Phospholipase D1 drives a positive feedback loop to reinforce the Wnt/β-catenin/TCF signaling axis

Activation of the Wnt signaling pathway occurs frequently in human cancers, but an understanding of the targets and regulation of this important pathway remains incomplete. In this study, we report that phospholipase D (PLD), a cell survival mediator that is upregulated in cancer, is an important target of the Wnt signaling pathway that functions in a positive feedback loop to reinforce pathway output. PLD1 expression and activity was enhanced by treatment with Wnt3a and glycogen synthase kinase-3 inhibitors, and the Wnt pathway-regulated transcription factors β-catenin and TCF-4 were required for this effect. Three functional TCF-4-binding sites were identified within the PLD1 promoter. Interestingly, suppressing PLD1 blocked the ability of β-catenin to transcriptionally activate PLD1 and other Wnt target genes by preventing β-catenin/TCF-4 complex formation. Conversely, tactics to elevate intracellular levels of phosphatidic acid, the product of PLD1 enzyme activity, enhanced β-catenin/TCF-4 complex formation as well as β-catenin- dependent TCF transcriptional activity. In cell-based assays, PLD1 was necessary for the anchorage-independent growth driven by Wnt/β-catenin signaling, whereas β-catenin/TCF-4 was necessary for the anchorage-independent growth driven by PLD1 activation. Taken together, our findings define a function for PLD1 in a positive feedback loop of Wnt/β-catenin/TCF-4 signaling that provides new mechanistic insights into cancer, with implications of novel strategies to disrupt Wnt signaling in cancer.