Valproic acid exhibits different cell growth arrest effect in three HPV-positive/negative cervical cancer cells and possibly via inducing Notch1 cleavage and E6 downregulation

We investigated the effect of valproic acid (VPA), a histone deacetylase (HDAC) inhibitor, and the mechanism of VPA-induced growth inhibition on three cervical Cancer cell lines with different molecular and genetic background. We found that VPA induced proliferation suppression, cell Apoptosis and cell cycle arrest in all tested cell lines, with an increase of Notch1 active form ICN1 as a tumor suppressor and its target gene HES1. Noteworthy, blocking of Notch signaling with DAPT resulted in growth inhibition in ICN1-overexpressing CaSki and HT-3 cells. Thus, endogenous Notch signaling may be necessary for survival of ICN1-overexpressing cervical Cancer cell lines. Furthermore, G1 phase arrest was induced in HeLa and CaSki cells by VPA while G2 phase arrest was induced in HT-3 cells, suggesting different mechanism in this cycle arrest. We also found VPA suppressed oncogene E6 in a Notch-independent manner, and induced significant Apoptosis in E6-overexpressing HPV positive CaSki cells. Cell morphological change was also observed in HeLa and HT-3 cell lines after VPA treatment with an upregulation of EMT transcription factor Snail1. Notch signaling inhibitor DAPT partly reversed VPA-induced Snail1 upregulation in HeLa cells. This discovery supports that VPA may induce EMT at least partly via Notch activation.