The Wnt antagonist secreted frizzled‐related protein‐1 controls osteoblast and osteocyte apoptosis

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Abstract
Mechanisms controlling human bone formation remain to be fully elucidated. We have used differential display‐polymerase chain reaction analysis to characterize osteogenic pathways in conditionally immortalized human osteoblasts (HOBs) representing distinct stages of differentiation. We identified 82 differentially expressed messages and found that the Wnt antagonist secreted frizzled‐related protein (sFRP)‐1 was the most highly regulated of these. Transient transfection of HOBs with sFRP‐1 suppressed canonical Wnt signaling by 70% confirming its antagonistic function in these cells. Basal sFRP‐1 mRNA levels increased 24‐fold during HOB differentiation from pre‐osteoblasts to pre‐osteocytes, and then declined in mature osteocytes. This expression pattern correlated with levels of cellular viability such that the pre‐osteocytes, which had the highest levels of sFRP‐1 mRNA, also had the highest rate of cell death. Basal sFRP‐1 mRNA levels also increased 29‐fold when primary human mesenchymal stem cells were differentiated to osteoblasts supporting the developmental regulation of the gene. Expression of sFRP‐1 mRNA was induced 38‐fold following prostaglandin E2 (PGE2) treatment of pre‐osteoblasts and mature osteoblasts that had low basal message levels. In contrast, sFRP‐1 expression was down‐regulated by as much as 80% following transforming growth factor (TGF)‐β1 treatment of pre‐osteocytes that had high basal mRNA levels. Consistent with this, treatment of pre‐osteoblasts and mature osteoblasts with PGE2 increased apoptosis threefold, while treatment of pre‐osteocytes with TGF‐β1 decreased cell death by 50%. Likewise, over‐expression of sFRP‐1 in HOBs accelerated the rate of cell death threefold. These results establish sFRP‐1 as an important negative regulator of human osteoblast and osteocyte survival.