Retinol increases beta-catenin-RXR alpha binding leading to the increased proteasomal degradation of beta-catenin and RXR alpha

Retinol utilizes a retinoid X receptor (RXR)-mediated degradation pathway to decrease beta-catenin protein in all-trans retinoic acid (ATRA)-resistant human colon cancer cells. In this study, we examined interactions between RXR alpha and beta-catenin in ATRA-resistant human colon cancer cells treated with retinol. Retinol treatment triggers relocation of beta-catenin and RXR alpha proteins. Cells treated with retinol for 8 and 24 h displayed increased cytosolic but decreased nuclear beta-catenin and RXRa. Retinol treatment increased beta-catenin and RXR alpha protein interaction. Previously, we showed that 24 h of retinol treatment increased RXR alpha protein. Here we show this increase in RXRct levels is due to increased RXR alpha messenger RNA. Treatment with 48 h with retinol decreased RXR alpha protein levels. Last, by transfecting HCT-116 cells with a RXR alpha construct lacking the activation function-1 and DNA binding domains, we show RXR alpha and beta-catenin binding is required for proteosomal degradation of beta-catenin. These results suggest retinol induces RXR alpha and beta-catenin binding and transport to the cytosol where they are proteasomally degraded.