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Abstract
Pleiotrophin (PTN) is a heparin-binding growth factor that can support tumor growth, invasion, angiogenesis, and metastasis, and is highly expressed in several types of cancers. The mechanism of action responsible for mediating its effects, however, is still controversial. Our laboratory has shown that PTN binds to and activates the anaplastic lymphoma kinase (ALK) receptor, a member of the insulin receptor superfamily. A parallel mechanism of ALK activation is through PTN binding and inactivation of the protein tyrosine phosphatase receptor ζ (Ptprz). Understanding the mechanism of PTN’s biological effect is of importance for the treatment of cancers that depend on this growth factor.
This dissertation demonstrates the role of the ALK receptor for the biological activity of PTN. Two different anti-ALK antibodies, a mouse monoclonal IgG and a human single-chain variable IgG fragment, inhibited the PTN-mediated effects on signaling in neuroblastoma and glioblastoma cells. Furthermore, an ALK kinase inhibitor blocked PTN-mediated cellular signaling and growth of adrenal carcinoma and neuroblastoma cells. The presence of Ptprz did not result in a significant effect on the PTN-ALK signaling axis. However, our findings suggest that the Ptprz pathway can crosstalk with the PTN-ALK axis at the level of p38 MAPK in glioblastoma cells.
We have described the significance for ALK in mediating the biological activity of PTN. Additionally, the ALK receptor has been described to be necessary for the growth of neuroblastoma cells, irrespective of its ligands. Thus, targeting the ALK receptor under both of these conditions may prove to be effective in the treatment of cancers utilizing the PTN-ALK axis. This dissertation further characterizes the ability of anti-ALK antibodies to block ALK activation, and identifies novel small molecule inhibitors capable of inhibiting ALK kinase activity. We show that the anti-ALK monoclonal IgG binds to the surface of ALK-expressing neuroblastoma cells and inhibits their growth. Furthermore, we characterize a previously developed kinase inhibitor as effective against ALK receptor activation. This drug inhibits PTN-mediated ALK activation and downstream signaling, resulting in the abrogation of neuroblastoma cell growth and PTN-induced adrenal carcinoma cell growth and soft agar colony formation.
