Commuting the death sentence: how oocytes strive to survive

Continuous loss of oocytes throughout meiotic prophase in the normal mouse ovary

The number of germ cells reaches the maximum just prior to entry into meiosis, yet decreases dramatically by a few days after birth in the female mouse, rat, and human. Previous studies have reported a major loss at the pachytene stage of meiotic prophase during fetal development, leading to the hypothesis that chromosomal pairing abnormalities may be a signal for oocyte death. However, the identification as well as the quantification of germ cells in these studies have been questioned. A recent study using Mouse Vasa Homologue (MVH) as a germ cell marker reached a contradictory conclusion claiming that oocyte loss occurs in the mouse only after birth. In the present study, we established a new method to quantify murine germ cells by using Germ Cell Nuclear Antigen-1 (GCNA-1) as a germ cell marker. Comparison of GCNA-1 and MVH immunolabeling revealed that the two markers identify the same population of germ cells. However, nuclear labeling of GCNA-1 was better suited for counting germ cells in histological sections as well as for double labeling with the antibody against synaptonemal complex (SC) proteins in chromosome spreading preparations. The latter experiment demonstrated that the majority of GCNA-1-labeled cells entered and progressed through meiotic prophase during fetal development. The number of GCNA-1-positive cells in the ovary was estimated by counting the labeled cells retained in chromosome spreading preparations and also in histological sections by using the ratio estimation method. Both methods demonstrated a continuous decline in the number of GCNA-1-labeled cells during fetal development when the oocytes progress through meiotic prophase. These observations suggest that multiple causes are responsible for oocyte elimination.

Phenoxodiol--an isoflavone analog--induces apoptosis in chemoresistant ovarian cancer cells

Interference with the innate apoptotic activity is a hallmark of neoplastic transformation and tumor formation. In this study we characterize the cytotoxic effect of phenoxodiol, a synthetic anticancer drug analog of genestein, and demonstrate the mechanism of action by which phenoxodiol affects the components of the Fas apoptotic pathway on ovarian cancer cells. Primary ovarian cancer cells, isolated from ascitic fluids of ovarian cancer patients, resistant to conventional chemotherapy, undergo apoptosis following phenoxodiol treatment. This effect is dependent upon the activation of the caspase system, inhibiting XIAP, an inhibitor of apoptosis, and disrupting FLICE inhibitory protein (FLIP) expression through the Akt signal transduction pathway. We suggest that phenoxodiol is an efficient inducer of cell death in ovarian cancer cells and sensitizes the cancer cells to Fas-mediated apoptosis. We identified FLIP and XIAP signalling pathways as key factors regulating the survival of ovarian cancer cells. These findings demonstrate a novel nontoxic drug that controls FLIP/XIAP function and has the potential to eliminate tumor cells through Fas-mediated apoptosis.

Ovarian responses to undernutrition in pregnant ewes, USA

In most mammals oogonia proliferate by mitosis and begin meiotic development during fetal life. Previous studies indicated that there is a delay in the progression to the first stage of meiotic arrest in germ cells of female fetuses of undernourished ewes. We report that underfeeding (50% NRC requirement beginning on Day 28 of pregnancy) provokes an increase in oxidative base lesions within DNA of mid-gestational (Day 78) fetal oogonia; this condition was associated with up-regulation of the tumor suppressor/cell-cycle arrest modulator p53, antiapoptotic factor Bcl-2, and base-excision repair polymerase beta. Fetal ovarian weights and germ cell concentrations were not altered by nutrient deprivation. Ovaries of ewes on control diets (100% NRC) contained more tertiary follicles than their restricted counterparts; however, peripheral venous estradiol-17beta was not different between groups. There was no effect of treatment on p53 accumulation in maternal oocytes. Luteal structure-function was not perturbed by undernutrition. No fetal losses were attributed to the dietary restriction. It is proposed that DNA of interphase fetal oogonia is vulnerable to oxidative insults perpetrated by a nutritional stress to the dam, and that multiple/integrated adaptive molecular response mechanisms of cell-cycle inhibition (providing the time required for base repairs) and survival hence sustain the genomic integrity and population stability of the germline.

Physiological and pathological cell deaths in the reproductive organs

Apoptosis of testicular germ cells and oocytes and their supporting cells in the gonads occurs at physiological and normal conditions or after exposure to pathological stimuli. Cell-death regulators, including Bcl-2 family members, caspases, Fas and p53 are thought to be involved in these processes. This article reviews the details of the apoptotic machinery in the reproductive organs by describing briefly the abnormal phenotypes observed in transgenic and gene-ablated mice.

Bax, caspase-2, and caspase-3 are required for ovarian follicle loss caused by 4-vinylcyclohexene diepoxide exposure of female mice in vivo

The industrial chemical, 4-vinylcyclohexene diepoxide (VCD), kills oocytes within immature follicles in the ovaries of mice and rats and is considered a potential occupational health hazard. It has been reported that VCD-induced follicle loss occurs via a cell death process involving elevated expression of Bax, a proapoptotic Bcl-2 family member, and increased caspase-3-like activity. We have previously shown that oocytes lacking acid sphingomyelinase (ASMase; an enzyme that generates the proapoptotic stress sensor ceramide), the aromatic hydrocarbon receptor (Ahr), Bax, or caspase-2 are resistant to apoptosis induced by other chemical toxicants. Therefore, this study was designed to investigate the functional importance of ASMase, Ahr, Bax, and caspase-2 as well as the related executioner enzyme caspase-3 to VCD-induced ovotoxicity in mice using gene knockout technology. For each gene mutant mouse line, wild-type and homozygous-null female siblings derived from heterozygous matings were given once-daily ip injections of either vehicle (sesame oil) or VCD (80 mg/kg body weight) for 15 d (three or four mice per treatment group per genotype). Ovaries were collected 24 h after the final injection and analyzed for the total number of nonatretic primordial and primary follicles remaining per ovary. No differences in the extent of primordial or primary follicle destruction resulting from VCD exposure were observed in wild-type vs. ASMase- or Ahr-deficient mice. By contrast, the extent of VCD-induced primordial follicle depletion in Bax-deficient mice (45 +/- 11%) was significantly (P < 0.05) lower than that in wild-type females (85 +/- 2%). The extent of primary follicle loss in bax-null mice exposed to VCD (3 +/- 22%) was also significantly (P < 0.05) lower than that in their wild-type sisters (86 +/- 4%). In caspase-2-deficient mice, significantly (P < 0.05) fewer oocyte-containing primary follicles were destroyed by VCD (17 +/- 19%) vs. wild-type controls (71 +/- 6%); however, no significant difference in the extent of VCD-induced primordial follicle destruction was observed in caspase-2-null vs. wild-type females. Finally, in caspase-3-deficient mice, significantly (P < 0.05) fewer oocyte-containing primary follicles were destroyed by VCD (33 +/- 3%) vs. wild-type controls (71 +/- 2%); however, no significant difference in the extent of VCD-induced primordial follicle destruction was observed in caspase-3-null vs. wild-type females. We conclude that Bax, caspase-2, and caspase-3, but not ASMase or Ahr, are functionally important in VCD-induced follicle loss. However, as a loss of Bax, caspase-2, or caspase-3 function conveyed only partial protection from the ovotoxic effects of VCD, other cell death pathways that either function independently of Bax, caspase-2, and caspase-3 or are not apoptotic in nature are also involved.

Sphingolipids, apoptosis, cancer treatments and the ovary: investigating a crime against female fertility

Premature ovarian failure and infertility are well-known side-effects observed in young girls and reproductive-age women treated for cancer. Although the need for tumor eradication in these patients is clear, the long-term consequences of chemotherapy and radiation on non-target tissues, such as the ovaries where large numbers of germ cells (oocytes) are also killed off, are substantial. Unfortunately, the mechanism mediating the undesirable toxicity of cancer therapies in the female gonads has only recently been explored. Nevertheless, some important insights into the role of ceramide and sphingosine-1-phosphate (S1P) as a mediator and suppressor, respectively, of cancer therapy-induced oocyte apoptosis have emerged over the past few years. Such findings are exciting in that a better understanding of the crime--how radiation and chemotherapy kill off this irreplaceable population of innocent cells in the ovaries--may finally allow for the development of novel lipid-based strategies to combat infertility and premature menopause in female cancer patients.

Fertility after treatment for Hodgkin's disease

BACKGROUND

The investigational endeavors of ovarian cryopreservation await the clinical experience of auto- or xenotransplantation, in vitro maturation of thawed primordial follicles, their in vitro fertilization and embryo transfer. Although promising, this experience is not yet available. Moreover, the risk of possible reimplantation of malignant stem cells with the thawed cryoperserved ovary has been raised following experimental animal observations. Therefore, until these innovative endeavors prove successful, we have attempted to minimize the gonadotoxic effect of chemotherapy by the co-treatment with a gonadotropin-releasing hormone agonistic analog (GnRH-a) to induce a temporary prepubertal milieu. The immunoreactive inhibin-A and -B in these patients was measured before, during and following the gonadotoxic chemotherapy.

METHODS

A prospective clinical protocol was undertaken in 60 women aged 15-40 years with lymphoma, 10 with leukemia and 10 undergoing chemotherapeutic treatments for non-malignant diseases such as systemic lupus erythematosus or other autoimmune diseases. A monthly injection of depot D-TRP(6)-GnRH-a was administered from before starting the chemotherapy until its conclusion, up to a maximum of 6 months. Hormonal profile [follicle-stimulating hormone (FSH), luteinizing hormone (LH), E2, T, P4, insulin-like growth factor (IGF)-1, IGF-BP3 and prolactin) was taken before starting the GnRH-a/chemotherapy co-treatment, and monthly thereafter until resumtion of spontaneous ovulation. This group was compared with a control group of 60 women who have been treated with similar chemotherapy.

RESULTS

Whereas all but three (40, 36 and 34 year old) of the surviving patients within the GnRH-a/ chemotherapy co-treatment group resumed spontaneous ovulation and menses within 12 months, less than half of the patients in the 'control' group (chemotherapy without GnRH-a co-treatment) resumed ovarian function and regular cyclic activity (P <0.05). The remaining 55% experienced premature ovarian failure (POF). Temporarily increased FSH concentrations were experienced by about one-third of the patients resuming cyclic ovarian function, suggesting reversible ovarian damage in a larger proportion of women than those experiencing POF. Inhibin-A and -B decreased during the GnRH-a/ chemotherapy co-treatment but increased to normal levels in patients who resumed regular ovarian cyclicity, and/or spontaneously conceived, as compared with low levels in those who developed POF.

CONCLUSIONS

If these preliminary data are consisent in a larger group of patients, GnRH-a co-treatment should be considered in every woman of reproductive age receiving chemotherapy, in addition to assisted reproductive technologies and the investigation into ovarian cryopreservation for future in vitro maturation, autotransplantation or xenotransplantation.

Investigation of KIT gene mutations in women with 46,XX spontaneous premature ovarian failure

BACKGROUND: Spontaneous premature ovarian failure presents most commonly with secondary amenorrhea. Young women with the disorder are infertile and experience the symptoms and sequelae of estrogen deficiency. The mechanisms that give rise to spontaneous premature ovarian failure are largely unknown, but many reports suggest a genetic mechanism in some cases. The small family size associated with infertility makes genetic linkage analysis studies extremely difficult. Another approach that has proven successful has been to examine candidate genes based on known genetic phenotypes in other species. Studies in mice have demonstrated that c-kit, a transmembrane tyrosine kinase receptor, plays a critical role in gametogenesis. Here we test the hypothesis that human KIT mutations might be a cause of spontaneous premature ovarian failure. METHODS AND RESULTS: We examined 42 women with spontaneous premature ovarian failure and found partial X monosomy in two of them. In the remaining 40 women with known 46,XX spontaneous premature ovarian failure we evaluated the entire coding region of the KIT gene. We did this using polymerase chain reaction based single-stranded conformational polymorphism analysis and DNA sequencing. We did not identify a single mutation that would alter the amino acid sequence of the c-KIT protein in any of 40 patients (upper 95% confidence limit is 7.2%). We found one silent mutation at codon 798 and two intronic polymorphisms. CONCLUSION: Mutations in the coding regions of the KIT gene appear not to be a common cause of 46,XX spontaneous premature ovarian failure in North American women.

A time to kill: viral manipulation of the cell death program

Many viruses have as part of their arsenal the ability to modulate the apoptotic pathways of the host. It is counter-intuitive that such simple organisms would be efficient at regulating this the most crucial pathway within the host, given the relative complexity of the host cells. Yet, viruses have the potential to initiate or stay the onset of programmed cell death through the manipulation of a variety of key apoptotic proteins. It is the intention of this review to provide an overview of viral gene products that are able to promote or inhibit apoptotic death of the host cell and to discuss their mechanisms of action. It is not until recently that the depth at which viruses exploit the apoptotic pathways of their host has been seen. This understanding may provide a great opportunity for future therapeutic ventures.

Preservation of fertility and ovarian function and minimalization of chemotherapy associated gonadotoxicity and premature ovarian failure: the role of inhibin-A and -B as markers

BACKGROUND

Following the improved long term survival in young women with lymphoma and leukemia undergoing chemotherapy, the preservation of future fertility has been the focus of recent interest. The investigational endeavors of ovarian cryopreservation awaits the clinical experience of in-vitro maturation of thawed primordial follicles, their in-vitro fertilization and ET. Although promising, this experience is not available yet. Moreover, the risk of possible reimplantation of malignant stem cells with the thawed cryopreserved ovary has been raised, following experimental animal observations. Therefore, until these innovative endeavors prove successful, and in parallel to them we attempted to minimize the gonadotoxic effect of chemotherapy by the co-treatment with a GnRH agonistic analogue to induce a temporary prepubertal milieu. Whereas, inhibin-B concentrations in serum may reflect the ovarian granulosa cell compartment, inhibin-A reflects luteal function. The immunoreactive inhibin-A and -B in these patients, before, during, and following the gonadotoxic chemotherapy were measured.

METHODS

A prospective clinical protocol was undertaken in 55 women with lymphoma, aged 15-40 years, ten with leukemia and eight undergoing chemotherapeutic treatments for non malignant diseases such as systemic lupus erythematosus (SLE) or other autoimmune diseases. A monthly injection of depot D-TRP6-GnRH-a was administered from before starting the chemotherapy until its conclusion, up to a maximum of 6 months. Hormonal profile (FSH, LH, E2, T, P4, IGF-1, IGF-BP3, and PRL) was taken before starting the GnRH-a/chemotherapy co-treatment, and monthly thereafter, until resuming spontaneous ovulation. This group was compared with a control group of 55 women who have been treated with similar chemotherapy. Inhibin-A and -B immunoactivity was measured by an ELISA commercial kit (Serotec).

RESULTS

Whereas, all but three (40- and 36-year-old) of the surviving patients with GnRH-a/chemotherapy co-treatment group resumed spontaneous ovulation and menses within 12 months, less than half of the patients in the control group (chemotherapy without GnRH-a co-treatment) resumed ovarian function and regular cyclic activity (P<0.05). The remaining 56% experienced premature ovarian failure (POF). Temporary increased FSH concentrations were experienced by about a third of the patients resuming cyclic ovarian function, suggesting a reversible ovarian damage in a larger proportion of women than those experiencing POF. Inhibin-A and -B decreased during the GnRH-a/chemotherapy co-treatment but increased to normal levels in patients who resumed regular ovarian cyclicity, and/or spontaneously conceived, as compared with low levels in those who developed POF.

CONCLUSIONS

If these preliminary data are consistent in a larger group of patients, inhibin-A or -B concentrations may serve as prognostic factors for predicting the resumption of ovarian function, in addition to the levels of FSH, LH and E2. The GnRH-a co-treatment should be considered in every woman in the reproductive age receiving chemotherapy, in addition to ART, and to the investigational attempts of ovarian cryopreservation for future in-vitro maturation, autotransplantation, or xenotransplantation.

Avian TVB (DR5-like) death receptor expression in hen ovarian follicles

TVB is an avian death domain-containing receptor belonging to the TNF receptor family and is proposed to be the ortholog to mammalian DR5. Although TVB receptor activation has been demonstrated to mediate apoptosis in chick embryo fibroblasts, there is essentially no information regarding TVB expression or regulation in the mature hen ovary, and in particular within the follicle granulosa layer where apoptosis is known to promote atresia. Significantly, the TVB receptor represents the fourth death domain-containing receptor (also including Fas, TNF-R1, and DR6) found to be expressed within hen granulosa cells. Levels of TVB expression are higher in prehierarchal follicles actively undergoing atresia compared to healthy follicles. However, increased TVB expression does not precede follicle death induced in vitro. Furthermore, TVB expression within granulosa cells is highest during the final stages of follicle development when follicles are not normally susceptible to undergoing atresia. These results provide evidence that TVB receptor signaling in the ovary may function in a capacity other than solely to mediate granulosa cell death and follicle atresia.

Ligand activation of the aromatic hydrocarbon receptor transcription factor drives Bax-dependent apoptosis in developing fetal ovarian germ cells

We recently reported that a targeted disruption of the gene encoding the aromatic hydrocarbon receptor (AHR) in mice reduces fetal oocyte apoptosis, leading to a 2-fold increase in the number of primordial follicles endowed at birth. Although the identity of the natural ligand(s) for the AHR remains to be unequivocally established, these findings indicate that the level of AHR function is an important physiological determinant of how many oocytes will succumb to apoptosis during development of the fetal ovaries. Furthermore, the AHR is a well established receptor for polycyclic aromatic hydrocarbons (PAHs), a class of ubiquitous environmental chemicals known to cause the death of female germ cells in fetal life. Given the possibility that the AHR serves as a key mediator of fetal oocyte death under both physiological and pathological situations, this study was conducted to more fully examine the impact of PAH-AHR interaction on fetal ovarian germ cells. In addition, experiments were designed to begin identification of the mechanism(s) by which ligand activation of the AHR induces prenatal oocyte depletion after transplacental exposure of fetuses to PAHs in vivo. Embryonic d 13.5 murine fetal ovaries cultured in the presence of PAHs exhibited a high level of germ cell loss via apoptosis that was prevented by the selective AHR antagonist, alpha-napthoflavone (ANF). Immunohistochemical analysis revealed an accumulation of Bax protein in germ cells of fetal ovaries exposed to PAHs before the onset of apoptosis, whereas cotreatment with ANF inhibited the induction of Bax expression. The functional importance of increased Bax expression to the cytotoxic response was confirmed by findings that fetal ovarian germ cell loss caused by in utero exposure of wild-type female fetuses to PAHs was not observed in Bax-deficient female fetuses exposed in parallel. We conclude that a central role exists for the AHR in transducing the actions of PAHs in fetal ovarian germ cells, and that the proapoptotic Bcl-2 family member, Bax, is a required mediator of PAH-induced oocyte loss in female fetuses exposed to PAHs in utero.