INHIBITION OF GRANULOCYTE DIFFERENTIATION BY G(1) CYCLINS D2 AND D3 BUT NOT D1

Growth factor-induced signals govern the expression of three D-type cyclins, which, in turn, function as regulatory subunits of cyclin-dependent kinases (cdks) to control cell cycle transitions during the late G1 interval. 32D myeloid cells, which self-renew as uncommitted precursors in interleukin 3 (IL-3), express cyclins D2 and D3 (but not D1) in complexes with cdk4 and cdk2. When transferred to granulocyte colony-stimulating factor (G-CSF), 32D cells stop dividing and terminally differentiate to mature neutrophils. Cyclin D and cdk4 expression ceased as cells underwent growth arrest in G-CSF, but cdk2 levels were sustained. 32D cells engineered to ectopically express D-type cyclins exhibited contracted G1 intervals with a compensatory lengthening of S phase but remained IL-3 dependent for cell growth; those overexpressing cyclins D2 and D3 (but not D1) were unable to differentiate and died in G-CSF. Cyclin D2 mutants, which cannot efficiently bind to, or functionally interact with, the retinoblastoma protein (pRb) or its relatives (p107) did not block differentiation. Conversely, the introduction of a catalytically inactive cdk4 mutant into cells overexpressing cyclin D2 restored their G-CSF response. The persistence of cdk2 and its predilection to functionally interact with cyclins D2 and D3 rather than D1 might explain the specificity of the differentiation blockade.