Molecular mechanisms and outcomes of arsenic-induced histone acetylation and microRNA regulation in cellular transformation

Chronic exposure to arsenic causes negative health outcomes, particularly malignant neoplasms of the skin, lung and bladder. Although epidemiological data has associated arsenic exposure to cancer, a clear molecular mechanism has remained elusive. This thesis studied the impact of arsenic trioxide (ATO) exposure on histone acetylation and microRNA expression at both tolerated and toxic levels in vitro to determine an epigenetic-based mechanism of carcinogenesis. This thesis outlines a framework for identifying tolerated and toxic ATO exposures, as a prerequisite to epigenetic characterisation. Tolerated ATO exposure increased cellular survival, anchorage-independent colony formation, cell-cycle progression and proliferation in HEK293T cells. HEK293T and UROtsa cells treated with tolerated ATO exhibited global H3K9 hyperacetylation at 3 hours and global H3K9 hypoacetylation at 72 hours. This was mediated by an imbalance in the intracellular HDAC2 to PCAF mRNA expression ratio. Global H3K9 hypoacetylation occurred for both tolerated and toxic exposures, giving poor mechanistic differentiation between these separated cellular outcomes. Chromatin immunoprecipitation identified PCAF recruitment, E2F1 binding and H3K9 acetylation at the FOS proto-oncogenic promoter leading to an elevation in FOS mRNA levels at the tolerated concentration only. This thesis also reports ATO-induced chromatin relaxation in HEK293T cells followed by a return to nominal levels for the tolerated concentration. This is in contrast to the toxic exposure, which leads to clear chromatin condensation and apoptosis. This thesis postulates that arsenic-induced global H3K9 hypoacetylation is caused by a miR-372 -mediated attenuation mechanism targeting PCAF mRNA, as predicted through bioinformatic analysis. In summary, tolerated ATO exposure resulted in measurable perturbations in both global and promoter-specific histone acetylation in addition to the aberrant expression of microRNAs, which led to cellular transformation over toxicity.Open Acces