Estrogen receptor α in osteocytes regulates trabecular bone formation in female mice

Abstract

Estrogens are well known steroid hormones necessary to maintain bone health. In addition, mechanical loading, in which estrogen signaling may intersect with the Wnt/β-catenin pathway, is essential for bone maintenance. As osteocytes are known as the major mechanosensory cells embedded in mineralized bone matrix, osteocyte ERα deletion mice (ERαΔOcy/ΔOcy) were generated by mating ERα floxed mice with Dmp1-Cre mice to determine the role of ERα in osteocytes. Trabecular bone mineral density of female, but not male ERαΔOcy/ΔOcy mice was significantly decreased. Bone formation parameters in ERαΔOcy/ΔOcy were significantly decreased while osteoclast parameters were unchanged. This suggests that ERα in osteocytes exerts osteoprotective function by positively controlling bone formation. To identify potential targets of ERα, gene array analysis of Dmp1-GFP osteocytes sorted by FACS from ERαΔOcy/ΔOcy and control mice was performed. Gene expression microarray followed by gene ontology analyses revealed that osteocytes from ERαΔOcy/ΔOcy highly expressed genes categorized in ‘Secreted’ when compared to control osteocytes. Among them, expression of Mdk and Sostdc1, both of which are Wnt inhibitors, was significantly increased without alteration of expression of the mature osteocyte marker Sost or β-catenin. Moreover, hindlimb suspension experiments showed that trabecular bone loss due to unloading was greater in ERαΔOcy/ΔOcy mice with no loss of cortical bone. These data suggest that ERα in osteocytes has osteoprotective functions in trabecular bone formation through regulating expression of Wnt antagonists, but conversely plays a negative role in cortical bone loss due to unloading.

RANKL (Receptor Activator of NFκB Ligand) Produced by Osteocytes Is Required for the Increase in B Cells and Bone Loss Caused by Estrogen Deficiency in Mice

“…Thus RANKL expression by osteocytes is permissive for the increase in osteoclast number and did not appear to be a direct target of estrogen. Consistent with this finding, deletion of ER␣ from osteocytes has no effect on osteoclast number (26,27). We and others have shown that the amount of soluble RANKL protein in bone marrow supernatants does increase with either estrogen or androgen deficiency (12,28,29).…”

The Role of Estrogen Receptor in Bone Cells

“…According to Windahl et al, only males with specific ERa deletion in osteocytes display a substantial reduction in femoral, tibiae and vertebral trabecular bone volume. Conversely, in Kondoh et al's [63] hands only proximal tibiae BMD of ERa f/f ;DMP1-cre animals exhibits lower trabecular bone mass in females, but not in males. Both DMP1 models, however, found no effect of ERa deletion from osteocytes on cortical bone.…”

“…Parameters related to osteoblastic bone formation were also decreased in both studies, indicating that lacking of ERa signaling in osteocytes induced bone loss by reducing osteoblast actions, instead of promoting osteoclast resorption [62,63]. Gene expression profile of osteoblastic cells obtained from ERa f/f ; DMP1-cre animals was analyzed and showed upregulation of RNAm of Mdk and Sostdc1 genes which are considered as Wnt inhibitors [63]. Trabecular bone mass in unloaded mice without ERa in osteocytes turned out to be much lower than in their controls, suggesting that osteocytic ERa might have a protective role under unloading circumstances [63].…”

“…Both DMP1 models, however, found no effect of ERa deletion from osteocytes on cortical bone. Parameters related to osteoblastic bone formation were also decreased in both studies, indicating that lacking of ERa signaling in osteocytes induced bone loss by reducing osteoblast actions, instead of promoting osteoclast resorption [62,63]. Gene expression profile of osteoblastic cells obtained from ERa f/f ; DMP1-cre animals was analyzed and showed upregulation of RNAm of Mdk and Sostdc1 genes which are considered as Wnt inhibitors [63].…”

“…Gene expression profile of osteoblastic cells obtained from ERa f/f ; DMP1-cre animals was analyzed and showed upregulation of RNAm of Mdk and Sostdc1 genes which are considered as Wnt inhibitors [63]. Trabecular bone mass in unloaded mice without ERa in osteocytes turned out to be much lower than in their controls, suggesting that osteocytic ERa might have a protective role under unloading circumstances [63]. However, ERa in osteocytes is not required for the osteogenic response to loading in female mice [62].…”