Alteration of brainstem auditory evoked potentials in diethylbenzene and diacetylbenzene-treated rats

Evaluating the toxic mechanism of 1,2-diacetylbenzene in neural cells/tissues: The favorable impact of silibinin

1,2-diacetylbenzene (1,2-DAB) is a neurotoxic component of aromatic solvents commonly used in industrial applications that induces neuropathological changes in animals. This study unraveled the toxic impact of 1,2-DAB in nerve tissues, explant cultures, and neuron-glial cultures, and explored whether herbal products can mitigate its toxicity. The effects of DAB on axonal transport were studied in retinal explant cultures grown in a micro-patterned dish. The mitochondrial movement in the axons was captured using time-lapse video recordings. The results showed that 1,2-DAB, but not 1,3-DAB inhibited axonal outgrowth and mitochondrial movement in a dose-dependent manner. The toxicity of 1,2-DAB was further studied in spinal cord tissues and cultures. 1,2-DAB selectively induced modifications of microtubules and neurofilaments in spinal cord tissues. 1,2-DAB also potently induced cell damage in both neuronal and glial cultures. Further, 1,2-DAB-induced cellular ATP depletion precedes cell damage in glial cells. Interestingly, treatment with the herbal products silibinin or silymarin effectively mitigated 1,2-DAB-induced toxicity in spinal cord tissues and neuronal/glial cultures. Collectively, the molecular toxicity of 1,2-DAB in neural tissues involves protein modification, ATP depletion, and axonal transport defects, leading to cell death. Silibinin and silymarin show promising neuroprotective effects against 2-DAB-induced toxicity.

Organic solvent metabolite, 1,2-diacetylbenzene, impairs neural progenitor cells and hippocampal neurogenesis

1,2-Diacetylbenzene (DAB) is a neurotoxic minor metabolite of 1,2-diethylbenzene or naphthalene reaction product with OH radical. DAB causes central and peripheral neuropathies that lead to motor neuronal deficits. However, the potent effects and molecular mechanisms of DAB on neural progenitor cells and hippocampus are unknown. In the current study, we report the DAB damage at lower doses (less than 50 μM) to neural progenitor cell (NPC) invitro and hippocampal neurogenesis invivo. DAB significantly suppressed NPC proliferation with increased reactive oxygen species (ROS) production in a dose-dependent manner. The suppression of NPC proliferation was effectively blunted by the action of an antioxidant, N-acetyl cysteine. Six-week-old male C57BL/6 mice were treated with 1 or 5 mg/kg DAB for 2 weeks. DAB significantly suppressed NPC proliferation in the dentate gyrus of the hippocampus, indicating impaired hippocampal neurogenesis. Increased ROS production and the formation of oxidative stress-associated dinitrophenyl adducts were detected in the hippocampal homogenates of DAB-treated mice. DAB activated Mac-1-positive immune cells which are involved in inflammatory process in the hippocampus. Taken together, these results confirm that oxidative stress by DAB might be cause of adverse effects in NPC proliferation and hippocampal neurogenesis.

Environmental metabolite, 1,2-diacetylbenzene, produces cytotoxicity through ROS generation in HUVEC cells

Organic solvents are ubiquitous in industrial and household surroundings, and thus individuals are easily exposed. 1,2-Diethylbenzene (DEB) is one of organic solvents contained in gasoline or jet fuels. DEB is absorbed by dermal or inhalation routes, metabolized by cytochrome P-450 in the liver, and ultimately affects mammalian functions. 1,2-Diacetylbenzene (1,2-DAB), which is a putative metabolite of 1,2-DEB, resulted in neuropathological effects on rodent central and peripheral nervous systems. To elucidate the possibility of 1,2-DAB effects on the vascular system, studies were undertaken to examine whether 1,2-DAB induces endothelial cytotoxicity through reactive oxygen species (ROS) generation. Incubation of human umbilical vein endothelial cells (HUVEC) with lower concentrations (4 or 8 microM) of 1,2-DAB induced inhibition of cellular growth and at higher amounts (16 or 32 microM) produced apoptosis. Endothelial cells cultured with 1,2-DAB also showed increased intracellular ROS production and morphological alterations indicative of senescence. Pretreatment with the well-known antioxidant glutathione or N-acetylcysteine (NAC) reduced cytotoxicity induced by 1,2-DAB. Taken together, the results provide evidence that cytotoxicity induced by 1,2-DAB in endothelial cells may be mediated by ROS generation.

Axonopathy-Inducing 1,2-Diacetylbenzene Forms Adducts with Motor and Cytoskeletal Proteins Required for Axonal Transport

The aromatic hydrocarbon 1,2-diacetylbenzene (1,2-DAB) is a protein-reactive gamma-diketone metabolite of the neurotoxic solvent 1,2-diethylbenzene (1,2-DEB). The effect of neurotoxic 1,2-DAB and its non-neurotoxic isomer 1,3-DAB has been studied on motor proteins and cytoskeletal proteins of rat spinal cord (SC). For in vitro studies, SC slices were incubated with 1, 2, 5, 10 mM of DAB isomers for 30 min at 37 degrees C. For in vivo studies, rats received (i.p.) 20 mg/kg/day of 1,2-DAB or 1,3-DAB, or vehicle (2% acetone in saline), 5 days a week for 2 weeks. Spinal cord and sciatic nerve proteins were subjected to Western blotting using monoclonal mouse antibodies to NF-M, kinesin, dynein, and tau. Proteins were quantified and paired mean comparisons performed to assess concentration-dependent changes in native protein bands. In vitro, 1,2-DAB produced a concentration-dependent decrease of motor and cytoskeletal proteins. While dynein and tau appeared similarly affected by 1,2-DAB, kinesin was most affected by the toxicant. In vivo, 1,2-DAB affected motor and cytoskeletal proteins of sciatic nerves and spinal cord differentially. In general, sciatic nerve proteins were much more affected than spinal cord proteins. The results show that motor proteins that drive axonal transport anterogradely (kinesin) and retrogradely (dynein), cytoskeletal protein NF-M, which is slowly transported in the anterograde direction, and microtubule-associated protein, tau, which is involved in axonal transport, are differentially impacted by 1,2-DAB. By contrast, non-neurotoxic isomer 1,3-diacetylbenzene (1,3-DAB), had no adverse effect on neural proteins either in vitro or in vivo. 2D-Differential in gel electrophoresis (2D-DIGE) of sciatic nerves from neurotoxic 1,2-DAB and non-neurotoxic 1,3-DAB treated rats revealed 197 and 304 protein spots, respectively.

Amino acid and protein targets of 1,2-diacetylbenzene, a potent aromatic gamma-diketone that induces proximal neurofilamentous axonopathy

The gamma-diketone analogs 1,2-diacetylbenzene (1,2-DAB) and 2,5-hexanedione (2,5-HD), but not the delta-diketone 1,3-diacetylbenzene (1,3-DAB) or the beta-diketone 2,4-hexanedione, induce neuropathological changes in the rodent central and peripheral nervous systems. The molecular targets of these neurotoxic aromatic and aliphatic gamma-diketones, and of their nonneurotoxic structural analogs and ninhydrin, are examined by assessing their differential reactivity with neural and nonneural amino acids and proteins in vitro and in vivo. Whereas 1,2-DAB is chromogenic and forms polymers with amino acids (notably lysine) and proteins (especially lysine-rich proteins), 1,3-DAB lacks these properties. Ninhydrin forms a chromophore without evidence of protein polymerization. 1,2-DAB preferentially targets neurofilament over microtubule protein in vitro and in situ. Based on protein reactivity, 1,2-DAB is three orders of magnitude more reactive than 2,5-HD. Lysine-rich neurofilament protein subunits NF-H and NF-M are more susceptible than lysine-poor NF-L and beta-tubulin to 1,2-DAB. These observations correlate with the development of proximal (1,2-DAB) and distal (2,5-HD) neurofilament-filled axonal swellings and segregated intact microtubules observed during systemic treatment with aromatic and aliphatic gamma-diketones.

Aromatic as well as aliphatic hydrocarbon solvent axonopathy

Superfund sites that contain mixtures of aromatic and aliphatic solvents represent an undefined health hazard. After prolonged exposure to relatively high levels of certain aliphatic solvents (e.g. n-hexane, 2-hexanone), humans and animals develop a dose-dependent neurodegeneration that occurs clinically as a symmetrical peripheral neuropathy. This is triggered by the action of 2,5-hexanedione (1,2-diacetylethane), a 1,4-diketone (gamma-diketone) metabolite that targets proteins required for the maintenance of neuronal (and testicular Sertoli cell) integrity. Certain aromatic solvents (1,2-diethylbenzene, 1,2,4-triethylbenzene) cause electrophysiological changes consistent with sensorimotor neuropathy in rodents, but the underlying mechanisms and pathogenesis are unclear. Our recent studies show that the o-diacetyl derivative and likely metabolite of 1,2-diethylbenzene, 1,2-diacetylbenzene, behaves as a neurotoxic (aromatic) gamma-diketone of high neurotoxic potency. Rats treated with 1,2-diacetylbenzene develop limb weakness associated with proximal, neurofilament-filled giant axonal swellings comparable to those seen in animals treated with the potent 3,4-dimethyl derivative of 2,5-hexanedione. The blue chromogen induced by treatment with 1,2-diacetylbenzene is under study as a possible urinary biomarker of exposure to aromatic solvents (e.g. 1,2-diethylbenzene, tetralin) with neurotoxic potential. Development and validation of sensitive new biomarkers, especially for non-cancer endpoints, will aid in assessing the health risk associated with exposure to hazardous substances at Superfund sites.

1,2-diacetylbenzene, the neurotoxic metabolite of a chromogenic aromatic solvent, induces proximal axonopathy

Several widely used aromatic hydrocarbon solvents reportedly induce blue-green discoloration of tissues and urine in animals and humans. The chomophore has been proposed to result from a ninhydrin-like reaction with amino groups in proteins. The present study examines the neurotoxic property of 1,2-diacetylbenzene (1,2-DAB), the active metabolite of the chromogenic and neurotoxic aromatic solvent 1,2-diethylbenzene. Rats treated with 1,2-DAB, but not with the nonchromogenic isomer 1,3-DAB or with ninhydrin developed blue discoloration of internal organs, including the brain and spinal cord. Only 1,2-DAB induced limb weakness associated with nerve fiber changes, which were most prominent in spinal cord and spinal roots. Changes began with the formation of proximal, neurofilament-filled axonal swellings of the type seen after treatment with 3,4-dimethyl-2,5-hexanedione, a potent derivative of the active metabolite of the neurotoxic aliphatic hydrocarbon solvents n-hexane and methyl n-butyl ketone. These compounds are metabolized to a gamma-diketone that forms pyrroles with target proteins, such as neurofilament proteins. A comparable mechanism is considered for 1,2-DAB, an aromatic gamma-diketone.

Assessment of the developmental toxicity and placental transfer of 1,2-diethylbenzene in rats

Sprague-Dawley rats were administered 1,2-diethylbenzene (1,2-DEB) by gavage on gestational days (GD) 6 through 20 at dose levels of 0 (corn oil), 5, 15, 25 or 35 mg/kg. The dams were euthanized on GD21 and the offspring were weighed and examined for external, visceral and skeletal alterations. Maternal toxicity, indicated by significant decreases in body weight gain and food consumption, was observed at doses of 15 mg/kg and above. Developmental toxicity, expressed as significantly reduced foetal body weights, was seen at doses of 15 mg/kg and higher. There was no evidence of embryolethal or teratogenic effects at any dose tested. The placental transfer of 1,2-DEB was examined after a single oral dose of 25 mg [14C]1,2-DEB/kg on GD18. Maternal and foetal tissues were collected at intervals from 1 to 48 hours. Placental and foetal tissues accounted for less than 0.35% of the administered dose. Levels of radiocarbon in foetuses were lower than those in maternal plasma and placenta at all time points. Analysis performed at 1, 2 and 4 hours indicated that ethyl acetate extractable (acidic) metabolites were predominant in the maternal plasma while n-hexane extractable (neutral) compounds represented the major part of radioactivity in the placenta and foetus. In conclusion, this study demonstrated that 1,2-DEB causes mild foetotoxicity at maternal toxic doses and that the exposure of the developing rat foetus to 1,2-DEB and/or metabolites after maternal administration of 1,2-DEB in late gestation is small.

Triethylbenzene-induced sensorimotor neuropathy in rats

1,2,4-Triethylbenzene (1,2,4-TEB) and 1,3,5-triethylbenzene (1,3,5-TEB) were administered orally to male Sprague-Dawley rats. Experimental and appropriate control rats were examined electrophysiologically for motor and sensory conduction velocities (MCV and SCV), and the amplitudes of the sensory (ASAP) and muscular action potentials (AMAP), at bi-weekly intervals. Oral administration of 1,2,4-TEB (200 or 400 mg kg-1, once daily, 4 days per week for 8 weeks) produced a time- and dose-dependent decrease in MCV, SCV, AMAP and ASAP. Rats treated with 1,2,4-TEB exhibited a bluish discoloration of the skin and the urine was greyish-greenish. No changes in MCV, SCV, AMAP and ASAP developed in rats given 1,3,5-TEB orally (200 or 400 mg kg-1, daily, 4 days per week for 8 weeks). The results indicate that 1,2,4-TEB is a neurotoxic isomer of TEB and that the presence of two ethyl radicals in the ortho-position on an aromatic ring could be a critical molecular arrangement resulting in chromogenic and neurotoxic properties.

Diethylbenzene inhalation-induced electrophysiological deficits in peripheral nerves and changes in brainstem auditory evoked potentials in rats

Motor and sensory conduction velocities (MCV and SCV), amplitude of the sensory action potential (ASAP) of the tail nerve and parameters of brainstem auditory evoked potentials (BAEP) were studied in male Sprague-Dawley rats after prolonged inhalation exposure to a commercial isomer mixture of diethylbenzene (DEB mixture) containing 6% 1,2-DEB. The MCV, SCV and ASAP were studied in one control group (10 rats) and three groups of 12 rats exposed to 500, 700 or 900 ppm DEB mixture for 6 h daily, 5 days per week, for 18 weeks. Rats used for recording BAEP (one control group and two other groups of 15 rats) were exposed to 600 and 800 ppm DEB mixture. The exposure time was the same. Rats exposed to DEB mixture exhibited a time- and concentration-dependent decrease in MCV, SCV and ASAP and a time- and concentration-dependent increase of both the peak latencies of all BAEP components and the interpeak (I-V) differences.