Frizzled Gene Expression and Negative Regulation of Canonical WNT-β-catenin Signaling in Mouse F9 Teratocarcinoma Cells

Mouse F9 cells differentiate into primitive endoderm (PrE) following the activation of the canonical WNT/β-catenin pathway. The upregulation of Wnt6 and activation of β-catenin-TCF/LEF-dependent transcription is known to accompany differentiation, but the Frizzled (FZD) receptor responsible for transducing the WNT6 signal is not known. Eight of the ten Fzd genes were found to be expressed in F9 cells, with Fzd7 being the most highly expressed, and chosen for further analysis. To alter steady-state Fzd7 levels and test the effect this has on differentiation, siRNA and overexpression approaches were used to knockdown and ectopically express the Fzd7 message, respectively. siRNA knock down of Fzd7 resulted in reduced DAB2 levels, and the overexpression activated a TCF-LEF reporter, but neither approach affected differentiation. Our focus turned to how canonical WNT6 signaling was attenuated to allow PrE cells to form PE. Dkk1, encoding a WNT antagonist, was examined and results showed that its expression increased in F9 cells treated with RA or overexpressing Wnt6. F9 cells overexpressing human DKK1 or treated with DKK1-conditioned medium and then treated with RA failed to differentiate, indicating that a negative feedback loop involving WNT6 and DKK1 attenuates canonical WNT/β-catenin signaling, thereby allowing PE cells to differentiate.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author