Down-regulation of CYP1A2 induction during the maturation of mouse cerebellar granule cells in culture: role of nitric oxide accumulation

Ahr and Cyp1a2 genotypes both affect susceptibility to motor deficits following gestational and lactational exposure to polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) are persistent organic pollutants known to cause adverse health effects and linked to neurological deficits in both human and animal studies. Children born to exposed mothers are at highest risk of learning and memory and motor deficits. We developed a mouse model that mimics human variation in the aryl hydrocarbon receptor and cytochrome P450 1A2 (CYP1A2) to determine if genetic variation increases susceptibility to developmental PCB exposure. In our previous studies, we found that high-affinity Ahr^bCyp1a2(-/-) and poor-affinity Ahr^dCyp1a2(-/-) knockout mice were most susceptible to learning and memory deficits following developmental PCB exposure compared with Ahr^bCyp1a2(+/+) wild type mice (C57BL/6J strain). Our follow-up studies focused on motor deficits, because human studies have identified PCBs as a potential risk factor for Parkinson's disease. Dams were treated with an environmentally relevant PCB mixture at gestational day 10 and postnatal day 5. We used a motor battery that included tests of nigrostriatal function as well as cerebellar function, because PCBs deplete thyroid hormone, which is essential to normal cerebellar development. There was a significant effect of PCB treatment in the rotarod test with impaired performance in all three genotypes, but decreased motor learning as well in the two Cyp1a2(-/-) knockout lines. Interestingly, we found a main effect of genotype with corn oil-treated control Cyp1a2(-/-) mice performing significantly worse than Cyp1a2(+/+) wild type mice. In contrast, we found that PCB-treated high-affinity Ahr^b mice were most susceptible to disruption of nigrostriatal function with the greatest deficits in Ahr^bCyp1a2(-/-) mice. We conclude that differences in AHR affinity combined with the absence of CYP1A2 protein affect susceptibility to motor deficits following developmental PCB exposure.

2,3,7,8-Tetrachlorodibenzo-p-dioxin induces apoptosis by disruption of intracellular calcium homeostasis in human neuronal cell line SHSY5Y

The persistent xenobiotic agent 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces neurotoxic effects that alters neurodevelopment and behavior both during development and adulthood. There are many ongoing efforts to determine the molecular mechanisms of TCDD-mediated neurotoxicity, the signaling pathways involved and its molecular targets in neurons. In this work, we have used SHSY5Y human neuroblastoma cells to characterize the TCDD-induced toxicity. TCDD produces a loss of viability linked to an increased caspase-3 activity, PARP-1 fragmentation, DNA laddering, nuclear fragmentation and hypodiploid (apoptotic) DNA content, in a similar way than staurosporine, a prototypical molecule of apoptosis induction. In addition, TCDD produces a decrease of mitochondrial membrane potential and an increase of intracellular calcium concentration (P < 0.05). Finally, based on the high lipophilic properties of the dioxin, we test the TCDD effect on the membrane integrity using sarcoplasmic reticulum vesicles as a model. TCDD produces calcium efflux through the membrane and an anisotropy decrease (P < 0.05) that reflects an increase in membrane fluidity. Altogether these results support the hypothesis that TCDD toxicity in SHSY5Y neuroblastoma cells provokes the disruption of calcium homeostasis, probably affecting membrane structural integrity, leading to an apoptotic process.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces microglial nitric oxide production and subsequent rat primary cortical neuron apoptosis through p38/JNK MAPK pathway

It has been widely accepted that microglia, which are the innate immune cells in the brain, upon activation can cause neuronal damage. In the present study, we investigated the role of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in regulating microglial nitric oxide production and its role in causing neuronal damage. The study revealed that TCDD stimulates the expression of inducible nitric oxide synthase (iNOS) as well as the production of nitric oxide (NO) in a dose- and time-dependent manner. Further, a rapid activation of p38 and JNK MAPKs was found in HAPI microglia following TCDD treatment. Blockage of p38 and JNK kinases with their specific inhibitors, SB202190 and SP600125, significantly reduced TCDD-induced iNOS expression and NO production. In addition, it was demonstrated through treating rat primary cortical neurons with media conditioned with TCDD treated microglia that microglial iNOS activation mediates neuronal apoptosis. Lastly, it was also found that p38 and JNK MAPK inhibitors could attenuate the apoptosis of rat cortical neurons upon exposure to medium conditioned by TCDD-treated HAPI microglial cells. Based on these observations, we highlight that the p38/JNK MAPK pathways play an important role in TCDD-induced iNOS activation in rat HAPI microglia and in the subsequent induction of apoptosis in primary cortical neurons.

Role of nitric oxide in cerebellar development and function: focus on granule neurons

More than 20 years of research have firmly established important roles of the diffusible messenger molecule, nitric oxide (NO), in cerebellar development and function. Granule neurons are main players in every NO-related mechanism involving cerebellar function and dysfunction. Granule neurons are endowed with remarkable amounts of the Ca(2+)-dependent neuronal isoform of nitric oxide synthase and can directly respond to endogenously produced NO or induce responses in neighboring cells taking advantage of the high diffusibility of the molecule. Nitric oxide acts as a negative regulator of granule cell precursor proliferation and promotes survival and differentiation of these neurons. Nitric oxide is neuroprotective towards granule neurons challenged with toxic insults. Nitric oxide is a main regulator of bidirectional plasticity at parallel fiber-Purkinje neuron synapses, inducing long-term depression (LTD) or long-term potentiation (LTP) depending on postsynaptic Ca(2+) levels, thus playing a central role in cerebellar learning related to motor control. Granule neurons cooperate with glial cells, in particular with microglia, in the regulation of NO production through the respective forms of NOS present in the two cellular types. Aim of the present paper is to review the state of the art and the improvement of our understanding of NO functions in cerebellar granule neurons obtained during the last two decades and to outline possible future development of the research.

Aryl hydrocarbon receptor-dependent induction of apoptosis by 2,3,7,8-tetrachlorodibenzo-p-dioxin in cerebellar granule cells from mouse

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a prototypical environmental contaminant with neurotoxic properties that alters neurodevelopment and behavior. TCDD is a ligand of the aryl hydrocarbon receptor (AhR), which is a key signaling molecule to fully understand the toxic and carcinogenic properties of dioxin. Much effort is underway to unravel the molecular mechanisms and the signaling pathways involved in TCDD-induced neurotoxicity, and to define its molecular targets in neurons. We have used cerebellar granule cells (CGC) from wild-type (AhR+/+) and AhR-null (AhR-/-) mice to characterize the cell death that takes place in neurons after TCDD toxicity. TCDD induced cell death in CGC cultures from wild-type mice with an EC(50) of 127±21 nM. On the contrary, when CGC neurons from AhR-null mice were treated with TCDD no significant cell death was observed. The role of AhR in TCDD-induced death was further assessed by using the antagonists resveratrol and α-naphtoflavone, which readily protected against TCDD toxicity in AhR+/+ CGC cultures. AhR+/+ CGC cultures treated with TCDD showed nuclear fragmentation, DNA laddering, and increased caspase 3 activity, similarly to what was found by the use of staurosporine, a well-established inducer of apoptosis. Finally, the AhR pathway was active in CGC because TCDD could induce the expression of the target gene cytochrome P450 1A2 in AhR+/+ CGC cultures. All together these results support the hypothesis that TCDD toxicity in CGC neurons involves the AhR and that it takes place mainly through an apoptotic process. AhR could be then considered a novel target in neurotoxicity and neurodegeneration whose down-modulation could block certain xenobiotic-related adverse effects in CNS.

2,3,7,8-Tetrachlorodibenzo-p-dioxin induces apoptosis in neural growth factor (NGF)-differentiated pheochromocytoma PC12 cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent toxicant that alters normal brain development and produces cognitive disability and motor dysfunction. However, after decades of intense study, the molecular mechanisms of TCDD-induced neurotoxicity, the signaling pathways involved and its molecular targets in neurons still remain unknown. TCDD acts as an exogenous ligand of the aryl hydrocarbon receptor (AhR) that becomes a key signaling molecule in the regulation of the toxic and carcinogenic properties of TCDD. We have used NGF-differentiated pheochromocytoma (dPC12) cells to determine the type of cell death that takes place by TCDD toxicity. TCDD induced cell death in dPC12 cultures with an EC(50) of 218+/-24 nM, similar to that obtained in undifferentiated PC12 cells, 171+/-31 nM. Nuclear fragmentation was observed after TCDD incubation in parallel to an increase in caspase-3 activity. Staurosporine, which readily induced apoptosis in dPC12 cells, showed a similar increase in caspase-3 activity and the characteristic pattern of nuclear fragmentation. Flow cytometry measurements showed that dPC12 cells in the presence of TCDD were positive for annexin V labeling but negative for propidium iodide staining. In addition, TCDD increased the area of the peak corresponding to hypodiploid (apoptotic) DNA content. All together these results support the hypothesis that TCDD toxicity in dPC12 cells takes place mainly through an apoptotic process.

Induction of Cyp1a1 Is a Nonspecific Biomarker of Aryl Hydrocarbon Receptor Activation: Results of Large Scale Screening of Pharmaceuticals and Toxicants in Vivo and in Vitro

Expression of Cyp1a1 and its related enzyme activity have long been used as a biomarker for aryl hydrocarbon receptor (AhR) activation and a warning of dioxin-like toxicity. As a result, induction of Cyp1a1 by pharmaceutical drug candidates or environmental contaminants raises significant concern in risk assessment. The current study evaluates the specificity of Cyp1a1 induction as a marker for AhR affinity and activation and provides context to assess the relevancy of AhR activation to risk assessment. In vivo experiments examined the expression of Cyp1a1 and other AhR-regulated genes in liver, kidney, and heart in response to 596 compounds. From this data set, a subset of 147 compounds was then evaluated for their ability to activate or bind to the AhR using a combination of gel shift, reporter gene, and competitive receptor binding assays. Whereas in vivo Cyp1a1 mRNA expression is a sensitive marker for AhR activation, it lacks specificity, because 81 (59%) of 137 compounds were found to significantly induce Cyp1a1 in vivo but were not verified to bind or activate the AhR in vitro. Combining in vivo and in vitro findings, we identified nine AhR agonists, six of which are marketed therapeutics and have been approved by the U.S. Food and Drug Administration, including leflunomide, flutamide, and nimodipine. These drugs do not produce dioxin-like toxicity in rats or in humans. These data demonstrate that induction of Cyp1a1 is a nonspecific biomarker of direct AhR affinity and activation and lend further support to the hypothesis that Cyp1a1 induction and/or AhR activation is not synonymous with dioxin-like toxicity.

Resveratrol-induced apoptosis in MCF-7 human breast cancer cells involves a caspase-independent mechanism with downregulation of Bcl-2 and NF-kappaB

Resveratrol (RES), a chemopreventive molecule, inhibits the proliferation of tumor cells of different etiologies. We previously showed that RES alters the cell cycle and induces apoptosis in MCF-7 breast tumor cells by interfering with the estrogen receptor (ERaalpha)-dependent phosphoinositide 3-kinase (PI3K) pathway. Here, we analyzed signaling downstream of PI3K, to understand the mechanisms of RES-induced apoptosis. Apoptotic death by RES in MCF-7 was mediated by Bcl-2 downregulation since overexpression of this protein abolished apoptosis. Decreased Bcl-2 levels were not related to cytochrome c release, activation of caspases 3/8 or poly(ADP-ribose) polymerase proteolysis. However, RES decreased mitochondrial membrane potential and increased reactive oxygen species and nitric oxide production. NF-kappaB, a regulator of Bcl-2 expression, and calpain protease activity, a regulator of NF-kappaB, were both inhibited by RES. The patterns for NF-kappaB and calpain activities followed that of PI3K and were inhibited by LY294002. NF-kappaB inhibition coincided with diminished MMP-9 activity and cell migration. These data suggest that RES-induced apoptosis in MCF-7 could involve an oxidative, caspase-independent mechanism, whereby inhibition of PI3K signaling converges to Bcl-2 through NF-kappaB and calpain protease activity. Therefore, Bcl-2 and NF-kappaB could be considered potential targets for the chemopreventive activity of RES in estrogen-responsive tumor cells.

Effect of inducible nitric oxide synthesis inhibitor on CYP1A2 protein expression in BCG-immune liver damage in mice

AIM: To study the effect of nitric oxide production on CYP1A2 protein expression in immune liver damage induced by Mycobacterium Calmette-Guerin (BCG) in mice.

METHODS: Immune liver damage was induced by intravenous injection of BCG (125 mg/kg) for 2 weeks in vivo. The hepatic tissues injury was estimated by histopathological H-E staining. The protein expression of CYP2E1 and iNOS in hepatic tissues was determined by the method of immunohistochemistry. The correlation between iNOS inducing and liver injury degree was observered by the method of demi-quantification image analysis.

RESULTS: Two weeks after of BCG injection, granuloma was easily observed, and over-expression of iNOS protein was detected in the granulomas. The decrease of CYP1A2 protein expression was observed in mice hepatic tissues. Aminoguanidine, a selective iNOS inhibitor, significantly inhibited iNOS protein expression, and reversed down-regulation of CYP1A2 protein induced by BCG-immune liver damage in mice.

CONCLUSION: Under the BCG-stimulated condition, nitric oxide production participates in the down-regulation of CYP1A2 protein expression induced by immune hepatic injury in mice.