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Estrogen receptor-? in osteocytes is important for trabecular bone formation in male mice.

ABSTRACT: The bone-sparing effect of estrogen in both males and females is primarily mediated via estrogen receptor-? (ER?), encoded by the Esr1 gene. ER? in osteoclasts is crucial for the trabecular bone-sparing effect of estrogen in females, but it is dispensable for trabecular bone in male mice and for cortical bone in both genders. We hypothesized that ER? in osteocytes is important for trabecular bone in male mice and for cortical bone in both males and females. Dmp1-Cre mice were crossed with ER?(flox/flox) mice to generate mice lacking ER? protein expression specifically in osteocytes (Dmp1-ER?(-/-)). Male Dmp1-ER?(-/-) mice displayed a substantial reduction in trabecular bone volume (-20%, P < 0.01) compared with controls. Dynamic histomorphometry revealed reduced bone formation rate (-45%, P < 0.01) but the number of osteoclasts per bone surface was unaffected in the male Dmp1-ER?(-/-) mice. The male Dmp1-ER?(-/-) mice had reduced expression of several osteoblast/osteocyte markers in bone, including Runx2, Sp7, and Dmp1 (P < 0.05). Gonadal intact Dmp1-ER?(-/-) female mice had no significant reduction in trabecular bone volume but ovariectomized Dmp1-ER?(-/-) female mice displayed an attenuated trabecular bone response to supraphysiological E2 treatment. Dmp1-ER?(-/-) mice of both genders had unaffected cortical bone. In conclusion, ER? in osteocytes regulates trabecular bone formation and thereby trabecular bone volume in male mice but it is dispensable for the trabecular bone in female mice and the cortical bone in both genders. We propose that the physiological trabecular bone-sparing effect of estrogen is mediated via ER? in osteocytes in males, but via ER? in osteoclasts in females.

SUBMITTER: Windahl SH

PROVIDER: S-EPMC3568324 | biostudies-literature | 2013 Feb

REPOSITORIES: biostudies-literature

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Estrogen receptor-α in osteocytes is important for trabecular bone formation in male mice.

Windahl Sara H SH Börjesson Anna E AE Farman Helen H HH Engdahl Cecilia C Movérare-Skrtic Sofia S Sjögren Klara K Lagerquist Marie K MK Kindblom Jenny M JM Koskela Antti A Tuukkanen Juha J Divieti Pajevic Paola P Feng Jian Q JQ Dahlman-Wright Karin K Antonson Per P Gustafsson Jan-Åke JÅ Ohlsson Claes C

Proceedings of the National Academy of Sciences of the United States of America 20130123 6

The bone-sparing effect of estrogen in both males and females is primarily mediated via estrogen receptor-α (ERα), encoded by the Esr1 gene. ERα in osteoclasts is crucial for the trabecular bone-sparing effect of estrogen in females, but it is dispensable for trabecular bone in male mice and for cortical bone in both genders. We hypothesized that ERα in osteocytes is important for trabecular bone in male mice and for cortical bone in both males and females. Dmp1-Cre mice were crossed with ERα(fl ...[more]

PMID: 23345419

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Project description:High-throughput microRNA sequencing was used to identify microRNAs (miRs) that regulate osteoblast differentiation. We establish that miR-101a, which targets the epigenetic enzyme Ezh2, is up-regulated in differentiated MC3T3 cells and robustly expressed in mouse calvaria. Our previous studies showed that loss of Ezh2 function enhances osteogenesis and bone formation. Thus, we tested the attractive mechanistic model that increased miR-101s expression may suppress Ezh2 to stimulate the osteogenic process. Transient miR-101a over-expression suppresses Ezh2 levels and reduces tri-methylation on lysine 27 of histone 3 (H3K27me3), a heterochromatic mark catalyzed by Ezh2. Importantly, over-expression of miR-101a stimulates osteogenic differentiation of MC3T3 cells as quantified by alizarin red staining. Therefore, we examined skeletal phenotypes of mice transgenic for miR-101a under the control of Col1a1promoer and doxycycline administration. Experimental controls and mir-101 over-expressing mice were exposed to doxycycline during pregnancy and postnatal stages (phenotyping at 8 weeks) to maximize penetrance of skeletal phenotypes. Our analyses revealed that miR-101a over-expressing male mice are bigger as measured by total body weight and exhibit an increase in long bone length. These mice exhibit significant increases in trabecular bone volume fraction, trabecular number, trabecular thickness, as well as a reduction in trabecular spacing based on microCT analysis. Histomorphometric examination established a significant reduction in osteoid volume to bone volume and osteoid surface to bone surface. These findings suggest that the enhancement in trabecular bone may be due to accelerated bone mineralization in miR-101a over-expressing male mice. Remarkably, while female mice exhibit a significant increase in bone length, no significant changes were noted by microCT (trabecular bone parameters) and histomorphometry (osteoid parameters). We conclude that miR-101a upregulation during osteoblast maturation enhances trabecular bone parameters in male mice. Thus, miR-101 and Ezh2 may form part of an intricate epigenetic feed-forward mechanisms to regulate bone formation.

Project description:Research backgroundThe role of osteocytes in maintaining bone mass has been progressively emphasized. Pip5k1c is the most critical isoform among PIP5KIs, which can regulate cytoskeleton, biomembrane, and Ca2+ release of cells and participate in many processes, such as cell adhesion, differentiation, and apoptosis. However, its expression and function in osteocytes are still unclear.Materials and methodsTo determine the function of Pip5k1c in osteocytes, the expression of Pip5k1c in osteocytes was deleted by breeding the 10-kb mouse Dmp1-Cre transgenic mice with the Pip5k1cfl/fl mice. Bone histomorphometry, micro-computerized tomography analysis, immunofluorescence staining and western blotting were used to determine the effects of Pip5k1c loss on bone mass. In vitro, we explored the mechanism by siRNA knockdown of Pip5k1c in MLO-Y4 cells.ResultsPip5k1c expression was decreased in osteocytes in senescent and osteoporotic tissues both in humans and mice. Loss of Pip5k1c in osteocytes led to a low bone mass in long bones and spines and impaired biomechanical properties in femur, without changes in calvariae. The loss of Pip5k1c resulted in the reduction of the protein level of type 1 collagen in tibiae and MLO-Y4 cells. Osteocyte Pip5k1c loss reduced the osteoblast and bone formation rate with high expression of sclerostin, impacting the osteoclast activities at the same time. Moreover, Pip5k1c loss in osteocytes reduced expression of focal adhesion proteins and promoted apoptosis.ConclusionOur studies demonstrate the critical role and mechanism of Pip5k1c in osteocytes in regulating bone remodeling.The translational potential of this articleOsteocyte has been considered to a key role in regulating bone homeostasis. The present study has demonstrated that the significance of Pip5k1c in bone homeostasis by regulating the expression of collagen, sclerostin and focal adhesion expression, which provided a possible therapeutic target against human metabolic bone disease.

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Estrogen receptor-? in osteocytes is important for trabecular bone formation in male mice.

Publications

Estrogen receptor-α in osteocytes is important for trabecular bone formation in male mice.

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