Effect of 2,5-Hexanedione on lipid biosynthesis in sciatic nerve and brain of the rat

Proapoptotic effects of 2,5‑hexanedione on pheochromocytoma cells via oxidative injury

N‑hexanes are prominent environmental pollutants that are able to cause neurotoxicity in vivo and in vitro. Central and peripheral neuropathies induced by n‑hexane exposure are a major health concern. 2,5‑Hexanedione (2,5‑HD) is the most significant neurotoxic metabolite of n‑hexane; however, little is known regarding the underlying mechanism of its neurotoxicity. Thus, the aim of the present study was to investigate the damaging effects of 2,5‑HD on pheochromocytoma PC12 cells, and to explore the underlying mechanism. Cell viability was tested using a Cell Counting Kit‑8 method, and the leakage of lactate dehydrogenase (LDH) from cells was measured using an LDH assay kit. Glutathione peroxidase (GSHPx) and superoxide dismutase (SOD) activities, and the level of malondialdehyde (MDA) were determined using corresponding assay kits. Apoptotic cells were detected using an annexin V‑fluorescein isothiocyanate/propidium iodide (PI) apoptosis kit, and were subsequently observed by fluorescence microscopy. The relative expression levels of cleaved‑caspase‑3, Bcl‑associated‑X protein (Bax) and Bcl‑2 were identified by western blotting. The results revealed that 2,5‑HD was able to decrease the viability of PC12 cells and promoted the leakage of LDH in a concentrationdependent manner. Further analysis demonstrated that 2,5‑HD decreased the activity of the antioxidative enzymes, SOD and GSHPx, and led to an increase in the levels of MDA in the supernatant of cultured PC12 cells. The annexin V/PI staining results revealed that the numbers of apoptotic cells were increased following treatment with 2,5‑HD. In addition, 2,5‑HD (5 and 10 mmol/l) led to significant increases in the expression levels of caspase‑3 and Bax, with the concomitant downregulation of Bcl‑2. The antioxidant N‑acetylcysteine was identified to antagonize 2,5‑HD‑stimulated cleaved‑caspase‑3 and Bax upregulation, and Bcl‑2 downregulation. Collectively, the results of the present study suggested that 2,5‑HD exerts proapoptotic effects on PC12 cells via oxidative injury. These findings may be applied in the development of novel therapeutic strategies to treat neurological disorders associated with nhexane exposure.

Pathogenetic studies of hexane and carbon disulfide neurotoxicity

Two commonly employed solvents, n-hexane and carbon disulfide (CS2), although chemically dissimilar, result in identical neurofilament-filled swellings of the distal axon in both the central and peripheral nervous systems. Whereas CS2 is itself a neurotoxicant, hexane requires metabolism to the gamma-diketone, 2,5-hexanedione (HD). Both HD and CS2 react with protein amino functions to yield initial adducts (pyrrolyl or dithiocarbamate derivatives, respectively), which then undergo oxidation or decomposition to an electrophile (oxidized pyrrole ring or isothiocyanate), that then reacts with protein nucleophiles to result in protein cross-linking. It is postulated that progressive cross-linking of the stable neurofilament during its anterograde transport in the longest axons ultimately results in the accumulation of neurofilaments within axonal swellings. Reaction with additional targets appears to be responsible for the degeneration of the axon distal to the swellings.

Neurophysiological and behavioural effects of combined exposure to 2,5-hexanedione and acetone or ethanol in rats

Rats were dosed with 2,5-hexanedione (2,5-HD), acetone, ethanol or combinations of these for 6 weeks. Nerve conduction velocity (NCV) was measured in the tails of the rats once a week from the third week of dosing. Behaviour was monitored during the whole experiment as performance on a rotarod, in 30 sec. sessions. 2,5-HD alone showed a significant effect on NCV and rotarod performance after 3 weeks of dosing. Acetone, but not ethanol, together with 2,5-HD had an additional significant effect on NCV and rotarod performance of the rats. This additional effect was seen from the 4th dosage week. The mechanism behind this effect is partly unknown.

Rat testis during 2,5-hexanedione intoxication and recovery. I. Dose response and the reversibility of germ cell loss

The histopathology of the testicular injury induced by 2,5-hexanedione (2,5-HD) exposure was examined in the rat. Charles River CD rats (200 g) were intoxicated by consuming 1% 2,5-HD in the drinking water or by intraperitoneal injection of the toxicant. Both neurotoxic and subneurotoxic exposures were studied, the total dose ranging from 40 to 211 mmol/kg. The following results were obtained: (1) there was a time delay between administration of the toxicant and development of the testicular injury, (2) Sertoli cell vacuolation in stages associated with the meiotic metaphase was the first histological sign of cellular injury at all doses, (3) subneurotoxic doses produced selective defects in germ cells in stages I-VIII of the spermatogenic cycle, (4) both subneurotoxic and neurotoxic doses produced germ cell necrosis and generalized sloughing of germ cells, and (5) intensive intoxication followed by a 17-week recovery period resulted in an absence of all postspermatogonial germ cells from the seminiferous epithelium of three of five treated rats. These data demonstrate that 2,5-hexanedione-induced testicular atrophy occurs at exposure levels below those producing clinical neurotoxicity and that, within the time frame of this study, the testicular injury is at least partially irreversible.

Electrophysiological investigation of 2,5-hexanedione neurotoxicity in rats

Rats were subcutaneously administered 0.8 ml/kg/day 2,5-hexanedione for periods of 18, 25, and 32 days, respectively, and were processed for electrophysiological observations. Action potential and membrane currents were recorded from the sciatic nerve under current- and voltage-clamp conditions. The contractile responses of gastrocnemius and diaphragm muscles following sciatic nerve and phrenic nerve stimulation, respectively, were observed. Animals developing peripheral neuropathy showed hindlimb weakness and depression of body weight gain. Current- and voltage-clamp experiments have shown that after 32 days treatment, the amplitude of the action potential recorded from single myelinated nerve fibers was reduced and the duration was slightly increased; peak sodium current was reduced by 15% and high potassium current density (12 nA) appeared. The twitch responses of diaphragm and gastrocnemius muscles following electrical stimulation of the phrenic and sciatic nerves, respectively, were strongly affected in the 32 day-treated rats. The results are discussed in terms of demyelination which should increase the amplitude of delayed current by exposing potassium channels which under physiological conditions are buried under the myelin sheath.

Molecular mechanisms of diketone neurotoxicity

The important industrial and commercial solvents n-hexane and methyl n-butyl ketone undergo metabolic conversion in experimental animals and man to the neurotoxic gamma-diketone 2,5-hexanedione. Several molecular mechanisms of action have been proposed to explain the pathogenesis of gamma-diketone neuropathy. Such a mechanism must account for the target organ specificity, neurofilament accumulation, structure/activity relationships, in vivo covalent binding, and apparent direct axonal toxicity encountered in this syndrome. It has been proposed that the gamma-diketones exert their effects by reaction with sulfhydryl moieties of energy-producing axonal glycolytic enzymes, with resultant disruption of axoplasmic transport. Others have suggested that reaction instead occurs with lysine moieties of axonal cytoskeletal proteins to form alkyl pyrrole adducts, leading to damaging physicochemical changes in these proteins. Additional hypotheses involve inhibition of axonal sterologenesis, alterations in nerve membrane properties, and reduced neurofilament proteolysis within the nerve terminal. Although a comprehensive mechanism of action for the gamma-diketones remains to be demonstrated, much progress has been made toward this goal. Ultimate success awaits elucidation of the interactions of the neurotoxic diketones with axonal components at the molecular level. Previous reviews have addressed the historical, pharmacokinetic, and neuropathological aspects of this neuropathy. The present critique will examine proposed molecular mechanisms for the gamma-diketones with regard to theoretical considerations and experimental evidence.

Effects of vitamin B12 and B6 on 2,5-hexanedione-induced neuropathy

The therapeutic effect of Vitamin B12 or Vitamin B6 on 2,5-hexanedione induced-neuropathy in animals was examined by using our electrodiagnostic technique in order to investigate the mechanism of the development of the neuropathy. Pyridoxal phosphate and two forms of Vitamin B12 were administered to rats intoxicated by the neurotoxin for a period of 18 weeks, and the sensory and motor fiber conduction velocity and the motor distal latency of the tail nerve were periodically determined. None of the groups treated with the therapeutic drugs exhibited a definite improvement in the nerve fiber conduction velocity and motor distal latency of the rat tail nerve, as compared with those of the 2,5-hexanedione-treated controls.

Peripheral nerve phospholipids in acrylamide neuropathy

Treatment of cats with acrylamide, either 7.5 or 15 mg/kg IM, once a day for 10 days, resulted in increases of 31 and 47% in the phospholipid content of sciatic nerve, respectively, from a control level of 41.1 +/- 2.7 mg/kg wet weight. Determination of the distribution of individual phospholipids indicated no significant differences between control cats and those receiving a cumulative dose of 150 mg/kg acrylamide. In a separate experiment, cats were treated with the 150 mg/kg dose of acrylamide and the sciatic nerve was divided into proximal and distal portions at the level of the triceps surae nerve. Significant increases in phospholipid content were observed in both the proximal and distal portions of peripheral nerve of the acrylamide-intoxicated cats. This effect was present even when the phospholipid content was expressed in terms of total protein, dry weight or total lipid. Total weight of nerve segments, however, was significantly decreased in the neuropathic animals. The data are consistent with a focal degeneration of axons with relative sparing of phospholipids.

Further observations on in vitro and in vivo effects of 2,5-hexanedione on glyceraldehyde-3-phosphate dehydrogenase

The in vitro and in vivo effect of aliphatic diketones has been studied on glyceraldehyde-3-phosphate dehydrogenase (GAPDH) D,L-glyceraldehyde-3-phosphate: NAD oxidoreductase (phosphorylating EC 1.2.1.12 activity). Neurotoxic diketone, 2,5-hexanedione (2,5-HD), but not 2,4-hexanedione (2,4-HD), a non-neurotoxic diketone, inhibited GAPDH in rat bran homogenate preincubated with 25 mM diketones for 20 min. If the preincubation period was increased to 2 h, approximately 25% and 55% inhibition of GAPDH activity was observed with 1 mM and 5 mM 2,5-HD respectively. The inhibition of GAPDH activity was also seen in sciatic nerves but not in the brain or liver homogenates of rats chronically intoxicated with 2,5-HD for 12 weeks. The inhibition of GAPDH by 2,5-HD appears to be selective, and thus confirms earlier data from this laboratory.

In vitro effect of n-hexane and its metabolites on selected enzymes in glycolysis, pentose phosphate pathway and citric acid cycle

The effect of n-hexane, 2-hexanol, 5-hydroxy-2-hexanone, 2,5-hexanediol, methyl n-butyl ketone ( MnBK ) and 2,5-hexanedione (2,5-HD) has been studied in vitro on crystalline glyceraldehyde-3-phosphate dehydrogenase (GAPDH), DL-glyceraldehyde-3-phosphate: NAD oxidoreductase (phosphorylating) EC. 1.2.1.12 and phosphofructokinase (PFK) ATP: D-fructose-6-phosphate-1-phosphotransferase; EC. 2.7.1.11 and lactic dehydrogenase (LDH) L-lactate: NAD+ oxidoreductase, EC. 1.1.1.27. MnBK and 2,5-HD both inhibited GAPDH and PFK activities selectively. n-Hexane and 2-hexanol had no effect on GAPDH and PFK activities; 5-hydroxy-2-hexanone and 2,5-hexanediol exhibited a slight inhibitory effect on these enzymes. Neither metabolites of n-hexane have any effect on LDH activity. 2,5-Hexanedione did not inhibit transketolase (D-sedoheptulose-7-phosphate: D-glyceraldehyde-3-phosphate glycolaldehyde transferase, EC. 2.2.1.1) and succinate dehydrogenase (succinate: 2,6-dichlorophenol-indophenol oxidoreductase, EC. 1.3.99.1) activities. The levels of ATP were reduced in 2,5-HD-treated cat sciatic nerves and returned to normal levels by exposing the nerve to sodium pyruvate.

The morphogenesis of testicular degeneration induced in rats by orally administered 2,5-hexanedione

The neurotoxic hexacarbon 2,5-hexanedione (2,5-HD) produces testicular atrophy in experimental animals. To examine the morphogenesis of the testicular lesion, 1.0% 2,5-HD was provided in the drinking water of adult F-344 rats for up to 6 weeks. After 3 weeks of administration, there were occasional large vacuoles in the basal region of the germinal epithelium. At 4 weeks, these vacuoles were much larger and more numerous; electron microscopy demonstrated that they were derived from the smooth endoplasmic reticulum. The vacuoles were preferentially associated with stages 12, 13, 14, and 1 of the spermatogenic cycle. Additionally, at 4 weeks there was a significant decrease in the number of tubules in stages 7 and 13, and a concomitant increase in the percentage of tubules in stages 3, 5, and 6. By Week 5, most Golgi-phase and cap-phase spermatids were visibly affected, showing margination of nuclear chromatin, and were becoming dissociated from Sertoli cells. Frequent multinucleated giant cells were seen and electron microscopy of these cells suggested that they were derived from fused spermatocytes or spermatids. After 6 weeks, fewer giant cells were present, most tubules contained cellular debris, and many showed empty lumina encircled by a thin ring of cytoplasm near the basement membrane. Interstitial tissue appeared unaffected. These studies indicate that the Sertoli cell is probably an initial target for 2,5-HD action in the testis.

Updating federal standards for toxicants: n-Hexane as the model

Regulatory agencies commonly set standards for a myriad of chemical and physical hazards, with a special focus on carcinogens. This focus can obscure the need to update previously set standards for non-carcinogenic hazards in response to new scientific data. This paper considers n-hexane as a representative chemical hazard: the occupational standard of 500 ppm over an 8 hr time weighted average for a 40 hr week as set in 1948. In the last 12 yr, n-hexane has been shown to cause adverse effects on the peripheral and central nervous systems, the eye, and the respiratory tract. Yet, these new data have not, as yet, been incorporated into a revised standard for n-hexane. The lack of scientific data on the carcinogenicity of n-hexane may be an underlying reason explaining the failure of regulatory agencies to revise their standards. Perhaps, the most troubling problem to regulatory agencies is the sheer magnitude of the chemical regulatory burden. Research should proceed on developing meaningful categories or groupings both for the initial setting of standards and for revising existing standards.