Effect of neonatal exposure to diethylstilbestrol and tamoxifen on pelvis and femur in male mice

A review of tamoxifen administration regimen optimization for Cre/loxp system in mouse bone study

Gene knockout is a technique routinely used in basic experimental research, particularly in mouse skeletal and developmental studies. Tamoxifen-induced Cre/loxp system is known for its temporal and spatial precision and commonly utilized by researchers. However, tamoxifen has been shown its side effects on affecting the phenotype of mouse bone directly. This review aimed to optimize tamoxifen administration regimens including its dosage and duration, to identify an optimal induction strategy that minimizes potential side effects while maintaining recombination efficacy. This study will help researchers in designing gene knockout experiments in bone when using tamoxifen.

Tamoxifen Treatment in the Neonatal Period Affects Glucose Homeostasis in Adult Mice in a Sex-Dependent Manner

Tamoxifen is a selective estrogen receptor modulator used to activate the CREERT2 recombinase, allowing tissue-specific and temporal control of the somatic mutagenesis to generate transgenic mice. Studies integrating development and metabolism require a genetic modification induced by a neonatal tamoxifen administration. Here, we investigate the effects of a neonatal tamoxifen administration on energy homeostasis in adult male and female C57BL/6J mice. C57BL/6J male and female mouse pups received a single injection of tamoxifen 1 day after birth (NTT) and were fed a high-fat/high-sucrose diet at 6 weeks of age. We measured weight, body composition, glucose and insulin tolerance, basal metabolism, and tibia length and weight in adult mice. The neonatal tamoxifen administration exerted long-term, sex-dependent effects on energy homeostasis. NTT female mice became overweight and developed impaired glucose control in comparison to vehicle-treated littermates. NTT females exhibited 60% increased fat mass, increased food intake, decreased physical activity and energy expenditure, impaired glucose and insulin tolerance, and fasting hyperglycemia and hyperinsulinemia. In contrast, NTT male mice exhibited a modest amelioration of glucose and insulin tolerance and long-term decreased lean mass linked to decreased bone weight. These results suggest that the neonatal tamoxifen administration exerted a marked and sex-dependent influence on adult energy homeostasis and bone weight and must therefore be used with caution for the development of transgenic mouse models regarding studies on energy homeostasis and bone biology.

New frontiers of developmental endocrinology opened by researchers connecting irreversible effects of sex hormones on developing organs

In the early 1960's, at Professor Bern's laboratory, University of California, Berkeley) in the US, Takasugi discovered ovary-independent, persistent vaginal changes in mice exposed neonatally to estrogen, which resulted in vaginal cancer later in life. Reproductive abnormalities in rodents were reported as a result of perinatal exposure to various estrogenic chemicals. Ten years later, vaginal cancers were reported in young women exposed in utero to the synthetic estrogen diethylstilbestrol (DES) and this has been called the "DES syndrome". The developing organism is particularly sensitive to developmental exposure to estrogens inducing long-term changes in various organs including the reproductive organs. The molecular mechanism underlying the persistent vaginal changes induced by perinatal estrogen exposure was partly demonstrated. Persistent phosphorylation and sustained expression of EGF-like growth factors, lead to estrogen receptor α (ESR1) activation, and then persistent vaginal epithelial cell proliferation. Agents which are weakly estrogenic by postnatal criteria may have major developmental effects, especially during a critical perinatal period. The present review outlines various studies conducted by four generations of investigators all under the influence of Prof. Bern. The studies include reports of persistent changes induced by neonatal androgen exposure, analyses of estrogen responsive genes, factors determining epithelial differentiation in the Müllerian duct, ESR and growth factor signaling, and polyovular follicles in mammals. This review is then expanded to the studies on the effects of environmental estrogens on wildlife and endocrine disruption in Daphnids.

Bone Manifestation of Faulty Perinatal Hormonal Imprinting: A Review

Hormonal imprinting takes place at the first encounter between the developing receptor and its target hormone and the encounter determines the receptor's binding capacity for life. In the critical period of development, when the window for imprinting is open, the receptor can be misdirected by related hormones, synthetic hormones, and industrial or communal endocrine disruptors which cause faulty hormonal imprinting with life-long consequences. Considering these facts, the hormonal imprinting is a functional teratogen provoking alterations in the perinatal (early postnatal) period. One single encounter with a low dose of the imprinter in the critical developmental period is enough for the formation of faulty imprinting, which is manifested later, in adult age. This has been justified in the immune system, in sexuality, in animal behavior and brain neurotransmitters etc. by animal experiments and human observations. This review points to the faulty hormonal imprinting in the case of bones (skeleton), by single or repeated treatments. The imprinting is an epigenetic alteration which is inherited to the progeny generations. From clinical aspect, the faulty imprinting can have a role in the pathological development of the bones as well, as in the risk of osteoporotic fractures, etc.

Endocrine-disrupting chemicals, epigenetics, and skeletal system dysfunction: exploration of links using bisphenol A as a model system

Early life exposures to endocrine-disrupting chemicals (EDCs) have been associated with physiological changes of endocrine-sensitive tissues throughout postnatal life. Although hormones play a critical role in skeletal growth and maintenance, the effects of prenatal EDC exposure on adult bone health are not well understood. Moreover, studies assessing skeletal changes across multiple generations are limited. In this article, we present previously unpublished data demonstrating dose-, sex-, and generation-specific changes in bone morphology and function in adult mice developmentally exposed to the model estrogenic EDC bisphenol A (BPA) at doses of 10 μg (lower dose) or 10 mg per kg bw/d (upper dose) throughout gestation and lactation. We show that F1 generation adult males, but not females, developmentally exposed to bisphenol A exhibit dose-dependent reductions in outer bone size resulting in compromised bone stiffness and strength. These structural alterations and weaker bone phenotypes in the F1 generation did not persist in the F2 generation. Instead, F2 generation males exhibited greater bone strength. The underlying mechanisms driving the EDC-induced physiological changes remain to be determined. We discuss potential molecular changes that could contribute to the EDC-induced skeletal effects, with an emphasis on epigenetic dysregulation. Furthermore, we assess the necessity of intact sex steroid receptors to mediate these effects. Expanding future assessments of EDC-induced effects to the skeleton may provide much needed insight into one of the many health effects of these chemicals and aid in regulatory decision making regarding exposure of vulnerable populations to these chemicals.

Developmental exposure to xenoestrogens at low doses alters femur length and tensile strength in adult mice

Developmental exposure to high doses of the synthetic xenoestrogen diethylstilbestrol (DES) has been reported to alter femur length and strength in adult mice. However, it is not known if developmental exposure to low, environmentally relevant doses of xenoestrogens alters adult bone geometry and strength. In this study we investigated the effects of developmental exposure to low doses of DES, bisphenol A (BPA), or ethinyl estradiol (EE(2)) on bone geometry and torsional strength. C57BL/6 mice were exposed to DES, 0.1 μg/kg/day, BPA, 10 μg/kg/day, EE(2), 0.01, 0.1, or 1.0 μg/kg/day, or vehicle from Gestation Day 11 to Postnatal Day 12 via a mini-osmotic pump in the dam. Developmental Xenoestrogen exposure altered femoral geometry and strength, assessed in adulthood by micro-computed tomography and torsional strength analysis, respectively. Low-dose EE(2), DES, or BPA increased adult femur length. Exposure to the highest dose of EE(2) did not alter femur length, resulting in a nonmonotonic dose response. Exposure to EE(2) and DES but not BPA decreased tensile strength. The combined effect of increased femur length and decreased tensile strength resulted in a trend toward decreased torsional ultimate strength and energy to failure. Taken together, these results suggest that exposure to developmental exposure to environmentally relevant levels of xenoestrogens may negatively impact bone length and strength in adulthood.

Multigenerational exposure to genistein does not increase bone mineral density in rats

Genistein has been shown to prevent bone loss in ovariectomized adult rats. However, the effects of genistein on bone in developing and reproductively-intact rats have not been examined. A large multigenerational experiment involved feeding 0, 5, 100, or 500 ppm genistein in the diet to intact male and female rats from conception until either weaning, postnatal day 140, or continuously for 2 years. Vertebrae (lumbar and caudal) were collected from these animals at necropsy at 2 years of age and subjected to dual-energy x-ray absorptiometry (DXA) scanning to measure bone mineral density (BMD), bone mineral content (BMC), and bone area. Femurs were collected, and length, cross-sectional area, and cortical bone area were measured directly. Serum was collected for measurement of pyridinoline (PYD) and alkaline phosphatase (ALP). BMD was not affected by genistein in any phase of the experiment. In female rats treated continuously with genistein, BMC and bone area were reduced in the 500 ppm group compared to the 5 ppm group in the lumbar vertebrae, and in all treatment groups compared to control in the caudal vertebrae. In both males and females treated continuously, the cross-sectional area of the femur was reduced in rats treated with 500 ppm compared to those treated with 5 ppm. In female rats treated continuously, PYD was higher in the 100 and 500 ppm groups than in the 0 and 5 ppm groups. In conclusion, the effects of genistein on reproductively-intact rats were not dramatic. High dose of genistein throughout the lifespan resulted in decreased bone size, which may reduce the force required to break the bone.

Early estrogen exposure induces abnormal development of Fundulus heteroclitus

Many chemicals released into the environment exhibit estrogenic activity, having the potential to disrupt development and the functioning of the endocrine system. In order to establish a model system to study the effects of such environmental chemicals on aquatic animals, we examined the effects of a natural estrogen, 17 beta-estradiol (E(2)), on early development of Fundulus heteroclitus. Embryos of F. heteroclitus were reared in seawater containing 10(-10), 10(-8), and 10(-6) M E(2) throughout the experiment. Hatching and survival rates decreased in a dose-dependent manner, and fry treated with 10(-6) M E(2) and 10(-8) M E(2) were dead by two weeks and 12 weeks after hatching, respectively. More than 85% of fry treated with 10(-8) M E(2) showed malformations: i.e., eye extrusion, crooked vertebral column, faded lateral-stripe pattern eight weeks after hatching. Body weight and head and body lengths were significantly reduced in E(2)-treated fry when compared to controls. Ossification was not completed in vertebrae, cranial bones, and other bones in fry treated with 10(-8) M E(2) even 12 weeks after hatching. Sex ratio of control fry was 57% male and 43% female, whereas fry treated with 10(-8) M E(2) were 100% female eight weeks after hatching. The present results demonstrate that exogenous estrogen induced death of embryos and fry, malformations, sex reversal, and incomplete ossification of vertebrae and cranial bones, which would result in shorter body and head lengths and in malformed vertebrae leading to a hunchback condition.

Developmental toxicity of estrogenic chemicals on rodents and other species

Antenatal sex-hormone exposure induces lesions in mouse reproductive organs, which are similar to those in humans exposed in utero to a synthetic estrogen, diethylstilbestrol. The developing organisms including rodents, fish and amphibians are particularly sensitive to exposure to estrogenic chemicals during a critical window. Exposure to estrogens during the critical period induces long-term changes in reproductive as well as non-reproductive organs, including persistent molecular alterations. The antenatal mouse model can be utilized as an indicator of possible long-term consequences of exposure to exogenous estrogenic compounds including possible environmental endocrine disruptors. Many chemicals released into the environment potentially disrupt the endocrine system in wildlife and humans, some of which exhibit estrogenic activity by binding to the estrogen receptors. Estrogen responsive genes, therefore, need to be identified to understand the molecular basis of estrogenic actions. In order to understand molecular mechanisms of estrogenic chemicals on developing organisms, we are identifying estrogen responsive genes using cDNA microarray, quantitative RT-PCR, and differential display methods, and genes related to the estrogen-independent vaginal changes in mice induced by estrogens during the critical window. In this review, discussion of our own findings related to endocrine distuptor issue will be provided.

Transient estrogen exposure of female mice during early development permanently affects osteoclastogenesis in adulthood

Estrogens modulate bone tissue turnover in both experimental animal models and postmenopausal women. Our previous studies have shown that exposure to diethylstilbestrol (DES) during the perinatal period increases peak bone mass in female mice in adulthood. We investigated whether developmental DES exposure can influence bone mass by affecting osteoclastogenesis. Female mice were injected with 100 microg/kg body weight DES from days 9-16 of gestation or, alternatively, pups received neonatal injections of 2 microg of DES from days 1-5 of life. Animals were weaned at 21 days of age and effects of estrogen on bone cells were evaluated in adulthood. A significant increase in bone mass in female mice was already observed at 2 months, with a maximal effect in older animals. Bone sections from DES-treated animals showed a significant decrease in osteoclast number and tartrate-resistant acid phosphatase (TRAP) enzymatic activity as compared with controls. To verify the importance of the estrogen surge at puberty in this event, a group of control and DES-treated mice were ovariectomized at 17 days to prevent puberty, and potential effect on osteoclastic cells was evaluated in adulthood. As expected, ovariectomy induced an increase of TRAP-positive cells. DES treatment blunted the ovariectomized-dependent increase of the total number of osteoclastic cells, suggesting a role of developmental DES exposure in the process of bone-cell imprinting. Our data indicate, for the first time, that transient changes in estrogen levels during development modulate bone turnover and osteoclastogenesis likely participating in bone-cell imprinting during early phases of bone development, and that this effect could be induced by direct alteration of bone microenvironment.

Tamoxifen attenuates the effects of exogenous glucocorticoid on bone formation and growth in piglets

Tamoxifen (Tam) has been shown to inhibit dexamethasone (Dex)-mediated effects on bone formation in vitro. Our objective was to determine whether Tam would block Dex-induced osteopenia and growth inhibition in growing piglets. Four-day-old male Yorkshire piglets were adapted to a liquid formula diet (400 ml/kg x day) and randomized to one of four groups (n = 5/group): Dex (0.5 mg/kg x day), Tam (1 mg/kg x day), Dex plus Tam, or placebo control (vehicle only). Both drugs were administered by orogastric gavage twice daily for 12 days. At baseline and at the end of treatment, whole body bone mineral density (BMD) was determined by dual energy x-ray absorptiometry (Hologic QDR1000W). Plasma osteocalcin and PTH were measured on days 0 and 12, and urinary N-telopeptide was measured on day 12. Changes in axial length and daily weight were also measured. Delta whole body BMD was 29% lower (P < 0.05) in Dex alone treated piglets than in controls (0.033 vs. 0.047 g/cm2, respectively), whereas the maximum change in BMD in Dex plus Tam group (0.046 g/cm2) was similar to that in controls. Concurrent Tam administration reduced the Dex-induced deficit in weight gain by 56% (P < 0.05) and the deficit in axial length gain by 72% (P < 0.01). In Dex alone treated piglets, PTH was significantly elevated (7-fold), whereas osteocalcin and N-telopeptide were significantly reduced compared with control values. These effects were prevented by Tam. These data suggest that the suppression of growth and other changes in parameters of bone metabolism induced by glucocorticoids in vivo can be attenuated by Tam.