The human PKP2/plakophilin‐2 gene is induced by Wnt/β‐catenin in normal and colon cancer‐associated fibroblasts

Colorectal cancer results from the malignant transformation of colonic epithelial cells. Stromal fibroblasts are the main component of the tumour microenvironment, and play an important role in the progression of this and other neoplasias. Wnt/β-catenin signalling is essential for colon homeostasis, but aberrant, constitutive activation of this pathway is a hallmark of colorectal cancer. Here we present the first transcriptomic study on the effect of a Wnt factor on human colonic myofibroblasts. Wnt3A regulates the expression of 1,136 genes, of which 662 are upregulated and 474 are downregulated in CCD-18Co cells. A set of genes encoding inhibitors of the Wnt/β-catenin pathway stand out among those induced by Wnt3A, which suggests that there is a feedback inhibitory mechanism. We also show that the PKP2 gene encoding the desmosomal protein Plakophilin-2 is a novel direct transcriptional target of Wnt/β-catenin in normal and colon cancer-associated fibroblasts. PKP2 is induced by β-catenin/TCF through three binding sites in the gene promoter and one additional binding site located in an enhancer 20 kb upstream from the transcription start site. Moreover, Plakophilin-2 antagonizes Wnt/β-catenin transcriptional activity in HEK-293T cells, which suggests that it may act as an intracellular inhibitor of the Wnt/β-catenin pathway. Our results demonstrate that stromal fibroblasts respond to canonical Wnt signalling and that Plakophilin-2 plays a role in the feedback control of this effect suggesting that the response to Wnt factors in the stroma may modulate Wnt activity in the tumour cells.
This work was supported by the Spanish Ministerio de Economía y Competitividad and the European Regional Development Fund (Fondos FEDER) (SAF2013–43468-R, and SAF2014–53819-R), the Agencia Estatal de Investigacion-Fondos FEDER (SAF2016–76377-R, AEI/FEDER, UE), the Comunidad de Madrid (S2010/BMD-2344 Colomics2), the Instituto de Salud Carlos III–Fondos FEDER (RD12/0036/0021, and CIBERONC-CB16/12/00273) and the Nuclear Receptors Network (Nurcamein, SAF2015–71878-REDT).