Retinol decreases β‐catenin protein levels in retinoic acid‐resistant colon cancer cell lines

Abstract

The beta-catenin signaling pathway is dysregulated in most cases of colon cancer resulting in an accumulation of nuclear beta-catenin and increased transcription of genes involved in tumor progression. This study examines the effect of retinol on beta-catenin protein levels in three all-trans retinoic acid (ATRA)-resistant human colon cancer cell lines: HCT-116, WiDr, and SW620. Each cell line was treated with increasing concentrations of retinol for 24 or 48 h. Retinol reduced beta-catenin protein levels and increased ubiquitinated beta-catenin in all cell lines. Treatment with the proteasomal inhibitor MG132 blocked the retinol-induced decrease in beta-catenin indicating retinol decreases beta-catenin by increasing proteasomal degradation. Multiple pathways direct beta-catenin to the proteasome for degradation including a p53/Siah-1/adenomatous polyposis coli (APC), a Wnt/glycogen synthase kinase-3beta/APC, and a retinoid "X" receptor (RXR)-mediated pathway. Due to mutations in beta-catenin (HCT-116), APC (SW620), and p53 (WiDr), only the RXR-mediated pathway remains functional in each cell line. To determine if RXRs facilitate beta-catenin degradation, cells were treated with the RXR pan-antagonist, PA452, or transfected with RXRalpha small interfering RNA (siRNA). The RXR pan-antagonist and RXRalpha siRNA reduced the ability of retinol to decrease beta-catenin protein levels. Nuclear beta-catenin induces gene transcription via interaction with T cell factor/lymphoid enhancer factor (TCF/LEF) proteins. Retinol treatment decreased the transcription of a TOPFlash reporter construct and mRNA levels of the endogenous beta-catenin target genes, cyclin D1 and c-myc. These results indicate that retinol may reduce colon cancer cell growth by increasing the proteasomal degradation of beta-catenin via a mechanism potentially involving RXR.

A direct protein-protein interaction is involved in the suppression of β-catenin transcription by retinoid X receptor α in colorectal cancer cells

“…[25][26][27] But, most studies reported that nuclear receptors repressed β-catenin signaling in the presence of ligand or agonist. 16,17,23,24 However, our data in the first time demonstrated that RXRα overexpression directly inhibited endogenous and exogenous β-catenin transcriptional activity and expression in the absence of RXR agonist. However, the inhibition was abrogated by targeted RXRα small RNA interfering.…”

“…21,22 Recently, several lines of evidence showed that nuclear receptors affected β-catenin/TCF/LEF-mediated gene transcription and β-catenin protein. For example, retinoid-activated RAR acts as a potent repressor of β-catenin/TCF signaling in retinoid-sensitive colorectal cancer cells, 16,17,23,24 and activation of the vitamin D receptor with its metabolite ligand, 1α,25(OH) 2 vitamin D 3 , could repress Wnt/β-catenin/TCF signaling. [25][26][27] But, most studies reported that nuclear receptors repressed β-catenin signaling in the presence of ligand or agonist.…”

Vitamin antioxidants, lipid peroxidation, tumour stage, the systemic inflammatory response and survival in patients with colorectal cancer

“…A number of reports have indicated that retinol can alter gene transcription via different mechanisms. [34][35][36][37][38] In particular, 2 classes of nuclear receptors for retinoic acid, RAR and RXR, were implicated in the effects of retinol on in vivo. 34 These receptors form heterodimers to bind to specific retinoic-acid-response elements, and have various cell-type dependent effects on cell signaling pathways, apoptosis and cell cycle arrests.…”

“…It was suggested that retinol can reduce beta-catenin protein levels, which is known to be able to increase transcription of genes involved in tumor progression. 35,36 In addition, retinol may also reduce matrix metalloproteinase mRNA, protein and activity levels. 37 The chemoprevention and therapeutic potential of retinoids was subsequently explored in some cancer types, with particular success in acute promyelocytic leukaemia.…”

Retinol Increases β-Catenin-RXRα Binding Leading to the Increased Proteasomal Degradation of β-Catenin and RXRα

“…The RXR agonist decreased both β-catenin and RXRα proteins independent of the GSK-3β/APC and p53/Siah-1 degradation pathways. Previously, we showed that retinol, which is not a RXR ligand, increases the proteosomal degradation of β-catenin via a RXR-mediated pathway, but the retinol-induced activation of the RXR pathway occurred independent of a RXR agonist (14). We also demonstrated that the RXR antagonist, PA452, and the knockdown of RXRα, using RXRα small interfering RNA, inhibited the ability of retinol to decrease total cellular β-catenin levels in both cell lines, suggesting RXRα facilitates the ability of retinol to decrease β-catenin protein levels (14).…”