¹Departamento de Bioquímica, Instituto de Investigaciones Biomédicas 'A. Sols', Universidad Autónoma-CSIC, Arturo Duperier, 4. 28029 Madrid, Spain

²Institute of Molecular Medicine, John Radcliffe Hospital, Headington, Oxford, OX3 9DS, UK

Correspondence to: C Calés, Departamento de Bioquímica, Instituto de Investigaciones Biomédicas 'A. Sols', Universidad Autónoma-CSIC, Arturo Duperier, 4. 28029 Madrid, Spain

^aP García and J Frampton contributed equally to this work

Megakaryocytes become polyploid by entering a truncated cell cycle, consisting of alternate S phases and abortive mitoses. We have investigated the regulation of the G1/S transition by comparing two megakaryoblastic cell lines, HEL and K562, which respectively do or do not become polyploid in response to phorbol esters. A pronounced downregulation of cyclin A, and to a lesser extent of cyclin E, occurred in K562 cells during the first 24 h after TPA treatment, in contrast with re-replicating HEL cells, in which both cyclins were present in individual G2/M cells. Transactivation experiments suggested that the absence of cyclin A in differentiated K562 cells could be due to a TPA-mediated inhibition of its transcription. To investigate the potential role of cyclin E in the establishment of re-replication cycles, we isolated K562 clones constitutively expressing cyclin E. The resulting clones, and also K562 cells transiently expressing cyclin E, entered re-replication cycles when treated with TPA. The transcriptional activity of the cyclin A promoter was not inhibited after TPA treatment, and although the levels of cyclin A fluctuated during further re-replication cycles, they never decreased below S phase levels. We conclude that the presence of cyclin E in megakaryoblastic G2/M cells determines cyclin A expression and allows the entrance into an extra S phase. Oncogene (2000) 19, 1820-1833

cyclins; cell cycle; re-replication; megakaryocytic differentiation; transcriptional regulation