Disruptive cell cycle regulation involving epigenetic downregulation of Cdkn2a (p16(Ink4a)) in early-stage liver tumor-promotion facilitating liver cell regeneration in rats

Cell cycle aberration was immunohistochemically examined in relation to preneoplastic liver cell foci expressing glutathione S-transferase placental form (GST-P) at early stages of tumor-promotion in rats with thioacetamide (TAA), a hepatocarcinogen facilitating liver cell regeneration. Immunoexpression of p16(Ink4a) following exposure to other hepatocarcinogens/promoters and its DNA methylation status were also analyzed during early and late tumor-promotion stages. GST-P(+) liver cell foci increased cell proliferation and decreased apoptosis when compared with surrounding liver cells. In concordance with GST-P(+) foci, checkpoint proteins at G(1)/S (p21(Cip1), p27(Kip1) and p16(Ink4a)) and G(2)/M (phospho-checkpoint kinase 1, Cdc25c and phospho-Wee1) were either up- or downregulated. Cellular distribution within GST-P(+) foci was either increased or decreased with proteins related to G(2)-M phase or DNA damage (topoisomerase IIα, phospho-histone H2AX, phospho-histone H3 and Cdc2). In particular, p16(Ink4a) typically downregulated in GST-P(+) foci and regenerative nodules at early tumor-promotion stage with hepatocarcinogens facilitating liver cell regeneration and in neoplastic lesions at late tumor-promotion stage with hepatocarcinogens/promoters irrespective of regenerating potential. Hypermethylation at exon 2 of Cdkn2a was detected at both early- and late-stages. Thus, diverse disruptive expression of G(1)/S and G(2)/M proteins, which allows for clonal selection of GST-P(+) foci, results in the acquisition of multiple aberrant phenotypes to disrupt checkpoint function. Moreover, increased DNA-damage responses within GST-P(+) foci may be the signature of genetic alterations. Intraexonic hypermethylation may be responsible for p16(Ink4a)-downregulation, which facilitates cell cycle progression in early preneoplastic lesions produced by repeated cell regeneration and late-stage neoplastic lesions irrespective of the carcinogenic mechanism.