Novel Links among Wnt and TGF-β Signaling and Runx2

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Abstract
Osteoblasts exhibit complex Wnt-induced effects that increase T cell factor (TCF)/lymphoid enhancing factor-dependent transcription in parallel with β-catenin stabilization and nuclear factor binding to TCF response element DNA. Here we show that Wnt-dependent gene expression increases during the early phase of osteoblast differentiation in vitro, is enhanced by prostaglandin E2 activation of transcription factor Runx2 (runt homology domain transcription factor 2), and is specifically suppressed in Runx2 antisense-depleted osteoblasts. Moreover, Wnt pathway induction increases expression of the Runx2-sensitive gene, TGF-β type I receptor, without increasing nuclear Runx2 levels or Runx2 binding to DNA. Rather, despite an increase in β-catenin levels, Wnt pathway induction enhances Runx2 transcriptional potential in a β-catenin-independent way. Runx2 functionally associates with TCF-4 that lacks a β-catenin-binding domain and is more fully activated in response to both prostaglandin E2 and Wnt pathway induction. Wnt pathway induction increases TGF-β type I receptor expression, yet regulates, both positively and negatively, TGF-β signaling. Furthermore, TGF-β signaling enhances TCF-4 and lymphoid enhancing factor-1 mRNA expression and increases TCF-4 transcriptional activity. Therefore, we propose that cross talk between the Wnt and TGF-β pathways, which converge on Runx2, both promotes and attenuates individual aspects of osteoblast maturation.

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