1,3-Dinitrobenzene metabolism and toxicity in seminiferous tubules isolated from rats of different ages

Oxidative stress response of rat testis to model prooxidants in vitro and its modulation

Understanding the effects of prooxidants on mammalian testis either in vitro or in vivo is important, since recent evidence shows that oxidative stress can play a vital role in the etiology of male infertility. In this investigation, we have examined the oxidative stress response of adult rat testis in vitro as induced by model prooxidants (tert-butyl hydroperoxide (t-bHP) and cumene hydroperoxide (cHP) deploying two models-testicular cell suspensions (TCS) and testicular explants (TE). Significant induction of oxidative stress was observed in both models as evidenced by increased thiobarbituric acid reactive substances (TBARS) levels on incubation with hydroperoxides. The response was both concentration and time dependent. At the highest concentration (200 microM), both hydroperoxides induced a 100% increase in the TE model, compared with a dramatic (380-560%) increase in the TCS model during a 30-min incubation. Further evidence of oxidative stress such as reduction in the GSH levels and alterations in the activity of antioxidant enzymes (catalase and glutathione peroxidase) were also obtained in the TE model. In the TE model, radical scavengers, namely thiourea, urea and mannitol, as well as antioxidants such as glutathione and catalase inhibited the t-bHP-induced lipid peroxidation response to varying degree. A similar degree of protection was also evident with known antioxidants such as ascorbic acid, butylated hydroxyanisole and butylated hydroxytoluene in the TE model. Further co-incubation of TE either with mercaptosuccinate (a potent glutathione peroxidase inhibitor) or 3-aminotriazole (an irreversible catalase inhibitor) resulted in a marked increase in t-bHP-induced lipid peroxidation, clearly suggesting the importance of both of these enzymic antioxidants in rat testis in vitro. These data suggest that the TE model may be further utilized to screen antioxidants in vitro and also investigate the prooxidant potency of xenobiotics in testicular cells.

1,3-Dinitrobenzene metabolism and protein binding

1,3-Dinitrobenzene is a testicular toxicant, which produces a lesion in the seminiferous tubules of the rat. In the present study, we investigated which subcellular fractions of the seminiferous tubules are capable of 1,3-dinitrobenzene metabolism and protein adduct formation. Subcellular fractions of the liver were used as positive controls and to further investigate potentially important binding proteins. Microsomes, cytosol, and mitochondria prepared from each tissue were incubated with 200 microM [(14)C]1,3-dinitrobenzene and 2 mM NADH or NADPH. Since nitroreduction is an oxygen sensitive metabolic pathway, incubations were carried out in the presence and absence of oxygen. Under anaerobic conditions, 1,3-dinitrobenzene was metabolized to nitroaniline and/or nitrophenylhydroxylamine. Metabolite formation was inhibited under aerobic conditions, suggesting the presence of an oxygen-dependent redox-cycle. For the seminiferous tubules, no metabolites were generated under aerobic conditions. In the absence of oxygen, only the mitochondria produced 1,3-dinitrobenzene metabolites. For the liver, under anaerobic conditions, all three subcellular fractions produced 1,3-dinitrobenzene metabolites with the microsomes containing the greatest activity. However, under aerobic conditions, only the microsomes generated metabolites. One-dimensional gel electrophoresis demonstrated that protein adduct formation within the liver and seminiferous tubule subcellular fractions correlated with metabolite formation. Addition of GSH to seminiferous tubule mitochondrial incubations decreased the amount of (14)C-labeled protein. Moreover, when seminiferous tubule mitochondria were incubated with 1,3-dinitrobenzene at an increased protein concentration, radioactive labeling of a 54 kDa protein became more prominent. Two-dimensional gel electrophoresis of liver mitochondrial protein incubated with [(14)C]1,3-dinitrobenzene and NADPH yielded three predominantly radiolabeled proteins of the same approximate size (54 kDa). Amino acid sequencing identified each of these proteins as rat mitochondrial aldehyde dehydrogenase.

1,3-Dinitrobenzene metabolism and GSH depletion

Previous work demonstrated that the mitochondrial fraction of rat seminiferous tubules is capable of metabolizing 1,3-dinitrobenzene, using NADPH as a cofactor. Moreover, 1,3-dinitrobenzene treatment of rat tubules caused a decrease in mitochondrial GSH levels. In situ mitochondrial metabolism of 1,3-dinitrobenzene may have caused this depletion through the production of reactive oxygen intermediates, generating oxidative stress and/or one or more metabolites of 1,3-dinitrobenzene which reacted nonenzymatically with GSH. The goal of this study is to investigate which of these two potential mechanisms may have caused the observed GSH depletion. Liver microsomes, known to rapidly metabolize 1,3-dinitrobenzene, generated the superoxide anion radical when incubated with 1,3-dinitrobenzene and NADPH. However, with the seminiferous tubule mitochondria, no oxygen radicals were detected. Hence, the aforementioned GSH depletion is unlikely due to the production of reactive oxygen intermediates from in situ mitochondrial metabolism of 1,3-dinitrobenzene. To investigate the ability of 1,3-dinitrobenzene metabolites to deplete seminiferous tubule mitochondrial GSH, mitochondria were incubated with 1,3-dinitrobenzene and NADPH. Loss of GSH correlated with the appearance of the 1,3-dinitrobenzene metabolites, nitrophenylhydroxylamine and nitroaniline. Subsequent investigation demonstrated that the metabolites, nitrosonitrobenzene, known to react nonenzymatically with nonprotein sulfhydryls, and nitrophenylhydroxylamine both oxidized seminiferous tubule mitochondrial GSH. Further studies suggested that nitrophenylhydroxylamine could deplete GSH via a free radical mechanism. In aqueous solution, this metabolite was shown to exist in equilibrium with a radical form, thought to be the hydronitroxide radical. The addition of GSH eliminated the signal, implying that the radical reacted nonenzymatically with GSH. In conclusion, the data in this study suggest that the decrease in mitochondrial GSH observed in DNB-treated seminiferous tubules is due to the formation of NPHA and NNB and not reactive oxygen intermediates.

Differential cellular regulation of the mitochondrial permeability transition in an in vitro model of 1,3-dinitrobenzene-induced encephalopathy

Exposure to 1,3-dinitrobenzene (DNB) is associated with neuropathologic changes in specific brainstem nuclei, mediated by oxidative stress and mitochondrial dysfunction. The expression of Bcl-2-family proteins as a function of sensitivity to 1, 3-dinitrobenzene (DNB)-induced mitochondrial permeability transition (MPT) was examined in C6 glioma and SY5Y neuroblastoma cells. Neuroblastoma cells were 10-fold more sensitive than glioma cells to DNB-induced decreases in mitochondrial reducing potential, measured by reduction of the tetrazolium compound, 3-[4, 5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT). The IC(50) values for DNB-related inhibition of MTT reduction were 107+/-25 microM in SY5Y cells and 1047+/-101 microM in C6 cells. Levels of reactive oxygen species (ROS) were increased in both SY5Y and C6 cells following DNB exposure by 4.6- and 6.0-fold above control, respectively. DNB caused abrupt depolarization of mitochondria in both neuroblastoma and glioma cells that was inhibited by trifluoperazine. The first order rate constants for mitochondrial depolarization were: C6, k=0.31+/-0.02 min(-1); SY5Y, k=0.14+/-0.01 min(-1). Onset of MPT occurred at 10-fold lower concentration of DNB in SY5Y cells than in C6 cells. The antioxidants, deferoxamine and alpha-tocopherol, effectively prevented DNB-induced MPT in C6 and SY5Y cells, suggesting involvement of ROS in the initiation of MPT. Exposure to DNB resulted in decreased cellular ATP content in SY5Y cells and efflux of mitochondrial calcium in both SY5Y and C6 cells, concurrent with onset of MPT. The expression of Bcl-2, Bcl-X(L), and Bax was evaluated in both cell types by Western blot analysis. C6 glioma cells strongly expressed Bcl-X(L) and only weakly expressed Bcl-2 and Bax, whereas SY5Y neuroblastoma cells expressed lower levels of Bcl-X(L) and higher levels of both Bcl-2 and Bax. Collectively, these results suggest that higher constitutive expression of Bcl-X(L), rather than Bcl-2, correlates with resistance to DNB-induced MPT in SY5Y and C6 cells and that differential regulation of the permeability transition pore may underlie the cell-specific neurotoxicity of DNB.

Germ cell apoptosis in rat testis after administration of 1,3-dinitrobenzene

The present study investigated the occurrence of apoptosis in rat testis 6, 12, 24, or 48 h after administration of 1,3-dinitrobenzene (DNB)(25 mg/kg, i.p.). Apoptotic cells were visualized using TUNEL in situ detection. In untreated control animals, occasional tubules contained a few apoptotic nuclei. After DNB treatment, more tubules were apoptotic and more germ cells within individual tubules were affected. The increase in incidence and severity of apoptosis was statistically significant 24 and 48 h after exposure. Spermatocytes were the primary cell population affected. Apoptosis was primarily present in Stages VI-VIII and IX-XIII. DNA ladders, characteristic of apoptosis, were clearly present by 24 h after DNB administration, faint at 12 h, and not present in the control or at 6 h. Although initiating mechanisms may be very different for different chemicals, apoptosis is a common and late manifestation of the testicular response to numerous toxicants.

Species difference in 1,3-dinitrobenzene testicular toxicity: in vitro correlation with glutathione status

It has been previously shown that 1,3-dinitrobenzene (DNB) causes testicular damage to the rat but not the hamster. The present study of DNB's mechanism of toxic action has exploited this species difference in susceptibility. Seminiferous tubules were isolated from Golden Syrian hamsters and incubated with 100 microM DNB or vehicle for 22 h. (A similar study with rat tubules has been published.) Formation of DNB metabolites were monitored over time; hamster tubules had a greater capacity than rat tubules for reductively metabolizing (activating) DNB. However, hamster tubules did not show the marked DNB-induced ATP depletion seen in rat tubules. Levels of mitochondrial glutathione and activities of enzymes that protect against oxidative stress were measured in both rat and hamster tubules. The observed differences in the capacity for detoxification of oxidants may underlie the difference in susceptibility to DNB-induced testicular toxicity between these species.