Activin A-induced differentiation in K562 cells is associated with a transient hypophosphorylation of RB protein and the concomitant block of cell cycle at G sub(1) phase

The human erythroleukemic cell line, K562, can be induced to differentiate by the addition of activin A, a newly purified protein belonging to the TGF- beta sub(1) family. The present studies used flow cytometric cell cycle analysis, indirect immunofluorescence staining of the proliferating cell nuclear antigen (PCNA), and thymidine incorporation assay of cell proliferation to study the effects of activin A on the cell cycle during differentiation in K562 cells. Activin A-treated K562 cells were found to undergo a transient block in cell cycle, temporarily halting progression from G sub(1) to S phase. The latter can be observed after approximately 24 hr of incubation with activin A and then disappears after this early stage of induction of differentiation. Cell cycle kinetics analysis using synchronized K562 cells also confirms that in the presence of activin A, K562 cells progress normally through various phases of cell cycle, except that there is prolongation of the G sub(1) phase between 10 to 24 hr of culture. Furthermore, this transient arrest in G sub(1) is correlated with dephosphorylation of a nucleoprotein, the RB gene product, which occurs within 9-24 hr of incubation with activin A; and phosphorylation of RB protein then develops afterward.