Phosphodiesterase 4 specific inhibitors control cell growth of human hepatoma HepG2 cells

Cyclic nucleotides, as part of the signal transduction system, play a key role in coordinating many cellular functions. eir involvement in the control of cell proliferation is widely recognized. Cyclic nucleotide levels are regulated by the action of cyclases as biosynthetic enzymes, and of phosphodiesterases (PDEs) as degradative enzymes. It is well known that in different cell types the increase of the intracellular concentration of cyclic nucleotides leads to many physiological changes, including blockade of cell proliferation and induction of cell differentiation. In this study, we investigated the effects of PDE inhibitors on the growth of human hepatoma HepG2 cells. Previous experiments showed a significant control of cell proliferation with the use of PDE4 inhibitors, such as rolipram and DCTA-46. Treatment of HepG2 cells with 100 μM rolipram or 1 μM DC-TA-46 caused a marked increase of cAMP, 6 and 2.7 fold respectively. e effects of a permanent increase of cyclic nucleotides were also investigated in these cells using dbcAMP as a membrane permeant-unhydrolysed cAMP analogue. Dose-response experiments showed dose- and time-dependent effects of PDE-inhibitors on cell growth. e concentrations of inhibitors that halved cell numbers after 96 h of treatment were used for subsequent experiments and were: 1 μM for DC-TA-46, 100 μM for rolipram, and 1mM for dbcAMP. Treatment with rolipram and especially with dbcAMP induced apoptosis as demonstrated by DNA fragmentation following DAPI staining and cytofluorimetric assays. In addition, all the inhibitors produced a decrease in cyclin D1 that was highly significant in dbcAMP-treated cells; dbcAMP also caused a slight decline of cyclin A. e results demonstrate an involvement of PDE inhibitors in cell cycle control and suggest that they might be useful as potential chemotherapeutic and/or chemopreventive agents in hepatocellular carcinoma.