Underlying Mechanisms by Which Estrogen Regulates Body Composition Including Bone and Muscle Mass

Ovariectomized (ovx) rats gain excess body weight (BW) even when they are pair-fed to sham animals. This higher BW, contrary to common belief, does not result in higher bone mineral density. The purpose of the present study was to evaluate the potential mechanisms underlying these paradoxical observations. We evaluated the effects of ovariectomy and 17β-estradiol (E2) using two age groups female Sprague-Dawley (SD) rats: 5- and 10-month old rats. Thirty-six female SD rats, 5- and 10-months old were acclimatized for 5 days. Animals in each category were divided into three groups: sham, ovx, and ovx+E2. There were 18 rats per age category with sample size of six rats per treatment group. Five- and ten-month old rats were chosen to model young and middle age women, respectively. Immediately after surgery, rats in all groups received a semi-purified control diet (AIN-93M). Animals in the ovx+E2 group were injected with E2 (10μ/kg BW subcutaneously, twice per week). Ovx control rats received solvent vehicle (sesame oil; subcutaneously, twice per week). Rats in ovx groups were pair-fed to the mean food intake of the sham group and their food intake was adjusted every three days. Rats' voluntary running activity was measured by activity wheel and distance counter in individual cages for 24 hours between one to two weeks before sacrifice. To confirm bone loss due to ovariectomy, whole body bone mineral density (BMD) was measured by dual energy x-ray absorptiometry (iDXA) at baseline, mid-point and at the end of study period. The losses of whole body BMD in five- and ten-month old ovx rats occurred after three and half- and five-months from the time of the removal of ovaries, respectively. In spite of pair feeding the ovx rats, the final body weights of ovx rats in both age categories were significantly (P<0.05) higher than those of sham-operated rats. Estrogen completely prevented the ovariectomy-induced weight gains in both age categories. Ovariectomy either significantly decreased (10-month old) or tended (P<0.1) to decrease (5-month old) voluntary wheel running activity and in comparison with sham animals. Estrogen treated rats voluntary wheel running activity was able to prevent this ovariectomy-induced decline in physical activity. Mean fat mass of ovx rats in comparison with sham animals was significantly higher in both age groups. Rats that received E2 did not experience gain in fat mass. The mean tibial and vertebral BMD values of ovx rats were significantly lower in comparison with sham rats in both age categories. Microstructural properties of tibiae and vertebrae, including bone volume/over total volume, connectivity density, structure model index were all unfavorably altered by ovariectomy in both age categories. In younger rats, E2 administration was able to prevent ovariectomy-induced alterations in lumbar vertebrae microstructural parameters such as bone volume/total volume (BV/TV), structure model index (SMI) and trabecular thickness (Tb. Th.). However, in this age group E2 administration was only able to prevent ovx-induced increase in tibial trabecular separation (Tb. Sp.). These findings suggest that E2 is more effective in preventing microstructural properties of trabecular rich bone such as vertebral bones. In older rats, in addition to the loss of BV/TV, ovx also caused significant decreases in connectivity density (Conn. D.) and trabecular number (Tb. N.) of both tibial and lumbar vertebral bones while increasing SMI and Tb. Sp. in the same bones. E2 administration was able to prevent changes in Conn. D. and SMI only in lumbar bones. E2 was also able to prevent the ovx-induced deleterious effects on tibial Tb.N. and Tb.Sp. by maintaining these values to those of sham levels. Overall, these observations suggest that while E2 is capable of preventing some structural properties as a result of ovx, it is not able to replace ovarian hormones as a whole. Ovariectomy significantly increased serum levels of both biomarker of bone resorption, C-telopeptides of type I collagen, and bone formation, alkaline phosphatase levels. These findings are in agreement with earlier reports that indicate ovariectomy increases the rate of bone turnover with resorption exceeding that of formation. Our findings do not suggest that the bone loss due to ovx is through inflammatory processes as the serum levels of tumor necrosis factor alpha (TNF-α) remained below the detectable level in ovx rats in both age categories. In spite of ovx-induced significant increases in systemic levels of insulin-like growth factor-I (IGF-I) in both age categories, its localized mRNA expressions were either significantly or tended (P < 0.1) to be reduced in gastrocnemius (GAS) and soleus (SOL) muscles of 10-month old rats. These local reductions in IGF-I mRNA expressions were in parallel with reductions in normalized to total body weight muscle mass, signifying the importance of IGF-I at the tissue level. Similar trends (P < 0.1) were also observed in 5-month old rats, albeit not significantly. Our findings for the first time, to our knowledge, suggest that elevated systemic IGF-I may down-regulate its synthesis in GAS and SOL of ovx rats. Overall, the findings of this study indicate that ovariectomy causes significant losses of BMD, microstructural properties and muscle mass. However, body fat increases as such that the weight of ovx animals exceeds those of sham and E2 treated animals, which indicates that ovarian hormone deficiency causes body weight gain mainly due to a gain in fat mass. Additionally, our data suggest that the detrimental effects of ovarian hormone deficiency are exacerbated as a result of aging. Our observations, especially in older rats, suggest that estrogen deficiency alters body composition which resembles that of osteosarcopenic obesity.