Cell cycle dependent phosphorylation and subnuclear organization of the histone gene regulator p220(NPAT) in human embryonic stem cells

The histone gene cell cycle regulator HiNF-P is a unique zinc finger transcription factor with a novel conserved auxiliary DNA-binding motif

Accumulation of histone proteins is necessary for packaging of replicated DNA during the S phase of the cell cycle. Different mechanisms operate to regulate histone protein levels, and induction of human histone gene expression at the G1-S phase transition plays a critical role. The zinc finger HiNF-P and coactivator p220 (NPAT) proteins are key regulators of histone gene expression. Here, we describe a novel HiNF-P-specific conserved region (PSCR) located within the C-terminus that is present in HiNF-P homologues of all metazoan species that have been examined. The PSCR motif is required for activation of histone H4 gene transcription and contributes to DNA binding of HiNF-P. Thus, the PSCR module represents an auxiliary DNA-binding determinant that plays a critical role in mediating histone gene expression during the cell cycle and defines HiNF-P as a unique cell cycle regulatory member of the zinc finger transcription factor family.

Expression and functional analysis of G1 to S regulatory components reveals an important role for CDK2 in cell cycle regulation in human embryonic stem cells

One of the characteristic features of human embryonic stem cells (hESCs) is the competence for self-renewal and pluripotency. To date, little is known about cell cycle regulation in these cells and how the cell cycle machinery influences hESCs properties. A common feature of human, murine and primate ESCs is the presence of a short G1 phase, which has been viewed as a time window during which stem cells are exposed to differentiation signals. We used the hESCs differentiation model and comparisons to human embryonic carcinoma (EC) cells to study the key regulators of G1 to S transition in hESCs. Our studies show that hESCs express all G1-specific CYCLINs (D1, D2, D3 and E) and cyclin-dependent kinases (CDK) (CDK2, CDK4 and CDK6) at variable levels. In contrast to murine ESCs, most of the cell cycle regulators in hESCs show cell cycle-dependent expression, thus revealing important differences in the expression of cell cycle regulatory components between these two embryonic cell types. Knockdown of CDK2 using RNA interference resulted in hESCs arrest at G1 phase of the cell cycle and differentiation to extraembryonic lineages. This suggests an important role for CDK2 in cell cycle regulation in hESCs that are likely to bear significant impacts on the maintenance of their pluripotent phenotype.