receptor on their surface. Because of the high levels of expression of PTN during the perinatal development of the nervous system, we hypothesized that fetal brain would most likely also express a PTN receptor. We therefore panned a human fetal brain cDNA phage display library over several rounds against purified PTN that had been tested for biological activity (15). From this we isolated a phage insert homologous to an amino acid sequence stretch in the ECD of the receptor tyrosine kinase anaplastic lymphoma kinase (ALK), a recently described orphan receptor with an apparent molecular mass of

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Glioblastoma multiforme is the most common highly aggressive human brain cancer, and receptor tyrosine kinases have been implicated in the progression of this malignancy. We have recently identified anaplastic lymphoma kinase (ALK) as a tyrosine kinase receptor for pleiotrophin, a secreted growth factor that is highly expressed during embryonic brain development and in tumors of the central nervous system. Here we report on the contribution of pleiotrophin-ALK signaling to glioblastoma growth. We found ALK overexpressed in human glioblastoma relative to normal brain and detected ALK mRNA in glioblastoma cell lines. We reduced the endogenous ALK in glioblastoma cells by ribozyme targeting and demonstrated that this prevents pleiotrophin-stimulated phosphorylation of the anti-apoptotic protein Akt. Furthermore, this depletion of ALK reduced tumor growth of xenografts in athymic nude mice and prolonged survival of the animals because of increased apoptosis in the tumors. These findings directly implicate ALK signaling as a rate-limiting factor in the growth of glioblastoma multiforme and suggest potential utility of therapeutic targeting of ALK.

Epidermal growth factor (EGF), insulin-like growth factor-I (IGF-I), and heregulin-b1 (HRG-b1), can modulate the expression and activity of the estrogen receptor-a (ER-a) via the phosphatidylinositol 3-kinase (PI 3-K)/Akt pathway in the ER-a-positive breast cancer cell line, MCF-7. Estradiol can also rapidly activate PI 3-K/Akt in these cells (nongenomic effect). The recent study examines whether Akt is involved in the ER-a regulation by estradiol (genomic effect). Stable transfection of parental MCF-7 cells with a dominant-negative Akt mutant, as well as the PI 3-K inhibitors wortmannin and LY 294,002, blocked the effect of estradiol on ER-a expression and activity by 70-80 and 55-63%, respectively. Stable transfection of MCF-7 cells with a constitutively active Akt mimicked the effect of estradiol. The changes in ER-a expression and activity were abrogated in response to estradiol by an arginine to cysteine mutation in the pleckstrin homology (PH) domain of Akt (R25C), suggesting the involvement of this amino acid in the interaction between Akt and ER-a. Experiments employing selective ErbB inhibitors demonstrate that the effect of estradiol on ER-a expression and activity is mediated by ErbB2 and not by EGFR. Moreover, anchorage-dependent and -independent growth assays, cell cycle and membrane ruffling analyses showed that Akt exerts estrogen-like activity on cell growth and membrane ruffling and that a selective ErbB2 inhibitor, but not anti-ErbB2 antibodies directed to the extracellular domain, can block these effects. In the presence of constitutively active Akt, tamoxifen only partially inhibits cell growth. In contrast, in cells stably transfected with either a dominant-negative Akt or with R25C-Akt, as well as in parental cells in the presence of a selective ErbB2 inhibitor, the effect of estradiol on anchorage-dependent and -independent cell growth was inhibited by 50-75 and 100%, respectively. Dominant-negative Akt inhibited membrane ruffling by 54%; however, R25C-Akt did not have any effect, suggesting that kinase activity plays an important role in this process. Scatchard analysis demonstrated a 67% reduction in estrogen-binding capacity in cells transfected with constitutively active Akt. No change in binding affinity of estradiol to the receptor was observed upon transfection with either Akt mutant. Taken together, our results suggest that estradiol treatment results in binding to membrane ER-a and interaction with a heterodimer containing ErbB2, leading to tyrosine phosphorylation. This results in the activation of PI 3-K and Akt. Akt, in turn, may interact with nuclear ER-a, altering its expression and activity.

The tumor suppressor gene Syk tyrosine kinase is absent or reduced in invasive breast cancer tissues and cell lines; its loss in breast tissues is linked to poor prognosis and metastasis. Also, evidence shows that in vitro Syk is involved in regulating proliferation. Here, we show by in situ hybridization on breast tissue sections that the loss of Syk expression is progressive during tumor development. Strikingly, Syk is already partially lost in normal epithelial tissue adjacent to the cancer lesion. In vivo, cell proliferation (as measured by the proliferative index Ki67) increased from normal to ductal carcinoma in situ to invasive, whereas Syk in situ staining in the same tissues decreased. In vitro, the presence of Syk was associated with reduced cell proliferation in an epidermal growth factor receptor-overexpressing breast cancer cell line, BT549, whereas changes in apoptosis were undetected. Concomitantly, the kinase activity of the proto-oncogene Src was reduced by ϳ30%. A 5-fold increase in abnormal mitoses was observed in the Syk-transfected cells compared with vector control. We propose that Syk is involved in the regulation of cell proliferation, possibly by controlling mechanisms of mitosis and cytokinesis via Src signal transduction pathway(s). Because of its progressive and early loss during tumor onset and development, monitoring of Syk loss in breast epithelial cells by noninvasive techniques such as ductal lavage may be a powerful tool for screening purposes.