title: Retinol decreases beta-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.
authors.affiliation: Department of Human Ecology, Division of Nutritional Sciences, The University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: Alice C
authors.initials: AC
authors.lastname: Dillard
authors.forename: Michelle A
authors.initials: MA
authors.lastname: Lane
mesh_terms: D045744:Cell Line, Tumor; D003110:Colonic Neoplasms / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D019008:Drug Resistance, Neoplasm; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D006801:Humans; D018168:Receptors, Retinoic Acid / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D014212:Tretinoin / Q000494:pharmacology; D014801:Vitamin A / Q000494:pharmacology* / Q000627:therapeutic use; D014815:Vitamins / Q000494:pharmacology* / Q000627:therapeutic use; D051176:beta Catenin / Q000037:antagonists & inhibitors*
chemical_list: D018168:Receptors, Retinoic Acid; D014815:Vitamins; D051176:beta Catenin; D014801:Vitamin A; D014212:Tretinoin