DIFFERENTIAL REGULATION OF CELL-CYCLE MACHINERY BY VARIOUS ANTIPROLIFERATIVE AGENTS IS LINKED TO MACROPHAGE ARREST AT DISTINCT G1 CHECKPOINTS

There is currently much interest in the mechanisms of action of antipr oliferative agents and their effects on cell cycle machinery. In the p resent study we examined the mechanisms of action of four unrelated ag ents known to inhibit proliferation of CSF-1-stimulated bone marrow-de rived macrophages (BMM). We report that 8-bromo-cAMP (8Br-cAMP) and li popolysaccharide (LPS) potently reduced CSF-1-stimulated cyclin D1 pro tein, and cyclin-dependent kinase (cdk) 4 mRNA and protein levels, whi le the inhibitory effects of the Na+/ H+ antiport inhibitor 5-(N',N'-d imethyl) amiloride (DMA) and interferon gamma (IFN gamma) were only we ak. All agents repressed CSF-1-stimulated retinoblastoma protein phosp horylation. Furthermore, 8Br-cAMP and to a lesser extent IFN gamma, al so reduced CSF-1-stimulated levels of E2F DNA binding activity in a ma crophage cell line, BAC1.2F5. An explanation for the different effects of the agents is that 8Br-cAMP and LPS were found to arrest BMM in ea rly/mid-G1, while IFN gamma and DMA arrested cells in late G1 or early S phase. These data indicate that (1) different antiproliferative age nts can arrest the same cell type at distinct checkpoints in G1 and (2 ) effects of antiproliferative agents on cell cycle machinery is Linke d to the position at which they arrest cells in G1.