Abstract 1968: Inactivation of the tumor suppressor Lgl via PTEN loss promotes the invasiveness of glioblastoma multiforme.
Authors: Gont Alexander , Hanson Jennifer , Soucie Mathieu , Kassam Amin , DaSilva Vasco , Woulfe John , Nicholas Garth , Lavictoire Sylvie , Restall Ian , Lorimer Ian A. 
Published: August 2014

Glioblastoma multiforme (GBM) is the most aggressive and invasive form of brain tumor. From diagnosis the average survival time is about one year. While other forms of brain cancer can be successfully removed by surgical means, the invasive nature of GBM results in frequent relapses at secondary sites within the brain. PTEN loss is very common in GBM and leads to aberrant activation of the phosphoinositide 3-kinase pathway. Here we show that this event results in the constitutive phosphorylation and inactivation of the tumor suppressor lethal giant larvae (LGL). In Drosophila, loss of LGL causes both over-proliferation and increased invasiveness of epithelial and brain tissue. Studies in yeast and mammals have also shown a role for Lgl in vesicle trafficking and cell motility. Its role in the proliferation and invasiveness in human brain cancer has not been investigated previously. In both cell culture and in vivo subcutaneous mouse models, expression of a non-phosphorylatable, constitutively active form of LGL (LGL-3SA) did not significantly affect the proliferation of human U87MG glioblastoma cells. However, LGL-3SA expression did lead to a reduction in the invasiveness of U87MG cells in vitro. This appeared to be mediated in part by effects on matrix metalloproteinase trafficking, as LGL-3SA prevented the delivery of matrix metalloproteinase 14 (MMP14) to the leading edge of U87MG cells. The role of Lgl in primary glioblastoma stem-like cells was also assessed, as these cells retain their invasive properties in mouse xenografts and are therefore a more clinically relevant model of glioblastoma. All primary glioblastoma stem-like cells expressed Lgl. MMP14 expression was also detected in primary glioblastoma stem-like cells, and Lgl-3SA impaired its trafficking in these cells as well. Experiments are underway to assess the affects of Lgl-3SA expression on the in vivo invasiveness of intracranial xenograft tumours generated using glioblastoma stem-like cells. Our current in vitro data suggest that inactivation of Lgl is an essential downstream step mediating the increased invasiveness of PTEN negative glioblastoma. Inactivation of Lgl enhances invasiveness by increasing the delivery of MMP-14 to the cell surface, where it can promote the degradation of extracellular matrix directly and also indirectly by activation of other matrix metalloproteinases.

Citation Format: Alexander Gont, Jennifer Hanson, Mathieu Soucie, Amin Kassam, Vasco DaSilva, John Woulfe, Garth Nicholas, Sylvie Lavictoire, Ian Restall, Ian A. Lorimer. Inactivation of the tumor suppressor Lgl via PTEN loss promotes the invasiveness of glioblastoma multiforme. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1968. doi:10.1158/1538-7445.AM2013-1968