PTEN gene transfer in human malignant glioma: sensitization to irradiationand CD95L-induced apoptosis

The tumor suppressor gene PTEN (MMAC1, TEP1) encodes a dual-specificity pho sphatase and is considered a progression-associated target of genetic alter ations in human gliomas. Recently, it has been reported that the introducti on of wild type PTEN into glioma cells containing endogenous mutant PTEN al leles (U87MC;, LN-308), but not in those which retain wild-type PTEN (LN-18 , LN-229), causes growth suppression and inhibits cellular migration, sprea ding and focal adhesion. Here, we show that PTEN gene transfer has no effec t on the chemosensitivity of the four cell lines. Further, a correlational analysis of the endogenous PTEN status of 12 human glioma cell lines with t heir sensitivity to seven different cancer chemotherapy drugs reveals no li nk between PTEN and chemosensitivity, In contrast, ectopic expression of wi ld type PTEN, but not the PTENG129R mutant, in PTEN-mutant gliomas markedly sensitizes these cells to irradiation and to CD95-ligand (CD95L)-induced a poptosis. PTEN-mediated facilitation of CD95L-induced apoptosis is associat ed with enhanced CD95L-evoked caspase 3 activity, Protein kinase B (PKB/Akt ), previously shown to inhibit CD95L-induced apoptosis in nonglial COS7 cel ls, is inactivated by dephosphorylation. Interestingly, both PTEN-mutant U8 7MG and PTEN-wild-type LN-229 cells contain phosphorylated PKB constitutive ly. Wild-type PTEN gene transfer promotes dephosphorylation of PKB specific ally in U87MG cells but not in LN-229 cells, Sensitization of U87NG cells t o CD95L-apoptosis by wild-type PTEN is blocked by insulin-like growth facto r-1 (IGF-1). The protection by IGF-1 is inhibited by the phosphoinositide 3 -OH (PI 3) kinase inhibitor, wort-mannin, Although PKB is a down-stream tar get of PI 3 kinase, the protection by IGF-1 was not associated with the rec onstitution of PKB phosphorylation. Thus, PTEN may sensitize human malignan t glioma cells to CD95L-induced apoptosis in a PI 3 kinase-dependent manner that may not require PKB phosphorylation.