Coexpression of EGFRvIII and PTEN by glioblastoma cells is associated with responsiveness to EGFR kinase inhibitors.

The epidermal growth factor receptor (EGFR) is commonly amplified, overexpressed, and mutated in glioblastoma, making it a compelling molecular target for therapy. We have recently shown that coexpression of EGFRvIII and PTEN protein by glioblastoma cells is strongly associated with clinical response to EGFR kinase inhibitor therapy. PTEN loss, by dissociating inhibition of the EGFR from downstream phosphatidylinositol 3-kinase (PI3K) pathway inhibition, seems to act as a resistance factor. Because 40% to 50% of glioblastomas are PTEN deficient, a critical challenge is to identify strategies that promote responsiveness to EGFR kinase inhibitors in patients whose tumors lack PTEN. Here, we show that the mammalian target of rapamycin (mTOR) inhibitor rapamycin enhances the sensitivity of PTENdeficient tumor cells to the EGFR kinase inhibitor erlotinib. In two isogenic model systems (U87MG glioblastoma cells expressing EGFR, EGFRvIII, and PTEN in relevant combinations, and SF295 glioblastoma cells in which PTEN protein expression has been stably restored), we show that combined EGFR/mTOR kinase inhibition inhibits tumor cell growth and has an additive effect on inhibiting downstream PI3K pathway signaling. We also show that combination therapy provides added benefit in promoting cell death in PTEN-deficient tumor cells. These studies provide strong rationale for combined mTOR/EGFR kinase inhibitor therapy in glioblastoma patients, particularly those with PTEN-deficient tumors.

Glioblastoma is the most common malignant brain tumor of adults and one of the most lethal cancers. The secreted growth factor pleiotrophin (PTN) promotes glioblastoma migration and proliferation, initiating its oncogenic activities through two cell surface receptors, the protein tyrosine phosphatase receptor (PTPRZ1) and the anaplastic lymphoma kinase (ALK), respectively. Here, we report on the presence and purification of two naturally occurring forms of PTN (18 and 15 kDa) that differentially promote glioblastoma migration and proliferation. Using a panel of glioblastoma cell lines, including low passage patient-derived cultures, we demonstrate that PTN15 promotes glioblastoma proliferation in an ALKdependent fashion, whereas immobilized PTN18 promotes haptotactic migration of glioblastoma cells in a PTPRZ1-dependent fashion. Mass spectrometric analysis indicated that PTN15 differs from PTN18 by processing of 12 C-terminal amino acids. To demonstrate clinical relevance, we show that PTN15, PTN18, and PTPRZ1 are significantly overexpressed in glioblastoma relative to normal brain at both mRNA and protein levels using microarray, Western blot, and tissue microarray analyses on human tumors. These results indicate that the PTN18-PTPRZ1 and the PTN15-ALK signaling pathways represent potentially important therapeutic targets for glioblastoma invasion and growth.Glioblastoma is the most frequent and lethal primary brain tumor of adults, with a median survival time of 12 months from the time of diagnosis despite aggressive surgery, radiation, and chemotherapy. Although refractory to standard treatments, glioblastomas are well suited for targeted molecular therapies (1). The characterization of molecules or pathways critical to glioblastoma pathogenesis may thus lead to the discovery of novel targets for therapy.The secreted growth factor PTN 1 (also called HARP, HB-GAM, HBNF, and OSF-1) may represent such a potential target.PTN is a heparin-binding, developmentally regulated protein that is expressed in the embryonic and postnatal nervous system (2-5) and has been shown to be overexpressed in a number of cancers, including glioblastomas (6 -11). A variety of biological activities have been attributed to PTN including mitogenesis, angiogenesis, neurite outgrowth, and cell migration (12). PTN promotes glioblastoma growth via the receptor tyrosine kinase ALK in a process that can be mediated via the phosphatidylinositol 3-kinase and MAPK signaling pathways (13,14). PTN also engages a second receptor, the receptor tyrosine phosphatase PTPRZ1 (also termed RPTP␤ and PTP) (15,16), and functional data suggest that this interaction may promote glioblastoma migration (17). Like PTN, both ALK and PTPRZ1 are predominantly expressed in the developing central nervous system (18 -21) and have also been shown to be overexpressed in glioblastomas (11,13,17).To date, the molecular mechanisms underlying PTN-mediated invasion and proliferation through these receptors have yet to be fully elucidated. Because the two receptor pathways ha...