Adenomatous Polyposis Coli (APC)-independent Regulation of β-Catenin Degradation via a Retinoid X Receptor-mediated Pathway

Abstract

␤-catenin is a component of stable cell adherent complexes whereas its free form functions as a transcription factor that regulate genes involved in oncogenesis and metastasis. Free ␤-catenin is eliminated by two adenomatous polyposis coli (APC)-dependent proteasomal degradation pathways regulated by glycogen synthase kinase 3␤ (GSK3␤) or p53-inducible Siah-1. Dysregulation of ␤-catenin turnover consequent to mutations in critical genes of the APC-dependent pathways is implicated in cancers such as colorectal cancer. We have identified a novel retinoid X receptor (RXR)-mediated APC-independent pathway in the regulation of ␤-catenin. In this proteasomal pathway, RXR agonists induce degradation of ␤-catenin and RXR␣ and repress ␤-catenin-mediated transcription. In vivo, ␤-catenin interacts with RXR␣ in the absence of ligand, but RXR agonists enhanced the interaction. RXR agonist action was not impaired by GSK3␤ inhibitors or deletion of the GSK3␤-targeted sequence from ␤-catenin. In APC-and p53-mutated colorectal cancer cells, RXR agonists still inactivated endogenous ␤-catenin via RXR␣. Interestingly, deletion of the RXR␣ A/B region abolished ligand-induced ␤-catenin degradation but not RXR␣-mediated transactivation. RXR␣-mediated inactivation of oncogenic ␤-catenin paralleled a reduction in cell proliferation. These results suggest a potential role for RXR and its agonists in the regulation of ␤-catenin turnover and related biological events.

Peroxisome Proliferator-activated Receptor γ Activation Can Regulate β-Catenin Levels via a Proteasome-mediated and Adenomatous Polyposis Coli-independent Pathway

“…A similar mechanism might be activated following stimulation of PPAR␥, because the PPAR␥2-mediated pathway of ␤-catenin degradation also involved the N-terminal (1-131 amino acids) fragment of ␤-catenin. Proteasome-mediated degradation of PPAR␥ was shown to be independent of the transcriptional activity of the receptor and was dependent upon the activation function 2 (AF2) domain (69) as was also reported in the case of RXR-mediated ␤-catenin degradation (52). It is still unclear whether the same domain of PPAR␥2 was involved in mediating ␤-catenin degradation.…”

“…It is thus possible that PPAR␥ stimulation leads to the activation of a novel proteasomal pathway, which is capable of degrading proteins involved in regulating important cellular events (example: growth and differentiation). In a very recent study a similar novel mechanism of ␤-catenin degradation has been reported to be activated by the retinoid X receptor (RXR) (52). Additionally, studies have shown that suppression of ␤-catenin-mediated signaling via nuclear recepassays were performed as in Fig.…”

“…Finally SDS sample buffer was added to the pellets, and the denatured protein was resolved on SDS-PAGE followed by Western blotting. To determine any interaction between PPAR␥2 and ␤-catenin, an in vivo cross-linking of the proteins using dithiobis(succinimidylpropionate) (Pierce, IL) was performed prior to immunoprecipitation as described earlier (52).…”

“…PPAR␥2 Interacted with ␤-Catenin in Vivo, Which Was Enhanced in the Presence of TZD-Recent studies by Xiao et al (52) have shown that degradation of ␤-catenin by the retinoic acid receptor RXR was also mediated by an APC-independent proteasomal degradation pathway involving an in vivo interaction of RXR with ␤-catenin. To determine whether PPAR␥2 and ␤-catenin also interacted with each other in vivo, we performed studies utilizing the P␥ and P␥-S37A-␤-catenin cells and subjected them to cross-linking prior to immunoprecipitation following treatment with TZD (see "Experimental Procedures").…”

Casein Kinase 1α Interacts with Retinoid X Receptor and Interferes with Agonist-induced Apoptosis

“…RXR␣ binds insulinlike growth factor-binding protein-3 and is necessary for the induction of apoptosis by insulin-like growth factor-binding protein-3 or RXR agonists in cancer cells (18). We have shown that RXR agonists affect the proliferation of cells by promoting interaction of RXR␣ with ␤-catenin and the degradation of both of these molecules (19). The variety of roles that RXRs can play in regulating cell survival leads to the possibility that tumor cells may circumvent RXR-driven apoptosis.…”