Abstract 5843: Truncated APC & AXIN2 expression contribute to maintaining β-catenin/WNT signaling at the “Just Right” level for CRC cell proliferation

Abstract Adenomatous polyposis coli (APC) mutation leads to increased WNT signaling and stem cell overpopulation, which drives development of colorectal cancer (CRC). APC mutations often encode a truncated protein that has loss of APC’s domains that mediate APC interaction with AXIN2. A substantial body of scientific evidence indicates that truncated APC is essential for CRC cell proliferation by maintaining beta-catenin levels and WNT signaling at the “just right” level (known as the “Just Right” Hypothesis). Thus, AXIN2 is of particular interest because both wild-type APC and AXIN2 are part of the beta-catenin destruction complex, which is essential for tumor-suppressing ability. Hypothesis: the degree of AXIN2 expression, in the context of truncated APC, is critical to maintain WNT-signaling activity at the “just right” level which is optimal for CRC cell proliferation. Accordingly, we evaluated the effect of AXIN2 levels in CRC cells that contain APC protein-truncating mutations on WNT signaling (Top Flash assay), expression of WNT-targeted genes, and CRC cell proliferation and differentiation. We found that modulating AXIN2 expression altered WNT signaling, expression of WNT-targeted genes, and CRC cell proliferation in CRC cell lines with different APC-truncation causing mutations. Indeed, western blot results showed that knock down of AXIN2 gene expression increased truncated-APC and AXIN1 protein expression, and decreased expression of several WNT-target genes (e.g. LGR5, CD44, c-Jun, c-Myc, Cyclin D1), particularly genes that have reported roles in cell cycle regulation. These findings were consistent with our independent bioinformatics analyses of NCI TCGA RNA-seq data on CRC patients. Overall, our study suggests that i) The beta-catenin destruction complex is still functional at a certain level despite APC-truncating mutations; ii) Perturbing AXIN2 expression leads to changes in WNT-signaling activity that deviate from a level that is “just right” for malignant cell proliferation, likely by affecting the function of the destruction complex; iii) Deviation from “just right” WNT signaling levels alters WNT-target gene expression and diminishes the ability of APC-mutant CRC cells to maintain uncontrolled proliferation. Citation Format: Chi Zhang, Caroline O. Facey, Katherine Funk, Lynn M. Opdenaker, Bruce M. Boman. Truncated APC & AXIN2 expression contribute to maintaining β-catenin/WNT signaling at the “Just Right” level for CRC cell proliferation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5843.