Direct stimulation of apoptotic signaling by soluble Apo2l/tumor necrosis factor-related apoptosis-inducing ligand leads to selective killing of glioma cells

Abstract

Apo2 ligand tumor necrosis factor-related apoptosis-inducing ligand (Apo2L/TRAIL) is a member of the tumor necrosis factor family that interacts with cell surface "death receptors" (DR4 and DR5) to initiate programmed cell death. Apo2L/TRAIL also binds to "decoy" receptors (DcR1 and DcR2) that can antagonize its interaction with DR4 and DR5. In recent studies, Apo2L/TRAIL has been noted to produce selective toxicity toward certain neoplastic cells versus normal cells. The decoy receptors may in part contribute to this selectivity, because they are expressed in various normal tissues but are present at low or undetectable levels in certain types of neoplastic cells. In the current study, we examined the potential therapeutic applicability of recombinant soluble Apo2L/TRAIL by investigating its effects in vitro and in vivo against a series of cell lines derived from malignant gliomas, which are often resistant to conventional treatment modalities. In cell proliferation assays, Apo2L/TRAIL produced a striking decrease in cell numbers, with a median inhibitory concentration of 30-100 ng/ml, in the TP53 wild-type high-grade glioma cell lines U87 and A172, the TP53-mutated T98G, and the TP53-deleted LN-Z308. In contrast, no significant effects were observed in non-neoplastic astrocytes at concentrations up to 3000 ng/ml. Clonogenic assays showed that exposure to Apo2L produced a time-dependent decrease in the viability of glioma-derived cell lines. This correlated with the induction of apoptosis as assessed by a terminal transferase-catalyzed in situ end-labeling assay. Pretreatment of the cells with the caspase inhibitors Acetyl-Asp-Glu-Val-L-aspartic acid aldehyde or Acetyl-Tyr-Val-Ala-Asp-chlormethylketone (200 microM) largely eliminated the effects of Apo2L/TRAIL. Administration of Apo2L/TRAIL (0.3, 1, 3, 10, and 30 mg/kg/day for 7 days via i.p. infusion) to nude mice harboring established intracranial U87 xenografts produced a significant, dose-dependent prolongation of survival versus control animals. Survival in the control group was 27 +/- 1.7 days, compared with more than 50 days in each of the treatment groups (P < 0.001). At the 30 mg/kg dose level, 100% of animals survived for 120 days without evidence of tumor, a substantial improvement in comparison with lower dose levels (P < 0.01). No overt toxicity was apparent even at the highest Apo2L dose. We conclude that soluble Apo2L/TRAIL is effective in inducing apoptosis in high-grade glioma cells in vitro. Because this ligand appears to exhibit selective cytotoxicity for glioma cells versus non-neoplastic cells in vitro and demonstrates significant activity in vivo when administered systemically in an otherwise uniformly fatal central nervous system glioma model system, Apo2L may constitute a useful therapeutic agent for these challenging tumors.