Histone H1^S-3 phosphorylation in Ha-ras oncogene-transformed mouse fibroblasts

Abstract

Phosphorylation of linker histone H1^S-3 (previously named H1b) and core histone H3 is elevated in mouse fibroblasts transformed with oncogenes or constitutively active mitogen-activated protein kinase (MAPK) kinase (MEK). H1^S-3 phosphorylation is the only histone modification known to be dependent upon transcription and replication. Our results show that the increased amounts of phosphorylated H1^S-3 in the oncogene Ha-ras-transformed mouse fibroblasts was a consequence of an elevated Cdk2 activity rather than the reduced activity of a H1 phosphatase, which our studies suggest is PP1. Induction of oncogenic ras expression results in an increase in H1^S-3 and H3 phosphorylation. However, in contrast to the phosphorylation of H3, which occurred immediately following the onset of Ras expression, there was a lag of several hours before H1^S-3 phosphorylation levels increased. We found that there was a transient increase in the levels of p21^cip1, which inhibited the H1 kinase activity of Cdk2. Cdk2 activity and H1^S-3 phosphorylated levels increased after p21^cip1 levels declined. Our studies suggest that persistent activation of the Ras-MAPK signal transduction pathway in oncogene-transformed cells results in deregulated activity of kinases phosphorylating H3 and H1^S-3 associated with transcribed genes. The chromatin remodelling actions of these modified histones may result in aberrant gene expression.