Reprogramming leukemic cells to terminal differentiation by inhibiting specific cyclin-dependent kinases in G1

Some tumor cells can be stimulated to differentiate and undergo terminal cell division and loss of tumorigenicity. The in vitro differentiation of murine erythroleukemia (MEL) cells is a dramatic example of tumor-cell reprogramming. We found that reentry of MEL cells into terminal differentiation is accompanied by an early transient decline in the activity of cyclin-dependant kinase (CDK) 2, followed by a decline of CDK6. Later, as cells undergo terminal arrest, CDK2 and CDK4 activities decline. By analyzing stable MEL-cell transfectants containing vectors directing inducible expression of specific CDK inhibitors, we show that only inhibitors that block the combination of CDK2 and CDK6 trigger differentiation. Inhibiting CDK2 and CDK4 does not cause differentiation. Importantly, we also show that reprogramming through inhibition of CDKs is restricted to G(1) phase of the cell cycle. The results imply that abrogation of normal cell-cycle controls in tumor cells contributes to their inability to differentiate fully and that restoration of such controls in G(1) can lead to resumption of differentiation and terminal cell division. The results also indicate that CDK4 and CDK6 are functionally distinct and support our hypothesis that the two CDKs regulate cell division at different stages of erythroid maturation.