Histone Deacetylase Inhibition Induces Apoptosis and Cell Cycle Dysregulation in Human and Murine Cancer Cell Lines

Abstract

Carcinogenesis is a complex multistep process that requires tumor cells to grow rapidly while overcoming growth inhibitory signals and sustained challenges from the host immune response. Mutations within promoter or enhancer regions, along with epigenetic changes, can induce aberrant expression of genes that regulate differentiation, cell cycle, and apoptosis, all of which enhance potential for cellular transformation. In recent years, our understanding of the biological processes that influence the activation and repression of transcription have evolved to highlight the role of chromatin architecture, and how chromatin remodeling may be utilized for the potential therapeutic benefit of genetic disease. Histone deacetylase inhibitors (HDACi) are small molecule drugs that affect the balance between acetylation and deacetylation of proteins, ultimately influencing cellular processes including gene transcription. There are currently three FDA-approved HDACi on the market (FK228, SAHA, Panobinostat) for cancer treatment, all of which have high systemic toxicity. Therefore, there is need for development of less toxic HDACi with improved tumor specificity. HDACi can exert anti-tumor effects by inducing transcriptional changes in tumor suppressors through modulating acetylation of histones and/or transcription factors. We hypothesize that the high systemic toxicity of the current HDACi is due to their non-selective HDAC activity and that more class-specific HDACi would result in less potent effects in cellular proliferation, death, and off target activity. The present studies tested this hypothesis by comparing effects of multiple HDACi on cell cycle progression and cell death in the murine breast 4T1, human prostate DU145, and human myelomonocytic U937 cancer cell lines. Systemic toxicity was assessed ex vivo using primary murine leukocyte populations. Further, cytotoxic effects of HDACi were tested in myeloid derived suppressor cell populations harvested from 4T1 tumor – bearing mice. Using flow cytometry and fluorescence microscopy, we demonstrate that FK228 and Panobinostat induced apoptosis and cell cycle dysregulation of cancer cell populations in vitro. Ongoing studies are elucidating the mechanisms of cell death and determining the off target effects of these HDACi.