G1/S arrest induced by histone deacetylase inhibitor sodium butyrate in E1A + Ras-transformed cells is mediated through down-regulation of E2F activity and stabilization of β-catenin

Abstract

Tumor cells are often characterized by a high and growth factor-independent proliferation rate. We have previously shown that REF cells transformed with oncogenes E1A and c-Ha-Ras do not undergo G1/S arrest of the cell cycle after treatment with genotoxic factors. In this work, we used sodium butyrate, a histone deacetylase inhibitor, to show that E1A + Ras transformants were able to stop proliferation and undergo G1/S arrest. Apart from inducing G1/S arrest, sodium butyrate was shown to change expression of a number of cell cycle regulatory genes. It down-regulated cyclins D1, E, and A as well as c-myc and cdc25A and up-regulated the cyclin-kinase inhibitor p21waf1. Accordingly, activities of cyclin E-Cdk2 and cyclin A-Cdk2 complexes in sodium butyrate-treated cells were decreased substantially. Strikingly, E2F1 expression was also down-modulated at the levels of gene transcription, the protein content, and the E2F transactivating capability. To further study the role of p21waf1 in the sodium butyrate-induced G1/S arrest and the E2F1 down-modulation, we established E1A + Ras transformants from mouse embryo fibroblast cells with deletion of the cdkn1a (p21waf1) gene. Despite the absence of p21waf1, sodium butyrate-treated mERas transformants reveal a slightly delayed G1/S arrest as well as down-modulation of E2F1 activity, implying that the observed effects are mediated through an alternative p21waf1-independent signaling pathway. Subsequent analysis showed that sodium butyrate induced accumulation of β-catenin, a downstream component of the Wnt signaling. The results obtained indicate that the antiproliferative effect of histone deacetylase inhibitors on E1A + Ras-transformed cells can be mediated, alongside other mechanisms, through down-regulation of E2F activity and stabilization of β-catenin.