Wnt Proteins Prevent Apoptosis of Both Uncommitted Osteoblast Progenitors and Differentiated Osteoblasts by β-Catenin-dependent and -independent Signaling Cascades Involving Src/ERK and Phosphatidylinositol 3-Kinase/AKT

Abstract

Genetic studies in humans and mice have revealed an important role of the Wnt signaling pathway in the regulation of bone mass, resulting from potent effects on the control of osteoblast progenitor proliferation, commitment, differentiation, and perhaps osteoblast apoptosis. To establish the linkage between Wnts and osteoblast survival and to elucidate the molecular pathways that link the two, we have utilized three cell models: the uncommitted bipotential C2C12 cells, the preosteoblastic cell line MC3T3-E1, and bone marrow-derived OB-6 osteoblasts. Serum withdrawal-induced apoptosis was prevented by the canonical Wnts (Wnt3a and Wnt1) and the noncanonical Wnt5a in all cell types. Wnt3a induced LRP5-independent transient phosphorylation and nuclear accumulation of ERKs and phosphorylation of Src and Akt. The anti-apoptotic effect of Wnt3a was abrogated by inhibitors of canonical Wnt signaling, as well as by inhibitors of MEK, Src, phosphatidylinositol 3-kinase (PI3K), or Akt kinases, or by the addition of cycloheximide to the culture medium. Wnt3a-induced phosphorylation of GSK-3␤ and downstream activation of ␤-catenin-mediated transcription required ERK, PI3K, and Akt signaling. Wnt3a increased the expression of the anti-apoptotic protein Bcl-2 in an ERK-dependent manner. ␤-Catenin-mediated transcription was permissive for the anti-apoptotic actions of Wnt1 and Wnt3a but was dispensable for the anti-apoptotic action of Wnt5a. However, Src, ERKs, PI3K, and Akt kinases were required for the anti-apoptotic effects of Wnt5a. These results demonstrate for the first time that Wnt proteins, irrespective of their ability to stimulate canonical Wnt signaling, prolong the survival of osteoblasts and uncommitted osteoblast progenitors via activation of the Src/ERK and PI3K/Akt signaling cascades.Wnts are secreted lipid-modified signaling proteins that influence cell proliferation, differentiation, and survival (1-3). Wnt proteins are divided into two classes. The Wnt1 class activates the canonical Wnt signaling pathway, which involves the formation of a complex between Wnt proteins, Frizzled, and LRP5 2 or LRP6 receptors (4, 5). This complex in turn leads to phosphorylation and inactivation of GSK-3␤, inhibition of ␤-catenin degradation, and subsequent accumulation of ␤-catenin in the nucleus (6, 7). Nuclear ␤-catenin binds the TCF/LEF family of transcription factors and induces target gene expression (8).The noncanonical Wnt5a class binds Frizzled proteins, activates heterotrimeric G proteins, and increases intracellular calcium via protein kinase C-dependent mechanisms or induces Rho-or c-Jun N-terminal kinase (JNK)-dependent changes in the actin cytoskeleton (9). Genetic studies in humans and mice have determined that LRP5 (LRP6)/Wnt signaling plays a major role in the control of bone mass. Mutations in LRP5 or LRP6 lead to disorders associated with either low (10) or high bone mass (11-13). In agreement with observations in humans, LRP5-deficient mice show decreased bone formation and osteoblast proliferation (14), wh...

Complex Expression Pattern of Wnt Ligands and Frizzled Receptors in Human Placenta and its Trophoblast Subtypes

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Down-regulation of frizzled-7 expression decreases survival, invasion and metastatic capabilities of colon cancer cells

“…Our previous data demonstrated that the expression levels of c-myc and cyclin-D mRNAs increased after FZD7 transfection into HCT-116 cells and a siRNA against FZD7 suppressed c-Myc protein expression (Ueno et al, 2008). Conversely, some Wnt signals were shown to promote viability in some cell types (Almeida et al, 2005;Railo et al, 2008). Wnt signalling through other FZDs might protect our transfectants from apoptosis.…”

Molecular and cellular mechanisms of the anabolic effect of intermittent PTH

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