Abstract 2428: Redox regulation of β-catenin in colorectal cancer

Colorectal cancer (CRC) is the second most newly diagnosed cancer in the United States. β-catenin is the downstream effector of the Wnt/β-catenin signaling pathway. Loss of function (LOF) mutations on adenomatous polyposis coli (APC), a critical member of the β-catenin degradation complex leads to constitutively elevated β-catenin as an early oncogenic event in 85% of CRCs and is associated with decreased CRC patient survival. Increased level of β-catenin augments the nuclear accumulation of β-catenin, where it acts a co-activator of a number of transcription factors leading to the expression of downstream target genes that promote cancer cell proliferation, survival, and migration. There is no effective therapy for β-catenin as a cause of cancer. In addition to CRC, elevated levels of Wnt signaling have been reported in leukemia, melanoma, and breast cancers highlighting the need for novel therapies that attenuate aberrant β-catenin mediated transcription. Here we report on the redox dependency of the Wnt/β-catenin signaling pathway, identified through a forward genetic screen in Drosophila, and identify thioredoxin reductase-1 (TXNR1) as a potential target for the inhibition of aberrant β-catenin transcriptional activity through in vivo studies with the APCmin/+ mouse model. Drosophila harboring a mutation in their thioredoxin reductase (trxr-1) gene and a partial lethality phenotype were used in an enhancer/suppressor forward genetic screen. Increased lethality was observed when the trxr-1 mutation (trxr-1481) was combined with heterozygous mutations and deletions in wnt2, while wnt2 deletion and mutations alone had no effect on lethality. To test the translational value of these findings, APCmin/+ mice were treated with PX-12, a human TXNR1 inhibitor, daily by gavage from day 40 to time of death. We observed a dose dependent reduction in the number and size of intestinal polyps by PX-12 treatment, and an increased survival in treated mice. Furthermore, mechanistic studies utilizing human CRC cell lines harboring LOF mutations to the β-catenin degradation complex revealed β-Catenin mediated transcription, as measured by TOP Flash luciferase assay, was diminished when TXNR1 was silenced by siRNA. Additionally, we observed decreased β-catenin protein level in CRC cell lines treated with PX-12 or siRNA targeting TXNRD1 and its functional partner thioredoxin (TXN). The addition of the pan caspase inhibitor ZVAD but not the proteasome inhibitor MG132 was able to stabilize β-catenin protein upon treatment with PX-12 suggesting β-catenin destabilization is occurring via a caspase dependent mechanism. These results suggest a conserved mechanism for the redox regulation of β-catenin involving TXNR1; furthermore, inhibition of TXNR1 activity destabilizes β-catenin protein and decreases β-catenin mediated transcription in human CRC cell lines and inhibits intestinal neoplasia development in an experimental mouse model for CRC. Citation Format: Alejandro D. Campos, Brian James, Petrus De Jong, Robert Lemos, Nikolas Marino, Garth Powis. Redox regulation of β-catenin in colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2428.