Truncation, deamidation, and oxidation of histone H2B in cells cultured with nickel(II)

Molecular mechanisms of nickel-induced carcinogenesis include interactions of Ni(II) cations with histones. Previously, we demonstrated in vitro and in cells that Ni(II) cleaved off the -SHHKAKGK C-terminal motif of histone H2A. In the present study, Western blotting of histones isolated from rat and human cell lines, cultured for 3-5 days with 0.05-0.5 mM Ni(II), revealed time- and dose-dependent appearance of a new band of histone H2B. This effect was also induced by Co(II), but not by Cu(II), Cd(II), and Zn(II). Mass spectrometry and amino acid sequencing of proteins from the new band allowed for identification of two derivatives of the major variant of histone H2B. The larger protein was histone H2B lacking 16 N-terminal amino acids. The smaller one was histone H2B which, in addition to being shortened at the N-terminus, had nine amino acids deleted from its C-terminus. At both termini, the truncation occurred between lysine and alanine in the two identical -KAVTK- repeats of histone H2B. Also, the truncated H2B proteins had their Q22 residues deamidated and M59 and M62 residues oxidized to sulfoxides, a signature of oxidative stress. The truncation did not concur with apoptosis. Its mechanism involved activation by Ni(II) treatment of specific nuclear proteolytic enzymes belonging to the calpain family. The terminal tails of core histones participate in structuring chromatin and regulating gene expression. Therefore, the observed truncation and other modifications of histone H2B may assist in Ni(II) carcinogenesis through epigenetic mechanisms.