Expression of estrogen receptors and enzymes involved in sex steroid metabolism in the rat tibia during sexual maturation

Abstract

Estrogens are essential for bone mass accrual but their role before sexual maturation has remained elusive. Using in situ hybridization and immunohistochemistry, we investigated the expression of both estrogen receptor (ER) and mRNA and protein as well as several mRNAs coding for enzymes involved in sex steroid metabolism (aromatase, type I and II 17 -hydroxysteroid dehydrogenase (17 -HSD), steroid sulfatase (STS) and type I 5 -reductase) on sections of tibial metaphyses before (1-and 4-week-old), during (7-week-old) and after (16-weekold) sexual maturation in female and male rats. ER and ER mRNA and protein were detected in metaphyseal bone in lining cells, osteoblasts, osteoclasts and some osteocytes with no apparent differences in expression during development or between the sexes. In contrast, aromatase, type I and II 17 -HSD and type I 5 -reductase mRNAs were first detected in osteoblasts, osteoclasts and occasionally in osteocytes from sexual maturation (7-week-old rat) and onwards. Only STS was present before sexual maturation. To study the significance of ER and expression in bone before sexual maturation when circulating sex steroid levels are low, 26-day-old female and male rats underwent gonadectomy or 17 -estradiol (E 2 ) supplementation (0·5 mg/21 days) during 3 weeks. Following gonadectomy, trabecular bone volume (TBV) was lower in males (P=0·03) and there was a trend towards reduction in females (P=0·057). E 2 supplementation increased tibial TBV compared with controls in both genders as assessed by Masson-Goldner staining. These data suggest that the presence of ERs in bone cells before sex maturation might be of significance for bone mass accrual. Furthermore, based on the mRNA expression of the crucial enzymes aromatase and type I 17 -HSD, we suggest that bone cells in the tibial metaphysis acquire the intrinsic capacity to metabolize sex steroids from sexual maturation onwards. This process may contribute to the beneficial effects of estrogen on bone mass accrual, possibly by intracrinology.

Methods to quantify sex steroid hormones in bone: applications to the study of androgen ablation and administration

“…Overall, our results appear to at least partially conflict with a predominant theory in the literature suggesting that T undergoes localized aromatization to E 2 within bone and then produces bone protection through subsequent ER activation (10). Thus, our results raise the distinct possibilities that 1) T does not undergo a significant degree of aromatization within bone or 2) T is aromatized to E 2 within bone but subsequently undergoes intraskeletal interconversion to less potent estrogens (8,20,21). Ultimately, determining the intraskeletal concentrations of E 1 and E 3 may assist in elucidating the intraskeletal metabolism of T and E 2 and may also assist in further defining the intracrine potential of bone.…”

“…In the present study, ORX induced an abrupt loss of circulating androgens and a concomitant reduction in total and trabecular BMD, whereas supraphysiological TE administration elevated serum androgens and E 2 concentrations and prevented the BMD loss. Androgens and estrogens may also undergo localized steroid interconversion within bone via the actions of 5␣-reductase, aromatase, 17␤-hydroxysteroid dehydrogenase, 16␣-hydroxylase, and/or steroid sulfatase (20,21), suggesting that localized enzyme expression influences the biological actions of sex hormones within bone. In our study, ORX did not lower the intraskeletal concentrations of any sex hormones, whereas TE administration elevated intraskeletal T and DHT, but not intraskeletal E 2 , indicating that the intraskeletal androgens represent a somewhat modifiable reservoir of sex hormones.…”

“…Skeletal protection induced by E 2 is well documented in humans of both sexes (7,16) and in various animal models (12,15,17,26,27) and occurs primarily via the activation of intraskeletal estrogen receptors (25). Skeletal tissue of both sexes also expresses aromatase and 5␣-reductase (25), along with several other enzymes (e.g., 17␤-hydroxysteroid dehydrogenase, 16␣-hydroxylase, and steroid sulfatases) that are capable of localized androgen and estrogen biotransformation (20,21). The ability of bone to interconvert androgens and estrogens suggests that intracrine action occurs within bone (20,21); however, the degree to which this interconversion occurs in bone and the roles that the circulating and intraskeletal reservoirs of sex steroids play in skeletal biology require further elucidation.…”

Mechanisms of Growth Plate Maturation and Epiphyseal Fusion

“…In addition to this endocrine route, estrogens can also be produced locally by aromatase in the growth plate (‘intracrinology’) [42]. Also other enzymes essential for estrogen production, including 17β-hydroxysteroid dehydrogenase, steroid sulfatase and type 1 5α-reductase, have been detected in epiphyseal chondrocytes and shown to be upregulated during sexual maturation in the rat growth plate suggesting a role for these enzymes and the steroids they produce during pubertal growth and growth plate maturation [43]. …”

Synergistic effect of bisphosphonate and docetaxel on the growth of bone metastasis in an animal model of established metastatic bone disease

“…The skin of the dissected hind legs was removed and the bones were fixed for 24 h in PBS with 4% formaldehyde; subsequently, the bones were decalcified in water containing 10% EDTA, pH 6.4 and embedded in paraffin and submitted to Masson-Goldner staining as previously described [26].…”