Heterologous expression of the transcriptional regulator escargot inhibitsmegakaryocytic endomitosis

Certain cell types escape the strict mechanisms imposed on the majority of somatic cells to ensure the faithful inheritance of parental DNA content. T his is the case in many embryonic tissues and certain adult cells such as m ammalian hepatocytes and megakaryocytes. Megakaryocytic endomitosis is char acterized by repeated S phases followed by abortive mitoses, resulting in m ononucleated polyploid. cells. Several cell cycle regulators have been prop osed to play an active role in megakaryocytic polyploidization; however, li ttle is known about upstream factors that could control endomitosis. Here w e show that ectopic expression of the transcriptional repressor escargot in terferes with the establishment of megakaryocytic endomitosis. Phorbol este r-induced polyploidization was inhibited in stably transfected megakaryobla stic HEL cells constitutively expressing escargot. Analysis of the expressi on and activity of different cell cycle factors revealed that Escargot affe cts the G(1)/S transition by influencing Cdk2 activity and cyclin A transcr iption. Nuclear proteins that specifically bind the Escargot-binding elemen t were detected in endomitotic and non-endomitotic megakaryoblastic cells, but down-regulation occurred only during differentiation of cells that beco me polyploid. As Escargot was originally implicated in ploidy maintenance o f Drosophila embryonic and larval cells, our results suggest that polyploid ization in megakaryocytes might respond to mechanisms conserved from early development to adult cells that need to escape normal control of the diploi d state.