INDUCTION OF POLYPLOIDIZATION IN THE HUMAN ERYTHROLEUKEMIA CELL-LINE (HEL) BY PROTEIN-KINASE INHIBITOR (K252A) AND THE PHORBOL-ESTER TPA

Endomitosis (polyploidization) is a distinctive feature of megakaryocy te differentiation. We examined this mechanism in an erythromegakaryoc ytic cell line, HEL, using a protein kinase inhibitor K252a or a phorb ol-ester TPA. HEL cells treated with K252a showed a marked increase in the proportion of CD41 positive cells and polyploid cells as well as in cellular size and nuclear size. TPA showed similar results but indu ced multi-nucleation instead of enlarge ment of nuclear size. K252a ad ded at the G1/S boundary phase did not inhibit the first and second ro und DNA synthesis, but inhibited cell division. K252a did not inhibit the expression of genes involved in mitosis such as cyclin B, cdc25B a nd cdc2, in the first round S phase. However, the cyclin B associated Cdc2 kinase activity needed for mitosis during the G2/M phase was redu ced by K252a. TPA delayed DNA synthesis and expression of these genes, and suppressed Cdc2 kinase activity in the second round G2/M phase. T hese results suggest that the polyploidization induced by K252a result s from inhibiting mitosis possibly caused by suppression of Cdc2 kinas e activity. TPA may induce the multi-nucleation through a different me chanism.