Auxiliary Wnt3A Signaling in Cell Fate Decisions of C3H10T1/2 Mesenchymal Stem Cells
Activation of Wnt signaling pathways is critical to a variety of developmental events across all animal taxa. These highly evolutionarily conserved pathways are also important in the adult organism for maintaining homeostasis of self-renewing tissues. Because of its role in such important physiological processes, deregulation of Wnt signaling can have severe consequences; indeed, inappropriate activation of this pathway has been implicated in multiple human diseases, including cancer.
Upon binding their cellular receptors, canonical Wnt ligands, like Wnt 3A, stimulate the stabilization, accumulation, and nuclear translocation of a multifunctional cellular protein βcatenin, the consequence of which is induction of βcatenin-dependent transcription. This work describes the identification and characterization of two Wnt3A-stimulated intracellular signaling pathways activated in parallel to βcatenin stabilization: the RhoA pathway and the ERK pathway. These two auxiliary pathways do not affect βcatenin stability, accumulation, or subcellular localization; rather, they modulate βcatenin -dependent transcriptional activity through other mechanisms. As a result of their influence on βcatenin-dependent transcription, these pathways instruct cell fate decisions in C3H10T1/2 mesenchymal stem cells, in particular inhibition of adipogenesis and promotion of osteoblastogenesis.
Expression microarray analysis and biochemical and pharmacological techniques were used to further characterize the two Wnt3A-stimulated auxiliary pathways in C3H10T1/2 cells. Remarkably, each pathway influences βcatenin function via a novel mechanism. In the Wnt3A/RhoA pathway, Wnt3A-stimulated trimeric G proteins activate a RhoA-ROCK-SRF cascade. Activated SRF can cooperate with βcatenin to enhance the induction of Wnt3A target genes, like Ctgf, that also contain SRF binding sites within regulatory elements. In the Wnt3A/ERK pathway, Wnt3A transactivates the EGFR in a concentration-dependent manner, leading ultimately to ERK activation, which interacts with and promotes βcatenin/Tcf4 interaction and enhances induction of βcatenin/Tcf4 target genes.
These data emphasize the complexity of Wnt signaling and have intriguing implications regarding cross-regulation of the pathway, especially in stem cells. Also, since not all cells are capable of responding to Wnt3A by activation of these auxiliary pathways, this work identifies novel mechanisms that could underlie cell type-specific responses to Wnts and provides mechanistic insight into cellular responses to Wnt concentration gradients. Moreover, this work identifies novel transcriptional mechanisms important for promoting osteogenic cell fate specification, which could ultimately provide new therapeutic targets in disease states with bone loss or ineffective bone formation.
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