Replication-dependent canonical histone messenger RNAs (mRNAs) do not terminate with a poly(A) tail at the 3′ end. We previously demonstrated that exposure to arsenic, an environmental carcinogen, induces polyadenylation of canonical histone H3.1 mRNA, causing transformation of human cells in vitro. Here we report that polyadenylation of H3.1 mRNA increases H3.1 protein, resulting in displacement of histone variant H3.3 at active promoters, enhancers, and insulator regions, leading to transcriptional deregulation, G2/M cell-cycle arrest, chromosome aneuploidy, and aberrations. In support of these observations, knocking down the expression of H3.3 induced cell transformation, whereas ectopic expression of H3.3 attenuated arsenic-induced cell transformation. Notably, arsenic exposure also resulted in displacement of H3.3 from active promoters, enhancers, and insulator regions. These data suggest that H3.3 displacement might be central to carcinogenesis caused by polyadenylation of H3.1 mRNA upon arsenic exposure. Our findings illustrate the importance of proper histone stoichiometry in maintaining genome integrity.

The use of electronic-cigarettes (e-cigs) has increased substantially in recent years, particularly among the younger generations. Liquid nicotine is the main component of e-cigs. Previous studies have shown that mice exposed to e-cig aerosols developed lung adenocarcinoma and bladder hyperplasia. These findings implicated a potential role for e-cig aerosols and nicotine in cancer development, although the underlying mechanisms are not fully understood. Here we report that exposure to liquid nicotine or nicotine aerosol generated from e-cig induces downregulation of Stem-loop binding protein (SLBP) and polyadenylation of canonical histone mRNAs in human bronchial epithelial cells and in mice lungs. Canonical histone mRNAs typically do not end in a poly(A) tail and the acquisition of such a tail via depletion of SLBP has been shown to causes chromosome instability. We show that nicotine-induced SLBP depletion is reversed by an inhibitor of α7-nicotinic acetylcholine receptors (α7-nAChR) or siRNA specific for α7-nAChR, indicating a nAChR-dependent reduction of SLBP by nicotine. Moreover, PI3K/AKT pathway is activated by nicotine exposure and CK2 and probably CDK1, 2 kinases well known for their function for SLBP phosphorylation and degradation, are shown to be involved, α7-nAChR-dependently, in nicotine-induced SLBP depletion. Importantly, nicotine-induced anchorage-independent cell growth is attenuated by inhibition of α7-nAChR and is rescued by overexpression of SLBP. We propose that the SLBP depletion and polyadenylation of canonical histone mRNAs via activation of α7-nAChR and a series of downstream signal transduction pathways are critical for nicotine-induced cell transformation and potential carcinogenesis.

H3K56 acetylation (H3K56Ac) was first identified in yeast and has recently been reported to play important roles in maintaining genomic stability, chromatin assembly, DNA replication, cell cycle progression and DNA repair. Although H3.1K56Ac has been relatively well studied, the function of H3.3K56Ac remains mostly unknown in mammals. In this study, we used H3.3K56Q and H3.3K56R mutants to study the possible function of H3.3K56 acetylation. The K-to-Q substitution mimics a constitutively acetylated lysine, while the K-to-R replacement mimics a constitutively unmodified lysine. We report that cell lines harbouring mutation of H3.3K56R exhibit increased cell death and dramatic morphology changes. Using a Tet-Off inducible system, we found an increased population of polyploid/aneuploid cells and decreased cell viability in H3.3K56R mutant cells. Consistent with these results, the H3.3K56R mutant had compromised H3.3 incorporation into several pericentric and centric heterochromatin regions we tested. Moreover, mass spectrometry analysis coupled with label-free quantification revealed that biological processes regulated by the H3.3-associating proteins, whose interaction with H3.3 was markedly increased by H3.3K56Q mutation but decreased by H3.3K56R mutation, include sister chromatid cohesion, mitotic nuclear division, and mitotic nuclear envelope disassembly. These results suggest that H3.3K56 acetylation is crucial for chromosome segregation and cell division in mammals.