Tumor necrosis factor (TNF) family ligand TNF-␣ and Fas ligand (FasL) can trigger apoptosis in solid tumors, but their clinical usage has been limited by hepatotoxicity. TNF-related apoptosis-inducing ligand (TRAIL) is a newly identified member of the TNF family, and its clinical application currently is under a similar debate. Here, we report a recombinant soluble form of human TRAIL (114 to 281 amino acids) that induces apoptosis in tumor cells but not human hepatocytes. We first isolated human hepatocytes from patients and showed that the human hepatocytes expressed Fas but no TRAIL death receptor DR4 and little DR5 on the cell surface. Antibody cross-linked FasL, but not TRAIL, triggered apoptosis of the human hepatocytes through cleavage of caspases. We then examined TRAIL hepatotoxicity in severe combined immunodeficient/Alb-uPA chimeric mice harboring human hepatocytes. Intravenous injection of FasL, but not TRAIL, caused apoptotic death of human hepatocytes within the chimeric liver, thus killing the mice. Finally, we showed that repeated intraperitoneal injections of TRAIL inhibited intraperitoneal and subcutaneous tumor growth without inducing apoptosis in human hepatocytes in these chimeric mice. The results indicate that the recombinant soluble human TRAIL has a profound apoptotic effect on tumor cells but is nontoxic to human hepatocytes in vitro and in vivo.

Interferon gamma (IFN-␥) is a cytokine predominantly involved in antiproliferative and antiviral responses, immune surveillance, and tumor suppression. However, it has been shown that IFN-␥ is also involved in central nervous system development. Here we studied the underlying mechanism for IFN-␥-induced neuronal differentiation using the human neuroblastoma Paju cell line. Our results indicate that IFN-␥ treatment led to neurite outgrowth followed by growth arrest in the G 1 phase of the cell cycle. IFN-␥ induced ERK1/2 phosphorylation and subsequently the transcription factor early gene response 1, which in turn up-regulated p35 expression and increased cyclin-dependent kinase 5 (Cdk5) activity. IFN-␥-induced neurite outgrowth was abolished by the treatment of MEK1/2 kinase inhibitors, such as U0126 and PD98059, which inhibit the ERK1/2 activation and subsequently prevent the up-regulation of p35 expression and Cdk5 activity. In agreement with the role of p35-Cdk5 in neuronal differentiation, small interfering RNA targeting Cdk5 abrogate the IFN-␥-induced neurite outgrowth. Together, these results demonstrate for the first time that IFN-␥-triggered neuronal differentiation mediated through the up-regulation of p35-associated Cdk5 depends on the activation of the ERK1/2 pathway. Therefore, the present study suggests that IFN-␥ is not only involved in tumorigenicity but also involved in neurogenesis by regulating cell proliferation and differentiation.

Many malignant glioma cells express death receptors for tumor necrosis factor‐related apoptosis‐inducing ligand (TRAIL), yet some of these cells are resistant to TRAIL. Here, we examined signaling events in TRAIL‐induced apoptosis and searched for therapeutic agents that could overcome TRAIL resistance in glioma cells. TRAIL induced apoptosis through death receptor 5 (DR5) and was mediated by caspase‐8‐initiated extrinsic and intrinsic mitochondrial pathways in sensitive glioma cell lines. TRAIL also triggered apoptosis in resistant glioma cell lines through the same pathways, but only if the cells were pretreated with chemotherapeutic agents, cisplatin, camptothecin and etoposide. Previous studies suggested that this was due to an increase in DR5 expression in wild‐type TP53 cells, but this mechanism did not account for cells with mutant TP53. Here, we show that a more general effect of these agents is to down regulate caspase‐8 inhibitor c‐FLIPS (the short form of cellular Fasassociated death domain‐like interleukin‐1‐converting enzyme‐inhibitory protein) and up‐regulate Bak, a pro‐apoptotic Bcl‐2 family member, independently of cell's TP53 status. Furthermore, we showed that TRAIL alone or in combination with chemotherapeutic agents, induced apoptosis in primary tumor cultures from patients with malignant gliomas, reinforcing the potential of TRAIL as an effective therapeutic agent for malignant gliomas.

Cyclin-dependent kinase-5 (Cdk5) is required for neuronal survival, but its targets in the apoptotic pathways remain unknown. Here, we show that Cdk5 kinase activity prevents neuronal apoptosis through the upregulation of Bcl-2. Treatment of SH-SY5Y cells with retinoid acid (RA) and brain-derived neurotrophic factor (BDNF) generates differentiated neuron-like cells. DNA damage triggers apoptosis in the undifferentiated cells through mitochondrial pathway; however, RA/BDNF treatment results in Bcl-2 upregulation and inhibition of the mitochondrial pathway in the differentiated cells. RA/BDNF treatment activates Cdk5-mediated PI3K/Akt and ERK pathways. Inhibition of Cdk5 inhibits PI3K/ Akt and ERK phosphorylation and Bcl-2 expression, and thus sensitizes the differentiated cells to DNA-damage. Inhibition of ERK, but not PI3K/Akt, abrogates Cdk5-medidated Bcl-2 upregulation and the protection of the differentiated cells. This study suggests that ERK-mediated Bcl-2 upregulation contributes to BDNF-induced Cdk5-mediated neuronal survival.

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