Ectopic expression of cyclin E allows non-endomitotic megakaryoblastic K562 cells to establish re-replication cycles

Megakaryocytes become polyploid by entering a truncated cell cycle, consist ing of alternate S phases and abortive mitoses, We have investigated the re gulation of the G1/S transition by comparing two megakaryoblastic cell line s, HEL and K562, which respectively do or do not become polyploid in respon se to phorbol esters, A pronounced downregulation of cyclin A, and to a les ser 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 cyc lins were present in individual G2/M cells, Transactivation experiments sug gested that the absence of cyclin A in differentiated K562 cells could be d ue to a TPA-mediated inhibition of its transcription. To investigate the po tential role of cyclin E in the establishment of re-replication cycles, we isolated K562 clones constitutively expressing cyclin E, The resulting clon es, and also K562 cells transiently expressing cyclin E, entered re-replica tion cycles when treated with TPA, The transcriptional activity of the cycl in A promoter was not inhibited after TPA treatment, and although the Level s of cyclin A fluctuated during further re-replication cycles, they never d ecreased 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.