The APC Tumor Suppressor Binds to C-Terminal Binding Protein to Divert Nuclear β-Catenin from TCF

TCF-4 isoforms absent in TCF-4 mutated MSI-H colorectal cancer cells colocalize with nuclear CtBP and repress TCF-4-mediated transcription

TCF-4 is the main effector of the Wnt/Wingless signalling pathway. As with other TCF/LEF factors, numerous alternative splicings at its 3' end affect its expression. Such a mechanism leads to the synthesis of numerous TCF-4 isoforms among which some contain binding domains for CtBP, an ubiquitous transcriptional corepressor. Of interest, we described a frequent TCF-4 frameshift mutation in mismatch-repair deficient colorectal cancers (MSI-H cancers) that leads to the selective loss of TCF-4 isoforms with CtBP binding abilities. We provide here data that argue for a partial colocalization of CtBP with TCF-4 isoforms containing CtBP binding domains in cellulo, and for a functional role of CtBP in repressing TCF-4 mediated transcription. We also demonstrate that such a colocalization is not observed in MSI-H colorectal cancer cells that harbour the TCF-4 frameshift mutation, and that CtBP is not able to repress TCF-4-mediated transcription in this context. Taken together, our results strongly suggest that CtBP would play a role in regulating TCF-4 mediated transcription upon its binding with some TCF-4 isoforms encoded by alternatively spliced mRNA. They also suggest a role for TCF-4 frameshift mutation during MSI-H colorectal tumour progression, by regulating the relative proportion of the different TCF-4 isoforms.

The APC tumor suppressor counteracts beta-catenin activation and H3K4 methylation at Wnt target genes

The APC tumor suppressor controls the stability and nuclear export of beta-catenin (beta-cat), a transcriptional coactivator of LEF-1/TCF HMG proteins in the Wnt/Wg signaling pathway. We show here that beta-cat and APC have opposing actions at Wnt target genes in vivo. The beta-cat C-terminal activation domain associates with TRRAP/TIP60 and mixed-lineage-leukemia (MLL1/MLL2) SET1-type chromatin-modifying complexes in vitro, and we show that beta-cat promotes H3K4 trimethylation at the c-Myc gene in vivo. H3K4 trimethylation in vivo requires prior ubiquitination of H2B, and we find that ubiquitin is necessary for transcription initiation on chromatin but not nonchromatin templates in vitro. Chromatin immunoprecipitation experiments reveal that beta-cat recruits Pygopus, Bcl-9/Legless, and MLL/SET1-type complexes to the c-Myc enhancer together with the negative Wnt regulators, APC, and betaTrCP. Interestingly, APC-mediated repression of c-Myc transcription in HT29-APC colorectal cancer cells is initiated by the transient binding of APC, betaTrCP, and the CtBP corepressor to the c-Myc enhancer, followed by stable binding of the TLE-1 and HDAC1 corepressors. Moreover, nuclear CtBP physically associates with full-length APC, but not with mutant SW480 or HT29 APC proteins. We conclude that, in addition to regulating the stability of beta-cat, APC facilitates CtBP-mediated repression of Wnt target genes in normal, but not in colorectal cancer cells.

Acetylation by p300 Regulates Nuclear Localization and Function of the Transcriptional Corepressor CtBP2

CtBP family members, CtBP1 and CtBP2, are unique transcriptional regulators that adapt a metabolic enzyme fold, and their activities are regulated by NAD(H)-binding. CtBP1 is both cytoplasmic and nuclear, and its subcellular localization is regulated by sumoylation, phosphorylation, and binding to a PDZ protein. In contrast, we showed that CtBP2 is exclusively nuclear. CtBP1 and CtBP2 are highly similar, but differ at the N-terminal 20 amino acid region. Substitution of the N-terminal domain of CtBP1 with the corresponding CtBP2 domain confers a dominant nuclear localization pattern to CtBP1. The N-terminal domain of CtBP2 contains three Lys residues. Our results show that these Lys residues are acetylated by the nuclear acetylase p300. Although all three Lys residues of CtBP2 (Lys-6, Lys-8, and Lys-10) appear to be acetylated, acetylation of Lys-10 is critical for nuclear localization. CtBP2 with a single amino acid substitution at Lys-10 (K10R) is predominantly localized in the cytoplasm. The cytoplasmic localization of the K10R mutant is correlated with enhanced nuclear export that is inhibited by leptomycin B. Furthermore, lack of acetylation at Lys-10 renders CtBP2 to be more efficient in repression of the E-cadherin promoter. Our studies have revealed the important roles of acetylation in regulating subcellular localization and transcriptional activity of CtBP2.

Endophilin and CtBP/BARS are not acyl transferases in endocytosis or Golgi fission

Endophilins have been proposed to have an enzymatic activity (a lysophosphatidic acid acyl transferase or LPAAT activity) that can make phosphatidic acid in membranes. This activity is thought to change the bilayer asymmetry in such a way that negative membrane curvature at the neck of a budding vesicle will be stabilized. An LPAAT activity has also been proposed for CtBP/BARS (carboxy-terminal binding protein/brefeldin A-ribosylated substrate), a transcription co-repressor that is implicated in dynamin-independent endocytosis and fission of the Golgi in mitosis. Here we show that the LPAAT activity associated with endophilin is a contaminant of the purification procedure and can be also found associated with the pleckstrin homology domain of dynamin. Likewise, the LPAAT activity associated with CtBP/BARS is also a co-purification artefact. The proposed locus of activity in endophilins includes the BAR domain, which has no catalytic site but instead senses positive membrane curvature. These data will prompt a re-evaluation of the molecular details of membrane budding.

Wnt signaling in the intestinal epithelium: from endoderm to cancer

The Wnt pathway controls cell fate during embryonic development. It also persists as a key regulator of homeostasis in adult self-renewing tissues. In these tissues, mutational deregulation of the Wnt cascade is closely associated with malignant transformation. The intestinal epithelium represents the best-understood example for the closely linked roles of Wnt signaling in homeostatic self-renewal and malignant transformation. In this review, we outline current understanding of the physiological role of Wnt signaling in intestinal biology. From this perspective, we then describe how mutational subversion of the Wnt cascade leads to colorectal cancer.