Multifarious effects of 17-β-estradiol on apolipoprotein E receptors gene expression during osteoblast differentiation in vitro

Apolipoprotein E (ApoE) regulated bone metabolism in mice might mediate uptake of lipid particles into target cells such as osteoblasts via receptor-mediated endocytosis by apoE receptors, which includes the low-density lipoprotein receptor (LDLR) family and heparan sulfate proteoglycans (HSPGs). There is no report regarding the expression of ApoE receptors mRNA induced by estrogen during osteoblast differentiation in vitro. Primary osteoblasts were collected from the calvaria of newborn mice and were subjected to osteoblast mineralization culture with serial concentrations of 17-beta-estradiol (E2) in vitro. RNA was isolated at days 0, 5 and 25 of differentiation. Real-time PCR was conducted to analyze apoE receptors mRNA levels. We found that most LDLR family members genes were induced during osteoblast differentiation in vitro. The effect of E2 on apoE receptors gene expression during osteoblast differentiation was multifarious. The most noted members of the LDLR family involved in the maintenance of bone metabolism were LRP5, LRP6, LRP4, and Apoer2. LRP6 was up-regulated, while LRP5, LRP4, and Apoer2 were down-regulated by E2. Given that LRP6 is required for early stages of differentiation, we speculate E2 promotes osteoblast differentiation mainly in the early stage.