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

• Published in: Molecular carcinogenesis 2007-04, Vol.46 (4), p.315-329
• Main Authors: Dillard, Alice C., Lane, Michelle A.
• Format: Article
• Language: English
• Subjects: beta Catenin - antagonists & inhibitors; Cell Line, Tumor; colon cancer; Colonic Neoplasms - drug therapy; Colonic Neoplasms - metabolism; Colonic Neoplasms - pathology; Drug Resistance, Neoplasm; Gene Expression Regulation, Neoplastic - drug effects; Humans; Receptors, Retinoic Acid - metabolism; RXR; Signal Transduction - drug effects; Tretinoin - pharmacology; ubiquitin; vitamin A; Vitamin A - pharmacology; Vitamin A - therapeutic use; Vitamins - pharmacology; Vitamins - therapeutic use; β-catenin
• ISSN: 0899-1987; 1098-2744
• DOI: 10.1002/mc.20280

The β‐catenin signaling pathway is dysregulated in most cases of colon cancer resulting in an accumulation of nuclear β‐catenin and increased transcription of genes involved in tumor progression. This study examines the effect of retinol on β‐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 β‐catenin protein levels and increased ubiquitinated β‐catenin in all cell lines. Treatment with the proteasomal inhibitor MG132 blocked the retinol‐induced decrease in β‐catenin indicating retinol decreases β‐catenin by increasing proteasomal degradation. Multiple pathways direct β‐catenin to the proteasome for degradation including a p53/Siah‐1/adenomatous polyposis coli (APC), a Wnt/glycogen synthase kinase‐3β/APC, and a retinoid “X” receptor (RXR)‐mediated pathway. Due to mutations in β‐catenin (HCT‐116), APC (SW620), and p53 (WiDr), only the RXR‐mediated pathway remains functional in each cell line. To determine if RXRs facilitate β‐catenin degradation, cells were treated with the RXR pan‐antagonist, PA452, or transfected with RXRα small interfering RNA (siRNA). The RXR pan‐antagonist and RXRα siRNA reduced the ability of retinol to decrease β‐catenin protein levels. Nuclear β‐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 β‐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 β‐catenin via a mechanism potentially involving RXR. © 2007 Wiley‐Liss, Inc.