Analysis of arsenic-induced gene expression in peripheral blood mononuclear cells (PMBCs) demonstrated a profound increase in the histone cluster gene expression: of the 50 most upregulated arsenic induced genes, 22 were replication-dependent canonical histone genes. The canonical histone genes are the only genes in multicellular organisms whose messenger RNA (mRNA) does not terminate at the 3' end with a poly (A) tail. Intriguingly, arsenic exposure induced polyadenylation of the canonical histone mRNA. This was accompanied by an increase in histone protein expression and a depletion of stem loop binding protein (SLBP). SLBP is a key factor in processing replication-dependent canonical histones pre-mRNA. It has been shown that depletion of SLBP results in histone mRNA misprocessing, generating canonical histone mRNAs with poly (A) tails. The mRNA coding for SLBP was also decreased in various cell types following arsenic exposure, whereas none of the other factors needed to process canonical histone mRNA were altered, suggesting that the reduction of SLBP expression is the major cause of arsenic-induced polyadenylation of canonical histone mRNA and the increase in histone protein expression. The addition of the poly (A) tail to the canonical histone mRNA will increase the mRNA stability, allowing for the polyadenylated histones to be present not only in the S phase, but in other phases of the cell cycle as well. In fact, after arsenic treatment canonical histone H3 with a poly (A) tail was 3- fod higher during mitosis compared to untreated cells. These effects of arsenic could be very disruptive to nucleosome assembly and transcription and may be involved in mediating arsenic carcinogenesis. Our research has focused on metals, epigenetics and cancer, with an emphasis on metal induced changes in histone modifications and how they impact transcription. However, we have never observed any metal that causes such a profound effect on the induction of histone genes and at such low concentrations (0.1-0.5 M). In this project, we will investigate how arsenic exposure results in the loss of SLBP, focusing on the activation of a phosphorylation dependent degradation process, and epigenetic changes in the SLBP promoter. We will also determine the consequences of arsenic-induced acquisition of poly (A) containing canonical histone H3 in terms of nucleosome assembly, transcription, cell cycle, and genomic stability. In addition, we will examine the impact of depletion and re-expression of SLBP in the absence and presence of arsenic exposure on cell transformation. Finally, we will study whether arsenic exposure in mice induces a loss of SLBP, increases poly (A) containing canonical histone mRNA and histone protein in vivo.
