Mutagen effects on rat seminiferous tubules in vitro: induction of meiotic micronuclei by adriamycin

Assuring safety without animal testing: the case for the human testis in vitro

From 15 to 17 June 2011, a dedicated workshop was held on the subject of in vitro models for mammalian spermatogenesis and their applications in toxicological hazard and risk assessment. The workshop was sponsored by the Dutch ASAT initiative (Assuring Safety without Animal Testing), which aims at promoting innovative approaches toward toxicological hazard and risk assessment on the basis of human and in vitro data, and replacement of animal studies. Participants addressed the state of the art regarding human and animal evidence for compound mediated testicular toxicity, reviewed existing alternative assay models, and brainstormed about future approaches, specifically considering tissue engineering. The workshop recognized the specific complexity of testicular function exemplified by dedicated cell types with distinct functionalities, as well as different cell compartments in terms of microenvironment and extracellular matrix components. This complexity hampers quick results in the realm of alternative models. Nevertheless, progress has been achieved in recent years, and innovative approaches in tissue engineering may open new avenues for mimicking testicular function in vitro. Although feasible, significant investment is deemed essential to be able to bring new ideas into practice in the laboratory. For the advancement of in vitro testicular toxicity testing, one of the most sensitive end points in regulatory reproductive toxicity testing, such an investment is highly desirable.

Functional in vitro model to examine cancer therapy cytotoxicity in maturing rat testis

Childhood cancer treatment can lead to infertility. Organ culture of early postnatal testicular tissue might provide a valuable approach to the study of acute testicular toxicity. The aim of the present study was to develop a functional in vitro organ culture method, in order to identify sensitive target cells to doxorubicin-induced cytotoxicity in immature rat testis during germ cell migration prior initiation of the first wave of spermatogenesis. Testicular tissue fragments from 5-day-old Sprague-Dawley rats were cultured in the absence or presence of doxorubicin (40 and 100ng/ml) and morphology, apoptosis, proliferation and testosterone secretion was analyzed. Postnatal testicular development proceeded normally in control samples for 48h in vitro. In these untreated culture conditions germ and Sertoli cell numbers and germ cell migration were comparable to in vivo. Germ cells were the primary, most sensitive targets for in vitro-induced doxorubicin (100ng/ml) toxicity and their death was not associated with any morphological defects in the Sertoli cells. Organ culture which reduces the need of animal experimentation can be used to study the cytotoxic effects of doxorubicin on the immature testis.

Local regulation of spermatogenesis: a living cell approach

The normal function of the testis is dependent on stimulation by pituitary gonadotrophins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Targets for these hormones are Leydig cells in the interstitial tissue, and Sertoli cells in the seminiferous epithelium, respectively. The effect of LH on the seminiferous epithelium is mediated by testosterone produced by the Leydig cells. Therefore, the two main hormones that influence the function of the seminiferous epithelium directly are FSH and testosterone. The preferential action of FSH in the adult seminiferous epithelium is associated with stages that involve meiotic divisions and early spermiogenesis. The parameters related to androgen action predominate at different stages during which the final maturation of the spermatids, spermiation and the onset of meiosis take place. The stage-dependent variation of the hormone responses in the seminiferous epithelium indicates the presence of local paracrine regulation and cell interaction mechanisms in the seminiferous epithelium, which are dependent on the spermatogenic cells associated with the Sertoli cells. Several growth factors have been suggested as mediators of this interaction. Owing to its highly complex structure, the seminiferous epithelium has been a difficult area for biochemical studies. New in vitro techniques have made these studies possible, and particular advances have been made using recombinant DNA techniques and transgene technology.

Differences in the number of micronucleated erythrocytes among young and adult animals including humans. Spontaneous micronuclei in 43 species

In our previous report we speculated about the possibility that some species had high levels of spontaneous micronucleated erythrocytes (MNE) just in a juvenile stage, this is, that the MNE diminish as the reticuloendothelial system matures. Here we show this effect in species including rat, rabbit, pig, dog, cat, gray squirrel, lion, giraffe, white-tailed deer, opossum and even human. The number of spontaneous MNE that we found in 43 species is shown, and the proportions of polychromatic and normochromatic. This is our third report on spontaneous MNE in different species. We obtained 189 peripheral blood samples of mammals, birds and reptiles. From 12 species we obtained only one sample, and 16 were reported previously, but now the size of the sample has been increased. The species with the highest spontaneous MNE were the Vietnamese potbelly pig (with the highest MNE number), Bengal tiger, capuchin monkey, puma, ferret, owl, hedgehog, squirrel monkey, pig and white-tailed deer. These species could be used as monitors for genotoxic events.

Apoptotic response of spermatogenic cells to the germ cell mutagens etoposide, adriamycin, and diepoxybutane

In testis, apoptosis is a way to eliminate damaged germ cells during their development. In this study, we evaluated the ability of three germ cell mutagens to induce apoptosis (or programmed cell death) at specific stages of rat seminiferous epithelial cycle. These chemicals include the cancer chemotherapy drugs etoposide and adriamycin and the butadiene metabolite diepoxybutane. According to our results, etoposide is a very potent inducer of apoptosis in male rat germ cells and the cell types most sensitive to it include all types of spermatogonia, zygotene, and early pachytene spermatocytes and meiotically dividing spermatocytes. Also, adriamycin causes an increase in apoptosis at specific stages of seminiferous epithelial cycle and the most sensitive cell types are type A3-4 spermatogonia, preleptotene, zygotene, and early pachytene spermatocytes. Diepoxybutane does not cause any significant increase in the frequency of apoptosis in rat testis. In addition, we studied whether p53 is taking part in the apoptotic response of spermatogenic cells by studying the levels of p53 protein in testis before and after chemical treatment. No accumulation of p53 in testis was seen after treatment with these three chemicals. The expression of two p53-regulated genes, p21WAF1 and mdm2, was also studied but no increase in the levels of mRNA of these genes was observed after treatment. The results indicate that apoptosis should be taken into consideration when the genotoxic effects of chemicals are evaluated in germ cells.

Genetic toxicities of human teratogens

Birth defects cause a myriad of societal problems and place tremendous anguish on the affected individual and his or her family. Current estimates categorize about 3% of all newborn infants as having some form of birth defect or congenital anomaly. As more precise means of detecting subtle anomalies become available this estimate, no doubt, will increase. Even though birth defects have been observed in newborns throughout history, our knowledge about the causes and mechanisms through which these defects are manifested is limited. For example, it has been estimated that around 20% of all birth defects are due to gene mutations, 5-10% to chromosomal abnormalities, and another 5-10% to exposure to a known teratogenic agent or maternal factor [D.A. Beckman, R.L. Brent, Mechanisms of teratogenesis. Ann. Rev. Pharmacol. Toxicol. 24 (1984) 483-500; K. Nelson, L.B. Holmes Malformations due to presumed spontaneous mutations in newborn infants, N. Engl. J. Med. 320 (1989) 19-23.]. Together, these percentages account for only 30-40%, leaving the etiology of more than half of all human birth defects unexplained. It has been speculated that environmental factors account for no more than one-tenth of all congenital anomalies [D.A. Beckman, R.L. Brent, Mechanisms of teratogenesis, Ann. Rev. Pharmacol. Toxicol. 24 (1984) 483-500]. Furthermore, since there is no evidence in humans that the exposure of an individual to any mutagen measurably increases the risk of congenital anomalies in his or her offspring' [J.F. Crow, C. Denniston, Mutation in human populations, Adv. Human Genet. 14 (1985) 59-121; J.M. Friedman, J.E. Polifka, Teratogenic Effects of Drugs: A Resource for Clinicians (TERIS). The John Hopkins University Press, Baltimore, 1994], the mutagenic activity of environmental agents and drugs as a factor in teratogenesis has been given very little attention. Epigenetic activity has also been given only limited consideration as a mechanism for teratogenesis. As new molecular methods are developed for assessing processes associated with teratogenesis, especially those with a genetic or an epigenetic basis, additional environmental factors may be identified. These are especially important because they are potentially preventable. This paper examines the relationships between chemicals identified as human teratogens (agents that cause birth defects) and their mutagenic activity as evaluated in one or more of the established short-term bioassays currently used to measure such damage. Those agents lacking mutagenic activity but with published evidence that they may otherwise alter the expressions or regulate interactions of the genetic material, i.e. exhibit epigenetic activity, have likewise been identified. The information used in making these comparisons comes from the published literature as well as from unpublished data of the U.S. National Toxicology Program (NTP).

Narrowing the embryologic window of the adriamycin-induced fetal rat model of esophageal atresia and tracheoesophageal fistula

We recently reported on a new fetal rat model of esophageal atresia (EA) with tracheoesophageal fistula (TEF) induced by prenatal exposure to adriamycin (1.75 mg/kg i. p. injected daily to the pregnant dam from the 6th to 9th gestational days). With this treatment regime, many fetuses were resorbed and the number of associated malformations was very high. The present study demonstrates that similar doses of the drug administered only on the 8th and 9th gestational days allow higher fetal survival (9.7 ∓ 3.9 vs. 6.8 ∓ 4.7 fetuses per litter, P < 0.01) with a similar incidence of EA-TEF (41.2% vs. 56.4%, n. s.) and decreased occurence of associated anorectal and genitourinary malformations. Since this model is an instrument for further investigation of the disturbed cellular and morphogenetic events leading to EA and TEF, the narrowing of the embryologic window obtained by the present study will allow better focusing of the research on the critical period of time involved.

Effects of vinblastine and colchicine on male rat meiosis in vivo: disturbances in spindle dynamics causing micronuclei and metaphase arrest

We have studied the effects of vinblastine sulfate (VBL) and colchicine (COL) on male rat in vivo and in vitro meiosis. A novel methodology based on isolating a segment of seminiferous tubules containing meiotically dividing spermatocytes was applied. During meiotic divisions at stage XIV of rat spermatogenesis, both chemicals induced only low frequencies of micronuclei (MN), 0.8-3.2 MN/1,000 spermatids. Fluorescence in situ hybridization experiments in mice with the mouse centromere-specific gamma-satellite DNA probe showed that 50.7% of VBL-induced MN and 56.6% of COL-induced MN were centromere positive, indicating that the MN induced by both chemicals contained detached chromosomes. The inhibition of cell proliferation was determined by counting the number of cells arrested at metaphase during the first meiotic (MI) or the second meiotic (MII) division. VBL was found to be a potent inducer of cell death while COL was not. The direct effects of VBL and COL on the meiotic spindles were evaluated using immunohistochemistry with anti-alpha-tubulin and confocal microscopy. In the control animals a significant difference was observed between the mean length of metaphase spindles of MI and MII. Both were dramatically decreased 6 hr after treatment with 2.0 mg/kg of VBL and 0.8 mg/kg of COL, respectively. At 18 hr after COL injection the spindles had about the same length as in the controls. However, the VBL-induced shortening was even more evident at 18 hr for both MI and MII. The possible reasons for observed differences between the two chemicals and between meiosis and mitosis are discussed.

The spermatid micronucleus test with the dissection technique detects the germ cell mutagenicity of acrylamide in rat meiotic cells

As a part of the development and validation of the spermatid micronucleus test (SMNT) in the project 'Detection of Germ Cell Mutagens' sponsored by the CEC we studied the mutagenicity of acrylamide (AA) and mitomycin C (MMC). Of two alternative techniques, we used the 'dissection technique' based on microdissection of seminiferous tubules offering a narrow window for evaluation of cell stage sensitivity, and including DNA-specific staining and scoring. AA given as a single injection of 50 or 100 mg/kg did not significantly increase MN frequencies. When a subchronic treatment (4 x 50 mg/kg) was given, a significant increase over background was observed 18 and 19 days after the last injection, indicating genotoxic activity in preleptotene spermatocytes and late spermatogonial stages. MMC given as single injections of 0.5 or 1.0 mg/kg increased MN frequencies significantly 17, 18, 19 and 20 days after treatment as a result of clastogenicity in S phase cells. DNA flow cytometry did not show cytotoxicity of AA to preleptotene spermatocytes, but a small decrease in the numbers of stem cells. If spindle disturbances are caused by AA, as suggested, they were not detectable by induction of spermatid MN in vivo 1 or 3 days after treatment or by treatment with AA of cultured segments of seminiferous tubules undergoing meiotic divisions in vitro. In conclusion, the SMNT with the dissection technique is able to show the germ cell clastogenicity of AA and MMC. AA was observed to have a much weaker MN inducing potency than MMC.

Germ-cell mutagenicity of etoposide: induction of meiotic micronuclei in cultured rat seminiferous tubules

Mutagen effects on male germ cells can be quantified by meiotic micronucleus induction in vitro. Late pachytene and diakinetic spermatocytes are able to differentiate through meiotic divisions in vitro and develop to round spermatids. In the presence of mutagens micronucleus induction reflects the potential of the chemical to induce chromosome breakage or uneven chromosome distribution. In this study we have investigated the mutagenicity of etoposide (VP-16) and its ability to induce micronuclei S-independently in meiosis by the meiotic micronucleus method in vitro. Our results indicate that etoposide is able to cause a statistically significant increase in the frequency of micronuclei at a concentration range as low as 0.5-8 mu mole/l. The meiotic micronucleus method in vitro seems to be a feasible and sensitive test system of male germ-cell mutagenesis.

Etoposide (VP-16) is a potent inducer of micronuclei in male rat meiosis: spermatid micronucleus test and DNA flow cytometry after etoposide treatment

The genotoxic and cytotoxic effects of etoposide (VP-16), a topoisomerase II inhibitor, on male rat spermatogenic cells were studied by analysing induction of micronuclei during meiosis. Micronuclei (MN) were scored in early spermatids after different time intervals corresponding to exposure of different stages of meiotic prophase. Etoposide had a strong effect on diplotene-diakinesis I cells harvested 1 day after exposure, and a significant effect also on late pachytene cells harvested 3 days after exposure. The effect at 18 days corresponding to exposure of preleptotene stage of meiosis (S-phase) was weaker but also statistically significant. Adriamycin was used as a positive control in this study. The results indicate a different mechanism of action of etoposide compared with adriamycin and other chemicals studied previously with the spermatid micronucleus test. DNA flow cytometry was carried out to assess cytotoxic damage at the same time intervals (1, 3, and 18 days after treatment) at stages I and VII of the seminiferous epithelial cycle allowing a study of cytotoxicity to different spermatogenic cell stages. Damage of differentiating spermatogonia was observed by a decrease in the cell numbers of the 2C peak 1 and 3 days after treatment and by a reduction of the number of 4C cells (primary spermatocytes) 18 d after etoposide treatment. Adriamycin also killed differentiating spermatogonia. Since the cell population which showed a high induction of MN by etoposide was not reduced in number, the genotoxic effect is remarkable. We conclude that etoposide is a potent inducer of genotoxicity and patients treated with this agent during cancer chemotherapy are at a risk of genetic damage.

Testicular toxicity and mutagenicity of steroidal and non-steroidal estrogens in the male mouse

The mutagenicity and toxicity of diethylstilbestrol (DES), 17 beta-estradiol and zeranol on the male mouse germ cells were investigated with meiotic micronucleus assays in vivo and in vitro, sperm-head abnormality test and morphometry. Further, the developmental effects of DES on testicular morphology were explored. Micronucleus induction was observed at 10(-7) M concentration of DES and 17 beta-estradiol in vitro, but other treatments yielded negative results. The micronucleus assay in vivo revealed a small number of micronuclei in early haploid spermatids 17 days after a single subcutaneous injection of DES 50 mg/kg, whereas estradiol and zeranol gave negative results. The sperm-head abnormality rates were significantly elevated 5 weeks after treatments with high doses of DES, 17 beta-estradiol and zeranol, and testicular morphometry revealed transient changes in the volume densities of testicular tissue components. Prenatal and neonatal estrogen administration resulted in permanent alterations in seminiferous epithelium and dilatation of the rete testis, but did not affect micronucleus or sperm-head abnormality rates. The mutagenicity and toxicity of hormones in the mouse testis paralleled the hormonal activity of these compounds. Early estrogenization was the most sensitive toxicity test, followed by in vitro meiotic micronucleus induction, whereas the sperm-head abnormality assay and morphological analysis did not reveal subtle changes.

Short-term morphological response of the rat testis to administration of five chemotherapeutic agents

As cancer survival rates improve, there is increasing concern about the adverse effects of chemotherapeutic agents on male fertility. Five chemotherapeutic agents (amethopterin, AP or methotrexate; doxorubicin, DX; cytoxan or cyclophosphamide, CP; cisplatinum, CDDP; and 5-fluorouracil, 5-FU) which belong to three different categories of chemotherapeutic agents (antimetabolite, antibiotic, alkylating agent, alkylating agent, antimetabolite, respectively) were given systemically to adult rats to determine the short-term morphological patterns of response in the testis, and the testes were examined by light microscopy. Morphological patterns of response were found to be highly characteristic for each agent, and some shared morphological responses were evident. All except one chemotherapeutic agent (5-FU) caused spermatogonial damage. Among the defects seen were probable degenerating meiotic spermatocytes (CDDP), presence of micronuclei (DX), "arrested" spermatid development (5-FU), and abnormally shaped step 15 spermatids (5-FU). Damage that could be due to the effect of an agent on the Sertoli cell was failure of sperm release (5-FU, CDDP, DX, and AP), increase in the Sertoli cell lipid (5-FU), and malorientation of step 8 spermatids (5-FU, DX). The varied patterns of damage observed are a possible explanation of why the reproductive recovery potential in cancer patients undergoing chemotherapy is variable and drug-specific.

Inhibition of meiotic divisions of rat spermatocytes in vitro by polycyclic aromatic hydrocarbons

The toxic effects of polycyclic aromatic hydrocarbons (PAH) on spermatogenic cells undergoing meiotic division were investigated in vitro. Toxicity was assayed as alterations in cell nucleus morphology and cell survival and by DNA flow cytometry. Benzo[a]pyrene (BP) and 7,12-dimethylbenz[a]anthracene (DMBA) inhibited the progression of spermatocytes through meiotic division and were highly cytotoxic at concentrations higher than 1 microM. These results were obtained upon addition of a drug-metabolizing system, indicating that the seminiferous tubules lack the enzymes required for the initiation of PAH metabolism. The spindle poisons, e.g., vincristine and Colcemid, a group of direct-acting agents, affected spermatogenesis during meiotic division in a manner similar to that observed with PAH. In contrast, adriamycin did not inhibit meiotic division, although it did induce the formation of meiotic micronuclei as a result of chromosome breakage. It is concluded that low concentrations, i.e., 0.1 microM of PAH, strongly inhibit meiotic division, presumably after metabolic activation to reactive molecules functionally resembling direct-acting alkylating agents. High concentrations of PAH are cytotoxic.

Mutagenicity of gossypol analyzed by induction of meiotic micronuclei in vitro

Chromosome breakage caused by mutagens in male germ cells can be analyzed by micronucleus induction during meiotic division. This can be followed in vitro by culturing seminiferous tubular segments from stages of the epithelial cycle that contain late pachytene and diakinetic primary spermatocytes. We studied the mutagenic potential of a male contraceptive, gossypol, in this test system using adriamycin (10 ng/ml) as a reference mutagen. A small but significant increase in the frequency of micronuclei was induced with concentrations of 10 and 20 micrograms/ml of gossypol, while cytotoxic effects appeared at concentration of 20 micrograms/ml and were evident at 50 micrograms/ml. Analysis of meiotic micronucleus induction in vitro seems to be a sensitive test system of male germ-cell mutagenesis, but further studies on the possible mutagenic effects of gossypol are needed.

In vitro analysis of germ cell genotoxicity in testis explant cultures: spermatid micronucleus assays

Explant cultures of testes from the frog Xenopus laevis have been employed to evaluate the application of testis culture to the routine screening of potential germ cell genotoxicants. Testis explants were incubated with varied concentrations of 3 model mutagens (9,10-dimethyl-1,2-benzanthracene, cyclophosphamide, adriamycin) and solvent controls. Round spermatids were isolated from testes cultured 2-30 days after exposure to each mutagen. The spermatids were then stained with Hoechst 33258 and spermatid micronuclei were scored with a fluorescence microscope. Acute exposure of testes to each mutagen resulted in a dose-dependent increase in spermatid micronuclei that was stage specific and proportional to the length of the exposure period. The assay sensitively detected clastogenic effects by 10(-7) M adriamycin (4-h exposure period) and 10(-6) M cyclophosphamide and dimethylbenzanthracene (24-h exposure). The results demonstrate the feasibility of in vitro approaches to the routine screening and investigation of genotoxicity in the premeiotic S through meiotic division stages of vertebrate spermatogenesis.