Mpl Ligand Enhances the Transcription of the Cyclin D3 Gene: A Potential Role for Sp1 Transcription Factor

An expansion phase precedes terminal erythroid differentiation of hematopoietic progenitor cells from cord blood in vitro and is associated with up-regulation of cyclin E and cyclin-dependent kinase 2

The dynamics of cell cycle regulation were investigated during in vitro erythroid proliferation and differentiation of CD34+cord blood cells. An unusual cell cycle profile with a majority of cells in S phase (70.2%) and minority of cells in G1 phase (27.4%) was observed in burst-forming unit-erythrocytes (BFU-E)–derived erythroblasts from a 7-day culture of CD34+ cells stimulated with interleukin 3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF), Steel factor, and Epo. Terminal erythroid differentiation was accompanied by a rapid increase of G0/G1 phase cells. Expression of cyclin E and cyclin-dependent kinase 2 (cdk2) correlated with the proportion of S phase cells. Cyclin D3 was moderately up-regulated during the proliferation phase, and both cyclin E and D3 were rapidly down-regulated during terminal differentiation. This suggests that the high proliferation potential of erythroblasts is associated with temporal up-regulation of cyclin E and cdk2.

Ectopic expression of the Aspergillus nidulans mitotic inducer, nimA kinase, in megakaryocytes: effect on polyploidization

Aspergillus nidulans nimA gene encodes a serine/threonine protein kinase (NIMA) whose activity is essential for mitotic entry and chromatin condensation. Both the activity and the abundance of NIMA protein increase at the G2/M transition of the fungal cell cycle. In this study, we report the effects elicited by ectopic expression of nimA on polyploidization in a mouse megakaryocytic line, Y10, which is undergoing an endomitotic cell cycle. A pool of Y10 stable transfectants that have been induced to express nimA displayed a decrease in cell number and an elevated DNA content per cell. NIMA also dramatically enhanced the activity of phorbal 12-myristate 13-acetate toward polyploidization. Analysis of individual nimA transfectants revealed that the DNA content per cell rose in cells expressing high levels of nimA and that the level of cyclin B was reduced as compared to the mock-transfected cells. These effects observed in polyploidizing megakaryocytes are in contrast to those found in A. nidulans and HeLa cells, in which induced nimA expression caused abnormal chromatin condensation and cell cycle arrest. We conclude that high-level expression of nimA in cells programmed to undergo endomitosis could potentiate polyploidization. The challenge now resides in the isolation of the authentic megakaryocyte counterpart of the fungal nimA.