Interaction and Functional Cooperation between the LIM Protein FHL2, CBP/p300, and β-Catenin

Transcriptional activation of gene expression by Wnt signaling is driven by the association of β-catenin with TCF/LEF factors and the recruitment of transcriptional coactivators. It has been shown that the LIM protein FHL2 and the acetyltransferase CBP/p300 individually stimulate β-catenin transactivating activity and that β-catenin is acetylated by p300. Here, we report that FHL2 and CBP/p300 synergistically enhanced β-catenin/TCF-mediated transcription from Wnt-responsive promoters and that the acetyltransferase activity of CBP/p300 was involved in the cooperation. CBP/p300 interacted directly with FHL2, predominantly through the CH3 domain but not the histone acetyltransferase domain, and different regions of CBP/p300 were involved in FHL2 and β-catenin binding. We provided evidence for the formation of a ternary complex by FHL2, CBP/p300, and β-catenin and for colocalization of the three proteins in the nucleus. In murine FHL2(−/−) embryo fibroblasts, the transactivation activity of β-catenin/TCF was markedly reduced, and this defect could be restored by exogenous expression of FHL2. However, CBP/p300 were still able to coactivate the β-catenin/TCF complex in FHL2(−/−) cells, suggesting that FHL2 is dispensable for the coactivator function of CBP/p300 on β-catenin. Furthermore, we found that FHL2 significantly increased acetylation of β-catenin by p300 in vivo. Finally, we showed that FHL2, CBP/p300, and β-catenin could synergistically activate androgen receptor-mediated transcription, indicating that the synergistic coactivator function is not restricted to TCF/LEF

Acetylation of β-Catenin by p300 Regulates β-Catenin-Tcf4 Interaction

Lysine acetylation modulates the activities of nonhistone regulatory proteins and plays a critical role in the regulation of cellular gene transcription. In this study, we showed that the transcriptional coactivator p300 acetylated β-catenin at lysine 345, located in arm repeat 6, in vitro and in vivo. Acetylation of this residue increased the affinity of β-catenin for Tcf4, and the cellular Tcf4-bound pool of β-catenin was significantly enriched in acetylated form. We demonstrated that the acetyltransferase activity of p300 was required for efficient activation of transcription mediated by β-catenin/Tcf4 and that the cooperation between p300 and β-catenin was severely reduced by the K345R mutation, implying that acetylation of β-catenin plays a part in the coactivation of β-catenin by p300. Interestingly, acetylation of β-catenin had opposite, negative effects on the binding of β-catenin to the androgen receptor. Our data suggest that acetylation of β-catenin in the arm 6 domain regulates β-catenin transcriptional activity by differentially modulating its affinity for Tcf4 and the androgen receptor. Thus, our results describe a new mechanism by which p300 might regulate β-catenin transcriptional activity

The LIM-Only Protein FHL2 Mediates Ras-Induced Transformation through Cyclin D1 and p53 Pathways

Background: Four and a half LIM-only protein 2 (FHL2) has been implicated in multiple signaling pathways that regulate cell growth and tissue homeostasis. We reported previously that FHL2 regulates cyclin D1 expression and that immortalized FHL2-null mouse embryo fibroblasts (MEFs) display reduced levels of cyclin D1 and low proliferative activity. Methodology/Principal Findings: Here we address the contribution of FHL2 in cell transformation by investigating the effects of oncogenic Ras in FHL2-null context. We show that H-RasV12 provokes cell cycle arrest accompanied by accumulation of p53 and p16 INK4a in immortalized FHL2 2/2 MEFs. These features contrast sharply with Ras transforming activity in wild type cell lines. We further show that establishment of FHL2-null cell lines differs from conventional immortalization scheme by retaining functional p19 ARF /p53 checkpoint that is required for cell cycle arrest imposed by Ras. However, after serial passages of Ras-expressing FHL2 2/2 cells, dramatic increase in the levels of D-type cyclins and Rb phosphorylation correlates with the onset of cell proliferation and transformation without disrupting the p19 ARF /p53 pathway. Interestingly, primary FHL2-null cells overexpressing cyclin D1 undergo a classical immortalization process leading to loss of the p19 ARF /p53 checkpoint and susceptibility to Ras transformation. Conclusions/Significance: Our findings uncover a novel aspect of cellular responses to mitogenic stimulation and illustrat

The LIM-only protein FHL2 regulates cyclin D1 expression and cell proliferation.

International audienceThe LIM-only protein FHL2 acts as a transcriptional modulator that positively or negatively regulates multiple signaling pathways. We recently reported that FHL2 cooperates with CBP/p300 in the activation of ss-catenin/TCF target gene cyclin D1. In this paper, we demonstrate that FHL2 is associated with the cyclin D1 promoter at the TCF/CRE site, providing evidence that cyclin D1 is a direct target of FHL2. We show that deficiency of FHL2 greatly reduces the proliferative capacity of spontaneously immortalized mouse fibroblasts which is associated with decreased expression of cyclin D1 and p16INK4a, and hypophosphorylation of Rb. Reexpression of FHL2 in FHL2-null fibroblasts efficiently restores cyclin D1 levels and cell proliferative capacity, indicating that FHL2 is critical for cyclin D1 activation and cell growth. Moreover, ectopic cyclin D1 expression is sufficient to override growth inhibition of immortalized FHL2-null fibroblasts. Gene expression profiling revealed that FHL2 deficiency triggers a broad change of the cell cycle program that is associated with downregulation of several G1/S and G2/M cyclins, E2F transcription factors and DNA replication machinery, thus correlating with reduced cell proliferation. This change also involves downregulation of the negative cell cycle regulators, particularly INK4 inhibitors, which could counteract the decreased expression of cyclins, allowing cells to grow. Our study illustrates that FHL2 can act on different aspects of the cell cycle program to finely regulate cell proliferation

Tbx3 Is a Downstream Target of the Wnt/β-Catenin Pathway and a Critical Mediator of β-Catenin Survival Functions in Liver Cancer

International audienceTbx3 encodes a transcriptional repressor that is important for diverse patterning events during development, and Tbx3 mutation in humans causes the ulnar-mammary syndrome. Here, we describe the identification of Tbx3 in array-based search for genes downstream Wnt/beta-catenin that are implicated in liver tumorigenesis. Overexpression of Tbx3 is closely associated with the mutational status of beta-catenin in murine liver tumors induced by Myc as well as in human hepatocellular carcinomas and hepatoblastomas. Moreover, Tbx3 transcription is activated by ectopic expression of beta-catenin in mouse liver and in human tumor cell lines. Evidence that Tbx3 transcription is directly regulated by beta-catenin is provided by chromatin immunoprecipitation and reporter assays. Although HepG2 cells stably transfected with Tbx3 display moderately enhanced growth rate, the dominant negative mutant Tbx3-Y149S drastically inhibits hepatoma cell growth in vitro and in vivo. Moreover, small interfering RNAs (siRNA) directed against Tbx3 inhibit anchorage-independent growth of liver and colon carcinoma cells. We further show that inhibition of Tbx3 expression by specific siRNAs blocks beta-catenin-mediated cell survival and renders cells sensitive to doxorubicin-induced apoptosis. Conversely, ectopic expression of Tbx3 inhibits apoptosis induced by beta-catenin depletion. Marked overexpression of Tbx3 in a subset of hepatoblastomas is associated with chemotherapy-resistant phenotype and unfavorable patient outcome. These results reveal an unsuspected role of Tbx3 as a mediator of beta-catenin activities on cell proliferation and survival and as an important player in liver tumorigenesis

Immortalized <i>FHL2^−/−</i> MEFs stably expressing cyclin D1 are susceptible to transformation by oncogenic Ras.

<p>(A) Increased cyclin D1 expression in immortalized <i>FHL2^−/−</i> MEFs after introduction of cyclin D1-HA transgene was confirmed by immunoblotting with anti-cyclin D1 and anti-HA antibodies. (B) Representative phase contrast images of immortalized wt and <i>cyclin D1-HA/FHL2^−/−</i> cell lines transduced with H-RasV12 or empty vector. (C) Soft agar assay. Cells were initially seeded at 5×10⁴ cells/plate and cultured for 10 days. Images are representative of three wt and two <i>cyclin D1-HA/FHL2^−/−</i> cell lines transduced with H-RasV12. (D) Western blot analysis of p53 induction by doxorubicin in the indicated cell lines as described in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0003761#pone-0003761-g003" target="_blank">Fig. 3A</a>.</p