Abstract
Deregulated Wnt signaling initiates most cases of colorectal cancer (CRC). Butyrate, a product of dietary fiber, hyperactivates Wnt signaling, resulting in induction of CRC cell apoptosis, which may in part explain the protective action of fiber. Nonsense mediated decay (NMD) of mRNAs containing premature stop codons (PTCs) affects tumorigenesis and upregulates Wnt signaling in human embryonic stem cells. However, it is unknown how NMD affects Wnt activity in CRC cells that exhibit constitutively activated Wnt signaling. We hypothesize that expression of genes that contain PTCs modulates Wnt signaling and response to butyrate in CRC cells. Amlexanox is a clinically utilized anti-allergic and anti-inflammatory drug that inhibits NMD and promotes PTC read-through. Therefore, Amlexanox is a relevant agent for assessing the role of NMD and PTC read-through in the response of CRC cells to butyrate. To test our hypothesis, we treated HCT-116 CRC cells with Amlexanox and determined effects on Wnt signaling levels, apoptosis, and response to butyrate. Amlexanox enhanced basal Wnt signaling levels; however, it repressed butyrate-induced Wnt signaling hyperactivation and suppressed apoptosis. To evaluate the contribution of NMD and altered expression of PTC-containing genes to these effects, we upregulated NMD by overexpression of up-frameshift protein 1 (UPF1), and observed effects opposite to these of Amlexanox (i.e., Wnt signaling hyperactivation by butyrate was enhanced and levels of apoptosis were increased). Our results support the possibility that altered expression of PTC-containing genes affects butyrate sensitivity of CRC cells.