SUPPRESSION OF CYCLIN-DEPENDENT KINASE-4 DURING INDUCED-DIFFERENTIATION OF ERYTHROLEUKEMIA-CELLS

Differentiation of murine erythroleukemia cells induced by hexamethyle ne bisacetamide (HMBA) is associated with accumulation of underphospho rylated retinoblastoma protein (pRB) and an increase in retinoblastoma (RB) gene expression. Here we show that HMBA causes a rapid decrease in the level of cyclin-dependent kinase 4 (cdk4) protein. This decreas e results from decreased stability of the protein, while the rate of s ynthesis of the protein is not affected by HMBA. The decrease in the l evel of cdk4 protein is followed by suppression of the pRB kinase acti vity associated with cdk4. Cyclin D3, which can bind and activate cdk4 , is increased in HMBA-induced cells and is found in complex with pRB and the transcription factor E2F. In uninduced cells cyclin D3 complex es with pRB and E2F are barely detected. At the later stages of differ entiation, MEL cells become arrested in G(1) and cdk2 kinase activity is suppressed; this is accompanied by a decrease in the level of cycli n A and cdk2 proteins. Cells transfected with cdk4, which continue to overexpress cdk4 protein during culture with HMBA, are resistant to HM BA-induced differentiation. In contrast, overexpression of cdk2 protei n does not inhibit induced differentiation. These findings suggest tha t suppression of cdk4 is a critical event in the pathway leading to te rminal differentiation of erythroleukemia cells.