Δ-Tetrahydrocannabinol induces cytotoxicity in macrophage J774-1 cells: involvement of cannabinoid receptor 2 and p38 MAPK.

Tetrahydrocannabinol (THC), a psychoactive component of marijuana, is known to exert cytotoxicity in immune cells. In the present study, we examined the cytotoxicity of Δ8-THC in mouse macrophage J774-1 cells and a possible involvement of cannabinoid receptors and stress-responsive mitogen-activated protein kinases (MAPKs) in the cytotoxic process. J774-1 cells were treated with Δ8-THC (0-20 µM) for up to 6 h. As measured by the MTT and LDH assays, Δ8-THC induced cell death of J774-1 cells in a concentration- and/or exposure time-dependent manner. Δ8-THC-induced cell damage was associated with vacuole formation, cell swelling, chromatin condensation, and nuclear fragmentation. The cytotoxic effect of Δ8-THC was significantly prevented by a caspase-1 inhibitor Ac-YVAD-cmk but not a caspase-3 inhibitor z-DEVD-fmk. The pretreatment with SR144528, a CB2 receptor-selective antagonist, effectively suppressed Δ8-THC-induced cytotoxicity in J774-1 cells, which exclusively expressed CB2 receptors as indicated by real-time polymerase chain reaction analysis. In contrast, AM251, a CB1 receptor-selective antagonist, did not affect the cytotoxicity. Pertussis toxin and α-tocopherol significantly attenuated Δ8-THC-induced cytotoxicity suggesting that G(i/o) protein coupling signal transduction and oxidative stress are responsible for the cytotoxicity. Δ8-THC stimulated the phosphorylation of p38 MAPK and c-Jun N-terminal kinase (JNK) in J774-1 cells, which were effectively antagonized by the pretreatment with SR144528. In addition, SB203580, a p38 MARK inhibitor, significantly attenuated the cytotoxic effect of Δ8-THC, whereas SP600125, a JNK inhibitor, significantly enhanced the cytotoxicity. These results suggest that the cytotoxicity of Δ8-THC to J774-1 cells is exerted mediated through the CB2 receptor followed by the activation of p38 MAPK.

Requirement of retinoic acid receptor ß for genipin derivative-induced optic nerve regeneration in adult rat retina.

Like other CNS neurons, mature retinal ganglion cells (RGCs) are unable to regenerate their axons after nerve injury due to a diminished intrinsic regenerative capacity. One of the reasons why they lose the capacity for axon regeneration seems to be associated with a dramatic shift in RGCs' program of gene expression by epigenetic modulation. We recently reported that (1R)-isoPropyloxygenipin (IPRG001), a genipin derivative, has both neuroprotective and neurite outgrowth activities in murine RGC-5 retinal precursor cells. These effects were both mediated by nitric oxide (NO)/S-nitrosylation signaling. Neuritogenic activity was mediated by S-nitrosylation of histone deacetylase-2 (HDAC2), which subsequently induced retinoic acid receptor ß (RARß) expression via chromatin remodeling in vitro. RARß plays important roles of neural growth and differentiation in development. However, the role of RARß expression during adult rat optic nerve regeneration is not clear. In the present study, we extended this hypothesis to examine optic nerve regeneration by IPRG001 in adult rat RGCs in vivo. We found a correlation between RARß expression and neurite outgrowth with age in the developing rat retina. Moreover, we found that IPRG001 significantly induced RARß expression in adult rat RGCs through the S-nitrosylation of HDAC2 processing mechanism. Concomitant with RARß expression, adult rat RGCs displayed a regenerative capacity for optic axons in vivo by IPRG001 treatment. These neuritogenic effects of IPRG001 were specifically suppressed by siRNA for RARß. Thus, the dual neuroprotective and neuritogenic actions of genipin via S-nitrosylation might offer a powerful therapeutic tool for the treatment of RGC degenerative disorders.

Neuritogenic activity of a genipin derivative in retinal ganglion cells is mediated by retinoic acid receptor ß expression through nitric oxide/S-nitrosylation signaling.

Genipin, a herbal iridoid, is known to have both neuroprotective and neuritogenic activity in neuronal cell lines. As it is structurally similar to tetrahydrobiopterin, its activity is believed to be nitric oxide (NO)-dependent. We previously proposed a novel neuroprotective activity of a genipin derivative, (1R)-isoPropyloxygenipin (IPRG001), whereby it reduces oxidative stress in RGC-5, a neuronal precursor cell line of retinal origin through protein S-nitrosylation. In the present study, we investigated another neuritogenic property of IPRG001 in RGC-5 cells and retinal explant culture where in we focused on the NO-cGMP-dependent and protein S-nitrosylation pathways. IPRG001 stimulated neurite outgrowth in RGC-5 cells and retinal explant culture through NO-dependent signaling, but not NO-dependent cGMP signaling. Neurite outgrowth with IPRG001 requires retinoic acid receptor ß (RARß) expression, which is suppressed by an RAR blocking agent and siRNA inhibition. Thereby, we hypothesized that RARß expression is mediated by protein S-nitrosylation. S-nitrosylation of histone deacetylase 2 is a key mechanism in chromatin remodeling leading to transcriptional gene activation. We found a parallelism between S-nitrosylation of histone diacetylase 2 and the induction of RARß expression with IPRG001 treatment. The both neuroprotective and neuritogenic activities of genipin could be a new target for the regeneration of retinal ganglion cells after glaucomatous conditions.

New physiological function of secoiridoids: neuritogenic activity in PC12h cells.

Previously, we have reported that geniposide isolated from an extract of Gardenia fructus has neuritogenic activity in PC12h cells, a subclone of rat pheochromocytoma cells. Furthermore, we have indicated that several geniposide-related iridoid compounds also had similar potent neuritogenic activity. In this study, we have examined the effects of various secoiridoid compounds [K-1, sweroside; K-2, swertiamarin; K-3, gentiopicroside; K-4, 6'-O-ß-D: -glucopyranosylsweroside; K-5, 6'-O-ß-D: -glucopyranosylgentiopicroside; K-6, 6'-O-ß-D: -glucopyranosylswertiamarin; K-7, 5'-O-ß-D: -glucopyranosylamarogentin; K-8, 5'-O-ß-D: -glucopyranosylamaroswertin; H-1, n-butyl vogeloside; H-2, n-butyl epivogeloside; H-3, (7S)-secologanin butyl methyl acetal; H-4, (7R)-secologanin butyl methyl acetal; H-5, secologanin dimethyl acetal] isolated from various medicinal herbs. The secoiridoids H-1, H-2, H-3, H-4, and H-5 induced significant neurite outgrowth. Among these H-series compounds, H-2 was the most potent neuritogenic compound. Among the K-series compounds, K-1, K-2, K-3, and K-8 showed the most potent activity. These results suggest that secoiridoids have neuritogenic activity in PC12h cells and that these secoiridoid compounds are promising starting compounds for the development of neurotrophic factor-like and iridoid compounds.

Long-acting genipin derivative protects retinal ganglion cells from oxidative stress models in vitro and in vivo through the Nrf2/antioxidant response element signaling pathway.

Previously, we reported that genipin, a herbal iridoid, had neuritogenic and neuroprotective actions on PC12 cells. Although nitric oxide (NO)-activated signalings were proposed to be neuritogenic, the neuroprotective action of genipin remains to be elucidated. From the standpoint of NO activation, we tested a possible protective mechanism through the nitrosative Kelch-like ECH-associated protein (Keap1)/NF-E2-related factor 2 (Nrf2)-antioxidant response element pathway in rat retinal ganglion cells (RGC-5 cells) in culture, and in vivo, against hydrogen peroxide and optic nerve injury (ONI), respectively, using a long-acting (1R)-isoPropyloxygenipin (IPRG001). IPRG001 induced NO generation and the expressions of antioxidative enzymes, such as heme oxygenase-1 (HO-1), in RGC-5 cells. The protective action of IPRG001 depended on HO-1 and NO induction. We found that S-nitrosylation of Keap1 by IPRG001 may contribute to translocation of Nrf2 to the nucleus and triggered transcriptional activation of antioxidative enzymes. Furthermore, apoptotic cells were increased and 4-hydroxy-2-nonenal was accumulated in rat retina following ONI. Pre-treatment with IPRG001 almost completely suppressed apoptosis and accumulation of 4-hydroxy-2-nonenal in RGCs following ONI accompanied by HO-1 induction. These data demonstrate for the first time that IPRG001 exerts neuroprotective action in RGCs in vitro and in vivo, through the Nrf2/antioxidant response element pathway by S-nitrosylation against oxidative stress.

Neuritogenic activities of 1-alkyloxygenipins.

We designed 1-alkyloxygenipins with the aim of improving the stability of genipins based on the structural and electronic properties of genipins, and prepared 1-alkyloxygenipins and examined their neuritogenic activities in PC12h cells. All genipin-derivatives exhibited electronic properties similar to those of genipin and induced significant neurite outgrowth. These compounds will be classified as nitric oxide synthase (NOS) activators (neuritogenic active compounds) since their lowest unoccupied molecular orbital (LUMO)-energies are similar to that of tetrahydrobiopterin (H4B). (1R)-isoPropyloxygenipin showed activity comparable to that of genipin, and unlike the parent compound genipin, it was found to be physiologically stable in rat liver homogenate.

Neuroprotective action of genipin on tunicamycin-induced cytotoxicity in neuro2a cells.

Accumulation of unfolding or misfolded proteins within the lumen of the endoplasmic reticulum (ER) triggers ER stress, and sustained ER stress ultimately leads to cell death. Both of these events are involved in the activation of glucose-regulated protein of 78 kDa (GRP78, also known as Bip), CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP, also known as growth arrest and DNA damage-inducible gene 153 (GADD153)), and caspase-12. ER stress has been shown to be involved in neurodegenerative disorders, such as Alzheimer, Parkinson, and polyglutamine diseases. We previously showed that genipin, a natural iridoid compound, has a protective effect against amyloid-beta (Abeta)-induced cytotoxicity. Here, we studied the protective effects of genipin on cytotoxicity induced in Neuro2a cells by the specific ER stress inducer tunicamycin (TM). TM treatment significantly reduced cell viability in a dose-dependent manner. Genipin dramatically rescued the cells against TM-induced cell death. In addition, genipin suppressed ER stress-induced upregulation of CHOP and GRP78. These data suggest that genipin is effective at protecting against neurodegenerative disorders.

Genipin suppresses A23187-induced cytotoxicity in neuro2a cells.

Genipin is an iridoid compound and an aglucon of geniposide isolated from Gardenia fructus. We have previously reported that genipin induces neurite outgrowth in PC12h and Neuro2a cells and protects against cytotoxicity induced by several conditions such as beta-amyloid peptide, serum deprivation, and oxidative stress in rat primary hippocampal neurons and Neuro2a cells. In this paper, we examined the protective effect of genipin on A23187 (a calcium ionophore)-induced cytotoxicity in Neuro2a cells. A23187 induced cytotoxicity in concentration- and time-dependent manners as assayed by measurements of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetazolium bromide (MTT) reduction activity and lactate dehydrogenase (LDH) release. The cytotoxicity was significantly suppressed by genipin in a concentration-dependent manner. A23187 also significantly activated caspase3/7, which is known to be the critical mediator of apoptosis, after 1 h, and the cytotoxicity was clearly blocked by an inhibitor of caspase 3/7. Furthermore, A23187 induced the expression of immunoglobulin-binding protein/glucose-regulated protein of 78 kDa (BiP/GRP78) protein, which is an endoplasmic reticulum (ER) stress marker protein, and the expression was suppressed by genipin. These results suggest that genipin protects Neuro2a cells from A23187-induced cytotoxicity mediated by caspase 3/7 and ER stress. Therefore, genipin may be effective in preventing neurodegeneration observed in Alzheimer's disease and Parkinson's disease involving ER stress.

Genipin exhibits neurotrophic effects through a common signaling pathway in nitric oxide synthase-expressing cells.

We have reported previously that genipin, a natural iridoid compound, induces neuritogenesis through a nitric oxide (NO)-cyclic GMP (cGMP)-cGMP-dependent protein kinase (PKG) signaling pathway in PC12h cells and that neuronal NO synthase (nNOS) is one of the target molecules of genipin in vitro. Recently, it has been suggested that the neurotrophic effects of NO are due to its direct activation of receptor-tyrosine kinase, especially TrkA. In this study, we investigated whether mouse neuroblastoma Neuro2a cells, which express nNOS but not TrkA, respond to genipin with neurite outgrowth through the mechanism observed in PC12h cells, to assess the involvement of TrkA in the mechanism. Neuro2a cells expressed all three types of NO synthase (NOS), and nNOS was detectable as the main component in Western blot analysis. Genipin significantly induced neurite outgrowth and activation of NADPH-diaphorase, which were significantly blocked by a non-selective NOS inhibitor. Both a soluble guanylate cyclase inhibitor and a PKG inhibitor also inhibited the genipin-induced neuritogenesis. Genipin induced sustained phosphorylation of mitogen-activated protein kinase (MAPK). In fact, the genipin-induced neurite outgrowth was completely inhibited by a specific MAPK kinase inhibitor. Moreover, a NOS inhibitor abolished MAPK phosphorylation as well as neurite outgrowth in genipin-treated cells. These results suggest that genipin induces neurite outgrowth through an NO-cGMP-PKG signaling pathway followed by MAPK phosphorylation without TrkA activation in Neuro2a cells and that PKG downstream to NOSs, which may be mainly nNOS, is very important for the signaling molecule to induce neuritogenesis by genipin.

The Neuroprotective and Neurotrophic Effects of Tremella fuciformis in PC12h Cells.

We examined the neuroprotective and neurotrophic effects of Tremella fuciformis. The neurotrophic effects of the hot water extract of T. fuciformis was evaluated by microscopically monitoring its potency to induce neurite outgrowth in PC12h cells. The hot water extract of T. fuciformis promoted neurite outgrowth in PC12h cells in this study, superior to other natural substances which was reported previously. When cells were treated with the hot water extract of T. fuciformis prior to ß-amyloid peptide treatment (active domain of A peptide 25~35 treated), toxicity was significantly diminished (p<0.01). These results suggest that T. fuciformis might potentially be used as a precautionary agent in neurodegenerative disease, such as Alzheimer's disease, etc.

Differences in neuritogenic response to nitric oxide in PC12 and PC12h cells.

We have demonstrated that a natural iridoid compound, genipin, induces neurite outgrowth through the nitric oxide (NO)-cGMP-protein kinase G signaling pathway in PC12h cells. PC12 cells, the parental cell line of PC12h cells, have been shown to carry out neurite extension that accompanies NO production in response to nerve growth factor (NGF). This neurite outgrowth was significantly inhibited by NG-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor, in both PC12 and PC12h cells, suggesting that the neuritogenesis is NO-dependent in both cells. In this report, we investigated whether genipin also induces neurite outgrowth in PC12 cells in order to determine the NO-dependent neurotrophic action of genipin in more than just one cell type. Genipin induced marked neurite outgrowth in PC12h cells but not in PC12 cells. The genipin-induced neurite outgrowth was significantly inhibited by L-NAME in PC12h cells. An NO donor, NOR4, also significantly induced neurite outgrowth in a concentration-dependent manner in PC12h cells but not in PC12 cells. On the other hand, NGF-primed PC12 cells exhibited significant neurite extension, which was inhibited by L-NAME, in response to genipin. Interestingly, NGF-primed PC12 cells responded to NOR4 extending neurites and expressed detectable neuronal NO synthase protein which is not detected in naive PC12 cells. These results suggest that genipin exerts a neuritogenic action on neuronal cells which are responsive to NO itself. Furthermore, the results also suggest that PC12h cells are more suitable for the study of NO-dependent neuronal function than PC12 cells which were not responsive to NO.

Fundamental role of nitric oxide in neuritogenesis of PC12h cells.

1 We investigated the neuritogenic action of nitric oxide (NO)-generating agents and their mechanisms of action in a subclone of rat pheochromocytoma, PC12h cells. 2 NO donors such as sodium nitroprusside (SNP, 0.05-1 microM), NOR1 (5-100 microM), NOR2 (5-20 microM), NOR3 (5-20 microM), NOR4 (5-100 microM), or S-nitroso-N-acetyl-DL-penicillamine (SNAP, 10-100 microM) significantly induced neurite outgrowth. 3 NOR4-induced neurite outgrowth was accompanied by expression of neurofilament 200 kDa subunit (NF200) protein, an axonal marker, and was significantly inhibited by an NO scavenger, a soluble GC inhibitor, and a PKG inhibitor: 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazole-1-oxyl-3-oxide (carboxy-PTIO, 20-100 microM), 1H-[1,2,4]oxadiazolo[4,3-a] quinoxalin-1-one (ODQ, 100 microM) and KT5823 (0.2-1 microM), respectively. 4 The intracellular cGMP concentration of cells was markedly increased by treatment with NOR4 (100 microM). 5 A mitogen-activated protein kinase (MAPK) kinase inhibitor, PD98059 (10-50 microM), abolished the NOR4-induced neurite outgrowth. In agreement with this observation, NOR4 did phosphorylate extracellular signal-regulated kinase (ERK) 1 and 2, substrates of MAPK kinase. 6 A membrane-permeable cGMP analog, 8-Br-cGMP (1 mM) also induced significant neurite outgrowth. The 8-Br-cGMP-induced neurite outgrowth was almost completely inhibited by both KT5823 (0.5 microM) and PD98059 (50 microM). Moreover, sustained ERK phosphorylation was observed in the 8-Br-cGMP-treated PC12h cells. 7 These results suggest that NO itself has the ability to induce neurite outgrowth and that NO-induced ERK activation involves the NO-cGMP-PKG signaling pathway in PC12h cells.

Cyclic GMP-dependent neurite outgrowth by genipin and nerve growth factor in PC12h cells.

We have demonstrated previously that a natural iridoid compound, genipin, induced neuritogenesis through activation of nitric oxide synthase (NOS) and mitogen-activated protein kinase (MAPK) in PC12h cells. In this paper, we investigated whether cyclic GMP (cGMP) and cGMP-dependent protein kinase (PKG) are involved in the neuritogenesis as a result of NOS activation. Furthermore, we also investigated the relationship between cGMP and MAPK activation in the signaling pathway. The genipin-induced neuritogenesis accompanied by induction of neurofilament was significantly inhibited by 1H-[1,2,4]oxadiazolo[4,3-a] quinoxalin-1-one (ODQ) and KT5823, inhibitors of soluble guanylate cyclase and PKG, respectively. Genipin-induced MAPK phosphorylation was also abolished by ODQ. These inhibitory effects of ODQ were similar to those observed for nerve growth factor (NGF)-induced neurite outgrowth and MAPK phosphorylation. The membrane-permeable cGMP analog, 8-Bromo-cGMP, had prominent neuritogenic activity, which was completely inhibited by a MAPK kinase inhibitor, PD98059. These results suggest that the soluble guanylate cyclase-PKG signaling pathway is important for MAPK activation by genipin as well as NGF during neuritogenesis in PC12h cells.

Propentofylline protects beta-amyloid protein-induced apoptosis in cultured rat hippocampal neurons.

beta-Amyloid protein 1-42 (beta42) can induce apoptosis in the cultured hippocampal neurons, suggesting that it plays an important role in causing neurodegeneration in Alzheimer's disease. Recently, propentofylline, a synthetic xanthine derivative, has been reported to depress ischemic degeneration of hippocampal neurons in gerbils. The present study investigated whether or not propentofylline affected the beta42-induced apoptosis of hippocampal neurons, and if so, which type of signaling machinery works in the neuroprotective action of propentofylline. Addition of propentofylline markedly attenuated the beta42-induced cell death of rat hippocampal neurons. The amyloid protein certainly induced apoptosis in the cultured hippocampal cells revealed by nuclear condensation, caspase-3 activation and an increase of Bax. Intriguingly, propentofylline blocked both the apoptotic features induced by beta42 and further induced an anti-apoptotic protein, Bcl-2, during a short time of incubation. The neuroprotective action of propentofylline was comparably replaced with dibutyryl cAMP (dbcAMP) and was completely suppressed by a low concentration of specific protein kinase A (PKA) inhibitor. Taken altogether, the data strongly suggest that the protection of propentofylline on the beta42-induced neurotoxicity is caused by enhancing anti-apoptotic action through cAMP-PKA system. Propentofylline as a therapeutic agent to Alzheimer's disease is discussed.

Neuritogenesis of herbal (+)- and (-)-syringaresinols separated by chiral HPLC in PC12h and Neuro2a cells.

Syringaresinol isolated from Epimedium koreanum NAKA1 and Magnolia officinalis REHD. et WILS. was subjected to optical resolution by chiral HPLC to give (+)- and (-)-enantiomers. The two syringaresinol enantiomers, as well as a mixture of their glucosides, showed dose-dependent neuritogenesis in a concentration range from 0.24 to 24 microM in PC12h cells.