Down-regulation of the Wnt/β-catenin signaling pathway by Cacnb4

The β₄ isoform of the β-subunits of voltage-gated calcium channel regulates cell proliferation and cell cycle progression. Herein we show that coexpression of the β₄-subunit with actors of the canonical Wnt/β-catenin signaling pathway in a hepatoma cell line inhibits Wnt-responsive gene transcription and decreases cell division, in agreement with the role of the Wnt pathway in cell proliferation. β₄-subunit-mediated inhibition of Wnt signaling is observed in the presence of LiCl, an inhibitor of glycogen synthase kinase (GSK3) that promotes β-catenin translocation to the nucleus. Expression of β₄-subunit mutants that lost the ability to translocate to the nucleus has no effect on Wnt signaling, suggesting that β₄-subunit inhibition of Wnt signaling occurs downstream from GSK3 and requires targeting of β₄-subunit to the nucleus. β₄-subunit coimmunoprecipitates with the TCF4 transcription factor and overexpression of TCF4 reverses the effect of β₄-subunit on the Wnt pathway. We thus propose that the interaction of nuclear β₄-subunit with TCF4 prevents β-catenin binding to TCF4 and leads to the inhibition of the Wnt-responsive gene transcription. Thereby, our results show that β₄-subunit is a TCF4 repressor and therefore appears as an interesting candidate for the regulation of this pathway in neurons where β₄-subunit is specifically expressed.