PTEN gene transfer in human malignant glioma: sensitization to irradiation and CD95L-induced apoptosis.

The tumor suppressor gene PTEN (MMAC1, TEP1) encodes a dual-specificity phosphatase and is considered a progression-associated target of genetic alterations in human gliomas. Recently, it has been reported that the introduction of wild type PTEN into glioma cells containing endogenous mutant PTEN alleles (U87MG, LN-308), but not in those which retain wild-type PTEN (LN-18, LN-229), causes growth suppression and inhibits cellular migration, spreading and focal adhesion. Here, we show that PTEN gene transfer has no effect on the chemosensitivity of the four cell lines. Further, a correlational analysis of the endogenous PTEN status of 12 human glioma cell lines with their sensitivity to seven different cancer chemotherapy drugs reveals no link between PTEN and chemosensitivity. In contrast, ectopic expression of wild type PTEN, but not the PTEN(G129R) mutant, in PTEN-mutant gliomas markedly sensitizes these cells to irradiation and to CD95-ligand (CD95L)-induced apoptosis. PTEN-mediated facilitation of CD95L-induced apoptosis is associated with enhanced CD95L-evoked caspase 3 activity. Protein kinase B (PKB/Akt), previously shown to inhibit CD95L-induced apoptosis in nonglial COS7 cells, is inactivated by dephosphorylation. Interestingly, both PTEN-mutant U87MG and PTEN-wild-type LN-229 cells contain phosphorylated PKB constitutively. Wild-type PTEN gene transfer promotes dephosphorylation of PKB specifically in U87MG cells but not in LN-229 cells. Sensitization of U87MG cells to CD95L-apoptosis by wild-type PTEN is blocked by insulin-like growth factor-1 (IGF-1). The protection by IGF-1 is inhibited by the phosphoinositide 3-OH (PI 3) kinase inhibitor, wortmannin. Although PKB is a down-stream target of PI 3 kinase, the protection by IGF-1 was not associated with the reconstitution of PKB phosphorylation. Thus, PTEN may sensitize human malignant glioma cells to CD95L-induced apoptosis in a PI 3 kinase-dependent manner that may not require PKB phosphorylation.