Overexpression of beta-catenin induces apoptosis independent of its transactivation function with LEF-1 or the involvement of major G1 cell cycle regulators.
beta-Catenin promotes epithelial architecture by forming cell surface complexes with E-cadherin and also interacts with TCF/LEF-1 in the nucleus to control gene expression. By DNA transfection, we overexpressed beta-catenin and/or LEF-1 in NIH 3T3 fibroblasts, corneal fibroblasts, corneal epithelia, uveal melanoma cells, and several carcinoma cell lines. In all cases (with or without LEF-1), the abundant exogenous beta-catenin localizes to the nucleus and forms distinct nuclear aggregates that are not associated with DNA. Surprisingly, we found that with time (5-8 d after transfection) cells overexpressing beta-catenin all undergo apoptosis. LEF-1 does not need to be present. Moreover, LEF-1 overexpression in the absence of exogenous beta-catenin does not induce apoptosis, even though some endogenous beta-catenin moves with the exogenous LEF-1 into the nucleus. TOPFLASH/FOPFLASH reporter assays showed that full-length beta-catenin is able to induce LEF-1-dependent transactivation, whereas Arm beta-catenin totally abolishes the transactivating function. However, Arm beta-catenin, containing deletions of known LEF-1-transactivating domains, has the same apoptotic effects as full-length beta-catenin. Overexpressed beta-catenin also induces apoptosis in cells transfected with nuclear localization signal-deleted LEF-1 that localizes only in the cytoplasm. Thus, the apoptotic effects of overexpressed exogenous beta-catenin do not rely on its transactivating function with nuclear LEF-1. Overexpressed delta-catenin, containing 10 Arm repeats, induces only minor apoptosis, suggesting that the major apoptotic effect may be due to domains specific to beta-catenin as well as to Arm repeats. The absence of p53, Rb, cyclin D1, or E2F1 does not affect the apoptotic effect of overexpressed beta-catenin, but Bcl-x(L) reduces it. We hypothesize that in vivo apoptosis of cells overexpressing beta-catenin might be a physiological mechanism to eliminate them from the population.
Kim
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Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115, USA.
Pang
K M
KM
Evans
M
M
Hay
E D
ED
D016475:3T3 Cells; D000818:Animals; D017209:Apoptosis; D015820:Cadherins / Q000235:genetics / Q000502:physiology; D002453:Cell Cycle / Q000502:physiology*; D002467:Cell Nucleus / Q000502:physiology / Q000648:ultrastructure; D003598:Cytoskeletal Proteins / Q000172:deficiency / Q000235:genetics / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D016193:G1 Phase; D016161:Genes, Retinoblastoma; D006367:HeLa Cells; D006801:Humans; D007700:Kinetics; D051821:Lymphoid Enhancer-Binding Factor 1; D051379:Mice; D018345:Mice, Knockout; D011994:Recombinant Proteins / Q000378:metabolism; D016160:Retinoblastoma Protein / Q000172:deficiency / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015534:Trans-Activators; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D014162:Transfection; D014407:Tumor Cells, Cultured; D051176:beta Catenin
C495270:CTNNB1 protein, human; C495267:CTNNB1 protein, mouse; D015820:Cadherins; D003598:Cytoskeletal Proteins; D004268:DNA-Binding Proteins; C497573:LEF1 protein, human; C497574:Lef1 protein, mouse; D051821:Lymphoid Enhancer-Binding Factor 1; D011994:Recombinant Proteins; D016160:Retinoblastoma Protein; D015534:Trans-Activators; D014157:Transcription Factors; D051176:beta Catenin