Abstract:
Malignant transformation of a cell is believed to involve the altered expression of specific genes involved in growth regulatory processes. This altered expression could be due to modification at the gene level or at the level of chromatin structure. It has been demonstrated that NIH-3T3 cells transformed with the ras oncogene have a more decondensed chromatin structure than the parental cells. Histone H1 and H3 are both required for stabilization of the chromatin fiber and phosphorylation of H1 and/or H3 could lead to chromatin decondensation which may function as a mechanism to regulate gene expression. We provide evidence that the level of phosphorylated H1b (pH1b) is elevated in cells that are transformed with 'ras, raf, fes, mos', or ' myc' oncogenes. We demonstrate that pH1b is localized near nuclear sites of transcription and splicing and phosphorylation of H1b is dependent upon ongoing transcription and replication. It has been reported that stimulation of serum starved mouse fibroblasts with growthfactors or phorbol esters results in a rapid phosphorylation of histone H3 that is concurrent with the transcriptional activation of c-' fos' and c-'jun'. We provide evidence that Ser-10 of H3 is the site phosphorylated in response to EGF (epidermal growth factor) or phorbol ester and TPA (12-O-tetradecanoylphorbol 13-acetate) stimulation. In addition, we demonstrated that pH3 is associated with TPA induced c-' fos' and c-'myc' genes. We observed an increased level of pH3 in oncogene-transformed cells that was not due to differences in cell cycle distributions. Furthermore, in an inducible-'ras' cell line we observed that induction of ' ras' oncogene expression resulted in increased levels of both pH1b and pH3. The H1b and H3 phosphatase activities of 'ras'-transformed (C3) and parental (10T[one half]) cells was similar, however, the activity of CDK2 (a candidate H1 kinase) appears to be elevated in the C3 cells. We found that the major H1b and H3 phosphatase in 10T[one half] cells is PP1 (protein phosphatase 1). We propose that the increased levels of the phosphorylated forms of H1b and H3 in the oncogene-transformed cells is a result of increased H1b and H3 kinase activities in these cells. Increased levels of pH1b and pH3 could lead to decondensation of chromatin and aberrant expression of genes (for example c-'fos' and c-'myc') thereby facilitating the process of malignant transformation.
