Effect of dibutyryl cyclic AMP on the cyclin-dependent kinase inhibitor p27(Kip1) in the human hepatoma cells PLC/PRF/5

The cyclin-dependent kinase (cdk) inhibitor p27(Kip1) is known to play a role in cell-cycle regulation at GI and G1/S phase. We investigated the effect of the putative growth-inhibiting agent dibutyryl cyclic AMP (DBcAMP) on the serial changes of p27(Kip1) expression in the human hepatoma cells PLC/PRF/5 in culture. The p27(Kip1) protein level increased at an early stage of G1 phase (2 hours) after a release from serum-starvation and subsequently maintained the level until the entry to S phase, whereas an addition of DBcAMP at 1mM increased the p27(Kip1) protein level during G1 phase. In contrast the relative expression levels of p27(Kip1) mRNA at 2 hours, 4 hours and 6 hours were lower in DBcAMP-added cells. The effects of DBcAMP on cell growth were, reduction of S-phase cells, inhibition of DNA synthesis, and accumulation of G2-phase cells. rn the presence of the antisense oligodeoxynucleotides against p27(Kip1) mRNA, DBcAMP-induced growth inhibition was partially abolished. These findings suggest that DBcAMP elevates p27(Kip1) protein expression during GI phase, which could be associated with growth inhibition. DBcAMP may inhibit the degradation of p27(Kip1) protein.