Reactivation of the silenced BASP1 gene suppresses oncogenic WNT signaling in human colorectal cancer cells
Starting from human colon cancer cells showing aberrant WNT/ß-catenin/TCF4 signaling, hyperactivated MYC and silenced BASP1, we generated stable cell lines overexpressing BASP1, either ectopically, or by reactivating the dormant BASP1 promoter using a lentiviral CRISPR-based system. BASP1 encodes a neuronal signaling protein and transcriptional co-repressor, from which a tumor-suppressive function has been described in avian cell systems and in multiple human cancer cell types. Proteome and transcriptome analyses revealed activation of several tumor and metastasis suppressors in BASP1-expressing cells, which also show strong repression of the transformed phenotype in terms of contact inhibition, anchorage-independent growth, and tumorigenesis. Cells with reactivated BASP1 display a flat and differentiated morphology with enhanced migratory potential, accompanied by expression of multiple genes implicated in actin polymerization, focal adhesion, and neuronal migration. Furthermore, MYC protein expression is substantially repressed due to BASP1-mediated transcriptional MYC downregulation involving BASP1 interaction with ß-catenin and binding to the MYC promoter. Upon BASP1 activation, multiple key proteins of the canonical WNT signaling pathway become suppressed. One of these BASP1 targets is the protein kinase TNIK catalyzing phosphorylation of TCF4, the latter required for transcriptional MYC activation. Results obtained with a preclinical TNIK inhibitor in human colorectal cancer cells show efficient abrogation of MYC expression and consequently impaired dimerization with its interaction partner MAX. The antagonistic BASP1 effect on MYC and the MYC dependency on TNIK could enhance the development of strategies to interfere with oncogenic functions of the cancer driver MYC. Overall design: RNA-seq profiling of SW480 colon cancer cells and comparison with SW480 expressing ectopic BASP1 and SW480 with CRISPR-activated endogenous BASP1.