K562 cells resistant to phorbol 12-myristate 13-acetate-induced growth arrest: dissociation of mitogen-activated protein kinase activation and Egr-1 expression from megakaryocyte differentiation.

The K562 cell line undergoes megakaryocytic differentiation in response to phorbol 12-myristate 13-acetate (PMA) stimulation. This event correlates with mitogen-activated protein kinase activation, cell cycle arrest, and expression of the Egr-1 transcription factor. We have isolated K562 cells that are resistant to the growth-inhibitory action of PMA. Molecular characterization demonstrates that PMA resistance is downstream from PMA-induced activation of the mitogen-activated protein kinase pathway. Although the levels of Egr-1 expression and cyclic AMP-responsive element-binding protein phosphorylation are comparable in wild-type and PMA-resistant clones in response to PMA, the expression of megakaryocytic cell surface marker CD41 is detected only in the wild-type cells. The lack of differentiation of the PMA-resistant clones correlates with a failure of the PMA-treated cells to induce dephosphorylation and down-regulation of the retinoblastoma protein. These cells may provide a useful model system to distinguish those events that are connected to cell cycle arrest from those involved in the differentiation program initiated by PMA.