Cyclic AMP and cyclic GMP suppress TNFalpha-induced hepatocyte apoptosis by inhibiting FADD up-regulation via a protein kinase A-dependent pathway.

Cyclic AMP (cAMP) and cyclic GMP (cGMP) suppress apoptosis in many cell types, including hepatocytes. We have previously shown that membrane-permeable cAMP and cGMP analogs attenuate tumor necrosis factor alpha plus actinomycin D (TNFalpha/ActD)-induced apoptosis in hepatocytes at a step upstream of caspase activation and cytochrome c release. Recently we have also shown that FADD levels increase 10 folds in response to TNFalpha/ActD. Therefore we hypothesized that cAMP and cGMP would inhibit FADD upregulation. We show here that cyclic nucleotide analogs dibutyryl cAMP (db-cAMP) and 8-bromo-cGMP (Br-cGMP) inhibit cell death and the cleavages of multiple caspases including caspase-10, -9, -8, -3, and -2, as well as suppress FADD protein up-regulation in TNFalpha/ActD-induced apoptosis. The inhibitory effects of cAMP were seen at lower concentrations than cGMP. Both cAMP and cGMP prevented FADD overexpression and cell death in hepatocytes transfected with the FADD gene. A protein kinase A (PKA) inhibitor, KT 5720, reversed the inhibition of FADD protein levels induced by cAMP or cGMP. In conclusion, our findings indicate that cAMP and cGMP prevent TNFalpha/ActD-induced apoptosis in hepatocytes and that this occurs in association with a near complete inhibition of the upregulation of FADD via a PKA-dependent mechanism.