Effects Of Follicle-Stimulating Hormone On Bone Metabolism Across The Menopausal Transition

ObjectiveThe Menopausal Transition（MT）is a specific period for women moving from reproductive to postmenopause.Studies have shown that there are dynamic changes of sex hormones,as well as various concomitant clinical symptoms and skeletal changes during the MT.At the same time,rapid bone mass loss during MT is closely associated with irreversible postmenopausal bone microstructure degeneration and increased risk of fracture.Therefore,MT is a critical time window for early intervention to prevent rapid bone loss and reduce the risk of fracture.It was found that during this period serum levels of estradiol（E₂）are relatively constant,while serum levels of follicle-stimulating hormone（FSH）begin to rise.Changes in serum FSH levels across MT were better predictors of bone loss than changes in E₂levels.At the same time,the changes of serum FSH and E₂levels in women crossing MT were not only different by race but also by region.However,there is still no systematic study on the relationship between serum level of FSH with the changes of bone turnover marker（BTMs）and bone mineral density（BMD）,as well as the risk of osteoporosis（OP）in native Chinese women across MT,and there is no suitable rat model for studying MT-related OP.Therefore,this study aims to further explore the effects of FSH on MT bone metabolism and explore a suitable rat model for studying MT-related OP by exploring the relationship between serum level of FSH with the changes of BTMs and BMD and the risk of OP in native Chinese women and longitudinal study of the effects of FSH on bone metabolism in female rats across MT.Thus laying a foundation for the early prevention and treatment strategy of MT-related OP in Chinese female.MethodsA total of 654 healthy women aged 35 to 66（321 were premenopausal and 333 were postmenopausal,with menopause years ranging from 1 to 12 years）,excluding disease and drug factors affecting bone metabolism,were recruited in the cross-sectional study.Their average age is 50.5（?）8.29.Fasting blood was taken from all subjects and morning urine was collected.Their serum levels of FSH and E₂were measured by radioimmunoassay,bone specific alkaline phosphatase（BAP）,osteocalcin（OC）,serum cross-linked N-terminal telopeptides of type I collagen（s NTX）,serum cross-linked C-terminal telopeptides of type I collagen（s CTX）,urinary excretion of NTX（u NTX）were measured by enzyme-linked immunosorbent assay（ELISA）,and BMDs at various sites of the skeleton（lumbar spine[LS],femur neck[FN],total hip[TH],the nondominant forearm ultradistal[FUD],and total 1/3 region of nondominant forearm distal[FD1/3T]）were measured.These healthy volunteers were grouped by menstrual status.The differences of BTMs and BMD among different groups were compared by ANOVA.The relationships of serum FSH with BTMs and BMD were analyzed by Person’s correlation analysis and partial correlation after adjusting for age and body mass index（BMI）.The effects of FSH and E₂on BTMs and BMD,and BTMs on BMD were analyzed by multiple linear regression.According to the World Health Organization（WHO）definitions,subjects with BMD of 2.5 SD lower（T-scores≤–2.5）than the peak BMD of the same gender were determined to be osteoporosis.FSH,E₂and BTMs were divided into three subgroups.The prevalence and risk of OP in different subgroups were compared byχ2 test.The effects of FSH on bone metabolism in female rats across MT were further studied by animal simulation,and the changes of bone mass,bone microstructure,biochemical indices and bone biomechanical properties with different concentrations of FSH were systematically observed.Ninety 1.4-month-old female SD rats were randomly divided into control group（group A,N=20）,FSH with low-dose intervention group（group B,N=20）,FSH with high-dose intervention group（group C,N=20）,ovarian toxicity model group with depleted follicles（group D,N=20）,and FSH high-dose intervention group in later phases（group E,N=10）.Group A,B and C were treated from 1.4 months old to the middle of the experiment（10.3-month age）when half of the rats in every group were randomly killed,and the remaining rats continued to be treated until the end of the experiment（20.3-month age）.Rats in group D were injected with the 4-vinylcyclohexene diepoxide（VCD）for 5 weeks from 1.4-month age,half of which were randomly killed at the middle of the experiment,and the remaining continued to be treated until the end of the experiment.Rats in group E were treated from 10.3-month age to the end of the experiment.BMD of whole body in rats and other related indicators were measured by dual-energy X-ray absorptiometry（DXA）with an interval 2 months.And the serum level of FSH,E₂,BAP,u NTX,and s CTX were analyzed by ELISA.The bilateral femur,tibia and the 3rd to 5th of lumbar spine were separated at the middle and end point of the experiment.BMD was measured by DXA,and microstructural parameters of proximal tibia and the 3rd lumbar vertebra were measured byμ-CT,and biomechanical properties of femur were measured by three-point bending test.The relevant data were analyzed statistically.ResultsThe epidemiological study suggested that serum FSH-level（7.4-fold）and BTMs（35.8%to 124%）increased,whereas BMDs of all bone sites（13.1%to 19.0%）decreased significantly in postmenopausal women compared with premenopausal women.The average level of E₂in perimenopausal Chinese women was the significantly higher than those in premenopausal and postmenopausal women（P<0.001）.The trend of fitting curve of serum FSH and BTMs with age was completely opposite to that of BMD at various sites of skeleton.Serum level of FSH and BTMs increased while BMD decreased with age.The variation trend of E₂with age was not similar to that of BTMs and BMD.Serum level of FSH were positively associated with BTMs and negatively correlated with BMDs of all bone sites,no matter being adjusted for age or BMI.After adjusted age and BMI,E₂levels were not significantly related with s CTX,s NTX and u NTX.Serum levels of E₂had no significant correlation with BMD.Multiple linear regression analysis showed that FSH and age were a negative determinant factor of BTMs and a positive determinant factor of BMDs.FSH can explain about 2.4%–33.3%of BTMs variability,and FSH level could only account for 1.1%–7.8%of BMD changes.The explanation of E₂for BAP and s CTX was slight,and there was almost no correlation of E₂with other BTMs and BMD at each bone site.After serum levels of FSH grouped into tertiles（T₁–T₃）from low to high,there were 218 cases in T₁group（<5.68 IU/L）,218 cases in T₂group（5.68–34.36 IU/L）and 218 cases in T₃group（>34.36 IU/L）.BTMs increased,BMDs decreased and prevalence and risk of osteoporosis increased with FSH tertile.When serum levels of E₂were divided into tertiles（T₁–T₃）,only BAP showed a significant stepwise increase.There was no significant difference in BMD levels among all groups.The results of whole body DXA measurement showed that BMD of group E was significantly lower than that of group D（all P<0.05）,and lean body weight（all P<0.05）and body weight（all P<0.05）were significantly lower than those of groups A,B and D between 12.3 months of age and 20.3 months of age.The results of BMD measurement in vitro showed that at the end of the experiment（20.3 months of age）,BMD of each region of femur in group E was significantly lower than that in group D（all P<0.05）,and that in group C（P<0.05）.The BMD of tibia in group E was significantly lower than that of group D（all P<0.05）.BMDs in 3 to 7,10 and 12 sections of tibia were significantly lower than those in group C（all P<0.05）.In the middle of the experiment,the cortical bone volume（BV）,bone volume/tissue volume（BV/TV）and cortical bone thickness（Ct.Th）in group B were significantly increased compared with group A（all P<0.05）,and total porosity（Po（TOT））was significantly decreased compared with group A（all P<0.05）.At the end of the experiment,trabecular number（Tb.N）of the proximal tibial cancellous bone in group E through MT was significantly lower than that in group D（P<0.05）.Po（TOT）was significantly higher in group E than that in groups A,C,and D（all P<0.05）,and Ct.Th was significantly lower than in groups C and D（P<0.05）.Trabecular thickness（Tb.Th）in group E was significantly lower than that in group D at the end of the experiment（P<0.05）.In the middle of the experiment,the mechanical parameters,elastic stage（ES）,fracture load（FL）and stiffness coefficient（SC）of femur in group B were significantly higher than those in group A（all P<0.05）,while fracture defletion（FD）was significantly lower than those in group A（P<0.05）.At the end of the experiment,femoral mechanical indexes,FD,maximum stress deflection（MSD）,ES,SC,and fracture strain（FS）in group E across MT,were significantly lower than those in group C（all P<0.05）.Conclusion1.Rapid increase of serum FSH levels across MT stimulates a high turnover bone loss and reduces bone deformability,strength,and rigidity,which primarily affecting the bone sites where cancellous bone predominates,such as lumbar.There was no significant correlation between serum level of E₂with the elevation in BTMs and bone loss across MT.2.Adult rat models of FSH with high-dose intervention during bone reconstruction can lead to a significant reduction in bone mass,which is expected to become an animal model for studying MT-related OP.3.Low-dose FSH can improve bone microstructure,increase bone strength and rigidity,and reduce bone deformation ability during bone modeling period.