Author Correction: Bisphosphonate enhances TRAIL sensitivity to human osteosarcoma cells via death receptor 5 upregulation.

After publication of this article, the authors noticed an error in the figure section.

SIRT1, a histone deacetylase, regulates prion protein-induced neuronal cell death.

Prion diseases associated with the conversion of the cellular prion protein (PrP(C)) to the misfolded isoform (PrP(Sc)), affect the central nervous system (CNS) of humans and animals. Resveratrol, an activator of class III histone deacetylase SIRT1, is important in attenuating cellular injury and oxidative stress. The present study investigated the effects of SIRT1 activation on prion protein-mediated neuronal cell death and examined its possible signals in intracellular apoptotic pathways. Resveratrol treatment significantly increased both SIRT1 protein expression and SIRT1 activity and protected neuronal cells against PrP (106-126)-induced cell death. Resveratrol-mediated SIRT1 activation decreased the acetylation of p53 and p65 induced by prion protein and SIRT1 inhibitor. SIRT1 activation also inhibited PrP (106-126)-mediated p38 mitogen-activating protein kinase (MAPK) activation and caspase-3 cleavage, and increased the expression of anti-apoptotic Bcl-xL protein. Furthermore, SIRT1 overexpression by using adenoviral vector protected neuronal cells against PrP (106-126). These results indicate that resveratrol inhibits PrP (106-126)-induced neuronal cell death by regulating SIRT1 activity and SIRT-related signaling, and suggest that prion-related disease may be attenuated by SIRT1 activation or by intake of SIRT1-activating molecules.

Hypoxia-inducible factor-1 α regulates prion protein expression to protect against neuron cell damage.

The human prion protein fragment, PrP (106-126), may contain a majority of the pathological features associated with the infectious scrapie isoform of PrP, known as PrP(Sc). Based on our previous findings that hypoxia protects neuronal cells from PrP (106-126)-induced apoptosis and increases cellular prion protein (PrP(C)) expression, we hypothesized that hypoxia-related genes, including hypoxia-inducible factor-1 alpha (HIF-1α), may regulate PrP(C) expression and that these genes may be involved in prion-related neurodegenerative diseases. Hypoxic conditions are known to elicit cellular responses designed to improve cell survival through adaptive processes. Under normoxic conditions, a deferoxamine-mediated elevation of HIF-1α produced the same effect as hypoxia-inhibited neuron cell death. However, under hypoxic conditions, doxorubicin-suppressed HIF-1α attenuated the inhibitory effect on neuron cell death mediated by PrP (106-126). Knock-down of HIF-1α using lentiviral short hairpin (sh) RNA-induced downregulation of PrP(C) mRNA and protein expression under hypoxic conditions, and sensitized neuron cells to prion peptide-mediated cell death even in hypoxic conditions. In PrP(C) knockout hippocampal neuron cells, hypoxia increased the HIF-1α protein but the cells did not display the inhibitory effect of prion peptide-induced neuron cell death. Adenoviruses expressing the full length Prnp gene (Ad-Prnp) were utilized for overexpression of the Prnp gene in PrP(C) knockout hippocampal neuron cells. Adenoviral transfection of PrP(C) knockout cells with Prnp resulted in the inhibition of prion peptide-mediated cell death in these cells. This is the first report demonstrating that expression of normal PrP(C) is regulated by HIF-1α, and PrP(C) overexpression induced by hypoxia plays a pivotal role in hypoxic inhibition of prion peptide-induced neuron cell death. These results suggest that hypoxia-related genes, including HIF-1α, may be involved in the pathogenesis of prion-related diseases and as such may be a therapeutic target for prion-related neurodegenerative diseases.

Sulforaphane blocks hypoxia-mediated resistance to TRAIL-induced tumor cell death.

Hypoxia occurs frequently in various solid tumors and elicits a cellular response designed to improve cell survival through adaptive processes, thereby accelerating cancer progression and the development of chemotherapy resistance. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), a member of the TNF superfamily, leads to tumor cell death via both intrinsic and extrinsic apoptotic signaling pathways. Hypoxia inhibits TRAIL-mediated apoptosis and attenuates the therapeutic activity of TRAIL in cancer management. Hypoxia-inducible factor-1α (HIF-1α) plays a central role in tumor hypoxia by up-regulating gene expression related to angiogenesis, cancer invasion and anti-apoptosis. Sulforaphane (SFN), a phenethyl isothiocyanate, elicits HIF-1α inactivation under hypoxia. This study investigated whether hypoxic inhibition of TRAIL-mediated tumor cell death is increased by SFN-mediated HIF-1α instability. SFN induced cell death in various tumor cells, including SK-N-SH, SNU-638, HeLa and A549 cells, and showed cell cytotoxicity in hypoxia-exposed tumor cells. Western blot analysis showed that SFN treatment increased p53 and activated caspase-3 proteins, and decreased HIF-1α activation under hypoxia. Under low-oxygen conditions, TRAIL-treated cells displayed inhibited apoptosis, while SFN-pre-treated cells exhibited stronger sensitization to TRAIL under the hypoxic conditions. SFN treatment enhanced TRAIL-induced activation of proteins, including caspase-3 and p53. SFN dose-dependently decreased HIF-1α protein levels in cancer cells, which was mediated by decreased protein stability. This study demonstrated that SFN recovered hypoxia-mediated resistance to TRAIL via instability of HIF-1α, and also suggests that combination therapy with SFN and TRAIL may provide a novel strategy for treating hypoxic solid tumors.

Bisphosphonate enhances TRAIL sensitivity to human osteosarcoma cells via death receptor 5 upregulation.

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), a member of the TNF superfamily of cytokines, is one of the most promising candidates for cancer therapeutics. However, many osteosarcomas are resistant to TRAIL. Bisphosphonates are very effective in the treatment of bone problems associated with malignancies; the antitumor effects are due to the inhibition of protein prenylation that is essential for cell function and survival. The purpose of this study was to determine the effects of bisphosphonates on TRAIL-resistant MG 63 human osteosarcoma cells. The cells showed no response to TRAIL alone; however, pre-treatment with bisphosphonates significantly increased TRAIL-mediated apoptosis and cellular activation of caspase-3. Bisphosphonates significantly induced mRNA and protein expression of the TRAIL receptor, DR5. Bisphosphonates induced protein unprenylation in MG 63 cells; in addition, co-treatment with TRAIL also significantly increased protein unprenylation. Blocking of protein unprenylation using geranylgeraniol attenuated the cellular responses, including cell apoptosis and protein unprenylation induced by bisphosphonates and TRAIL. This is the first study to demonstrate that bisphosphonates markedly enhanced TRAIL-induced apoptosis in human osteosarcoma cells. These findings suggest that bisphosphonates may be a new and effective anticancer treatment with TRAIL proteins for TRAIL-resistant cancer cells.

Prion peptide-mediated cellular prion protein overexpression and neuronal cell death can be blocked by aspirin treatment.

Prion diseases are infectious neurodegenerative disorders characterized by the conversion of the cellular prion protein (PrPc) to the misfolded isoform (PrPsc). Prion peptide PrP 106-126 [PrP (106-126)] shares many physiological properties with PrPsc; it is neurotoxic in vitro and in vivo. PrP (106-126) induces neurotoxicity by the overexpression of PrPc and activation of the mitogen-activated protein (ERK1/2). Aspirin, an anti-inflammatory drug, is a known ERK inhibitor and prevents neurodegenerative disorders including prion diseases. The influence of aspirin treatment on prion protein-mediated neurotoxicity and expression of PrPc were the focus of this study. Cell viability and apoptosis were assessed by crystal violet staining and the TUNEL and DNA fragmentation assays. Apoptosis-associated protein expression of PrPc, p-53, p-ERK1/2, p-p38, Bcl-2 and cleaved-caspase-3 was examined by Western blotting and immunocytochemistry. Aspirin treatment inhibited PrP (106-126)-induced neuronal cell death in SH-SY5Y neuroblastoma cells. In addition, the PrP (106-126)-mediated increase of p-p38, p53, cleaved-caspase-3 and decrease of Bcl-2 expressions were blocked by aspirin and the ERK inhibitor, PR98059. Furthermore, we showed that the PrP (106-126)-mediated increase of PrPc and p-ERK1/2 were inhibited by PD98059 and aspirin. Taken together, these results demonstrate that ERK1/2 is a key modulator of the protective effect of aspirin on PrP-106-126-mediated cellular prion protein overexpression and neurotoxicity and also suggest that aspirin may prevent neuron cell damages caused by the prion peptide.

Cellular cholesterol enrichment prevents prion peptide-induced neuron cell damages.

The prion diseases are neurodegenerative disorders characterized by the conversion of the PrPc (normal cellular prion) to the PrPsc (misfolded isoform). The accumulation of PrPsc within the central nervous system (CNS) leads to neurocytotoxicity by increasing oxidative stress. In addition, many neurodegenerative disorders including prion, Parkinson's and Alzheimer's diseases may be regulated by cholesterol homeostasis. The effects of cholesterol balance on prion protein-mediated neurotoxicity and ROS (reactive oxygen species) generation were the focus of this study. Cholesterol treatment inhibited PrP (106-126)-induced neuronal cell death and ROS generation in SH-SY5Y neuroblastoma cells. In addition, the PrP (106-126)-mediated increase of p53, p-p38, p-ERK and the decrease of Bcl-2 were blocked by cholesterol treatment. These results indicated that cellular cholesterol enrichment is a key regulator of PrP-106-126-mediated oxidative stress and neurotoxicity. Taken together, the results of this study suggest that modulation of cellular cholesterol appears to prevent the neuronal cell death caused by prion peptides.

Hypoxia inducing factor-1alpha regulates tumor necrosis factor-related apoptosis-inducing ligand sensitivity in tumor cells exposed to hypoxia.

Hypoxia is a common environmental stress. Particularly, the center of rapidly-growing solid tumors is easily exposed to hypoxic conditions. Hypoxia is well known to attenuate the therapeutic response to radio and chemotherapies including tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) protein. HIF-1alpha is a critical mediator of the hypoxic response. However, little is known about the function of hypoxia-inducible factor-1alpha (HIF-1alpha) on hypoxic inhibition of TRAIL-mediated apoptosis. In this study, we investigated whether hypoxic inhibition of TRAIL-mediated apoptosis can be regulated by modulating HIF-1alpha protein. Hypoxia- and DEF-induced HIF-1alpha activation inhibited the TRAIL-mediated apoptosis in SK-N-SH, HeLa, A549 and SNU-638 cells. And also, HIF-1alpha inactivating reagents including DOX increased the sensitivity to TRAIL protein in tumor cells exposed to hypoxia. Furthermore, knock-down of HIF-1alpha using lentiviral RNA interference sensitized tumor cells to TRAIL-mediated cell death under hypoxic condition. Taken together, these results indicate that HIF-1alpha inactivation increased TRAIL sensitivity in hypoxia-induced TRAIL-resistant tumor cells and also suggest that HIF-1alpha inhibitors may have benefits in combination therapy with TRAIL against hypoxic tumor cells.

Protective effect of hypoxia on bisphosphonate­related bone cell damage.

Bisphosphonates (BPs) are widely used for the prevention and treatment of osteoporosis. However, there have been numerous reports of side effects of BPs, including osteonecrosis of the jaw. In the present study, we investigated whether hypoxia inhibits BP-induced apoptosis, and examined the mechanisms of this inhibition. The cell viability of the MG 63 human osteoblast-like cell line treated with the nitrogen-containing (N)-BPs alendronate, risedronate and zoledronate was investigated, and hypoxia was assessed by crystal violet staining and the MTT assay, and by observing cell morphology. The effect of N-BPs and hypoxia on apoptotic cell signaling was evaluated using Western blotting, immunocytochemistry and the TUNEL assay. The results of crystal violet staining and the MTT and TUNEL assays showed that the N-BPs inhibited proliferation and induced apoptosis in MG 63 cells. Hypoxia significantly prevented N-BP-induced MG 63 cell apoptosis, and also attenuated BP-induced c-Jun N-terminal kinase (JNK) phosphorylation and BCL-xL reduction. Hypoxia prevented BP-induced cell damage by blocking JNK phosphorylation and by regulating the BCL-xL protein. Thus, hypoxia or hypoxia-related genes, including hypoxia-inducible factor 1α, may be a potential therapy for BP-related side effects such as osteonecrosis of the jaw.

Hypoxia protects neuronal cells from human prion protein fragment-induced apoptosis.

Prion diseases are neurodegenerative disorders characterized by the accumulation of an abnormal isoform of the prion protein PrP(Sc). Human prion protein fragment, PrP (106-126) (prion protein peptide 106-126), may contain most of the pathological features associated with PrP(Sc). Hypoxic conditions elicit cellular responses adaptively designed to improve cell survival and have an important role in the process of cell survival. We investigate the effects of hypoxia on PrP (106-126)-induced apoptosis in the present study. Human neuroblastoma and glioblastoma cells were incubated with varied doses of PrP (106-126) under both normoxic or hypoxic conditions, in order to determine the regulatory effects of hypoxia on PrP (106-126)-induced apoptosis. The results indicate that hypoxia protects neuronal cells against PrP (106-126)-induced cell death by activating the Akt signal, which is inactivated by prion proteins, and inhibiting PrP (106-126)-induced caspase 3 activation. Low oxygen conditions increase the Bcl-2 protein, which is associated with anti-apoptotic signals, and recover the PrP (106-126)-induced reduction in mitochondrial transmembrane potential. This study demonstrates that hypoxia inhibits PrP (106-126)-induced neuron cell death by regulating Akt and Akt-related signaling, and it also suggests that prion-related neuronal damage and disease may be regulated by hypoxia or by hypoxic-inducing genes.