Effects of delaying puberty on bone mineralization in female rats

The Utility of Preclinical Models in Understanding the Bone Health of Transgender Individuals Undergoing Gender-Affirming Hormone Therapy

PURPOSE OF REVIEW

To summarise the evidence regarding the effects of gender-affirming hormone therapy (GAHT) on bone health in transgender people, to identify key knowledge gaps and how these gaps can be addressed using preclinical rodent models.

RECENT FINDINGS

Sex hormones play a critical role in bone physiology, yet there is a paucity of research regarding the effects of GAHT on bone microstructure and fracture risk in transgender individuals. The controlled clinical studies required to yield fracture data are unethical to conduct making clinically translatable preclinical research of the utmost importance. Novel genetic and surgical preclinical models have yielded significant mechanistic insight into the roles of sex steroids on skeletal integrity. Preclinical models of GAHT have the potential inform clinical approaches to preserve skeletal integrity and prevent fractures in transgender people undergoing GAHT. This review highlights the key considerations required to ensure the information gained from preclinical models of GAHT are informative.

Evaluation of long-term effects of the gonadotrophin-releasing-hormone antagonist acyline on domestic-cat growth

Acyline contraception has been described in cats, but few data are available on the drug's long-term effect on growth. The relevant data cover until puberty with no radiographic description. We investigated the radiographic parameters throughout bone growth in order to more completely determine the drug's safety. Thirteen male and 12 female cats were studied, with the kittens being randomly assigned to one of the following groups within the first 24 h of birth: ACY, subcutaneous acyline, 33 µg/100 g, which injection was repeated weekly until age 3 months; or CO, untreated control. Body measurements were recorded weekly and radiographic parameters obtained from monthly radiographs of the antebrachium. In the ACY and CO male and female kittens, the body weight, withers height, and body length plus the age at the end of body growth and radial growth remained similar throughout the study (p>0.05). Both female groups finished radial growth before the males (p<0.05). The ACY females evidenced a longer radial length between the 8th and 28th weeks (p<0.05). All groups closed their proximal and distal physes within the normal ranges described for the species. The bone-cortex width was lower in the ACY vs. the CO animals at weeks 52 and 60 in the males and at weeks 24, 48, 52, and 56 in the females (p <0.05) The transient greater radial length and lower bone-cortex thickness observed in the treated cats were compensated for at the end of growth with no adverse clinical effects being observed. In conclusion, acyline as a contraceptive did not evidence a permanent or severe effect on domestic-cat growth.

Acute hypothalamic suppression significantly affects trabecular bone but not cortical bone following recovery and ovariectomy surgery in a rat model

Background. Osteoporosis is “a pediatric disease with geriatric consequences.” Bone morphology and tissue quality co-adapt during ontogeny for sufficient bone stiffness. Altered bone morphology from hypothalamic amenorrhea, a risk factor for low bone mass in women, may affect bone strength later in life. Our purpose was to determine if altered morphology following hypothalamic suppression during development affects cortical bone strength and trabecular bone volume (BV/TV) at maturity.

Methods. Female rats (25 days old) were assigned to a control (C) group (n = 45) that received saline injections (.2 cc) or an experimental group (GnRH-a) (n = 45) that received gonadotropin releasing hormone antagonist injections (.24 mg per dose) for 25 days. Fifteen animals from each group were sacrificed immediately after the injection protocol at Day 50 (C, GnRH-a). The remaining animals recovered for 135 days and a subset of each group was sacrificed at Day 185 ((C-R) (n = 15) and (G-R) (n = 15)). The remaining animals had an ovariectomy surgery (OVX) at 185 days of age and were sacrificed 40 days later (C-OVX) (n = 15) and (G-OVX) (n = 15). After sacrifice femurs were mechanically tested and scanned using micro CT. Serum C-terminal telopeptides (CTX) and insulin-like growth factor 1 (IGF-1) were measured. Two-way ANOVA (2 groups (GnRH-a and Control) X 3 time points (Injection Protocol, Recovery, post-OVX)) was computed.

Results. GnRH-a injections suppressed uterine weights (72%) and increased CTX levels by 59%. Bone stiffness was greater in the GnRH-a groups compared to C. Ash content and cortical bone area were similar between groups at all time points. Polar moment of inertia, a measure of bone architecture, was 15% larger in the GnRH-a group and remained larger than C (19%) following recovery. Both the polar moment of inertia and cortical area increased linearly with the increases in body weight. Following the injection protocol, trabecular BV/TV was 31% lower in the GnRH-a group compared to C, a similar deficit in BV/TV was also measured following recovery and post-OVX. The trabecular number and thickness were lower in the GnRH-a group compared to control.

Conclusion. These data suggest that following a transient delay in pubertal onset, trabecular bone volume was significantly lower and no restoration of bone volume occurred following recovery or post-OVX surgery. However, cortical bone strength was maintained through architectural adaptations in the cortical bone envelope. An increase in the polar moment of inertia offset increased bone resorption. The current data are the first to suppress trabecular bone during growth, and then add an OVX protocol at maturity. Trabecular bone and cortical bone differed in their response to hypothalamic suppression during development; trabecular bone was more sensitive to the negative effects of hypothalamic suppression.

Different effects on bone strength and cell differentiation in pre pubertal caloric restriction versus hypothalamic suppression

Hypothalamic amenorrhea and energy restriction during puberty affect peak bone mass accrual. One hypothesis suggests energy restriction alters hypothalamic function resulting in suppressed estradiol levels leading to bone loss. However, both positive and negative results have been reported regarding energy restriction and bone strength. Therefore, the purpose of this study was to investigate energy restriction and hypothalamic suppression during pubertal onset on bone mechanical strength and the osteogenic capacity of bone marrow-derived cells in two models: female rats treated with gonadotropin releasing hormone antagonists (GnRH-a) or 30% energy restriction. At 23 days of age, female Sprague Dawley rats were assigned to three groups: control group (C, n=10), GnRH-a group (n=10), and Energy Restriction (ER, n=12) group. GnRH-a animals received daily injections for 27 days. The animals in the ER group received 70% of the control animals' intake. After sacrifice (50 days of age), body weight, uterine and muscle weights were measured. Bone marrow-derived stromal cells were cultured and assayed for proliferation and differentiation into osteoblasts. Outcome measures included bone strength, bone histomorphometry and architecture, serum IGF-1 and osteocalcin. GnRH-a suppressed uterine weight, decreased osteoblast proliferation, bone strength, trabecular bone volume and architecture compared to control. Elevated serum IGF-1 and osteocalcin levels and body weight were found. The ER model had an increase in osteoblast proliferation compared to the GnRH-a group, similar bone strength relative to body weight and increased trabecular bone volume in the lumbar spine compared to control. The ER animals were smaller but had developed bone strength sufficient for their size. In contrast, suppressed estradiol via hypothalamic suppression resulted in bone strength deficits and trabecular bone volume loss. In summary, our results support the hypothesis that during periods of nutritional stress the increased vertebral bone volume may be an adaptive mechanism to store mineral which differs from suppressed estradiol resulting from hypothalamic suppression.

Effect of maturational timing on bone mineral content accrual from childhood to adulthood: evidence from 15 years of longitudinal data

A higher bone mass may reduce the risk of osteoporosis and fractures. The role of maturational timing for optimizing bone mass is controversial due to the lack of prospective evidence from childhood to adulthood. The purpose of this study was to examine the long term relationship between the onset of maturation and bone mineral content (BMC) development. Two hundred thirty individuals (109 males and 121 females) from the Saskatchewan Pediatric Bone Mineral Accrual Study (PBMAS) were classified into maturity groups based on age of peak height velocity. BMC was serially assessed using dual energy X-ray absorptiometry (DXA). Multilevel models were constructed to examine the independent development of BMC by maturity group. When age, body size, and body composition were controlled early maturing females had on average 3-4%, 62.2 ± 16.8g (p<0.05), more total body BMC than their average maturing peers by 20 years of age. In contrast, late maturing females had 50.7 ± 15.6g less total body BMC. No maturational effects were found at either the lumbar spine or femoral neck (p>0.05) in females. There were no significant differences in BMC development at any site among male maturational groups (p>0.05). In this group of healthy participants, there appears to be a sex-dependent effect on the relationship between maturational timing and total body BMC development. Early, average and late maturing males displayed similar BMC development. Late maturing females had compromised BMC accrual compared to their early and average maturing peers.

Delayed pubertal development by hypothalamic suppression causes an increase in periosteal modeling but a reduction in bone strength in growing female rats

The timing of the pubertal growth is a critical event in skeletal development. A delay in the onset of puberty has been correlated with increased stress fracture incidence in young women and as a result, suboptimal skeletal development may affect long-term bone strength. Gonadotropin releasing hormone antagonist (GnRH-a) injections were used to delay the onset of puberty in growing female rats. 23-day-old female rats were injected with a GnRH-antagonist at 2 dosage levels (n=15/group). The Low Dose group (1.25 mg/kg/dose) received daily injections for 27 days (sacrifice 49 days). The High Dose group received (5.0 mg/kg/dose) only 5 days per week over a 26 day period (sacrifice 48 days). Calcein injections measured bone formation activity on the periosteal and endocortical surfaces. Standard histomorphometric and biomechanical analyses were performed on the femora and ash content was measured on the tibiae of all animals. Serum estradiol and insulin-like growth factor (IGF)-1 levels were assayed. Significant delays in pubertal development occurred in the two GnRH-a groups as evidenced by delayed vaginal openings, decreased uterine and ovarian weights and suppressed estradiol levels compared to control. Femoral lengths were significantly shorter in the experimental groups and serum IGF-1 levels were higher than control. Bone strength and stiffness were significantly lower in the GnRH-a groups. Cortical bone area was decreased and total area was not different between groups. There was a significant decrease in % Ct.Ar/T.Ar. The decreased bone strength may have resulted from a decrease in the amount and distribution of bone, however, stress and Young's modulus were also decreased. There was a different response between endocortical formation indices and periosteal formation indices to the GnRH-a protocol. Endocortical bone formation rates decreased and there was an increase in periosteal labeled surface. A dose response between bone strength and GnRH-a dosage was found. The data suggest that hypothalamic suppression during pubertal development resulted in decreased bone strength which may result in fracture development.

The effect of a short-term delay of puberty on trabecular bone mass and structure in female rats: a texture-based and histomorphometric analysis

Accrual of bone mass and strength during development is imperative in order to reduce the risk of fracture later in life. Although delayed pubertal onset is associated with an increased incidence of stress fracture, evidence supports the concept of "catch up" growth. It remains unclear if deficits in bone mass associated with delayed puberty have long-term effects on trabecular bone structure and strength. The purpose of this study was to use texture-based analysis and histomorphometry to investigate the effect of a delay in puberty on trabecular bone mass and structure immediately post-puberty and at maturity in female rats. Forty-eight female Sprague-Dawley rats (25 days) were randomly assigned to one of four groups; (1) short-term control (C-ST), (2) long-term control (C-LT), (3) short-term GnRH antagonist (G-ST) and (4) long-term GnRH antagonist (G-LT). Injections of either saline or gonadotropin-releasing hormone antagonist (GnRH-a) (100 microg/day) (Cetrotide, Serono, Inc.) were given intraperitoneally for 18 days (day 25-42) to both ST and LT. The ST groups were sacrificed after the last injection (day 43) and the LT groups at 6 months of age. Pubertal and gonadal development was retarded by the GnRA antagonist injections as indicated by a delay in vaginal opening, lower ovarian and uterine weights and suppressed estradiol levels in the short-term experimental animals (G-ST). Delayed puberty caused a transient reduction in trabecular bone area as assessed by histomorphometry. Specifically, the significant deficit in bone area resulted from a decreased trabecula number and an increase in trabecular separation. Texture analysis, a new method to assess bone density and structural anisotropy, correlated well with the standard histomorphometry and measured significant deficits in the density measure (M(Density)) in the G-ST group that remained at maturity (6 months). The texture energy deficit in the G-ST group was primarily in the 0 degrees orientation (-13.2%), which measures the longitudinal trabeculae in the proximal tibia. However, the deficit in the G-LT group was in the 45 degrees and 135 degrees orientations. These results suggest that any "catch-up" growth following the cessation of the GnRH-antagonist injection protocol may be directed in trabeculae oriented perpendicular to 0 degrees at the expense of trabeculae in other orientations.

Growth hormone normalizes pubertal onset in children with cystic fibrosis

Children with cystic fibrosis (CF) have a high incidence of delayed puberty and poor growth. We retrospectively reviewed pubertal maturation data from 105 children with CF who had participated in studies on growth hormone (GH). As part of the GH study, participants were randomized into two cohorts, one of which was treated with GH for 1 year, and then followed off GH, and the other group was first followed off GH, and then treated with GH for 1 year. Pubertal staging was obtained throughout these studies and we have retrospectively analyzed the data.

RESULTS

In prepubertal females, GH treatment resulted in a normalized onset of breast development as compared to delayed onset in non-treated females. Females treated during puberty had a normal tempo of breast development. In prepubertal males, GH treatment resulted in a normalized onset of testicular volume compared to non-treated males. Testicular size progression was not accelerated in pubertal boys treated with GH.

CONCLUSION

GH treatment normalizes pubertal onset in prepubertal children with CF.

Short-term delay of puberty causes a transient reduction in bone strength in growing female rats

Multiple factors affect the structural development of the skeleton; in particular, estrogen levels during growth are an important factor in the pathogenesis of bone fragility. The delay of menarche and infrequent menstrual cycles decrease estrogen levels during adolescence and decrease peak bone mass. The aim of this study was to determine if delayed puberty through administration of a GnRH antagonist initiated prior to the onset of the first estrus cycle would delay the increase in estrogen levels and impede bone strength development in female rats. Twenty-three-day-old female Sprague-Dawley rats were randomly assigned to one of four groups; 1) short-term control group (C-ST) (n = 12), 2) long-term control (C-LT) (n = 12), 3) short-term GnRH antagonist group (G-ST) (n = 12) and 4) long-term GnRH antagonist group (G-LT) (n = 12). Injections (0.2 ml) of either saline or GnRH antagonist (100 microg/day) (Cetrotide, Serono, Inc) were given intraperitoneally for a duration of 18 days. Pubertal and gonadal development was retarded as indicated by a delay in vaginal opening (an indicator of pubertal onset), lower ovarian and uterine weights and lower estradiol levels in the short-term experimental animals (G-ST). However, at maturity (G-LT), there were no significant differences found in these measures. A delay in the timing of puberty significantly attenuated the development of femoral bone strength at 6 weeks of age. Peak moment, yield moment and stiffness in the G-ST group were all significantly less than the C-ST group. Cortical width was significantly attenuated due to the increased percentage of marrow area per total bone area in the G-ST group. However, femoral bone strength was recovered at maturity (G-LT). In summary, a transient delay in pubertal timing has short-term effects on bone strength development. In the current animal model of delaying puberty through GnRH antagonist injections, there appears to be no long-term effects on bone strength.

Effect of oral contraceptive treatment on bone mass acquisition in skeletally immature young female rats

The objective of the present study was to investigate the effect of oral contraceptive (OC) treatment on bone mass accrual in skeletally immature young female rats. Animals in the baseline group were killed at the beginning of the experiment and were subjected to bone density assessment by peripheral quantitative computerized tomography (pQCT). The control group was fed a base diet free of phytoestrogens, while animals in the contraceptive group received the same base diet mixed with 2.67 microg desogestrel/100 g body weight and 0.0533 microg ethinyl estradiol/100 g body weight. The duration of the treatment period was 16 weeks. Densitometric measurements by dual energy x-ray absorptiometry and serum bone markers assessment were carried out at baseline, at 8 weeks and at 16 weeks, while pQCT densitometry took place after sacrifice. All bone mineral density and bone mineral content indices measured by dual energy x-ray absorptiometry increased significantly throughout the study period in both the OC and control group. Concerning pQCT measurements, animals in both the OC and the control group had significantly higher cortical density compared with baseline (midtibia: p=.0003 and .0003, respectively). Total area and periosteal circumference were significantly higher in OC group, both in proximal (p=.003 and .003, respectively) and midtibia (p=.048 and .042, respectively) compared with baseline. Osteoprotegerin serum levels increased in both groups, and at the end of the experiment, circulating osteoprotegerin was significantly higher in the OC group compared with controls (p=.032). At the end of the experiment, carboxyl-terminal telopeptides of collagen type I levels were significantly lower in the OC-treated animals compared with controls (p=.046). Our results suggest that OC administration to skeletally immature female rats allows normal bone accrual and may even improve bone geometry. This effect may be mediated through enhanced inhibition of bone resorption.

Estrogen and bone--a reproductive and locomotive perspective

The primary function of the skeleton is locomotion, and the primary function of estrogen is reproduction. When the skeleton is considered within this locomotive context, the onset of estrogen secretion at puberty leads to packing of mechanically excess mineral into female bones for reproductive needs. Accordingly, the unpacking of this reproductive safety deposit at menopause denotes the origin of type I osteoporosis.

INTRODUCTION

According to the prevailing unitary model of involutional osteoporosis, female postmenopausal bone loss can be described as having an initial accelerated, transient phase (type I), followed by a gradual continuous phase (type II). Estrogen withdrawal is generally accepted as the primary cause of the type I osteoporosis. Thus, the quest to uncover the origin of type I osteoporosis has focused on the estrogen withdrawal-related skeletal changes at and around the menopause. However, considering that the cyclical secretion of estrogen normally begins in early adolescence and continues over the entire fertile period, one could argue that focusing on perimenopause alone may be too narrow.

MATERIALS AND METHODS

This is not a systematic review of the literature on the skeletal function of estrogen(s), but rather, an introduction of a novel structure- and locomotion-oriented perspective to this particular issue through pertinent experimental and clinical studies.

RESULTS AND CONCLUSIONS

When considering locomotion as the primary function of the skeleton and integrating the classic findings of the pubertal effects of estrogen on female bones and the more recent hypothesis-driven experimental and clinical studies on estrogen and mechanical loading on bone within this context, a novel evolution-based explanation for the role of estrogen in controlling female bone mass can be outlined: the onset of estrogen secretion at puberty induces packing of mechanically excess bone into female skeleton for needs of reproduction (pregnancy and lactation). Accordingly, the unpacking of this reproductive safety deposit of calcium at menopause denotes the accelerated phase of bone loss and thus the origin of type I osteoporosis.

Effects on bone mineral density of gonadotropin releasing hormone analogs used in the treatment of central precocious puberty

The aim of this study was to compare vertebral bone mass values of patients with central precocious puberty (CPP) with healthy age and puberty matched controls and to determine the effect of gonadotropin releasing hormone (GnRH) analogs on bone mass in patients who had been treated at least for 1 year. Girls with idiopathic CPP, 11 pretreatment, 14 post-treatment, and 19 pubertal girls as controls were enrolled in the study. The mean ages of the controls and the patients with CPP pre- and post-treatment were 10.25 +/- 1.06, 8.23 +/- 1.11, and 10.36 +/- 1.82 years, respectively. Leuprolide acetate (Lucrin) 3.75 mg was administered s.c. monthly. Bone measurements were performed by dual energy X-ray absorptiometry (DEXA) (Norland) at the anterior-posterior vertebrae (L2-L4). The post-treatment group's mean BMD value was 0.66 +/- 0.12; Z scores according to CA and BA were 0.32 +/- 10 and 0.30 +/- 1.1, respectively. In the study group, BMD values compared to the control group were normal. No significant change in BMD values was observed after treatment. Neither osteopenia nor osteoporosis was observed in patients taking GnRH analog.