cAMP exerts an antiproliferative effect on a number of cell types including lymphocytes. This effect of cAMP is proposed to be mediated by its ability to inhibit G1/S transition. In this report, we provide evidence for a new mechanism whereby cAMP might inhibit cellular proliferation. We show that elevation of intracellular levels of cAMP inhibits DNA replication and arrests the cells in S phase. The cAMP-induced inhibition of DNA synthesis was associated with the increased binding of p21Cip1 to Cdk2-cyclin complexes, inhibition of Cdk2 kinase activity, dephosphorylation of Rb, and dissociation of PCNA from chromatin in S phase cells. The ability of cAMP to inhibit DNA replication and trigger release of PCNA from chromatin required Rb and p21Cip1 proteins, since both processes were only marginally affected by increased levels of cAMP in Rb Ϫ/Ϫ and p21 Cip1Ϫ/Ϫ 3T3 fibroblasts. Importantly, the implications of cAMP-induced inhibition of DNA synthesis in cancer treatment was demonstrated by the ability of cAMP to reduce apoptosis induced by S phase-specific cytotoxic drugs. Taken together, these results demonstrate a novel role for cAMP in regulation of DNA synthesis and support a model in which activation of cAMP-dependent signaling protects cells from the effect of S phase-specific antitumor agents. INTRODUCTIONThe second messenger cyclical AMP (cAMP) plays an important role in regulation of numerous cellular functions. Elevation of intracellular cAMP levels either stimulates or inhibits proliferation of cells depending on the cell type (Pastan et al., 1975;Dumont et al., 1989). Generally, cAMP has an antiproliferative effect on most cells of mesenchymal origin (Blomhoff et al., 1988;Dugan et al., 1999). We have observed that increase in intracellular cAMP leads to accumulation of lymphoid cells in the G1 phase and correlates with rapid dephosphorylation of the retinoblastoma tumor suppressor protein (Rb;Blomhoff et al., 1987Blomhoff et al., , 1988Christoffersen et al., 1994;Naderi and Blomhoff, 1999;Gutzkow et al., 2002). Furthermore, this antiproliferative effect of cAMP in lymphocytes has been shown to be mediated through protein kinase A type I (Skalhegg et al., 1992;Tasken et al., 1994).Rb, is an important inhibitor of cell cycle progression (Wang et al., 1994;Harbour and Dean, 2000). In its hypophosphorylated active form, Rb inhibits transition of cells from G1 into S phase of the cell cycle. In addition, Rb has also been shown to negatively regulate DNA replication and block S phase progression (Chew et al., 1998;Knudsen et al., 1998). The negative effect of Rb on cell cycle progression is countered by its phosphorylation by cyclin-dependent kinases (Cdks). When complexed to their regulatory cyclin subunits, Cdks phosphorylate and thereby inactivate the antiproliferative function of Rb (Mittnacht, 1998). Specifically, inactivation of Rb in G1 is achieved through its sequential phosphorylation by Cdk4/6-cyclin D, Cdk2-cyclin E, and Cdk2-cyclin A complexes (Zarkowska and Mittnacht, 1997;Lundberg and Weinberg...

Growth arrest induced by DNA damage in mammalian cells requires the function of the retinoblastoma tumor suppressor protein (RB). RB-deficient cells cannot undergo G1, mid-S or G2 arrest following DNA damage, although they can activate the G2 checkpoint, which is reversible. RB-deficient cells are also hypersensitive to DNA damage-induced apoptosis. Induction of apoptosis in RB wild-type cells is associated with the loss of RB protein through cleavage by caspase. Two substitution mutations in exon 25 of the Rb gene have been created in the mouse germline to generate the Rb-MI allele that codes for a caspase-resistant RB protein. The RB-MI protein desensitizes cells to apoptosis. Taken together, these results suggest that RB plays a critical role in determining the cell fate following DNA damage. Growth arrest is dependent on RB and apoptosis is activated following RB degradation.