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

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 preosteocytes, 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 E 2 (PGE 2 ) 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)-b1 treatment of preosteocytes that had high basal mRNA levels. Consistent with this, treatment of pre-osteoblasts and mature osteoblasts with PGE 2 increased apoptosis threefold, while treatment of pre-osteocytes with TGF-b1 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.

Age-dependent Wnt gene expression in bone and during the course of osteoblast differentiation

“…Further, our data indicate that sFRP-1 is also expressed highest in late osteoblasts, suggesting that this protein may also have a role in mechanotransduction similar to sclerostin by suppressing osteoblast activity and enabling the transition into an osteocyte. Notably, another group observed sFRP-1 peak expression during the transition from preosteoblast to osteoblast (Bodine et al 2005). Despite the early expression of this protein in the osteoblast ontogenesis, the authors observed a differentiation-inhibitory and proapoptotic role for sFRP-1.…”

Accelerated aging phenotype in mice with conditional deficiency for mitochondrial superoxide dismutase in the connective tissue

“…In humans, osteoporosis causes kyphosis through the deformity of the vertebral bodies (Cummings & Melton, 2002). The current concept suggests that osteoporosis represents a continuum with multiple underlying mechanisms contributing to loss of bone mass and mineral density and overall microarchitectural deterioration of bones (Ferrari et al, 2004;Tasker et al, 2004;Bodine et al, 2005;Horowitz & Lorenzo, 2007;Perrini et al, 2010). These factors are causally related to increased risk of falls, contributing to high incidence of fragility fractures in osteoporotic patients (Raisz, 2005;Ralston & de Crombrugghe, 2006).…”

Wnt signaling and skeletal development

“…Expression of Sfrp-1 increased with advancing osteoblast differentiation and peaked in the preostecyte stage [78]. Sfrp-1 −/− mice exhibit increased trabecular bone mineral density and volume.…”