Cyclic Nucleotides Suppress Tumor Necrosis Factor α-Mediated Apoptosis by Inhibiting Caspase Activation and Cytochrome cRelease in Primary Hepatocytes via a Mechanism Independent of Akt Activation
Cyclic nucleotides have been previously shown to modulate cell death processes in many cell types; however, the mechanisms by which cyclic nucleotides regulate apoptosis are unclear. In this study, we demonstrated that cAMP as well as cGMP analogs suppressed tumor necrosis factor alpha (TNFalpha) plus actinomycin D (ActD)-induced apoptosis in a dose-dependent manner in cultured primary hepatocytes. Furthermore, forskolin, which increases intracellular cAMP levels, also effectively suppressed TNFalpha+ActD-induced apoptosis. Activation of multiple caspases was suppressed in cells exposed to TNFalpha+ActD in the presence of cAMP or cGMP analogs. TNFalpha+ActD-induced cytochrome c release from mitochondria was also inhibited by cAMP or cGMP, reinforcing our conclusion that cyclic nucleotides interfere with the early signaling events of TNFalpha-mediated apoptosis. We evaluated the possibility that cAMP and cGMP inhibit apoptosis by activating the serine/threonine kinase Akt, which is known to promote cell survival. Both cAMP- and cGMP-elevating agents led to marked increases in Akt activation that was inhibited by the phosphatidylinositol 3'-kinase inhibitors, LY294002 and wortmannin. However, complete inhibition of cyclic nucleotide-induced Akt activation had little effect on cyclic nucleotide-mediated cell survival, indicating the existence of other survival pathways. Interestingly, the specific inhibitor of protein kinase A (PKA), KT5720, blocked cGMP-mediated protection but only partially prevented the anti-apoptotic effect of cAMP, indicating that both PKA-dependent and -independent mechanisms are involved in cAMP-mediated suppression of apoptosis signaling. Our data suggest that multiple survival signaling pathways coexist in cells and that cyclic nucleotides delay apoptosis by interfering with apoptosis signaling by both PKA-dependent and -independent mechanisms.