L1 stimulated glioma cell migration and invasion associates with FAK activation

Glioma is the most common primary tumor that occurs in adult human brain. High-grade glioma unfortunately has the worst prognosis and any current clinical treatment is not effective. The most severe form of glioma, glioblastoma multiforme (GBM) is practically incurable. The main reason of this deadly disease is due to the extensive and diffusive migration of glioma cells. By moving away from the tumor mass and infiltrating into the surrounding normal brain tissue, those cells escape surgical resection and cause recurrence. Other therapeutic strategies, besides surgical resection, for instance alkylating agents and radiation mainly target glioma cell mitosis. However, after those treatments chemo- and radiation-resistant cells are still able to migrate away from the bulk of the tumor and come back to cause recurrence. Elucidating the mechanism responsible for driving cell motility can potentially lead to development of therapeutic strategies which may be combined with existing therapies to better battle high grade glioma.;The function of the neural recognition molecule L1 (L1CAM, CD171) has been intensively studied in human neural system development. Its function in neuronal migration, axon guidance, and growth cone stimulation during CNS development is well established. Its expression is restricted to post mitotic neurons in normal adult brain. L1 undergoes highly regulated proteolysis by ADAM family member ADAM10 to release a large ectodomain fragment during neural development, which is thought to facilitate neuronal migration and axon outgrowth. This proteolysis is undetectable in adult CNS. However, L1 is found to be abnormally expressed and frequently proteolyzed in several types of cancer, including human glioma. Although it is suggested that L1 and its abnormal proteolysis plays a role in facilitating cell migration in several malignancies, its function and possible signaling pathways involved in human glioma cells is still not clear. The focus of my study has been to investigate L1 function in human glioma cell migration and invasion and to reveal the mechanism of this L1-stimulated glioma cell motility. The hypothesis of my dissertation work therefore is: L1 stimulated glioma cell motility correlates with FAK activation and changes in focal complexes.;The collective data supports a model of autocrine/paracrine stimulation of cell migration and invasion in human glioma cells by releasing the L1 ectodomain and/or L1 containing exosomal vesicles. L1 expression and its ectodomain shedding, possibly by ADAM10, are found in rat and human glioma cell lines as well as glioma human surgical samples. This ADAM10 mediated L1 proteolysis was observed in migrating glioma cells as opposed to non-migrating cells. L1 ectodomain was found to regulate cell motility in vitro, and L1 expression promotes cell invasion in vivo. This L1 ectodomain-stimulated glioma cell migration correlates with FAK activation and alteration of focal complexes. The mechanism, to certain extent, explains the aggressively migratory nature of high-grade human glioma cells in the brain. These data demonstrated that it is the released soluble L1 (from ADAM10 proteolysis or exosomal L1) and not the full length cell membrane-bound L1 that stimulates glioma cell migration in an autocrine/paracrine manner, possibly through binding to cell surface integrin receptors. The interaction between soluble L1 and integrins activates FAK at focal complexes thereby controlling cell migration. By using a novel chick embryonic brain microinjection model, I further confirmed the function of L1-facilitated human glioma cell invasion in vivo.;The findings of my study provide possible therapeutic targets for glioma treatment. Directly interrupting L1 expression, L1-integrin interaction, its proteolysis and the downstream signaling molecule FAK can be considered as future targets in the development of glioma treatment by battling glioma migration. Further treatment strategies, for example using ADAM10 and FAK inhibition to interfere with L1 ectodomain shedding and focal complex turnover, respectively, still need to be investigated.