Structure-activity relationships in the induction of DNA-protein cross-links by hematotoxic ring-opened benzene metabolites and related compounds in HL60 cells.

Previous studies from our laboratory have demonstrated the formation of DNA-protein cross-links (DNAPC), a potentially cytotoxic and genotoxic lesion induced by many leukemogenic agents, in bone marrow cells of mice administered benzene, however, reactive benzene metabolites involved in DNAPC formation by benzene have not been characterized. The present studies examined DNA PC formation in HL60 cells treated with trans,trans-muconaldehyde (MUC), a hematotoxic ring-opened metabolite of benzene, as well as with MUC metabolites and structurally related compounds. Using a K(+)/SDS precipitation assay for DNAPC determination, concentration- and time-dependent increases in DNAPC formation were observed 2 and 4 h after treatment of HL60 cells with 50, 75 and 100 microM MUC. No increases in DNAPC levels were measured in HL60 cells 4 h after treatment with the MUC metabolites 6-hydroxy-trans,trans-2,4-hexadienal (HO-M-CHO), 6-oxo-trans,trans-2,4-hexadienoic acid (CHO-M-COOH), or trans,trans-muconic acid (HOOC-M-COOH), each at 100 microM. Significant increases in DNAPC levels were observed 4 h after treatment with 500 and 1000 microM HO-M-CHO, but not CHO-M-COOH. No effect on DNAPC levels was observed 4 h after treatment with 100 microM for trans, trans-2,4-hexadienal, trans-2-hexenal, hexanal, trans,trans-2, 4-hexadiene, glutaraldehyde, or acrolein. DNAPC induced by MUC and HO-M-CHO may be cytotoxic lesions, as increases in DNAPC levels by these compounds correlated with decreases in cell viability. Except for acrolein, compounds not inducing DNAPC at 100 microM also did not affect cell viability. These studies suggest that both aldehydic carbons of MUC contribute to DNAPC induction, and that the presence of alpha,beta-unsaturated double bonds conjugated with the aldehyde groups increases the ability of MUC to induce DNAPC relative to the saturated dialdehyde glutaraldehyde.

DNA-protein cross-link levels in bone marrow cells of mice treated with benzene or trans,trans-muconaldehyde.

Increased levels of DNA-protein cross-links (DNAPC) have been observed in vitro and in vivo following treatment with a number of chemotherapeutic alkylating agents and topoisomerase II inhibitors, that is, agents that have also been associated with the development of bone marrow depression and acute myelogenous leukemia. The current studies were undertaken to examine the effect of benzene, a bone marrow toxin and human leukemogen, on DNAPC levels in mouse bone marrow cells. Using a K+/sodium dodecyl sulfate (SDS) precipitation assay for DNAPC determination, the results indicate increased DNA-protein cross-link levels in mouse bone marrow cells at 2 and 4 but not 8 h after a single ip injection of 440 mg/kg benzene. Following the administration of multiple hematotoxic benzene doses (440 or 880 mg/kg, 2x/d for 2 d), increases in DNA-protein cross-link levels were either slight or not present. These results suggest that DNAPC induced by benzene are neither cumulative nor persistent lesions. The toxicity of benzene is mediated by a number of number of ring-hydroxylated and ring-opened compounds; therefore the present studies also examined DNAPC levels in mice administered trans,trans-muconaldehyde (MUC), a ring-opened hematotoxic and genotoxic metabolite of benzene. No marked increases in DNAPC levels were observed in CD- mouse bone marrow cells 1-12 h following a single ip injection of 3 mg/kg muconaldehyde. It is possible that multiple doses of MUC are required to induce elevated DNAPC levels in bone marrow cells of mice, since multiple doses are required for MUC-induced hematotoxicity. Other reactive metabolites and/or an interaction of reactive intermediates may also be involved in DNAPC induced by benzene.

Effect of CYP2E1 induction by ethanol on the immunotoxicity and genotoxicity of extended low-level benzene exposure.

Potential additive effects of ethanol consumption, a common life-style factor, and low-level benzene exposure, a ubiquitous environmental pollutant, were investigated. Ethanol is a potent inducer of the cytochrome P-450 2E1 (CYP2E1) enzyme, which bioactivates benzene to metabolites with known genotoxicity and immunotoxicity. A liquid diet containing 4.1% ethanol was used to induce hepatic CYP2E1 activity by 4-fold in female CD-1 mice. Groups of ethanol-treated or pair-fed control mice were exposed to benzene or filtered air in inhalation chambers for 7 h/d, 5 d/wk for 6 or 11 wk. The initial experiment focused on immunotoxicity endpoints based on literature reports that ethanol enhances high-dose benzene effects on spleen, thymus, and bone marrow cellularity and on peripheral red blood cell (RBC) and white blood cell (WBC) counts. No statistically significant alterations were found in spleen lymphocyte cellularity, subtype profile, or function (mitogen-induced proliferation, cytokine production, or natural killer cell lytic activity) after 6 wk of ethanol diet, 0.44 ppm benzene exposure, or both. This observed absence of immunomodulation by ethanol alone, a potential confounding factor, further validates our previously established murine model of sustained CYP2E1 induction by dietary ethanol. Subsequent experiments involved a 10-fold higher benzene level for a longer time of 11 wk and focused on genotoxic endpoints in known target tissues. Bone marrow and spleen cells were evaluated for DNA-protein cross-links, a sensitive transient index of genetic damage, and spleen lymphocytes were monitored for hprt-mutant frequency, a biomarker of cumulative genetic insult. No treatment-associated changes in either genotoxic endpoint were detected in animals exposed to 4.4 ppm benzene for 6 or 11 wk with or without coexposure to ethanol. Thus, our observations suggest an absence of genetic toxicity in CD-1 mice exposed to environmentally relevant levels of benzene with or without CYP2E1 induction.

Spermatogenesis by Sisyphus: proliferating stem germ cells fail to repopulate the testis after 'irreversible' injury.

2,5-Hexanedione is the toxic metabolite resulting from oxidation of the commonly used solvents n-hexane and methyl n-butyl ketone. Exposure to 2,5-hexanedione or its precursors results in a slowly progressive peripheral polyneuropathy and testicular injury. The chemical basis of the injury involves reaction of 2,5-hexanedione with protein amines, such as the epsilon-amine of lysine, to form pyrroles which further react to form protein-protein crosslinks. The target cell of injury in the testis is the supportive cell in the seminiferous epithelium, the Sertoli cell. A major function of the Sertoli cell is to nurture the dependent germ cell population by secreting seminiferous tubule fluid. 2,5-Hexanedione-induced crosslinking of the microtubule subunit protein, tubulin, leads to altered Sertoli cell microtubule-dependent transport and deficient formation of seminiferous tubule fluid, compromising germ cell viability. In an established model of testicular injury, rats are exposed to 1% 2,5-hexanedione in the drinking water for a period of 3-5 weeks. Three weeks after initiating exposure, decreased seminiferous tubule fluid secretion initiates a wave of germ apoptosis which peaks during the 5th week. The germ cell content of the injured testis continues to decline after cessation of the exposure, reaching a nadir during the 12th week. From this time onward, the testis is severely atrophic with less than 1% of seminiferous tubules in a testicular cross section containing germ cells more advanced than spermatogonia. Interestingly, this persistent state of post-injury 'irreversible' atrophy in the rat is characterized by the presence of a proliferating stem germ cell population which produces differentiating spermatogonia which then die by apoptosis. Serial cross sections of bromodeoxyuridine-labeled testis were analyzed to determine the kinetics of stem germ cell proliferation. Approximately 40% of stem cells (identified as single cells in the seminiferous epithelium) were actively proliferating with a cell cycle time of 8-14 days. Analysis of the total germ cell population present and modeling using the known cell cycle times of differentiating spermatogonia indicated a block in differentiation at the level of type A3/A4 spermatogonia. Quantitation of the frequency of apoptosis indicated that all of the germ cells died prematurely by this mechanism. Leuprolide is a gonadotropin-releasing hormone agonist which produces a profound suppression of testosterone levels with chronic administration. When delivered as a series of 3 depot injections 24 days apart, leuprolide resulted in a partial reversal of the 2,5-hexanedione-induced persistent atrophy. The reinitiation of spermatogenesis follows a lowering of the intratesticular testosterone concentration, indicating that intratesticular testosterone is at least partially responsible for the persistent atrophy. The efficacy of leuprolide-induced reversal of the persistent atrophy decreases with time after injury, suggesting that atrophic seminiferous tubules are initially capable of recovery and then enter a state of irreversible injury. Injection of ethane dimethane sulfonate at the beginning of leuprolide treatment eliminated Leydig cells during therapy and ablated the recovery of spermatogenesis, indicating that a Leydig cell-associated paracrine factor is required to restart spermatogenesis. The rat, therefore, has multiple states of testicular germ cell proliferation: normal spermatogenesis and at least two forms of persistent atrophy (leuprolide reversible and leuprolide non-reversible). Partial reversal of the persistent atrophy can be achieved by lowering intratesticular testosterone. Ongoing experiments are designed to address the role of the Leydig cell in post-injury recovery, and to further characterize the molecular events contributing to the different states of persistent atrophy.

Continuously proliferative stem germ cells partially repopulate the aged, atrophic rat testis after gonadotropin-releasing hormone agonist therapy.

Aging in the male human is accompanied by testicular atrophy, although relatively little is known about the mechanisms underlying germ cell loss. Testicular atrophy in the aged Brown Norway rat, an animal model for studies of aging in the human, has been attributed to a loss of spermatogonial stem cells. However, examination of testicular cross-sections from 27-mo-old Brown Norway rats indicated that approximately 14% of type A spermatogonia were stem cells. Furthermore, using bromodeoxyuridine labeling, we found that approximately 47% of these stem cells were actively dividing, with a cell cycle time of approximately 12.6 days. Both serum and testicular interstitial fluid testosterone levels were depressed in the aged rat. Therapy with the GnRH agonist, leuprolide, which has been empirically shown to reverse testicular atrophy in other models of germ cell loss, also partially restored spermatogenesis in the aged Brown Norway rat. The extent of testicular atrophy varied considerably, not only within the control and leuprolide-treatment groups but also between the left and right testes of the same animals. No significant difference was found between the mean percentage of populated tubules in 31-mo-old control animals (16.2 +/- 28%, mean +/- SD) and 31-mo-old leuprolide-treated animals (20.9 +/- 19.8%), but categorical comparisons showed that significantly fewer leuprolide-treated animals and testes contained < or = 1% populated tubules, indicating that GnRH agonist therapy stimulates differentiation of type A spermatogonia. An increase in the ratio of soluble to membrane stem cell factor mRNA levels was present in aged rats and partially reversed following leuprolide therapy.

Role of Sertoli cells in injury-associated testicular germ cell apoptosis.

This review examines experimental models of Sertoli cell injury resulting in germ cell apoptosis. Since germ cells exist in an environment created by Sertoli cells, paracrine signaling between these intimately associated cells must regulate the process of germ cell death. Germ cell apoptosis may be signaled by a decrease in Sertoli cell pro-survival factors, an increase in Sertoli cell pro-apoptotic factors, or both. The different models of Sertoli cell injury indicate that spermatogenesis is susceptible to disruption, and that targeting critical Sertoli cell functions can lead to rapid and massive germ cell death.