ANCHORAGE-DEPENDENT TRANSCRIPTION OF THE CYCLIN-A GENE

NIH 3T3 cells cultured in suspension fail to express cyclin A and henc e cannot enter S phase and divide, We show that loss of cell adhesion to substratum abrogates cyclin A gene expression by blocking its promo ter activity through the E2F site that mediates its cell cycle regulat ion in adherent cells, In suspended cells, G(0)-specific E2F complexes remain bound to the cyclin A promoter, Overexpression of cyclin D1 re stores cyclin A transcription in suspended cells and rescues them from cell cycle arrest, In suspended cells, cyclin D1 and cyclin E accumul ate normally upon serum stimulation, but their associated kinases rema in inactive; their substrates, pRb and p107, are not hyperphosphorylat ed. Concomitantly, the cyclin-dependent kinase Inhibitor, p27/(KIP1), is stabilized, Ectopic expression of p27/(KIP1) blocks cyclin A promot er activity through its E2F binding site, These data suggest that the block to cyclin A transcription in nonadherent NIH 3T3 cells results f rom stabilization of p27/(KIP1) and subsequent inactivation of the spe cific E2F moiety required for its induction.