MCF-7 breast cancer cells stably overexpressing protein kinase C-α (MCF-7-PKC-α cells) exhibit reduced cell-cell adhesion and increased tumorigenicity in nude mice. We investigated the possibility that alterations in E-cadherin and catenins contribute to the unique phenotype of MCF-7-PKC-α cells. Northern and Western blotting indicated that MCF-7-PKC-α cells express abnormally low amounts of plakoglobin mRNA and protein, and undetectable levels of E-cadherin mRNA and protein. In contrast, even though MCF-7-PKC-α cells express low levels of β-catenin mRNA, they express undetectable levels of β-catenin protein, suggesting that post-transcriptional events further diminish β-catenin expression in these cells. Pulse-labeling of the cells with [35S]methionine showed that the half-life of β-catenin is less than 15 min in MCF-7-PKC-α cells, compared to over 2 h in MCF-7-Vector cells [MCF-7 cells transfected with pSV2M(2)6 vector only]. Incubation with LiCl to inactivate glycogen synthase kinase-3 (GSK-3) significantly prolonged the half-life of β-catenin in MCF-7-PKC-α cells, suggesting that the GSK-3-dependent degradation of β-catenin contributes to β-catenin instability in these cells. Northern and Western blotting indicated that Wnt-1, which also inhibits GSK-3 activity, is expressed by MCF-7-Vector cells, but not by MCF-7-PKC-α cells. Transfection of (S37A)β-catenin, which is resistant to GSK-3-dependent degradation, stimulated TCF/LEF-dependent luciferase expression from the pTOPFLASH reporter plasmid by 753-fold in MCF-7-PKC-α cells, and by 268-fold in MCF-7-Vector cells. Inactivation of GSK-3 by LiCl stimulated luciferase expression from the pTOPFLASH reporter plasmid by 12.4-fold in MCF-7-PKC-α cells, and by 4.8-fold in MCF-7-Vector cells. These results suggest that degradation of β-catenin by GSK-3 contributes to β-catenin instability in MCF-7-PKC-α cells, diminishing the ability of -catenin to act as a transcriptional co-activator. Reduced Wnt-1 expression by MCF-7-PKC-α cells may promote β-catenin degradation by enhancing GSK-3 activity. Loss of β-catenin-dependent cell-cell adhesion and transcription may contribute to the aggressive phenotype of MCF-7-PKC-α cells.

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